Endogenous opioid peptides in the control of food intake in cats.
In this report, we investigated the role of exogenous and endogenous enkephalins on food intake in the cat, using, respectively, exogenous [D-Ala2-Met5]-enkephalin (DAME) and acetorphan (Ac) in order to inhibit the degradation of endogenous enkephalins. In addition, the selective peripheral antagonist naltrexone methylbromide (NTxMB) and the nonselective antagonist naloxone (Nx) were used in an attempt to discriminate central and peripheral opioid receptors. In 18-hours food-deprived animals, Ac (5 mg/kg IV) increased milk intake during sham feeding (+18%, p less than 0.05), but did not modify it in feeding conditions. Nx (1 mg/kg SC) reduced milk intake in sham-feeding experiments (-67%, p less than 0.01) more than in milk-feeding conditions (-30%, p less than 0.01). NTxMB (1 mg/kg SC) did not modify milk intake in sham-feeding but decreased it in feeding experiments. In nonfasted animals, Ac did not modify food intake. IV infusion of DAME (50 micrograms/kg) resulted in a reduction of daily food intake (-32%, p less than 0.01). Nx (1 mg/kg SC) decreased the earlier 30 min intake followed by reduction of daily intake (-30%, p less than 0.01). NTxMB (1 and 4 mg/kg SC) increased the 30-min intake dose dependently, without significant change in daily intake. In conclusion, Ac increases food intake in sham-feeding conditions, suggesting that endogenous enkephalins are likely to be involved in the stimulation of food intake. The effects of Nx and NTxMB furthermore suggest both a central activation, and a peripheral inhibition of food intake by opiates when food is allowed to proceed normally through the digestive tract.