Population activity in the dorsal hippocampal CA1 encoding the surrounding environment is absent during contextual fear memory expression

Persistent representation of the environment in the hippocampus

In the hippocampus, environmental changes elicit rearrangement of active neuronal ensembles or remapping of place cells. However, it remains elusive how the brain ensures a consistent representation of a certain environment itself despite salient events occurring there. Here, we longitudinally tracked calcium dynamics of dorsal hippocampal CA1 neurons in mice subjected to contextual fear conditioning and extinction training. Overall population activities were significantly changed by fear conditioning and were responsive to footshocks and freezing. However, a small subset of neurons, termed environment cells, were consistently active in a specific environment irrespective of experiences. A decoder modeling study showed that these cells, but not place cells, were able to predict the environment to which the mouse was exposed. Environment cells might underlie the constancy of cognition for distinct environments across time and events. Additionally, our study highlights the functional heterogeneity of cells in the hippocampus.

Differential Phosphorylation of the Glucocorticoid Receptor in Hippocampal Subregions Induced by Contextual Fear Conditioning Training

Aversive events induce the release of glucocorticoid stress hormones that facilitate long-term memory consolidation, an effect that depends on the activation of glucocorticoid receptors (GRs). GRs are distributed widely in the hippocampus. The dorsal region of the hippocampus has been related to cognitive functions and the ventral region to stress and emotion. GR acts as a transcription factor which after hormone binding becomes phosphorylated, affecting its cellular distribution and transcriptional activity. Two functionally well-described GR phosphorylation sites are serine 232 (pSer232), which enhances gene expression, and serine 246 (pSer246), having the opposite effect. Since gene expression is one of the plastic mechanisms needed for memory consolidation, we investigated if an aversive learning task would induce GR phosphorylation in the dorsal (DH) and the ventral (VH) hippocampus. We trained rats in contextual fear conditioning (CFC) using different foot-shock intensities (0.0, 0.5, or 1.5 mA). One subgroup of animals trained with each intensity was sacrificed 15 min after training and blood was collected to quantify corticosterone (CORT) levels in serum. Another subgroup was sacrificed 1 h after training and brains were collected to evaluate the immunoreactivity (IR) to GR, pSer232 and pSer246 by SDS-PAGE/Western blot in DH and VH, and by immunohistochemistry in dorsal and ventral CA1, CA2, CA3, and dentate gyrus (DG) hippocampal regions. The conditioned freezing response increased in animals trained with 0.5 and 1.5 mA during training and extinction sessions. The degree of retention and CORT levels were directly related to the intensity of the foot-shock. Although total GR-IR remained unaffected after conditioning, we observed a significant increase of pSer246-IR in the dorsal region of CA1 and in both dorsal and ventral DG. The only region in which pSer232-IR was significantly elevated was ventral CA3. Our results indicate that fear conditioning training is related to GR phosphorylation in specific subregions of the hippocampus, suggesting that its transcriptional activity for gene expression is favored in ventral CA3, whereas its repressor activity for gene-silencing is increased in dorsal CA1 and in both dorsal and ventral DG.

Immediate-early gene transcriptional activation in hippocampus CA1 and CA3 does not accurately reflect rapid, pattern completion-based retrieval of context memory

No studies to date have examined whether immediate-early gene (IEG) activation is driven by context memory recall. To address this question, we utilized the context preexposure facilitation effect (CPFE) paradigm. In CPFE, animals acquire contextual fear conditioning through hippocampus-dependent rapid retrieval of a previously formed contextual representation. Despite differences in behavior, we did not find any difference in CA1 or CA3 IEG activity associated with this rapid recall phase when comparing context preexposed and non-pre-exposed groups. These findings indicate that IEG activation in CA1 and CA3 is not an accurate readout of the neural activity associated with hippocampus-dependent rapid memory retrieval.