Vagal drive to the heart is a key determinant of resting heart rate and respiratory sinus arrhythmia. Cardiac vagal tone is an important indicator of cardiovascular health, and loss of cardiac vagal tone is a feature of many cardiovascular disorders, including hypertension and heart arrhythmias. The cardiac ganglia are an important site for regulation of vagal drive to the heart, where depression of transmission may translate to loss of vagal tone. Attenuated ganglionic transmission has been shown in models of heart failure (1) and hypertension (2). As cardiac vagal postganglionic neurons (CVNs) are located in a ganglionic plexus on the surface of the beating heart, it has hitherto been difficult to investigate how CVNs process ongoing and reflex evoked vagal activity. We have extended the working heart-brainstem preparation (WHBP, 3) and developed a model whereby we can make stable, intracellular recordings from CVNs. To begin to address any deficit in cardiac ganglionic transmission in disease states we first characterised how CVNs respond to ongoing cardiac vagal tone and activation of a number of vagally-mediated cardiorespiratory reflexes in Wistar rats (4). Male Wistar rats (P18-32, n=39) were anaesthetised with 5% Halothane (until decerebration) for surgical set up of the working heart-brainstem preparation. The atria were opened, stabilised and the cardiac vagal ganglia exposed. Intracellular recordings were made from CVNs with sharp microelectrodes (65-170MΩ, 0.5M KCl). CVNs were classified as tonic or phasic depending on whether they produced tonic firing or single spikes in response to depolarising current pulses (500ms, 0.3Hz, 1-10nA), respectively. CVN responses to activation of chemo- (50μl, 0.03% NaCN i.a.), baro- (+20-50mmHg perfusion pressure ramps), diving (100μl, ~10°C aCSF to the nose) and von Bezold-Jarisch (phenylbiguanide; PBG, 200μg i.a.) reflexes were assessed. Examples of phasic (n=6) and tonic (n=6) CVNs were filled with biocytin (4%), fixed in 4% formaldehyde and then incubated with Streptavidin conjugated to Alexa Fluor 594 (red) dye (1:1000, 12-24hr). The cells were then morphologically characterised using fluorescence microscopy. Stable intracellular recordings were made from 61 cardiac vagal ganglion cells with preserved central connections. Cardiac ganglion cells with vagal synaptic inputs (spontaneous, n=21; or electrically evoked from the vagus, n=3) were identified as principal neurones and showed tonic firing responses to injected current pulses as described previously (5). Cells lacking vagal inputs (n=37, presumed interneurones) were quiescent but showed phasic firing responses to depolarising current pulses. In principal cells the ongoing action potentials and EPSPs exhibited respiratory modulation, with peak frequency in post-inspiration. Action potentials arose from unitary EPSPs and autocorrelation of those events showed that each ganglion cell received inputs from a single active preganglionic neurone. Peripheral and cardiac chemoreceptor, arterial baroreceptor and diving response activation all evoked high frequency synaptic barrages in these cells, as well as a concomitant bradycardia. Principal cells responded to activation of all reflexes indicating a convergence of reflex pathways. EPSP amplitudes showed frequency dependent depression, leading to more spike failures at shorter inter-event intervals. Marked temporal summation of EPSPs occurred only during activation of an intense von Bezold-Jarisch reflex response where EPSP frequency was so high (151Hz) that they summated to produce a depolarising envelope (+10mV). All imaged cells had identifiable processes, which were divided into three classes: monopolar (5/12), dipolar (4/12) and multipolar (3/12). Most cells had a long, recognisable axon (9/12), and none had extensive dendritic arrangements. Morphometrically, there was no significant difference between tonic and phasic cells as assessed by their mean soma area, length or diameter. These data show that activation of vagally-mediated reflexes excite a common pool of vagal postganglionic neurones shared by numerous cardiorespiratory reflexes, with no evidence of recruitment of ganglionic interneurons during reflex activation. There do not appear to be marked morphological differences between ‘tonic’ (putative principal ganglion neurones) or ‘phasic’ (putative interneurones) cells, despite markedly different synaptic and intrinsic electrophysiological properties. These findings indicate that rather than integrating convergent inputs, cardiac vagal postganglionic neurones can gate preganglionic inputs, so regulating the proportion of central parasympathetic tone that is transmitted on to the heart. We plan to assess how this is altered in cardiovascular disease states.