Convergence properties of solitary tract neurones driven synaptically by cardiac vagal afferents in the mouse

1. Cardiac vagal receptors are chemically and/or mechanically sensitive but it is unknown if this information is preserved centrally within the nucleus of the solitary tract (NTS). The present study had two aims: first, to investigate qualitatively whether both mechanically and chemically sensitive cardiac vagal encoding were preserved within the NTS, and second, to determine the patterns of convergence from other cardiorespiratory afferents to NTS neurones receiving cardiac vagal inputs. 2. The extracellular activity of single NTS neurones was investigated during stimulation of both chemically and mechanically sensitive cardiac vagal receptors in a working heart-brainstem preparation of mouse. 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 the left ventricle was distended to activate cardiac mechanoreceptors. 3. Forty-three NTS neurones were activated both synaptically by electrical stimulation of the ipsilateral vagus nerve (latency, 35 +/- 3 ms), and by intra-left ventricular injection of veratridine and also, in some cases, by bradykinin and/or PGE(2). These NTS neurones were delineated into two populations based on their response to left ventricular distension and convergence properties. Left ventricular distension-insensitive 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 mainly by baroreceptors (86 %) rather than peripheral chemoreceptors (14%; i.e. n = 1 neurone). 4. The data reveal two distinct populations of NTS neurones receiving cardiac vagal inputs: (a) cells responsive to veratridine stimulation only, and (b) neurones activated by both veratridine and mechanical stimuli. The specific convergence pattern of baroreceptors and chemoreceptors to these cardioreceptive NTS neurones is discussed in relation to a common afferent modality integration within the NTS.