a Swim velocity and b number of complete pool circlings. Times during which rats were not actively swimming (i.e., when floating) were excluded from the velocity calculations. For other details, see legend of Fig. 1

Spatial memory retention: proportion of time spent by rats in the quadrants of the Morris water maze during probe trials. See legend of Fig. 2 for details with exception of the following: rats were treated with either almorexant alone, both almorexant and scopolamine (“combo”), or scopolamine alone. In the examples of individual swim paths, the start location was in the south–west quadrant of the pool, and the platform had formerly been located in the north–east quadrant of the pool

Test of passive avoidance learning: an avoidance memory retention test was conducted 24 h after rats had been trained once on a task of passive avoidance learning under either vehicle, almorexant, combo (both almorexant and scopolamine), or scopolamine treatment. Avoidance memory retention was intact in both vehicle and almorexant-treated rats (see text). Asterisks indicate step-through latency of the respective drug treatment group was statistically different from step-through latency of the vehicle group. Data are presented as mean ± standard error of the mean with 14–16 rats per treatment group

a Experimental design illustrating the 15-day training and assessment protocol of Morris water maze (MWM) experiments. A tick mark indicates that rats received drug treatment on the respective day. b Spatial learning: latency in the MWM task to localize, approach, and climb an escape platform that was visible (cued trials, C1–C2; C3) or hidden (uncued trials, U1–U8). Rats were treated with vehicle only (open circles), almorexant (triangles), or scopolamine (squares) before daily spatial training sessions. Trial blocks C1–C2 and U1–U8 include three trials per rat; trial block C3 includes two trials. Note that retention tests (probe trials P1 and P2, in open triangles) are depicted only to illustrate the order of trial blocks (for results on probe trials, see Fig. 2). Data are presented as mean ± standard error of the mean with eight rats per treatment group

Spatial memory retention: proportion of time spent by rats in the quadrants of the Morris water maze (MWM) during probe trials. After rats had been repeatedly trained on a MWM task under either vehicle, almorexant, or scopolamine treatment (see Fig. 1b), spatial memory retention tests were conducted 24 ± 2 h after a the first week's (P1) and b the second week's (P2) last uncued trial block. No drug treatment was given at P1; drug treatment was given at P2 as on preceding days of spatial training (for a timeline, see Fig. 1a). Black bars represent the target quadrant (the quadrant that had contained the escape platform during the spatial training period); the bar on the left and the two bars on the right of each black bar represent the quadrants to the left, right, and opposite the target quadrant, respectively. Asterisks indicate that the proportion of time spent in the target quadrant was significantly above the chance rate of 25% (p < 0.05, T test for single means against a constant), i.e., spatial memory had been established. Data are presented as mean ± standard error of the mean with eight rats per treatment group. Inserts show representative plots (top view) of individual swim paths of one rat for each treatment group and probe trial. In these examples, the start location was in the north–west quadrant of the pool, and the platform had formerly been located in the south–east quadrant of the pool (indicated by small circle for illustration purposes only; note that no platform was present in probe trials)