Gentle mechanical skin stimulation inhibits the somatocardiac sympathetic C-reflex elicited by excitation of unmyelinated C-afferent fibers

Abstract

The effects of gentle mechanical skin stimulation on reflex discharges in cardiac sympathetic nerve evoked by somatic afferent stimulation were studied in anesthetized rats. Mass discharges were recorded from cardiac sympathetic efferent nerve while somatocardiac sympathetic A- and C-reflexes were elicited by single electrical stimuli to myelinated A- and unmyelinated C-afferent fibers of the tibial nerve. Continuous touch was applied to inner thigh skin with a force of 0.12 N for 10 min periods by a soft elastomer "brush" (1.1cm in diameter with 417 microcones). When touch was applied ipsilateral to the stimulated tibial nerve, the C-reflex was inhibited by up to 40% of its pre-touch amplitude, whereas the A-reflex was unaffected. Inhibition of the C-reflex started during the touch period and lasted for 15 min after cessation of touching. Contralateral touch did not inhibit the C-reflex. The opioid receptor antagonist naloxone attenuated the C-reflex inhibition, but did not abolish it. The C-reflex inhibition was abolished after severing cutaneous nerves innervating inner thigh. We recorded unitary afferent activity from thigh branches of the saphenous nerve and found fibers excited by touch were low-threshold mechanoreceptive Abeta, Adelta and C fibers that have rapidly or slowly adapting properties. In all units tested, average discharge rates during touch period were less than 4 Hz. The results suggest that touch-induced excitation of low threshold cutaneous mechanoreceptive fibers inhibits nociceptive transmission conveyed by C-primary-afferents, via the release of both opioid and non-opioid inhibitory mediators.