Responsivity to pain in rats changed by the ingestion of flavored water

Sprague-Dawley male rats drank flavored water and subsequent pain responsivity was assessed using a hot plate first after isotonic saline and later after either 2.75 (Experiment 1) or 2.5 (Experiment 2) mg/kg of morphine hydrochloride was injected. A 48-h exposure to any one of several different flavors resulted in a reduction of the analgesic effects of morphine as shown by an attenuation of the increased latency to paw lick caused by the morphine. This effect was independent of the amount consumed. A 26-h exposure to a flavor decreased pain reactivity before and after morphine as shown by an overall increase in latency to paw lick. This effect was not influenced by whether the normally preferred flavor was made aversive by a previous pairing with lithium chloride-induced illness. These findings are consistent with the idea that many flavors, independently of palatability and amount consumed, increase the release and utilization of endogenous opioids.