The effects of μ, δ- and ±-opioid receptor antagonists on the pain threshold increase following muscle stimulation in the rat

In a previous study, prolonged low-frequency muscle stimulation, inducing dynamic contractions in the hind leg of unanaesthetized rats, was shown to give rise to a hypoalgesia. The increase in pain threshold, measured as squeak threshold to noxious electric pulses, lasted 3 h. In the present study, the involvement of the endogenous opioid system in the post-stimulatory analgesia was investigated using selective opioid receptor antagonists. The post-stimulatory analgesia was completely reversed back to prestimulatory control levels by naloxone, 1 mg kg-1. ICI 154, 129 and MR 2266 BS, selective δ- and ±-receptor antagonists respectively, did not significantly influence the post-stimulatory analgesia, although ICI 154, 129 had a minor pain threshold-lowering effect. Rats pretreated with β-funaltrexamine, a μ-preceptor antagonist, did not exhibit any post-stimulatory analgesia. These results suggest that opioid systems are involved in the increase in pain threshold after muscle stimulation and that the analgesic response is both elicited and maintained by the μ-preceptor.