Normal spatial reference, working, and episodic-like memory of M
R KO mice in the Morris water maze. (A-E) No significant differences in the conventional hidden platform version of the Morris water maze were detected between genotypes. Escape latency (A), swimming speed (B), and time spent in the perimeter of the pool (C) did not differ significantly between the two genotypes during either original or reversal learning. M4R KO mice, n = 13; wild-type mice, n = 14. In probe trials, both M4R KO mice and wild-type mice selectively searched the location where the platform had been located. M4R KO mice, n = 11; wild-type mice, n = 12. (D) Both genotypes spent significantly more time in the training quadrant (black bars) compared with the other quadrants (opposite quadrant, white bars; right quadrant, dark gray bars; left quadrant, light gray bars) in the probe trials conducted after original training. (E) Also, both genotypes crossed the training site significantly more often than the equivalent sites in the other three quadrants in the probe trials. (F) No significant difference in the delayed matching-to-place (DMP) task of the Morris water maze was detected between genotypes. M4R KO mice showed normal latency in the first five trials of new platform training, averaged over six training sessions. M4R KO mice n = 13; wild-type mice, n = 14.