Immediate-early gene Homer1a intranuclear transcription focus intensity as a measure of relative neural activation.

Immediate-early genes (IEGs) exhibit a rapid, transient transcription response to neuronal activation. Fluorescently labeled mRNA transcripts appear as bright intranuclear transcription foci (INF), which have been used as an all-or-nothing indicator of recent neuronal activity; however, it would be useful to know whether INF fluorescence can be used effectively to assess relative activations within a neural population. We quantified the Homer1a (H1a) response of hippocampal neurons to systematically varied numbers of exposures to the same places by inducing male Long-Evans rats to run laps around a track.

Nonuniform allocation of hippocampal neurons to place fields across all hippocampal subfields.

The mechanisms governing how the hippocampus selects neurons to exhibit place fields are not well understood. A default assumption in some previous studies was the uniform random draw with replacement (URDWR) model, which, theoretically, maximizes spatial "pattern separation", and predicts a Poisson distribution of the numbers of place fields expressed by a given cell per unit area. The actual distribution of mean firing rates exhibited by a population of hippocampal neurons, however, is approximately exponential or log-normal in a given environment and these rates are somewhat correlated across multiple places, at least under some conditions.