Parabrachial area as mediator of bradycardia in rabbits.

This study examined the role of the parabrachial nucleus (PBN) in the mediation of bradycardia and in the reception of barosensory information. The 82 rabbits in the investigation were anesthetized with ethyl carbamate. Train stimulation of medial or lateral PBN produced primary bradycardia (mean peak change: -74 beats/min) associated with a pressor response (average peak mean change: +10 mm Hg) of longer latency. Section of the cervical vagus nerves indicated that the bradycardia was mediated primarily by the parasympathetic nervous system. Heart rate and blood pressure responses to train stimulation did not vary systematically as a function of respiratory pattern; paralyzing animals with decamethonium hydrochloride and artificially ventilating them also did not influence the cardiovascular responses to stimulation. Single-pulse stimulation of PBN in conjunction with extracellular single neuron recording established that neurons originating in or projecting through PBN project to the commissural region of nucleus tractus solitarius (NTS) where synapse is made neurons receiving barosensory input. In addition to establishing the existence of descending functional projections passing from PBN to NTS, injections of HRP into PBN revealed direct descending anatomical projections to PBN from regions of the forebrain previously implicated in the mediation of bradycardia. These included central nucleus of amygdala, lateral preoptic region, medial, forebrain bundle, bed nucleus of stria terminalis, anterior and lateral hypothalamus and zona incerta. The present investigation also indicated that PBN receives barosensory information. Single-pulse electrical stimulation of the aortic nerve (AN) activated neurons in NTS at an average latency of 7.5 ms and in PBN at a mean latency of 12.1 ms. Mean latency of 9 neurons in NTS activated antidromically by PBN stimulation was 3.4 ms. Conduction velocity of the monosynaptic fibers projecting from NTS to PBN was approximately 3.5 m/s, which would be characteristic of finely myelinated fibers. Injection of HRP into PBN confirmed the existence of direct ascending projections to PBN from regions of NTS (e.g. lateral commissural area) previously shown to receive primary barosensory input. However, the finding that only 1 of 9 NTS neurons antidromically activated by PBN stimulation also received barosensory stimulation, indicates that additional study is needed of the mono- and oligosynaptic functional projections from NTS to PBN. The present study did provide evidence that PBN both receives barosensory information at short latency over a direct route, and serves as a relay for descending projections mediating bradycardia.