Roles of the Na+/H+-Exchanger Nhe1 in the Regulation of [Ca2+]I and Glutamate and Gaba Responses in the Central Clock of the Rat Suprachiasmatic Nucleus

The central clock of the hypothalamic suprachiasmatic nucleus (SCN) coordinates the peripheral oscillators to control circadian rhythms in mammals. On the one hand, the SCN neurons exhibit circadian rhythms in spontaneous firing rate, [Ca2+]i, and Na+/Ca2+ exchanger NCX1 activity, and also metabolic rhythms in glucose uptake, cytochrome oxidase activity, and Na/K pump activity. On the other hand, the SCN neurons are metabolically active and sensitive to metabolic perturbation. While evidence suggests that metabolic regulation of the circadian clock is mediated by NPYergic inputs onto the VIP-positive SCN neurons, we recently demonstrate that the AVP-positive, but not VIP-positive, SCN neurons express KATP channels to act as a glucosensor to respond to glucose shortage. Most recently, we show that mitochondria preferentially uptake Ca2+ entering via nimodipine-insensitive Ca2+ channels, suggesting that the VIP-positive SCN neurons, which do not express KATP channels, might still be sensitive to glucose shortage by compromising mitochondrial Ca2+ uptake. Energy metabolism produces H+, which may cause intracellular and extracellular acidifications to impact H+ targets to regulate cellular activity. As the SCN is densely packed with neurons and has higher level of metabolic activity than extra-SCN areas, we hypothesized there be a standing extracellular acidification in the SCN in hypothalamic slices. Indeed, our unpublished result shows a standing extracellular acidification ~0.3 pH unit in the center of SCN, but not in extra-SCN areas. Furthermore, the standing extracellular acidification is partly mediated by H+ extrusion via the Na+/H+ exchanger NHE1. The constitutive activation of NHE1 maintains a more alkaline intracellular pH, which plays an important role in the regulation of nimodipine-sensitive [Ca2+]i. Consistently, immunofluorescent staining indicate high levels of colocalization between NHE1 and CaV1.2 both in the soma and in the presynaptic structures including at least the serotonergic input. The combined results of Ca2+ imaging and immunostaining suggest that NHE1 could regulate local pH around the CaV1.2 channel to influence its activity and thus associated Ca2+ signaling. Further support for a role of the NHE1/CaV1.2 couple in the SCN came from our preliminary results showing that both CaV1.2 and NHE1 contribute to shape the firing and Ca2+ responses to glutamate. This three-year proposal aims to investigate the role of NHE1, as well as energy metabolism, in the central clock of the SCN, focusing on the regulation of glutamate and GABA responses. In the first and second year, we will characterize the Ca2+ dynamics of the glutamate-evoked Ca2+ response and determine its regulation by NHE1, as well as energy metabolism. In the third year, we will determine the role of NHE1 in the regulation of GABAergic signaling.

Project ID:PC10901-0409
External Project ID:MOST108-2320-B182-017-MY3