Diurnal rhythms in cocaine sensitization and in Period1 levels are common across rodent species.

Circadian and seasonal rhythms in psychostimulant-induced behaviors have been reported in different species including humans. Using inbred mice, we recently reported that both cocaine sensitization and striatal "clock" gene Period1 (PER1 for protein) levels demonstrate a diurnal pattern that is maintained by the rhythm of pineal products N-acetylserotonin (NAS) and melatonin. It is well known that genetic background differences in inbred mice affect their behavioral traits. Therefore, to test whether our initial observations were limited to these mouse strains or whether these traits are common across rodent species we have tested additional strains of mice (CBA/J and AKR/J) and rats (Sprague-Dawley). We found that regardless of the species/strains, subjects with regular NAS and melatonin rhythms present diurnal cocaine sensitization and striatal PER1 rhythm. Since there is a growing interest in clock gene-mediated circadian mechanisms, these results may be important in designing experiments (e.g., time of day and subject strain) to study the role of these genes in psychiatric disorders such as addiction and depression.