Baclofen-induced modification of conditioned discriminative avoidance behaviour and contraversive turning in the rat

Benzodiazepines and GABA hypothesis of schizophrenia

Clinical and experimental studies pertinent for demonstrating the antipsychotic potential of benzodiazepines (BDZ) and the involvement of γ-aminobutyric acid (GABA) in the origin of schizophrenia are reviewed. It is shown that, due to severe methodological problems and pitfalls, placebo-controlled, double-blind studies do not permit unequivocal conclusions on the efficacy of BDZs, but neither do they completely disprove it. Furthermore, at first glance, confusing and controversial findings in animal models indicate a bi-directionality of effects of full BDZ agonists on dopamine-mediated functions, which may perhaps be explained by (i) anatomical and functional organization of the GABA-dopamine system in the nigro-striatal and ventro tegmental area, and (ii) the regional non-selectivity of action of these drugs. The recent demonstration of structural polymorphism of the GABA(A)-BDZ receptor complex and heterogeneous distribution of sets of subunits of the GABA( A)-BDZ receptor in the brain, suggests possibilities for development of partial BDZ agonists showing greater regional selectivity of action and thus potentially more specific antipsychotic action. Initial clinical results with bretazenil (Ro 16-6028), a partial BDZ agonist, in acute schizophrenia are, in this respect, an encouraging lead to be followed further.

Effect of beta-adrenoceptor agonists on apomorphine-induced turning in rats

The beta-adrenoceptor agonists clenbuterol, salbutamol and formoterol were found to inhibit apomorphine-induced turning behavior in rats with unilateral nigrostriatal lesions. The inhibitory effect of clenbuterol was antagonized by (-)-propranolol, but unaffected by practolol (which does not cross the blood-brain barrier), indicating a central localization of the beta-adrenoceptors mediating the inhibitory effect. A dopamine-releasing activity of clenbuterol did not seem to be responsible for the effect, because known dopamine-releasing agents (amphetamine, methylphenidate) did not antagonize apomorphine-induced turning behavior. Furthermore alpha-methyl-p-tyrosine pretreatment did not prevent the inhibitory effect of clenbuterol. Since L-5-hydroxytryptophan did not mimic the inhibitory effect of clenbuterol, it was concluded that the inhibition of turning behavior is not mediated by the interaction of clenbuterol with the serotonin system. The finding that clenbuterol depressed locomotor activity in the same dose range as it inhibited apomorphine-induced turning tentatively suggests that clenbuterol inhibits apomorphine-induced turning behavior by its central beta-adrenoceptor mediated sedative action.

Modulation of dopamine-mediated behavioural responses by antidepressants: effects of single and repeated treatment

The study concerned the effects of maprotiline, imipramine, clomipramine and amitriptyline on the stereotyped and turning behaviour induced by apomorphine in rats. At either single or repeated doses of 25 mg/kg i.p. neither maprotiline nor imipramine changed the stereotyped responses induced by apomorphine. Clomipramine showed in contrast an inhibitory effect which increased after 7 daily injections of the drug. Moderate suppression of stereotypies was also observed after repeated administration of amitriptyline. In rats with unilateral 6-OHDA lesions of the s.nigra apomorphine-induced contralateral turning was markedly suppressed (70%) after a single 25 mg/kg i.p. injection of maprotiline. A tolerance to this effect developed after 7 daily injections of the drug. Clomipramine and amitriptyline caused an inhibition of turning which was markedly increased after repeated treatment. These results suggest that antidepressants do not uniformly affect the behavioural responses mediated by dopamine. Clomipramine and amitriptyline appear to possess dopamine receptor blocking properties which may become more pronounced after chronic treatment. In contrast, the dopamine receptor blockade by maprotiline diminished and disappeared under such conditions. Among the drugs investigated imipramine was the one which seemed to have the weakest influence on dopaminergic receptors.