SOMATOSTATIN-IMMUNOREACTIVE NEURONS IN THE HYPOTHALAMIC VENTROMEDIAL NUCLEUS POSSESS ESTROGEN-RECEPTORS IN THE MALE AND FEMALE RAT

Neurones containing oestrogen receptors (ERs) in the ventrolateral division of the hypothalamic ventromedial nucleus (VMNvI) are believed to play an important role in mediating oestrogen's regulatory influence on reproductive behaviour. As somatostatin (SOM)-immunoreactive neurones are found exclusively within the ventrolateral division of VMN, this study has used double-labelling immunocytochemistry techniques to evaluate whether ER-immunoreactive cells in the VMNvI synthesize SOM in both the male and female rat. Rats gonadectomized 1 week earlier were perfused and brain sections through the mediobasal hypothalamus processed for ER and SOM immunoreactivity using the H222 monoclonal rat ER antibody and a polyclonal rabbit SOM antiserum. Within the VMN, cells immunoreactive for SOM were found predominantly in the rostral portion of the VMNvI while ER-immunoreactive cells were distributed throughout the VMNvI. Sequential double-labelling studies revealed that many ER-containing cells in the rostral VMNvI were also immunoreactive for SOM. A semi-quantitative analysis of double-labelled cells in the rostral VMNvI of male and female rats, respectively, estimated that 52 +/- 2% and 55 +/- 8% of SOM-IR cells possess ERs while 35 +/- 1% and 28 +/- 3% of ER-IR cells synthesise SOM in the rostral VMNvI. No sex differences were detected at this level. These results show that approximately half of the SOM-synthesizing neurones in the rostral VMN possess ERs and indicate that SOM should now be considered alongside enkephalin and Substance P as a putative mediator of oestrogen's regulatory influence on reproduction through the VMN of the rat.