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The Journal of Neuroscience, October 15, 1999, 19(20):8839-8848
The weaver Mouse gain-of-function Phenotype of Dopaminergic Midbrain Neurons Is Determined by Coactivation of wvGirk2 and K-ATP Channels
Birgit Liss, Axel Neu, and Jochen Roeper Medical Research Council, Anatomical Neuropharmacology Unit, Department of Pharmacology, Oxford University and Institute for Neural Signaltransduction, Center for Molecular Neurobiology 20246, Hamburg, Germany
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 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. The gain-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 the gain-of-function weaver phenotype of dopaminergic substantia nigra neurons is mediated by coactivation of wvGirk2 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 weaver phenotype of surviving calbindin-positive dopaminergic SN neurons will help to understand the molecular mechanisms of selective neurodegeneration of dopaminergic SN neurons in the weaver mouse and might be important in Parkinson's disease.
Key words: weaver; dopamine; substantia nigra; Girk2; K-ATP channel; single-cell RT-PCR; neurodegeneration; Parkinson's disease
Copyright © 1999 Society for Neuroscience 0270-6474/99/19208839-10$05.00/0
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