Dietary cadmium exposure alters characteristics of training, substitution, and tolerance when morphine is used as a discriminative stimulus.

This study examined the possibility that cadmium, a toxicant in high concentration in all tobacco products, may alter the stimulus properties of morphine. Adult male rats were exposed to regular laboratory chow (Group Control) or chow containing 100 ppm added cadmium chloride (Group Cadmium). Following an initial 30 day exposure period, control and cadmium-exposed animals were trained to discriminate between i.p. injections of 3.00 mg/kg morphine sulfate and vehicle (distilled water) in a two-choice drug discrimination task. Subsequently, the morphine dose-effect generalization function (0.75-6.00 mg/kg) was determined for control and cadmium-exposed animals. Additional substitution tests were conducted with increasing doses of the high efficacy mu agonist fentanyl (0.0016-0.04 mg/kg), the intermediate efficacy mu agonist (-)-metazocine (0.60-5.00 mg/kg), and the kappa agonist (+/-)-bremazocine (0.03-0.12 mg/kg). Also, increasing doses of the selective mu antagonist naloxone (0.0008-0.50 mg/kg) were presented against the training dose of morphine (3.00 mg/kg) and 0.02 mg/kg fentanyl. Finally, training was discontinued, and control and cadmium-exposed animals were injected with 8.00 mg/kg morphine in the home cage every 12 hr for 2 weeks, prior to redetermining the morphine dose effect function. Following a 1 week recovery period where morphine injections were discontinued, a final determination of the morphine dose-effect function was made. The results of the investigation indicated that cadmium exposure, without affecting the rate-changing properties of the drugs, slowed initial acquisition of the morphine discrimination, decreased the potency of selective doses of naloxone with respect to antagonizing the stimulus effects of morphine and fentanyl, and blocked the development of tolerance to morphine. Morphine, fentanyl, and (-)-metazocine generalized (substituted) equally across both groups, while (+/-)-bremazocine failed to substitute for the morphine stimulus in either group. These findings add to the growing literature on the interaction between metal poisoning and drug selection/abuse.