Expression profiles of PER2 immunoreactivity within the shell and core regions of the rat suprachiasmatic nucleus: lack of effect of photic entrainment and disruption by constant light.

The circadian clock cells of the mammalian suprachiasmatic nucleus (SCN) generate oscillations in physiology and behavior that are synchronized (entrained) by the external light/dark (LD) cycle. The mechanisms that mediate the effect of light on the core molecular mechanism of the clock are not well understood, but evidence suggests that the Period2 gene, which encodes a key clock regulator (PER2), might be involved. We assessed the expression of PER2 immunoreactivity in the retinorecipient core and shell compartments of the SCN of rats entrained to cycles of discrete light pulses presented at the early subjective day (dawn) or night (dusk), or housed in constant light. We found that in animals entrained to a 0.5 h:23.5-h LD cycle (light falls near dawn), PER2 expression is rhythmic both in the shell and in the core regions of the SCN and indistinguishable from that seen in the SCN of control rats housed in complete darkness. Similarly, the pattern of PER2 expression in the SCN of rats entrained to a 0.5-h:25.5-h LD cycle (light falls near dusk) resembled that in dark-housed controls. We also found that presentation of a discrete light pulse in the early subjective night did not induce PER2 protein expression in the SCN, even 6 h after photic stimulation. Finally, in constant light-housed, behaviorally arrhythmic rats, PER2 expression in the SCN was low and nonrhythmic. These results show that rhythmic PER2 expression occurs both in the shell and core regions of the rat SCN. Furthermore, they show that the expression of PER2 in the SCN is not regulated by entraining light. Finally, constant light-induced behavioral arrhythmicity is associated with a disruption of rhythmic PER2 expression in the whole SCN. Together, the results are consistent with a proposed role for PER2 in the core mechanism of the circadian clock but argue against an important role for PER2 in the mechanism mediating photic entrainment.