Effects of barbiturates on the decerebrate rigidity in rats

Effects of barbiturates (phenobarbital, mephobarbital, pentobarbital, hexobarbital and amobarbital) on the tonic and phasic responses of forelimbs were studied in the rats in which rigidity was induced by means of anemic decerebration. The inhibitory pattern of the tonic and phasic responses to phenobarbital was different from those to the other barbiturates, i.e. phenobarbital gradually inhibited both tonic and phasic responses, whereas other barbiturates suppressed significantly only the tonic response immediately after the administration of a relatively low dose (10 mg/kg) but hardly inhibited the phasic response unless at higher doses (20 and 40 mg/kg). Possible involvement of GABA-system in the inhibitory action of phenobarbital and pentobarbital on the rigidity was investigated. Since inhibition of the phasic response to pentobarbital was potentiated by the pretreatment of animals with carboxymethoxylamine hemihydrochloride (AOAA) and suppressed by the pretreatment with semicarbazide, it was suggested that GABAergic mechanism may be involved in the particular inhibition of phasic response to pentobarbital. However, the inhibitory action of phenobarbital appeared to be independent of GABA-system.