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 div ision 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 neuron es are found exclusively within the ventrolateral division of VMN, thi s study has used double-labelling immunocytochemistry techniques to ev aluate 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 proces sed for ER and SOM immunoreactivity using the H222 monoclonal rat ER a ntibody and a polyclonal rabbit SOM antiserum. Within the VMN, cells i mmunoreactive 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 differen ces 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 an d Substance P as a putative mediator of oestrogen's regulatory influen ce on reproduction through the VMN of the rat.