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The Journal of Neuroscience, March 15, 2003, 23(6):2102

Voltage-Gated Sodium Channels and AnkyrinG Occupy a Different Postsynaptic Domain from Acetylcholine Receptors from an Early Stage of Neuromuscular Junction Maturation in Rats

Sarah J. Bailey, Mark A. Stocksley, Alexandra Buckel, Carol Young, and Clarke R. Slater

School of Neurology, Neurobiology Psychiatry, The Medical School, University of Newcastle upon Tyne, Newcastle upon Tyne NE2 4HH, United Kingdom

Spatial segregation of membrane proteins is a feature of many excitable cells. In skeletal muscle, clusters of acetylcholine receptors (AChRs) and voltage-gated sodium channels (Na_V1s) occupy distinct domains at the neuromuscular junction (NMJ). We used quantitative immunolabeling of developing rat soleus muscles to study the mechanism of ion channel segregation and Na_V1 clustering at NMJs. When Na_V1s can first be detected, at birth, they already occupy a postsynaptic domain that is distinct from that occupied by AChRs. At this time, Na_V1s are expressed only in a diffuse area that extends 50-100 µm from the immature NMJ. However, in the region of the high-density AChR cluster at NMJ itself, Na_V1s are actually present in lower density than in the immediately surrounding membrane. These distinctive features of the Na_V1 distribution at birth are closely correlated with the distribution of ankyrinG immunolabeling. This suggests that an interaction with ankyrinG plays a role in the initial segregation of Na_V1s from AChRs. Both Na_V1 and ankyrinG become clustered at the NMJ itself 1-2 weeks after birth, coincident with the formation of postsynaptic folds. Syntrophin immunolabeling codistributes with AChRs and never resembles that for Na_V1 or ankyrinG. Therefore, syntrophin is unlikely to play an important part in the initial accumulation of Na_V1 at the NMJ. These findings suggest that the segregation of Na_V1 from AChRs begins early in NMJ formation and occurs as a result of the physical exclusion of Na_V1 and ankyrinG from the region of nerve-muscle contact rather than by a process of active clustering.

Key words: neuromuscular junction; sodium channel; ankyrin; syntrophin; rat; development

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Copyright © 2003 Society for Neuroscience  0270-6474/03/2362102-10$05.00/0

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