Thyrotropin-releasing hormone (TRH) stimulates the secretion of both thyrotropin (TSH) and prolactin. We have found that electrical stimulation (ES) of the medial basal hypothalamus in rhesus macaques elicits a prompt rise in serum prolactin levels. Exogenous TRH causes a rise in serum prolactin that is similar in magnitude and time course to that induced by ES. Administration of specific serotoninergic and dopaminergic neural blockers inhibits the electrically induced prolactin release, but the serotoninergic blocker methysergide has no effect on the trh-induced release of prolactin. To examine the potential physiological role of TRH in prolactin secretion, we will compare changes in TSH and thyroxine with those in prolactin after either ES or the hypothalamus or TRH administration. Continuous and intermittent pulses of TRH will be infused for several days into rhesus macaques to determin if the nature of TRH stimulation affects pituitary thyrotropes and lactotropes differently. We will also continue our studies on the role of opioid peptides in prolactin secretion. We hope these studies will provide insights into the neurological mechanisms that regulate prolactin secretion in primates.