MEDULLARY PATHWAYS MEDIATING DEPRESSOR RESPONSES FROM NA-SENSITIVE SITES IN NUCLEUS OF THE SOLITARY TRACT()

Two series of experiments were done in male Wistar rats to investigate the medullary pathways that mediate the depressor responses from sodi um-sensitive sites in the nucleus of the solitary tract (NTS). In the first series, the anterograde tract tracer Phaseolus vulgaris leucoagg lutinin (PHA-L) was iontophoresed unilaterally at sites in the NTS at which microinjections (20 nl) of a 154-175 mM NaCl solution elicited d epressor responses. PHA-L injection sites were found to be localized w ithin the medial subnucleus of the NTS (Sm). In the medulla, PHA-L-lab eled fibers and presumptive terminal boutons were observed bilaterally , but with an ipsilateral predominance, throughout the rostrocaudal ex tent of the NTS, the dorsal motor nucleus of the vagus, area postrema, the ventrolateral medulla (VLM), and nucleus ambiguus. The pontine re gion, containing the A5 catecholaminergic cell group and the parabrach ial nucleus, also received projections from Sm. In the second series o f experiments, the effect of blocking synaptic transmission in VLM wit h cobalt chloride (CoCl2; 5 mM, 100 nl) on the cardiovascular response elicited by microinjection (20 nl) of hypertonic saline (154-175 mM) into the ipsilateral Sm was investigated in the alpha-chloralose-anest hetized, paralyzed, and artificially ventilated rat. Microinjection of CoCl2 into VLM, at sites shown in the previous study to receive effer ent projections from Sm, significantly attenuated the depressor (60%) and bradycardic (80%) responses to stimulation of Sm. These data indic ate that the sodium-sensitive region of the caudal Sm innervates VLM n eurons and suggest that these VLM neurons are involved in mediating th e depressor and bradycardic responses elicited by changes in the extra cellular concentration of sodium.