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.

Differences in the brain expression of c-fos mRNA after restraint stress in Lewis compared to Sprague-Dawley rats.

In order to study the contribution of genetic factors to the pattern of stress-induced brain activation, we studied the expression of c-fos mRNA, a marker of neuronal activity, in male Sprague-Dawley and Lewis strains, the latter being known to have a deficient responsiveness of the hypothalamic-pituitary-adrenal (HPA) axis. Immobilization (IMO) alone or combined with the immersion into water at 21 degrees C was applied for 15 or 60 min. The expression of c-fos mRNA was quantified by in situ hybridization in those brain areas that represent important parts of neuronal circuits activated by stress: medial prefrontal cortex, medial amygdala, lateral septum ventral part, paraventricular nucleus of the hypothalamus and locus coeruleus. While in controls, c-fos mRNA was not detectable in tested brain areas, both types of stressors induced a strong expression of this immediate early gene. There were only small differences in c-fos mRNA expression related to the type of stressor or the length of exposure to them. However, there were remarkable differences in the expression between the two rat strains. When compared to Sprague-Dawley rats, Lewis rats showed a reduced c-fos mRNA expression after both stressors in most brain areas, which may be related to the reduced responsiveness of HPA axis and also with other abnormal responses in this strain. However, this hyporesponsiveness was not observed in all brain areas studied, suggesting that there is not a generalized defective c-fos response to stress in Lewis rats and that some responses to stress may be normal in this strain.

Effects of two types of restraint stress on the spontaneous behaviour in rats.

Our previous findings suggested the existence of stressor-specific behavioural and cognitive responses in rats. In the present study, restraint stressor (immobilization, IMO) and restraint stressor combined with partial immersion of rats into water (IMO+C) were applied for 1 hour to Wistar male rats and their spontaneous behaviour was examined in the open field test. The classic behavioural parameters were recorded: crossing, rearing, and resting. When tested 1 and 4 hours after IMO+C, animals exhibited strong suppression of locomotor and exploratory activity (crossing and rearing); partial inhibition of both behavioural variables was found after IMO. Thus, substantial differences were observed in dependence on the length of period between the end of stressor application and the start of testing. In testing performed one week later, the locomotor and exploratory activity levels of both IMO and IMO+C animals corresponded to the control ones. These data suggest a differential behavioural response to both used stressors that may result from their different proportion of psychical and physical components. In conclusion, our results provide other data for the support of differential effects of two types of restraint stressors on spontaneous behaviour of animals exposed to a novel environment.