Diurnal modulation of the Na⁺/K⁺-ATPase and spontaneous firing in the rat retinorecipient clock neurons

The ventral "core" suprachiasmatic nucleus (vSCN) neurons are the retinorecipient neurons in the mammalian circadian clock and maintain a diurnal firing rhythm in reduced preparations. We tested the possibility that daily changes in Na⁺/K⁺-ATPaSe accompany diurnal variation in spontaneous electrical activity. In control, bath application of 9 μM strophanthidin increased the spontaneous firing both at day and night but to different extents. In the presence of 1 mM Ni²⁺ to block spontaneous firing, addition of 9 μM strophanthidin, but not higher concentrations (6.5-20 mM) of external K⁺, induced the silenced cells to fire action potentials in a diurnal rhythm, suggesting a diurnal change in Na ⁺/K⁺-ATPase activity. Consistently, voltage-clamp recordings demonstrated that the pump current blocked by 9 μM strophanthidin was approximately three times larger in daytime than nighttime and was little affected by the presence of 1 mM Ni²⁺. Experiments with various concentrations of strophanthidin further suggests day-night differences in maximum Na⁺/K⁺-ATPase activity, amounting to 6 pA of pump current at day and down to 3.5 pA at night, and in its half-block concentrations, changing from a daytime value of 4 μM to a nighttime value of 8 μM. Our results indicate that the vSCN neurons exhibit a diurnal rhythm in the Na⁺/K⁺-ATPase the activity of which is higher during the day when the firing rate is also higher. Mechanistically, the modulation could be accounted for in terms of changes in the maximum activity of Na ⁺/K⁺-ATPase and its ability to block by strophanthidin.