Contingent suppression of tolerance to haloperidol: a dose-response analysis.

Rats were given injections of haloperidol (HAL; 0.625 or 2.5 mg/kg) either before (Before groups) or after (After groups) access to sweetened milk on alternate days. Controls (Saline groups) were given injections of saline. At biweekly intervals ("test days"), all groups were given pretest injections of the drug in order to monitor the development of tolerance in the After and Saline groups. Rats in the Before groups showed no tolerance to the initial suppression of milk intake. In contrast, rats in the After groups had greater intakes, although the level of intake declined on subsequent test days in the group given the lower dose. Rats in the Saline groups drank less on the test days than any of the other groups, suggesting that sensitization occurred. These results are consistent with previous findings (29) that tolerance to HAL is suppressed following pretest injections of the drug. The degree of suppression appears to be inversely related to the frequency of such injections.