Chronic cadmium exposure attenuates ethanol-induced hypoalgesia in the adult rat.

Adult male rats were exposed to a diet containing either 100 ppm added cadmium (Cadmium-Diet), or a control diet containing no added chemicals (Control-Diet) for 67 days prior to pain reactivity testing using a tail-flick procedure. Rats were placed in restraining tubes for a 20-min acclimation period, then baseline tail-flick latencies in response to a radiant heat source were measured. Subsequently, half the animals from each group were serially injected intraperitoneally with either 0.5, 1.0, 1.5, or 2.0 g/kg body weight of a 20% v/v ethanol solution, and the other half of the animals were injected with an equivalent volume of saline. Tail-flick latencies were reassessed at 20-min intervals over the next 2 hr. Results indicated dose-dependent ethanol-induced hypoalgesia in the Control-Diet animals for the two highest doses, but ethanol-induced hypoalgesia was evident only at the highest dose for the Cadmium-Diet animals. Further, the magnitude of this hypoalgesic effect was significantly lower for the Cadmium-Diet animals than the Control-Diet animals at the 2.0 g/kg dose. Results are discussed in terms of an attenuation of the pharmacological properties of ethanol by cadmium.