Prodynorphin and proenkephalin gene expression in the anteroventral periventricular nucleus of the rat: Sexual differentiation and hormonal regulation.

Opioid peptides are generally thought to exert hormone-dependent regulatory influences on gonadotropin secretion and the anteroventral periventricular nucleus (AVPv) has been shown to play a critical role in the neural control of this sexually dimorphic function. In the present study we used in situ hybridization to compare the numbers of proenkephalin (PENK) and prodynorphin (PDYN) mRNA-containing neurons in the AVPv of male and female rats and to evaluate the influence of circulating sex steroid hormones on the development and mature regulation of PENK and PDYN gene expression in these neurons. In agreement with earlier immunohistochemical observations, the number of PENK mRNA-containing neurons in the AVPv of male rats was found to be twice that of female animals. In contrast, the AVPv contains over four times the number of PDYN mRNA-containing cells in female rats, relative to intact males. Treatment of newborn female rats with testosterone increases the number of PENK mRNA-containing neurons in the AVPv, but decreases the number of PDYN mRNA-containing neurons in the AVPv compared with untreated females. Treatment of adult ovariectomized female rats with estradiol significantly increased PDYN mRNA levels in the AVPv; however, comparable changes in levels of PENK mRNA were not detected. In adult male rats, neither PDYN, nor PENK mRNA were significantly altered by orchidectomy or testosterone treatment. Thus, the maintenance of enkephalinergic neurons and the loss of hormone-sensitive dynorphin-containing neurons in the AVPv may represent important developmental influences of neonatal androgens on the sexually differentiated neural circuitry controlling gonadotropin secretion.