The central depressant effects of ceruletide (CER, 0.04 mg/kg s.c.) and cholecystokinin octapeptide (CCK-8, 0.25 mg/kg s.c.) were compared with those of clonidine (0.04 mg/kg s.c.). At doses that were nearly equipotent with respect to motor inhibition (catalepsy, reduction in ambulation and exploratory rearing), only the peptides produced ptosis. Yohimbine (1 mg/kg s.c., 30 min) antagonized the effect of clonidine but not of the peptides. Clonidine (0.07-0.2 mg/kg s.c., 30 min) antagonised the ptotic action of the peptides, and this effect was abolished by yohimbine (0.2-1 mg/kg i.p.) but resistant to haloperidol (0.05 and 0.15 mg/kg i.p.). These results separate the behavioural effects of the peptides from those of clonidine and also the ptotic effect of the peptides from their effect on motor activity. The antiptotic effect of clonidine may originate from activated adrenergic autoreceptors. © 1984.
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