Local antinociception induced by endothelin-1 in the hairy skin of the rat's back.

UNLABELLED: Subcutaneous injection of endothelin-1 (ET-1) into the glabrous skin of the rat's hind paw is known to produce impulses in nociceptors and acute nocifensive behavioral responses, such as hind paw flinching, and to sensitize the skin to mechanical and thermal stimulation. In this report, we show that in contrast to the responses in glabrous skin, ET-1 injected subcutaneously into rat hairy skin causes transient antinociception. Concentrations of 1 to 50 microM ET-1 (in 0.05 mL) depress the local nocifensive response to noxious tactile probing at the injection site with von Frey filaments for 30 to 180 minutes; distant injections have no effect at this site, showing that the response is local. Selective inhibition of ET(A) but not of ET(B) receptors inhibits this antinociception, as does coinjection with nimodipine (40 muM), a blocker of L-type Ca(2+) channels. Local subcutaneous injection of epinephrine (45 microM) also causes antinociception through alpha-1 adrenoreceptors, but such receptors are not involved in the ET-1-induced effect. Both epinephrine and ET-1, at antinociceptive concentrations, reduce blood flow in the skin; the effect from ET-1 is largely prevented by subcutaneous nimodipine. These data suggest that ET-1-induced antinociception in the hairy skin of the rat involves cutaneous vasoconstriction, presumably through neural ischemia, resulting in conduction block. PERSPECTIVE: The pain-inducing effects of ET-1 have been well documented in glabrous skin of the rat, a frequently used test site. The opposite behavioral effect, antinociception, occurs from ET-1 in hairy skin and is correlated with a reduction in blood flow. Vasoactive effects are important in assessing mechanisms of peripherally acting agents.

Cutaneous endothelin-A receptors elevate post-incisional pain.

The contribution of endothelin-1 (ET-1), acting via endothelin-A receptors (ET(A)), on post-incisional pain was examined in a rat model of incision through the hairy skin of the lumbar dorsum. Post-incisional mechanical hyperesthesia was evaluated by cutaneous trunci muscle reflexes (CTMR) of subcutaneous muscles responding to stimulation with von Frey filaments near the wound (primary responses) and at a distance, especially on the contralateral dorsum (secondary responses, involving spinal circuits). The role of ET(A) was determined by pre-incisional, subcutaneous injection of the selective receptor antagonist BQ-123 at the incision site, 15 min or 24h before surgery. Control incisions showed both primary tactile allodynia and hyperalgesia, and a weaker secondary hyperesthesia, peaking 3-4h after surgery and lasting at least 24h. Primary allodynia, but not hyperalgesia, was dose-dependently suppressed by 15 min pre-incisional BQ-123. In contrast, both secondary allodynia and hyperalgesia were inhibited by local BQ-123. The suppression of primary allodynia by local antagonist disappeared in 24h, but that of secondary hyperesthesia remained strong for at least 24h. Systemically delivered BQ-123 was without effect on any post-incisional hyperesthesia, and if the antagonist was locally injected 24h before surgery there was no difference on hyperesthesia compared to vehicle injected at that time. We conclude that ET-1, released from skin by incision, activates nociceptors to cause primary allodynia and to sensitize spinal circuits through central sensitization. Blockade of ET(A) in the immediate peri-operative period prevents the later development of central sensitization.

Reduction of postincisional allodynia by subcutaneous bupivacaine: findings with a new model in the hairy skin of the rat.

BACKGROUND: An incision of hairy skin of the rat's back provides a new model for postincisional pain to determine the importance of cutaneous anesthesia. METHODS: Male Sprague-Dawley rats were anesthetized with sevoflurane and given a 0.6-ml subcutaneous injection of bupivacaine (0.25%) under the incision site or the medial lumbar dorsum or at the nuchal midline, 30 min before a 1.0-cm skin incision. Mechanical stimuli (von Frey hairs, 18-250 mN) were applied to measure nociception, indicated by twitching of local subcutaneous muscles, the cutaneus trunci muscle reflex. A graded response score, averaging the twitches weighted by their vigor, or a population response score, measuring the fraction of rats that showed any response, was assessed for 3 days before and over 7 days after incision. von Frey hairs were applied 0.5 cm from the incision to test primary hyperalgesia and 2.0 cm contralateral to the incision for secondary hyperalgesia. RESULTS: Incision induced responses to stimuli that had no effect on intact skin (allodynia) and also enhanced responses to forces that normally gave less than the full reflex (hyperalgesia). Hyperalgesia was present 30 min after surgery, peaked at 3-6 h, and persisted through the week; allodynia had a similar onset but was briefer. Both changes were transiently reversed by subcutaneous morphine (2.5 mg/kg intraperitoneal). Subcutaneous bupivacaine (0.25%), injected preoperatively at the incision site and anesthetizing skin for 2-3 h, suppressed primary allodynia for 1 week but had no effect on hyperalgesia. Secondary allodynia was obliterated, and secondary hyperalgesia attenuated by this treatment. Bupivacaine injected subcutaneously at the nuchal midline before surgery was also effective in abbreviating primary and secondary allodynia, with no signs of sedation, ataxia, or preconvulsive behavior. CONCLUSIONS: Incision of rat hairy skin changes pain responses, similar to pain in humans. Preincisional subcutaneous bupivacaine selectively suppresses and shortens allodynia for times far outlasting its local anesthesia, an effect largely from systemic actions.