Food deprivation and methadone effects on fixed-interval performance by pigeons

Five adult male White Carneaux pigeons, maintained at approximately 70, 80 or 90% free-feeding weight, were administered methadone (0.5, 1.5, and 2.5 mg/kg) prior to sessions of key pecking maintained under 5-min fixed-interval schedules of food presentation. Increasing deprivation levels attenuated the rate-decreasing effects of methadone. Methadone effects on pause time were unchanged. Three pigeons, maintained at 80% free-feeding weight were then administered methadone hydrochloride (0.62, 1.25 and 2.5 mg/kg) and naloxone hydrochloride (.0001, .0003, .001, .003 or .01 mg/kg) alone and in combination while responding under these same contingencies. Naloxone had no systematic behavioral effects alone but antagonized methadone's effects on fixed-interval responding and pausing. Naloxone also altered methadone's rate-dependent effects. These results suggest the behavioral effects of methadone are differentially altered following food-deprivation and opiate antagonist manipulations.