Hyperprolactinemia suppresses the luteinizing hormone responses to N-methyl-D-aspartate, epinephrine, and neuropeptide-Y in male rats.

This study characterizes the responses of LHRH neurons to N-methyl-D-aspartate (NMDA), norepinephrine, epinephrine (E), and neuropeptide-Y (NPY), as evidenced indirectly by the measurement of circulating LH titers, and investigates whether neurons using these compounds as neurotransmitters might be involved in mediating hyperprolactinemic (HP) suppression of LH release. Male rats were orchidectomized, adrenalectomized, and implanted with a testosterone-containing Silastic capsule, a 50% corticosterone pellet, and third cerebroventricular and right atrial cannulae at time zero. Rats received sc injections of ovine PRL (2400 micrograms/250 microliters) in a polyvinylpyrrolidone depot or vehicle every 12 h for 48 h when experiments were performed. The mean maximal LH increments (delta LH) in response to two doses of LHRH (0.4 and 0.8 ng/100 g BW) were not altered in HP rats, indicating that ovine PRL did not cause a change in pituitary responsiveness. NMDA (20 mg/kg BW, iv)-induced LH release peaked 5 min after injection. The delta LH (0-5 min) in HP rats was suppressed by 53% compared with the control value. Epinephrine [5, 10, and 15 micrograms/2 microliters, intracerebroventricularly (icv)], but not norepinephrine (20 and 40 micrograms/2 microliters, icv), produced dose-dependent LH responses that peaked at 10 min. The delta LH (0-10 min) in HP rats in response to 10 micrograms/2 microliters E was suppressed by 68% compared with the control value. Two doses of NPY (2 and 10 micrograms/2 microliters, icv) produced dose-dependent LH increments that peaked at 10 min. In HP rats, the delta LH (0-10 min) in response to 10 micrograms/2 microliters NPY was suppressed 52% compared with the control value. The combined administration of E (10 or 16 micrograms) and NPY (5 or 10 micrograms) produced mean maximal LH responses that significantly exceeded the additive responses of these compounds individually. This synergistic effect may be mediated by separate adrenergic and NPYergic afferents to the LHRH neurons or may, in fact, reflect corelease of these two neurotransmitters from the same neurons. The LH responses to NMDA, E, and NPY were all inhibited in HP rats. This suggests that elevated PRL levels act on the LHRH neurons, either directly or indirectly through an inhibitory afferent neuronal system, to decrease their responsivity to all stimuli.