Mixing and Matching NMDA Subunits
Rana A. Al-Hallaq, Thomas P. Conrads, Timothy D. Veenstra, and Robert J. Wenthold
Much has been written about the subunit composition of hippocampal NMDA receptors and their differential contributions to functions ranging from learning to neurodegeneration. Although NMDA receptors can be di-heteromeric, two NR1 subunits and two identical NR2 subunits, they can also be tri-heteromeric, containing NR1, NR2A, and NR2B subunits. Al- Hallaq et al. set out to examine receptors according to subunit content. The authors performed serial immunoprecipitations on membrane fractions from rat hippocampus. After depleting NR2A or NR2B, they quantified the remaining NR2-containing receptors. About two-thirds of total NR2A and NR2B subunits resided in di-heteromeric receptors, and one-third in tri-heteromeric receptors. Membrane-associated guanylate kinases bound equally to receptors containing NR2A or NR2B, but several synapse-associated proteins associated preferentially with NR2A, whereas collapsin response mediator protein 2 favored NR2B. End of story" Probably not, because only a fraction of NMDA receptors in mature animals could be extracted; thus synaptic receptors may have been underrepresented.
Intrinsic Membrane Properties and Homeostatic Regulation
Kara G. Pratt and Carlos D. Aizenman
Remaining flexible (plastic) in response to ongoing neuronal activity is not limited to synapses, as Pratt and Aizenman demonstrate this week. The authors examined the intrinsic membrane excitability of tadpole optic tectal neurons during the period from early synapse formation (stage 42/43) to stage 49, when tectal field receptive fields become more refined. Measured sodium current amplitudes increased slightly between stage 42/43 and 4446 and then declined by stage 49. The depolarization required to reach spike threshold was also lower at stage 4446 than at stage 49. This
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Society for Neuroscience