TRPV1 channel antagonist capsazepine alleviates morphine tolerance and morphine-induced neurotoxicity by preventing mitochondrial damage and apoptosis: an in vivo and in vitro study

Morphine is one of the drugs frequently used for severe pain in chronic diseases such as cancer, but long-term use leads to morphine tolerance. The mechanism of morphine tolerance is not yet fully understood. This study aimed to investigate the effects of the TRPV1 channel antagonist capsazepine (CPZ) on morphine tolerance and morphine-induced neurotoxicity, mitochondrial damage, and apoptosis by in vivo and in vitro methods. Thirty-six male Wistar Albino rats, aged 12-14 weeks (weight 230-250 g), were included in the study. To evaluate the effect of morphine on mitochondrial damage and apoptosis, cytochrome c, apoptosis-inducing factor (AIF), caspase-9, and caspase-3, Bax, and Bcl-2 levels were determined from tissue samples by ELISA and immunohistochemical (IHC) methods. For in vitro analysis, CPZ C6 glioma cells were treated for 1 h, and then neurotoxic morphine (4 mM) was added to the cell medium. Cell viability was measured by the XTT method. Biochemical methods and immunofluorescence staining were used to evaluate mitochondrial damage and apoptosis. The findings indicated that co-administration of CPZ with morphine significantly reduced morphine tolerance (p < 0.05). Furthermore, in vivo and in vitro tests showed that CPZ administration decreased the levels of mitochondrial markers cytochrome c and AIF and proapoptotic markers caspase-3, caspase-9, and Bax and significantly increased the expression of antiapoptotic Bcl-2 (p < 0.01). In conclusion, both in vivo and in vitro test findings demonstrated that CPZ ameliorated morphine-induced mitochondrial dysfunction and attenuated apoptosis, reducing morphine-induced toxicity and tolerance.