Dynamics of immediate early gene and neuropeptide gene response to prolonged immobilization stress: evidence against a critical role of the termination of exposure to the stressor

Stress-induced expression of immediate early genes (IEGs) appears to be transient even if the exposure to the stressor persists. However, there are some exceptions which suggest that particular characteristics of stressors can affect the dynamics of IEG expression. We studied in selected telencephalic, diencephalic and brainstem regions the mRNA levels of two clearly distinct IEGs (c-fos and arc) during prolonged exposure to a severe stressor such as immobilization (IMO) and after releasing the rats from the situation. Although regional differences were observed with the two IEGs, overall, c-fos mRNA levels progressively declined over the course of 4 h of continuous exposure to IMO, whereas arc mRNA levels were maintained at high levels in the brain regions that express this gene under stress (telencephalon). Levels of CRF hnRNA in the hypothalamus paraventricular nucleus only slightly declined during prolonged exposure to IMO. Surprisingly, termination of exposure to IMO did not modify CRF gene expression in the paraventricular nucleus or the pattern of IEGs expression, with the exception of c-fos in the lateral septum. Thus, putative signals associated to the termination of exposure to IMO were unable to modify either IEG expression in most brain areas or CRF gene expression in the paraventricular nucleus.     

Phospholipase C-gamma1 is required for the induction of immediate early genes by platelet-derived growth factor

To explore the functional role of phospholipase C-gamma1 (PLC-gamma1) in the induction of immediate early genes (IEGs), we have examined the influence of Plcg1 gene disruption on the expression of 14 IEG mRNAs induced by platelet-derived growth factor (PDGF). Plcg1-null embryos were used to produce immortalized fibroblasts genetically deficient in PLC-gamma1 (Null cells), and retroviral infection of those cells was used to derive PLC-gamma1 re-expressing cells (Null+ cells). In terms of PDGF activation of PDGF receptor tyrosine phosphorylation as well as the mitogen-activated protein kinases Erk1 and Erk2, Null and Null+ cells responded equivalently. However, the PDGF-dependent expression of all IEG mRNAs was diminished in cells lacking PLC-gamma1. The expression of FIC, COX-2, KC, JE, and c-fos mRNAs were most strongly compromised, as the stimulation of these genes was reduced by more than 90% in cells lacking PLC-gamma1. The combination of PMA and ionomycin, downstream analogs of PLC activation, did provoke expression of mRNAs for these IEGs in the Null cells. We conclude that PLC-gamma1 is necessary for the maximal expression of many PDGF-induced IEGs and is essential for significant induction of at least five IEGs.     

Induction of immediate early genes in the mouse auditory cortex after auditory cued fear conditioning to complex sounds

Immediate early genes (IEGs) are widely used as markers to delineate neuronal circuits because they show fast and transient expression induced by various behavioral paradigms. In this study, we investigated the expression of the IEGs c-fos and Arc in the auditory cortex of the mouse after auditory cued fear conditioning using quantitative polymerase chain reaction and microarray analysis. To test for the specificity of the IEG induction, we included several control groups that allowed us to test for factors other than associative learning to sounds that could lead to an induction of IEGs. We found that both c-fos and Arc showed strong and robust induction after auditory fear conditioning. However, we also observed increased expression of both genes in any control paradigm that involved shocks, even when no sounds were presented. Using mRNA microarrays and comparing the effect of the various behavioral paradigms on mRNA expression levels, we did not find genes being selectively upregulated in the auditory fear conditioned group. In summary, our results indicate that the use of IEGs to identify neuronal circuits involved specifically in processing of sound cues in the fear conditioning paradigm can be limited by the effects of the aversive unconditional stimulus and that activity levels in a particular primary sensory cortical area can be strongly influenced by stimuli mediated by other modalities.