Like other endogenous enkephalinase inhibitors, human opiorphin peptide (QRFSR) attenuates catabolism of enkephalins and appears to be a promising therapeutic, displaying antinociceptive action in several pain models. However, its opioid-like side-effect profile is insufficiently characterized. In the present set of experiments, acute intraperitoneal administration of opiorphin produced an antinociceptive effect in the tail-flick test in mice (0.3mg/kg) and this action was inhibited by opioid receptor antagonist naloxone (3mg/kg). Repeated treatment with opiorphin changed the antinociceptive response neither to itself nor to morphine, suggesting the lack of tolerance and morphine cross-tolerance, respectively. Repeated treatment with opiorphin (3mg/kg) also failed to produce opioid dependence. Opiorphin (0.3 or 1mg/kg) produced no rewarding effects in the conditioned place preference test. However, at the dose of 3mg/kg, the peptide produced antidepressant-like effect in the forced swim test, and it did not affect locomotor activity. The present set of results confirms the beneficial effects of opiorphin in pain management, and suggests a lack of opioid-like side effects as well as the presence of antidepressant-like actions of this peptide.
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