Quantitative fluorescence imaging was used to study the regulation of acetylcholine receptor (AChR) number and density at neuromuscular junctions in living adult mice. At fully functional synapses, AChRs have a half-life of about 14 days. However, 2 hours after neurotransmission was blocked, the half-life of the AChRs was now less than a day; the rate was 25 times faster than before. Most of the lost receptors were not quickly replaced. Direct muscle stimulation or restoration of synaptic transmission inhibited this process. AChRs that were removed from nonfunctional synapses resided for hours in the perijunctional membrane before being locally internalized. Dispersed AChRs could also reaggregate at the junction once neurotransmission was restored. The rapid and reversible alterations in AChR density at the neuromuscular junction in vivo parallel changes thought to occur in the central nervous system at synapses undergoing potentiation and depression.

Nanometre-level analyses of the movements of membrane glycoproteins tagged with gold particles demonstrate that diffusing particles are not under the influence of a lipid flow, although a subset of particles which appear attached to the cytoskeleton are moving rearward.

yosin II pulls growth cones in the right direction, as shown by Stephen Turney and Paul Bridg-man (Washington University, St. Louis, MO). Growing neurons in the developing embryo are directed by guidance cues such as laminin-1 (LN1), which steer the extension of neurite growth cones. Bridgman had previously noticed that neuronal growth cones contain high levels of myosin II. As this motor M Neurites turn at the edge of LN1 (red), but crossover without myosin II (right). generates force on the cytoskeleton, he figured it might be involved in turning neurites in response to guidance cues. Such was the case for LN1, as shown by the growth of neurites at borders between LN1 and polyornithine substrates. Normally, growing neurites rapidly retreat from polyornithine and turn back into the laminin surface. But when myosin II activity was inhibited, the neurites ignored the change in substrate and grew over polyornithine. Turning depended on the activation of integrins-the LN1 receptors. The subsequent activation of focal adhesion kinases might activate or recruit myosin II. On polyornithine, both myosin II and focal complexes are randomly distributed. On LN1, however, myosin IIB concentrated in the transitional domain of the growth cone-intermingled with or just behind the new front of focal complexes. Myosin placement in relation to adhesion sites might pull neurites toward more LN1 and away from unwanted substrates.

Competition between neurons for the same synaptic sites at the developing neuromuscular junction drives synaptic rearrangements.

Large-scale microscopy approaches are transforming brain imaging, but currently lack efficient multicolor contrast modalities. We introduce chromatic multiphoton serial (ChroMS) microscopy, a method integrating one‐shot multicolor multiphoton excitation through wavelength mixing and serial block-face image acquisition. This approach provides organ-scale micrometric imaging of spectrally distinct fluorescent proteins and label-free nonlinear signals with constant micrometer-scale resolution and sub-micron channel registration over the entire imaged volume. We demonstrate tridimensional (3D) multicolor imaging over several cubic millimeters as well as brain-wide serial 2D multichannel imaging. We illustrate the strengths of this method through color-based 3D analysis of astrocyte morphology and contacts in the mouse cerebral cortex, tracing of individual pyramidal neurons within densely Brainbow-labeled tissue, and multiplexed whole-brain mapping of axonal projections labeled with spectrally distinct tracers. ChroMS will be an asset for multiscale and system-level studies in neuroscience and beyond.