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Maturation of the Hypothalamic Control of Pulsatile Gonadotropin-Releasing Hormone Secretion at Onset of Puberty: II. Reduced Potency of an Inhibitory Autofeedback^*

Department of Pediatrics (J.P.B.) and Radioimmunoassay Laboratory (J.P.B., A.G., P.F.), University of Liege Liege, Belgium

Address all correspondence and requests for reprints to: Jean-Pierre Bourguignon, M.D., Ph.D., Department of Pediatrics, CHU Sart-Tilman, B4000 Liege, Belgium.

Abstract

At the time of onset of puberty in the male rat, between 15 and 25 days of age, we have reported that pulsatile GnRH secretion by hypothalamic explants showed an increased frequency as indicated by the reduction of the mean interpulse interval from 66 to 40 min (P < 0.05). This study aimed to evaluate whether these changes in GnRH secretion involved a self-regulatory mechanism. A 7.5-min exposure of explants obtained at 50 days to 0.1 /*M superagonist D-TRP⁶-PRO⁹-JV-Et-GnRH (GnRH-A) resulted in a delay of the next GnRH secretory pulse so that the mean interpulse interval increased from 35 to 67 min (P < 0.001). In addition, after a 7.5-min exposure to GnRH-A, there was a 15-min period with absent or reduced release of GnRH in response to 50 (iM veratridine, a depolarizing agent. A similar refractory period of 15 min was observed using explants obtained at 25 and 50 days whereas, at 15 days, the period of refractoriness lasted for 52.5 min. The inhibitory effect of GnRH-A on the subsequent response to veratridine occurred at similar concentrations of GnRH-A at the three studied ages and the inhibition was prevented using an antagonist of GnRH together with GnRH-A. The involvement of GnRH itself in an autofeedback mechanism was evaluated by studying the period of refractoriness separating two GnRH pulses elicited by 7.5-min exposures to veratridine. The initial responsiveness to veratridine was recovered after a refractory period of 52.5, 22.5, and 15 min when studied at 15, 25, and 50 days, respectively. While refractoriness occurred during repeated depolarization with K+ or veratridine, such an effect was not observed using AT-methyl- D-aspartate (NMDA). During exposure to GnRH-A, the NMDAinduced release of GnRH was only reduced by 38% whereas veratridine-induced secretion showed a 94% reduction. Thus, exogenous activation of NMDA-sensitive receptors could bypass the inhibitory autofeedback. We conclude that: 1) pulsatile GnRH secretion is controlled by an inhibitory autofeedback involving NMDA sensitive receptors, 2) the increased frequency of pulsatile GnRH secretion at onset of puberty may be related to a reduced sensitivity of the hypothalamic pulse generator to an inhibitory autofeedback. (Endocrinology 127: 2884–2890,1900

Footnotes

^* This work was supported by Grant 3.4574.87 from the Belgian Fonds de la Recherche Scientifique Medicale and by a grant from the Faculty of Medicine, University of Liege.

Received January 29, 1990.

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