[A pharmacological study of the participation of the peripheral endings of primary afferent neurons in the inflammatory response evoked by heat and mechanical noxious stimulation].

To investigate the participation of neuropeptides present in the peripheral endings of primary afferent neurons in the inflammatory response, immunoreactive substance P (iSP), calcitonin gene-related peptide (iCGRP) and neurokinin A (iNKA) levels in the s.c. perfusate, and inflammatory response (edema and plasma protein extravasation) evoked in rat paw by noxious stimulation were determined. The effects of these peptides on plasma protein extravasation in the skin of the hind paw of mice were also examined with the pontamine sky blue protein labelling method. The following results were obtained. 1) Immersion of the rat hind paw for 30 min into hot water adjusted to 47 degrees C led to a marked increase in the release of iSP and iCGRP in the subcutaneous perfusate with the formation of thermal edema. 2) Mechanical stimulation (600 g, 10 min) to the hind paw or electrical stimulation of the saphenous and sciatic nerves (10 V, 2 Hz, 1msec duration, 10 min) evoked the increase of iSP release in the perfusate with plasma protein extravasation. 3) iNKA release was not affected by neither heat nor mechanical stimulation. 4) Intraplantar injection of SP, CGRP and NKA induced plasma protein extravasation, the order of potencies being SP greater than CGRP greater than NKA. The action of SP was antagonized by spantide, an SP antagonist. The injection of CGRP with SP produced a synergistic action on plasma protein extravasation. 5) Neonatal pretreatment with capsaicin, which is known to degenerate small-diameter primary afferent neurons, caused the decrease in amount of iSP and iCGRP released during noxious heat stimulation. 6) Pretreatment with Compound 48/80, or stem bromelain and emorphazone, or des-Arg9-[Leu8]-BK, inhibited the iSP release evoked by noxious heat stimulation. 7) Opioids such as morphine (mu-agonist) and ethylketocyclazocine (kappa agonist) inhibited the heat stimulus-evoked iSP release and thermal edema, and the inhibitory effects were antagonized by pretreatment with their antagonists. 8) Morphine or ethylketocyclazocine or [D-Ala2,D-Leu5]-enkephalin (delta-agonist) inhibited the release of iSP evoked by electrical stimulation of the saphenous and sciatic nerves. These results indicate that SP and CGRP present in peripheral endings of small-diameter primary afferent neurons play an important role in the inflammatory response, and that opioids are involved in the regulation of inflammatory response through the inhibition of SP release.