WHAT ARE THE ROLES OF SUBSTANCE-P AND NEUROKININ-1 RECEPTORS IN THE CONTROL OF NEGATIVE CHRONOTROPIC OR NEGATIVE DROMOTROPIC VAGAL MOTONEURONS - A PHYSIOLOGICAL AND ULTRASTRUCTURAL ANALYSIS

Recent data indicate that there is a cardiotopic organization of negat ive chronotropic and negative dromotropic neurons in the nucleus ambig uus (NA). Negative dromotropic neurons are found in the rostral ventro lateral NA (rNA-VL), negative chronotropic neurons are found in the ca udal ventrolateral NA (cNA-VL), and both types of neurons are found in an intermediate level of the ventrolateral NA (iNA-VL). Substance P ( SP) immunoreactive nerve terminals synapse upon negative chronotropic vagal motoneurons in the iNA-VL, and SP microinjections in the NA caus e bradycardia. In the present report we have attempted to: (1) define the type of tachykinin receptor which mediates the negative chronotrop ic effect of SP microinjections into the iNA-VL; (2) define the physio logical effect of microinjections of a selective SP agonist into the r NA-VL on atrioventricular (AV) conduction; and (3) find ultrastructura l evidence for synaptic interactions of SP-immunoreactive nerve termin als with negative dromotropic vagal motoneurons in the rNA-VL. Microin jections of the excitatory amino acid glutamate (Glu) into the iNA-VL to activate all local vagal preganglionic neurons caused both bradycar dia and a decrease in the rate of AV conduction. Injections of the sel ective neurokinin-1 (NK-1) receptor agonist drug GR-73632 also caused bradycardia, however the rapid onset of agonist induced desensitizatio n prevented an evaluation of potential effects on AV conduction in the INA-VL. These data suggest that the SP-induced bradycardia which can be elicited from the NA is mediated, at least in part, by NK-1 recepto rs. Microinjections of Glu into the rNA-VL caused a decrease in AV con duction without an effect on cardiac rate. On the other hand, GR-73632 microinjections into rNA-VL did not affect AV conduction. Following i njections of the beta subunit of cholera toxin conjugated to horseradi sh peroxidase (CTB-HRP) into the left atrial fat pad ganglion which se lectively mediates changes in AV conduction, retrogradely labeled neur ons were histochemically visualized in the rNA-VL. These tissues were subsequently processed for the simultaneous immunocytochemical visuali zation of SP, and examined by electron microscopy. Histochemically lab eled neurons were large, multipolar, with abundant cytoplasm containin g large masses of rough endoplasmic reticulum, and exhibited distincti ve dendritic and somatic spines. Unlabeled nerve terminals were noted to form either asymmetric or symmetric synapses with dendrites, dendri tic spines, and perikarya of histochemically labeled neurons. SP-immun oreactive nerve terminals were also detected in the rNA-VL. SP termina ls typically contained numerous small pleomorphic vesicles, multiple l arge dense core vesicles, and several mitochondria, and they synapsed upon unlabeled dendritic profiles. A total of 154 SP-immunoreactive ne rve terminals were observed on photomicrographs of tissues which also contained histochemically labeled profiles. None made an identifiable synapse with a retrogradely labeled profile on the sections examined. In summary, both physiological and ultrastructural data indicate that SP terminals in the iNA-VL do modify the output of negative chronotrop ic vagal motoneurons. This effect is mediated by NK-1 receptors. On th e other hand both physiological and ultrastructural data indicate that SP terminals in the rNA-VL do not modify the output of negative dromo tropic vagal motoneurons. Therefore different mechanisms (neurotransmi tters or receptors) mediate the central vagal control of cardiac rate and AV conduction.