Types of Skin Afferent Fibers and Spinal Opioid Receptors that Contribute to Touch-Induced Inhibition of Heart Rate Changes Evoked by Noxious Cutaneous Heat Stimulation

Abstract

BackgroundIn anesthetized rats and conscious humans, a gentle touch using a soft disc covered with microcones (with a texture similar to that of a finger), but not with a flat disc, inhibits nociceptive somatocardiac reflexes. Such an inhibitory effect is most reliably evoked when touch is applied to the skin ipsilateral and closest to nociceptive inputs. However, the mechanism of this inhibition is not completely elucidated. We aimed to clarify the types of cutaneous afferent fibers and spinal opioid receptors that contribute to antinociceptive effects of microcone touch.ResultsThe present study comprised two experiments with urethane-anesthetized rats. In the first experiment, unitary activity of skin afferent fibers was recorded from the saphenous nerve, and responses to a 10-min touch using a microcone disc and a flat disc (control) were compared. Greater discharge rate during microcone touch was observed in low-threshold mechanoreceptive Aδ and C afferent units, whereas many Aβ afferents responded similarly to the two types of touch. In the second experiment, the effect of an intrathecal injection of opioid receptor antagonists on the inhibitory effects of microcone touch on heart rate responses to noxious heat stimulation was examined. The magnitude of the heart rate response was significantly reduced by microcone touch in rats that received saline or naltrindole (δ-opioid receptor antagonist) injections. However, such an inhibition was not observed in rats that received naloxone (non-selective opioid receptor antagonist) or Phe-Cys-Tyr-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP; μ-opioid receptor antagonist) injections.ConclusionsMicrocone touch induced greater responses of low-threshold mechanoreceptive Aδ and C afferent units than control touch. The antinociceptive effect of microcone touch was abolished by intrathecal injection of μ-opioid receptor antagonist. These results suggest that excitation of low-threshold mechanoreceptive Aδ and C afferents produces the release of endogenous μ-opioid ligands in the spinal cord, resulting in the inhibition of nociceptive transmission that contributes to somatocardiac reflexes.

Gentle Mechanical Skin Stimulation Inhibits Micturition Contractions via the Spinal Opioidergic System and by Decreasing Both Ascending and Descending Transmissions of the Micturition Reflex in the Spinal Cord

“…There are direct symmetric synaptic connections between endomorphin 2-containing primary afferent terminals and μ-opioid receptor-expressing neurons in the parasympathetic nucleus [ 34 ]. Our recent study also suggested that spinal μ-opioids are involved in the segmental inhibitory mechanism in which noxious heat stimuli-induced cardiac reflexes are inhibited by gentle skin touch [ 35 ]. Such an endogenous opioid system in the sacral cord would inhibit the ascending and/or descending limb of the micturition reflex pathway.…”

Gentle Perineal Skin Stimulation for Control of Nocturia

“…Natural stimulation such as gentle touch is able to activate these non‐noxious nerve fibers without noxious fiber activation. While little is known about the physiological functions of low‐threshold mechanoreceptive Aδ and C fibers, we found that these afferents may contribute to autonomic regulation (Hotta et al, , , ; Watanabe et al, , ). In the following section, the effects of natural stimulation (gentle stimulation) on urinary bladder function are introduced.…”

Interpersonal touch interventions for patients in intensive care: A design‐oriented realist review

“…Dynamic touch may reduce stress and pain more effectively than static touch Proximity to nociceptive input Ascending inhibition of pain signals at the neural gate in the spinal cord. Inhibition (2015), Melzack and Wall (1965), Watanabe, Piché, and Hotta (2015) Reduced pain signalling to the brain, inhibition of somatocardiac reflexes…”