Effect of melatonin administration on the male viscacha adrenal cortex proliferative activity

Abstract

The adrenal cortex, under control of the hypothalamic-pituitary-adrenal axis, produces specific steroid hormones which are key to ensure physiological adaptation and improvement of survival chances when facing homeostatic challenges. The viscacha (Lagostomus maximus maximus) is a hystricomorph rodent that lives in burrows and has nocturnal habits. The adult male viscacha exhibits an annual reproductive cycle synchronized by the environmental photoperiod through the pineal gland and its main hormone, melatonin. The aim of this study was to assess the effect of melatonin administration on the adrenocortical cells proliferative activity in adult male viscachas. For this study, animals were divided in two groups: an experimental group (n=4) that receive two daily injections of melatonin; and a control group (n=4) that receive two daily injections of the diluent. After 9 weeks, animals were sacrificed and adrenal glands processed for light microscopy. Adrenal cortex proliferation activity was assessed trough immunohistochemical detection of the proliferating cell nuclear antigen (PCNA), an S-phase related cell proliferation marker. The number of PCNA-immunoreactive adrenocortical cells (PCNA-ir) was counted per reference area. Differences between the two groups were evaluated using Mann–Whitney U test. In both experimental and control animals, cell proliferation was observed mainly in the outermost portion of the adrenal cortex, near the zona Glomerulosa and outer zona Fasciculata. In the innermost portion of the cortex PCNA-ir cells were scarce, except in some control animals where proliferation could also be observed in the inner zona Fasciculata. In melatonin administered animals, the number of PCNA-ir was (1.31  0.2) significantly lower than in control animals (2.31  0.16). These results indicate that proliferation in the adrenal cortex of the viscacha occurs mainly in the outermost portion of the cortex as described in other mammal species. Furthermore, melatonin is capable of modulating the adrenal cortex proliferative activity, reducing proliferation under stressful conditions of daily manipulation and subcutaneous injections.