pharmacological characterization of dopamine, norepinephrine and serotonin release in the rat...
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Brain Research 990 (2003) 203–208
Research report
Pharmacological characterization of dopamine, norepinephrine and
serotonin release in the rat prefrontal cortex by neuronal nicotinic
acetylcholine receptor agonists
Tadimeti S. Rao*, Lucia D. Correa, Pamala Adams, Emily M. Santori, Aida I. Sacaan1
Merck Research Laboratories, 3535 General Atomics Court, San Diego, CA 92121, USA
Accepted 4 August 2003
Abstract
Neuronal nicotinic acetylcholine receptors (nAChRs) modulate synaptic transmission by regulating neurotransmitter release, an action
that involves multiple nAChRs. The effects of four nAChR agonists, nicotine (NIC), 1,1-dimethyl-4-phenylpiperzinium iodide (DMPP),
cytisine (CYT) and epibatidine (EPI) were investigated on [3H]-norepinephrine (NE), [3H]-dopamine (DA) and [3H]-serotonin (5-HT) release
from rat prefrontal cortical (PFC) slices. All four agonists evoked [3H]-DA release to a similar magnitude but with a differing rank order of
potency of EPIHDMPPcNICcCYT. Similarly, all four agonists also increased [3H]-NE release, but with a differing rank order of
potency of EPIHCYTcDMPP>NIC. NIC-induced [3H]-NE and [3H]-DA release responses were both calcium-dependent and attenuated
by the sodium channel antagonist, tetrodotoxin (TTX) and by the nAChR antagonists mecamylamine (MEC) and dihydro-h-erythroidine(DHhE), but not by D-tubocurare (D-TC). The modulation of [3H]-5-HT release by nAChR agonists was distinct from that seen for
catecholamines. DMPP produced robust increases with minimal release observed with other agonists. DMPP-induced [3H]-5-HT release was
neither sensitive to known nAChR antagonists nor dependent on external calcium. The differences between nicotinic agonist induced
catecholamine and serotonin release suggest involvement of distinct nAChRs.
D 2003 Elsevier B.V. All rights reserved.
Theme: D-Neurotransmitters, modulators, transporters and receptors
Topic: Acetylcholine receptors, nicotinic
Keywords: Nicotine; Neuronal nicotinic acetylcholine receptor; Prefrontal cortex; Catecholamine and serotonin release
1. Introduction limited number of studies have evaluated nAChR regulation
Neuronal nicotinic acetylcholine receptors (nAChRs)
regulate catecholaminergic and cholinergic neurotransmis-
sion in several brain regions ([4,7,11,13,17–19,20,23,27–
29], reviewed in Ref. [37]). The nAChR regulation of
dopamine (DA) release has been extensively studied in
the projection areas of the nigrostriatal pathway (e.g.,
striatum [4,7,11,13,23,28]) and/or in the mesolimbic path-
way (e.g., nucleus accumbens [27]) as these pathways
contain nAChRs [3]. The nAChR regulation of NE release
was also examined in the hippocampus [4,17,28,35]. A
0006-8993/$ - see front matter D 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0006-8993(03)03532-7
* Corresponding author. Kalypsys, Inc., 11099 North Torrey Pines
Road, La Jolla, CA 92037, USA. Tel.: +1-858-754-3300.
E-mail address: [email protected] (T.S. Rao).1 Arizeke Pharmaceuticals Inc., 6828 Nancy Ridge Dr. Suite 400, San
Diego, CA 92121, USA.
of 5-HT release in the striatum and hippocampus
[10,12,15,35,36]. Several lines of evidence suggest a dif-
ferential regulation by nAChRs of DA in the striatum and
NE release in the hippocampus [4,28–30]. In addition, the
pharmacology of nAChR regulation of hippocampal 5-HT
appears to be quite different from that of NE release [15].
These results suggest that multiple subtypes of nAChRs are
involved in regulating neurotransmission in different brain
regions and are consistent with the molecular diversity of
nAChRs [26,31,39]. To date, investigation of nAChR reg-
ulation of multiple neurotransmitters in a single brain region
has not been reported and this was the focus of our
investigation.
The role of nAChRs in prefrontal cortical (PFC) function
is of considerable interest as this region receives multiple
neuronal inputs susceptible to modulation by nAChRs. The
extensive cholinergic projections play an important role in
T.S. Rao et al. / Brain Research 990 (2003) 203–208204
higher brain functions, such as cognition [1,6]. Ligand
binding and functional studies demonstrate nAChR locali-
zation in the PFC [16,17,34] and modulation of excitatory
synaptic transmission [14,34]. Studies in which nAChR
antagonists are directly injected into the rat PFC indicate a
critical role of nAChR neurotransmission in information
processing [8]. In addition to cholinergic projections, the
PFC also receives DA projections from the ventral tegmen-
tal area, NE projections from the locus coeruleus and 5-HT
projections from the raphe nucleus (dorsal and median
nuclei) [5,33]. These monoaminergic systems appear to play
an important role in modulation of memory fields [37]. ACh
release in the PFC and subsequent activation of nAChRs
can therefore differentially influence the release of a number
of neurotransmitters. These activities may be of importance
in understanding the cognitive enhancing activities of nic-
otine (NIC) and other nAChR agonists in rodents and
humans [1,22].
The results have appeared in abstract form [25,30].
Fig. 1. Concentration-related effects of NIC, EPI, DMPP and CYT on [3H]-
DA release (Panel A) or [3H]-NE release (Panel B) from rat PFC slices.
Data are normalized to NIC (30 AM) and represent meansF S.E.M. (n= 3–
5 experiments each with two to three replicates).
2. Materials and methods
1-[7,8-3H] Norepinephrine ([3H]-NE, 40 Ci/mmol) was
purchased from Amersham (Arlington Heights, IL). 3,4-
[7-3H] dihydroxyphenylethylamine ([[3H]-DA, 20 Ci/mmol)
and, [3H]-5-hydroxytryptamine (28 Ci/mmol; 5-HT or Se-
rotonin) were purchased from NEN (Boston, MA). (� )-
NIC hydrogen tartrate, mecamylamine (MEC) HCl, D-TC,
1,1-dimethyl-4-phenylpiperzinium iodide (DMPP), desipra-
mine HCl, cytisine (CYT), tetrodotoxin (TTX) HCl were
purchased from Sigma (St. Louis, MO). Dihydro-h-eryth-roidine (DHhE) HCl and (F ) epibatidine (EPI) 2 HCl were
purchased from Research Biochemical (RBI, Natick, MA).
All other reagents were of the highest purity commercially
available.
2.1. Animals
Male Sprague–Dawley rats (250–300 g) purchased from
Harlan (San Diego, CA) were used throughout the study.
The rats were acclimated to the vivarium (temperature: 22–
24 jC, humidity: 50–55%, with 12-h light–dark cycle) for
3–5 days before use in experiments. All the experiments
were conducted as per institutionally approved animal care
guidelines.
2.2. Neurotransmitter release assays
Superfusion release assays in the various brain areas
were conducted as previously described [28,29]. Briefly,
rats were decapitated and the brain rapidly dissected on ice.
The PFC slices were cross chopped (300 Am) in a McIlwain
tissue chopper and equilibrated in Kreb’s buffer (in mM:
sodium chloride, 119.5; potassium chloride, 3.3; calcium
chloride, 1.3; potassium dihydrogen phosphate, 1.2; mag-
nesium sulfate, 1.2; EDTA, 0.03 and glucose 11.0; pargy-
line, 0.01) that was continuously gassed with 95–5% O2/
CO2 mixture for 10 min. The PFC slices were loaded with
one of the following tritiated neurotransmitters (60 nM [3H]-
DA, 50 nM [3H]-NE and 57 nM of ([3H]-5-HT) for 30 min
at 37 jC in Kreb’s buffer. For the [3H]-DA release assay,
PFC slices were pre-incubated for 5 min at 37 jC in Kreb’s
buffer containing desipramine (1 AM) prior to the addition
of [3H]-DA. Inclusion of 5-HT uptake inhibitor during this
labeling procedure did not affect the uptake of [3H]-DA to
any significant extent. Therefore, all subsequent experi-
ments for [3H]-DA release were conducted only in the
presence of desipramine. Similar experiments suggested
limited influence of specific DA uptake inhibitors or 5-HT
inhibitors on [3H]-NE uptake or DA/NE uptake inhibitors
on [3H]-5HT uptake, respectively. Therefore, PFC slices
were loaded with [3H]-NE or [3H]-5HT in the absence of
any uptake inhibitors. At the end of the loading period,
tissue was rinsed with fresh warm Kreb’s buffer, transferred
to chambers and continuously superfused with oxygenated
buffer for 60 min. Following collection of superfusates to
establish basal release, slices were exposed to test com-
pounds for 3-min intervals. For antagonist sensitivity
experiments, either MEC, DHhE or D-TC was added 3
T.S. Rao et al. / Brain Research 990 (2003) 203–208 205
min prior to the agonist stimulation and included during the
agonist stimulation. The fractional efflux of tritium was
estimated as the amount of radioactivity in the superfusate
fraction relative to the total amount in the tissue, multiplied
by 100.
3. Results
The nAChR agonists tested increased [3H]-DA and [3H]-
NE release from rat PFC slices in a concentration-dependent
manner (Fig. 1).
Fig. 2. Pharmacological characterization of NIC-induced [3H]-DA or [3H]-NE
experiments with two to three replicates). (A or E) Effect of presence or absence of
or D) Effect of different nAChR antagonists (*p< 0.05 vs. NIC alone). (C or F)
In the [3H]-DA release assay, EPI was the most
potent nAChR agonist among the four agonists examined
(F [3,15] = 12.4, p < 0.001; Fig. 1, panel A). All four
agonists evoked [3H]-DA release to a similar magnitude.
Similarly, all four nAChR agonists also increased [3H]-NE
release from PFC slices in a concentration-related manner
with a similar magnitude of increase (Fig. 1, panel B). EPI
was the most potent agonist among the four agonists exam-
ined while NIC was the least potent agonist (F [3,8] = 25.3,
p < 0.001).
NIC-induced [3H]-DA or [3H]-NE release was largely
calcium-dependent (Fig. 2, panels A and D). NIC-evoked
release from rat PFC slices. Data represent meansF S.E.M. (n= 3–4
2.4 mM calcium in the superfusion buffer (*p< 0.05 vs. calcium present). (B
Effect of TTX (*p< 0.05 vs. NIC alone).
Fig. 3. Concentration-related effects of NIC, EPI, DMPP and CYT on [3H]-
5-HT release from rat PFC slices. Data are normalized to DMPP (30 AM)
and represent meansF S.E.M. (n= 3 experiments with two to three
replicates in each experiment).
T.S. Rao et al. / Brain Research 990 (2003) 203–208206
catecholamine release was sensitive to the nAChR antago-
nists, MEC and DHhE, but not to D-TC (Fig. 2, panels B
and E). In addition, NIC-evoked catecholamine release was
sensitive the sodium channel blocker, TTX (1 AM; Fig. 2,
panels C and F).
In contrast to their effects on [3H]-NE and [3H]-DA
release in PFC, the nAChR agonists showed a differential
profile on [3H]-5-HT release (Fig. 3). DMPP elicited robust
increases in [3H]-5-HT release. In contrast, EPI did not
evoke any noticeable increase in [3H]-5-HT release at
concentrations as high as 100 AM. DMPP-evoked [3H]-
5HT release was largely insensitive to the nAChR antago-
nists, MEC, DHhE or D-TC or to the removal of external
calcium or TTX (data not shown).
4. Discussion
The nAChRs are proposed to play an important role in
modulating synaptic transmission by their effects on neuro-
transmitter release [38]. The nAChRs have a potentially
enormous structural and functional diversity due to their
pentameric structure with multiple genes encoding the alpha
(a2�a9) and beta (h2�h4) subunits involved in the
formation of heteromeric or homooligomeric receptors
[26,31,39]. Neurotransmitter release studies in rodent brain
slices or synaptosomes not only provide a useful means of
defining function of native nAChRs, but also provide some
insight into the composition of the receptors. If release is
regulated by different receptors, this should reflect in
differential agonist rank order of potency and efficacy, as
well as differential antagonist sensitivity. It appears that
different nAChRs regulate the release of a given neurotrans-
mitter in different brain regions [4,23,38]. The results from
this study demonstrate differential regulation of catechol-
amine and 5-HT release in the rat PFC by nAChR agonists.
EPI was the most potent in evoking catecholamine
release, yet it was least effective at evoking [3H]-5HT
release. The potency of EPI on PFC catecholamine release
is consistent with reports that is the most potent nAChR
agonist in the release of catecholamines from striatal or
hippocampal preparations [23,28–30,38] as well as in in
vivo assays [24]. The nAChR agonists did not show marked
potency and efficacy differences in eliciting catecholamine
release. Puttafarken et al. [23] reported the rank order of
potency of EPI>CYT<NIC>DMPP in evoking [3H]-DA
release from PFC. These differences may be related to
methodological differences such as the inclusion of NE
uptake inhibitor during the loading of PFC slices with
[3H]-DA in the present investigation.
Since both a and h subunits have been shown to
contribute to agonist pharmacology [16], it is conceivable
that subtle differences in the heteromeric assembly of
nAChR subunits, and relative distribution of these recep-
tors may contribute to differences in agonist rank order of
potency and/or antagonist sensitivity. In situ hybridization
studies indicated that DA and NE cell bodies have distinct
nAChR subunit mRNA distribution (reviewed in Refs.
[25,39]), and therefore, the respective projections are also
likely to possess distinct nAChRs. This reflected in re-
gional differences in nAChR-evoked catecholamine release
[4,28,38].
The nAChR pharmacology of 3H]-DA and [3H]-NE
release in PFC slices provides some evidence as to which
nAChR subunits may be involved. Thus, at h2-containingheteromeric nAChRs, CYT functions as a partial agonist.
The competitive antagonist, DHhE, is relatively more potent
at inhibiting heteromeric h2-containing nAChR responses
relative to those from h4-containing nAChRs [2,9]. In slice
superfusion assays, DHhE significantly attenuates nAChR-
mediated striatal DA release without significant effects on
nAChR-mediated hippocampal NE release [4,28–30]. On
the other hand, another nAChR antagonist, D-tubocurare (D-
TC) is more effective at attenuating NIC-induced hippo-
campal NE release than NIC-induced striatal DA release.
These data suggest that D-TC may have an opposite nAChR
subunit selectivity to that of DHhE. The use of highly
subtype selective antagonists such as alpha-conotoxin AuIB
and alpha-conotoxin MII further support the notion that
striatal DA release and hippocampal NE are differentially
regulated by nAChRs [11,13,17]. The pharmacological
sensitivity of nicotine-induced [3H]-DA and [3H]-NE re-
lease in PFC to DHhE, but not to D-TC, suggests the likely
involvement of h2-containing nAChRs. However, the full
agonist activity of cytisine in [3H]-DA and [3H]-NE release
from the rat PFC slices implies that the h2 subunit combi-
nations in the PFC are different from those in the striatum.
Similarly, the lack of D-TC sensitivity of [3H]-DA and [3H]-
NE in PFC suggest that nAChR combinations in the PFC
are also different from those involved in the hippocampal
NE release.
The pharmacology of nAChR agonist-induced [3H]-5-
HT release in the PFC is distinctly different from [3H]-NE
or [3H]-DA release in the same region. Surprisingly, EPI
T.S. Rao et al. / Brain Research 990 (2003) 203–208 207
was the least effective in [3H]-5-HT release with DMPP
being the most efficacious and most potent of the four
ligands examined. This profile does not match pharmacol-
ogy of any known recombinant nAChRs. In addition,
DMPP-induced [3H]-5-HT release appeared to have atypi-
cal nAChR pharmacology in that the release was insensitive
to putative nAChR antagonists and was independent on
extracellular calcium. In this respect, nAChR agonist-in-
duced [3H]-5-HT release from PFC shows considerable
similarities to that seen the hippocampal slices. Thus,
Lendvai et al. [15] observed a lack of effect of EPI, NIC
or CYT on electrically evoked hippocampal [3H]-5-HT
release while DMPP evoked a robust release. DMPP-
evoked [3H]-5-HT release in the hippocampus was also
calcium-insensitive and showed only modest attenuation by
MEC at concentrations as high as 10 and 20 AM [15].
Kenny et al. [12] also reported a complex influence of
nAChRs on hippocampal 5-HT release. Although consid-
erable evidence exists for presynaptic localization of
nAChRs on the cortical catecholamine terminals [3], defin-
itive evidence for the localization of nAChRs on seroto-
nergic terminals exists only for the striatum [21] and
hypothalamus [32]. Thus, the destruction of 5-HT neurons
markedly decreased [3H]-ACh binding to nAChRs in the
striatum and hypothalamus, without significant reductions
in the thalamus or cortex [32]. [3H]-5-HT release in this
context argues that mechanisms other than receptor-medi-
ated exocytosis are operative.
In summary, the present investigation demonstrates a
differential regulation of catecholamine and 5-HT release
in rat PFC by nAChRs. Pharmacological manipulations that
either directly activate nAChRs in PFC or indirectly activate
through increases in ACh are likely to influence DA and NE
neurotransmission and, to a lesser extent, 5-HT neurotrans-
mission in the PFC. These activities, combined with in-
creased glutaminergic neurotransmission observed with
nAChR activation [34] strongly suggest that nAChRs in
the PFC subserve the various components of excitatory
functions such as focus/attention, abstract thinking and
decision processes [34,37]. The results from this investiga-
tion implies that nicotinic agonists can differentially affect
neurotransmitter release in a given brain region and that the
magnitude of such responses will largely be determined by
the subtype selectivity of the agonist.
Acknowledgements
The authors acknowledge support and critical input from
Dr. G. Kenneth Lloyd during the course of the investigation.
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