intrinsic electrophysiology of neurons in thalamorecipient layers of developing rat auditory cortex

Ž .Developmental Brain Research 115 1999 131–144www.elsevier.comrlocaterbres

Research report

Intrinsic electrophysiology of neurons in thalamorecipient layers ofdeveloping rat auditory cortex

Raju Metherate ), V. Bess AramakisDepartment of Neurobiology and BehaÕior and Center for the Neurobiology of Learning and Memory, UniÕersity of California, IrÕine, 2205 Biological

Sciences II, IrÕine, CA 92697-4550, USA

Accepted 6 April 1999

Abstract

During early postnatal life, several critical events contribute to the functional development of rat sensory neocortex. Thalamocorticalinnervation of sensory cortex is completed during the first postnatal week and extrathalamic innervation develops over the first severalweeks. In auditory cortex, acoustic-evoked potentials first occur in week 2 and develop most rapidly over weeks 2–3. Thus, rapidfunctional maturation of cortical circuits in sensory cortex occurs during the second and third postnatal weeks. The electrophysiologicalproperties of cortical neurons that receive afferent inputs during this time may play an important role in development and function. In thisstudy we examined the intrinsic electrophysiology, including spiking patterns, of neurons in layers IIrIII and IV of auditory cortex during

Žpostnatal weeks 2 and 3. Many neurons displayed characteristics consistent with previous descriptions of response classes regular.spiking, fast spiking, intrinsic bursting . In addition, we identified two groups, Rectifying and On-spiking neurons, that were characterized

Ž . Ž .by i brief spike trains in response to maintained intracellular depolarizations, and ii striking outward rectification upon depolarization.Ž . Ž .Unusually brief spike trains 1–2 spikes and short spike latencies -10 ms further distinguished On-spiking from Rectifying cells.

Biocytin labeling demonstrated that On-spiking and Rectifying cells could be either pyramidal or nonpyramidal neurons. The intrinsicphysiology of these cell groups may play an important role in auditory cortex function. q 1999 Elsevier Science B.V. All rights reserved.

Keywords: Sensory cortex; Onset cell; Intrinsic property; Postnatal development; Thalamocortical; Outward rectification

1. Introduction

A convergence of several events during the second andthird weeks after birth likely plays an important role in thedevelopment of rat sensory cortex. The first postnatal weekwitnesses the arrival of thalamocortical afferents in layer

Ž .IV of sensory cortex around postnatal days P 3–4w x8,29,56,69 . Extrathalamic afferents from the cholinergicbasal forebrain that are thought to regulate cortical devel-

w xopment 17,55 also begin to innervate the cortex at thisw xtime 6,13,36 . By the end of the first week, thalamocorti-

cal terminals have reached their adult laminar pattern anddensity, and cholinergic afferents innervate all corticallayers. Cholinergic terminations continue to mature in

w xterms of laminar patterns and density 6,36 . In the audi-tory system, functional development proceeds rapidly inpostnatal weeks 2 and 3. Acoustic stimuli first elicit imma-

w xture cochlear potentials on P8–9 12 and cortical evoked

) Corresponding author. Fax: q1-949-824-2447; E-mail:[email protected]

w xpotentials on P10–13 19,41 . The amplitude, latency andcomplexity of cortical evoked potentials change rapidlyduring the third week and then mature fully at a slower

w xrate over several weeks 19,41 . Thus, the second and thirdweeks of postnatal life witness massive synaptogenesisŽ w x.see also Ref. 1 and rapid functional maturation thatlikely involves experience-dependent formation of synapticcircuitry. As part of ongoing studies of cortical and cholin-

w xergic function during this period 3,18,37,40 , we haveexamined the intrinsic electrophysiological properties ofneurons in auditory cortex with the ultimate goal of under-standing how responses to sensory inputs develop. Wehave focused on neurons in layers IIrIII and IV as theyare the main recipients of lemniscal thalamocortical projec-

w xtions 58,67,68 .Intrinsic electrophysiology generally refers to whole-cell

electroresponsiveness resulting from the types and distribu-tions of ion channels in the membrane. These propertiesdetermine postsynaptic responses, including patterns ofspike discharge, to afferent inputs. A widely-used systemof classifying cortical neurons by their intrinsic properties

0165-3806r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0165-3806 99 00058-9

( )R. Metherate, V.B. AramakisrDeÕelopmental Brain Research 115 1999 131–144132

w x10,32 focuses primarily on the patterns and character-istics of spike discharge elicited by depolarizing currentpulses delivered via intracellular recording electrodesw x2,21,26,46 . Neurons generally are classified as regular-

Ž . Ž . Ž .spiking RS , fast-spiking FS , and intrinsic-bursting IB .Ž w x.Additional cell types e.g., ‘chattering’ cells 15 and

Ž w x.subtypes of existing classes e.g., RS1–2 2 have beendescribed since. In each case, the pattern of spike dis-charge is thought to reflect a functional contribution of thecell type to cortical circuits.

To fully understand how auditory cortex processes sen-sory information, we must determine the degree to whichintrinsic properties contribute to acoustic-evoked responsesw x38,65 . For example, cortical neurons generally respondtransiently to acoustic stimuli, falling silent after discharg-

w xing only one or a few spikes 7,51 , whereas some neuronsin the subcortical auditory system can respond in a more

w xsustained manner 11,49,52 . Explanations for the transientdischarge of cortical neurons generally revolve around the

w xstrength of intracortical inhibition 7,42,54,66 and the useof anesthesias that depress cortical excitability andror

w xenhance GABAergic inhibition 43,44 . Such factors un-doubtedly contribute to neuronal excitability, but intrinsicproperties must also contribute to responsiveness, sinceboth synaptic and intrinsic mechanisms shape evoked re-sponses. Here, we continue the task of determining thecellular bases of acoustic responsiveness by examining theintrinsic electrophysiology of neurons in layers IIrIII andIV of developing auditory cortex.

2. Materials and methods

Details of the slice preparation and electrophysiologicalw xrecordings are as published recently 3 . Briefly, Sprague–

Dawley rats of either sex and age 8–23 days were anes-thetized with barbiturate or halothane and decapitated.Brains were removed into cold artificial cerebrospinal fluidŽ . Ž .ACSF containing in mM : NaCl 125.0, KCl 2.5,NaHCO 25.0, KH PO 1.25, MgSO 1.2, CaCl 2.0,3 2 4 4 2

dextrose 10.0, bubbled with 95% O , 5% CO . Coronal2 2Ž .slices 300–350 mm containing auditory cortex were ob-

w xtained based on landmarks visible in coronal sections 48Žand acetylcholinesterase histochemistry which delineates

w x.primary sensory cortex in juvenile rats; see Refs. 3,56 ,and placed in a holding chamber containing oxygenatedACSF at room temperature. For recordings, each slice wastransferred to a submersion chamber on a fixed-stage

Ž .microscope Zeiss Axioskop and maintained at 348C.Ž .Infrared differential interference contrast IR-DIC optics

enabled visualization of individual neurons and precisew xplacement of recording electrodes 63 .

Whole-cell recordings were obtained with patch pipettesŽ . Ž .4–6 MV filled with in mM : KMeS0 125.0, CaCl3 2

0.05, NaCl 0.5, Mg-ATP 2, Na-GTP 0.5, EGTA 0.16,ŽHEPES 10, and 0.3–0.5% biocytin pH 7.3 with KOH,.final osmolality 270–280 mosMrkg . Neural responses

Žwere amplified dc-2 kHz, Axoclamp 2B, Axon Instru-.ments , viewed on an oscilloscope and chart recorder, and

digitized at 5 kHz for viewing and storage on computer.Series resistance was typically less than ;15 MV andwas fully compensated using the bridge balance. Note thatthe low resistance patch electrodes did not rectify evenupon passage of positive or negative currents much greater

Ž .than those used here. Membrane potential V was notm

corrected for junction potential which was estimated to beŽ;13 mV range 12–14 mV, ns3, measured by replacing

bath ACSF with pipette filling solution while using a.free-flowing KCl reference electrode . Software

Ž .AXODATA, AXOGRAPH, Axon Instruments controlleddata acquisition and analysis.

Measurements of intrinsic electrophysiology were madewithin ;5 min of establishing whole-cell recordings, afterV stabilization but before significant washout was likelymŽobservations repeated after 30–90 min revealed reducedmembrane rectification andror reduced spike-frequencyadaptation over time for some cells, but not all, presum-

.ably due to washout of cellular contents . Measurements ofpassive and spike properties were obtained by passingrectangular current pulses ranging approximately from

Žy0.2 to q0.2–0.7 nA in steps of 0.02–0.1 nA typically. Ž .0.05 nA . Apparent input resistance R and membranei

Ž .time constant t at resting V were estimated from themŽ .initial response peak hyperpolarizing deflection to a y0.1

nA current pulse. Time constants were estimated by curveŽ .fitting single exponential function . Spike threshold, height

and width were determined from the first spike elicited at aŽthreshold intensity current pulse note that ‘threshold’ in

the text below can refer either to spike threshold, in mV,or to the minimum effective current stimulus intensity, innA, for generating a spike; threshold current intensity was

.determined typically with 0.05 nA resolution . Spike heightŽ .was measured from threshold not resting V and spikem

width was measured at half height. The latency to spikeŽ .onset at threshold was determined from the onset of the

current step.After each recording, the distance between the soma

and the pia was measured using a microscope reticuleŽ .resolution 2.5 mm . The cell was assigned to a layer bydetermining its depth relative to a cortical width of 1500

Ž .mm mean cortical width 1501"24 mm, ns14 slices ,and relating the value obtained to published laminar

w xboundaries for rat auditory cortex 57 .Slices with biocytin-filled neurons were placed in 4%

paraformaldehyde overnight, then in 0.1 M phosphateŽ .buffer PB for 1–7 days. Slices were washed in PB,

Ž .incubated in ABC complex Vector Labs for 2–18 h, thenrinsed in PB and in Tris buffer. The tissue was then

Žpreincubated in 0.02% diaminobenzidine DAB, in Tris.with 0.25% nickel ammonium sulfate for 20 min, then

reacted in 0.006% H O qDABrnickel for 4–5 min.2 2

Sections were washed in Tris and in PB, then mounted onslides, dried, dehydrated, cleared and coverslipped. Recov-

( )R. Metherate, V.B. AramakisrDeÕelopmental Brain Research 115 1999 131–144 133

ered neurons were viewed on a microscope equipped withŽ .a drawing tube Zeiss Axioskop and drawn with a 40= or

100= oil-immersion objective.All mean values reported are "1 S.E.M. Statistical

Ž .comparisons StatView software of passive and spikeproperties among the main cells groups were performedusing ANOVA and Scheffe’s F-post hoc procedures.Comparisons of OS cell characteristics with those of RS

Ž .and RECT cells see below for definitions of cell typesused one- or two-tailed t-tests, where appropriate.

3. Results

The data derive from 233 cells recorded in layers IIrIIIand IV. The cells had stable resting V more negative thanm

Ž .y50 mV mean y67"0.4 mV and overshooting actionŽpotentials amplitude 69"0.6 mV measured from spike

threshold of y40"0.3 mV; note that V values are notm

corrected for junction potential of ca. y13 mV; see Sec-.tion 2 . Of these, 187 cells could be assigned to one of five

categories based on intrinsic electrophysiology. These fivecategories are the conventional RS, FS, and IB groups, as

Ž .well as two additional groups, rectifying RECT andŽ . Ž .on-spiking OS cells defined below . Of the 46 cells that

were not assigned to one of these groups, most hadŽcharacteristics that overlapped two groups usually RS and

.RECT and could not be placed easily into one group oranother. A very few cells had characteristics that did notresemble those of any group or were insufficiently charac-terized to classify. Data from unclassified cells are notexplicitly discussed but contribute to the analysis of age-

Ž .related changes in neuronal properties Fig. 6 . Each groupof cells will be described in turn.

3.1. RS, FS and IB cells

The descriptions of RS, FS, and IB cells presented forŽ .other cortical areas see Section 1 also apply to cells in

auditory cortex. The descriptions here serve only to con-firm the distinctive features of each cell type.

RS cells were by far the most common cell type,Ž .comprising 160 of 187 86% classified cells. In response

to 600 or 800 ms depolarizing current pulses ofsuprathreshold intensity, RS cells spiked for the duration

Ž .of the pulse Fig. 1A, Ci . The firing rate of RS cellsadapted at the beginning of a pulse and then maintained a

Ž .constant rate RS1 or continued to adapt throughout theŽpulse RS2; RS1,2 refer to subtypes of Agmon and Con-

w x.nors 2 . While some RS cells adapted completely beforeŽ .the end of a near-threshold current pulse Fig. 1Ai , they

generally spiked for at least 200–300 ms, and at higherintensities they always spiked for the duration of the entire

Ž .pulse Fig. 1Aii . Rates of adaptation for RS cells appearŽ .to form a continuum Fig. 1Ci , rather than delineating

distinct RS1 and RS2 subclasses, thus, such distinctions

were not made routinely. In response to large hyperpolariz-ing current pulses, RS cells often displayed a delayed

Ždepolarizing ‘sag’ suggestive of inward rectification notshown; similar responses are evident in other cell types,

.see Fig. 3D, open arrowhead . The passive membrane andspike characteristics for RS cells are in Table 1. Cells thatwere judged qualitatively to belong to RS1 and RS2subgroups did not differ significantly in any of the proper-ties listed in Table 1.

Ž .FS cells were encountered in six cases Fig. 1B . FSŽ .cells have narrow spikes Table 1 that typically are termi-

nated by large, fast-onset and short-duration afterhyperpo-Ž .larizations AHPs, see first spike in Fig. 1Bi . FS cells can

discharge at high frequencies and their firing does notŽ .adapt during a maintained stimulus Fig. 1B, Cii .

IB cells respond to depolarizing current pulses with ashort burst of spikes, and sometimes burst repetitively and

w xrhythmically throughout the pulse 2,39 . These cells canw xbe found in layer IV 32 but more often are found in layer

w xV 9,10,31 . We encountered only one IB cell in thisŽ .population not shown , located in layer IV.

Twenty-five neurons filled with biocytin were recov-Žered. Each of 11 RS cells were pyramidal neurons Fig.

.1Aiii . Only one FS cell was filled with biocytin, and thisŽ .cell was nonpyramidal Fig. 1Biii . The sole IB cell was

recovered and found to be pyramidal. The other filledŽneurons either could not be classified physiologically ns

.6 or belong to one of the two cell types described nextŽ .ns6 .

( )3.2. Rectifying RECT cells

Two groups of neurons could not be placed into theprevious categories. The first group, RECT cells, com-prised 16 neurons with three related defining character-

Ž .istics: i RECT cells spiked early in response to currentŽpulses and then stopped, typically within ;250 ms Fig.

. Ž .2A,B . ii With increasing-amplitude depolarizing steps,Ž .the duration of the spike train either decreased Fig. 2Aii

Ž .or remained relatively constant Fig. 2Bii , even as theŽ .firing frequency increased. iii Increasing-amplitude depo-

larizing steps produced decreasing increments of mem-Žbrane depolarization see traces in Fig. 2Ai, Bi, current

step increments are of equal magnitude; see also I–V plots.in Fig. 2Bv and C .

Membrane depolarization in RECT cells could activatean outward current to suppress spiking behavior and re-duce V deflections. Evidence for such a phenomenon canm

be seen in subthreshold voltage depolarizations. For thecell in Fig. 2B, depolarizing current steps of sufficientamplitude produced an initial rapid depolarization that‘sagged’ within tens of ms to a more negative potentialŽ .Fig. 2Biv, open arrowhead . Such a sag could result from

Ž .activation of an outward hyperpolarizing current, or fromŽ .inactivation of an inward depolarizing current. However,

the suppression of longer-latency action potentials with


Fig. 1. Characteristics of pyramidal RS cell and nonpyramidal FS cell in rat auditory cortex. A. RS intrinsic physiology in a neuron from a P11 animal.Ž . Ž . Ž . Ž .Current pulses not shown were y0.2 to q0.1 nA in steps of 0.1 nA in i and q0.2 nA in ii . Resting V y68 mV. iii Biocytin-filled layer IIrIIIm

Ž . Ž .pyramidal neuron whose physiology is shown in i and ii . B. FS intrinsic physiology in a P14 neuron. Current pulses were y0.1 to q0.3 nA in steps ofŽ . Ž . Ž . Ž .0.1 nA in i response to subthreshold 0.2 nA stimulus not shown, for clarity of viewing spikes and q0.4 nA in ii . Resting V y68 mV. iii Layerm

Ž . Ž . Ž .IIrIII nonpyramidal neuron whose physiology is shown in i and ii . C. i Plots of spike number vs. time of occurrence indicate varying degrees ofŽ .adaptation to just-suprathreshold current pulses in 13 RS neurons. Asterisk indicates plot based on response in Aii. ii Plots of spike number vs. time of

Ž .occurrence are given for five stimulus intensities 0.3–0.7 nA for interneuron in B. Asterisks indicate plots based on responses in Bi and Bii.

Ž .ever-greater depolarizing steps e.g., Fig. 2Aii stronglyimplies the presence of hyperpolarizing current. Also, thepresence of an AHP following the end of the subthreshold

Ž .current pulse Fig. 2Biv, closed arrowhead whose ampli-

tude increased with increasing membrane depolarizationcould reflect the tail of an outward current. Thus, depolar-ization-induced outward rectification likely contributes tothe characteristic physiology of RECT cells.

Table 1Ž .Passive and spike characteristics of cells mean"S.E.M.

aŽ . Ž . Ž .V mV R MV t ms Spike threshold Spike height Spike widthm ia bŽ . Ž . Ž .mV mV ms

cRS, ns160 y67"0.5 216"7.1 19.7"0.4 y40"0.3 71"0.6 1.1"0.02FS, ns6 y60"3.3 197"37.1 19.0"4.4 y37"2.2 53"4.0 0.6"0.03RECT, ns16 y68"1.3 223"9.6 23.8"1.3 y38"1.5 64"1.8 1.1"0.08OS, ns4 y62"1.4 202"66.7 20.4"4.6 y45"2.2 59"1.1 1.0"0.17

a Values not corrected for junction potential of ;y13 mV, see Section 2.bSpike heights, especially for FS cells, are underestimated due to 5 kHz sampling rate.cActual number of RS cells for each mean ranged from 155 to 160.


Ž .Fig. 2. Characteristics of RECT cells in auditory cortex. A. i Superimposed voltage responses of a P14 neuron to current steps of "0.2 nA in incrementsŽ . Ž .of 0.05 nA; V y66 mV. Suprathreshold responses in i are depicted separately in ii ; note that the decrease in spike amplitude during each responsem

Ž . Ž . Ž . Ž . Ž .occurred only infrequently in RECT cells cf. response in B . iii Layer III nonpyramidal neuron whose responses are shown in i and ii . B. iŽ .Responses of a P12 neuron to current steps of y0.2–0 nA in increments of 0.05 nA, and 0.04–0.32 nA in steps of 0.04 nA; V y70 mV. ii Plot of spikem

Ž . Ž . Ž . Ž .number vs. time of occurrence for each suprathreshold response in i . iii Layer IV pyramidal neuron whose responses are in i . iv SubthresholdŽ . Ž .responses current steps of 0.05 and 0.1 nA in same cell demonstrate hyperpolarizing sag open arrowhead upon sufficient membrane depolarization, and

Ž . Ž . Ž .afterhyperpolarization closed arrowhead following the stimulus. v I–V function based on records in i ; measurements made at end of current pulses. C.Mean I–V function for the 16 RECT neurons.

The strong spike frequency adaptation in RECT cellsmight also involve a Ca2q-dependent AHP activated by

w xspike-induced depolarization 28,59 . In general, increas-ing-amplitude depolarizing steps produced increased num-

Žber of spikes although the suppression of longer-latencyaction potentials often meant that the increased number of

.spikes occurred within a shorter duration train and in-Žcreased amplitude AHPs note, however, that not all RECT

.cells had obvious AHPs; see Fig. 2Ai . Thus, the AHPmagnitude increased both with the number of spikes andwith depolarization. However, in a few cases the effects ofspike number and depolarization could be dissociated. Asshown in Fig. 2Bii, increased depolarization did not al-

Žways elicit an increased number of spikes current steps of0.2 and 0.24 nA each elicited four spikes, and steps of 0.28

.and 0.32 nA each elicited five spikes . In each case thatthe number of spikes remained constant with increased

depolarization, the larger depolarization produced an ap-Ž .proximately 0.5 mV larger amplitude AHP not shown .

Thus, spike-induced depolarization likely contributes to,but does not entirely account for, AHP magnitude, consis-tent with the data obtained from subthreshold depolarizing

Ž .responses Fig. 2Biv suggesting that a separate outwardcurrent is active in RECT cells.

Fig. 2C depicts the mean I–V response for the 16RECT cells, obtained by measuring voltage responses nearthe end of each current pulse. The dashed line is extrapo-lated from the data points near the resting potential, andemphasizes the nonlinear portion of the I–V function thatbegins near y55 mV. Note also the lesser rectification at

Ž .hyperpolarized potentials -y80 mV that is also visiblein the voltage traces in Fig. 2A,B. Such rectification oftenwas associated with a depolarizing sag following an initial

Ž .large hyperpolarization e.g., Fig. 2Bi .


Four of the 16 RECT cells were filled with biocytin andŽrecovered. Of these, two were nonpyramidal neurons Fig.

. Ž .2Aiii and two were pyramidal Fig. 2Biii .

( )3.3. On-spiking OS cells

The second group of nonconventional cells resembledRECT cells in that they showed evidence for outwardrectification upon membrane depolarization. However, theydiffered sufficiently in two respects to be considered sepa-

Ž .rate from RECT cells: i OS cells fired only one or two.spikes in response to depolarizing pulses, and ii the

Ž .latency to the spike onset threshold was unusually short,i.e., -10 ms. We encountered four of these cells and referto them as on-spiking cells.

Data from three of the four OS cells are shown in Fig. 3and illustrate their defining characteristics. In each case,depolarizing current pulses of threshold intensity elicited a

Ž .single spike within 10 ms mean 7.8"1.0 ms, ns4 .With increased-amplitude depolarization, spike latency de-creased slightly, and in one case, a second spike appearedŽFig. 3Dii; a third level of depolarization in this cell again

.elicited two spikes at even shorter latency . Overlappingtraces in Fig. 3Ai, Ci and Di demonstrate the temporal

Žconsistency of evoked spikes there is one spike at each oftwo levels in Fig. 3Ai, and 1–2 spikes at each of three

.levels in Di and evidence for membrane rectification inŽboth depolarizing and hyperpolarizing directions current

.steps increments are of equal magnitude . Traces for eachcell are shown at higher resolution in Fig. 3Aii, Cii, and

Ž .Fig. 3. OS cells in layer IV of auditory cortex. A. i Superimposed voltage responses of a P13 neuron to current steps of y0.2 to q0.4 nA in incrementsof 0.1 nA; V y60 mV. Note that the cell fired a single spike in response to each of the two largest current steps. Responses to depolarizing current stepsm

Ž . Ž . Ž .in i are shown at higher resolution in ii ; traces with truncated spikes are offset for clarity. B. Mean I–V function for the four OS neurons. C. iŽ .Responses of a P11 neuron to current steps of "0.2 nA in increments of 0.05 nA; V y66 mV. Responses to steps of y0.05 to q0.2 nA in i are shownm

Ž .at higher resolution in ii ; note the rapid activation of outward rectification upon subthreshold depolarizing steps. Although this cell spiked at only oneŽ .level, it was included in the OS group because of its single spike, short spike latency and outward rectification. iii Layer IV pyramidal neuron whose

Ž . Ž . Ž .responses are in i and ii . D. i Responses of a P12 neuron to "0.2 nA in increments of 0.05 nA; V y60 mV. Open arrowhead indicates depolarizingmŽ .sag in response to large hyperpolarization. Higher resolution responses in ii are in response to steps of 0.00, 0.02, 0.04, 0.06, 0.1, 0.15 nA. Closed

Ž . Ž . Ž .arrowhead indicates robust spike AHP. iii Layer IV nonpyramidal neuron whose responses are in i and ii .


Dii. Note the short latency to spikes, the large, fast spikeŽAHPs arrowhead in Fig. 3Dii; traces with truncated spikes

.are offset for clarity , and in subthreshold responses, therapid activation of rectification similar to that observed in

Ž .RECT cells cf. Fig. 2Biv . Note the similarity of spikingand rectifying properties within the OS group of cells, aswell as the different degrees of hyperpolarization-activated

Žrectification and depolarizing sag e.g., arrowhead in Fig..3Di .

Fig. 3B depicts the average I–V function for OS cells,obtained by measuring voltage responses near the end ofeach current pulse. Note the similarity of this function to

Ž .that for RECT cells Fig. 2C in that the deviation from alinear function in the depolarizing direction begins aroundy55 mV.

Two of the four OS neurons were filled with biocytin.Ž .Of these, one was pyramidal Fig. 3Ciii and one was

Ž .nonpyramidal Fig. 3Diii .

3.4. Comparison of features among cell groups

The passive and spike properties listed in Table 1 werecompared to identify possible functional distinctions among

Žthe four cell groups ANOVA and Scheffe’s F-post hoc.procedure . Neither resting V nor R differed among them iŽ .cell types ps)0.05 . Membrane time constants did not

differ among groups, except for the RECT vs. RS compari-

Ž .son p-0.05 . Thus, passive properties largely did notdiffer among the cell groups.

Comparison of spike properties produced unremarkabledistinctions except in one case. The mean FS spike widthof 0.6 ms was significantly less than the ;1 ms width of

Žspikes from the other cell classes one-tailed t-test vs. RS.and RECT, ps-0.001 vs. OS, p-0.05 . While spikes

heights also differed among cell groups, note that the 5kHz digitization rate used in this study produced truncatedFS spikes, since these spikes are unusually fast. Thisresulted in underestimated spike heights and overestimated

Žspike durations since duration is measured at half ampli-.tude . Thus, while the narrower widths of FS cells are

Ž .undoubtedly real and even narrower than reported , thelesser spike heights are at least partly artifactual. Measuresfor the other cell classes, with spike durations nearly twiceas long, were less affected by digitization rate. Spike

Ž .thresholds differed significantly p-0.05 between FSand OS cells and between OS and RECT cells, but ingeneral appeared similar among the groups. The moststriking distinction among spike features, therefore, is theshorter spike duration of FS cells.

An important criterion used to distinguish among celltypes was the duration of the spike train elicited bysuprathreshold current pulses. To demonstrate the useful-ness of this criterion, Fig. 4A displays examples for each

Žcell class the analyses in Fig. 4 utilized all 10 FS and OS

Fig. 4. Comparison of firing durations and latencies to first spike among cell groups. Analyses in A and B utilized all 10 FS and OS cells, as well as all RSŽ .and RECT cells nine cells each with data from suprathreshold stimulus intensities at three or more current levels. Each line depicts the effect of changes

Ž . Ž .in current intensity on firing duration A or latency to first spike B for a single cell.


Ž .cells, as well as all RS and RECT cells nine cells eachwith data from suprathreshold stimulus intensities at three

.or more current levels . For each cell, lines connect pointsobtained at different current levels. While RS and FS cellssometimes did not spike throughout a current pulse atthreshold, they always spiked throughout the pulse at

Ž .higher intensities Fig. 4A . In contrast, RECT cells tendedto spike only for the first 100–300 ms at threshold, and theduration of spiking either decreased or stayed relativelyconstant with increases in intensity. The single OS cell that

Žproduced two spikes allowing measurement of a spike.train duration is shown for comparison. Thus, the duration

of cell spiking effectively distinguished among some cellgroups.

One criterion that defined OS cells was the spike la-tency at threshold current intensity. The latency to first

Ž .spike for OS cells 7.8"1.2 ms was significantly lessŽthan that for RS cells 85"6.4 ms, one-tailed t-test,

. Ž .p-0.05 and RECT cells 57"11.6 ms, p-0.05 , butŽnot less than the latency for FS cells 116"52.7 ms,

.p)0.05 , due to the large variability in the FS group. Datafor each cell group showing the change in first spike

Žlatency with current intensity are in Fig. 4B for clarity,only latenciesF40 ms are shown; data are from same cell

.sample as in Fig. 4A . The faster onset of OS spikesclearly distinguishes that group from the others.

It is possible that the shorter spike latency of OS cellsresults from outward rectification suppressing first spikesthat otherwise would appear at longer latencies. If so, thenonly stimuli that were sufficiently strong to elicit spikesbefore the onset of outward rectification would be effec-tive. We would predict, therefore, that threshold currentintensities for OS cells would be higher than for other cellgroups. Consistent with this possibility, the threshold stim-ulus intensity for OS cells was 0.19"0.04 nA; this was

Žsignificantly higher than the threshold for RS cells 0.05". Ž0.01 nA, p-0.001 and RECT cells 0.09"0.01 nA,

. Žp-0.01 , but not FS cells 0.12"0.04 nA, p)0.05;.comparisons based on the 28 cells in Fig. 4 . RECT cells

also displayed outward rectification which might be ex-pected to raise their thresholds, and, in fact, thresholdintensities for RECT cells were higher than for RS cellsŽ .p-0.01 . Thus, outward rectification may suppresslonger latency spikes, leading to shorter-latency thresholdresponses at higher stimulus intensities. While these datasuggest that the short spike latencies of OS cells resultfrom outward rectification, other factors may also con-

Žtribute since at a fixed suprathreshold current level 0.2.nA , OS cells still spiked at shorter latencies than RS and

ŽRECT cells OS 5.6"1.6 ms vs. RS 11.3"0.9 ms,.p-0.01 vs. RECT 15.7"2.5 ms, p-0.05 . These data

suggest mechanistic bases for the unusual characteristics ofOS and RECT cells.

Ž .OS and RECT cells appeared similar in terms of iŽ .outward rectification, ii a lack of spikes at longer laten-

Ž .cies, and iii higher threshold stimulus intensities. We

therefore looked for mechanisms that might account fortheir differences. Specifically, we predicted that the lesser

Ž .number of spikes one or two elicited at shorter latenciesŽ .-10 ms in OS cells could result from faster activation of

Ž .outward rectification. In 11 cells four OS, seven RECTwhere a clear hyperpolarizing sag implied the onset ofoutward rectification at a subthreshold membrane potentialŽ .e.g., Fig. 2Biv, open triangle , we measured the latency tothe peak depolarization from which the sag began. Thepeak latency for OS cells was 21.8"5.8 ms, significantlyfaster than the latency of 46.6"6.9 ms for RECT cellsŽ .one-tailed t-test, p-0.02 and consistent with our predic-tion. There was no difference between the V of the peakm

Ždepolarization OS y48"2.0 mV, RECT y45"2.5 mV,.p)0.05 , indicating that the rate of depolarization was

faster in OS cells than in RECT cells. Thus, faster outwardrectification may distinguish OS from RECT cells andcould contribute to functional differences between the twogroups; however, voltage-clamp studies will be necessaryto demonstrate this conclusively.

Rectification in OS and RECT cells could also influ-ence cell responsiveness by modifying passive membrane

Fig. 5. Membrane depolarization strongly reduces R and t for OS andiŽ .RECT cells combined . Values for R and t are based on measurementsi

made at the end of current pulses, to ensure full activation of voltage-de-pendent currents, except for measures attributed to resting V which werem

made at the beginning of a y0.1 nA pulse. Data points were grouped byŽ .current level "0.2 nA in 0.05 nA steps and values for V , R and tm i

were averaged for each level. Except for data at y101 mV, error bars forV are smaller than symbols used.m


properties. Fig. 5 illustrates how R and t varied with Vi mŽfor the 20 OS and RECT cells group data combined due. Žto similar degrees of rectification . From resting V y67m

.mV to approximately y45 mV, R decreased 47% fromiŽ .219"14.2 to 116"6.0 MV paired t-test, p-0.001 ,

and t decreased 42% from 23.1"1.3 to 13.4"1.2 msŽ .p-0.001 . Such changes could strongly influence neu-ronal responses by reducing the amplitude, time course,and spatial integration of synaptic inputs.

3.5. Correlation of cell types defined morphologically andphysiologically

Of the 25 cells filled with biocytin and recovered, 19were unambiguously classified with respect to intrinsicphysiology. As reported above, each of the 11 RS cellsfilled with biocytin was pyramidal. These findings are

consistent with assessments of neuron morphology madeupon visualization with IR-DIC optics during experiments.The vast majority of RS neurons had prominent apicaldendrites that ascended towards the pia, i.e., these neuronsclearly were pyramidal. The single biocytin-filled FS cellwas nonpyramidal, consistent with the round somas andlack of apical dendrites of FS cells visualized with IR-DICoptics. OS and RECT cells filled with biocytin were foundto be either pyramidal or nonpyramidal, indicating thatthese physiological response types are not associated ex-clusively with either broad morphological category. It is of

Žinterest, however, that in the case of two cells one OS,.one RECT , a preliminary classification during the experi-

Žment of ‘potentially nonpyramidal’ based upon soma.shape and the lack of a prominent apical dendrite was

demonstrated incorrect by the biocytin results. It is possi-

Fig. 6. Changes in passive membrane and spike properties during development. Data from all neurons recorded in this study are included. Correlationcoefficients indicate significant correlation of resting V , R , t , spike height and spike width, but not spike threshold, with age over the range of P8–21.m i

Ž . Ž . Ž . Ž . Ž . Ž .Data were grouped for P8–9 ns11 neurons , P10–11 34 , P12–13 89 , P14–15 52 , P16–17 17 , P19–21 23; no data from P18 and meanvalues"S.E.M. were plotted at the first day for each age range.


Žble that finer morphological distinctions e.g., less promi-.nent apical dendrites may distinguish OS and RECT

pyramidal neurons from other pyramidal neurons. Largersamples will be required to resolve this issue.

It would be useful to determine the laminar distributionof the cell types described here, however, a likely sam-pling bias precludes definitive answers. The majority ofRS cells were found in layer IIrIII of the cortex, whereasnearly all OS and RECT cells were located in layer IV ornear the border between layers III and IV. However, wemade no attempt to sample different cell types in eachlayer equally, and post hoc analyses suggest that experi-ments were biased towards recording either from pyrami-dal neurons in layers IIrIII or from smaller, putativenonpyramidal neurons in layer IV. Thus, while OS andRECT cells appear to lie preferentially in layer IV, theymay instead tend to be smaller neurons which were mostlysampled in layer IV. Nonetheless, one implication of these

Ždata, along with IR-DIC and biocytin results previous.paragraph , is that OS and RECT cells may include non-

pyramidal neurons as well as small pyramidal neuronswith slender apical dendrites.

3.6. Changes in neuron characteristics with deÕelopment

Neurons examined in the present study were obtainedfrom animals aged P8–23, a period of significant func-

Ž .tional development see Section 1 . Several physiologicalparameters were found to vary with age, as shown in Fig.6. Resting V , R and t all decreased with age. Spikem i

amplitude increased with age and spike width decreased.Spike threshold did not change, nor did the latency to first

Ž .spike at threshold stimulus intensity not shown; p)0.05 .These changes in passive and spike properties may con-tribute to the functional maturation of auditory cortex.

The relative prevalence of cell types may also changewith age. In the interval between ages P11–15, we encoun-tered 111 RS cells and all 20 OS and RECT cells. Incontrast, during P16–20 there were 24 RS cells and no OSor RECT cells. The distribution of cell types across these

Žintervals differs significantly chi-squared, p-0.05; OSand RECT cells were combined because of their similar

.rectification properties and low numbers . Interestingly,whereas outward rectification became less prevalent with

Ž .development, some cells from older slices e.g., P20displayed clear inward rectification upon depolarizationŽ .not shown . It is possible that OS andror RECT cells mayfunction only during development, however, the low num-ber of cells studied at greater ages precludes strong conclu-sions. Additional studies on adult tissue will be required toresolve this issue.

4. Discussion

In this study we report on the intrinsic electrophysiol-ogy, including spiking patterns, of neurons in rat auditory

cortex during the second and third weeks after birth. Manyneurons display intrinsic physiology and morphology con-

Žsistent with previous descriptions of response classes RS,.FS, IB neurons . In addition, we identify two groups,

RECT and OS neurons, that are characterized by brieferspike trains in response to maintained depolarizations, andby strong depolarization-induced outward rectification.

Ž . ŽUnusually brief 1–2 spikes and fast -10 ms spike.latency responses further distinguish OS cells from RECT

cells. Biocytin labeling demonstrated that RECT and OScells can be either pyramidal or nonpyramidal neurons.The intrinsic electrophysiology of different cell groupsmay contribute to auditory cortex functions.

4.1. Mechanisms of intrinsic electrophysiology

The intrinsic electrophysiology of a neuron reflects thetypes and distributions of membrane ion channels. Thecollective behavior of these channels determines how aneuron responds to depolarizing and hyperpolarizing cur-

w x Žrent pulses and, presumably, to synaptic inputs 10,38 butw x.see Ref. 45 . Intrinsic electrophysiology is therefore likely

to be an important determinant of neuronal responsiveness.

4.1.1. Regular spiking neuronsThe spike discharge of RS cells adapts to a maintained

depolarizing current pulse. The adaptation is thought toinvolve spike-induced activation of a Ca2q-dependent Kq

w xcurrent 28,30,59 . Previous studies have subdivided RSw xcells depending on their rate of adaptation 2 . However, in

the present and previous studies, rates of adaptation appearto form a continuum with the RS1 and RS2 subdivisionsrepresenting extremes. For some whole-cell recordings inthe present study, rates of adaptation decreased over timeŽ .tens of minutes , probably reflecting ‘washout’ of an

Ž 2q 2q.intracellular constituent e.g., Ca buffer or free Ca .Similarly, varying rates of adaptation in different neuronsmay reflect varying intracellular levels of the same con-

Ž .stituent s .Some RS cells adapt completely, well before the end of

a near-threshold current pulse, and in this respect theyresemble RECT cells. However, with an increase in stimu-lus intensity RS cells always spiked for the duration of thecurrent pulse. Further, they did not display membranerectification with increasing depolarization. These charac-teristics distinguished RS and RECT cells, and indicatethat different mechanisms underlie the two types of cell

Ž .properties see below .

4.1.2. Fast spiking neuronsThe narrow spike width of FS cells that gives rise to

their name results from the unusually rapid repolarizationof the action potential. Previous studies have shown thatthe rate of rise of FS spikes is similar to those of other cell

w xtypes, but the falling rate is significantly faster 32 . Thespike’s fast AHP also permits FS cells to discharge at a


high rate, up to several hundred Hz, in response to main-tained depolarization. The lack of adaptation in FS cellsmay indicate the lack of a Ca2q-dependent AHP, or per-

2q Ž w x.haps an intracellular Ca buffer e.g., parvalbumin 22that prevents activation of a slow AHP.

The relative lack of FS cells in the present sample ispuzzling, for although their scarcity in previous studies has

w xbeen attributed to their small size 10 , size is not a majorfactor in these experiments due to the IR-DIC optics. Otherfactors might explain their low numbers, such as an in-creased vulnerability to injury so that FS cells do notsurvive near the cut surface of the slice where most cellsare recorded. Also, in the present study many recordingswere targeted at pyramidal rather than nonpyramidal neu-

Ž .rons see Section 3 . Given their probable GABAergicw x Žnature 23,64 but note that some nonadapting cells with

w x.fast spikes are pyramidal neurons 14,62 , the scarcity ofFS cells in the present study is unlikely to reflect eithertheir true numbers or their functional importance.

4.1.3. Rectifying neuronsThe major defining feature of RECT cells was striking

outward rectification that began within ;50 ms of adepolarizing stimulus pulse and appeared to strongly sup-press spike discharge. Examination of the AHP after sub-threshold responses indicated that the magnitude of the

Ž .outward hyperpolarizing current increased with increas-ing amplitude depolarizing steps. This voltage-dependenceof the outward current may underlie the observation thatthe duration of spiking decreased with larger depolarizingsteps, i.e., greater depolarization could produce a strongeroutward current that would suppress firing more strongly.Since rectification and the AHP occurred in subthreshold

Ž 2qresponses, spike-induced mechanisms e.g., Ca -depen-.dent AHP cannot wholly account for the observed effect.

Many cells that were not classified physiologically be-cause they could not be placed easily into a response class

Žhad properties that overlapped the RECT and RS specifi-.cally, RS2 groups, indicating that the frequency of the

rectifying current is likely underestimated. Further, it maybe that the degree of rectification across neurons forms acontinuum, rather than defining two distinct classes. Thefunctions of outward rectification, however, should bemost clearly discerned in RECT neurons.

Outward Kq currents have been implicated in strongspike adaptation in previous studies of cortical neurons. Insensorimotor cortex slices from rats 1- to 4-weeks-old,many neurons spike only for the first 100–250 ms of a 1 s

Ždepolarizing step designated ‘completely adapting cells’;w x.Ref. 28 . The discharge is followed by a slow AHP that

lasts for several hundred ms and produces a sag in theresponse to the depolarizing step. At times, the AHP endsbefore the end of the current pulse and additional spikesmay recur. The addition of Ca2q channel blockers orneuromodulators blocked both the AHP and spike adapta-

tion. In visual cortex neurons isolated from 2-week-old ratsand maintained in culture, depolarizing steps revealed

w xrobust spike frequency adaptation 27 . In 24% of neurons,obtained from pupsGP11, spike discharge adapted com-pletely. The duration of spiking increased with increasingamplitude depolarizing steps, but after cessation of spikingthe V held steady for the remaining duration of them

current pulse, i.e., the response did not resemble a slowAHP. Pharmacological and voltage-clamp manipulationsimplicated Ca2q-independent Kq channels in the spikefrequency adaptation. Thus, in other preparations of sen-sory cortical neurons, both Ca2q-dependent and Ca2q-inde-pendent Kq currents can produce spike frequency adapta-tion and rectification similar to that seen in RECT cells.

Other studies of neurons from sensorimotor cortex ofŽ .young 1–4 week rats have emphasized inward, rather

than outward, rectification upon depolarization from restw x25,34 . This inward rectification grows more prominentwith age and is reduced by Naq channel blockade, impli-cating activation of a persistent Naq current. Interestingly,in those studies, Naq channel blockade reveals strikingoutward rectification. In the present study, outward rectifi-cation was only observed at agesFP15, and inward recti-

Ž .fication was most apparent in older neurons ;P20 .Thus, channels mediating depolarization-activated persis-tent inward and outward currents may coexist in the sameneuron and may be regulated differently during develop-ment.

4.1.4. On-spiking neuronsThe major defining feature of OS cells was their short

Ž . Ž .latency -10 ms and brief spike response 1–2 spikes todepolarizing pulses. Similar features are found in some

w xbrainstem auditory neurons 47,65 , but to our knowledgehave not been described previously for cortical neurons.OS cells also displayed depolarization-activated outwardrectification that began within ;20 ms of stimulus onset,significantly faster than the nearly 50 ms latency to activa-tion in RECT cells. While other aspects of the rectificationin OS and RECT cells appeared similar, it seems unlikelythat OS cells are simply an extreme case of RECT cells.For example, individual OS cells demonstrate weaker rec-

Ž . Žtification albeit faster than some RECT cells cf. Figs. 2.and 3 , which is inconsistent with the notion of OS cells

being extreme cases. Further, the duration of RECT cellŽ .firing e.g., 100–250 ms was always considerably longer

than OS cells, i.e., there was no continuum of firingdurations between OS and RECT cells. Finally, even athigh stimulus intensities RECT cells generally did notspike at as short a latency as did OS cells at their thresh-old. Thus, other factors, possibly morphological as well asphysiological, determine OS firing behavior.

Some brainstem auditory neurons exhibit rectifying Kq

currents that limit to 1–2 spikes the neuron’s responses toeither depolarizing current pulses or afferent EPSPs


w x4,47,53,65 . One mechanism underlying this behavior is alow threshold Kq current that activates rapidly, within afew ms upon depolarization from rest, and thereby limits

w xspike discharge 4,47 . Pharmacological reduction of thiscurrent results in multiple spikes being elicited at a lowercurrent step intensity. It has been suggested that the rele-vant Kq channels may be clustered near the neuron’s spikeinitiation zone to more effectively regulate spike dischargew x q4 . Similar K currents in cortical neurons may contributeto the fast, brief spiking in OS cells and to rectification inboth OS and RECT cells.

OS cells comprised a very small group within thepresent study, however, it is unclear whether this reflects asampling bias or is a reflection of their true numbers. OScells may be preferentially distributed, e.g., among smallerneurons in layer IV, and therefore difficult to sampleequally with larger pyramidal neurons. Other factors mayalso contribute to the scarcity of OS cells. As describedabove, recordings from FS cells are also scarce, yet theseputative GABAergic neurons have an undisputed func-tional importance. Similarly, the importance of OS cellsmay be disproportionate to their numbers in the currentstudy. Finally, it should be noted that OS and RECT cellsare not an artifact of the particular recording conditionsused here, since they are encountered in slices maintainedin an interface-type recording chamber and in recordings

Žusing either patch pipettes or sharp microelectrodes S..Cruikshank and R. Metherate, unpublished data .

4.2. Intrinsic currents actiÕated by membrane hyperpolar-ization

Numerous cells exhibited nonlinear behavior uponmembrane hyperpolarization. Within tens of millisecondsfollowing large hyperpolarizing steps, in particular, a depo-larizing sag occurred frequently in RS, RECT, and OS

Ž .cells e.g., Fig. 2B, Fig. 3D . This behavior has beenstudied extensively in thalamic and cortical neurons and

w xrepresents activation of the mixed cation current, I 33,60 .h

This current likely contributes to spontaneous slow oscilla-tions in the intact animal as well as to recovery fromlong-lasting Kq-dependent IPSPs. Intrinsic currents acti-vated by membrane hyperpolarization were not a focus ofthis study, and more extensive discussions of their func-

w xtions are found elsewhere 61 .

4.3. Functions of intrinsic electrophysiology in auditorycortex

The most straightforward prediction of the functionalrelevance of intrinsic properties described here is thatdifferent cell classes will respond differently to sustaineddepolarizing inputs. Thus, RS pyramidal neurons wouldfire moderately upon depolarization and excite nearby and

w xdistant neurons via glutamatergic axon collaterals 35,64 .

Inhibitory FS cells would fire strongly upon excitation andthereby produce a barrage of IPSPs on nearby cellsw x5,24,64 . RECT cells could fire strongly, but only for alimited time, after which their output would be suppressed,and further depolarization opposed by membrane rectifica-tion.

The intrinsic physiology of OS cells suggest severalfunctional implications. In brainstem auditory nuclei, neu-rons with similar electrophysiology are able to precisely

w xencode the timing of afferent inputs 47,65 . It is intriguingto consider similar roles for cortical OS cells. For example,OS cells may signal the timing of stimulus onsets. Recentstudies of cortical function stress the importance of detect-

w xing stimulus transients 16,50 , and OS cells are ideallysuited to mediate such a function. Appropriate synapticcontacts with other cells could contribute to the wellknown tendency for cortical neurons in vivo to fire tran-

Ž .siently to maintained stimuli see Introduction . Similarly,if OS cells receive afferent inputs from diverse sources,their single spike discharge could signal synchronous orcoincident inputs, and even enhance synchrony by sup-pressing nonsynchronous inputs. For example, following

Ž .spike discharge, outward rectification in OS and RECTcells would reduce the impact of subsequent EPSPs as aresult of the greater than 40% decrease in R and t . Thei

faster time course of afferent EPSPs might also be usefulfor timing the onsets of repetitive inputs.

4.4. Changes in intrinsic electrophysiology during deÕel-opment

ŽSeveral intrinsic and spike properties e.g., V , R , t ,m i.spike height and width changed over the developmental

Ž .period studied P8–23 , as seen in previous studies ofw xcortical neurons 25,28,34 . Electrophysiological properties

w xchange little between P20 and P30 34 , suggesting thatphysiological maturation is largely complete by the end ofthe period covered in the present study. Firing patterns of

w xcortical neurons are also regulated developmentally 25,28 .However, the impact of such changes, and the responsepatterns per se, on cortical development and function areunclear. Given that sensory experience and patterns ofcortical activity are thought to guide the formation of

w xfunctional synaptic circuitry during development 20 , OSand RECT cells may promote the influence of auditorytransients in the shaping of cortical circuitry. The implica-tions of their actions should be determined in future stud-ies.

Acknowledgements

We thank Dr. S. Cruikshank for helpful discussions andcomments on the manuscript, and Ms. N. Patel for histo-logical processing. This work was supported by the NSFŽ . Ž .IBN 9510904 and the NIH NIDCD, DC02967 .


References

w x1 G.K. Aghajanian, F.E. Bloom, The formation of synaptic junctionsin developing rat brain: a quantitative electron microscopic study,

Ž .Brain Res. 6 1967 716–727.w x2 A. Agmon, B.W. Connors, Correlation between intrinsic firing

patterns and thalamocortical synaptic responses of neurons in mouseŽ .barrel cortex, J. Neurosci. 12 1992 319–329.

w x3 V.B. Aramakis, R. Metherate, Nicotine selectively enhances NMDAreceptor-mediated synaptic transmission during postnatal develop-

Ž .ment in sensory neocortex, J. Neurosci. 18 1998 8485–8495.w x q4 H.M. Brew, I.D. Forsythe, Two voltage-dependent K conduc-

tances with complimentary functions in postsynaptic integration at aŽ .central auditory synapse, J. Neurosci. 15 1995 8011–8022.

w x5 E.H. Buhl, K. Halasy, P. Somogyi, Diverse sources of hippocampalunitary inhibitory postsynaptic potentials and the number of synaptic

Ž .release sites, Nature 368 1994 823–828.w x6 C.A. Calarco, R.T. Robertson, Development of basal forebrain

projections to visual cortex: DiI studies in rat, J. Comp. Neurol. 354Ž .1995 608–626.

w x7 M.B. Calford, M.N. Semple, Monaural inhibition in cat auditoryŽ .cortex, J. Neurophysiol. 73 1995 1876–1891.

w x8 S.M. Catalano, R.T. Robertson, H.P. Killackey, Early ingrowth ofthalamocortical afferents to the neocortex of the prenatal rat, Proc.

Ž .Natl. Acad. Sci. U.S.A. 88 1991 2999–3003.w x9 Y. Chagnac-Amitai, H.J. Luhmann, D.A. Prince, Burst generating

and regular spiking layer 5 pyramidal neurons of rat neocortex haveŽ .different morphological features, J. Comp. Neurol. 296 1990 598–

613.w x10 B.W. Connors, M.J. Gutnick, Intrinsic firing patterns of diverse

Ž .neocortical neurons, Trends Neurosci. 13 1990 99–104.w x11 O. Creutzfeldt, F.-C. Hellweg, C. Schreiner, Thalamocortical trans-

formation of responses to complex auditory stimuli, Exp. Brain Res.Ž .39 1980 87–104.

w x12 D.E. Crowley, M.-C. Hepp-Reymond, Development of cochlearfunction in the ear of the infant rat, J. Comp. Physiol. Psych. 62Ž .1966 427–432.

w x13 A. Dinopoulos, L.A. Eadie, I. Dori, J.G. Parnavelas, The develop-ment of basal forebrain projections to the rat visual cortex, Exp.

Ž .Brain Res. 76 1989 563–571.w x14 R.W. Dykes, Y. Lamour, P. Diadori, P. Landry, P. Dutar, So-

matosensory cortical neurons with an identifiable electrophysiologi-Ž .cal signature, Brain Res. 441 1988 48–58.

w x15 C.M. Gray, D.A. McCormick, Chattering cells: superficial pyramidalneurons contributing to the generation of synchronous oscillations in

Ž .the visual cortex, Science 274 1996 109–113.w x16 P. Heil, Auditory cortical onset responses revisited: I. First-spike

Ž .timing, J. Neurophysiol. 77 1997 2616–2641.w x17 C.F. Hohmann, A.R. Brooks, J.T. Coyle, Neonatal lesions of the

basal forebrain cholinergic neurons result in abnormal cortical devel-Ž .opment, Dev. Brain Res. 42 1988 253–264.

w x18 C.Y. Hsieh, S.J. Cruikshank, R. Metherate, Cholinergic agonistdifferentially suppresses auditory thalamocortical and intracortical

Ž .synaptic transmission, Soc. Neurosci. Abstr. 24 1998 1879.w x19 H. Iwasa, W.P. Potsic, Maturational change of early, middle, and

late components of the auditory evoked responses in rats, Otolaryn-Ž .gol. Head Neck Surg. 90 1982 95–102.

w x20 L.C. Katz, C.J. Shatz, Synaptic activity and the construction ofŽ .cortical circuits, Science 274 1996 1133–1138.

w x21 Y. Kawaguchi, Groupings of nonpyramidal and pyramidal cells withspecific physiological and morphological characteristics in rat frontal

Ž .cortex, J. Neurophysiol. 69 1993 416–431.w x22 Y. Kawaguchi, Y. Kubota, Correlation of physiological subgroup-

ings of nonpyramidal cells with parvalbumin- and calbindin -im-D28k

munoreactive neurons in layer V of rat frontal cortex, J. Neurophys-Ž .iol. 70 1993 387–396.

w x23 Y. Kawaguchi, Y. Kubota, GABAergic cell subtypes and theirŽ .synaptic connections in rat frontal cortex, Cereb. Cortex 7 1997

476–486.w x24 U. Kim, M.V. Sanchez-Vives, D.A. McCormick, Functional dynam-

Ž .ics of GABAergic inhibition in the thalamus, Science 278 1997130–134.

w x25 A.R. Kriegstein, T. Suppes, D.A. Prince, Cellular and synapticphysiology and epileptogenesis of developing rat neocortical neu-

Ž .rons in vitro, Dev. Brain Res. 34 1987 161–171.w x26 A. Larkman, A. Mason, Correlations between morphology and

electrophysiology of pyramidal neurons in slices of rat visual cortex:Ž .I. Establishment of cell classes, J. Neurosci. 10 1990 1407–1414.

w x q27 R.E. Locke, J.M. Nerbonne, Role of voltage-gated K currents inmediating the regular-spiking phenotype of callosal-projecting rat

Ž .visual cortical neurons, J. Neurophysiol. 78 1997 2321–2335.w x28 N.M. Lorenzon, R.C. Foehring, The ontogeny of repetitive firing

and its modulation by norepinephrine in rat neocortical neurons,Ž .Brain Res. 73 1993 213–223.

w x29 R.D. Lund, M.J. Mustari, Development of the geniculocortical path-Ž .way in rats, J. Comp. Neurol. 173 1977 289–306.

w x30 D.V. Madison, R.A. Nicoll, Noradrenaline blocks accommodation ofŽ .pyramidal cell discharge in the hippocampus, Nature 299 1982

636–638.w x31 A. Mason, A. Larkman, Correlations between morphology and

electrophysiology of pyramidal neurons in slices of rat visual cortex:Ž .II. Electrophysiology, J. Neurosci. 10 1990 1415–1428.

w x32 D.A. McCormick, B.W. Connors, J.W. Lighthall, D.A. Prince,Comparative electrophysiology of pyramidal and sparsely spiny

Ž .stellate neurons of the neocortex, J. Neurophysiol. 54 1985 782–806.

w x33 D.A. McCormick, H.C. Pape, Properties of a hyperpolarization-activated cation current and its role in rhythmic oscillation in

Ž . Ž .thalamic relay neurones, J. Physiol. London 431 1990 291–318.w x34 D.A. McCormick, D.A. Prince, Post-natal development of electro-

physiological properties of rat cerebral cortical pyramidal neurones,Ž . Ž .J. Physiol. London 393 1987 743–762.

w x35 B.A. McGuire, C.D. Gilbert, P.K. Rivlin, T.N. Wiesel, Targets ofhorizontal connections in Macaque primary visual cortex, J. Comp.

Ž .Neurol. 305 1991 370–392.w x36 N. Mechawar, L. Descarries, Early postnatal development of the

cholinergic innervation of the rat cerebral cortex, Soc. Neurosci.Ž .Abstr. 24 1998 1338.

w x37 R. Metherate, Auditory thalamocortical and intracortical synapticŽ .transmission in vitro, Soc. Neurosci. Abstr. 23 1997 2070.

w x38 R. Metherate, Synaptic mechanisms in auditory cortex function,Ž .Front. Biosci. 3 1998 494–501.

w x39 R. Metherate, C.L. Cox, J.H. Ashe, Cellular bases of neocorticalactivation: modulation of neural oscillations by the nucleus basalis

Ž .and endogenous acetylcholine, J. Neurosci. 12 1992 4701–4711.w x40 R. Metherate, S.J. Cruikshank, Thalamocortical inputs trigger a

propagating envelope of gamma-band activity in auditory cortex, inŽ .vitro, Exp. Brain Res. 126 1999 160–174.

w x41 J. Mourek, W.A. Himwich, J. Myslivecek, D.A. Callison, The roleof nutrition in the development of evoked cortical responses in rat,

Ž .Brain Res. 6 1967 241–251.w x42 C.M. Muller, H. Scheich, Contribution of GABAergic inhibition to

the response characteristics of auditory units in the avian forebrain,Ž .J. Neurophysiol. 59 1988 1673–1689.

w x43 R.A. Nicoll, E.T. Iwamoto, Action of pentobarbital on sympatheticŽ .ganglion cells, J. Neurophysiol. 41 1978 977–986.

w x44 R.A. Nicoll, D.V. Madison, General anesthetics hyperpolarize neu-Ž .rons in the vertebrate central nervous system, Science 217 1982

1055–1057.w x45 L.G. Nowak, M.V. Sanchez-Vives, D.A. McCormick, Influence of

low and high frequency inputs on spike timing in visual corticalŽ .neurons, Cereb. Cortex 7 1997 487–501.


w x46 A. Nunez, F. Amzica, M. Steriade, Electrophysiology of cat associa-tion cortical cells in vivo: intrinsic properties and synaptic responses,

Ž .J. Neurophysiol. 70 1993 418–430.w x47 D. Oertel, Encoding of timing in the brain stem auditory nuclei of

Ž .vertebrates, Neuron 19 1997 959–962.w x48 G. Paxinos, C. Watson, The Rat Brain in Stereotaxic Coordinates,

2nd edn., Academic Press, New York, 1986.w x49 R.R. Pfeiffer, Classification of response patterns of spike discharges

for units in the cochlear nucleus: tone-burst stimulation, Exp. BrainŽ .Res. 1 1966 220–235.

w x50 D.P. Phillips, Central auditory processing: a view from auditoryŽ .neuroscience, Am. J. Otolaryngol. 16 1995 338–352.

w x51 D.P. Phillips, S.S. Orman, A.D. Musicant, G.F. Wilson, Neurons inthe cat’s primary auditory cortex distinguished by their responses to

Ž .tones and wide-spectrum noise, Hearing Res. 18 1985 73–86.w x52 J.O. Pickles, An Introduction to the Physiology of Hearing, Aca-

demic Press, London, 1982, 341 pp.w x53 A.D. Reyes, E.W. Rubel, W.J. Spain, Membrane properties underly-

ing the firing of neurons in the avian cochlear nucleus, J. Neurosci.Ž .14 1994 5352–5364.

w x54 F.d. Ribaupierre, M.H. Goldstein, G. Yeni-Komshian, IntracellularŽ .study of the cat’s primary auditory cortex, Brain Res. 48 1972

185–204.w x55 R.T. Robertson, K.A. Gallardo, K.J. Claytor, D.H. Ha, K.-H. Ku,

B.P. Yu, J.C. Lauterborn, R.G. Wiley, J. Yu, C.M. Gall, F.M.Leslie, Neonatal treatment with 192 IgG-saporin produces long-termforebrain cholinergic deficits and reduces dendritic branching andspine density of neocortical pyramidal neurons, Cereb. Cortex 8Ž .1998 142–155.

w x56 R.T. Robertson, F. Mostamand, G.H. Kageyama, K.A. Gallardo, J.Yu, Primary auditory cortex in the rat: transient expression ofacetylcholinesterase in developing geniculocortical projections, Dev.

Ž .Brain Res. 58 1991 81–95.w x57 M. Roger, P. Arnault, Anatomical study of the connections of the

Ž .primary auditory area in the rat, J. Comp. Neurol. 287 1989339–356.

w x58 D.K. Ryugo, H.P. Killackey, Differential telencephalic projections

of the medial and ventral divisions of the medial geniculate body ofŽ .the rat, Brain Res. 82 1974 173–177.

w x59 P.C. Schwindt, W.J. Spain, R.C. Foehring, C.E. Stafstrom, M.C.Chubb, W.E. Crill, Multiple potassium conductances and their func-tions in neurons from cat sensorimotor cortex in vitro, J. Neurophys-

Ž .iol. 59 1988 424–449.w x60 W.J. Spain, P.C. Schwindt, W.E. Crill, Anomalous rectification in

neurons from cat sensorimotor cortex in vitro, J. Neurophysiol. 57Ž .1987 1555–1576.

w x61 M. Steriade, D.A. McCormick, T.J. Sejnowski, ThalamocorticalŽ .oscillations in the sleeping and aroused brain, Science 262 1993

679–685.w x62 M. Steriade, I. Timofeev, N. Durmuller, F. Grenier, Dynamic prop-

erties of corticothalamic neurons and local cortical interneuronsŽ .generating fast rhythmic 30–40 Hz spike bursts, J. Neurophysiol.

Ž .79 1998 483–490.w x63 G.J. Stuart, H.-U. Dodt, B. Sakmann, Patch-clamp recordings from

the soma and dendrites of neurons in brain slices using infraredŽ .video microscopy, Pflugers Arch. 423 1993 511–518.

w x64 A.M. Thomson, J. Deuchars, Synaptic interactions in neocorticallocal circuits: dual intracellular recordings in vitro, Cereb. Cortex 7Ž .1997 510–522.

w x65 L.O. Trussell, Cellular mechanisms for preservation of timing inŽ .central auditory pathways, Curr. Opin. Neurobiol. 7 1997 487–492.

w x66 I.O. Volkov, A.V. Galazjuk, Formation of spike response to soundtones in cat auditory cortex neurons: interaction of excitatory and

Ž .inhibitory effects, Neuroscience 43 1991 307–321.w x67 F.H. Willard, D.K. Ryugo, Anatomy of the central auditory system,

Ž .in: J.F. Willott, C.C. Thomas Eds. , The Auditory Psychobiology ofthe Mouse, Springfield, 1983, pp. 201–304.

w x68 J.A. Winer, The functional architecture of the medial geniculatebody and the primary auditory cortex, in: D.B. Webster, A.N.

Ž .Popper, R.R. Fay Eds. , The Mammalian Auditory Pathway: Neu-roanatomy, Springer-Verlag, New York, 1992, pp. 222–409.

w x69 S.P. Wise, E.G. Jones, Developmental studies of thalamocortical andcommissural connections in the rat somatic sensory cortex, J. Comp.

Ž .Neurol. 178 1978 187–208.

intrinsic electrophysiology of neurons in thalamorecipient layers of developing rat auditory cortex

Documents