In the present experiments SPS enhanced GR expression in the hippocampus and prefrontal cortex, replicating findings previously reported by ourselves and others
[16, 18, 19]. EH, on the other hand, attenuated SPS-induced enhancement of GR in the prefrontal cortex, suggesting that EH may be protective against some of the SPS-induced changes implicated in PTSD pathophysiology. Interestingly, while affecting GR in prefrontal cortex, EH did not attenuate SPS-induced enhancement of GR in the hippocampus suggesting that there are regional differences in GR expression following combined effects of early life environment and stressors experienced in adulthood. It has long been suggested that early life experiences may lead to developmental changes that result in stable alterations to HPA axis and potentially confer resilience to later stress. To our knowledge, this is the first report of the combined effects of early life experiences and later “traumatic” stress on GR expression. Given the established role of GR in HPA axis regulation and stress reactivity
[30, 31] as well as in the pathophysiology of the SPS model of PTSD
, these findings may have some implications for understanding mechanisms of resilience to traumatic stress, and of the regional differences that may be critical in moderating the protective effect of early life experiences to later life stressors. While intriguing, the functional implications of these GR expression changes will need to be examined in order to further establish the significance of this finding. In addition, given the heterogeneity of the prefrontal cortex, in particular with respect to regulation of stress reactivity conferred by environmental influences
, it would be interesting to further examine whether different subregions of the prefrontal cortex contribute differentially to the effect reported herein.
Differential changes in GR expression between frontal cortex and hippocampus following stress manipulations have been previously reported by our laboratory
 as well as by others. Indeed, in Meaney’s seminal paper in which the effect of EH on GR in the frontal cortex and hippocampus is first described, GR in the hippocampus was increased in EH animals compared to non-handled controls irrespective of post-weaning housing conditions
. However, this was not the case in the frontal cortex in which post-weaning housing condition moderated GR expression. These data suggest that hippocampal changes in GR may be more stable and enduring than those in the frontal cortex, resonating with our own finding, in which hippocampal GRs were found to be less sensitive to environmental effects than GRs in the prefrontal cortex.
The precise functional role of hippocampal and prefrontal cortex GRs are not known, although a wealth of data suggests that receptor sites within the hippocampus and prefrontal cortex play an important role in the regulation of HPA axis activity
[2, 5]. Recent data from our own laboratory, in which full SPS (comprised of all three stressors) was compared to partial SPS procedures (e.g., restraint + ether or forced swim + restraint), demonstrated that exposure to ether alone was sufficient to alter prefrontal GR levels, while multiple combined stressors were required to alter GR levels in the hippocampus. Moreover, the behavioral data from this study indicated that the combined effect of serial exposure to all three stressors (restraint, forced swim and ether) was required in order to observe extinction retention deficits. These results suggest that the mere enhancements in GR expression in the hippocampus and prefrontal cortex might be insufficient to lead to PTSD-relevant behavioral deficits, but “threshold” change in these regions is required for SPS-induced extinction retention deficits to manifest. Together with the present data, these findings suggest that the ability of EH to attenuate SPS-induced enhancement in prefrontal GR levels should be interpreted with caution as they may not necessarily translate to resilience in PTSD-relevant behavioral outcomes. Addressing this question directly, for instance by examining the effect of EH on extinction retention deficits in SPS animals, will be an important goal of future studies.
Interestingly, in these experiments we did not detect effects of EH alone on total GR expression. This is in contrast to the findings reported by Meaney et al., in which EH was found to increase baseline levels of unbound cytoplasmic GR. There are several possibilities that may explain this apparent discrepancy; EH effects are known to be mediated by FKBP5 protein modulating GR sensitivity to ligands
. Thus, when FKBP5 is bound to GR, binding of glucocorticoids to GRs is reduced. It is therefore possible that EH could increase GR sensitivity by attenuating FKBP5-GR binding. Because radioimmunoassays are typically used to assay unbound cytoplasmic GR, these assays rely on protein-ligand binding and therefore a treatment that increases GR sensitivity could be interpreted as an increase in unbound cytoplasmic GR. Thus, the differing approaches to measuring GR levels may explain these apparently contradictory findings. Alternatively, there were a number of other methodological differences that may underlie the difference in baseline EH findings between the two studies. For example, different strains of rat were used and the age at which GR was measured was different, as were post-weaning housing conditions, all of which have been suggested to impact GR expression
Interestingly, in Meaney’s model
, increases in GR expression are interpreted as functionally beneficial, with EH increasing GR expression and conferring later resilience to stress. Accordingly, prolonged maternal separation, which reduces GR expression, is proposed to have adverse consequences, resulting in vulnerability to later stress. Conversely, our data suggest that GR increases following SPS relate to greater functional impairment
. The differences in the developmental stages at which GR changes are initiated may be critical to the behavioral impact of GR changes, explaining the seemingly conflicting results. The present data shows that EH prevents trauma-induced increases in GR in adult fully-grown animals, thus suggesting that early life EH protects against later increases in GR, possibly because of a more efficient negative feedback system which clamps down the HPA axis response following traumatic stress. Critically, both studies confirm EH results in changes in GR expression that likely result in resilience but further research is clearly needed to examine the precise mechanisms by which EH modulates GR expression following different stressors and in different brain regions.