Neuroimaging

Imaging input and output of neocortical networks in vivo. Kerr, J. N. D., Greenberg, D. & Helmchen, F. Proc. Natl Acad. Sci. USA 102, 14063–14068 (2005)

A new study has opened the way for detailed imaging of neocortical activity at the single-cell level. Kerr et al. used in vivo two-photon calcium imaging of bulk-labelled tissue in the rat neocortex, and observed strong calcium signals in axonal structures, which are indicative of input activity, and local spike patterns, which reflect output activity. Using these measures, they found that spontaneous neocortical activity is sparse and heterogeneously distributed across neuronal populations.

Memory

Linking new information to a reactivated memory requires consolidation and not reconsolidation mechanisms. Tronel, S. et al. PLoS Biol. 3, e293 (2005)

New memories become stabilized by the process of consolidation. But they can become labile again if they are reactivated by recall, and require another phase of RNA and protein synthesis to be maintained. This process of reconsolidation was a strong candidate for linking new information to reactivated memories. Surprisingly, Tronel et al. show that reconsolidation does not contribute to this process. Instead, molecular mechanisms similar to those underlying consolidation are used to form an association between new and reactivated information.

Synaptic transmission

Heterogeneity in synaptic transmission along a Drosophila larval motor axon. Guerrero, G. et al. Nature Neurosci. 8, 1188–1196 (2005)

To address the question of how transmission strength is distributed among the many connections between a motor neuron and its muscle partner, Guerrero et al. created synapcam, a version of the Ca2+ reporter cameleon. Synapcam can detect Ca2+ influx through postsynaptic glutamate receptors with single-impulse and single-bouton resolution. This approach was combined with presynaptic imaging to show that transmission strength is determined mainly presynaptically, varying in a gradient along the length of axonal branches, with distal connections making stronger functional associations.

Vision

Urochordate βγ-crystallin and the evolutionary origin of the vertebrate eye lens. Shimeld, S. M. et al. Curr. Biol. 15, 1684–1689 (2005)

The formation of the eye lens and the high concentration of crystallins expressed by lens cells are crucial for the ability of the eye to form a clear, focused image on the retina, which is unique to vertebrates. Shimeld and colleagues have now identified a gene in the primitive visual system of the vertebrates' nearest relative — the sea squirt — that is highly homologous to βγ-crystallin. This suggests that the vertebrate βγ-crystallin gene might have evolved from its counterpart in sea squirts.