Voltage-gated calcium channels mediate intracellular calcium increase in weaver dopaminergic neurons during stimulation of D2 and GABA B receptors

The weaver (wv) mutation affects the pore-forming region of the inwardly rectifying potassium channel (GIRK) leading to degeneration of cerebellar granule and midbrain dopaminergic neurons. The mutated channel (wvGIRK) loses its potassium selectivity, allowing sodium (Na⁺) and possibly calcium ions (Ca²⁺) to enter the cell. Here we performed whole cell patch-clamp recordings combined with microfluorometry to investigate possible differences in calcium ([Ca²⁺]_i) dynamics in native dopaminergic neurons (expressing the wvGIRK2 subunits) in the midbrain slice preparation from homozygous weaver (wv/wv) and control (+/+) mice. Under resting conditions, [Ca²⁺]_i was similar in wv/wv compared with +/+ neurons. Activation of wvGIRK2 channels by D₂ and GABA _B receptors increased [Ca²⁺]_i in wv/wv neurons, whereas activation of wild-type channels decreased [Ca²⁺] _i in +/+ neurons. The calcium rise in wv/wv neurons was abolished by antagonists of the voltage-gated calcium channels (VGCC); voltage clamp of the neuron at -60 mV; and hyperpolarization of the neuron to -80 mV or more, in current clamp, and was unaffected by TTX. Therefore we propose that wvGIRK2 channels in native dopamine neurons are not permeable to Ca²⁺, and when activated by D₂ and GABA_B receptors they mediate membrane depolarization and an indirect Ca²⁺ influx through VGCC rather than via wvGIRK2 channels. Such calcium influx may be the trigger for calcium-mediated excitotoxicity, responsible for selective neuronal death in weaver mice.