The Na,K pump regulates decreases in the cholinosensitivity of neurons in the common snail to a cellular analog of habituation: the role of cellular calcium.

The effects of the Na,K pump inhibitor ouabain on the depth of depression of cholinosensitivity of defensive behavior command neurons LPa2, LPa3, RPa3, and RPa2 were studied in the common snail using a cellular analog of habituation; the role of intracellular Ca2+ in these effects was analyzed. Integral acetylcholine-evoked transmembrane currents (ACh currents) were recorded by two-electrode membrane voltage clamping. In one group of neurons, extracellular application of ouabain (0.1 mM) by addition to the bathing solution evoked increases in the depression of the ACh current evoked by rhythmic application of mediator (with interstimulus intervals of 1-3 min) while neurons of the other groups responded with decreases in depression. After spontaneous diffusion of the Ca2+ ion chelator BAPTA (1 mM) from the intracellular microelectrode for 60-150 min, ouabain only increased the level of depression of the ACh current. After intracellular injection of CaCl2 (100 mM), ouabain only decreased the level of depression of the ACh current. It was concluded that inhibition of the Na,K pump modifies depression of the cholinosensitivity of neurons in the cellular model of habituation. The direction of the effect depends on the basal concentration of intracellular Ca2+.