Androgen receptor-like immunoreactivity in the developing Brazilian opossum brain and pituitary: Ontogeny and effects of exogenous testosterone administration

Indirect immunohistochemistry and an androgen receptor antibody, PG 21, was used to characterize the ontogeny of cells containing androgen receptor-like immunoreactivity (AR-ir) and the effects of exogenous testosterone administration on AR-ir in the developing Brazilian opossum brain. Cells containing AR-ir were first seen in the anterior pituitary at day 5 of postnatal life (PN). Between 10 and 15 PN, high numbers of immunoreactive cells were detected in the dorsomedial hypothalamic and ventral premammillary nuclei, and a few cells were seen in the arcuate nucleus of the male brain. The female brain of the same age had a low number of moderately immunostained cells in the dorsomedial hypothalamic and ventral premammillary nuclei. No AR-ir was observed in other areas of the brain until 45 PN, when a low number of immunoreactive cells were seen in the ventral- nucleus of the lateral septum of the male: In males, between 60 and 80 PN, cells containing AR-ir were present in additional areas of the forebrain and resembled that of the adult. In the midbrain and brainstem, AR-ir was first seen at 80 PN in the male in the central gray, mesencephalic trigeminal nucleus, medullary reticular formation, and the nucleus of the solitary tract. In the female, no AID-ir was seen in any of these areas at 60 and 80 PN. Exogenous testosterone injection 2 hours prior to tissue collection did not result in any change in AR-ir in animals at 1, 3, and 5 PN. However, between 10 and 15 PN, testosterone exposure resulted in an increase in the number and intensity of AR-ir in cells in the dorsomedial hypothalamic nucleus, arcuate nucleus, arcuate- median eminence region; ventral premammillary nucleus, and anterior pituitary gland. Testosterone treatments also resulted in the expression of AR-ir in cells in several of the forebrain and midbrain areas in both sexes. Testosterone injection at 25 PN resulted in expression of AR-ir in cells in similar brain areas as was observed at 15 PN in both the male and female. These findings suggest that the sex differences in AR-ir observed under physiological conditions in the opossum brain are possibly due to differences in plasma androgen levels in the male and female during postnatal development. Further, androgen receptors are present in several brain areas of the opossum during early postnatal development, which can be detected immunohistochemically using PG-21 after exogenous testosterone administration. The early presence of androgen receptors indicate that androgens may be involved in sexual differentiation and neuroendocrine regulation in the opossum brain. Further work will define the significance of these results.