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

Abstract

Although immediate-early gene expression analyses using fluorescent in situ hybridization is an effective method to identify recently activated neurons; and non-Boolean variations in transcription foci have been documented, it remains unclear whether there is a systematic relationship between magnitude of neural activation and corresponding RNA signal. Here, we quantified the Homer1a response of hippocampal neurons in rats that ran laps on a familiar track to induce consistent cell firing. A strong linear trend (r2 > 0.9) in INF intensity (brightness) was observed between 1 and 25 laps, after which INF signal dispersed within the nucleus. When the integrated intranuclear fluorescence was considered instead, the linear relationship extended to 50 laps. But there was only an approximate doubling of Homer1a RNA detected for this 50-fold variation in total spiking. Thus, this low-gain dynamic range likely precludes INF intensity as a precise quantitative readout of neural activation, albeit a useful qualitative tool.