Inducing rat brain CYP2D with nicotine increases the rate of codeine tolerance; predicting the rate of tolerance from acute analgesic response
Repeated opioid administration produces analgesic tolerance, which may lead to dose escalation. Brain CYP2D metabolizes codeine to morphine, a bioactivation step required for codeine analgesia. Higher brain, but not liver, CYP2D is found in smokers and nicotine induces rat brain, but not liver, CYP2D expression and activity. Nicotine induction of rat brain CYP2D increases acute codeine conversion to morphine, and analgesia, however the role of brain CYP2D on the effects of repeated codeine exposure and tolerance is unknown. Rats were pretreated with nicotine (brain CYP2D inducer; 1 mg/kg subcutaneously) or vehicle (saline; 1 ml/kg subcutaneously). Codeine (40–60 mg/kg oral-gavage) or morphine (20–30 mg/kg oral-gavage) was administered daily and analgesia was assessed daily using the tail-flick reflex assay. Nicotine (versus saline) pretreatment increased acute codeine analgesia (1.32-fold change in AUC0–60min; p < 0.05) and the rate of loss of peak analgesia (11.42%/day versus 4.20%; p < 0.006) across the first four days of codeine administration (time to negligible analgesia). Inducing brain CYP2D with nicotine did not alter acute morphine analgesia (1.03-fold; p > 0.8), or the rate of morphine tolerance (8.1%/day versus 7.6%; p > 0.9). The rate of both codeine and morphine tolerance (loss in peak analgesia from day 1 to day 4) correlated with initial analgesic response on day 1 (R = 0.97, p < 001). Increasing brain CYP2D altered initial analgesia and subsequent rate of tolerance. Variation in an individual's initial response to analgesic (e.g. high initial dose, smoking) may affect the rate of tolerance, and thereby the risk for dose escalation and/or opioid dependence.