Chronic effects of methadone on a line tilt generalization gradient in the pigeon

Using an adjusting interval schedule, pigeons were trained to key peck at a relatively constant rate during 45 degrees line tilt presentations and at an extremely low rate in the presence of 0 degrees and 90 degrees line angles. Subsequently, the birds received intramuscular injections of methadone hydrochloride every 8 hr in doses beginning at 3 mg/kg/day and increasing progressively to 90 mg/kg/day. Stimulus generalization tests were run on the first day and on the last day a given dose was administered to assess the development of tolerance. Although chronic methadone treatment was associated with an overall decrement in key peck rate, the gradients of line tilt generalization were indistinguishable from saline control gradients at doses as high as 90 mg/kg/day. Though the birds exhibited gross behavioral incoordination, visual stimulus control appeared unaffected.