Evaluation of tuberoinfundibular dopamine function by neuropharmacologic means in old male rats

Baseline prolactin (PRL) levels and the PRL-lowering effect of nomifensine (Nom), an indirect dopamine (DA) agonist, were evaluated in young adult (3-5 months) and old (21 and 27 months) male rats. In addition, PRL responsiveness to acute or repeated administration of haloperidol (Hal), a DA receptor antagonist, was assessed in either young or old rats. Baseline PRL levels did not differ significantly between young and aged rats either when rats were killed by decapitation or underwent sampling from the retro-orbital venous plexus. Administration of Nom (10.0 mg/kg i.p.), a drug which inhibits PRL levels in normal rats and humans but lacks any action in conditions of impaired tuberoinfundibular DA (TIDA) function, affected PRL levels in aged rats not differently than in young rats. The PRL inhibition induced by the drug was greater in both groups when basal PRL levels were higher. Acute administration of Hal (0.5 mg/kg i.p.) increased significantly and to the same extent as PRL levels in aged and young rats. In aged rats treated chronically with Hal (0.5 mg/kg i.p., twice daily for 14 days) and sampled at the 8th day, 1 h after the first daily Hal injection, plasma PRL rose to levels about 3-fold as high as those after the first injection; in young rats, instead, the PRL-releasing effect of Hal was similar to that evoked by the first administration. 3 days after Hal withdrawal, baseline PRL levels were significantly higher in aged Hal-treated rats than in vehicle-injected or young Hal-treated rats and so were pituitary concentrations of PRL.(ABSTRACT TRUNCATED AT 250 WORDS)