A projection from the ventral tegmental area to the periaqueductal gray involved in cardiovascular regulation

Experiments were done in alpha-chloralose-anesthetized rats to determine a pathway mediating the cardiovascular depressor responses elicited from stim ulation of the ventral tegmental area (VTA). The magnitude of the depressor responses elicited by glutamate stimulation (0.1 M/30 nl) of the VTA was e xamined after neuronal block produced by microinjections of lidocaine into ascending fiber bundles leaving the VTA to innervate the forebrain and thal amus. Bilateral microinjections of 1 mu l of 4% lidocaine in the medial for ebrain bundle (n = 6) and in the periventricular fibers of the midbrain (n = 5) did not attenuate the depressor response from stimulation of the VTA. Experiments were done using the anterograde tracer biotinylated dextran ami ne to identify descending projections from the VTA to cardiovascular center s in the brain stem. Examination of the nucleus of the solitary tract, vent rolateral medulla, and A5 catecholaminergic cell group revealed few or no f ibers or terminals. Occasional fibers and some terminals were observed in t he nucleus of raphe magnus, parabrachial nucleus, and locus ceruleus. A ver y dense bilateral projection was found to the ventrolateral periaqueductal gray (PAGvl) and dorsal raphe nucleus adjacent to the PAGvl. Bilateral inje ctions of 4% lidocaine (n = 4) or 10 mM cobalt chloride (n = 5) into the PA Gvl region attenuated the depressor responses elicited by stimulation of th e VTA by similar to 50%. These experiments indicate that the depressor resp onses elicited from activation of the VTA are mediated in part by a pathway to a cardiovascular depressor area located in the PAGvl.