Sleep

Hippocampal population activity during the small-amplitude irregular activity state in the rat.Jarosiewicz, B. et al. J. Neurosci. 22, 1373–1384 (2002)

The authors describe a third sleep state in the rat, which is distinct from rapid eye movement (REM) sleep and normal slow-wave sleep. This state, which follows nearly every REM episode and accounts for around 20% of total sleep, is characterized by a few seconds of low-amplitude electroencephalographic activity and is called S-SIA (sleep small-amplitude irregular activity). During S-SIA, only a few hippocampal cells are active. The authors provide evidence that many of these cells have place fields that include the location where the rat is sleeping.

Neurotechniques

Visualization of functionally activated circuitry in the brain.Wilson, Y. et al. Proc. Natl Acad. Sci. USA 99, 3252–3257 (2002)

The authors developed a transgenic model to visualize activated neurons. As the c-fos gene is rapidly induced in neurons after activation, they used its promoter to drive the expression of a tau–lacZ fusion gene. Neuronal activation was accompanied by the expression of β-galactosidase — the lacZ product — which diffused to dendrites and axons. They tested whether physiological stimuli could induce β-galactosidase and found that water deprivation led to protein expression in circuits that participate in water homeostasis. This method might have general applications in tracing studies.

Synaptic physiology

Dependence of EPSP efficacy on synapse location in neocortical pyramidal neurons.Williams, S. R & Stuart, G. J. Science 295, 1907–1910 (2002)

The effect of dendritic location on the potency of a synapse has been the subject of many recent studies. Here the authors show that, in contrast to what has been found in the hippocampus, the size of excitatory potentials in neocortical neurons decreases as distance from the soma increases. However, if distal inputs are co-activated within a narrow time window, then they can provide a significant depolarizing stimulus. So, in neocortical neurons, distal inputs might act as coincidence detectors.