Pentobarbital stimulates the activity of the GnRH pulse generator interacting with opioid neurons in rats in proestrus

In order to investigate the possibility that i.p. injection of pentobarbita l sodium (PB, 32 mg/kg bw) potentiates the GnRH pulse generator activity, e ffects of i.v. infusions of an opiate receptor antagonist naloxone (NAL, 2 mg/h) on the pulsatile LH secretion were compared in saline (SAL)- and PB-i njected rats in proestrus and diestrus 1. In SAL-injected rats in proestrus , NAL infusions significantly increased both the frequency and amplitude of LH pulses, and also the overall mean LH concentration. In PB-injected rats in proestrus, all the parameters of the pulsatile LH secretion were simila r to those in SAL-injected rats in proestrus. The NAL infusion in PB-inject ed rats caused an increase in the frequency, but it was similar to that in SAL-injected rats. But, increases in the amplitude and the overall mean LK observed during NAL infusions in PB-injected rats were greater than in SAL- injected rats. In SAL-injected rats in diestrus 1, NAL infusions increased all the parameters, as in rats in proestrus. In PB-injected rats in diestru s 1, LH secretion was severely suppressed. NAL infusions recovered the puls atile LH secretion, but the frequency and the overall mean LH of the secret ion were smaller than those obtained during NAL infusions in SAL-injected r ats. In addition, characteristic increases in the MUA (volleys), which occu r in association with the initiation of an LH pulse and thus are considered to represent an increased activity of the GnRH pulse generator, appeared m ore frequently during NAL infusions in PB-injected rats in proestrus than i n SAL-injected rats. These results suggest that the GnRH pulse generator in rats in proestrus, but not in rats in diestrus 1, is refractory to PB and further is potentiated by PB in the response to NAL. Together with the fact that this dosage of PB blocks the surge of LH secretion in rats in proestr us, the concept of the existence of separate neuronal mechanisms responsibl e for the surge and pulsatile secretion of LH are supported. (C) 2000 Elsev ier Science Ltd. All rights reserved.