Na⁺/Ca²⁺ Exchanger 2 (NCX2) in the Circadian Clock of the Rat Suprachiasmatic Nucleus: Colocalization with Neuropeptides and Daily Profiles of Gene Expression and Protein Levels.

The plasmalemmal Na⁺/Ca²⁺ changer (NCX) regulates intracellular Ca²⁺ by exchanging 3 Na⁺ for 1 Ca²⁺ in either the Ca²⁺ exit or Ca²⁺ entry mode. All three NCX isoforms NCX1, NCX2, and NCX3 are expressed in the rat brain, with isoform-specific differential distribution. In the central clock of suprachiasmatic nucleus (SCN), intracellular Ca²⁺ controls the circadian release of major neuropeptides, which are the arginine vasopressin (AVP), vasoactive intestinal peptide (VIP) and gastrin releasing peptide (GRP), and the NCX, most likely NCX1, rapidly clears depolarization-induced somatic Ca²⁺ influx. However, the role of NCX2 in the SCN remains unknown. This study aimed to investigate the colocalization of NCX2 with neuropeptides and daily expression profiles of NCX2 in mRNA and protein levels. Consistent with the restricted distribution of NCX2 in the retinorecipient ventral SCN, the immunostaining results showed colocalization of NCX2 with VIP, GRP and VIP/GRP in the ventral SCN, but not with AVP in the dorsal SCN, or markers for astrocyte and major input pathways. Importantly, the presynaptic marker Bassoon was found to colocalize with NCX2/GRP and NCX2/ VIP, indicating localization of both VIP/NCX2 and GRP/NCX2 at the presynaptic sites. Furthermore, real-time PCR and western blotting revealed no day-night difference in NCX2 mRNA and protein levels, in contrast to a robust circadian rhythm in the expression of clock genes Per1 and Per2. Together the results suggest a role of NCX2 in the regulation of the release of VIP and GRP.