lsd and 2-bromo-lsd: comparison of effects on serotonergic neurones and on neurones in two...

h’ul,rophnrmacoloy?, 1976. 15. 521-528. Pergamon Press. Printed m Gt. Britain

LSD AND 2-BROMO-LSD: COMPARISON OF EFFECTS ON SEROTONERGIC NEURONES AND ON NEURONES

IN TWO SEROTONERGIC PROJECTION AREAS, THE VENTRAL LATERAL GENICULATE

AND AMYGDALA

G. K. ACHAJANIAN Departments of Psychiatry and Pharmacology, Yale University School of Medicine and

Connecticut Mental Health Center, New Haven, CT 06508

(Accrptrd 21 Drcrrnhvr 1975)

Summary-o-Lysergic acid diethylamide (LSD) and its psychotropically weak 2-bromo derivative (BOL) were compared in terms of their relative potencies in inhibiting serotonergic (5hydroxytryptamine) neurones versus neurones in two representative areas receiving an identified serotonergic input-the amygdala and ventral lateral geniculate. Drug ejection and unit recording were accomplished by means of 5-barrelled micropipettes. For each neurone tested, the ejection currents of LSD and BOL were standardized in relation to an ejection current of serotonin that produced 50% inhibition of firing within about 20 sec. Serotonin had a high inhibitory potency in all the areas studied. At equal ejection currents. serotonin and LSD (the latter ionically diluted 100 fold with NaCl) were equipotent in inhibiting serotonergic neurones in the dorsal raphe nucleus; BOL had substantially less inhibitory activity at equal or even higher ejection currents. In the amygdala and ventral lateral geniculate both LSD and BOL were considerably less potent than serotonin in inhibiting neuronal firing. Moreover. neither BOL nor LSD produced any marked blockade of serotonin in these areas. It is concluded that dttfer- ences in direct actions upon serotonergic (raphe) neurones discriminates best between LSD and BOL. o-lysergic acid diethylamide but not BOL had a highly potent serotonin agonist-like inhibitory action on serotonergic neurones, while both drugs had relatively weak inhibitory actions (in comparison with serotonin) on postsynaptic cells in the ventral lateral geniculate and amygdala. The greater potency of LSD in inhibiting serotonergic neurones parallels the greater hallucinogenic potency of LSD compared with BOL

Recently, it has been reported that the 2-bromo derivative (BOL) of D-lysergic acid diethylamide (LSD) is as potent as LSD in displacing D-C3H]LSD binding to presumed postsynaptic serotonin (S-hydroxytryptamine; 5-HT) receptors in brain (BENNETT and SNYDER, 1975). This contrasts with the fact that BOL has considerably less hallucinogenic activity than LSD (SCH- NECKLOTH, PAGE, DEL GRECO and CORCORAN, 1957; BERTINO, KLEE and WEINTRAUB, 1959; ISBELL, MINER and LOGAN, 1959). Thus, the similar effects of these drugs on serotonin binding in vitro are not paralleled by their markedly different psychotropic potencies. On the other hand, a number of physiological and biochemical studies indicate a greater potency for LSD than its 2-bromo derivative. For example, when it 15 given by the systemic route, LSD is consider-

ably more potent than BOL in inhibiting the firing of serotonergic neurones located in the midbrain raphe nuclei of the rat (AGHAJANIAN, F~XJTE and SHEARD, 1970). Activation of the firing of neurones in the lateral geniculate nucleus of the cat is also more sensitive to LSD than BOL when these drugs are given by the systemic route (HORN and MCKAY.

1973). Similarly, in microiontophoretic studies on netlrones in the brainstem (BOAKES, BRADLEY, BRIGGS and DRAY. 1970) and cerebral cortex (ROBERTS and

STRAUGHAN, 1967) excitatory effects of 5-HT are more readily blocked by LSD than BOL. Finally. in biochemical studies, LSD has considerably more potency in increasing brain 5-HT levels and reducing 5-HT turnover (FREEDMAN, 1961; AND~N, CORRODI, FUXE and HBKFELT, 1968).

The in h-o serotonin receptor binding studies are presumably dealing with postsynaptic 5-HT receptors, since chronic destruction of serotonergic neurones by raphe lesions does not alter the amount of binding (BENNETT and AGHAJANIAN, 1974; BENNETT and SNYDER, 1975). In previous studies it has been shown that LSD has a considerably more potent direct in-

hibitory action on serotonergic neurones than on

neurones receiving an identified serotonergic input

(e.g. in amygdala, ventral lateral geniculate, and optic

tectum; HAIGLER and AGHAJANIAN, 1974). In contrast, 5-HT itself was approximately equipotent in its ability to inhibit serotonergic neurones and identified postsynaptic neurones. It has been suggested that the hal- lucinatory and other effects of LSD may in part be due to its ability to inhibit selectively serotonergic neurones, thereby releasing postsynaptic areas from a tonic 5-HT inhibitory influence (AGHAJANIAN and HAIGLER, 1974). Thus, it is possible that the physiological effects of LSD and BOL do not differ greatly

521

522 G. K. AGHAJANIAN

at certain postsynaptic sites but that they differ mainly in their effects upon the presynaptic (serotonergic) neurone. Therefore, in the present study LSD and BOL were compared with respect to their relative potencies in inhibiting serotonergic neurones and neurones in two representative areas receiving an identified serotonergic input, the amygdala and ventral lateral geniculate (FUXE, 1965; KUHAR, AGHA- JANIAN and ROTH, 1972; AGHAJANIAN, KUHAR and ROTH, 1973).

METHODS

Male albino rats (Charles River Laboratories, Wilmington, MA), weighing approx. 250 g were used. Recordings in the postsynaptic areas (i.e. ventral lateral geniculate and amygdala) were carried out in unanaesthetized, low cerueau isole’ preparations. For this purpose the animals (under temporary halothane anaesthesia) were mounted in a stereotaxic instrument and a burr hole drilled 1 mm posterior to lambda, 4-5 mm lateral to the midline. The brain was then transected at the petrigeminal level (ZERNICKI, 1968) by means of a stereotaxically positioned retractable wire knife (SCLAFANI and GROSSMAN, 1969), which when extended was 8-9 mm in overall length. Haloth-

ane was then discontinued and a local anesthetic (mepivacaine HCl, 2%) was injected at all pressure points (external auditory canals and snout). Animals were then remounted in a stereotaxic instrument for unit recording from the ventral lateral geniculate and amygdala. Recordings from raphe neurones were made from chloral hydrate anaesthetized animals after it was determined in preliminary experiments that the administration of chloral hydrate to unanaesthetized (low cerueau isole’) preparations did not alter significantly either the spontaneous rate of firing of raphe neurones or their response to 5-HT, LSD or BOL. In the ventral lateral geniculate and amygdala, the use of unanaesthetized (low cerveau isole’) animals was necessitated by the fact that the administration of chloral hydrate markedly reduced or entirely elim- inated ongoing unit activity in these areas. In the case of raphe neurones, the use of chloral hydrate was pre- ferable to the low cerueau isok preparation in terms of recording stability, presumably because of the anatomical proximity of the knife cuts to the midbrain raphe nuclei.

Five barrel micropipettes were prepared using a Narishige pipette puller as described previously (HAIGLER and AGHAJANIAN, 1974). To improve recording quality the pipettes were pulled with relatively low heat and a delayed magnet pull to produce a wide tip angle rather than a filamentous tip. The tips were broken back to 4-5 pm under microscopic control and then directly filled by injection of solutions into the various barrels; the presence of fibre- glass filaments (placed in each barrel prior to pulling) allowed the tips to become filled rapidly by capillary action (TASAKI, TSUKAHARA, ITO, WAYNER and Yu,

1968). The central (recording) barrel was filled with 2 M NaCl saturated with fast green dye. The impe- dences of recording barrels were typically between 3 and 5 MR. One side barrel was filled with 4 M NaCl and this channel was used for automatically balancing

(neutralizing) tip current (SALMOIRAGHI and WEIGHT, 1967). The remaining 3 side barrels were filled with serotonin creatinine sulphate (0.04 M, pH 4.0), LSD (0.001 M in 0.1 M NaCl, pH 4.0), of BOL (0.001 M in 0.1 M NaCl, pH 4.0). The LSD was diluted ionically with NaCl in order to produce a systematic reduction

in its transport number (HAIGLER and AGHAJANIAN, 1974). The latter reduction was necessary because of the extreme sensitivity of raphe neurones to LSD, making an undiluted solution difficult to use in a con- trolled fashion. The BOL solutions were diluted ioni- tally to the same degree.

During periods of unit recording, a negative retain- ing current of 10 nA was applied to each drug barrel. The response of cells to 5-HT was tested first. An ejecting (positive) current of 5-HT that produced a 50% inhibition in spontaneous firing rate within 20-30 set was determined. The effects of LSD and BOL were then tested on each cell at the same ejection current used for 5-HT. By this means the ejection current for each cell was standardized in relation to 5-HT; this procedure allowed comparisons of relative inhibitory potencies within areas as well as between areas. The data were analyzed according to the method of HILL and SIMMONDS (1973): :;‘, inhibition was plotted against linear time of ejection at a constant current. The time required for 5076 inhibition (tso) was then used as a measure of inhibitory potency. Electrical signals of spike activity were fol- lowed by a high input-impedence amplifier, displayed on an oscilloscope, and led into an electronic counter whose threshold was set so that it was triggered by the individual spike of the single neurone under study. Counts of unit activity were then transformed into integrated rate histograms with an interval time of 1 or 10 sec. A mercury vapour lamp oscillograph was used to record directly spike activity to monitor for constancy of spike amplitude.

The anatomical location of all the units tested was determined by passing a 20 PA negative current through the recording barrel for IO min. This resulted in the deposition of fast green in a discrete spot (THOMAS and WILSON, 1965). Animals were anaesthetized and perfused with 10% formalin (phosphate buf- fered, pH 7.0). Serial sections were then cut, mounted and counterstained with neutral red. Although the identity of all cells was retrospectively determined by histological examination after each experiment, there were certain practical guidelines which were useful for locating units in the various regions during the actual recording procedure. For the raphe recordings, a burr hole was placed in the midline 0.5%1.0 mm anterior to lambda. The dorsal raphe nucleus was usually found 5.5-6.5 mm ventral to the skull surface, just ventral to a zone of relative electrical silence cor-

LSD, 2-bromo-LSD and serotonin 523

responding to the cerebral aqueduct. Serotonergic neurones can be tentatively identified by their regular

rhythm and slow rate (0.5-2 spikes/set). Only units with such characteristics have been specifically identified by histochemical methods to be serotonergic (AGHAJANIAN and HAIGLER, 1975). To record from the ventral lateral geniculate, a burr hole was placed 4.1 mm anterior to lambda and 4.1 mm lateral to the midline. Cells showing a distinct response to light flashes or movement usually could be detected 4.0-5.0 mm ventral to the skull surface. Cells in the ventral portion of this area exhibiting stable baseline rates under constant illumination were used. To record from the amygdala, a burr hole was placed 4.8 mm anterior to lambda and 4.4 mm lateral to the midline. The amygdala (basalateral, cortical and medial nuclei) was usually found between 7.8-9.2 mm ventral to the skull surface. These guidelines obviously are only approximations and would require some adjustment in rats of strains or weights other than used in this study. Fluorescence histochemical analysis of the distribution of 5-HT terminals in the lateral geniculate and amygdala was performed by the specific trypto-

phan-ldading method, as previously described (AGHA- JANIAN et a[., 1973).

RESULTS

Histology and histochemistry

All the single unit recordings included within the study were from neurones histologically verified by fast green marking to be either in the dorsal raphe nucleus of the midbrain, the amygdala (basolateral, cortical, or medial nucleus) and the ventral portion of the lateral geniculate nucleus. As can be seen from Figure lA, there is a marked increase in the density of 5-HT terminals in passing from the dorsal lateral to the ventral lateral geniculate nucleus. In the amygdala, there is an increase in the density of 5-HT terminals in passing from the lateral posterior (or anterior) nuclei to the basolateral (Fig. lB), cortical, and medial nuclei (not shown). Neurones within these zones of high 5-HT terminal density were almost in- variably responsive to the microiontophoretic application of 5-HT. Moreover, responses in these areas were always depressant in nature. In many cases, neurones in areas outside the zones of high 5-HT terminal density (e.g. dorsal lateral geniculate or posterior lateral amygdala) also showed depressant responses to the microiontophoretic application of 5-HT. However, in the low terminal areas, recovery from the depressant effects of 5-HT usually occurred more slowly than the high terminal areas; presumably because in the latter case the action of 5-HT could be terminated more efficiently by selective active uptake mechanisms (KUHAR and AGHAJANIAN, 1973). In any event, only neurones confirmed to be within the high 5-HT-terminal areas were used for single unit data analysis.

Single unit recording and microiontophoresis

In line with the previous results (AGHAJANIAN. HAICLER and BLOOM, 1972; HAIGLER and AGHA- JANIAN, 1974) single units in the dorsal raphe nucleus with the firing characteristics of 5-HT neurones were rapidly and totally inhibited by 5-HT and LSD ejected at low iontophoretic currents (Fig. 2). In contrast, BOL at equal or higher currents (or durations) usually failed to inhibit firing completely (Fig. 2). In addition, during periods of BOL application, the effects of 5-HT and LSD were intensified. This can

be seen from the last portion of the trace in Figure 2, where during the concurrent application of BOL, an extremely short pulse of 5-HT produced an almost instantaneous inhibition and the effect of LSD was prolonged.

Neurones in the ventral lateral geniculate were also

rapidly and completely inhibited by 5-HT but were relatively resistant to LSD (Fig. 3). The effects of pulses of 5-HT during periods of prolonged LSD (Fig. 3) or BOL application were not attenuated as indicated by the fact that the time to 50% inhibition (tso) during control 5-HT periods (n = 15) was not significantly different (P > 0.1) from paired 5-HT periods during concurrent LSD (n = 8) or BOL ejection (n = 7).

Neurones in the amygdala were also highly sensitive to 5-HT but were relatively insensitive to LSD and BOL even with prolonged periods of ejections (Fig. 4). In partial contrast to the results in the ventral lateral geniculate, at the same ejection currents LSD was generally more potent than BOL in producing direct depressant responses in the amygdala (Fig. 4). As in the ventral lateral geniculate, concurrent application of BOL (n = 7) or LSD (n = 7) did not alter significantly (P > 0.1) the inhibitory tsO of pulses of 5-HT in comparison to control pulses of 5-HT (n = 14). However occasionally the maximal effect of 5-HT was slightly reduced by LSD (e.g. 3rd to last

application of 5-HT in Fig. 4). This apparent blocking effect occurred only at times when there already was considerable depression of firing rate produced by the prolonged application of LSD itself. Thus, any 5-HT blocking action of LSD on amygdala neurones occurred after periods of LSD ejection considerably beyond that required to inhibit raphe neurones (see above). Moreover the prolonged application of LSD tended to reduce spike amplitude, indicating some degree of local anaesthetic action. The early portion of the record shown in Figure 5 again illustrates a reduction in maximal response to 5-HT induced by the concurrent application of LSD. Previous studies have shown that LSD given intravenously in doses which inhibit raphe firing tend to increase rather than decrease the rate of firing of postsynaptic neurones in the amygdala and elsewhere, perhaps through a release from a tonic raphe inhibitory influence (HAIGLER and AGHAJANIAN, 1974). In the latter study, the inhibitory response to 5-HT was not diminished after low intravenous doses of LSD. Similar results

Fig. 1. A: Hisrofluorescence micrograph of the tateral geniculate nucleus near junction of dorsal (DLG) and ventral (VLG) nuclei. As can be seen, there is a strikingly greater number of 5-HT terminals (small dots) II? the W_G and border zone than in the body of the DLG. Experimental conditions were such as to specitieally enhance the fluorescence of 5-HT terminals (see ~~~AJ~~I~~ et aL, 1973). OT: optic tract. ScaIe: 20 pm. 8: HistoHuorescence micrograph showing border zone between anterola- terai (ALP) and basolateral (ABL) nucler of amygdala, There is a much higher density of 5-HT terminals (small dots) in ARL than in ALP. Larger bright dots in ventro-lateral portion of ABL represent

catecholamlne terminals. V: lateral cerebral ventricle. Scale: 20 ym.

5.24

LSD, 2-bromo-LSD and serotonin

BOL 10

SHT LSD BOL BOL 5HT LSDSHT BOL BOL 5HT LSD I,0 l_o 10 20 10 I,0 E 10 wm- 20 l_o l,o

CJ 101

525

t 10 min

I

Fig. 2. Integrated rate recording comparing effects of microiontophoretically applied 5-HT, LSD and BOL on the rate of firing of a neurone in the dorsal raphe nucleus, Note that both 5-HT and LSD at the same ejection current (10 nA) rapidly produced a total inhibition of firing. Although the rate at which inhibition was produced by LSD is approximately the same as for 5-HT, recovery from LSD was somewhat slower than from 5-HT. BOL up to 20 nA applied for prolonged periods failed to produce total inhibition; with some recovery beginning even prior to termination of ejection. Concur- rent application of BOL failed to block 5-HT or LSD (latter part of records). Sample time, 10 set for integrated rate recording (see Methods). Duration of ejection is indicated by horizontal bars below

values for ejection currents (in nA).

5HT LSD BOL 5HT

I_0 10 ID’ 5HT \

AL lo - Lo lo 251

t 5 min

I

Fig. 3. Comparison of the effects of 5-HT, LSD and BOL on the rate of firing of a neurone in the ventral lateral geniculate nucleus. As can be seen, 5-HT (10 nA) induced a rapid and complete inhibition of firing but LSD and BOL were relatively ineffective even with prolonged periods of ejection. Concurrent application of LSD (latter portion of record) failed to block 5-HT given in repeated pulses.

BOL LSD 16 10

5HT BOL LSD SHT 5HT 5HT5HT 5HTSHT 5Hf 16 16 16 16 16 16 16 -v- -_ -- 1616 12

r 10 min ’

Fig. 4. Comparison of the effects of 5-HT. LSD and BOL on the rate of firing of a neurone in the basolateral nucleus of the amygdala. Note that 5-HT rapidly pr0duced.a complete inhibition of firing but that BOL was relatively ineffective. LSD was intermediate in activity but required a substantially longer period of ejection than did 5-HT to reach the 509:, level of inhibition (see Fig. 6). Also note the reduction in maximal S-HT inhibition during concurrent LSD ejection (third to last 5-HT

pulse record).

G. K. AGHAJANLAN

r Fig. 5. A comparison of the influence of microiontaphoreticalty applied LSD versus a small intravenous dose of LSD (10 fig/kg) on responses of a neurone in the basolateral amygdah to 5-HT. The prolonged ejection of LSD (6 nA) slightly attenuated the maximal response ta 5-HT. However, no attenuation

of S-HT was seen after intravenous LSD, despite some activation in baseline firing rate.

were found in the present experiments: small intravenous doses of LSD (10-20 pg/kg) which tended to accelerate the rate of firing in amygdala (n = 6) neurones, did not attenuate the response to iontophoretic S-HT {Fig. 5).

The combined data comparing responses of neurones to 5-HT, LSD and BOL in the raphe* ventral lateral geniculate. and amygdala are summarized

graphically in Figure 6. It can be seen that raphe neurones responded identically to 5-HT and LSD given at the same ejection currents (Fig. 6A); the t,, in both cases being 18 sec. In a previous study it was shown that because of the 100 fold ionic dilution of LSD by NaCl, at equal ejection currents 100 times as many molecules of 5-HT as LSD are being ejected under these experimental conditions (HAIGLER and AGHAIANIAN, 1974). The ejection of BOL onto raphe cells at the same currents used for LSD and 5-HT produced much less of a response: the 507; level of inhibition was not reached even after 180 set of ejection. In the ventral lateral geniculate, 5-IIT produced its usual potent inhibitory response (tsO = 18 set; Fig. 6B). However, in this region both LSD and BOL had a considerably weaker action than 5-HT, and neither reached the SOY/, level of inhibition. As can be seen from Figure 6C, the inhibitory effect of LSD in the amygdala was intermediate between that of S-HT and BOL. Nevertheless, the mean inhibitory effect of LSD did not exceed the 50% level of inhibition even though the action of WIT in the amygdala was as potent as in the raphe (tso = 18 set at IO nA of ejection current).

In this study, the effects of BOL and LSD on serotonergic neurones of the dorsal raphe nucleus were compared with effects on neurones in two areas which receive a prominent serotonergic input (i.e. ventral lateral geniculate and amygdala). The most striking difference between the two drugs was the fact that LSD had a highly potent .5-WT agonist-like inhibitory

action on raphe (serotonergic) neurones, while BOL was relatively inactive at this site. In contrast, both BOL and LSD (relative to 5-HT) had weak inhibitory actions on postsynaptic cells in the ventral lateral geniculate and amygdab. Thus, the difference in their direct action upon serotonergic (raphe) neurones per se appears to distinguish best between LSD and BOL.

50 “_ P -~-r-______-- w---x 2

= $0 - -----~-.-- __-_ 9 k 2

is z o

”

’ ’ 8 0 60 120 I%0

WRBTlON OF EJECTION lsecf

Fig. 6. Percentage inhibition of firing rate (ordinate) plotted against linear time (abcissa) comparing responses of 5-HT. LSD and BQL in the raphe (A), ventral lateral geniculate (B), and amygdala (C). Levels of statistical aignifi- cance (paired r-test) between 5-HT responses and responses to BOL or LSD are indicated by asterisks (* P < 0.005; ** P r O.OOf). For each cell tested, an ejection current for 5-HT that produced 50% inhibition within about 20 set was established first. The same ejection current was

then used for LSD and B5L for each cell.

LSD. t-bromo-LSD and serotonin 527

This difference parallels the marked differences in the hallucinogenic or psychotogenic actions of these drugs. Although BOL was first believed to have no psychological effects (CERLETTI and ROTHLIN, 1955), later studies showed that very large doses of BOL (e.g. > 64 pg/kg) could induce a partial LSD-like syn- drome except for visual hallucinations (SCH~ECK~TH

rt al., 1957; BERTINO rt al., 19.59; ISBELL et al., 1959). Similarly, very large intravenous doses of BOL are able to produce at least a partial inhibition of raphe

neurones (AGHAJANIA~ et al.. 1970). The similar potency of LSD and BOL in the amyg-

dala and ventral lateral geniculate (i.e. postsynaptic areas) corresponds to the in vitro studies of BENNETT and SNYDER (1975) on presumed S-HT receptor binding in which LSD and BOL were virtually equipotent

in their ability to displace bound D-C3H]LSD. It is puzzling, however, that neither BOL nor LSD greatly potentiated or antagonized S-HT responses in the terminal fields studied. This raises the question as to whether the binding of C3H]-LSD can be taken to provide a reliable index to the presence of 5-HT receptors, at least in a physiological sense. In any event, it has been shown that in z&-o LSD binding primarily reflects binding at postsynaptic sites, since the destruction of serotonergic projections by place- ment of fesions in the raphe nuclei does not reduce the number of binding sites in postsynaptic areas (BENNETT and ACHAJANIAN, 1974; BENNETT and SNYDER, 1975). Thus the in citro binding studies are not likely to reveal the differences between LSD and BOL with respect to the potency of their actions on rhe presynaptic (serotonergic) neurones,

In the present experiments, the inability to demon- strate any striking 5-HT blocking action by LSD or BOL is consistent with most previous reports dealing with the depression of neuronal firing by S-HT in various areas of brain (CURTIS and DAVIS, 1962; KRNJEVIC’ and PH~LIS, 1963; LEGGE, RANDIC’ and STRAUGHAN, 1966; ROBERTS and STRAUGIIAN, 1967; BOAKES et at., 1970; BLOOM, H~FFER, SIGGIN$ BARKER and NICOLL, 1972). In one study, performed on olfactory bulb neurones, LSD and .BOL were found to block 5-HT-induced depressions of firing, but these effects were regarded as being non-specific since norepinephrine depressions were also blocked (BLOOM, COSTA and SALMOIRAGHI, 1964). In the present study, although time to 50”/, inhibition by 5-HT was not increased by LSD or BOL, occasionally in the amyg- Idala the maximal extent of S-HT inhibition was

‘reduced by LSD. This result may correspond to the report of SEGAL and BLOOM (1974) showing that depressant responses to 5-HT in the hippocampus were sometimes attenuated by LSD. However, the amounts of LSD required to produce antagonism of 5-HT depressant effects are probably much higher than required to directly inhibit raphe neurones (see Results and HAICLER and AGNAJANIAN, 1974).

In contrast to its lack of pronounced effect on 5-HT depressions. excitatory responses to 5-HT in the cere-

bra1 cortex (ROBERTS and STRAUGHAN, 1967) and brainstem (BOAKES et al., 1970) are consistently blocked by LSD. Moreover. LSD is distinctly more

potent than BOL in blocking such 5-HT excitations (BRAKES et al., 1970; ROBERTS and STRAUGHAN, 1967). Thus, the greater potency of LSD in the latter studies also parallels the greater psychotogenic action of LSD as compared with BOL. It remains to be established that the excitatory effects of S-HT described in the above studies are upon neurones which in fact receive a 5-HT synaptic input. Nevertheless, it can be postu- lated that the ultimate effect of LSD on 5-HT transmission is the same no matter whether a direct inhibition of presynaptic cells or a postsynaptic blockade of 5-HT receptors is the primary mechanism

involved. Thus, if LSD directly inhibited serotonergic neurones, an impairment of serotonergic transmission would occur at all postsynaptic sites (inhibitory or excitatory) due to a reduction in impulse-flow depen- dent 5-HT release (GALLAGER and AGHAJANIAN, 1975). Similarly, if LSD blocked 5-HT excitatory (or inhibitory) responses, then this would simply reinforce the effects of raphe inhibition. Thus, both of the

mechanisms suggested above for the action of LSD would lead to a failure of serotonergic transmission. In parallel with its weak psychotropic effects, BOL is less potent than LSD in both of the actions pro- posed above.

Acknowlrc!qements-The author is grateful for useful dis- cussions with Dr. SOLOMON SNYDER and Professor P. B. BUDLEY and his associates. Supported in part by P.H.S. Grants MH-17871, MH-14459 and the State of Connecti- cut.

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lsd and 2-bromo-lsd: comparison of effects on serotonergic neurones and on neurones in two...

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