Effects of long-term orchidectomy on in vitro pulsatile gonadotropin-releasing hormone release from the medial basal hypothalamus of the adult guinea pig.

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There is ample evidence provided by in vivo studies that orchidectomy increases the frequency of episodic GnRH release from the hypothalamus, but the effects on other aspects of pulsatile GnRH secretion are less well defined. We used a continuous superfusion system to study the effects of long term (4 weeks) orchidectomy or sham castration on in vitro release of GnRH from the retrochiasmatic medial basal hypothalamus (MBH) of the adult male guinea pig. RIA of GnRH in 5-min fractions (750 microliters) of superfusion medium collected during a 540-min observation period revealed a clearly episodic pattern (Ultra pulse analysis algorithm) of GnRH secretion for the MBHs of both orchidectomized (n = 15) and sham-operated animals (n = 15). Pulse frequency was significantly higher (P < or = 0.01) after orchidectomy than after sham castration (mean +/- SEM pulse intervals of 32.5 +/- 2.7 and 57.8 +/- 5.1 min, respectively; pulse analysis at 3 coefficient of variation threshold). The mean amplitude of all pulses (15.8 +/- 0.8 vs. 15.2 +/- 1.1 pg/750 microliters) and mean GnRH output (8.2 +/- 1.3 vs. 6.7 +/- 1.3 pg/750 microliters) were not significantly different between the two groups. Exposure for 30 min to 20 microM veratridine, a depolarizing agent, toward the end of the experiment resulted in a significant increase in GnRH output (P < or = 0.01) in both groups. The latter response was somewhat greater for the MBHs of orchidectomized animals (P < or = 0.01 for absolute response; nonsignificant for percent response), notwithstanding a 50% reduction of the total GnRH content of the MBH explants (P < or = 0.0001). The results of the present study provide further support for the view that retardation of the GnRH pulse generator frequency is the major mechanism by which the testicular hormones exert, at the hypothalamic level, their negative feedback action on gonadotropin secretion.