Diurnal, circadian and photic regulation of calcium/calmodulin-dependent kinase II and neuronal nitric oxide synthase in the hamster suprachiasmatic nuclei.

Mammalian circadian rhythms are entrained by light pulses that induce phosphorylation events in the suprachiasmatic nuclei (SCN). Ca(2+)-dependent enzymes are known to be involved in circadian phase shifting. In this paper, we show that calcium/calmodulin-dependent kinase II (CaMKII) is rhythmically phosphorylated in the SCN both under entrained and free-running (constant dark) conditions while neuronal nitric oxide synthase (nNOS) is rhythmically phosphorylated in the SCN only under entrained conditions. Both p-CaMKII and p-NOS (specifically phosphorylated by CaMKII) levels peak during the day or subjective day. Light pulses administered during the subjective night, but not during the day, induced rapid phosphorylation of both enzymes. Moreover, we found an inhibitory effect of KN-62 and KN-93, both CaMKII inhibitors, on light-induced nNOS activity and nNOS phosphorylation respectively, suggesting a direct pathway between both enzymes which is at least partially responsible of photic circadian entrainment.