Measurement of immediate-early gene activation-c-fos and beyond

Immediate-early genes (IEG) are powerful tools for identifying activated neurosecretory neurones and extended circuits that affect neuroendocrine functions. The generally acknowledged scenario is when cells became activated, IEGs expressed and IEG-encoded transcription factors affect target gene expression. However, there are several examples in which: (i) neuronal activation occurs without induction of IEGs; (ii) IEG induction is not related to challenge-induced neuropeptide expression; and (iii) markers of neuronal activation are not expressed in chronically activated neurones. In spite of these limitations, the use of c-Fos and other regulatory- or effector transcription factors as markers of neuronal activation will continue to be an extremely powerful technique. Recently-developed models, including transgenic mice expressing different marker genes under the regulation of IEG promoters, will help to monitor neuronal activity in vivo or ex vivo and to reveal connection between activated neurones. Furthermore, combinations between novel imaging techniques, such as magnetic resonance and IEG-based mapping strategies, will open new means with which to study functional activity in the neurosecretory systems.