Oxycodone is a semi-synthetic opioid originally developed as a safer alternative to morphine. It is commonly prescribed for its pain-relieving effects, but has recently been implicated as a major underlying cause of the current opioid epidemic due to its clinical limitations that include tolerance, dependence and a high abuse liability. Simultaneous consumption of opioids and ethanol has been shown to increase the risk of overdose and death from opioids in opioid-tolerant individuals. We hypothesized that ethanol reversed opioid tolerance and previous studies showed that ethanol reversed morphine tolerance. This dissertation investigated whether ethanol reversed tolerance to other opioids in mice, primarily oxycodone. We found that tolerance developed to the antinociceptive effects of both oxycodone and hydrocodone, and that the same dose of ethanol (1 g/kg i.p.) reversed that tolerance. Oral ethanol (2 g/kg) also effectively reversed oxycodone tolerance. Ethanol did not significantly alter either acute or chronic oxycodone brain concentrations, suggesting that the reversal effect was mediated by neuronal mechanisms. DRG neurons were isolated from adult mice and the effects of oxycodone were assessed using whole-cell patch clamp electrophysiology experiments. Oxycodone [3µM] acutely reduced neuronal excitability as measured by a shift in threshold potentials to a more positive value. DRG neurons incubated overnight with 10µM oxycodone did not respond to the 3µM oxycodone challenge, indicating tolerance developed within these neurons. To test if ethanol was reversing tolerance through neuronal mechanisms, we incubated DRG neurons overnight with 10µM oxycodone and applied 20mM ethanol to the media prior to recording. Tolerance was robustly reversed in these neurons, as indicated by a response to 3µM oxycodone. The PKC inhibitor, Bis XI, also reversed oxycodone tolerance.
In these studies we have clearly shown that tolerance develops to oxycodone in both the whole animal in an isolated neuronal preparation. In addition we have shown that the tolerance produced in these two preparations was reversed by ethanol at blood levels similar to those seen in humans. Further we have also included preliminary data that suggest that this reversal of oxycodone tolerance by ethanol may well be due to its actions on PKC
