Calbindin expression in the hamster suprachiasmatic nucleus depends on day-length

The mammalian circadian clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus controls many physiological and behavioral rhythms. The SCN is compartmentalized in two functionally distinct subregions: a dorsomedial subregion that rhythmically expresses clock genes, and a ventrolateral subregion which, in contrast, mainly expresses clock genes at a constant level. In the golden hamster, this ventrolateral part of the SCN contains a subpopulation of neurons expressing calbindin D28k. This subpopulation has recently been implicated in the control of locomotor rhythmicity. Because both the pattern and level of locomotor activity are affected by day-length, we investigated whether photoperiod also affects calbindin expression. We show that calbindin expression is negatively correlated to the day-length. The number of calbindin immunopositive neurons and calbindin mRNA levels were markedly increased in hamsters exposed to short photoperiods (light/dark cycle [LD] 6:18 and LD10:14) when compared with hamster exposed to long photoperiods (LD18:6 and LD14:10). This suggests that calbindin neurons are involved in the encoding of seasonal information by the SCN.