The weaver mouse gain-of-function phenotype of dopaminergic midbrain neurons is determined by coactivation of wvGirk2 and K-ATP channels

The phenotype of substantia nigra (SN) neurons in homozygous weaver (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 S N neurons, which were unaffected in homozygous weaver mice (wv/wv), dopamin ergic SN neurons possessed a dramatically altered phenotype with a depolari zed membrane potential and complete loss of spontaneous pacemaker activity. The gain-of-function phenotype was mediated by a large, nonselective membr ane conductance exclusively present in (wv/wv) dopaminergic SN neurons. Thi s constitutively activated conductance displayed a sensitivity to external QX-314 (IC50 = 10.6 mu M) very similar to that of heterologously expressed wvGirk2 channels and was not further activated by G-protein stimulation. Si ngle-cell Girk1-4 expression profiling suggested that homomeric Girk2 chann els were present in most dopaminergic SN neurons, whereas Girk2 was always coexpressed with other Girk family members in GABAergic SN neurons. Surpris ingly, acute QX-314 inhibition of wvGirk2 channels did not induce wild-type -like pacemaker activity but instead caused membrane hyperpolarization. Add itional application of a blocker of ATP-sensitive potassium channels (100 m u M tolbutamide) induced wild-type-like pacemaker activity. We conclude tha t the gain-of-function weaver phenotype of dopaminergic substantia nigra ne urons is mediated by coactivation of wvGirk2 and SUR1/Kir6.2-mediated ATP-s ensitive K+ channels. We also show that in contrast to wildtype neurons, al l (wv/wv) dopaminergic SN neurons expressed calbindin, a calcium-binding pr otein that marks dopaminergic SN neurons resistant to neurodegeneration. Th e identification of two ion channels that in concert determine the weaver p henotype of surviving calbindin-positive dopaminergic SN neurons will help to understand the molecular mechanisms of selective neurodegeneration of do paminergic SN neurons in the weaver mouse and might be important in Parkins on's disease.