Effect of the norepinephrine transporter (NET) inhibition on μ-opioid receptor (MOR)-induced anti-nociception in a bone cancer pain model

Although norepinephrine transporter (NET) inhibition has an additional effect on μ-opioid receptor (MOR)-mediated anti-nociception in inflammatory and neuropathic Pain, its effect on Cancer pain is not well characterized. We investigated the additional effect of NET inhibition on MOR activation using a mouse femur bone Cancer (FBC) Pain model by comparing the anti-nociceptive effect of the dual-acting opioids tramadol and tapentadol and the clinically used MOR-targeted opioids oxycodone and morphine. The anti-nociceptive effects of subcutaneously administered opioids were assessed using the von-Frey filament test. Oxycodone (1 - 10 mg/kg) and morphine (5 - 50 mg/kg) dose-dependently exhibited potent anti-nociceptive effects, whereas tramadol (10 - 56 mg/kg) and tapentadol (10 - 30 mg/kg) exhibited partial effects. Rota-rod analyses of tapentadol at a higher dose (> 30 mg/kg) showed a significant decrease in motor coordination, which was partially recovered by pretreatment with MOR or α(1)-adrenoceptor antagonists. The partial anti-nociceptive effect of tapentadol (30 mg/kg) was completely suppressed by a MOR antagonist, but not by α(1)- or α(2)-adrenoceptor antagonists, suggesting that neither α(1)-adrenoceptor- nor α(2)-adrenoceptor-mediated pathways are involved in anti-nociception in the FBC model. We conclude that addition of NET inhibition does not contribute to MOR-mediated anti-nociception in bone Cancer pain.