Drug-induced home cage conspecifics' behavior can potentiate behavioral sensitization in mice.

The effect of home cage conspecifics' behavior on locomotor sensitization to amphetamine (AMP) or ethanol (ETOH) were investigated. Female mice were repeatedly treated with saline or AMP (2.0 mg/kg for 13 days--Experiment 1) or saline or ETOH (1.8 g/kg for 21 days--Experiment 2) in home cages where all the animals had the same treatment (homogeneous home cages--HOM-HC) or in home cages where half of the animals were drug-treated and half of them were saline-treated (heterogeneous home cages--HET-HC). Behavioral sensitization was evaluated by the quantification of open-field locomotor activity after AMP or ETOH challenge injection, respectively. In both experiments, behavioral sensitization was potentiated in HOM-HC maintained animals. These results suggest that the behavioral sensitization phenomenon can be modified by home cage conspecifics' behavior.

Effects of long-term continuous exposure to light on memory and anxiety in mice.

The studies on the relationship between the light/dark cycle and memory function mostly used protocols of acute disruption of the circadian rhythm. The aim of the present study is to verify the effects of long-term continuous exposure to light on memory, anxiety and motor parameters of mice tested in the plus-maze discriminative avoidance task. Mice were conditioned to choose between the two enclosed arms (one aversive and one non-aversive) while avoiding the open arms of a modified elevated plus-maze apparatus. Memory was evaluated by the time spent in the aversive enclosed arm, anxiety was evaluated by the time spent in the open arms and locomotor behavior was evaluated by number of entries in the arms of the maze. The results showed that long-term (35-42 days) continuous light exposure did not modify memory or anxiety parameters but increased locomotor activity. While the increase in locomotor behavior is in line with previous studies, the unexpected absence of alterations in memory and anxiety (reported to be influenced by the circadian rhythm) is discussed.

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

The cataleptic behavior induced by the dopamine D1 antagonist SCH 23390 (SCH) has proven to be a useful assay for investigating the sensitivity of D1-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 D1 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 D1-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 repeated treatment with the dopamine D2 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.