Panicolytic-like effect of tramadol is mediated by opioid receptors in the dorsal periaqueductal grey

Tramadol is a synthetic opioid prescribed for the treatment of moderate to severe pain, acting as agonist of μ-opioid receptors and serotonin (5-HT) and noradrenaline (NE) reuptake inhibitor. This study evaluated the effects of tramadol in rats submitted to the elevated T-maze (ETM), an animal model that evaluates behavioural parameters such as anxiety and panic. Male Wistar rats were intraperitoneally (i.p.) treated acutely with tramadol (16 and 32 mg/kg) and were submitted to the ETM. Tramadol (32 mg/kg) promoted a panicolytic-like effect. Considering that dorsal periaqueductal grey (dPAG) is the main brain structure related to the pathophysiology of panic disorder (PD), this study also evaluated the participation of 5-HT and opioid receptors located in the dPAG in the panicolytic-like effect of tramadol. Seven days after stereotaxic surgery for implantation of a cannula in the dPAG, the animals were submitted to the test. To assess the involvement of 5-HT1A receptors on the effect of tramadol, we combined the 5-HT1A receptor antagonist, WAY100635 (0.37 nmol), microinjected intra-dPAG, 10 min prior to the administration of tramadol (32 mg/kg, i.p.). WAY100635 did not block the panicolytic-like effect of tramadol. We also associated the non-selective opioid receptor antagonist, naloxone, systemically (1 mg/kg, i.p.) or intra-dPAG (0.5 nmol) administered 10 min prior to tramadol (32 mg/kg, i.p.). Naloxone blocked the panicolytic-like effect of tramadol in both routes of administrations, showing that tramadol modulates acute panic defensive behaviours through its interaction with opioid receptors located in the dPAG.