Critical role of B3 serotonergic cells in baroreflex inhibition during the defense reaction triggered by dorsal periaqueductal gray stimulation

The present study was designed to identify the serotonergic pathway causing baroreflex inhibition associated with the defense reaction in rats. Under conditions that produce physiological responses typical of the defense reaction, electrical stimulation of the dorsal periaqueductal gray (dPAG) was found to double c-Fos immunoreactive serotonergic neurons within the midrostrocaudal extent of the B3 group (which comprises the raphe magnus and the lateral paragigantocellular reticular nuclei) in anesthetized rats. Local blockade of neuronal activity by microinjection of muscimol (a GABAA receptor agonist) directly into the B3 region prevented the inhibitory effect of dPAG activation on the cardiac baroreflex. Conversely, neuron activation by local application of D,L-homocysteic acid into B3 region caused baroreflex inhibition that was suppressed by microinjection of granisetron (a 5-HT3 antagonist) into the nucleus tractus solitarius. These results show that activation of serotonergic cells in the mid-portion of B3 group is critical to trigger baroreflex inhibition occurring during the defense reaction evoked by dPAG stimulation. J. Comp. Neurol. 506:108 –121, 2008. © 2007 Wiley-Liss, Inc. Indexing terms: heart rate; serotonin; c-Fos; tractus solitarius; periaqueductal; ventromedial medulla