TheweaverMousegain-of-functionPhenotype of Dopaminergic Midbrain Neurons Is Determined by Coactivation ofwvGirk2 and K-ATP Channels

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Abstract
The phenotype of substantia nigra (SN) neurons in homozygousweaver (wv/wv) mice was studied by combining patch-clamp and single-cell RT-multiplex PCR techniques in midbrain slices of 14-d-old mice. In contrast to GABAergic SN neurons, which were unaffected in homozygous weaver mice (wv/wv), dopaminergic SN neurons possessed a dramatically altered phenotype with a depolarized membrane potential and complete loss of spontaneous pacemaker activity. Thegain-of-function phenotype was mediated by a large, nonselective membrane conductance exclusively present in (wv/wv) dopaminergic SN neurons. This constitutively activated conductance displayed a sensitivity to external QX-314 (IC50 = 10.6 μm) very similar to that of heterologously expressed wvGirk2 channels and was not further activated by G-protein stimulation. Single-cell Girk1–4 expression profiling suggested that homomeric Girk2 channels were present in most dopaminergic SN neurons, whereas Girk2 was always coexpressed with other Girk family members in GABAergic SN neurons. Surprisingly, acute QX-314 inhibition of wvGirk2 channels did not induce wild-type-like pacemaker activity but instead caused membrane hyperpolarization. Additional application of a blocker of ATP-sensitive potassium channels (100 μm tolbutamide) induced wild-type-like pacemaker activity. We conclude that thegain-of-function weaver phenotype of dopaminergic substantia nigra neurons is mediated by coactivation ofwvGirk2 and SUR1/Kir6.2-mediated ATP-sensitive K+ channels. We also show that in contrast to wild-type neurons, all (wv/wv) dopaminergic SN neurons expressed calbindin, a calcium-binding protein that marks dopaminergic SN neurons resistant to neurodegeneration. The identification of two ion channels that in concert determine the weaverphenotype of surviving calbindin-positive dopaminergic SN neurons will help to understand the molecular mechanisms of selective neurodegeneration of dopaminergic SN neurons in theweaver mouse and might be important in Parkinson’s disease.