Nociceptive afferents selectively modulate the cardiac component of the peripheral chemoreceptor reflex via actions within the solitary tract nucleus
Section snippets
The working heart–brainstem preparation (WHBP)
The experimental procedures were approved by the Home Office. Sprague–Dawley rats (University of Bristol colony; 75–150 g male/female) were anaesthetised deeply in a saturated environment of halothane, and the WHBP prepared according to the original description by Paton (1996). Every effort was made to minimise the number of animals used. Prior to surgery, the level of anaesthesia was assessed as deep by a complete absence of withdrawal reflexes following a pinch to a paw or the tail. The rat
Control values
In 13 WHBP the mean HR and PNA cycle length were 279±8.4 b.p.m. and 3.6±0.3 s respectively.
Peripheral chemoreceptor reflex and effect of noxious stimulation
Aortic injection of CN to stimulate peripheral chemoreceptors elicited a reflex bradycardia (−121±12 b.p.m.) and decreased PNA cycle length (Table 1 and Fig. 1; n=13; P<0.01). In these preparations, mechanical noxious stimulation of either forelimb elicited a tachycardia (17.5±3 b.p.m.) and decreased PNA cycle length (Table 1 and Fig. 1; n=13; P<0.01); this is quantitatively similar to our recent report (
Discussion
Our results show that the reflex bradycardia following stimulation of peripheral chemoreceptors is attenuated by activation of forelimb nociceptors. This attenuation occurs at the level of the NTS and involves a GABAAergic mechanism. Furthermore, electrical stimulation of somatic afferents in the brachial nerve (to include Aδ and C fibres) can depress the synaptic response of some NTS neurones activated orthodromically by peripheral chemoreceptors. The latter is also mediated by GABAA
Acknowledgements
We thank Tony MacDonald for building the strain gauge used to measure forces generated during mechanical noxious stimulation. P.B. is supported by a University of Bristol postgraduate scholarship. The work was supported by the British Heart Foundation (BS/93003).
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