CONTROL OF NEGATIVE INOTROPIC VAGAL PREGANGLIONIC NEURONS IN THE DOG - SYNAPTIC-INTERACTIONS WITH SUBSTANCE-P AFFERENT TERMINALS IN THE NUCLEUS AMBIGUUS

Previous research from this laboratory has shown that substance P-immu noreactive (SP) terminals synapse upon negative chronotropic vagal pre ganglionic neurons (VPNs), but not upon negative dromotropic VPNs, of the ventrolateral nucleus ambiguus (NA-VL). Moreover, SP agonists inje cted into NA-VL cause bradycardia without decreasing AV conduction. In the current study, we have: (1) defined the electron microscopic char acteristics of the SP neurons of NA-VL in dog; and (2) tested the hypo thesis that SP nerve terminals synapse upon negative inotropic VPNs of NA-VL, retrogradely labeled from the cranial medial ventricular (CMV) ganglion. Numerous SP terminals and a few SP neurons were observed in the vicinity of retrogradely labeled neurons. SP terminals were obser ved forming synapses with unlabeled dendrites and with SP dendrites, b ut never with the retrogradely labeled neurons. Together, these result s and earlier findings suggest that SP agonists may be able to induce bradycardia without decreasing AV conduction or ventricular contractil ity. (C) 1998 Elsevier Science B.V. All rights reserved.