Effects of selective dopamine D1 and D2 receptor agonists on the rate of GABA synthesis in mouse brain

European Journal of Pharmacofogy, 191 (1990) 19-27

19

Elsevier EJP 51586

Effects of selective dopamine D, and D, receptor agonists on the rate of GABA synthesis in mouse brain Anne-Fraqoise Steulet, Raymond Bemasconi, Therese Leonhardt, Pierre Martin, Christine Griinenwald, Serge Bischoff, Micheline Heir&h, Viviane Bandelier and Laurent Maftre Research Department, Pharmaceuticals Divisiotr. Ciba-Geigy Ltd.. 4002 Bare& Switzerland

Received 2 April 1990, revised MS received18 July 1990,accepted28 July 1990

The effects of dopamine D, and Dz receptor agonists and antagonists on the rate of GABA synthesis in four regions of mouse brain (corpus striatum, cerebellum, cortex and hippocampus) were examined after irreversible inhibition of 4-aminobutyrate: Zoxoglutarate aminotransferase (EC 2.6.1.19; GABA-T) by gabaculine. The dopamine 4 receptor agonists PPHT, LY 171555 and RU 24213 exerted a dose-related inhibitory effect on GABA synthesis in these four regions. The decreases in the rate of GABA formation were prevented by the dopamine Dz receptor antagonist q-)-sulpiride. The dopamine D1 receptor agonists SKF 77434 and SKF 38393 augmented gab~u~e-~~ duced GABA accumulation in the corpus striatum only, and this effect was blocked by the dopamine D, receptor antagonist SCH 23390. However, SKF 81297 and SKF 82958, two other dopamine D, receptor agonists, did not affect or only marginally altered the rate of GABA synthesis. Stimulation of D, receptors thus induces a decrease in the rate of GABA formation in the four brain areas examined, whereas stimulation of D, receptors either increases GABA synthesis in the CCJ~XIS striatum or does not alter it. This effect appears to be independent of the degree of receptor occupancy. Dopamine D, receptors; Dopamine D2 receptors; GABA synthesis (rate of); Gabaculine; Brain (mouse)

1. ~n~~ucti~

There is sufficient anato~cal evidence to warrant the conclusion that GABAergic neurones in the rat neost~atum receive dopaminergic inputs (Kubota et al., 1987). Moreover, certain biochemical interactions between the neuro~ans~tters dopamine and GABA have been well documented. Apomorphine, a mixed dopamine DI/D2 receptor agonist, has been shown to exert an inhibitory effect on GABA release in the rat caudate puta-

Correspondence to: A.F. Steulet, K.136.295. Ciba-Geigy Ltd., 4002 Basei, Switzerland. ~~~2999/90/$03.SO

men (Brase, 1980; De Belleroche and Gardiner, 1983; Caudill et al., 1985; Umeda and Sumi, 1989) and substantia nigra (Arbilla et al., 1981; Kelly et al., 1985). In vivo, stimulated GABA overflow in the rat striatum is decreased by dopamine (Van der Heyden et al., 1980) and by the doparnine D2 receptor agonist pergolide (Tossman and Ungerstedt, 1986). Dopaminergic afferents to the rat striatum exercise an inhibitory control over striatal GABAergic interneurones, and the interruption of dopamine function (by lesioning with (i-hydroxydopamine or chronic administration of neuroleptics) augments the rate of GABA synthesis {Marco et al., 1976; Gale and Casu, 1981). However, these findings are restricted to the st~atonigral circuit and are o~~ion~ly con-

0 1990 Elsevier Science Publishers B.V. (Biome~cal Division)

tradicted. Tossman and Wngerstedt (1986). for instance. dnd not observe any effect of apomorabide on GABA retease in viva. According to dgren (1987). the rate of GABA synthesis is unaffected; however, Perez de la Mora and Fuxe ~~9~7~ reported an increase in the rate of GABA synthesis after administration of apomorTn an attempt to. clarify previous findings on dopa~n~GABA interactions, we measured the effects of dopamine agonists and antagonists on the rate of GABA synthesis, which is an index of cell activity. Dopamine is known to act on at least two subtypes of receptor in the mammalian CNS. These have been classified as D, and D, on the basis of the pi~armacological effects mediated and according to their association with the enzyme adenyh&e cyclase (Kebabian and Calne, 1979). Apomorphine. which acts at both these receptor sites. markedly decreases the rate of GABA synthesis. This effect is completely antagonized by S( - )-sulpiride, a selective dopamine Dz receptor antagonist (Bemascom et al.. 1989). indicating that the action of apomorphine is due to stimulation of D7 and not D, receptors. To test the validity of this assumption, we investigated the effects of selective dopamine D, receptor agonists (PPHT (N-0434), LY 171555, RU 24213), dopamine D, receptor agonists (SKF 77434, SKF 38393, SKF 81297 and SKF 82958), a dopamine D, receptor antagonist (S( - )-sulpiride) and a dopamine D, receptor antagonist (SCH 23390) cm the rate of GABA synthesis in four regions of mouse brain: the corpus sttiatum, which contains a very high density of dopa~ne receptors, and the cortex. bippocampus and cerebellum, which have a low density of dopamine receptors (Boyson et al., 1986). The rate of GABA synthesis was estimated after inhibition of 4-aminobutyrate: 2-oxoglutarate ~notransferase; E.C. 2.6.1.19; GABA-T, the e-nzyme responsible for GABA catabolism, by a selective and irreversible inhibitor, ( f)-gabaculine (Steulet et al., 1989). In previous studies with selective dopamine agonists and antagonists, attempts have been made to quantify dopamine-GABA interactions in terms

of GABA release, either in vitro or in vivo, but there have not as yet been any reports on the rate of GABA synthesis. The present study was intended to test the hypothesis that both subtypes of dopamine receptor are involved in the regulation of GABA synthesis and to examine the possibilities that (a) stimulation of D, receptors may elicit a change in the rate of GABA synthesis opposite to that caused by activation of D, receptors; and (b) the effects of selective dopamine D, and D, reeeptor agonists may be specifically prevented by selective dopamine D, and DZ receptor antagonists, respectively.

2. Materials and methods 2.2. Drugs and chemicals

PPHT hydrochloride (2-(N-phenylethyl-N-propyl)amino-5-hydroxy-tetralin, HCl; N-0434), R( + )-SCH 23390 hydrochloride (7-chloro2,3,4,5-tetr~ydr~3-methyl-S-phenyl-lH-3-be~zepine-7-01, HCl), R( +)-SKF 38393 hydrochloride (2,3,4,5-tetrahydro-7,8-dihydroxy-l-phenylIH-3-benzazepine, HCl) and SKF 82958 hydrobromide (3N-allyl-6-chloro-SKF 38393, HBr) (new catalogue number: C-130) were purchased from Research Biochemical Inc., USA. SKF 81297 hydrobromide (6-&Joro-SKF 38393, HBr) and SKF 77434 hydrobromide (3N-allyl-SKF 38393, HBr) (Smith Kline & French, USA) were a gift from Dr. S. Erma. S( - )-Sulpiride (Ravizza, Italy), RU 24213 (N-propyl-N-phenethyl-m-tyramine) (Roussel-Uclaf, France) and LY 171555 (quinpirole hydro~~o~de~ (Lilly Research Laboratories, USA) were also gifts. ( + )-Gabaculine hydrochloride was synthesized in our laboratories by Dr. W. Bencze. ~-A~nobuty~c acid (GABA) and S-aminovaleric acid were obtained from Fluka, Switzerland. Trifluoroacetic anbydride and hexafluoropropanol were purchased from Pierce Chemical Company, USA. 2.2. Animals Male Tif : MAGf (SPS) mice (Tierfarm, Sisseln, Switzerland), weighing 24-26 g, were used in this

21

study. They were kept six to a cage in an animal room at a constant temperature of 21-22’ C with a 12-h light/dark cycle for 5 days before the experiments. Food and water were freely available.

chromatography under the conditions by Steulet et al. (1989).

2.3. Drug administration

Results are expressed as means of six animals per group + S.E.M. Dumrett’s test {Wirier, 1971) was used to assess significance.

The dopamine D2 receptor agonists were injected 5 min before and the dopamine D, receptor agonists 5 min (SKF 77434, SKF 81297 and SKF 82958) or 60 min (SKF 38393) before administration of gabacuiine. The different intervals between injection and killing correspond to the time of the maximal effect of the D, receptor agonists in behavioural experiments (Amt and Perregaard, 1987; Amt et al,, 1988). S( - )-sulpiride was injected 5 min before the dopamine D2 receptor agonists, and SCH 23390 15 min before the dopamine D, receptor agonists. The doses used in this study are in the same range as those used in behavioural experiments with rats and mice for D, receptor agonists (Starr and Starr, 1987; Amt et al., 1988; Vasse et al., 1988; Murray and Wadd~gton, 1989) and D, receptor agonists (Boissier et al., 1980; Van Oene et al., 1984; Dall’Olio et al., 1988; Starr and Starr, 1989). Except for S( - )-sulpiride and PPHT, to which we first added 10 ~1 of acetic acid, all drugs were dissolved in 0.9% w/v physiological saline. The pH was then adjusted to approximately 5 with 2 N NaOH. All solutions were prepared just before i-p. injection in a volume of 100 pl.

described

2.5. Statistics

3. Results 3.1. Effects of sclectiue D, dopamke agonists on the rate of GABA synthesis

receptor

The rate of GABA synthesis can be inferred from the difference in GABA levels between gabacu~e-treated mice and control mice, expressed as pm01 GABA/g per h. PPHT significantly and dose d~endently reduced the accumulation of GABA induced by gabaculine in the cortex and ~pp~~pus (fig. 1). The highest dose of PPHT (1.0 mg/kg) induced a maximal decrease in the rate of GABA synthesis from 7.53 to 3.04 pm01 GABA/g per h in the cortex (P < 0.001) and from 7.31 to 2.95 pmol GABA/g per h in the GABA pmollg

a

12

cortex ~tppocam~us

7 10

8

2.4. Tissue extraction and gas-chromatographic dete~inativn of GABA

6 4

One hour after gabaculine injection, the mice were killed by microwave irradiation (2450 MHz for 2.5 s), The brain was removed, cooled to 4°C and dissected into the desired regions. After homogenization by sonication of the cerebral parts in 4 M hydrochloric acid containing the internal standard S-aminovaleric acid, aliquots of the homogenates were filtered through SA-2 strong acid cation-exchange paper. The eluate was processed as described previously (Bernasconi et al., 1982; 1985). The volatile derivatives of GABA and the internal standard were separated by capillary gas

2 0 PPHT

-

1.0

-

0.3

1.0

Gc

-

-

150

150

150

Fig. 1. GABA levels in the cortex and hippocampus after administration of PPHT (0.3 and 1.0 mg/kg i.p.) and gabaculine (150 mg/kg i.p.) either alone or in combination. Mice were killed by microwave irradiation 1 h after gabaculine injection. Each bar represents the mean f S.E.M. (n = 6). GC = gabaculine. Doses are expressed in mg/kg i.p. * * P < 0.01, * * * P < O.OOlrelative to the gabaculine group.

~~~~~~~~ CS the rate of G4BA synthesis induced by several &Q of three & recqator agonists (PPHT. LY 171555 and rebellum and corpus striatum (the comin thn~ separate expetiments similar to the I), A11 Dz receptor agonists were adminre gabaculine (150 rug/kg i.p.). Mice were Howard ~~dia~~ 1 h after gabaculine injection. v&es represent the inhibition of the gabaculine-induced GAIL% ~~rnu1atio~ caused by Dz receptor agonists, expd in prcent of the total GABA a~umulation induced by gabaczne alone (6.69&O% and 5.88 f0.23 pmol GABA/g per h for cerebellum and corpus striatum. respectively) and are the means from six mice _i SEM.

ra, agonists

Dose (mg/kg i-p.)

% inhibition of the rate of GABA synthesis+ S.E.M. Cerebellum

Corpus striatum

IQ.8 ltO.2 29.0+ 1.0 a 32.6 * 1.7 = 66.6 rt 2.2 h 22.3 +O.l a 42.4* 2.8 b 58.5 * 3.9 h 17.2 +o.s 3 33.4+ 1.2 b 51.4*3.7 b

17.9+0.1

b

29.4*1.2h 39.1+ 1.7 b 69.9 + 2.4 b 16.4 * 0.2 b 42.71t1.9 b 65.6* 5.1 b X2+0.2 24.3 f 0.8 = 49.2kO.6 b

’ P c 0.01, b P c 0.001 relative to the gabaculine group.

lum and by 65.6% (P c 0.001) in the corpus striaturn. RU 24213 (5.0 mg/kg) caused approximately 50% i~bition (P c 0.001) in rhe cerebellum and corpus striatum.

3.2. Effects of S( - ~-suip~r~deon the ~Q~~rn~ne D2 recepior agonist-induced inhibition of the rate of GABA synthesis As indicated in fig. 2,10 mg/kg of ( - )-sulp~de completely prevented any decrease in the rate of GABA synthesis in the four brain areas after injection of 0.3 mg/kg of PPHT. This D, receptor antagonist also inhibited the effects of RU 24213 (5.0 mg/kg) and LY 171555 (0.5 mg/kg) in the corpus striatum (table 2). The same antagonistic effect was found in the cerebellum, cortex and hippocampus (data not shown). ( - )-Sulpiride alone did not modify the accumulation of GABA induced by gabaculine.

cerebellum

GABA

pmollg

cortex

hippocampus

q

c. striatum

12 10

hippocampus (P < 0.001); it did not modify the GABA condensation in the two brain regions extied. Lower doses of PPHT (range: 0.03-1.0 mg/kg) also inhibited GABA accusation in the cerebellum and corpus striatum (table I). The threshold of approximately 0.03 mg/kg caused 10.8 and 17.9% inhibition of the rate of GABA synthesis in the cerebellum and corpus striatum, respectively. Tbe effects of the other two, chemically unrelated, D-, receptor agonists LY 171555 and RU 24213 on gabaculine-induced GABA accumulation are shown in table 1. These compounds also inhibited tbe rate of GABA synthesis in the cerebellum and corpus striatum in a dose-dependent manner. Similar results were found with the cortex and ~pp~~pus (data not shown). LY 171555 (0.5 mg/kg) maximally inhibited the rate of GABA accumulation by 58.5% (P K 0.001) in the cerebel-

8 6 4 2 0 (-)Sulp.

-

10.0

-

10.0

PPHT

*

-

0.3

0.8

150

150

150

Gc

0

150

Fig. 2. Antagonism by S( - )-sulpiride (10.0 mg/kg i.p.) of the decrease in gabacu~ne-induct GABA a~umuiation in the cerebellum, cortex, hippocampus and corpus striatum elicited by 0.3 mg/kg i.p. of PPHT. Mice were killed by microwave irradiation 1 h after gabaculine ad~~s~ation. Each bar represents the mean& S.E.M. (n = 6). (-)Sulp. = S( -)-sulpiride; Gc = gabaculine. Doses are expressed in mg/kg i.p. * * P c 0.01, *** P < 0.001 relative to the gabaculine group; Ac P < 0.01, AAA P -Z0.001 relative to the S( - )-sulpiride + PPHT+ gabaculine group.

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3.3. Effects oj selective DI dopamine receptor agonists OHthe rate of GABA synthesis

GABA

umollg 10

Since preliminary tests indicated that significant and dose-dependent increases in the rate of GABA synthesis induced by D, receptor agonists only occurred in the corpus striatum (unpublished observations), the data presented are restricted to this brain area. As shown in fig. 3, 15.0 mg/kg of SKF 77434 had no effect on the GABA content. However, 10.0 and 15.0 mg/kg of this agonist significantly and dose dependently increased the rate of GABA TABLE 2 Panels A and B: effects of several doses of D, receptor agonists (SKF 38393 and SKF 81297) on the GABA accumulation elicited by 150 mg/kg i.p. of gabaculine in the corpus striatum. Panels C and D: antagonism by S( -)-sulpiride (10 mg/kg i.p.) of the decrease in the gabaculine-induced GABA accumulation caused by the selected doses of the Dz receptor agonists RU 24213 (5.0 mg/kg i.p.) and LY 171555 (0.5 mg/kg i.p.) in the corpus striatum. Mice were killed by microwave irradiation 1 h after gabaculine injection. Results are expressed as means of GABA levels from six mice + S.E.M. Drug treatment

Dose (mg/kg i.p.)

GABA levels in the corpus striatum PmoVg + S.E.M.

% of gabaculine group + S.E.M.

(A) Gabaculine + SKF 38393

150 10.0 20.0 40.0

6.11kO.29

100.0~4.7 112.6 f 3.5 121.4rt3.3 b 119.4k4.9 a

(B) Gabaculine + SKF 81297

150 1.0 5.0 10.0

7.46rt0.22

100.0~2.9 100.3 f 4.9 116.3 +4.0 a 110.8+7.1

(C) Gabaculine + RU 24213 + Sulpiride + RU 24213 + sulp.

150 5.0 10.0

8.41 kO.24

100.0 f 3.2 63.6 f 2.8 =*’ 95.1 f 4.8

(D) Gabaculiie +LY 171555 + Sulpiride +LY 171555 + sulp.

150 0.5 10.0

88.6 f 3.1 a 7.09 kO.26

100.0 f 3.6 74.7 * 3.1 c-l 108.8 + 2.2 102.1 f 2.9

a P -z 0.05, b P < 0.01, ’ P c 0.001 relative to the gabaculine group; ’ P < 0.001 relative to the S( -)-sulpiride + Dz agonist + gabaculine group.

AA

SCH 23390 -

-

-

-

-

-

1.5

1.5

SKF 77434 _ Gc

15.0 _

150

5.0 150

10.0 150

15.0 150

150

15.0 150 17

Fig. 3. GABA levels in corpus striatum after administration of SKF 77434 (5.0,lO.O and 15.0 mg/kg i-p.) and gabaculine (150 mg/kg i.p.) either alone or in combination. Inhibition by SCH 23390 (1.5 mg/kg i.p.) of the effect of SKF 77434 (15.0 mg/kg i.p.). Mice were killed by microwave irradiation 1 h after gabaculine injection. Each bar represents the mean+S.E.M. (n = 6). Gc = gabacuhne. Doses are expressed in mg/kg i.p. * * P c 0.01, * * * P -z 0.001 relative to the gabacuhne group. Ap P < 0.01 relative to the SCH 23390 + SKF 77434+ gabaculine group.

accumulation (123% of GABA level relative to the gabaculine group (P < 0.001) after administration of 15.0 mg/kg = 47% increase in the rate of GABA synthesis). The lowest dose tested, 5.0 mg/kg, did not alter the rate of GABA formation. As shown in table 2,10.0 mg/kg of SKF 38393, another dopamine D, receptor agonist, led to a slight, but not significant, increase in the rate of GABA synthesis. Doses of 20.0 and 40.0 mg/kg, however, increased GABA accumulation to the same extent (approximately 120% of GABA level relative to the gabaculine group), suggesting that the maximal effect was already reached with 20.0 mg/kg. SKF 81297 induced a statistically significant increase (P -C0.05) in the rate of GABA synthesis at a dose of 5.0 mg/kg (table 2). A similar slight and not dose-related elevation was observed in another experiment, but not in a third experiment, nor in a time course analysis when GABA synthesis was measured 65, 90 and 120 min after injection of this D, receptor agonist (unpublished observations).

ly, as with SKF 51297. SKF 82958 (5.0 i.p.) slightly increased the rate of GABA in some experiments and had no effect ers (results not-shown in this study).

SCH 23390 of the increase in synthesis induced by SKF 77434

The dopamine D, receptor antagonist SCH (1.5 mg/kg) si~~c~tly inhibited the inin GABA accumulation induced by 15.0 of SKF 77434 (P < 0.01) (fig. 3) and 20 rq/kg of SKF 38393 (results not shown), without itself affecting the rate of GABA synthesis.

The two major findings of this study are that summation of Dz dopamine receptors decreases the rate of G.4BA synthesis and, conversely, that D, receptor agonists, such as SKF 38393 and SKF 77434, increase it. Most of the neurobiochemical and behavioural data related to the dopaminergic systems have been obtained with rats and are therefore not strictly comparable with the present findings with mice. Biological differences may exist between the rat and mouse. For example, the various agonists and antagonists could act on other receptor types or even be metabolized ~fferently by these two species. The dose-dependent inhibition of the rate of GABA synthesis caused by the selective Dz dopamine receptor agonists (PPHT, LY 171555 and RU 24213) in the four brain areas examined (cortex, cerebellum, corpus striatum and hippocampus) was reversed by S( - )-sulpiride, demonstrating that the inhibition was due to specific activation of DZ receptors. Surprisingly, the ~bito~ effect was just as strong in the cerebellum, cortex and hippocampus as in the corpus striatum, although the density of

DZ receptors and that of dopaminergic afferents are much more important in the rat corpus striaturn (Kizer et al., 1976; Moore and Bloom, 1978; Simon et al., 1979; Scatton et al., 1980; Vemey et al., 1985; Boyson et al., 1986; Bruinink and Bischoff, 1986). Little information is available concerning the dopaminergic innervation of the mouse brain. However, it has been reported that the density of dopamine uptake sites is approximately 30-fold higher in the corpus striatum than in the hippocampus (Donnan et al., 1989). The lack of correlation between the dopamine innervation and the decrease in the rate of GABA synthesis is not well documented. However, we may assume that more D, receptors are located presynaptically on dopaminergic nerve terminals in the corpus striatum than in the three other areas. This hypothesis is supported by the study of Wolf and Roth (1987) and that of Hoffmann et al. (1988) who reported the presence of functional autoreceptors on dop~er~c terminals in the rat prefrontal cortex, but fewer than on striatal dopaminergic terminals. Therefore, it could be possible that equal numbers of & receptors are located postsynaptically on GABAergic cells in the fourbrain ureas examined, and hence comparable effects on the rate of GABA synthesis are observed in these regions. However, the effects induced by the D, receptor agonists in the cortex, cerebellum and hippocampus could be mediated by polys~aptic pathways, including an efferent dopaminergic system originating in the basal ganglia. These dopaminergic neurones might innervate neurones of a neurotransmitter X, which in turn synapse with GABAergic intemeurones located in the above-mentioned areas, as has been proposed for the cerebellum (P&ez de la Mora and Fuxe, 1977; Wood et al,, 1988). Thus, the intensity of the dopamine-GABA interactions mediated by DZ receptors does not seem to depend on the density of dopamine receptors. Other factors such as the localization of the D2 receptors and polysynaptic orga~zation may be more important. In contrast, the effects of some dopamine D1 receptor agonists appear to correlate with the autoradiographic distribution of binding sites of

25

t3H]SKF 38393 in the mouse forebrain (Juhasz et al., 1987) where the density of D, receptors is greatest in the corpus striatum. In fact, selective stimulation of D, receptors by SKF 77434 and SKF 38393 led to an increase in the rate of GABA synthesis in corpus striatum only. This increase was blocked by the D, receptor ~tago~st SCH 23390, indicating that is was due to specific activation of D, receptors. The two 6-cbloro-substituted compounds, SKF 81297 and SKF 82958, only slightly increased the rate of GABA synthesis. Each of these selective D, dopamine receptor agonists increased striatal CAMP levels in vivo (unpub~sh~ results): demunstrating that SKF 81297 and SKF 82958 penetrate into the brain, and that they effectively stimulate D, receptors without significantly modifying the rate of GABA synthesis. One possible explanation for this unexpected finding is that SKF 81297 and SKF 82958 are potent and effective agonists of one sub~pulation of D, receptors (Andersen et al., 1990), coupled to adenylate cyclase. This would be compatible with their marked effect on CAMP levels. On the other hand, they would presumably only be partial agonists (the 6chloro substitution would modify their intrinsic activity and affinity) of another subpop~ation of D, receptors, activation of which leads to an elevated rate of GABA synthesis. A second possibility might be that SKF 81297 and SKF 82958 act on other neurotransmitter systems (e.g. cy- and /%adrenergic or serotoninergic systems), activation of which might diminish the rate of GABA synthesis (Bemasconi, 1981; Bernasconi et al., 1986; Steulet et al., unpublished results). The results of in vitro binding assays do not support this hypothesis. In fact, the IC,,s of SKF 38393, SKF 77434, SKF 81297 and SKF 82958 for 5-HT, receptors were 2.2 + 0.2, 5.1 -i_0.7, 5.2 f 0.4 and 2.3 f 0.2 PM, respectively (results not shown; for materials and methods, see Bischoff et al, 1986). The D, receptor agonists also have weak affinities for 5-HT, and q-adrenoceptors: they show no activity at P-adrenoceptors in concentrations as high as 1 mM, but are moderately active at qbinding sites (0.3-l @I) (K. Hauser, personal communication). However, the I&s of the D, receptor agonists for 02-

adrenoceptors are 300-1000 times higher than their affinity for D, receptors (Seeman and Niulik, 1988). Moreover, SKF 81297 and SKF $2958 are not more active than the other two dopamine D, receptor agonists at these crysites, stimulation of which effectively reduces the rate of GABA synthesis (~rn~o~, 1981). This hypothesis may still be valid for other types of receptors (e.g. histamine, peptide, purine receptors). The opposite roles of D, and D2 receptors in regulating the rate of GABA synthesis in the mouse corpus striatum have not been demonstrated until now. We did, however, suggest in a previous paper (Steulet et al., 1989) that the action of dopamine agonists on the rate of GABA synthesis could be due to a modification of GABA release, followed by feed-back control on the rate of GABA synthesis. The opposite effects of D, and D2 receptor stimulation on [ ‘H]GABA release have been reported in vivo by Girault et al. (1986) in the rat neost~atum and in vitro by Bernath and Zigmond (1989) in the rat corpus striatum and by Starr (1987) in the rat substantia nigra. It has been reported that selective D, and D, dopamine receptor agonists increase and decrease, respectively, body temperature in mice (Sanchez, 1989; Vasse et al., 1990). It is thus possible that the changes in body temperature induced by the dopamine agonists play a crucial role in the rate of GABA synthesis. However, this hypothesis seems unlikely since 15 mg/kg of SKF 38393 increases body temperature by only 0.2OC 90 min after injection (Vasse et al., 1990). Moreover, no temperature modification was detected after the administration of 2.5 cmol/kg (approximately 06 mg/kg) of LY 171555 (Sanchez, 1989). In conclusion, the foregoing results indicate that: (1) both subtypes of dopamine receptor are involved in the processes governing the rate of GABA synthesis in the corpus striatum and they exert opposing actions: stimulation of D2 receptors decreases the rate of GABA synthesis in the four brain regions examined, whereas activation of D, receptors by SKF 38393 and SKF 77434 increases GABA formation in the corpus striatum only; (2) the effects of selective D, and D, re=ptor agonists are specifically antagonized by selective D1 and D, receptor antagonists.

The authors wish to thank Prof. M. Schorderet, Dr. K. Hausrr and Dr. P. WaIdmeier for their helpful comments and suggestions during the preparation of the manuscript and wish to express their gratitude to Dr. S. Enna for the gift of the SKF S11297 and SKF 82958. and to Professor John Newmeyer (Northeastern Wniversity) for synthesizing SKF 82958. Other gifts from various sources (see Materisfs and methods) were alse greatly appreciated. This work forms part of the Ph.D. thesis of A.F. Steulet (School of Pharmacy. University of Lausanne. !&itzerland).

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