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.     

Studies of the role of gamma-aminobutyric acid in the hypothalamic control of feed intake in sheep

Fourteen sheep were used to study the role of gamma-aminobutyric acid (GABA) on the hypothalamic control of feed intake. Injections (1 microL) of pentobarbital (262 nmol) into preoptic and paraventricular areas induced feeding in satiated sheep. Injections of GABA into the same loci gave variable results, probably because the neuronal and glial uptake of GABA limits its effects. Muscimol, a GABA agonist with a higher affinity for postsynaptic GABA receptors than GABA, injected at doses from 0 to 0.750 nmol, gave a cubic dose-response curve; the highest feed intake was measured at 0.5 nmol. The response induced by muscimol was blocked by preinjections of two GABA antagonists, picrotoxin and bicuculline, with picrotoxin being more effective than bicuculline. Muscimol responsive loci were identified mainly in the preoptic, paraventricular, and anterior hypothalamus. The data suggests that neurons sensitive to gamma-aminobutyric acid may be implicated in the control of feed intake in sheep.     

Effects of intra-amygdala injection of GABA(A)-receptor agonist and antagonist on behavior of active and passive rabbits in negative emotional situations

The effects of right-side or left-side intra-amygdala injections of the GABA(A)-receptor agonist muscimol hydrobromide (0.1 microg/1 microl) and antagonist bicuculline methiodide (0.05, 0.1, 0.5 microg/1 microl) on the behavior of active and passive rabbits were studied in open field, light-dark test and during presentation of emotionally significant stimuli. The effect of compounds injection was differed in active and passive rabbits. The active rabbits were more sensitive to bicuculline injection and the passive rabbits to muscimol administration. Bicuculline induced anxiolytic-like effects on the active animals in open field, light-dark test and motor-activating effects during emotionally significant stimuli. Muscimol induced anxiolytic-like effects on the passive rabbits and sedative effects on the both groups of rabbits. The differences in effectiveness of right-side and left-side intra-amygdala injections on behavior of rabbits were revealed: more powerful changes were during injection of bicuculline in left and muscimol in right amygdala. Anxiolytic-like effects were revealed during injection of bicuculline into left amygdala of active rabbits and muscimol into right amygdala of passive rabbits. These findings indicate that there are individual-typological and interhemispheric differences in functioning of GABAergic system of amygdala.     

Effect of GABAergic substances on firing rate and thermal coefficient of hypothalamic neurons in the juvenile chicken

The goal of the study is to investigate the GABAergic action on firing rate (FR) and temperature coefficient (TC) on hypothalamic neurons in the juvenile chicken. Extracellular recordings were obtained from 37 warm-sensitive, 32 cold-sensitive and 56 temperature-insensitive neurons in brain slices to determine the effect of GABA(A)-receptor agonist muscimol, GABA(A)-receptor antagonist bicuculline, GABA(B)-receptor agonist baclofen and GABA(B)-receptor antagonist CGP 35348. Muscimol and baclofen in equimolar concentrations (1 microM) significantly inhibited FR of the neurons, regardless of their type of thermosensitivity. In contrast, bicuculline, as well as CGP 35348 (10 microM) increased FR of the majority of the neurons. The TC of most chick hypothalamic neurons could not be estimated during muscimol application because FR was completely inhibited. GABA(B)-receptor agonist specifically increased TC. This effect was restricted to cold-sensitive neurons, which were determined in a high number. The TC was significantly increased (p<0.05) by baclofen and significantly decreased (p<0.05) by CGP 35348. The effects of muscimol and baclofen on FR and TC were prevented by co-perfusion of the appropriate antagonists bicuculline and CGP 35348. The results suggest that the fundamental mechanisms of GABAergic influence on temperature sensitive and insensitive neurons in the chicken PO/AH are conserved during evolution of amniotes.     

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.     

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.     

Physiological and behavioral studies with muscimol

Muscimol has been used to increase our knowledge of central GABAergic systems, CNS physiology, and behavior. Some studies concerning the neurophysiological and behavioral effects of muscimol and its analogs have been reviewed and analyzed. In vivo iontophoretic studies have greatly increased our knowledge of the active conformation(s) adopted by GABA during its interaction with neuronal synaptic (or extrasynaptic) receptors, and behaviors. studies have supported the notion that central GABAergic systems might be involved in convulsions, extrapyramidal functions, and other behaviors. However, behavioral studies with muscimol remain difficult to interpret in terms of central GABAergic systems, especially since muscimol is extensively metabolized and since it appears to interact with membrane sites other than GABA receptors. Muscimol does not appear to be useful for reversing human neurologic-psychiatric disorders.     

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.     

MUSCIMOL UPTAKE, RELEASE AND BINDING IN RAT BRAIN SLICES

Abstract— The GABA analogue, muscimol, was taken up relatively inefficiently compared to GABA by slices of rat cerebral cortex at 37 C. Muscimol uptake followed saturation kinetics (K_m ImM. V_m 0.1 μmol g mini and showed an absolute dependence on sodium ions. The relative susceptibilities of muscimol uptake and GABA high affinity uptake to a variety of inhibitors, including (-)-nipecotic acid. (+)-2.4-diaminobutyric acid and arecaidine, and the stimulation of muscimol efflux by 50μM-GABA, suggest that muscimol and GABA share some common transport carriers. Since L-histidine inhibited muscimol uptake hut not GABA high affinity uptake, at least part of the observed muscimol uptake may be mediated by the 'small basic'amino acid transport system. Muscimol appeared to he taken up into nerve terminals, since uptake was inhibited by the neuronal uptake inhibitor cis -3-aminocyclohexanecarboxylic acid but not by the glial uptake inhibitor β-alanine. Muscimol efflux was stimulated in a calcium-dependent manner by an increased potassium ion concentration.
Sodium-independent binding of muscimol was observed in slices of rat cerebral cortex at 4 C. Binding could be inhibited by a variety of substances. including GABA, isoguvacine and (+)-bicuculline methochloride, which are known to inhibit the binding of muscimol to putative GABA receptors associated with synaptic membranes purified from rat brain.     

Ventrobulbular hypotensive action of muscimol]

Muscimol, a rigid analogue of GABA has been injected in the CNS of urethane anesthetized, normotensive cats. Injected either intracisternally (1 or 2 microgram/kg, 0.05 ml) or directly by microinjection in a restricted ventrolateral region of the brain stem (0.5 or 1 microgram/kg, 0.5 microliter), muscimol induced hypotension and bradycardia. These central cardiovascular effects of muscimol were antagonized by bicuculline, a "specific" GABA antagonist agent. These data emphasize the involvement of gabergic mechanisms in the central cardiovascular control, at least in the ventrolateral part of the medulla oblongata.     

A benzodiazepine antagonist action of CL 218,872

The effect of the Type I benzodiazepine (BDZ) receptor agonist, CL 218,872, on convulsions generated by low doses of methyl beta-carboline-3-carboxylic acid ( MBCC ), bicuculline, picrotoxin and pentylenetetrazole (PTZ) in mice was examined. Low doses of CL 218,872 enhanced the convulsions produced by all agents except PTZ. An anticonvulsant action of CL 218,872 was observed at higher doses. Since CL 218,872 exhibits proconvulsive effects at low doses, and a proconvulsant action is a characteristic of compounds classified as BDZ antagonists, it appears that CL 218,872 has some antagonist action.     

Photoaffinity Labeling of the GABAA Receptor with [3H]Muscimol

Muscimol is one of the most potent agonist ligands at the gamma-aminobutyric acidA (GABAA) receptor. Analysis of its chemical structure showed it to be a candidate for photoaffinity labeling. In practice, UV irradiation at 254 nm both changed the UV spectrum of muscimol and induced an irreversible binding of [3H]-muscimol to rat cerebellar synaptosomal membrane. After 10 min of irradiation, using 10 nM [3H]muscimol, the specific portion of this binding was 270 fmol/mg protein. (Nonspecific binding was defined as that arising in the presence of 1 mM GABA.) Specific binding increased asymptotically up to 100 nM [3H]muscimol. Irradiation of the membranes themselves did not significantly alter the KD or Bmax of reversible [3H]muscimol binding. However, irradiation of [3H]muscimol reduced its capacity subsequently to photolabel the membranes by 86 +/- 3%. Dose-dependent inhibition of binding was observed with muscimol, GABA, and bicuculline methiodide; with 10 nM [3H]muscimol maximum inhibition was 70% of total labeling and the order of potencies of these three compounds was characteristic of labeling to the GABAA receptor. Baclofen, l-glutamate, and diazepam exerted no effect at high concentrations. SDS-PAGE of the photolabeled membranes indicated specific incorporation of radioactivity into two molecular-weight species. One failed to enter the separating gel, implying a molecular weight greater than 250,000 daltons (250 kD). The molecular weight of the other was identified by fluorography to be about 52,000 daltons (52 kD).     

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.     

[3H]Muscimol Binding Site on Purified Benzodiazepine Receptor

The presence of a [3H]muscimol binding site on the purified benzodiazepine receptor was demonstrated. The purified protein was apparently homogeneous as shown by sodium dodecyl sulfate polyacrylamide gel electrophoresis (stained with silver), with a molecular weight of 60,000 +/- 3000. The benzodiazepine binding sites were characterized as being of the central type and the [3H]flunitrazepam binding was enhanced by GABA. This activation was antagonized by bicuculline. [3H]Muscimol specifically binds to the benzodiazepine receptor. The Scatchard plot indicates a Kd of 23 nM and the ratio [3H]flunitrazepam/[3H]muscimol is approximately unity.     

Sensitivities of Achatina giant neurones to putative amino acid neurotransmitters

1. GABA receptors in Achatina identifiable giant neurones were classified into the muscimol I, muscimol II and baclofen types. Muscimol I and II type GABA receptors were sensitive to GABA and muscimol but insensitive to baclofen, whereas baclofen type receptors were sensitive to GABA and baclofen but insensitive to muscimol. Muscimol I and baclofen types were associated with the inhibition caused by GABA, while muscimol II type with the GABA excitation.2. GABA, muscimol and TACA produced a transient outward current (I_out) with an increase in membrane conductance (g) of an Achatina neurone, TAN, having the muscimol I type GABA receptors. Their relative potency values (RPV) at GABA ed₅₀ (approximately 10⁻⁴ M) were: GABA: muscimol: TACA = 1:0.6:0.3. The GABA effects were potentiated by pentobarbitone, antagonized competitively by pitrazepin and non-competitively by picrotoxin and diazepam, and unaffected by bicuculline. The ionic mechanism of effects of GABA and its two analogues was the increase in membrane Cl⁻ conductance (g_Cl).3. GABA and (±)-baclofen produced a slow I_out with a g increase of another Achatina neurone, RPeNLN, having the baclofen type GABA receptors. The two compounds were almost equipotent (ed₅₀: approximately 3 × 10⁻⁴ M). The ionic mechanism of their effects was the increase in g_k. The two compounds hardly affected the voltage-gated and slowly inactivating calcium current. I_out produced by GABA and (±)-baclofen were reduced by TEA, but unaffected by 4-AP, bicuculline, pitrazepin and picrotoxin.4. β-hydroxy-l-glutamic acid (l-BHGA) showed the marked effects on the Achatina giant neurones; the two neurones were excited by the compound, whereas the three inhibited. D-BHGA, l-Glu, d-Glu and NMDA were less effective than l-BHGA or almost ineffective. Erythro-l-BHGA was more or less effective than threo-l-BHGA according to the neurones tested.5. α-Kainic acid and domoic acid excited the two neurones, which were excited by l-BHGA. l-Quisqualic acid showed the similar effects to l-BHGA, which were mostly much stronger than l-BHGA. Erythro-l-tricholomic acid and dl-ibotenic acid showed the effects similar to l-BHGA selectively on some neurones.6. It was pointed out that the pharmacological features of GABA on the Achatina neurones are simpler than those of l-BHGA, due to the simpler structure of the former compound having less binding sites than the latter.     

Effects of muscimol and flurazepam on the sleep EEG in the rat

In order to assess the possible role of GABA receptor function in the hypnotic property of benzodiazepines, we have examined the sleep EEG in rats given the GABA agonist muscimol, alone and in combination with flurazepam. Muscimol 0.05 and 0.1 mg/kg IP failed to alter sleep latency or total sleep time, and did not interact with the sleep-enhancing properties of flurazepam 20 mg/kg IP. These observations, in conjunction with a previous study of bicuculline, suggest that the hypnotic property of benzodiazepines may not be mediated by alteration of GABAergic activity.     

Evidence that GABA in the nucleus dorsalis raphé induces stimulation of locomotor activity and eating behavior

Recent electrophysiological studies have provided evidence that GABA controls inhibitorily the activity of the serotonin containing cell bodies in nucleus dorsalis raphé (NDR). The present investigation shows that local injection of baclofen or the GABA agonist muscimol (25–100 ng) into the NDR strongly increased locomotor activity and stimulated eating in satiated rats. These effects are antagonized by the GABA antagonists bicuculline or picrotoxin given systemically or locally. Muscimol injected in NDR also decreased serotonin and 5-hydroxyindole acetic acid in hypothalamus but not in striatum. These findings support a transmitter role of GABA in NDR and may be interpreted related to a decreased activity of serotonin.     

Effect of intraventricular injection of muscimol on appetite in rats kept at high and temperate ambient temperatures

The central mechanism controlling food intake in response to the change in environmental temperature has been little examined. The GABA agonist, muscimol, was injected into the lateral ventricle of rats which were acclimated to temperate (26 degrees C) and hot (33 degrees C) environments. Muscimol obviously stimulated the feeding behavior of rats in both environments. However, when muscimol was administered at doses of 100 and 250 ng, the food intake at 26 degrees C was greater than that at 33 degrees C. In addition, the stimulating effect of muscimol (250 ng) on food intake at 26 degrees C lasted longer than that at 33 degrees C. These findings suggested that there might be a difference in muscimol metabolism at the two temperatures.     

GABAergic contribution to rat bladder hyperactivity after middle cerebral artery occlusion

To evaluate the influences of gamma-aminobutyric acid (GABA) mechanisms on bladder hyperactivity after left middle cerebral artery occlusion, cystometric recordings were obtained from unanesthetized female rats. Intracerebroventricular administration of both muscimol (GABA(A) receptor agonist; 0.1-10 nmol) and baclofen (GABA(B) receptor agonist; 0.1-3 nmol) produced dose-dependent inhibitions of micturition with increases in bladder capacity (BC). The effects of high doses (1-10 nmol) were similar in sham-operated (SO) and cerebral-infarcted (CI) rats. However, lower doses of muscimol (0.1 or 0.3 nmol) and baclofen (0.1 nmol) reduced BC in CI rats. After bicuculline (GABA(A) receptor antagonist; 1 or 3 nmol) administration, BC in both SO and CI rats first decreased and subsequently increased. An increase in urethral pressure was observed after administration of bicuculline (3 nmol) but not with either muscimol or baclofen. Infarct volumes in muscimol-, bicuculline-, or baclofen-treated rats were not significantly different from those of vehicle-treated rats. These results suggest that GABAergic mechanisms inhibit the micturition reflex at the supraspinal level but that this can change as a result of CI.     

Role of cholinergic, opioidergic and GABAergic neurotransmission of the dorsal hippocampus in the modulation of nociception in guinea pigs

AIMS: Several physiological, pharmacological and behavioral lines of evidence suggest that the hippocampal formation is involved in nociception. The hippocampus is also believed to play an important role in the affective and motivational components of pain perception. Thus, our aim was to investigate the participation of cholinergic, opioidergic and GABAergic systems of the dorsal hippocampus (DH) in the modulation of nociception in guinea pigs. MAIN METHODS: The test used consisted of the application of a peripheral noxious stimulus (electric shock) that provokes the emission of a vocalization response by the animal. KEY FINDINGS: Our results showed that, in guinea pigs, microinjection of carbachol, morphine and bicuculline into the DH promoted antinociception, while muscimol promoted pronociception. These results were verified by a decrease and an increase, respectively, in the vocalization index in the vocalization test. This antinociceptive effect of carbachol (2.7 nmol) was blocked by previous administration of atropine (0.7 nmol) or naloxone (1.3 nmol) into the same site. In addition, the decrease in the vocalization index induced by the microinjection of morphine (2.2 nmol) into the DH was prevented by pretreatment with naloxone (1.3 nmol) or muscimol (0.5 nmol). At doses of 1.0 nmol, muscimol microinjection caused pronociception, while bicuculline promoted antinociception. SIGNIFICANCE: These results indicate the involvement of the cholinergic, opioidergic and GABAergic systems of the DH in the modulation of antinociception in guinea pigs. In addition, the present study suggests that cholinergic transmission may activate the release of endorphins/enkephalin from interneurons of the DH, which would inhibit GABAergic neurons, resulting in antinociception.