Seasonal changes in plasma testosterone and cortisol suggest an androgen mediated regulation of the pituitary adrenal axis in the Tarabul's gerbil Gerbillus tarabuli (Thomas, 1902)

In the desert gerbil Gerbillus tarabuli (Thomas, 1902), cortisol is the main glucocorticosteroid produced by the adrenal glands. Plasma cortisol concentrations show highest values when testosterone is reduced and lowest values during the breeding season which occurs from early winter to late spring. In order to specify the implication of testicular androgens in these corticosteroid seasonal variations we investigated the effects induced by gonadectomy performed during the breeding season on the pituitary adrenal axis. The animals collected in winter were assessed into three groups: sham-operated (Controls; n=13), gonadectomised (GDX; n=13) and testosterone replaced gonadectomised (GDX+T; n=13). Physiological replacement of testosterone enanthate (75µg/100gb.w./twice daily) was applied during one week, while GDX group received the vehicle (40µL sesame oil) alone. The right adrenal glands removed from euthanized animals were fixed for histomorphometry and androgen receptors (ARs) immunohistochemistry and the left ones were frozen with plasma samples until hormonal assays. Gonadectomy induces the enlargement of the adrenal cortex essentially due to that of zonae fasciculata (ZF) and reticularis (ZR) and perimedullary connective tissue which is abundant in the gerbil adrenals. The ARs immunostaining present at both cytoplasmic and nucleus level, is enhanced intensely in the ZR and moderately in the ZF and zona glomerulosa (ZG) cells. GDX group shows reduced plasma ACTH concentration (p=0.0126) by 61% despite the increase in cortisol concentration occurring both in plasma (+216%; p=0.0436) and adrenal tissue (+117%; p=0.0348). Plasma aldosterone is also enhanced significantly (p=0.0147) by 189% but androstenedione synthesis increased in adrenal tissue (p=0.0459) by 65% instead a decrease at circulatory level (p=0.0355) by 58% due to lack of testicular origin. So, testosterone deprivation activates corticosteroidogenesis also evidenced by the adrenal structure changes and the gonadectomy-induced increase in the plasma cholesterol. All of the gonadectomy-induced responses are reversible after physiological testosterone replacement. We conclude that the assessment of circulating adrenocorticotropic hormone (ACTH) concentrations together with cortisol levels essentially, reflecting the hypothalamic-pituitary adrenal (HPA) axis feedback loop control during the annual endogenous changes of testosterone secretion, represents a well-adapted response of this desert species living in an extreme environment.