Independent gabaergic and cholinergic modulation of apomorphine-induced stereotyped rearing in the rat

Neuropharmacology Vol. 27, No. 3, pp. 281-285, 1988 Printed

in Great

Britain.

Copyright 0

All rights reserved

0028-3908/88 $3.00 + 0.00 1988 Pergamon Journals Ltd

INDEPENDENT GABAERGIC AND CHOLINERGIC MODULATION OF APOMORPHINE-INDUCED STEREOTYPED REARING IN THE RAT L. DECSI~ and JULIA NAGY* Institute

of Pharmacology,

University (Accepled

Medical

School,

H-7643

PCcs, Hungary

10 September 1987)

Summary-The injection of GABA into the caudate nucleus inhibited the stereotyped rearing induced by apomorphine in a dose-related manner. Muscimol, a potent GABAergic agonist shared this effect. The inhibitory effect of GABA was easily counteracted by bicuculline but not by pretreatment with atropine. Injection of carbachol into the caudate nucleus inhibited the stereotyped rearing induced by systemically-applied apomorphine in a dose-related manner. This inhibitory effect was easily abolished by atropine but not bicuculline. Thus, the stereotyped rearing induced by apomorphine, an effect due to an increased excitatory state of the dopaminergic system in the caudate nucleus, could be modified (inhibited) by augmentation of either the GABAergic or of the cholinergic state excitation. The two modulatory systems did not appear to be interlinked; most probably, they influence the dopaminergic effect independently of one another. Key words:

apomorphine,

stereotyped

rearing,

The stereotypy induced by apomorphine in the rat is known to be caused by stimulation of dopaminergic receptors located in the caudate nucleus (Ernst and Smelick, 1966; Fuxe and Ungerstedt, 1970; Nagy and Decsi, 1977; Decsi, Nagy and Zgmb6, 1978). In addition to the dopaminergic system, cholinergic and y-aminobutyric acid (GABAergic) (and most probably other) systems play an important part in the organization of the normal-function of this nucleus. Data exists in the literature on the role of, and interaction between, various neurotransmitter systems; these data are sometimes not quite clear. It was the aim of the present study to check in a direct, simple, reliable and nonsophisticated way, two questions: (a) how would alterations in GABAergic or cholinergic transmission within the caudate nucleus influence the stereotypy induced by apomorphine (APO) in the rat, (b) what relationships, if any, could be demonstrated between the dopaminergic system on the one hand and the GABAergic and cholinergic systems on the other? The best approach to investigate this problem is to intervene in the above systems in the caudate nucleus itself, leaving all other parts of the brain unaffected. This can easily be performed by using the direct intracerebral injection technique. METHODS

Long 22&240

Evans

*To whom N P 27i--E

hooded

rats

of both

sexes

g were used. They were housed

correspondence

should

weighing

in groups of

be addressed.

caudate

nucleus,

neurotransmitter

interactions.

5 with food and water ad libitum except 1 hr before the experiment. A 12/12 hr light-dark period (light from 6 a.m. to 6 p.m.) was used; the temperature was 22°C. First, the stereotyped rearing effect of an intraperitoneal injection of apomorphine (1.25 or 2.5 mg/kg) was checked in each animal. Thereafter, chronic cannulae were implanted stereotaxically in the caudate nucleus on both sides, under pentobarbital anaesthesia (40 mg/kg i.p.). The guide cannula was 0.6 mm in diameter while the inner one, serving for the injection proper, 0.3 mm. The cannulae were implanted corresponding to the coordinates as follows: A = 7.4; L = 4.0; V = +2.0. The exact location of the cannulae was checked on 50 p frozen sections. The apomorphine-induced stereotypy consists of several components (sniffing, licking, gnawing, rearing) (Ernst and Smelick, 1966; Worm, Depoortere and Lloyd, 1979; Scheel-Kriiger, 1982), of which the rearing is characteristic very of RAmsterdam x Long Evans F, hibrid and Long Evans rats (Nagy and Decsi, 1977; Decsi, GBcs, Z9mb,6 and Nagy, 1979; Bryan and Ellison, 1975). This stereotyped rearing (without exploratory activity) was measured by means of a device developed in this laboratory. Details of the apparatus have been described earlier (Decsi et al., 1979). The apparatus works on the principle of capacitance changes due to postural changes, in this case rearing, of the animal. Even though this method only measures one component (rearing) of stereotyped behaviour (and it is not at all sure that every component of stereotypy runs quantitatively parallel), but it does so automatically and in 281

L. DECSIand JULIA NAGY

282

a fully objective and quantitative way. The animals were placed in the apparatus immediately after the injection of the drug. The first 5 min served for the habituation of the rat to the environment; then the time spent in rearing position was measured within the next 15 min. The 5S6 days after the control values (i.p. APO alone) were taken, the same dose of apomorphine was injected intraperitoneally with concomittant administration of drug into the caudate. The percentage inhibition of the means was determined after 3-1 doses of drugs into the caudate, then the 50% inhibition of the rearing time was calculated

(ID,,). Carbachol and atropine sulphate were dissolved in containing 0.9% sodium chloride, 0.003 M NaH,PO,-Na,HPO, buffer (pH 7.3). Muscimol and GABA were dissoived in distilled water. Apomorphine was dissolved in physiological saline containing 0.1% ascorbic acid. Bicuculline was dissolved in 0.1 N HCI; to this solution 1 N NaOH was added up to adjust the pH to 5.5. All solutions were used immediately after preparation. The drugs were injected in a volume of 2~11 over 45 set (i.c.) and 0.1 ml/lOOg (i.p.). Statistical analysis was made according to Student’s “t”-test (one-tailed). RESULTS

GABArrgic neurotransmission Direct injection of GABA in the caudate nucleus inhibited apomorphine-stereotyped rearing in a doserelated manner. As seen in Figure 1, the ID,, value was 226 pg. A similar inhibitory effect was seen after the application of muscimol, a potent GABAergic agonist into the caudate. In a dose of 100 ng it inhibited the stereotyped rearing by 47.2% and 84.1% inhibition was observed after injection of 200ng (Fig. 2). As the next step, the specificity of this effect of the GABA agonists was checked. First, the effect of bicuculline, a potent GABA receptor blocking agent was investigated in untreated rats after intra-

Fig. 2. Inhibition of stereotyped rearing induced by apomorphine by injection of muscimol in the caudate nucleus. Column I = saline in caudate nucleus; column 2 = 2.5 mgikg apomorphine (i.p.); column 3 = saline in caudate nucleus and simultaneously + 2.5 mg/kg apomorphine (i.p.); column 4 = 100 ng muscimol in caudate nucleus + 2.5 mg/kg apomorphine (i.p.); x = P < 0.05 when compared with saline + apomorphine treated group; column 5= 200 ng muscimol in caudate nucleus + 2.5 mg/kg apomorphine (i.p.); xx = P < 0.01 when compared with saline f apomorphine-treated group. The bars represent the mean & SEM. IO animals in each group.

~riton~al and int~dcaudate application. Intraperitoneal injection of 1 mg/kg of bicuculline faited to produce any behavioural changes; as seen in Table 1, the rearing was identical with that of saline-treated rats. The same was the case upon injection of 5 or 1Opg bicuculline into the caudate. Injection of IOgg of bicuculline into the caudate did not influence the stereotyped rearing either; the drug was also ineffective when given in a dose of 1 mg/kg by the intraperitoneal route, 15 min prior to the injection of apomorphine. Bicuculline, however, fully antagonized the inhibition of stereotyped rearing induced by apomorphine caused by injection of GABA into the caudate. This is shown in Figure 3. As seen, the 68% inhibition (P < 0.001) was reduced to a mere 9..5%, a value not differing statistically from that found after administration of apomorphine alone. Even being

though

bicuculline

a specific antagonist

is generally

at GABA

regarded

receptors,

as

this

lnh,brtion%

4/

60

XX’

500 pg

GABA

Fig. 1,Inhibition of apomorphine-induced stereotyped rearing (APO-SR) by injection of GABA in the caudate nucleus. The stereotyped rearing was induced by 2.5 mg/kg apomorphine (i.p.) (control value = 332.7 + 42.8 sec/l5 min). xx = P i: 0.01 when compared with apomorphine treated group, xxx = P < 0.001 when compared with apomorphinetreated group, 15 animals in each group.

Fig. 3. Antagonism by bicuculline of the inhibitory effect of GABA. Column I = saline (i.p.); column 2 = 2.5 mg/kg apomo~hine (i.p.); column 3 = 5OOpg GABA in caudate nucleus and simultaneously + 2.5 mg/kg apomorphine (i.p.); xx = P <: 0.01 when compared with apomorphine-treated group, column 4 = 1 mg/kg bicuculline (i.p.) 15 min later 500 pg GABA in caudate nucleus + 2.5 mg/kg apomorphine (ip.); xx = P ~0.01 when compared with GABA + apomorphine-treated group.

Transmitter modulation of apomorphine induced stereotypy I

Table

Effect

of bicuculline

on apomoruhine-induced

stereotvoed

283

rearine

Rearing Treatment

sec/l5min

N

31.5k7.1

II

I.

Saline

2.

I mg/kg

(i.p.)

3.

IO pg bicuculline

bicuculline

4. 2.5 mg/kg 5.

(i.p.)

IO pg bicuculline

6. 2.5 mg/kg 7.

APO

I

ma/kg

APO

APO

29.5 f

(i.p.) on caudate in caudate

+ 2.5 mg/kg

APO

(i.p.)

(i.p.)

bicuculline

(i.p.)

+ 2.5 mg/kp,

APO

(i.p.)

P

7.8

9

NS

(vs I)

23.3 k 7.2

IO

NS

(vs I)

330.2 f 45. I

IO


304.7 f

32.7

IO

NS

(vs 4)

340.1 f

36.3

8

410.1 f 65.7

I2

NS

(vs 6)

(vs I)

= apomorphine

specificity was also checked in another way that, as will be seen later, served another purpose too. First 5 pg of atropine was injected in the caudate nucleus and, 5 min later, 500 pg of GABA in the same locus and simultaneously apomorphine intraperitoneally. As seen in Figure 4, the GABA-induced inhibition of stereotyped rearing was left unaltered by either intracaudate or intraperitoneal pretreament with atropine. All the above findings mean that: (a) GABAergic stimulation of the caudate nucleus inhibits the stereotyped rearing induced by apomorphine in a specific way; (b) atropine, applied either locally or systemically, does not influence the inhibitory effect of GABA, a fact precluding any participation of muscarinic receptors in the GABA-induced inhibition of stereotyped rearing.

apomorphine in a dose-related manner. As seen in Figure 5, the ID,,, value was 5.2pg. Intraperitoneal pretreatment with 5 mg/kg atropine fully counteracted the inhibitory effect of carbachol (Table 2). Intraperitoneal pretreatment with bicuculline, however, did not influence the inhibitory effect exerted on stereotyped rearing by carbachol, applied to the caudate nucleus. This is shown in Figure 6. Ten pg of carbachol, injected in the caudate nucleus, caused a 12.1% inhibition of stereotyped rearing; this inhibition remained unaltered after intraperitoneal pretreatment with 1 mg/kg of bicuculline or after injection of 10 pg of bicuculline into the caudate. Finally, it should be noted that the doses of carbachol used in the experiments never caused any visible change in the gross behaviour of the animals per se; the same was the case with GABA, bicuculline or atropine. All the above findings mean that: (a) cholinergic stimulation of the caudate nucleus inhibits the stereo-

Cholinergic neurotransmission

Injection of carbachol (CCh) in the caudate nucleus inhibited the stereotyped rearing induced by

50"

Fig. 4. Failure of atropine to influence GABA-induced inhibition. Columns I and 5 = 2.5 mg/kg apomorphine (i.p.); column 2 = 500 peg GABA in caudate nucleus and simultaneously + 2.5 mg/kg apomorphine (i.p.); xxx = P < 0.001 when compared with apomorphine-treated group; column 3 = 5 pg atropine in caudate nucleus 5 min later 500 pg GABA in caudate nucleus + 2.5 mg/kg apomorphine (i.p.); column 4 = 5 mg/kg atropine (i.p.) 15 min later 500 pg GABA in caudate nucleus + 2.5 mg/kg apomorphine (i.p.).

Table

2. Effect

of atropine,

carbachol

and atropine

25

5P

2QO pg CCh

lo,0

Fig. 5. Inhibition of stereotyped rearing induced by apomorphine by injection of carbachol in the caudate nucleus. Stereotyped rearing evoked by 2.5 mg/kg apomorphine (control value = 343.1 & 38.8 sec/l5 min); xx = (i.p.); P
+ carbachol

on apomorphine-induced

stereotyped

rearing

Rearing Treatment I.

Saline

(i.p.)

2. 5 mg/kg

atropine

3. 2.5 mg/kg 4.

APO

IO pg carbachol

5. 5 mg/kg

atropine

+ 2.5 mg/kg

APO

(i.p.) (i.p.)

nucleus + 2.5 mg/kg

+ IO pg carbachol

APO

in caudate

N

31.7 f 6.6

IO

28.5 f

5.2

P

IO

NS

400. I f 48.2

IO


(vs I)

(i.p.)

120.1 i

26.6

IO

co.001

(vs 3)

nucleus

356.2 f

53.3

IO

NS

(i.p.) (Lp.) in caudate

sec/lSmin

(vs I)

(vs 3)

L. DECSIand Jli’t~~ NAGY

284

Fig. 6. Failure of bicuculline to influence carbachoi-induced inhibition. Columns 1 and 5 = 2.5 mg/kg apomorphine (i.p.); column 2 = IOpg carbachol in caudate nucleus and simultaneously + 2.5 mg/kg apomorphine (i.p.); xxx = P < 0.001 when compared with apomorphine-treated group; column 3 = 1mg/kg bicuculline (i.p.f 15 min later IOflg carbachol in caudate nucleus + 2.5 mg/kg apomorphine (i.p.); column 4 = IOpg bicuculiine in caudate nucleus 5 min later 10 pg carbachol in caudate nucleus + 2.5 mg/kg apomorphine (i.p.); 12 animals in each group.

typed rearing induced by apomorphine in a specific way, through influencing muscarinic receptors; (b) bicuculline fails to influence the inhibitory effect of carbachol, a fact precluding any participation of GABAergic receptors in the carbachol-evoked inhibition of stereotyped rearing. DISCUSSION

It is not the aim of the present paper to review the very complex problem of transmitter interactions within the caudate nucleus. In this connection only some papers are referred to which deal in detail, partly with the pharmacological and partly with anatomical and biochemical aspects of this problem (for mainly GABAergic transmission see Worm et al., 1979; De Feudis, 1980; Kaakkola and Kaarianen. 1980; McKenzie and Hansen, 1980; Scheel-Kriiger, Magelund and Olianas, 198 I; Scheel-Kriiger and Magelund, 198 1; Kuschinsky, 198 1; Scheel-Kriiger, 1982; Turski, Havemann and Kuschinsky, 1984; Kemal, Gauchy, Glowinski and Besson, 1984; for mainly cholinergic transmission see Scheel-Kriiger, 1970; Bryan and Ellison, 1975; Ladinsky, Consolo, Bianchi and Jori, 1976; Neil], 1976; de La Mora and Fuxe, 1977; Yamada and Furukawa, 1980; Schallert, De Ryck and Teizelbaum, 1980; Ward, Kosh, Freeman, 1981; Ushijima, Noda, Mizuki and Yamada, 1984; for presynaptic or non-synaptic modulation see Vizi, 1975; Vizi, R6nai, HQrsing and Knoll, 1977; Hgrsing and Vizi, 1978; Vizi, 1980; Vizi, H;irsing and Zsilla, 1981; for some further general information see Fog, 1972; Roth and Bunney, 1976; Cuello and lversen, 1978: Bartholini, 1980). Using the direct intracerebral injection technique, it was intended only to shed some more light on the possible interaction between GABAergic and cholinergic systems with dopaminergic function of the caudate nucleus. Many attempts have been made at clarifying the essence of such interactions in experi-

ments with various methods, the majority of them, however, only allow indirect conclusions to be drawn. The development of stereotyped behaviour is due to a disturbance in the normal neurotransmitter equilibrium state within the caudate nucleus. It has been demonstrated that an increase in the dopaminergic excitatory state (induced by APO) will lead to stereotypy. In the experiments presented here, topical application of GABA or muscimol to the caudate nucleus inhibited the stereotyped rearing induced by intraperitoneal injection of apomorphine in a dose-related manner. This inhibition was fully antagonized by either topical or systemic application of bicuculline, but not by atropine. Injection of carbachoi in the caudate nucleus also inhibited the stereotyped rearing in a dose-related manner. The inhibitory effect of carbachol was fully antagonized by either topical or systemic application of small doses of atropine, but not by bicuculline. Thus, both GABAergic and cholinergic stimulation was capable of counteracting the response to dopaminergic stimulation. Thus, the question arises of whether or not these two neurotransmitters are interlinked, for instance in such a way that GABAergic stimulation finally acts through cholinergic synapses or vice uersu. This question was checked in experiments where GABAergic stimulation was performed after blockade of the cholinergic receptors and, respectively, cholinergic stimulation after blockade of GABAergic receptors. These experiments unequivocally showed that, in the case of GABAergic stimulation there was no need for free muscarinic receptors for the inhibitory effect to occur, and that, in the case of cholinergic stimulation there was no need for free GABAergic receptors for the inhibitory effect. To put it in another way: the two types of inhibition were independent of one another. The interactions described above can most simply be explained as follows: administration of apomorphine increases the dopaminergic excitatory state in the caudate nucleus and disturbs, thereby, the normal transmitter equilibrium necessary for proper functioning of this region. The shift of the transmitter balance in direction to dopaminergic preponde~n~e can theoretically be counteracted in either of the following two ways: (a) by impeding dopaminesensitive cells to the response to chemical stimulation, either through occupation of the receptors (by antidopamine drugs, see Decsi et ul., l979), or through a direct inhibition of neuronal activity, which may well be the case upon application of GABAergic agonists; (b) by increasing the activity of another, oppositelyacting neurotransmitter system to such an extent as to keep pace with the augmentation of the dopaminergic excitatory state, i.e. to restore the original equilibrium state of the neurotransmitter systems involved, be it even at a higher level than the original one; that may well be the case upon application of cholinergic stimulants.

Transmitter modulation of apomorphine induced stereotypy Acknowledgements-The

authors thank Mrs Katalin GoglKeserii for her excellent technical assistance. This work was supported by the Scientific Research Council, Ministry of Public Health, Hungary, No. 3-~-0101-02-2/V and No. 911-11-92.

REFERENCES

Bartholini G. (1980) Interaction of striatal dopaminergic, cholinergic and GABA-ergic neurons: relati& to extranvramidal function. TIPS 1: 138-140. B&n K. S and Ellison G. (1975) Cholinergic modulation of an opposed effect of d-amphetamine and methylphenidate on the rearing response. Psychopharmacologia 43: 1699173. Cue110 A. C. and Iversen L. L. (1978) Interactions

of dopamine with other neurotransmitters in the rat substantia nigra: A possible functional role of dendritic dopamine. In: Inleructions Between Putative Neurorransmitters in the Brain (Garattini S., Pujol J. F. and Samanin R., Eds), pp. 127-149. Raven Press, New York. Decsi L., Gacs I?., Zambo K. and Nagy J. (1979) A simple device to measure stereotyped rearing of the rat in an objective and quantitative way. Neuropharmacology 18: 723-725.

Decsi L., Nagy J. and Zambo K. (1978) Stereotyped behaviour after cholinergic, but not dopaminergic, stimulation of the substantia nigra in rats. L& Sci. %?z 187331878. DeFeudis F. V. (1980) Phvsiolosical and behavioural studies with muscimol. ‘Neuiochem. Res. 5: 104771068. Ernst A. M. and Smelick P. G. (1966) Site of action of dopamine and apomorphine on compulsive gnawing behaviour in rats. E.~per~ent~a22: 837-838. Fog R. (1972) On Stereotypy and Catalepsy: Studies on the Effect of Amphetamines and Neuroleptics in Rats.

Munskgaard, Copenhagen. Fuxe K. and Ungerstedt U. (1970) Histochemical, biochemical and functional studies on central monoamine neurons after acute and chronic amphetamine administration. In: Amphetamine and Related Compounds (Costa E. and Garattini S., Eds), pp. 257-288. Raven Press, New York. H&sing L. G. Jr and Vizi S. E. (1978) Different sensitivitv of p&- and postsynaptic dopamink receptors in the rat striatum. In: Modulation of Neurochemical Transmission (Vizi E. S., Ed.), Vol. II, pp. 181-188. Pergamon Press, Akademiai Kiado, Budapest. Kaakkola S. and Kaarianen I. (1980) Circling behavior induced by intranigral injections of GABA and muscimol in rats. P.~ye~~opharmaeo~ogy68: 31-36. Kemal M. L., Gauchy C., Glowinski J. and Besson M. J. (I 984) In vioo release of [‘HIGABA in cat caudate nucleus and substantia nigra: Involvement of different thalamic nuclei in the bilateral changes induced by a nigral application of muscimol. Brain Res, 303: 2033213. Kuschinsky K. (1981) Psychic dependence on opioids: mediated by dopaminergic mechanisms in the striatum? TZPS 2: 2877289.

Ladinsky H., Consolo S., Bianchi S. and Jori A. (1976) Increase in striatal acetylcholine by picrotoxin in the rat: Evidence for a gabaergic-dopaminergic-cholinergic link. Brain Res. 108~351-361. de La Mora M. P. and Fuxe K. (1977) Brain, GABA, dopamine and acetylcholine interactions. I. Studies with oxotremorine. Brain Res. 135: 107-122.

285

McKenzie G. M. and Hansen E. L. (1980) GABA agonists dissociate striatal unit activity from drug-induced stereotyped behaviour. Neuropharmacology 19: 957-962. Nagy J. and Decsi L. (1977) Studies on the site of action of a~morphine by intra~erebral injection technique in the rat. Pal. J. Pharmac. Pharm. 29: 195-196. Neil1 D. B. (1976) Frontal-striatal control of behavioral inhibition in the rat. Brain Res. 105: 89-103. Roth R. H. and Bunney B. S. (1976) Interaction of cholinergic neurons with other chemically defined neuronal systems in the CNS. in: Biology of Cholinergic Function (Goldberg A. M. and Hanin L., Eds), pp. 3799394. Raven Press, New York. Schallert T., De Ryck M. and Teizelbaum P. (1980) Atropine stereotypy as a behavioral trap: a movement subsystem and electroencephalographic analysis. J. camp. physiol. Psychol. 94: l-24.

Scheel-Kruger J. (1970) Central effects of anticholinergic drugs measured by the apomorphine gnawing test in mice. Aria pharmac. tax. 28: l-16.

Scheel-Kruger J. (1982) GABA: An essential moderator and mediator in the basal ganglia system of dopamine related functions. ACZUneurol. stand. suppl. 90: 4&45. Scheel-Kruger J. and Magelund G. (1981) GABA in the entopeduncular nucleus and the subthalamic nucleus participates in mediating dopaminergic striatal output functions. Life Sci. 29: 15551562. Scheel-Kruger J., Magelund G. and Olianas M. C. (1981) Role of GABA in the striatal output system: Globus pallidus, nucleus entopeduncularis, substantia nigra and nucleus subthalamicus. Adv. Biochem. Psychopharm. 30: 165-186.

Turski L., Havemann V. and Kuschinsky K. (1984) GABAergic mechanisms in mediating muscular rigidity, catalepsy and postural asymmetry in rats: Differences between dorsal and ventral striatum. Brain Res. 322: 49-57. Ushijima J., Noda Y., Mizuki Y. and Yamada M. (1984) Modification of apomorphine, physostigmine and pilocarpine-induced yawning after long-term treatment with neuroleptic or cholinergic agents. Archs int. Pharmacodyn. Ther. ,271: 180-188. Vizi S. E. (1975) The role of Na+-K*-activated ATPase in transmitter release: acetylcholine release from basal ganglia and its inhibition by dopamine and noradrenaline. In: Subcortical Mechanisms and Sensorimoror Activities (Frigyesi T. L., Ed.), pp. 63-87. Hans Huber, Berne. Vizi E. S. (1980) Non-synaptic modulation of transmitter release: pharmacological implication. TIPS 1: I72- 175. Vizi E. S., HBrsing L. G. Jr and Zsilla G. (1981) Evidence of the modulatory role of serotonin in a~tylchoIine release from striatal interneurones. Brain Res. 212: 89-99. Vizi E. S., Ronai A. Z., Harsing L. G. Jr and Knoll J. (1977) Inhibitory effect of dopamine on acetylcholine release from caudate nucleus. Pal. J. Pharmac. Pharm. 29: 201-211.

Ward H. E. Jr, Kosh J. W. and Freeman J. J. (1981) Antagonism of methylphenidate-induced behaviour by neostigmine or hemicholinium-3. Neuropharmacoiogy M: 703-709.

Worm P., Depoortere H. and Lloyd K. G. (1979) Neuropharmacological spectrum of muscimol. Life Sci. 25: 6077614. Yamada K. and Furukawa T. (1980) Direct evidence for involvement of dopaminergic inhibition and chobnergic activation of yawning. Psychopharmaeo~ogy 67: 39-41.