In vivo supersensitivity of the anterior pituitary of old female rats to dopaminergic inhibition of prolactin secretion.

Abstract

Old female rats (20-27 months) were given acute administration of an indirectly acting dopamine (DA) agonist, nomifensine or scalar doses of the direct DA receptor agonist, bromocriptine. Young female rats (4-9 months) were used as controls. Nomifensine (10 mg/kg i.p.) decreased significantly basal prolactin (PRL) levels in young rats as in old rats. In young rats, bromocriptine decreased significantly basal PRL levels only at the dose of 0.5 mg/kg intraperitoneally, the doses of 0.1 and 0.02 mg/kg being ineffective. In contrast, in old rats administration of 0.02 mg/kg of bromocriptine consistently inhibited basal PRL levels and the maximum PRL-lowering effect was already evident at the dose of 0.1 mg/kg. These data indicate that the pituitary of old rats, due to the age-related removal of dopaminergic inputs from the tuberoinfundibular system, becomes supersensitive to direct dopaminergic stimulation. This phenomenon may explain the normal PRL responsiveness of old rats to nomifensine, despite defective tuberoinfundibular dopaminergic function.