CHEMOSENSITIVITY AND SENSITIZATION OF NOCICEPTIVE AFFERENTS THAT INNERVATE THE HAIRY SKIN OF MONKEY

1. A large proportion of the cutaneous nociceptor population in monkey either does not respond to mechanical stimuli or has very high mechan ical thresholds (>6 bar). The goal of this study was to determine whet her these mechanically insensitive nociceptive afferents (MIAs) differ from mechanically sensitive nociceptive afferents (MSAs) with regard to responses to chemical stimuli. 2. Teased-fiber techniques were used to record from 28 Adelta-fiber (16 MIAs and 12 MSAs) and 23 C-fiber ( 10 MIAs and 13 MSAs) nociceptors in hairy skin of pentobarbital sodium -anesthetized monkeys. An electrocutaneous search technique was used t o locate the putative receptive fields of the MIAs. The response to me chanical and heat stimuli was determined before and after intradermal injection of a standard mixture of algesic/inflammatory mediators (bra dykinin, histamine, serotonin, and prostaglandin E1). 3. All 25 MSAs, but only 65% of the MIAs, responded to the chemical stimulus. The Adel ta-fibers, both MSAs and responsive MIAs, and the responsive C-fiber M IAs gave a robust discharge. In contrast, the C-fiber MSAs (the conven tional polymodal C-fiber nociceptors) exhibited a significantly weaker response. Three MIAs responded only to the chemical mixture and not t o mechanical or heat stimuli. 4. Before injection of the chemical mixt ure, a significantly smaller proportion of C-fiber MIAs (50%) than of C-fiber MSAs (92%) responded to heat stimuli, whereas a similar propor tion (38%) of Adelta-fiber MIAs and MSAs were heat sensitive. 5. Appro ximately one-half of the MIAs and MSAs were sensitized to mechanical s timuli after the chemical injection, as manifest by a decreased thresh old and / or an enlarged receptive field. 6. The chemical injection se nsitized 90% of Adelta-fiber MSAs, but only 8% of Adelta-fiber MIAs, t o heat stimuli. In contrast, 38% of C-fibers were sensitized. 7. In 14 fibers, the chemical stimulus resulted in sensitization to mechanical stimuli without sensitization to heat stimuli, or vice versa. This di ssociated sensitized state suggests that the molecular mechanisms of s ensitization to heat and mechanical stimuli differ. 8. In conclusion, a large proportion of primate cutaneous nociceptors respond to intrade rmal injection of algesic/inflammatory mediators and may also become s ensitized to mechanical and/or heat stimuli.