Cardiorespiratory sensory afferent inputs terminate in the NTS where information is integrated before transmission to neuronal pools controlling autonomic outflow. Some convergence between different cardiorespiratory inputs has been shown in NTS (e.g. Boscan et al. 2002). We used stimulation of the aortic depressor nerve (AN) to identify barosensitive neurones in the NTS and examined their convergence with lung inflation inputs.

Sprague-Dawley rats (320-380 g) were anaesthetized (pentobarbital sodium, 60 mg kg^-1, I.P.), neuromuscularly blocked (α-bungarotoxin, 140 µg kg^-1, I.V.) and artificially ventilated (Jones et al. 2002). Anaesthesia and neuromuscular block were monitored and maintained (both I.V.) until humane termination with anaesthetic (I.V.). Putative barosensitive neurones were identified on the basis of their response to ipsilateral AN stimulation. Responses to phenylephrine (PE, 5-10 µg, I.V.) were recorded and correlations of neuronal activity with tracheal pressure (TP) and blood pressure made. Neurones were categorised on the standard deviation of the latency of their response to AN stimulation (Doyle & Andresen, 2001). Group A neurones (presumed to be 2nd order) had S.D. values < 0.5 ms. Group C neurones had S.D. values > 1 ms (presumably higher order). Those with S.D. values 0.5-1 ms were designated Group B. Following identification some neurones were labelled juxtacellularly, visualised and where possible their axons traced (Pinault, 1996; Jones et al. 2002).

In total 101 NTS neurones responding to AN stimulation were recorded. 44 were classed as Group A, 19 Group B and 38 Group C. PE excited 19 neurones and inhibited three. Only two were unaffected, indicating that the majority of the neurones were barosensitive. Group A neurones were significantly more active (8.51 ± 2.14 Hz, n = 23; mean ± S.E.M.) than Group C (3.15 ± 0.59 Hz, n = 22, P < 0.05; Student’s unpaired t test) with Group B falling between (5.30 ± 2.98 Hz, n = 8). Activity in 71 % Group A neurones, 63 % of Group B neurones but only 27 % of Group C neurones were strongly correlated with TP. Cessation of artificial ventilation inhibited firing in 11/12 tests. Pulse related modulation was identified in 71 % of Group A, 63 % of Group B and 59 % of Group C neurones. These populations showed considerable overlap with those synchronised to TP.

Thus convergence of lung inflation with aortic nerve inputs appears to occur most commonly at presumed 2nd order neurones within the NTS.

This work was supported by The Wellcome Trust (057296).