(B) Chronic naltrindole administration (0,3mg/kg) transiently (session 2) restores motor skill learning deficit in mice lacking GPR88 receptors. (C) In the Y-maze, this treatment increases significantly their number of same arm returns. (D) Naltrindole had no effect on exploration ratios during the elevated plus-maze test in mutants, but normalized their number of stretched attend postures, (E) as well as their latency to feed in the center of the open field in the novelty-suppressed feeding test. More parameters are displayed in Figure S4 in Supplement 1. Data are presented as mean ± SEM. Open stars: *p < 0.05; **p< 0.01; ***p < 0.001 (upper panel: genotype effect; lower panel: treatment effect). Solid stars: *p < 0.05, **p < 0.01, ***p < 0.001 (post-hoc: Newman– Keuls test).