Pituitary responses to gonadotrophin-releasing hormone (GnRH) and prolactin and steroid secretory profiles were examined in two populations of adult, female lions in the Serengeti (one outbred in the Serengeti Plains and one inbred in the Ngorongoro Crater) to determine whether reductions in genetic variability adversely affected endocrine function. GnRH-induced gonadotrophin secretion was also examined after adrenocorticotrophic hormone (ACTH) treatment to determine whether acute increases in serum cortisol altered pituitary function. Anaesthetized lions were administered (i) saline i.v. after 10 and 100 min of blood sampling; (ii) saline at 10 min and GnRH (1 μg kg−1 body weight) after 100 min; or (iii) ACTH (3 μg kg−1) at 10 min and GnRH after 100 min of sampling. Basal serum cortisol and basal and GnRH-induced gonadotrophin secretion were similar (P > 0.05) between females of the Ngorongoro Crater and Serengeti Plains. After ACTH, serum cortisol increased two-to threefold over baseline values and the response was unaffected (P > 0.05) by location. ACTH-induced increases in serum cortisol had no effect on subsequent basal or GnRH-stimulated luteinizing hormone (LH) or follicle-stimulating hormone (FSH) secretion. Overall mean serum progesterone concentrations ranged from 0.2 to 5.4 ng ml−1 with the exception of four females (two in the Serengeti and two in the Crater; progesterone range, 18.4–46.5 ng ml−1) that were presumed pregnant (three of these females were observed nursing cubs several weeks later). There were no differences (P > 0.05) between Serengeti and Crater lions in mean serum progesterone, oestradiol or prolactin concentrations, and hormone secretion was not influenced (P > 0.05) by GnRH or ACTH treatment. Although Ngorongoro Crater lions have decreased genetic variability, the reproductive–endocrine system of females appears functionally normal compared with outbred counterparts living in the Serengeti Plains. Furthermore, the acute rise in serum cortisol after ACTH administration in lions fails to alter subsequent GnRH-induced gonadotrophin release, suggesting that short-term changes in adrenal activity do not markedly affect pituitary responsiveness in this species.
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