Muscimol differentially facilitates stereotypy but antagonizes motility induced by dopaminergic drugs: a complex GABA-dopamine interaction

Muscimol is the most potent and specific GABA agonist presently available. The influence of muscimol on two behavioral parameters, dependent on dopamine was studied: locomotor activity and stereotyped gnawing induced by apomorphine, cocaine or methylphenidate. In mice pretreated with a non-sedative subcutaneous dose of muscimol, a sedative effect was seen a few minutes after the injection of a stimulant dose of the dopaminergic drugs; the combination muscimol - apomorphine being most sedative. Contrastingly, muscimol strongly facilitates the development of stereotyped gnawing induced by higher doses of cocaine, methylphenidate or apomorphine. Pretreatment with α-methyltyrosine, an inhibitor of the catecholamine synthesis given before muscimol, did not antagonize the stereotyped gnawing after cocaine or methylphenidate. This finding suggests that the muscimol effect primarily depends on a direct GABA-ergic mechanism facilitating stereotyped gnawing.     

Pharmacological effects of a specific dopamine D-1 antagonist SCH 23390 in comparison with neuroleptics

Neuroleptics such as thioxanthenes (cis($\text{Z}$)-flupentixol and cis($\text{Z}$)-clopenthixol) and phenothiazines (fluphenazine and perphenazine), which block both dopamine (DA) D-1 and D-2 receptors and the butyrophenones (haloperidol and spiroperidol), which block D-2 receptors only, are equipotent both behaviorally and clinically. A new compound SCH 23390 which selectively blocks DA D-1 receptors, resembles many neuroleptics in its pharmacological profile: antistereotypic effects in mice, rats and dogs, cataleptogenic effect and inhibitory effect on amphetamine circling. In contrast SCH 23390 has no effect on apomorphine-induced vomiting in dogs and little effects on 6-OHDA-denervated supersensitive DA receptors, stimulated by the DA agonist 3-PPP. In a series of experiments where methylphenidate-induced stereotyped gnawing in mice was inhibited by neuroleptics, it was shown that concomitant treatment with scopolamine or diazepam attenuated the effect of butyrophenones (D-2 antagonists). The same treatment attenuated the effect of phenothiazines, to a lesser extent, and hardly attenuated the effect of thioxanthenes and SCH 23390 at all. It is concluded that DA D-1 receptors are as important as D-2 receptors for the expression of neuroleptic activity in most animal models believed to be predictive of antipsychotic and extrapyramidal side-effect potential. However, the D-1 antagonist is less sensitive than D-2 antagonists to antimuscarinic compounds and benzodiapines.     

Influence of GABA-agonists and antagonists on neuroleptic-induced catalepsy in rats.

Direct (muscimol) or indirect (aminooxyacetic acid, diaminobutyric acid, pyrrolidinone) GABA-mimetic compounds significantly potentiate neuroleptic induced catalepsy in rats. In contrast at subconvulsant doses, direct (bicuculline, picrotoxinin and indirect (allylglycine) GABA antagonists antagonized haloperidol-induced catalepsy. The effect of bicuculline and picrotoxinin was biphasic with the lowest doses increasing catalepsy. These results indicate that GABA mechanisms are involved in the induction of catalepsy by neuroleptics.     

Muscimol a GABA-agonist injected into the nucleus accumbens increases apomorphine stereotypy and decreases the motility

Muscimol, a highly potent GABA agonist was injected in the nucleus accumbens. Muscimol (10 and 100 ng) was found to facilitate the development of stereotyped licking and gnawing, but contrastingly to depress the locomotion induced by subcutaneously injected apomorphine (0.25 mg/ kg). The local injection of muscimol induces per se no stereotypy. These results indicate that GABA in the nucleus accumbens differentially influences behavior dependent on dopaminergic mechanisms.     

Significance of the dopamine-blocking and m-choline-blocking components in the antiapomorphine action of neuroleptics

In experiments on rats and mice the correlation between the ability of neuroleptics to antagonize apomorphine induced stereotypy and to block central dopamine and muscarinic acetylcholine receptors was studied. The analysis showed significant correlation (v = 0.76; P less than 0.05) between antistereotypic effects of drugs and their ability to inhibit 3H-spiperone binding to rat striated tissue. However, no correlation was found between antistereotypic effect of neuroleptics and their ability to block 3H-quinuclidinyl benzylate binding or arecoline-induced tremor.     

Antagonism of AP-5-induced sniffing stereotypy links umespirone to atypical antipsychotics.

Blockade of glutamatergic transmission in the striatum (using the NMDA-antagonist DL-2-amino-5-phosphonovaleric acid AP-5) was recently shown to induce stereotyped sniffing in rats. Comparable stereotyped behaviour is well known to be elicited by stimulation of dopamine activity, which since long was the basis for experimental models to check for possible antipsychotic activity of new compounds. However, whereas dopamine-induced stereotypies are antagonized only by classical neuroleptics, stereotypies induced by blockade of glutamatergic transmission are antagonized by classical as well as by atypical antipsychotics. Umespirone, a novel psychotropic which has been reported to exhibit behavioural effects predictive for antipsychotic as well as anxiolytic potential was evaluated for antagonistic effects against AP-5-induced behaviour. The profile of umespirone was compared with the profile of a non-benzodiazepine anxiolytic buspirone as well as with previously published data of neuroleptics. Umespirone like clozapine specifically antagonized AP-5-induced sniffing, i.e. did not impair spontaneous sniffing but reversed AP-5-induced excessive sniffing. In contrast, buspirone impaired spontaneous and AP-5-induced sniffing to about the same extend. These results are in accordance with the glutamate hypothesis of schizophrenia and again give evidence that umespirone should have antipsychotic potential and a very low liability to exhibit unspecific sedative action.     

Phenothiazine antagonism of the noradrenergic inhibition of cerebellar Purkinje neurons.

Phenothiazine derivatives were examined as potential antagonists of the inhibitory noradrenergic synapses from the nucleus locus coeruleus to rat cerebellar Purkinje cells. Fluphenazine, and its thioxanthine analogue, flupenthixol, antagonized the inhibitory action of norepinephrine, when iontrophoretically applied to single cells. Alpha-flupenthixol was generally more active than the beta isomer. Fluphenazine had no appreciable effect on inhibitions induced by iontophoresis of GABA or cyclic AMP. Parenteral fluphenazine also blocked the inhibition of Purkinje cells produced by the stimulation of the noradrenergic pathway from locus coeruleus, but basket and stellate cell inhibitory inputs to Purkinje cells were unaffected. These data suggest that fluphenazine can specifically block a known central adrenergic inhibitory pathway.     

Phenothiazine antagonism of the noradrenergic inhibition of cerebellar purkinje neurons

Phenothiazine derivatives were examined as potential antagonists of the inhibitory noradrenergic synapses from the nucleus locus coeruleus to rat cerebellar Purkinje cells. Fluphenazine, and its thioxanthine analogue, flupenthixol, antagonized the inhibitory action of norepinephrine, when iontrophoretically applied to single cells. Alpha-flupenthixol was generally more active than the beta isomer. Fluphenazine had no appreciable effect on inhibitions induced by iontophoresis of GABA or cyclic AMP. Parenteral fluphenazine also blocked the inhibition of Purkinje cells produced by the stimulation of the noradrenergic pathway from locus coeruleus, but basket and stellate cell inhibitory inputs to Purkinje cells were unaffected. These data suggest that fluphenazine can specifically block a known central adrenergic inhibitory pathway.     

Muscimol and γ-hydroxybutyrate: Similar interactions with convulsant agents

Muscimol has been shown to be a potent GABA agonist in several preparations. After systemic administration, muscimol is rapidly metabolized in the periphery and little, if any, unchanged muscimol gains access to the brain. A major metabolite of muscimol may be structurally analogous to γ-hydroxybutyric acid. In this study it is shown that both muscimol and γ-hydroxybutyrate antagonize convulsions induced by 3-mercaptopropionate, an inhibitor of GABA synthesis, and strychnine, a glycinergic antagonist, while potentiating convulsions induced by bicuculline, a putative GABA antagonist, and pentylenetetrazol, a generalized excitant and possible GABA antagonist. Although these results apparently contradict previously reported data, it is proposed that these anomalies reflect differences dependent upon varying dose regimens of muscimol. The differential effects of low vs. high doses of muscimol may reflect differences in the accessibility to, or affinity of, morphologically distinct GABA receptors that mediate different pharmacological functions.     

Antagonism of AP-5- and amphetamine-induced behaviour by timelotem as compared with clozapine and haloperidol

Bilateral intrastriatal injection of DL-2-amino-5-phosphonovaleric acid (AP-5), that blocks glutamatergic transmission at the N-methyl-d-aspartate preferring receptor, induces sniffing and body turns and reduces grooming in rats. Timelotem, a representative of the newly developed chemical class of anellated benzodiazepines antagonized specifically AP-5-induced sniffing and body turns. Classical (haloperidol) as well as atypical (clozapine) neuroleptics had recently been shown to antagonize AP-5-induced sniffing; clozapine, like timelotem, but not haloperidol, additionally antagonized AP-5-induced body turns. Further, timelotem antagonized amphetamine-induced stereotyped behaviour in rats, but was found less active than haloperidol in this test. Comparing the activity of drugs in both paradigms revealed that haloperidol inhibited AP-5-induced sniffing and amphetamine-induced stereotypies within the same dose range, but timelotem and clozapine were found more potent in the AP-5 test than in the amphetamine test. Thus, detailed drug profiles discriminate timelotem and clozapine from haloperidol, linking timelotem again to atypical antipsychotic compounds.     

Differential effects of acute and chronic treatment with typical and atypical neuroleptics on c-fos mRNA expression in rat forebrain regions using non-radioactive in situ hybridization.

The regional difference in the expression of c-fos mRNA in rat forebrain after either acute or chronic administration of typical (haloperidol and fluphenazine) and atypical neuroleptics (clozapine and (+/-)-sulpiride) was investigated. Rats were injected intraperitoneally with vehicle or neuroleptics daily for 14 days. Twenty-four hours after the last injection, the rats were challenged with vehicle or neuroleptics. C-fos mRNA expression was determined by non-radioactive in situ hybridization. Acute treatment with typical neuroleptics induced a remarkable induction of c-fos mRNA in the dorsolateral striatum, whereas this induction was greatly attenuated by chronic administration. All neuroleptics examined induced c-fos mRNA in the shell region of N. accumbens by acute administration and this expression was still elevated after chronic treatment. Since chronic neuroleptics do not induce tolerance to their antipsychotic activities, our study suggests that the shell region of N. accumbens is an important target site for antipsychotic effects of neuroleptics.     

Intra-accumbens baclofen, but not muscimol, increases second order instrumental responding for food reward in rats

Stimulation of either GABA(A) or GABA(B) receptors within the nucleus accumbens shell strongly enhances food intake in rats. However the effects of subtype-selective stimulation of GABA receptors on instrumental responses for food reward are less well characterized. Here we contrast the effects of the GABA(A) receptor agonist muscimol and GABA(B) receptor agonist baclofen on instrumental responding for food using a second order reinforcement schedule. Bilateral intra-accumbens administration of baclofen (220-440 pmol) stimulated responding but a higher dose (660 pmol) induced stereotyped oral behaviour that interfered with responding. Baclofen (220-660 pmol) also stimulated intake of freely available chow. Muscimol (220-660 pmol) was without effect on responding for food on this schedule but did stimulate intake of freely available chow. Unilateral administration of either baclofen or muscimol (220 pmol) induced similar patterns of c-fos immunoreactivity in several hypothalamic sites but differed in its induction in the central nucleus of the amygdala. We conclude that stimulation of GABA(A) or GABA(B) receptors in the nucleus accumbens shell of rats produces clearly distinguishable effects on operant responding for food.     

Neuropharmacological spectrum of muscimol

Muscimol was tested in comparison with a series of reference compounds in a variety of situations in which GABA-related drugs are known to have an effect. Muscimol blocked the convulsions and/or lethality due to picrotoxinin, strychnine and a low dose of bicuculline. It was inactive against higher dosis of bicuculline, metrazole or electroshock convulsions. Muscimol reduced both the basal and the picrotoxin-induced multi-unit activity of the neurons of the dorsal Deiters' nucleus ; although active at low doses, the maximum effect of muscimol was relatively weak. Muscimol potentiated neuroleptic-induced catalepsy, and this effect was bicuculline sensitive ; it did not induce catalypsy in the presence of sulpiride. At high doses muscimol blocked apomorphine-induced stereotyped behaviour. It is proposed that muscimol is a GABA agonist of high affinity but of relatively low efficacy as based on its spectrum of neuropharmacological activities “in vivo”.     

Modulation of acetylcholine release from rat striatal slices by the GABA/benzodiazepine receptor complex

GABA, THIP and muscimol enhance spontaneous and inhibit electrically induced release of tritium labelled compounds from rat striatal slices which have been pre-labelled with 3H-choline. Baclofen is inactive in this model. Muscimol can inhibit electrically induced release of tritiated material by approximately 75% with half maximal effects at 2 microM. The response to muscimol can be blocked by the GABA antagonists bicuculline methobromide, picrotoxin, anisatin, R 5135 and CPTBO (cyclopentylbicyclophosphate). Drugs which act on the benzodiazepine receptor (BR) require the presence of muscimol to be effective and they modulate the effects of muscimol in a bidirectional manner. Thus BR agonists enhance and inverse BR agonists attenuate the inhibitory effects of muscimol on electrically induced release. Ro15-1788, a BR antagonist, does not modulate the inhibitory effects of muscimol but antagonizes the actions of clonazepam, a BR agonist, and of DMCM, an inverse BR agonist. These results demonstrate that a GABA/benzodiazepine receptor complex can modulate acetylcholine release from rat striatal slices in vitro.     

NE-100, a novel sigma receptor ligand: In vivo tests

It has been suggested that sigma receptor antagonists may be useful as antipsychotic drugs. N, N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl]-ethylamine monohydrochloride (NE-100) is a novel compound with high affinity for the sigma receptor (IC50 = 4.16 nM), but low affinity (IC50 > 1000 nM) for D1, D2, 5-HT1A, 5-HT2 and phencyclidine (PCP) receptors. The head-weaving behavior induced by either (+)SKF10047 or PCP was dose-dependently antagonized by NE-100 with oral ED50 at 0.27 and 0.12 mg/kg, respectively. NE-100 did not affect dopamine agonists-induced stereotyped behavior and/or hyperactivity. NE-100 failed to induce catalepsy in rats. These findings indicate that NE-100 may have antipsychotic activity without the liability of motor side effects typical of neuroleptics.     

Modulation of Extracellular γ-Aminobutyric Acid in the Ventral Pallidum Using In Vivo Microdialysis

Intracranial microdialysis was used to investigate the origin of extracellular gamma-aminobutyric acid (GABA) in the ventral pallidum. Changes in basal GABA levels in response to membrane depolarizers, ion-channel blockers, and receptor agonists were determined. Antagonism of Ca2+ fluxes with high Mg2+ in a Ca(2+)-free perfusion buffer decreased GABA levels by up to 30%. Inhibition of voltage-dependent Na+ channels by the addition of tetrodotoxin also significantly decreased basal extracellular GABA concentrations by up to 45%, and blockade of Ca2+ and Na+ channels with verapamil reduced extracellular GABA by as much as 30%. The addition of either the GABAA agonist, muscimol, or the GABAB agonist, baclofen, produced a 40% reduction in extracellular GABA. GABA release was stimulated by high K+ and the addition of veratridine to increase Na+ influx. High K(+)-induced release was predominantly Ca(2+)-dependent, whereas the effect of veratridine was potentiated in the absence of extracellular Ca2+. Both high K(+)- and veratridine-induced elevations in extracellular GABA were inhibited by baclofen, whereas only veratridine-induced release was antagonized by muscimol. These results demonstrate that at least 50% of basal extracellular GABA in the ventral pallidum is derived from Ca(2+)- or Na(+)-dependent mechanisms. They also suggest that Na(+)-dependent release of GABA via reversal of the uptake carrier can be shown in vivo.     

Evidence that the nigrotegmental GABAergic projection mediates stereotypy induced by apomorphine and intranigral muscimol

The substantia nigra plays a pivotal role in the relay of output from the striatum. One neural pathway from substantia nigra projects GABAergic fibers to the caudal mesencephalic tegmentum, terminating in the vicinity of the pedunculopontine nucleus (PPN). To evaluate the functional importance of this projection in the mediation of stereotyped behaviors of striatal and nigral origin, we microinjected low doses of the GABA agonist, muscimol, bilaterally into the vicinity of the PPN. This muscimol treatment resulted in a total blockade of all stereotyped behaviors normally elicited by systemic apomorphine or by intranigral muscimol. Blockade was not observed in animals microinjected with muscimol into the dorsal reticular formation, 1 mm above the level of the PPN. Our results indicate that the nigrotegmental projection may play a crucial role in the expression of stereotyped and dyskinetic behaviors of basal ganglia origin.     

Growth hormone secretion of the neonatal rat pituitaries is stimulated by gamma-aminobutyric acid in vitro

Growth hormone secretion from pituitaries of neonatal rats was stimulated by gamma-aminobutyric acid (GABA) and the GABA agonist muscimol $\text{in vitro}$. This response to GABA was absent after the 9th postnatal day. The stimulation of growth hormone secretion by GABA was antagonized by bicuculline-methiodide and by picrotoxin. Diazepam stimulated while baclophen had no effect on growth hormone secretion. This stimulatory GABA effect might be related to a certain developmental stage of the pituitary GABA receptors or to the lack of hypothalamic regulatory influence(s) in the newborn.     

Benzodiazepines modulate striatal enkephalin levels via a gabaergic mechanism

As measured by a highly specific radioimmunoassay, diazepam treatment of rats results in a rapid decrease of enkephalin levels in the striatum whilst these are increased in the hypothalamus. This striatal effect is mimicked by the GABA agonist muscimol and the GABA-transaminase inhibitor aminooxyacetic acid (AOAA). It is furthermore blocked by the GABA antagonist bicuculline and is thus GABAergic in nature. Further, the diazepam effect upon striatal enkephalin levels is antagonized by low doses of naloxone (1.0 mg/kg, i.p.). In the hypothalamus, diazepam effects were neither mimicked nor modulated by any of a variety of agonists and antagonists tested, suggesting that benzodiazepine effects on enkephalin levels in this structure are not mediated via a GABAergic mechanism.     

Effects of taurine and a taurine antagonist on some respiratory and cardiovascuar parameters

Respiratory performance, heart rate and blood pressure were studied in halothane anesthetized rats after administration of taurine and the putative taurine antagonist 6-aminomethyl-3-methyl-4H-1,2,4-benzothiadiazine-1, 1-dioxide hydrochloride (TAG). Intracerebroventricular (i.c.v.) taurine depressed ventilation due to decreased inspiratory neural drive and depression of respiratory timing mechanisms. I.c.v. administration of 1–100 μg TAG caused no changes in the respiratory adn circulatory parameters studied except at the highest dose interval where respiratory frequency and minute ventilation were depressed. The respiratory depression induced by taurine (0.2 mg) or β-alanine (1 mg) was antagonized by administration of TAG (100 μg). However, TAG did not antagonize the respiratory effects induced by i.c.v. glycine or γ-aminobutyric acid (GABA) in equipotent respiratory depressant doses. The decline in inspiratory neural drive as well as in “respiratory timing” after i.c.v. taurine was restituted toward control values by TAG. The hypotension and bradycardia induced by taurine were also antagonized by TAG. It is concluded that TAG seems to antagonize the depressant action of taurine and β-alanine but not of GABA and glycine on respiratory performance. TAG might also possess some partial agonist activity in higher doses.