The Acute Effects of Different doses of Tramadol on Neuronal Activity of Medial Prefrontal Cortex in Rats.

Background: Tramadol is an opioid analgesic with monoamine reuptake inhibitory effects. Although tramadol has been widely used to control pain, there is controversy about the risk of abuse. Therefore, in the present study, the acute effects of tramadol on neuronal activity in the medial prefrontal cortex (mPFC), which is one of the important centers of the reward system, were investigated electrophysiologically. Materials and Methods: Tramadol was injected interperitoneally (12.5 and 25 mk/kg) or subcutaneously (40 mg/kg) and its effect on the firing of mPFC neurons was investigated, using in vivo extracellular single unit recording. Results: Tramadol could not significantly affect neural activity in mPFC, suggesting no acute and rapid effect on mPFC. Conclusions: The present results showed that neural activity in mPFC was not rapidly affected by acute application of tramadol. Since the role of mPFC in tramadol addiction has been elucidated, it can be concluded that these effects may be due to delayed responses or chronic use of tramadol.

Effects of selective orexin receptor-2 and cannabinoid receptor-1 antagonists on the response of medial prefrontal cortex neurons to tramadol.

Tramadol is widely used to control pain in various diseases, but the relevant mechanisms are less known despite the severe risks of abuse. The medial prefrontal cortex (mPFC) is one of the critical centers of the reward system. Studies have shown that orexins and endocannabinoids are likely to play an important role in addiction. In this study, the effect of orexin receptor-2 (OX2R) and endocannabinoid receptor-1 (CB1R) blockade on the neuronal activity of mPFC was investigated in response to tramadol in male rats. Tramadol was injected intraperitoneally, and its effects on the firing of mPFC pyramidal neurons were investigated using in vivo extracellular single-unit recording. Tramadol affected the pyramidal neuronal activity of the mPFC. AM251 (18 nmol/4 µl), as a selective CB1R antagonist, and TCS-OX2-29 (50 nmol/4 µl), as a selective OX2R antagonist, individually or simultaneously were microinjected into the lateral ventricle of the brain (intracerebroventricular, ICV). The results showed that the ratio of neurons with the excitatory/inhibitory or no responses was significantly changed by tramadol (p < .05). These changes were prevented by blockade of CB1Rs alone or blockade of OX2Rs and CB1Rs simultaneously (p < .05). However, blockade of these receptors in the vehicle group had no significant effect on neuronal activity. The findings of this study indicate the potential role of orexin and endocannabinoid systems in mediating the effects of tramadol in mPFC and the possible interaction between the two systems via OX2 and CB1 receptors. However, further studies are needed to identify these effects by examining intracellular signaling.