Prior maze experience required to alter midazolam effects in rats submitted to the elevated plus-maze

In rodents, prior maze experience increases open arm avoidance (OAA) and comprises the anxiolytic effects of benzodiazepines in a subsequent exposure to the evaluated plus-maze (EPM), a phenomenon referred to as "one trial tolerance" (OTT). Nevertheless, a possible correlation between these intriguing events remains unclear. Using maze-naive and maze-experienced (free exploration of the EPM for 5 min) rats, Experiment 1 confirmed the anxiolytic effects of midazolam (MDZ; 0.125 - 1.0 mg/kg) in maze-naive rats, while both increased OAA and OTT to the MDZ anxiolytic effects were observed in maze-experienced rats. However, our results from Experiment 2, designed to assess whether open, enclosed or both arms experience is involved in increased OAA and OTT, showed that MDZ retained its efficacy in rats confined either to an open or enclosed arm, where no significant changes in open arm exploration were observed when compared to the maze-naive group, therefore suggesting that prior experience in the whole apparatus may be involved in the loss of the anxiolytic MDZ effects. Results are discussed in terms of a possible correlation between increased OAA and the OTT phenomenon elicited in a subsequent exposure to the EPM. (C) 2002 Elsevier Science Inc. All rights reserved.