Descending Bradycardia Pathways Between The Parabrachial Nucleus And The Medulla In The Rabbit

Previous research in our laboratory (Hamilton, et al. in press) has shown that electrical stimulation (10 sec; 100 pulses/sec; 0.25 msec pulse duration; 100-150 uA) elicited primary, short latency (1.0 sec) bradycardia (-100 beats/min) from the parabrachial nucleus (PBN) of the pons in ethyl carbomate anesthetized rabbits. Also, single pulse stimulation (0.4 pulses/sec) of PBN was shown to orthodromically activate barosensory-sensitive neurons in the nucleus tractus solitarius (NTS). The present study replicated these findings and also showed that cardioinhibitory vagal pre-ganglionic neurons in the dorsal vagal nucleus (DVN) could be orthodromically driven by single pulse stimulation of PBN at a latency of 8.4(+7.8)msec. However, because (1) extensive attempts to antidromically activate barosensory sensitive neurons in PBN by stimulating NTS or DVN were unsuccessful, (2) latencies and other characteristics of NTS and DVN neuronal responses to PBN suggested the presence of an oligosynaptic pathway, and (3) only sparse retrograde labeling was seen in PBN after injections of horseradish peroxidases (HRP) were made into the dorsal medulla (NTS/DVN complex), attempts were made to identify alternative putative synapses in the central bradycardia pathway between PBN and cardioinhibitory motor neurons. Anterograde and retrograde labeling using HRP conjugated with wheat germ agglutinin revealed the presence of direct projections from PBN and NTS and from PBN to nucleus ambiguus (NA). Indirect projections from PBN to NA synapsed in either the A5 region or the nucleus reticularis gigantocellularis (NRG). Direct projections were traced from NA to DVN, and reciprocal connections were observed between NTS and NA. Extensive exploration of the caudal pons-rostral medulla indicated that train stimulation of the A5 region and ventral NRG elicited bradycardia responses that were similar to those elicited by stimulating PBN. These responses were eliminated by bilateral vagotomy, but not by artificial ventilation in decamethonium paralyzed animals. Bradycardia responses were also elicited by stimulating NTS, DVN, and NA. The results suggest that the bradycardia pathway(s) descending through PBN may have their out-flow from DVN with intermediate synapses in NTS, and from NA, either without intervening synapse or with intermediate synapse in the A5 region or ventral NRG.In addition this study showed that train electrical stimulation of the principle nucleus of V (Nvp) also elicited bradycardia responses. The form and characteristics of these bradycardia responses were similar to the trigeminal depressor response described by Kumada, et al. (1979). Injection of HRP into Nvp showed that the descending trigeminal depressor response pathway is anatomically distinct from the descending PBN bradycardia pathway