Pharmacology of (+)- and (-)-baclofen: GABA-dependant rotational behaviour and [3H]GABA receptor binding studies

Involvement of GABAB receptors in the antinociception induced by baclofen in the formalin test

1. The effect of GABA receptor antagonists on baclofen-induced antinociception was examined in rats using the formalin test. Intraperitoneal (IP) administration of different doses of baclofen (2.5-10 mg kg-1) to rats induced antinociception in both phases of the test. 2. The response was dose-dependent and the maximum response was observed with 10 mg kg-1 of the drug. Intracerebroventricular (ICV) injection of baclofen (0.5-20 micrograms/rat) also induced dose-dependent antinociception in the second phase of the formalin test. 3. The GABAB antagonist, phaclofen (1 mg kg-1, IP) but not the GABAA antagonists picrotoxin (1 mg kg-1, IP) and bicuculline (1.5 mg kg-1, IP), decreased the antinociception induced by both ICV and IP administration of baclofen. 4. It is concluded that baclofen antinociception in the formalin test is mediated through GABAB receptor activation.

Food intake suppressant effect of baclofen in rats

1. Baclofen given intraperitoneally (i.p.) to rats caused a dose-dependent decrease in food intake. 2. Bicuculline or picrotoxin (GABAA-antagonist) and methergoline (5-HT antagonist) decreased the anorectic effect of baclofen. 3. Pimozide (dopamine receptor blocker), phenoxybenzamine and propranolol (alpha and beta adrenergic blockers) did not diminish the baclofen effect, but even increased the anorexia induced by the drug. 4. It can be postulated that, at least partially, GABAA receptor mechanism, GABA-5HT receptor complex and/or 5-HT mechanism may be involved in baclofen induced anorexia.

GABAA-antagonists and baclofen analgesia

1. Intraperitoneal (i.p.) injection of different doses of baclofen (5, 7.5 and 10 mg/kg) induced analgesia in tail-flick test. The effect was dose-dependent. 2. The antinociception induced by baclofen (10 mg/kg, i.p.) was decreased in animals pretreated with bicuculline (1.5 mg/kg, i.p., 30 min), but not with naloxone (1.5 mg/kg, i.p., 30 min). 3. In picrotoxin (1 mg/kg, i.p., 15 min) pretreated mice, baclofen (5 mg/kg, i.p.) showed a significant analgesic effect. 4. Morphine (6 mg/kg, s.c.) induced analgesia which was antagonized by naloxone pretreatment (1.5 mg/kg, i.p.), while bicuculline or picrotoxin did not alter the morphine response. 5. These data suggest that a part of analgesic effect of baclofen may be mediated through GABAA receptor sites, and differs from that of morphine.

A possible neurochemical basis for the neuropsychiatric aspects of baclofen therapy

Baclofen, commonly used to reduce severe muscle spasms in patients with spinal cord injuries, is also active in the brain. A patient with pre-existing bipolar affective disorder developed increased depression while on baclofen, which progressed to a delusional depression when baclofen and haloperidol were rapidly decreased. When the dose of haloperidol was increased to a previously well tolerated dose to deal with the depressive delusion, a pseudoparkinson's state developed. This case demonstrates the interactive effects of baclofen and haloperidol on central noradrenergic and dopaminergic systems and suggests a possible neurochemical basis for the difference between delusional and nondelusional depression that is consistent with the different therapeutic response to psychotropic drugs of patients with these illnesses. The paradoxical appearance of the pseudoparkinson state in this patient when much higher doses of haloperidol had been free of such side effects, may reflect baclofen-induced alterations in receptor sensitivity. It appears that baclofen should be used with caution in patients with neuropsychiatric problems and that, when used, the withdrawal of baclofen should be continued over several weeks to allow receptor sensitivity to return to normal levels.

Biphasic dose-response effect of baclofen on haloperidol catalepsy in the rat

Haloperidol-induced catalepsy in rats is reduced by low dose baclofen and is potentiated by high dose baclofen. This biphasic behavioral effect is consistent with reported differences between presynaptic and postsynaptic GABA receptors. Low dose baclofen could have an anti-cholinergic effect by activating presynaptic GABA receptors that inhibit the release of acetylcholine. High dose baclofen could have an anti-dopamine effect by activating postsynaptic GABA receptors that inhibit the firing of dopamine neurons.

Baclofen selectively inhibits excitatory synaptic transmission in the hippocampus

The effect of baclofen was investigated on mossy fiber, Schaffer collateral and perforant path synaptic transmission in hippocampal slices. Baclofen completely inhibits mossy fiber and Schaffer collateral synaptic transmission with an IC50 of 3.8 microM. The lateral perforant path is insensitive to baclofen, while the response in the medial zone was partially blocked. Baclofen does not appear to act in a GABA-like manner.

GABA agonists. Development and interactions with the GABA receptor complex

This review describes the development of GABA receptor agonists with no detectable affinity for other recognition sites in GABA-mediated synapses. The key compounds are THIP, isoguvacine, and piperidine-4-sulphonic acid (P4S), developed via extensive structural modifications of the potent but not strictly specific GABA agonist muscimol. The structural parameters, which have to be considered in the design of GABA agonists are discussed on the basis of the structures and biological activities of these GABA agonists and a number of related compounds. A model, which summarizes our present knowledge of the structure of the postsynaptic GABA receptors complex, is presented, and the interaction of GABA agonists with various sites in this complex is discussed. Of particular interest are the effects of GABA agonists on the binding of diazepam to the benzodiazepine binding site, assumed to be a structural unit of the GABA receptor complex. While rigid molecules like THIP are capable of activating the GABA receptors, a certain degree of conformational mobility of GABA agonists apparently is a prerequisite for stimulation of diazepam binding in vitro at 0 degree C. The findings suggest that GABA receptor functions involve conformational changes of certain elements, including the attempts to develop GABA agonists with desirable pharmacokinetic and toxicological characteristics. While muscimol is a toxic compound, THIP is well tolerated by animals, and in contrast to isoguvacine, THIP penetrates into the brain after systemic administration to animals, a difference which can be explained on the basis of their protolytic properties. The attempts to develop pro-drugs of isoguvacine capable of penetrating the blood-brain barrier with subsequent decomposition in the brain tissue to isoguvacine are described.

GABA and baclofen potentiate the K+-evoked release of methionine-enkephalin from rat striatal slices

GABA potentiates the potassium-evoked release of methionine-enkephalin (ME) from slices of rat corpus striatum. This potentiation is observed only when a submaximal concentration (30 mM) of K+ is used to evoke release. The effect of GABA is dose-dependent between 100 and 100 micrometers. The basal release of ME is not altered by these concentrations of GABA. Before, but not muscimol, mimics the effect of GABA on the evoked release of ME. This effect is not stereoselective as both the (+)- and (-)-isomers of baclofen enhance ME release. Picrotoxin (100 micrometers) blocks the enhancement of ME release produced by both GABA and baclofen. Bicuculline methiodide (100 micrometers) does not block the effect of GABA. The effect of GABA on ME release may be mediated by an atypical GABA receptor which is activated by baclofen.

Effects of electrolytic and 6-hydroxydopamine lesions of the lateral hypothalamus on rotation evoked by electrical stimulation of the substantia nigra in rats

Contralateral rotation evoked by electrical stimulation of the left substantia nigra was studied in rats before and after electrolytic or 6-hydroxydopamine (6-OHDA) lesions of the lateral hypothalamus. Electrolytic lesions (2 mA DC, 15 sec) which produced mean ipsilateral striatal dopamine depletion of 58% significantly reduced the rotation at 2 h to 14 days postlesion. 6-OHDA (8 microgram in 4 microliter) which produced mean ipsilateral striatal dopamine depletion of 93% significantly increased the rotation at 3 to 14 days postlesion. Haloperidol 0.1 and 0.5 mg/kg i.p. partially reduced rotation in both control and lesioned rats in a dose-related manner. Control and lesioned rats showed no significant differences in haloperidol sensitivity. If stimulus induced rotation were mediated by activation of dopaminergic neurons, one would have expected lesion effects in the present experiments to parallel those on rotation caused by pharmacologically evoked release of dopamine. The lesions effects we obtained on stimulus induced rotation, however, parallel those on rotation evoked by the predominantly directly acting dopamine agonist, apomorphine, rather than those on rotation evoked by the indirect (presynaptic) action of amphetamine. We suggest that contralateral rotation evoked by electrical stimulation of the substantia nigra may reflect direct activation of neurons postsynaptic to the dopaminergic nigrostriatal neurons.

Effect of dopaminergic and GABA-ergic drugs given alone or in combination on the anticonvulsant action of phenobarbital and diphenylhydantoin in the electroshock test in mice

In the electroshock test--taking hind-limb tonic extension as the end point--apomorphine (10 mg/kg) exerted no effect on the anticonvulsant action of phenobarbital (PB; 20 mg/kg) or diphenylhydantoin (DPH; 8 mg/kg); amantadine (25 and 100 mg/kg) decreased that of DPH, while L-DOPA (500 mg/kg) and d,l-amphetamine (10 mg/kg) potentiated the action of both anticonvulsants. Fluphenazine (4 mg/kg) had no influence on the effects of the two anticonvulsants, but haloperidol lessened that of DPH. All GABA-ergic stimulants used, i.e., gamma-hydroxybutyric acid (GHBA/250 mg/kg), baclofen (2.5 and 10 mg/kg) and aminooxoacetic acid (AOAA; 15 and 20 mg/kg) potentiated the action of PB; the action of DHP was unaffected by these drugs except for AOAA (20 mg/kg). The combined treatment with dopaminergic and GABA-ergic stimulants, being ineffective in terms of anticonvulsant activity, resulted in a marked potentiation of the action of the anticonvulsants tested in this study. The most distinct potentiation was noted in the case of PB, baclofen (1 mg/kg), and amantadine (25 mg/kg).