VENTRAL SUBICULAR INTERACTION WITH THE HYPOTHALAMIC PARAVENTRICULAR NUCLEUS - EVIDENCE FOR A RELAY IN THE BED NUCLEUS OF THE STRIA TERMINALIS

The axonal projections of the ventral subiculum to the bed nucleus of the stria terminalis (BST) were examined in the rat with the anterogra de neuronal tracer Phaseolus vulgaris-leucoagglutinin (PHA-L). Axons o riginating in the ventral subiculum coursed to the BST through either the fimbria-fornix, or a pathway involving the stria terminalis via th e amygdala. Ventral subicular axons gave rise to dense terminal networ ks that were preferentially distributed in medial and ventral subregio ns of the BST. The distribution of subicular fibers and terminals was examined in relation to BST neurons that project to the hypothalamic p araventricular nucleus (PVN). In these cases, discrete iontophoretic i njections of the retrograde tracer Fluoro-gold were made in the PVN, w ith PHA-L delivered to the ipsilateral ventral subiculum. An immunocyt ochemical double-labeling protocol was then employed for the simultane ous detection of PHA-L and Fluoro-gold, and provided light microscopic evidence for subicular input to PVN-projecting cells located within t he BST. In a second series of experiments, the gamma-amino butyric aci d (GABA)ergic nature of the BST was examined by in situ hybridization histochemistry for detection of transcripts encoding GAD67 mRNA. The s tudies revealed that a high proportion of BST neurons express GAD67 tr anscripts. Also, experiments combining Fluoro-gold tracing with GAD67 in situ hybridization suggested that a proportion of PVN-projecting ne urons in the BST are GABAergic. Taken together, the results of these s ets of studies suggest that the inhibitory influences of the hippocamp us on the PVN might be relayed through specific portions of the BST. T hese findings may have important implications for our understanding of the neural regulation of the hypothalamic-pituitary-adrenal axis.