NEURAL CONTROL OF LEFT-VENTRICULAR CONTRACTILITY IN THE DOG HEART - SYNAPTIC-INTERACTIONS OF NEGATIVE INOTROPIC VAGAL PREGANGLIONIC NEURONSIN THE NUCLEUS AMBIGUUS WITH TYROSINE-HYDROXYLASE IMMUNOREACTIVE TERMINALS

Recent physiological evidence indicates that vagal postganglionic cont rol of left ventricular contractility is mediated by neurons found in a ventricular epicardial fat pad ganglion, In the dog this region has been referred to as the cranial medial ventricular (CMV) ganglion [J.L . Ardell, Structure and function of mammalian intrinsic cardiac neuron s, in: J.A. Armour, J.L. Ardell (Eds.), Neurocardiology, Oxford Univ, Press, New York, 1994, pp. 95-114; B.X. Yuan, J.L. Ardell, D.A. Hopkin s, A.M. Losier, J.A. Armour, Gross and microscopic anatomy of the cani ne intrinsic cardiac nervous system, Anat. Rec., 239 (1994) 75-87]. Si nce activation of the vagal neuronal input to the CMV ganglion reduces left ventricular contractility without influencing cardiac rate or AV conduction, this ganglion contains a functionally selective pool of n egative inotropic parasympathetic postganglionic neurons. In the prese nt report we have defined the light microscopic distribution of pregan glionic negative inotropic neurons in the CNS which are retrogradely l abeled from the CMV ganglion. Some tissues were also processed for the simultaneous immunocytochemical visualization of tyrosine hydroxylase (TH: a marker for catecholaminergic neurons) and examined with both l ight microscopic and electron microscopic methods. Histochemically vis ualized neurons were observed in a long slender column in the ventrola teral nucleus ambiguus (NA-VL). The greatest number of retrogradely la beled neurons were observed just rostral to the level of the area post rema. TH perikarya and dendrites were commonly observed interspersed w ith vagal motoneurons in the NA-VL. TH nerve terminals formed axo-dend ritic synapses upon negative inotropic vagal motoneurons, however the origin of these terminals remains to be determined. We conclude that s ynaptic interactions exist which would permit the parasympathetic preg anglionic vagal control of left ventricular contractility to be modula ted monosynaptically by catecholaminergic afferents to the NA-VL. (C) 1998 Elsevier Science B.V. All rights reserved.