Opioid Receptors Contribute to Antinociceptive Effect of Tianeptine on Colorectal Distension-Induced Visceral Pain in Rats.

Tianeptine is a clinically effective atypical antidepressant with distinct neurochemical properties. In this study, we aimed to investigate the contribution of opioid receptors in the antinociceptive effect of tianeptine on visceral pain in awake rats and to differentiate the subtype and the localization (central and/or peripheral) of these opioid receptors involved in this antinociception. Visceromotor response to noxious colorectal distension (CRD) was quantified with electromyographic recordings, obtained from previously implanted electrodes into the external oblique musculature of rats under anesthesia, before and after tianeptine administration. The opioid receptor antagonist naloxone hydrochloride (NLX) and peripherally restricted opioid receptor antagonist naloxone methiodide (NLXM) were administered intravenously 10 min before tianeptine (10 mg/kg, i.v.). The antinociceptive effect of tianeptine was abolished by NLX (1 and 2 mg/kg, i.v.), but was partially reduced by NLXM (1 and 2 mg/kg, i.v.). A µ-opioid receptor-selective dose (0.03 mg/kg, i.v.) of NLX, but not NLXM, significantly inhibited the antinociceptive effect of tianeptine. Our results suggest that antinociceptive effect of tianeptine on CRD-induced visceral nociception in rats involves the activation of both central and peripheral opioid receptors.