Benzamil inhibits neuronal and heterologously expressed small conductance Ca²⁺-activated K⁺ channels

Small conductance Ca²⁺-activated K⁺ (SK) channels are expressed throughout the soma and dendrites of pyramidal neurons in the neocortex and hippocampal formation, where they participate in the local regulation of membrane excitability and synaptic signals. Through their inter-play with Ca²⁺ channels, SK channels regulate Ca²⁺ influx triggered by back-propagating action potentials in dendrites. Inhibition of SK channels affects both the amplitude and duration of Ca²⁺ transients, but the role of Ca²⁺ clearance mechanisms and their link to SK channel activity has not been established. Here we report the effect of the Na⁺/Ca²⁺ exchanger (NCX) inhibitor benzamil on Ca²⁺ extrusion and SK channels in the regulation of dendritic Ca²⁺ signals. Benzamil increased the duration and amplitude of dendritic Ca²⁺ transients elicited by back-propagating action potentials in hippocampal pyramidal neurons. This data is consistent with previous studies with SK channel blockers and suggests that benzamil inhibits SK channels in addition to the Na⁺/Ca²⁺ exchanger. Here we show that indeed both the neuronal SK-mediated I_AHP current and the currents mediated by heterologously expressed SK channels were inhibited by benzamil. The inhibition of recombinant SK channels was seen with different K⁺ concentration gradients, and was stronger at negative voltages. The suppression of SK channels by benzamil is consistent with previous findings on the modulation of Ca²⁺ signals by SK channels in neurons. We additionally show that benzamil inhibits neuronal voltage-gated calcium currents. The results prompt a careful reassessment of the effects of benzamil on Ca²⁺ transients in native systems, given the spectrum of ion channels and exchangers this compound targets within a similar range of concentrations.