Prolactin-induced activation of tuberoinfundibular dopaminergic neurons: evidence for both a rapid 'tonic' and a delayed 'induction' component.

Results of previous studies have revealed that prolactin causes a delayed (12-16 h) increase in the rate of synthesis and turnover of dopamine (DA) in terminals of tuberoinfundibular (TI) neurons in the median eminence. Attempts to demonstrate a rapid in vivo action of prolactin on these neurons has been frustrated because pharmacological manipulations needed to make the biochemical measurements of TIDA neuronal activity (i.e., administration of alpha-methyltyrosine or NSD 1015) inhibit DA synthesis and thereby remove the tonic inhibitory control of DA on prolactin secretion. Thus, 'control' rates of synthesis and turnover of DA in terminals of TIDA neurons are actually values obtained in the presence of high circulating concentrations of prolactin. Results of the present in vivo studies demonstrate that there are two components to the activation of TIDA neurons by prolactin: a rapid 'tonic' component, which is responsive to acute changes in prolactin concentrations, and a delayed 'induction' component, which is activated by long-term changes in prolactin concentrations. Experimental observations which support this proposal are described below. Hypophysectomy or treatment with bromocriptine (a DA agonist) reduce circulating levels of prolactin and reduce the rate of DA synthesis in the median eminence. Intracerebroventricular (i.c.v.) administration of prolactin to these animals increases the rate of DA synthesis in the median eminence within 4 h (rapid 'tonic' component) and then causes a further increase after 12 h (delayed 'induction' component); only the latter component is blocked by treatment with cycloheximide, indicating the involvement in protein synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)