Effects of Chronic Treatment with an LHRH Agonist on the Secretion of LH, FSH and Testosterone in the Ram

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Abstract
Four adult Soay rams were given a daily i.v. injection of 10 .mu.g LHRH [luteinizing hormone releasing hormone] agonist [D-Ser(But)6 des Gly10 LHRH ethylamide] for 8 days and blood samples were collected at 20 min intervals for 6-7 h on the 1st and the last day of injection and 1 and 23 days after stopping treatment. Four untreated animals acted as controls. The 1st injection of agonist induced a supraphysiological increase in the circulating levels of LH [luteinizing hormone] and FSH [follicle-stimulating hormone], but this response was markedly reduced by the last injection. Treatment with the LHRH agonist also resulted in a reduction in the testicular response to LH. Twenty-four h after the last injection there were changes in the endogenous secretion of LH and testosterone [T]; the number of LH pulses in the agonist treated rams was significantly greater (P < 0.001) than in controls while the pulse amplitude was significantly lower (P < 0.02). At the same time there was no longer any increase in the plasma concentration of T in response to the spontaneous episodic pulses of LH in the agonist treated rams. Also the LH response to a physiological dose of 200 ng LHRH was reduced and was largely ineffective in provoking a testicular response. This testicular refractoriness was overcome within 6 days of stopping treatment with the agonist, although the effects on pituitary responsiveness to LHRH were more sustained.

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