Conditioning to injection procedures and repeated testing increase SCH 23390-induced catalepsy in mice

The cataleptic behavior induced by the dopamine D-1 antagonist SCH 23390 (SCH) has proven to be a useful assay for investigating the sensitivity of D-1-like dopamine receptor-mediated effects during chronic drug administration. A fundamental flaw in most of these studies may be the involvement of the "repeated measures effect," a behavioral phenomenon well demonstrated for neuroleptic-induced catalepsy but not yet investigated for dopamine D-2 antagonists. In this study, mice exposed for various sessions to the bar test presented a strong sensitization to the cataleptic behavior induced by repeated SCH treatment. Conversely, single tested animals exhibited a trend toward decreased catalepsy after repeated SCH treatment, which was in line with the development of a D-1-like dopamine receptor supersensitivity suggested by an increase in SKF 38393-induced grooming behavior. Surprisingly, a challenge intraperitoneal saline injection increased the cataleptic behavior of single tested mice after long-term SCH treatment. This "injection-conditioned catalepsy" was also observed after repented treatment with the dopamine D-2 antagonists, haloperidol and metoclopramide. While these findings seem to explain some important contradictory data in the literature, they provide a new and simple animal model of the placebo effect. [Neuropsychopharmacology 21:670-678,1999] (C) 1999 American College of Neuropsychopharmacology. Published by Elsevier Science Inc.