ULTRASTRUCTURAL CIRCUITRY OF CARDIORESPIRATORY REFLEXES - THERE IS A MONOSYNAPTIC PATH BETWEEN THE NUCLEUS OF THE SOLITARY TRACT AND VAGAL PREGANGLIONIC MOTONEURONS CONTROLLING ATRIOVENTRICULAR-CONDUCTION IN THE CAT

We have tested the hypothesis: (1) that presumptive negative dromotrop ic vagal preganglionic neurons in the ventrolateral nucleus ambiguus ( NA-VL) can be selectively labelled from the heart, by injecting one of two fluorescent tracers into the two intracardiac ganglia which indep endently control sino-atrial (SA) rate or atrioventricular (AV) conduc tion; i.e., the SA and AV ganglia, respectively. The NA-VL was examine d for the presence of single and/or double labelled cells. Over 91% of vagal preganglionic neurons in the NA-VL projecting to either intraca rdiac ganglion did not project to the second, ganglion. Consequently, we also tested the hypothesis: (2) that there is a monosynaptic connec tion between neurons of the medial, and/or dorsolateral nucleus of the solitary tract (NTS), rostral to obex, and negative dromotropic neuro ns in the NA-VL. An anterograde tracer was injected into the NTS, and a retrograde tracer into the AV ganglion. The anterograde marker was f ound in both myelinated and unmyelinated axons in the NA-VL, as well a s in nerve terminals. Axo-somatic and axo-dendritic synapses were dete cted between terminals labelled from the NTS, and retrogradely labelle d negative dromotropic neurons in the NA-VL. This is the first ultrast ructural demonstration of a monosynaptic pathway between neurons in th e NTS and functionally associated (negative dromotropic) cardioinhibit ory neurons. The data are consistent with the hypothesis that the neur oanatomical circuitry mediating the vagal baroreflex control of AV con duction may be composed of as few as four neurons in series, although interneurons may also be interposed within the NTS. (C) 1998 Elsevier Science B.V.