PLUS-MAZE RETEST PROFILE IN MICE - IMPORTANCE OF INITIAL-STAGES OF TRIAL-1 AND RESPONSE TO POSTTRIAL CHOLINERGIC RECEPTOR BLOCKADE

Recent research has shown that a single undrugged prior experience of the elevated plus-maze produces significant behavioural changes upon 2 4-h retest in rats and mice. Typically, when reexposed to the maze, an imals display an increased avoidance of the open arms and a correspond ing preference for the enclosed sections of the apparatus. Using ethol ogical analyses, the present series of experiments sought to further c haracterize this phenomenon in mice and to determine whether or not it involves cholinergic receptor mechanisms. Results confirmed that beha viour during Trial 2 is markedly different to that seen on initial exp osure, and that such changes are independent of the duration of Trial 1 (2 vs. 5 min). Retest behavioural changes included reduced entry lat encies, reduced open arm entries, less time on the open arms and centr e platform, lower levels of exploratory head-dipping, and increased en tries into and time spent in the closed arms. The importance to the re test phenomenon of the first few minutes of initial exposure was furth er suggested by min-by-min analyses of the behaviour of animals naive to the maze. Results showed that behaviour during the first min is cha racterized by high levels of risk assessment from the centre platform and relatively low, but equal, levels of open- and closed-arm explorat ion. From min 2 onwards, however, behaviour showed a marked change wit h increasing open arm/centre platform avoidance, increasing closed-arm preference, and decreasing levels of risk assessment and exploratory head-dipping. Thus, it would appear that this within-session aversive learning transfers between sessions to account for behavioural profile s on retest. Irrespective of the duration of Trial 1 (2 or 5 min), pos ttrial administration of the muscarinic antagonist, scopolamine (0.1-1 .0 mg/kg), failed to significantly alter the behavioural changes seen between trials. Data are discussed in relation to the apparent sensiti zation of fear produced by plus-maze exposure, its possible relation t o phobia acquisition, and the need for further research on underlying mechanisms.