Satiated as well as 24 h food-deprived pigeons showed an attenuated consumption of food in response to peripheral injection of the opiate antagonist naloxone. The minimal effective amount of the drug was found to be 1.5 mg in satiated birds and 0.5 mg in fasted birds. The anorexic effect of naloxone was observed for no longer than 3 h in both situations, and it did not differ at doses of either 1.5 mg or 5 mg of the antagonist. By contrast, doses of up to 5 mg of naloxone failed to depress the water intake of pigeons which were either 24 h water-deprived or were loaded with hypertonic saline. Together with previous studies, these results suggest that, in pigeons, endorphinergic naloxone-sensitive mechanisms are involved in the regulation of feeding but play no major role in the control of drinking. Possible modes of action of opiate antagonists on ingestive behaviour are discussed.
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