Direct projections from caudal ventrolateral medullary depressor sites to the subfornical organ.

Experiments were performed in the male Wistar rat to investigate the projections from cardiovascular responsive sites in the ventrolateral medulla (VLM) to the subfornical organ (SFO). Unilateral iontophoretic injections of Phaseolus vulgaris leucoagglutinin (PHA-L) were made into either caudal VLM (CVLM) sites at which microinjection of l-glutamate (10 nl; 0.25 M) elicited decreases in mean arterial pressure or into rostral VLM (RVLM) sites at which l-glutamate microinjection elicited increases in arterial pressure. After a survival period of 7-10 days, transverse sections of the forebrain and brainstem were processed for PHA-L immunoreactivity. After injections of PHA-L into the CVLM, axonal and presumptive terminal labeling was found bilaterally throughout the rostrocaudal extent of the SFO, although most of the projections were observed within the rostral half of the nucleus. Within the SFO, labeling was found primarily in the lateral aspects of the nucleus, often in close proximity to blood vessels. In addition, CVLM injections resulted in labeling within the organum vasculosum of the laminae terminalis (OVLT) and within the ventral and dorsal components of the median preoptic nucleus (MnPO) bilaterally, but with an ipsilateral predominance. In contrast, PHA-L injections into the RVLM did not result in axonal labeling in the SFO or OVLT, although a few labeled axons were found to course through the region of the ventral component of MnPO. These data have demonstrated that neurons within the cardiovascular responsive region of the CVLM send direct axonal projections to the SFO and other structures of the laminae terminalis, and suggest that the CVLM may function in the modulation of the activity of neurons of circumventricular organs to intra- and extracellular signals of body fluid balance.