CONVERGENCE PROPERTIES OF SOLITARY TRACT NEURONS DRIVEN SYNAPTICALLY BY CARDIAC VAGAL AFFERENTS IN THE MOUSE

1. Cardiac vagal receptors are chemically and/or mechanically sensitiv e but it is unknown if this information is preserved centrally within the nucleus of the solitary tract (NTS). The present study had two aim s: first, to investigate qualitatively whether both mechanically and c hemically sensitive cardiac vagal encoding were preserved within the N TS, and second, to determine the patterns of convergence from other ca rdiorespiratory afferents to NTS neurones receiving cardiac vagal inpu ts. 2. The extracellular activity of single NTS neurones was investiga ted during stimulation of both chemically and mechanically sensitive c ardiac vagal receptors in a working heart-brainstem preparation of mou se. Chemically sensitive cardiac receptors were stimulated using intra -left ventricular injections of either veratridine (1-3 mu g kg(-1)), bradykinin (0.25-1 mu g) or prostaglandin E-2 (100-200 ng), whereas th e left ventricle was distended to activate cardiac mechanoreceptors. 3 . Forty-three NTS neurones were activated both synaptically by electri cal stimulation of the ipsilateral vagus nerve (latency, 35 +/- 3 ms), and by intra-left ventricular injection of veratridine and also, in s ome cases, by bradykinin and/or PGE(2). These NTS neurones were deline ated into two populations based on their response to left ventricular distension and convergence properties. Left ventricular distension-ins ensitive neurones (n = 30) were excited by stimulation of carotid body chemoreceptors (81%) but not arterial baroreceptors (3%; i.e. n = 1, neurone), whereas distension-sensitive cells (n = 13) were activated m ainly by baroreceptors (86 %) rather than peripheral chemoreceptors (1 4%; i.e. n = 1 neurone). 4. The data reveal two distinct populations o f NTS neurones receiving cardiac vagal inputs: (a) cells responsive to veratridine stimulation only, and (b) neurones activated by both vera tridine and mechanical stimuli. The specific convergence pattern of ba roreceptors and chemoreceptors to these cardioreceptive NTS neurones i s discussed in relation to a common afferent modality integration with in the NTS.