SEXUALLY DIMORPHIC EXPRESSION OF ANDROGEN RECEPTOR IMMUNOREACTIVITY BY SOMATOSTATIN NEURONS IN RAT HYPOTHALAMIC PERIVENTRICULAR NUCLEUS AND BED NUCLEUS OF THE STRIA TERMINALIS

Gonadal steroids exert important regulatory actions on the hypothalamic neurones regulating growth hormone secretion and are believed to play a role in generating its sexually dimorphic pattern of secretion. Recent evidence indicates that estrogen actions on one of these neural populations, the periventricular somatostatin (SOM) neurones, are likely to be indirect as they do not possess nuclear estrogen receptors in either sex although androgen receptors (ARs) have been reported within these cells in male rats. The present study has used double-labelling immunocytochemistry procedures to examine whether sex differences exist in AR expression by SOM neurones located in the periventricular nucleus and bed nucleus of the stria terminalis (BNST). Within the hypothalamus, SOM-immunoreactive neurones were found concentrated in the periventricular nucleus while both anterior and posterior divisions of the BNST contained scattered populations of SOM cells. Cells immunoreactive for the AR were detected in all of these areas. Although the intensity of AR cell nuclei staining was equivalent in males and females in regions such as the lateral septum, the intensity of AR staining in many individual cells of the periventricular nucleus and posterior BNST of the female was reduced when compared with the male. Double-labelling experiments revealed that approximately 40% of periventricular SOM neurones expressed AR immunoreactivity in the male compared with significantly ; (P < 0.01) fewer cells in the female (similar to 7%). In the BNST, double-labelled cells were only detected within the principle encapsulated, interfascicular and transverse nuclei of its posterior division. Approximately 60% of SOM cells in these nuclei expressed AR immunoreactivity in the male while significantly (P < 0.01) fewer did so in the female (similar to 25%). These results indicate that substantial sex differences exist in AR expression by SOM neurones in both the periventricular nucleus and BNST. Such differences in AR expression by periventricular SOM cells may contribute to their sexually dimorphic nature and, consequently, sex differences in growth hormone secretion.