SUBSTANCE-P NERVE-TERMINALS SYNAPSE UPON NEGATIVE CHRONOTROPIC VAGAL MOTONEURONS

Previous data indicate that there are anatomically segregated and phys iologically independent parasympathetic ganglia on the surface of the heart which are capable of selective control of sino-atrial rate, atri o-ventricular conduction, and atrial contractility. We have injected a retrograde tracer into the cardiac ganglion which selectively regulat es heart rate (the SA ganglion). Medullary tissues were processed for the histochemical visualization of retrogradely labeled neurons and fo r the immunohistochemical detection of the neurotransmitter substance P (SP) by dual labeling light and electron microscopic methods. Negati ve chronotropic retrogradely labeled cells were found in a long slende r column in the ventrolateral nucleus ambiguus (NA-VL) which enlarged somewhat at the level of the area postrema. These cells were found bil aterally, but they were asymmetrically distributed. Half the animals s howed a pronounced right side predominance in retrograde labeling, whi le the other half of the animals showed a lesser left side predominanc e. These observations may help to explain some of the controversy in t he literature concerning the relative influence of the right and left vagus nerves on sinus rate. Ultrastructural examination demonstrated a xo-somatic and axo-dendritic contacts between SP nerve terminals and r etrogradely labeled negative chronotropic NA-VL neurons. SP immunoreac tivity was often associated with large dense-core vesicles in terminal s forming either symmetric or asymmetric synapses. These observations provide a potential anatomical substrate for the centrally mediated br adycardia elicited by microinjections of SP into the NA, SP immunoreac tive terminals were also observed to make axe-somatic, axo-dendritic, and axo-axonic synapses with unlabeled neurons in NA-VL. These data su ggest that SP may also modulate the activity of other vagal preganglio nic neurons.