Opioidergic modulation of in vitro pulsatile gonadotropin-releasing hormone release from the isolated medial basal hypothalamus of the male guinea pig.

Endogenous opiates exert a tonic restraint on LH release, which is centrally mediated through inhibition of hypothalamic GnRH secretion. The aim of the present study was, using an in vitro superfusion approach, to perform a detailed study of the effects of opioid receptor activation and blockade on the dimensions of pulsatile GnRH release from the isolated medial basal hypothalamus of the adult male guinea pig. The mu-receptor antagonist naloxone at 10(-3) M (n = 12), 10(-6) M (n = 12), and 10(-9) M (n = 12), the opioid agonist morphine sulfate at the same concentrations (n = 14 for each concentration), or morphine and naloxone together at 10(-3) M (n = 14), was added to the medium alternately either during the first or the second of two consecutive 3-h observation periods. Five-minute fractions (750 microliters) were collected for measurement of GnRH by RIA. Morphine suppressed total GnRH output from the medial basal hypothalami (P < or = 0.01 at 10(-3) M and 10(-6) M; P = NS at 10(-9) M), with mean pulse frequency, mean amplitude of all pulses, and mean sum of pulse amplitudes being significantly inhibited at all morphine concentrations, whether pulse analysis (Ultra pulse analysis algorithm) was performed at a two-coefficient of variation (2CV; P < or = 0.05, P < or = 0.01, and P < or = 0.01, respectively) or a 3CV (P < or = 0.05, P < or = 0.05, and P < or = 0.01, respectively) threshold. A different set of experiments (n = 14), with exposure to 10(-3) M morphine during the second of three consecutive 3-h observation periods, indicated that after morphine withdrawal a gradual recovery of GnRH output and pulse frequency to pretreatment values occurred, whereas pulse amplitude remained significantly suppressed during the posttreatment observation period. The inhibitory actions of 10(-3) M morphine were effectively prevented by the presence of 10(-3) M naloxone. Total GnRH output (P < or = 0.05), mean pulse frequency (P < or = 0.05), mean amplitude of all pulses (P < or = 0.05), and mean of the sum of pulse amplitudes (P < or = 0.01) were increased during opioid blockade with 10(-3) M and 10(-9) M naloxone (either 2CV or 3CV threshold for pulse analysis). A similar trend for 10(-6) M naloxone was only apparent after the exclusion of two outliers with unusually high basal GnRH release.(ABSTRACT TRUNCATED AT 400 WORDS)