HYPOTHALAMIC DOPAMINE D1 RECEPTORS ARE INVOLVED IN THE STIMULATION OFPROLACTIN SECRETION BY HIGH ENVIRONMENTAL-TEMPERATURE IN THE FEMALE SHEEP

Recent evidence suggests that dopamine, acting via its D1 receptors, m ay function as a neurotransmitter in intrahypothalamic pathways involv ed in the stimulation of prolactin secretion. Functional dopamine D1 r eceptors are present in the ventromedial hypothalamic nucleus (VMH) an d we hypothesized that they might be part of a prolactin-stimulatory p athway activated by stress. We tested this hypothesis in a series of e xperiments on sheep involving two different forms of stressors, audiov isual (barking dog) and high environmental temperature. We attempted t o block the stimulation of prolactin secretion by infusion into the VM H of an antagonist specific for the D1 receptor. Ovariectomised, oestr adiol-implanted merino ewes were surgically implanted with bilateral g uide tubes directed at the VMH. After a 180 min pretreatment period, t he ewes either were or were not exposed to a stressor (30 min of barki ng dog or 120 min at 35 degrees C, 65% relative humidity). D1 receptor antagonist, SCH23390 or vehicle (0.9% saline) was infused into the VM H (1.7 mu l/h, 120 nmol/h) for 60 min prior to and during the stressor period. Blood was sampled every 15 min via jugular cannulae and the p lasma was assayed for prolactin, cortisol and growth hormone (GH). Bot h stressors significantly increased prolactin concentrations over cont rol levels. SCH23390 infusion significantly attenuated the prolactin r esponse to high environmental temperature, but had no effect on the pr olactin response to audiovisual stress. Cortisol concentrations were s ignificantly increased by audiovisual stress only and were not affecte d by SCH23390, GH concentrations were not changed by either stressor o r infusion. Drug infusion alone did not affect the concentration of th e hormones. The data suggest that the VMH D1 receptors are involved in a prolactin stimulatory pathway in response to high environmental tem perature. The inability of the D1 antagonist to affect the response to the barking dog indicates that this pathway is stress-specific, imply ing that there is more than one mechanism or pathway involved in the p rolactin response to different stressors.