TOLERANCE, WITHDRAWAL, AND SUPERSENSITIVITY TO DOPAMINE MEDIATED CUES IN A DRUG-DRUG DISCRIMINATION

Rats were trained to discriminate between 0.25 mg/kg amphetamine (AMPH) and 0.03 mg/kg haloperidol (HAL) in a two-lever drug discrimination task. In order to test for a drug-induced withdrawal state, animals were assigned to one of three chronic treatment groups and given injections of AMPH, HAL, or distilled water (DW) for 10 consecutive days. Subjects from each treatment condition were then tested at 24, 48, or 72 h after the final injection. At the 24 h retest interval, subjects injected with AMPH responded as though administered an acute dose of HAL (0.028 mg/kg) and subjects injected with chronic HAL responded as though administered an acute dose of AMPH (0.15 mg/kg). By 72 h choice behavior had returned to pretreatment values. To determine whether the rebound observed after 10 days of drug treatment was present after a single injection, independent groups of subjects were injected with single doses of either 10 mg/kg AMPH or 1.0 mg/kg HAL and then retested from 4 h to 48 h later. Single doses of both AMPH and HAL produced significant rebounds that peaked between 20 h (AMPH) and 24 h (HAL) following administration. In a third experiment, animals were tested with or without acute doses of drug following pretreatment with either HAL or AMPH. Receptor supersensitivity accounts for the tolerance observed to HAL 24 h after treatment with 1.0 mg/kg HAL, whereas receptor subsensitivity accounts for the tolerance observed 20 h after treatment with 10 mg/kg AMPH.