Daily variation in melatonin receptor expression, cocaine efficacy and ability of melatonin to counteract the effects of cocaine are preserved during early postnatal period in zebrafish. (A) Normal daily pattern of mRNA expression for six melatonin receptors on Day 5, Night 5 and Day 6 post-fertilization in Control group, fold change expressed relative to Day 5 (=1). (B) Change in melatonin receptor expression following Daytime cocaine administration alone (Coc) or following melatonin pre-treatment (MLT+Coc) at 6 dpf, relative to Daytime Control group (=1); (C) Change in melatonin receptor expression following Nighttime cocaine administration alone (Coc) or following melatonin pre-treatment (MLT+Coc) at 6 dpf, relative to Nighttime Control group (=1). Cocaine (0.3 μM, 20 min), melatonin pre-treatment (100 nM, 30 min prior to cocaine). Y axis: fold change relative to corresponding control (=1). N=3–4 samples/treatment group, 20 larvae per sample. Mean±SEM, t-test (vs. control), * p<0.05, ** p<0.01, *** p<0.001 vs. cocaine-only group; One way ANOVA.