Food intake: opioid/purine interactions.

The exogenous opioids butorphanol tartrate (BT) and ethylketocyclazocine (EKC) have been reported to stimulate feeding in rats. In this study we evaluated the effects of purines (known to suppress feeding) and the adenosine antagonist, caffeine, on opioid induced feeding. Adenosine and inosine significantly suppressed BT and EKC induced feeding at various doses and time points. Caffeine enhanced food consumption was suppressed by various doses of naloxone, but was not suppressed by adenosine or inosine. Although caffeine itself induced further feeding, it did not enhance BT induced food consumption. Adenosine and inosine failed to suppress BT induced feeding when 12.5 mg/kg of caffeine was administered to the rats suggesting blockade of the adenosine receptor by caffeine. In contrast to 12.5 mg/kg caffeine, high dose caffeine (50 mg/kg) suppressed BT induced feeding over a 4 hour time period. Adenosine (50 mg/kg) and inosine (50 mg/kg) injected one hour after injection of BT and caffeine (50 mg/kg) reversed the suppressive effect of high dose caffeine in BT induced feeding. These studies indicate that opioid induced feeding can be suppressed by adenosine and inosine. Also, caffeine can reverse the suppressive effect of adenosine and inosine on feeding and vice versa. Naloxone's suppression of caffeine enhanced food consumption indicate that at least part of caffeine's effect on food intake may be mediated through an opioid mechanism.