Effects of hallucinogenic drugs on serotonergic neuronal systems

Pharma~olog) Biochemistry & Behavior, Vol 24, pp 359-363 1986 e Ankho International Inc Printed m the U S A

0091-3057/86 $3 00 + 00

Effects of Hallucinogenic Drugs on Serotonergic Neuronal Systems ROBERT B McCALL

Cardtova3~ ular D i s e a s e s Re.search, The U p / o h n C o m p a n y , 7243-209-3, K a l a m a z o o , M i 49001

McCALL, R B Lffe~ t~ oJ hallu~tnogent~ drug~ on ~erotonergl~ neuronal ~v~tem~ PHARMACOL BIOCHEM BEHAV 24(2)359-363, 1986 --Studies indicate that hallucinogens markedly suppress the discharge of serotonln containing neurons in the dorsal raphe nucleus Forebraln neurons receiving a major serotonerglc input, are relatively insensitive to hallucinogens These actions of hallucinogens are not sufficient to explain the psychoactive effects of these drugs Ewdence is presented to indicate that hallucinogens sensitize serotonln and noreplnephrme receptors in the facial nucleus This receptor sensitizing effect is common to all, and specific for, hallucinogens It is suggested that a mechanism of receptor sensitization might account for the altered perceptual reactlvlly produced by hallucinogens Serotonln

Hallucinogens

d-Lyserglc acid dlethylamlde

BA SE D upon the structural similarity between the hallucinogen d-lyserglc acid dlethylamlde (LSD) and serotonm (5-HT), Gaddum [14] and Wooley and Shaw [34] independently proposed that the potent psychoactive effects of LS D might be mediated by an action on 5-HT receptors in the central nervous system However, it was not until Dahlstrom and Fuxe localized 5-HT containing penkarya to the brain stem raphe nuclei and identified the areas of projection of these neurons using fluorescence hlstochemlcal techniques that the effects of hallucinogens on 5-HT pathways could be studied directly [11,13] Using mlcroelectrode recording techniques, Aghajanlan and co-workers were the first to examine the effects of intravenous (IV) LSD on 5-HT neuronal finng recorded from the dorsal raphe nucleus [2,3] They found that LSD (10-50 ~g/kg) produced a complete but reversible inhibition of 5-HT neuronal discharges Similarly, the mdoleamlne hallucinogens psdocln and N,Ndlmethyltryptamlne (DMT) inhibited raphe firing [3,4] In contrast, the non-hallucinogenic analog of LSD, brom-LSD, faded to reduce 5-HT unit activity More recently. Mosko and Jacobs [ 2 6 ] demonstrated that 5-methoxy-N,Ndlmethyltryptamme (5-MeODMT), an ldoleamlne with weak hallucinogenic properties, also depressed raphe neuronal firing In order to determine if hallucinogens acted directly on 5-HT neurons to suppress their firing, Aghajanlan and coworkers studied the effect of mlcrolontophoretically applied LSD on raphe cells [1, 4, 6] They found that small ejecting currents of dilute LSD profoundly suppressed the firing of serotonerglc neurons Direct application of 5-HT also inhibited raphe finng (Fig 1, upper trace), although L SD was much more than 5-HT in its inhibitory effects lontophoretlc application of psllocin, DMT and 5-MeODMT also dramatically inhibited the discharges of 5-HT cells [4,12] These data indicate that hallucinogenic lndoles act as agonlsts at receptors located on 5-HT perlkarya and dendntes (t e , autoreceptors [7]) in order to produce their inhibitory effects

Mescaline

Facial motor nucleus

The inhibitory effect of LSD on 5-HT cell firing ts not sufficient to explain its psychedelic actions, since 5-HT also suppresses raphe neuronal discharges However, Aghajaman and Halgler [4,17] found that forebram neurons receiving a major serotonerglc input (e g , ventral lateral genlculate, the basolateral and cortical amygdala and the optic rectum) are relatively insensitive to LSD at lontophoretlc ejection currents that are highly effective m the raphe (Fig 1) In contrast, lontophoretic 5-HT is approximately equlpotent in depressing activity of 5-HT neurons and neurons in postsynapttc areas which receive a major serotonerglc input (Fig 1) Psllocln, DMT and 5-MeODMT also preferentially Inhibit raphe firing as compared to the discharge of neurons In postsynaptlc forebraln areas [4,12] In these studies, the degree of preference to the 5-HT autoreceptor correlated well with hallucinogenic potency The preferential inhibitory action of hallucinogens on raphe cells suggests that these agents could release postsynaptlc forebram neurons from a tonic inhibitory influence In agreement with this prediction, low intravenous doses of LSD accelerates the discharge rate of neurons in postsynaptlc areas which receive an inhibitory 5-HT input (Fig 1, [15, 18, 27]) These studies led to the hypothesis that lndole hallucinogens produced their psychoactive effects by acting preferentially upon 5-HT autoreceptors in the dorsal raphe, allowing postsynaptlc neurons to escape from the tonic inhibitory action of 5-HT neurons Since the visual and hmblc systems are densely innervated by 5-HT axons, the hallucinogen-induced dlsmhlbltlon in these areas could account for two of the major consequences ofhallucmogemc action visual hallucinations and alterations of affect Although the '~dlslnhlbitlon" hypothesis laid the framework for much Investigation, certain problems exist with the theory [22] First, the inhibitory action of LSD on raphe neurons is relatively transient, while the psychedelic effects of the drug are prolonged [9,32] Moreover, the time course of the behavioral and raphe neuronal effects of LSD in the free-moving cat do not correlate [19,29] Second, al-

359

360

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Amygdala I

5 MIN

FIG 1 Comparison of the response of a neuron m the dorsal raphe nucleus with that of neurons m the ventral lateral gemculate and the basolateral nucleus of the amygdala to mlcro~ontophoreUc administration of 5-HT and LSD LSD preferentmlly inhibited 5-HT neurons 5-HT mh~blted 5-HT neurons and postsynapt~c neurons to a slmdar degree In this and the following figures, the horizontal bar indicates the ~ontophoret~c ejection period, the number over the bar represents the ejecting current The ordinate represents the rotegrated finng rate m spikes per second Repnnted by permission of The Wllhams and Wdklns Co , Baltimore, MD Copyright 1974

though tolerance develops to the behavioral effects of LSD [9, 29, 32], no such tolerance develops to the inhibitory effect of LS D on 5-HT neurons [30] Third, the phenethylamlne hallucinogen mescahne and L S D are thought to act at the same receptor site to produce their psychoactive effects [9, 32, 33] However, mescahne does not directly suppress the firing of 5-HT neurons in the dorsal raphe [16] Finally, the ergohne derivative llsunde, produces a direct, rapid dosedependent suppression of the spontaneous activity of 5-HT neurons [20,28] In fact, hsurlde is approximately 5-10 times as potent as LSD in inhibiting raphe finng Importantly, however, hsurtde does not possess halluclnogemc properties Taken together, these data suggest that an agonlst action at 5-HT autoreceptors and the subsequent release from tonic Inhibition in postsynaptlc forebraln areas may be insufficient to explain the mode of action of hallucinogens An alternative posstblhty ts that hallucinogens act on postsynaptic 5-HT receptors to produce their psychedelic effects In this regard, McCall and Aghajanlan have characterized a third type of 5-HT receptor in the central nervous system located on facial motoneurons [5, 23-25] As illustrated in Fig 2, they found that mlcrolontophoretlc apphcatlon of 10-200 nA pulses of 5-HT lasting from 1 to 10 mln failed to activate the normally quiescent facial motoneurons However, 5-HT markedly potentmted the excitatory effect of lontophoretlcally applied glutamate on these neurons (Fig 2A) One minute pulses of iontophoretlc 5-HT also enhanced the subthreshold (Fig 2B) and threshold (Fig 2C) excitatory effects of glutamate In addition, 5-HT (5-10 hA) dramatically facilitated the excitatory effects of afferent nerve stimulation Like 5-HT, noreplnephnne (NE) also markedly facilitated excitatory inputs to facial motoneurons [23] Peripheral 5-HT antagonists (1 e , methyserglde, metergohne,

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FIG 2 Facdltatlon of glutamate-reduced excitation of facial motoneurons by 5-HT A 5-HT (10 hA) reduced the threshold reqmred for glutamate-reduced activation ofumt 5-HT (200 nA) faded to directly excite neuron B Faohtatmg effect of l-mm pulses of 5-HT on glutamate-mduced exotatlon (G2 nA) of motoneuron C Faclhtatlon of the subthreshold exotatory effect of glutamate (G7 nA) by 5-HT

cyproheptadine and clnanserln) which fall to block the inhibitory effects of 5-HT in the raphe and forebraln areas [7], antagonized the facilitating effects of 5-HT but not NE [25] Thus, in the facial nucleus, 5-HT functions in a manner that is not analogous to direct excitation, but rather acts as a gain setter to enhance the effects of excitatory inputs Furthermore, the data indicate that the receptor mediating the facdltatlng effects of 5-HT in the facial nucleus is distinct from those found in the dorsal raphe and postsynaptlc forebrain areas More recently, McCall and Aghajanlan studied the effects of hallucinogens on the faclhtatlng action of 5-HT and N E in the facial nucleus [24] Intravenous administration of LSD (10-100 /zg/kg) had no effect by itself on the glutamateinduced excitation of facial motoneurons In contrast, the faclhtatlon of facial motoneuron excitation by lontophoretlcally applied 5-HT and N E was enhanced approximately 10fold by low doses of LS D (5-10 txg/kg, IV, Fig 3A, Table 1) Similarly, lontophoresls of LSD at low currents (2-7 hA) failed to effect the glutamate-induced excitation of motoneurons, but markedly enhanced the facilitation of fa-

HALLUCINOGENS

AND SEROTONERGIC NEURONS

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2 MIN FIG 3 LSD potentmtes the facthtatmg effect of lontophoret~cally apphed 5-HT and NE on glutamate (G)-mduced facial motoneuron excltatmn A Intravenous LSD (10 /xg/kg) ~mmedmtely enhances the response to 5-HT and NE, an effect lasting at least 4 hours (slashed basehne) B Iontophoretlcally apphed LSD (5 nA) also enhances the effect of 5-HT C Higher ejecting currents of LSD (20 nA) temporarily depress the response to 5-HT

FIG 4 Mescahne potentmtes the facdltatlng effect of mntophoretlcally apphed 5-HT and NE on facml motoneurons A A low dose of mescahne (1 mg/kg, IV) enhances the effects of 5-HT and NE without directly activating the cell The actmn of mescahne lasts at least 2 hours B Metergohne (100/xg/kg, IV) blocks the enhancement of glutamate excltatmn which is produced by higher doses of mescahne (6 mg/kg, IV), metergohne also blocks 5-HT but the response to NE is enhanced C Iontophoretlcally apphed mescaline (10 nA) potentmtes the facd~tatmg effect of NE on facml motoneurons

TABLE 1 INFLUENCE OF INTRAVENOUS HALLUCINOGENS AND NONHALLUCINOGENS ON THE FACILITATING EFFECT OF 5-HT AND NE ON FACIAL MOTONEURONS

Drug LSD Mescahne Psdocm DMT~: Llsunde Methysergzde

Number of Ammals*

Dose

% Control 5-HT

8 6 5 5 5 5

10 txg/kg 1 mg/kg 1 mg/kg I mg/kg 40/zg/kg 1 mg/kg

1026 ± 93t 763 ± 120t 497 ± 46t 102 ± 0

4

% ControlNE 976 913 491 469 102 98

± ± ± ± ± ±

109t 168t 72f 48 2 6

*One neuron was tested per ammal +p<0 02, Student's t-test ~Ammals were pretreated with metergohne (100 ~g/kg, IV)

clal m o t o n e u r o n excatataon by 5-HT (Fag 3B) H i g h e r ejectmg c u r r e n t s o f L S D (15-40 nA) p r o d u c e d a t r a n s i e n t block o f the effect to 5-HT whach was f o l l o w e d by an e n h a n c e d res p o n s e (Fig 3C) The m o s t r e m a r k a b l e f e a t u r e o f L S D ' s effect was that e v e n a b n e f e x p o s u r e o f the hallucinogen senSatlzed r e s p o n s e s to 5-HT and N E for several h o u r s (Fag 3A) L o w d o s e s o f m e s c a l i n e (0 5-1 0 mg/kg, IV) also poten-

tlated the effect o f 5-HT and N E , but did not alter the g l u t a m a t e - r e d u c e d e x c i t a t i o n o f facial m o t o n e u r o n s Again, the p o t e n t m t l n g e f f e c t s o f m e s c a l i n e lasted several h o u r s (Fag 4A) Hagher d o s e s o f m e s c a l i n e (6-10 mg/kg, IV) acted darectly to faclhtate the g l u t a m a t e - i n d u c e d excatatmn o f factal m o t o n e u r o n s This effect w a s b l o c k e d by the 5-HT a n t a g o m s t m e t e r g o h n e , indicating that at hagh d o s e s mes-

362

McCALL

callne acts as a 5-HT agonlst in the facial n u c l e u s (Fig 4B) In animals p r e t r e a t e d with m e t e r g o h n e , i o n t o p h o r e t i c mescaline p o t e n t i a t e d the facilitating effect o f N E (Fig 4C) T h u s , the data in Figs 3 and 4 indicate that low d o s e s o f L S D and m e s c a l i n e , in the a b s e n c e o f an effect o f t h e i r o w n , m a r k e d l y i n c r e a s e sensitivity to b o t h 5-HT and N E in the facial n u c l e u s To d e t e r m i n e if the sensitizing action o f L S D and mesc a h n e is specific for h a l l u c i n o g e n s , the e f f e c t s o f t w o i n d o l e a m l n e h a l l u c i n o g e n s , psllocln and DMT, and two n o n h a l l u c l n o g e n l c ergot d e r i v a t i v e s , hsurlde and m e t h y s e r glde, w e r e studied Like L S D , psllocln (0 5-2 0 mg/kg, IV) m a r k e d l y p o t e n t i a t e d the effect o f 5-HT and N E on m o t o n e u r o n s but had no direct action w h e n g~ven alone (Table 1) In c o n t r a s t , D M T (0 5-2 0 mg/kg, IV) facilitated the g l u t a m a t e - i n d u c e d e x c i t a t i o n o f m o t o n e u r o n s by itself This effect was b l o c k e d by m e t e r g o l l n e D M T was unlike m e s c a l i n e in that a 5-HT sensitizing action could not be diff e r e n t i a t e d e v e n at low d o s e s from the 5-HT agonlst a c t w l t y N e v e r t h e l e s s , in animals p r e t r e a t e d with m e t e r g o h n e , D M T (like m e s c a l i n e ) p o t e n t i a t e d the facilitating effect o f N E (Table 1) T h u s , it a p p e a r s that the ablhty to p o t e n t i a t e the f a c l h t a t m g effect o f 5-HT and N E may be c o m m o n to all o f the p s y c h e d e l i c hallucinogens I m p o r t a n t l y , the nonhall u c i n o g e n s h s u r l d e and m e t h y s e r g l d e failed to p o t e n t i a t e the facilitating e f f e c t s o f 5-HT and N E (Table 1)

The hallucinogens a p p e a r to p o t e n t m t e the e f f e c t s o f m o n o a m l n e s on facial m o t o n e u r o n s by increasing the senSltWlty o f 5-HT and N E r e c e p t o r s [24] The m e c h a n i s m by w h i c h hallucinogens sensitize 5-HT and N E r e c e p t o r s is not k n o w n H o w e v e r . the fact that L S D and 5-HT a p p e a r to brad to different 5-HT r e c e p t o r sites 110] s u g g e s t s the possibility that drug induced c h a n g e s in r e c e p t o r sensitivity could o c c u r t h r o u g h interactions b e t w e e n 5-HT and L S D binding sites In any c a s e . a sensitization o f 5-HT and N E r e c e p t o r s loc a t e d on m o t o n e u r o n s [23,31] likely a c c o u n t s for the e n h a n c e m e n t o f spinal r e f l e x e s p r o d u c e d by hallucinogens [8,21] If the s e n s l h z l n g e f f e c t s o f hallucinogens d e s c r i b e d a b o v e o c c u r s in s e n s o r y p a t h w a y s in the central n e r v o u s s y s t e m , then a m e c h a n i s m o f r e c e p t o r sensitization, in dJst l n c h o n to dlslnh~bltlOn, m~ght a c c o u n t for the altered perceptual reactivity p r o d u c e d by t h e s e drugs S u p p o r t for a r e c e p t o r sensitizing effect o f hallucinogens s t e m s from the fact that l n d o l e a m l n e and p h e n e t h y l a m l n e hallucinogens act in a similar fashion in the facial nucleus, while hsurlde has no effect F u r t h e r m o r e , the time c o u r s e o f the L S D e n h a n c e d e f f e c t s o f 5-HT and N E in the facml nucleus parallels the p r o l o n g e d b e h a w o r a l e f f e c t s o f this drug in the free m o w n g cat [29] T h e s e o b s e r v a t i o n s raise the interesting p o s s l b l h t y that relatively long-term c h a n g e s in the s e n s l t w l t y o f n o r a d renerglc a n d / o r serotonerglc r e c e p t o r s may be revolved in the p s y c h e d e l i c actions o f hallucinogens

REFERENCES 1 Aghajanlan, G K LSD and 2-Bromo-LSD Comparison ot effects on serotonerglc neurones and on neurones in two serotonerglc projections areas, the ventral lateral genlculate and amygdala Neutophatma
12 De Monttgny, C and G K Aghajanlan Preferential action of 5-methoxytryptamlne and 5-methoxydlmethyltryptamlne on presynaptlc serotonln receptors A comparative iontophoretzc study with LSD and serotonln Net#opltarma¢olog~ 16" 811818. 1977 13 Fuxe, K Ewdence tor the existence ot monoamlne-contalnlng neurons in the central nervous system IV The distribution of monoamlne terminals m the central nervous system A¢ta Phx s . d A¢ and [Suppl] 247: 37-85. 1965 14 Gaddum. J H Antagonism between lyserglc acid dlethylam~de and 5-hydroxytryptamlne J Phvstol 121. 15, 1953 15 Gullbaud, G , J M Besson, J L OllverasandJ C Llebesklnd Suppression by LSD of the inhibitory effect exerted by dorsal raphe stimulation on certain spinal cord lnterneurons m the cat Brain Res 61 417-422. 1973 16 Halgler, H J and G K Aghajanlan Mescaline and LSD Direct and indirect effects on serotonln-contalnlng neurons m brain Em I Pharma¢ ol 21: 53-60, 1973 17 Halgler, H J and G K Aghajanlan Lyserglc acid dlethylamlde and serotonln A comparison of effects on serotonerglc neurons and neurons receiving a serotonerg~c input I Pha;m t , ol Lrp lhet 188 688--699. 1974 18 Horn. G and J M McKay Effects of lyserglc acid dlethylamlde on the spontaneous activity and visual receptive fields of cells in the lateral genlculate nucleus of the cat E~p Btam Res 17" 271-284. 1973 19 Jacobs. B L . M E Trulson and W C Stern An animal behavioral model for studying the actions of LSD and related hallucinogens 3~ t~.tl¢ t. 194. 741-743 1976 20 Laurent. J P and L Pleri Llsurlde and LSD decrease spontaneous firing of neurons in the raphe L ~pet tentta 34" 926, 1978 21 Maj, J . W Pahder and A Rawlow The influence ot mescaline on the flexor reflex of the hmdhmb of the spinal rat J PhaJm Pharma¢ ol 29" 177-178, 1977 22 McCall, R B Neurophyslologlcal effects of hallucinogens on serotonerglc neuronal systems Neuro.sct Blobehal Rel 6 50% 514, 1982

HALLUCINOGENS

AND SEROTONERGIC

NEURONS

23 McCall, R B and G K A g h a j a m a n Serotonerglcfacd~tatlonof facml m o t o n e u r o n excitation Btam Re~ 169" 11-27, 1979 24 McCall. R B and G K Aghajanlan Hallucinogens p o t e n h a t e r e s p o n s e s to s e r o t o m n and n o r e p m e p h r m e m the facial motor nucleus LlJe St t 26: 1149-1156, 1980 25 McCall. R B and G K A g h a j a m a n Pharmacological characterization ol serotonln receptors m the facml motor nucleus A microlontophorehc study Eur l Pharma~ol 65: 175-183, 1980 26 Mosko, S S and B L Jacobs Electrophyslologlcal evidence against negative neuronal feedback from the forebram controlling mKdbraln raphe umt activity Blare Res !19 291-303, 1977 27 Mourlz-Garc~a, A , R Schm~dt and A Arlazoroff Effects of LSD on the s p o n t a n e o u s and evoked acttv~ty o f retinal and gemculate ganghon cells P~chophalma~ologla 15" 382-391, 1969 28 RogawskJ, M A and G K Aghajanlan R e s p o n s e ot central m o n o a m m e r g l c n e u r o n s to h s u n d e C o m p a r i s o n w~th LSD Ltfe S~t 24. 1289-1298 1979

363

29 Trulson, M E and B L Jacobs Dissociations between the effects of L S D on b e h a w o r and raphe umt actlwty m freely m o v m g c a t s 5 ¢ w m e 205: 515-518, 1979 30 Trulson, M E , C A Ross and B L Jacobs Lack of tolerance to the depression of raphe umt activity by lyserg~c acid dlethylamlde Neuropharmacologv 16" 771-774. 1977 31 White, S R and R S N e u m a n Faclhtatlon of spmal m o t o n e u r o n excJtablhty by '~-hydroxytryptamme and norepln e p h r m e B t a t n R e s 188 119-127, 1980 32 W | n t e r s J C Tolerance to a behavioral effect of lyse|glc acid d~ethylam~de and cross-tolerance to m e s c a h n e m the rat Absence of a metabohc c o m p o n e n t 1 Phatma~ol t:~p 7her 178: 625-630 1971 33 Wolbach, A B , E J Miner and H Isbell C o m p a r i s o n of ps|locm and psflocybln, m e s c a h n e , and LSD-25 Psv~hopharma~ olog, ta 3: 219-223, 1962 34 Wooley, D W and E Shaw A biochemical and pharmacological suggestion about certain mental disorders Pro~ Natl A~ ad S~t UAA 40 228-2~,1, 1954