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Episodic memory, an essential element of orderly thinking, requires the organization of serial events into narratives about the identity of cues along with their locations and temporal order (what, where, and when). The hippocampus plays a central role in the acquisition and retrieval of episodes with two of its subsystems being separately linked to what and where information. The substrates for the third element are poorly understood. Here we report that in hippocampal slices field CA3 maintains self-sustained activity for remarkable periods following a brief input and that this effect is extremely sensitive to minor network perturbations. Using behavioral tests, that do not involve training or explicit rewards, we show that partial silencing of the CA3 commissural/associational network in mice blocks acquisition of temporal order, but not the identity or location, of odors. These results suggest a solution to the question of how hippocampus adds time to episodic memories., Brittney Cox, Conor Cox, Ben Gunn et al. show that brief stimulation of the commissural/associational fibers elicits extended periods of reverberating activity in the CA3 hippocampal region ex vivo. Partial silencing of the CA3 network in vivo inhibits the acquisition of temporal, but not identity or location, information in mice.

The developmental origins of stress-related mental illnesses are well-established, and early-life stress/adversity (ELA) is an important risk factor. However, it is unclear how ELA impacts the maturation of salient brain circuits, provoking enduring vulnerability to stress and stress-related disorders. Here we find that ELA increases the number and function of excitatory synapses onto stress-sensitive hypothalamic corticotropin-releasing hormone (CRH)-expressing neurons, and implicate disrupted synapse pruning by microglia as a key mechanism. Microglial process dynamics on live imaging, and engulfment of synaptic elements by microglia, were both attenuated in ELA mice, associated with deficient signaling of the microglial phagocytic receptor Mer. Accordingly, selective chemogenetic activation of ELA microglia increased microglial process dynamics and reduced excitatory synapse density to control levels. Selective early-life microglial activation also mitigated the adrenal hypertrophy and prolonged stress responses in adult ELA mice, establishing microglial actions during development as powerful contributors to experience-dependent sculpting of stress-related brain circuits.

Hippocampal inhibitory interneurons comprise an anatomically, neurochemically and electrophysiologically diverse population of cells that are essential for the generation of the oscillatory activity underlying hippocampal spatial and episodic memory processes. Here, we aimed to characterize a population of interneurons that express the stress-related neuropeptide corticotropin-releasing hormone (CRH) within existing interneuronal categories through the use of combined electrophysiological and immunocytochemical approaches. Focusing on CA1 strata pyramidale and radiatum of mouse hippocampus, CRH interneurons were found to exhibit a heterogeneous neurochemical phenotype with parvalbumin, cholecystokinin and calretinin co-expression observed to varying degrees. In contrast, CRH and somatostatin were never co-expressed. Electrophysiological categorization identified heterogeneous firing pattern of CRH neurons, with two distinct subtypes within stratum pyramidale and stratum radiatum. Together, these findings indicate that CRH-expressing interneurons do not segregate into any single distinct subtype of interneuron using conventional criteria. Rather our findings suggest that CRH is likely co-expressed in subpopulations of previously described hippocampal interneurons. In addition, the observed heterogeneity suggests that distinct CRH interneuron subtypes may have specific functional roles in the both physiological and pathophysiological hippocampal processes.

Transgenic mice, including lines targeting corticotropin-releasing factor (CRF or CRH), have been extensively employed to study stress neurobiology. These powerful tools are poised to revolutionize our understanding of the localization and connectivity of CRH-expressing neurons, and the crucial roles of CRH in normal and pathological conditions. Accurate interpretation of studies using cell type-specific transgenic mice vitally depends on congruence between expression of the endogenous peptide and reporter. If reporter expression does not faithfully reproduce native gene expression, then effects of manipulating unintentionally targeted cells may be misattributed. Here, we studied CRH and reporter expression patterns in 3 adult transgenic mice: Crh-IRES-Cre;Ai14 (tdTomato mouse), Crfp3.0CreGFP, and Crh-GFP BAC. We employed the CRH antiserum generated by Vale after validating its specificity using CRH-null mice. We focused the analyses on stress-salient regions, including hypothalamus, amygdala, bed nucleus of the stria terminalis, and hippocampus. Expression patterns of endogenous CRH were consistent among wild-type and transgenic mice. In tdTomato mice, most CRH-expressing neurons coexpressed the reporter, yet the reporter identified a few non-CRH-expressing pyramidal-like cells in hippocampal CA1 and CA3. In Crfp3.0CreGFP mice, coexpression of CRH and the reporter was found in central amygdala and, less commonly, in other evaluated regions. In Crh-GFP BAC mice, the large majority of neurons expressed either CRH or reporter, with little overlap. These data highlight significant diversity in concordant expression of reporter and endogenous CRH among 3 available transgenic mice. These findings should be instrumental in interpreting important scientific findings emerging from the use of these potent neurobiological tools.

Highlights • GABAA receptors (GABAARs) curtail stress-induced activation of the HPA axis. • Stressful challenges evoke de novo brain synthesis of GABAAR-active neurosteroids (NS). • NS inhibit the output of CRF-releasing neurones of the hypothalamus. • NS actions in the hypothalamus are blunted in rodent models of early-life adversity. • NS may be important molecular messengers in the programming of the stress-response., Regulation of hypothalamic–pituitary–adrenocortical (HPA) axis activity by stress is a fundamental survival mechanism and HPA-dysfunction is implicated in psychiatric disorders. Adverse early life experiences, e.g. poor maternal care, negatively influence brain development and programs an abnormal stress response by encoding long-lasting molecular changes, which may extend to the next generation. How HPA-dysfunction leads to the development of affective disorders is complex, but may involve GABAA receptors (GABAARs), as they curtail stress-induced HPA axis activation. Of particular interest are endogenous neurosteroids that potently modulate the function of GABAARs and exhibit stress-protective properties. Importantly, neurosteroid levels rise rapidly during acute stress, are perturbed in chronic stress and are implicated in the behavioural changes associated with early-life adversity. We will appraise how GABAAR-active neurosteroids may impact on HPA axis development and the orchestration of the stress-evoked response. The significance of these actions will be discussed in the context of stress-associated mood disorders.

Memory is strongly influenced by stress but underlying mechanisms are unknown. Here, we used electrophysiology, neuroanatomy, and network simulations to probe the role of the endogenous, stress-related neuropeptide corticotropin-releasing hormone (CRH) in modulating hippocampal function. We focused on neuronal excitability and the incidence of sharp waves (SPWs), a form of intrinsic network activity associated with memory consolidation. Specifically, we blocked endogenous CRH using 2 chemically distinct antagonists of the principal hippocampal CRH receptor, CRHR1. The antagonists caused a modest reduction of spontaneous excitatory transmission onto CA3 pyramidal cells, mediated, in part by effects on IAHP. This was accompanied by a decrease in the incidence but not amplitude of SPWs, indicating that the synaptic actions of CRH are sufficient to alter the output of a complex hippocampal network. A biophysical model of CA3 described how local actions of CRH produce macroscopic consequences including the observed changes in SPWs. Collectively, the results provide a first demonstration of the manner in which subtle synaptic effects of an endogenously released neuropeptide influence hippocampal network level operations and, in the case of CRH, may contribute to the effects of acute stress on memory.

Pyramidal cells express various GABA(A) receptor (GABA(A)R) subtypes, possibly to match inputs from functionally distinct interneurons targeting specific subcellular domains. Postsynaptic anchoring of GABA(A)Rs is ensured by a complex interplay between the scaffolding protein gephyrin, neuroligin-2 and collybistin. Direct interactions between these proteins and GABA(A)R subunits might contribute to synapse-specific distribution of GABA(A)R subtypes. In addition, the dystrophin-glycoprotein complex, mainly localized at perisomatic synapses, regulates GABA(A)R postsynaptic clustering at these sites. Here, we investigated how the functional and molecular organization of GABAergic synapses in CA1 pyramidal neurons is altered in mice lacking the GABA(A)R α2 subunit (α2-KO). We report a marked, layer-specific loss of postsynaptic gephyrin and neuroligin-2 clusters, without changes in GABAergic presynaptic terminals. Whole-cell voltage-clamp recordings in slices from α2-KO mice show a 40% decrease in GABAergic mIPSC frequency, with unchanged amplitude and kinetics. Applying low/high concentrations of zolpidem to discriminate between α1- and α2/α3-GABA(A)Rs demonstrates that residual mIPSCs in α2-KO mice are mediated by α1-GABA(A)Rs. Immunofluorescence analysis reveals maintenance of α1-GABA(A)R and neuroligin-2 clusters, but not gephyrin clusters, in perisomatic synapses of mutant mice, along with a complete loss of these three markers on the axon initial segment. This striking subcellular difference correlates with the preservation of dystrophin clusters, colocalized with neuroligin-2 and α1-GABA(A)Rs on pyramidal cell bodies of mutant mice. Dystrophin was not detected on the axon initial segment in either genotype. Collectively, these findings reveal synapse-specific anchoring of GABA(A)Rs at postsynaptic sites and suggest that the dystrophin-glycoprotein complex contributes to stabilize α1-GABA(A)R and neuroligin-2, but not gephyrin, in perisomatic postsynaptic densities.