LHRH pulse generator is stimulated by naloxone in the pentobarbital-blocked proestrous rat

Previous studies by us and others led us to hypothesize that there are separate LHRH pulse and surge generators in the rat brain. The present study was designed to detect the activity of LHRH pulse generator by checking changes in LH secretion and the multiunit activity (MUA) of the arcuate-median eminence region of the hypothalamus during infusions of naloxone (NAL, 2 mg/h) in the proestrous rat in which the LHRH surge generator activity was blocked by pentobarbital sodium (PB, 32 mg/kg bw, ip). The animals were subjected to blood sampling in the morning (1000-1300 h) or afternoon (1400-1700), and injected with PB at 09.45 or 13.45, respectively. During saline infusions in the rat given PB injection at either 09.45 or 13.45, serum LH levels were low but fluctuated significantly, suggesting a pulsatile secretion in either the morning or the afternoon period. The pulse intervals were an average 28.2 min in the morning and 42.2 min in the afternoon. NAL infusions decreased the pulse interval significantly, to 22.0 min in the morning and to 27.0 min in the afternoon. In the electrophysiological experiment, characteristic increases in the MUA (volleys), which occur in association with the initiation of an LH pulse and therefore are considered to represent an increased activity of the LHRH pulse generator, appeared during NAL (5 mg/h) infusions in either the morning or the afternoon. These results strongly suggest that separate LHRH pulse and surge generators exist in the brain, and that, even during the critical period of proestrus, the activity of LHRH pulse generator is disclosed by PB, which, on the other hand, arrests the surge generator.