SELECTIVE OPIOID INHIBITION OF SMALL NOCICEPTIVE NEURONS

Opioid analgesia, the selective suppression of pain without effects on other sensations, also distinguishes between different types of pain: severe, persistent pain is potently inhibited by opioids, but they fa il to conceal the sensation of a pinprick. The cellular basis for this specificity was analyzed by means of patch-clamp experiments performe d on fluorescently labeled nociceptive neurons (nociceptors) that inne rvate rat tooth pulp. Activation of the mu opioid receptor inhibited c alcium channels on almost all small nociceptors but had minimal effect on large nociceptors. Somatostatin had the opposite specificity, pref erentially inhibiting calcium channels on the large cells. Because per sistent pain is mediated by slow-conducting, small nociceptors, opioid s are thus likely to inhibit neurotransmitter release only at those pr imary synapses specialized for persistent pain.