NEUROPEPTIDE-Y POTENTIATES LUTEINIZING-HORMONE (LH)-RELEASING HORMONE-STIMULATED LH SURGES IN PENTOBARBITAL-BLOCKED PROESTROUS RATS

Recent evidence suggests that hypothalamic neurosecretion of neuropeptide Y (NPY) may be required for the preovulatory LH surge in female rats. Results of immunoneutralization and portal blood collection studies have suggested that NPY may serve to enhance the response of gonadotropes to the stimulatory action of LHRH. To directly test this hypothesis, the effects of NPY on LHRH-stimulated LH secretion were assessed in proestrous rats that were anesthetized with pentobarbital (PB) to block endogenous LHRH neurosecretion. Female rats were fitted with atrial catheters on diestrus. On proestrus, hourly blood samples were collected from 0900-2100 h. At 1330 h, rats received PB (40 mg/kg BW) or saline. Every 30 min from 1400-1800 h, PB-treated rats received iv pulses of LHRH (15, 150, or 1500 ng/pulse) or saline along with concurrent pulses of NPY (1 or 10-mu-g/pulse). Plasma samples were analyzed by LH RIA. In PB-treated rats receiving vehicle pulses only, LH surges were completely blocked. Pulsatile LHRH treatments at 15, 150, and 1500 ng/pulse produced subphysiological, physiological, and supraphysiological LH surges, respectively. Simultaneous administration of NPY pulses with 15 ng/pulse LHRH produced significant dose-related potentiations of LHRH-stimulated LH surges (P < 0.0001). Administration of NPY pulses with 150 ng LHRH/pulse also significantly enhanced LHRH-induced LH surges (P < 0.05). NPY RIA of plasma confirmed NPY increments after treatments. These results demonstrate that NPY administration can potentiate pituitary responsiveness to LHRH stimulation, and are consistent with the hypothesis that one function of NPY is to operate as a neurohormonal modulator at the level of the gonadotrope during generation of the preovulatory LH surge.