Functional Roles of the Na+/H+ Exchanger 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. These clock neurons exhibit circadian rhythms in spontaneous firing rate, [Ca2+]i, and Na+/Ca2+ exchanger NCX1 activity. They also exhibit metabolic rhythms, with higher glucose uptake, cytochrome oxidase activity, and Na/K pump activity during the day. The SCN neurons are metabolically active and sensitive to metabolic perturbation. Most recently we show that hypoglycemia selectively inhibits KATP-positive SCN neurons via opening KATP channels. ATP hydrolysis during energy metabolism produces H+, which may cause extracellular acidification to impact external H+ targets to regulate neuronal excitability. While steady pH gradient between the living tissue and the superfusate has been demonstrated in various neural tissues, it is not known if this exists in the SCN. Such knowledge is particularly important, as the SCN neurons are very sensitive to mild extracellular acidification and express acid-sensing ion channels (ASIC), which contain high pH sensitivity of ASIC3 and ASIC1a subunits. Furthermore, the SCN also expresses pH-sensitive two-pore domain K+ channels including TASK1 and TASK3. Other pH-sensitive membrane conductance involved in neurotransmission such as NMDA receptors, GABAA receptors, and voltage-gated calcium channels also play important roles in the regulation of circadian functioning. Together, extracellular acidifications may have impact on ion channels/transporters implicated in the regulation of neuronal excitability and neurotransmission in the circadian clock. 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. For this purpose, we used ion-selective electrodes to measure the extracellular pH (pHo) values in the hypothalamic slice containing the SCN. Our unpublished result shows a standing extracellular acidification ~0.3 pH unit in the center of SCN but not in extra-SCN areas. The standing extracellular acidification in the SCN in hypothalamic slices indicates continuous production and extrusion of H+ out of cells into the extracellular space. We hypothesized that H+ produced during energy metabolism may be extruded by the Na+/H+ exchanger (NHE) to cause standing extracellular acidification in the SCN in hypothalamic slices. This three-year project aims to investigate the functional roles of the NHE in the central clock of the SCN. In the first year, we will determine the role of NHEs in extruding H+ to mediate standing extracellular acidification in the SCN in hypothalamic slices. In the second year, we will determine the role of NHE1 in the regulation of intracellular pH (pHi), Ca2+, and neuronal excitability in the SCN neurons in reduced reparations. In the third year, we will investigate the possible involvements of NHE1 (and NHE5) in the regulation of circadian functioning in the SCN.

Project ID:PC10607-0367
External Project ID:MOST106-2320-B182-014