Role of the solitary tract nucleus in mediating nociceptive evoked cardiorespiratory responses.

We compared the cardiorespiratory reflex responses evoked by noxious stimulation of the forelimb and cornea. Due to the depressant effects of anaesthesia on visceral reflexes we compared data from an unanaesthetised decerebrate rat model--the working heart-brainstem preparation (WHBP), with the anaesthetised rat. In both experimental models stimulation of the forelimb (mechanical pinch) evoked a tachycardia (WHBP: 19 +/- 2 bpm) and a decrease in respiratory cycle length (WHBP: from 4.1 +/- 0.2 to 2.3 +/- 0.1 s). The magnitude of response in anaesthetised animals depended on anaesthetic depth. Mechanical stimulation of the cornea evoked a bradycardia (-49.2 +/- 4.8 bpm) and an increase in respiratory cycle length from 4 +/- 0.36 to 5.88 +/- 0.2 s which was only present in the WHBP. In the WHBP activation of forelimb and corneal nociceptors both elicited significant pressor effects; in anaesthetised rats there were inconsistent changes in arterial pressure. To determine a role for the nucleus of the solitary tract (NTS) in mediating nociceptive evoked responses in the WHBP, synaptic transmission was blocked reversibly following bilateral microinjections of cobalt chloride. The heart rate responses evoked from either forelimb or corneal nociceptors were attenuated by approximately 50% (P < 0.05). A similar effect was observed using isoguvacine, a GABAA receptor agonist, to hyperpolarise NTS neurones. In conclusion, activation of forelimb and corneal nociceptors evoked contrasting patterns of cardiorespiratory response in the WHBP while in the anaesthetised rat the magnitude of the cardiorespiratory response to forelimb stimulation was quantitatively dependent on anaesthetic dose. In the WHBP, NTS neurones appear important for mediating the cardiac component of the reflex response following stimulation of nociceptive reflex pathways.