INHIBITION OF PROLACTIN-RELEASE IN CONSCIOUS SHEEP FOLLOWING STIMULATION OF NUCLEUS ACCUMBENS AND CAUDATE-NUCLEUS

Two sites in the nucleus accumbens and 1 site in the head of the caudate nucleus were electrically stimulated in conscious sheep with unilaterally implanted bipolar electrodes. Plasma concentrations of prolactin (Prl) were quantified before, during and after 30- or 60-min periods of electrical or sham stimulation. Relative to Prl concentrations during sham procedures, electrical stimulation of each site in the basal forebrain significantly decreased plasma Prl. Electrical stimulation was also performed during periods of elevated plasma Prl resulting from continuous infusion of pentobarbital or of thyrotropin-releasing hormone (TRH). During these 2 situations, electrical stimulation failed to decrease plasma Prl. In addition, bolus injections of TRH were administered during sham procedures and during brain stimulation. The increase in plasma Prl due to bolus injections of TRH was not significantly reduced by stimulation of the Prl-inhibitory site in the caudate nucleus. Since TRH propably releases PRl by a direct action on the sheep pituitary, the failure of electrical stimulation to antagonize the Prl-releasing effects of injected TRH suggests that brain stimulation did not increase the action of PRl-inhibitory substances on the pituitary. On the contrary, stimulation of the basal forebrain probably decreased the release of unknown Prl-releasing subtances by the hypothalamus.