Neurons in the parabrachial nuclei respond to hemorrhage

To examine the organization of pathways in the dorsal rostral pons that process information from cardiovascular receptors, 62 neurons in the parabrachial nuclei were tested, in rats anesthetized with urethane, for their response to hemorrhage (10 ml/kg.min) and reinfusion. Of these neurons, 21 exhibited patterns of activity in response to hemorrhage that were significantly different from those seen prior to hemorrhage. The patterns of activity appeared to depend, at least in part, on the location of the neurons within the parabrachial nuclei. The activity of 8 neurons in the external lateral and central lateral parabrachial nuclei (LT) and adjacent Kolliker-Fuse nucleus (KF) increased during hemorrhage and remained elevated during reinfusion. Multivariate regression analysis indicated that the activity of these neurons was best predicted by mean arterial pressure. In contrast, the activity of 8 neurons in the dorsal cap of the central lateral parabrachial nucleus (DR) and in the caudal medial parabrachial nucleus (MD) increased during hemorrhage and decreased during reinfusion, and appeared to be best predicted by blood volume. The activity of 5 neurons in the region between the caudal medial parabrachial nucleus and the locus coeruleus (BT) responded inversely to those in the caudal medial parabrachial nucleus, and was predicted well both by blood volume and by mean arterial pressure. Together, these data reveal a complex processing of hemodynamic signals within the parabrachial nuclei which may play a critical role in the control of neuroendocrine and sympathetic responses in relation to regulation of arterial pressure, blood volume and fluid balance.