Afterhyperpolarization potential modulated by local [K+]o in K+ diffusion-restricted extracellular space in the central clock of suprachiasmatic nucleus

Abstract

Background: Intercellular coupling is essential for the suprachiasmatic nucleus (SCN) to serve as a coherent central clock. Synaptic release of neurotransmitters and neuropeptides is critical for synchronizing SCN neurons. However, intercellular coupling via non-synaptic mechanisms has also been demonstrated. In particular, the abundant perikaryal appositions with morphological specializations in the narrow extracellular space (ECS) may hinder molecular diffusion to allow for ion accumulation or depletion.

Methods: The SCN neurons were recorded in the whole-cell current-clamp mode, with pipette filled with high (26 mM)-Na⁺ or low (6 mM)-Na⁺ solution.

Results: Cells recorded with high-Na⁺ pipette solution could fire spontaneous action potentials (AP) with peak AHP more negative than the calculated value of K⁺ equilibrium potential (E_K) and with peak AP more positive than calculated E_Na. Cells recorded with low-Na⁺ pipette solution could also have peak AHP more negative than calculated E_K. In contrast, the resting membrane potential (RMP) was always less negative to calculated E_K. The distribution and the averaged amplitude of peak AHP, peak AP, or RMP was similar between cells recorded with high-Na⁺ and low-Na⁺ solution pipette. In a number of cells, the peak AHP could increase from more positive to become more negative than calculated E_K spontaneously or after treatments to hyperpolarize the RMP. TTX blocked the Na⁺ -dependent APs and tetraethylammonium (TEA), but not Ba²⁺ or Cd²⁺, markedly reduced the peak AHP. Perforated-patch cells could also but rarely fire APs with peak AHP more negative than calculated E_K.

Conclusion: The result of peak AHP negative to calculated E_K indicates that local [K⁺]_o sensed by the TEA-sensitive AHP K⁺ channels must be lower than bulk [K⁺]_o, most likely due to K⁺ clearance from K⁺ diffusion-restricted ECS by the Na⁺/K⁺-ATPase. The K⁺ diffusion-restricted ECS may allow for K⁺-mediated ionic interactions among neurons to regulate SCN excitability.

Keywords: Action potential; Afterhyperpolarization potential; Extracellular space; Na(+)/K(+)-ATPase; Resting membrane potential; Suprachiasmatic nucleus.