title: 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: BACKGROUND
  In 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.
  
  
  RESULTS
  The 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.
  
  
  CONCLUSIONS
  Microcone 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.
authors.affiliation: Department of Autonomic Neuroscience, Tokyo Metropolitan Institute of Gerontology, 35-2 Sakaecho, Itabashi-ku, Tokyo, 173-0015, Japan. watanobu@tmig.or.jp.
authors.forename: Nobuhiro
authors.initials: N
authors.lastname: Watanabe
authors.affiliation: Department of Chiropractic, Université du Québec à Trois-Rivières, 3351 Boul. Des Forges, C.P 500, Trois-Rivières, Québec, G9A 5H7, Canada. Mathieu.Piche@uqtr.ca.
authors.forename: Mathieu
authors.initials: M
authors.lastname: Piché
authors.affiliation: Department of Autonomic Neuroscience, Tokyo Metropolitan Institute of Gerontology, 35-2 Sakaecho, Itabashi-ku, Tokyo, 173-0015, Japan. hhotta@tmig.or.jp.
authors.forename: Harumi
authors.initials: H
authors.lastname: Hotta
mesh_terms: D000701:Analgesics, Opioid / Q000494:pharmacology; D000818:Animals; D006339:Heart Rate / Q000502:physiology; D006358:Hot Temperature; D008297:Male; D009270:Naloxone / Q000494:pharmacology*; D009271:Naltrexone / Q000031:analogs & derivatives / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology*; D010146:Pain; D059408:Pain Management; D017208:Rats, Wistar; D011957:Receptors, Opioid / Q000378:metabolism*; D012018:Reflex / Q000187:drug effects; D012867:Skin / Q000294:innervation*; D013116:Spinal Cord / Q000187:drug effects; D014110:Touch / Q000502:physiology*
chemical_list: D000701:Analgesics, Opioid; D009292:Narcotic Antagonists; D011957:Receptors, Opioid; D009270:Naloxone; D009271:Naltrexone; C055382:naltrindole