Feedback Effects of the Testis on Pituitary Responsiveness to Luteinizing Hormone-Releasing Hormone Infusions in the Ram*

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Abstract
The present studies examined the effects of both short term and long term alterations in testicular hormone levels on pituitary gonadotropin responses during 4–6 h of constant infusions of LHRH (0.5 μg/min) in the ram. Short term changes in testicular hormone levels were produced either by testosterone (T) infusions [three animals (6.7 μg/min) and 3 animals (83 /xg/min)] beginning 4 h before the LHRH infusion or by castration (n = 4) 2 h before the LHRH infusions. Long term alterations in testicular hormone levels were induced by T injections (T enanthate, 250 mg im twice weekly for 2 weeks) or castration (n = 4) performed 3–4 months before the LHRH infusion. Control LHRH infusions were performed in the same animals 2 weeks before the T studies. For the acute castrations, control studies were undertaken in four animals acutely sham castrated; for chronic castration normal animals were used as controls (n = 5). Acute changes in testicular feedback induced either by castration 2 h before LHRH administration or by several hours of T infusion did not alter the normal patterns of LH and FSH increase and decrease during LHRH infusions. The normal biphasic pattern of increase in plasma LH levels and the monophasic pattern of FSH increase persisted, as did the development of pituitary refractoriness to LHRH stimulation in the last 2–3 h of LHRH administration. In contrast, more chronic changes in testicular feedback induced either by T injections for 2 weeks or by castration 3–4 months before LHRH administration produced definite alterations in pituitary responsiveness. LH secretion from the immediately releasable pool was markedly increased in the chronically castrated animals and decreased by 2 weeks of T administration. LH secretion from the second pool was less affected by 2 weeks of T therapy and was apparently unaffected by chronic castration. Chronic castration led to markedly increased basal FSH levels, but no further increase was seen during LHRH administration. These results demonstrate that in the ram 1) alterations in testicular hormone production require longer than 2–4 h to affect pituitary responsiveness to LHRH, 2) testicular feedback exerts a greater effect on the immediately releasable pool of pituitary LH than on the pool requiring longer LHRH stimulation for release, and 3) the pituitary refractoriness that develops after 2–3 h of LHRH stimulation in normal rams is not caused by the increasing plasma T levels but is an intrinsic pituitary phenomenon that could be due to an adverse effect of LHRH on its own receptors. (Endocrinology106: 329, 1980)