Calcium buffering and clearance in spider mechanosensory neurons

Spider VS-3 mechanoreceptor neurons have a low-voltage-activated Ca²⁺ current that raises intracellular calcium concentration [Ca²⁺] when they are depolarized by agonists of GABA_A receptors or fire action potentials. The Ca²⁺ rise produces negative feedback by modulating the mechanoreceptor current and regulates Ca²⁺- and voltage-activated K⁺ currents. However, nothing is known about Ca²⁺ buffering in VS-3 neurons. Dynamic changes in VS-3 neuron intracellular [Ca²⁺] were measured using the fluorescent Ca²⁺ indicator Oregon Green BAPTA-1 (OG488) to understand Ca²⁺ buffering and clearance. Intracellular OG488 concentration increased slowly over more than 2 h as it diffused through a sharp intracellular microelectrode and spread through the cell. This slow increase was used to measure endogenous Ca²⁺ buffering and clearance by the added buffer technique, with OG488 acting as both added exogenous buffer and Ca²⁺ indicator. [Ca²⁺] was raised for brief periods by regular action potential firing, produced by pulsed electric current injection through the microelectrode. The resulting rise and fall of [Ca²⁺] were well fitted by the single compartment model of Ca²⁺ dynamics. With earlier ratiometric [Ca²⁺] estimates, these data gave an endogenous Ca²⁺ binding ratio of 684. Strong Ca²⁺ buffering may assist these neurons to deal with rapid changes in mechanical inputs.