Sculpting the Hippocampus from within: Stress, Spines, and CRH

Why study the effects of stress on the hippocampus?

The hippocampal formation is a complex and highly organized brain structure [1] involved in encoding, storing and retrieving information, i.e., in learning and memory [2,3]. Afferent inputs into the hippocampus provide information from both within and outside of the brain, and this information is parsed and processed through specific molecular, functional and structural synaptic mechanisms [4]. Given that these incoming signals convey messages about a changing and evolving environment, it is teleologically reasonable for the hippocampus to be endowed with mechanisms to recognize the salience of incoming new messages and distinguish critical signals from trivial ones. Of specific, paramount importance is the ability to identify, store and react to potentially life-threatening signals.

Stress is an external or internal signal indicating potential or perceived threat [5–8]. Stress is ubiquitous and is biologically important because it enables both rapid and delayed adaptive processes to a changing environment [5–7]. Indeed, the mammalian brain is equipped with numerous sensing devices to identify stress, as well as mechanisms to respond to--and be influenced by--stressful signals [6,9]. The hippocampus appears to be particularly vulnerable to the effects of stress [6,9–11], although the relationship between stress and hippocampal function is complex, depending on the context and nature of the stress [6,7,12,13]. Mild or short-lasting stress often enhances hippocampal function by augmenting synaptic plasticity, perhaps reflecting the adaptive importance of remembering threatening or dangerous circumstances [6,7,14]. However, these same mechanisms, when activated intensely or for a prolonged period, may render the hippocampus susceptible to detrimental effects of chronic or severe stress [7,14,15]. In fact, chronic stress impairs learning and memory function in both humans and experimental animals [7,10,12–15].

Structural foundations of the actions of stress on hippocampal functions

Stress impacts learning and memory processes, at least in part, through altering the structure of hippocampal neurons [11,15]. These stress-induced changes take place at several levels, ranging from rapid modifications of the synaptic machinery to the eventual restructuring (remodeling) of dendritic branches [9–11]. Indeed, one of the most consistently observed effects of chronic stress on the hippocampus is a reduction (retraction) in the branching of pyramidal cell dendrites [11,16–18]. Dendritic integrity is governed by the presence of functional excitatory synapses, which are located primarily on specialized structures known as dendritic spines [19–21]. Rapid, stress-induced dendritic spine loss has been found in the distribution of eventual dendritic atrophy in adult hippocampus [22–24], suggesting that they are related. Importantly, the number and shape of synapse-bearing spines are dynamic [19–21] and are regulated by factors including neurotransmitters, growth factors and hormones that, in turn, are governed by environmental signals, including stress [25,26]. Thus, a derangement of spine dynamics may provide a mechanism for stress-evoked changes in synaptic function, followed by dendritic loss and cognitive impairments.

Multiple stress mediators shape the hippocampus

Because of the significant impact of stress on hippocampal structure and function, the mechanisms by which stress exerts these effects have been intensely studied [6,7,10]. Multiple mediators regulate the effects of stress on hippocampus (Box 1), and these molecules influence the brain along a continuum of spatial and temporal domains (Figure 1). Glucocorticoids, which are released peripherally in response to stress, can have broad impacts on brain function [5,8,9,27–30], whereas the local release of neurotransmitters and neuropeptides within the hippocampus itself provides for more spatially restricted modulation of specific synaptic populations [6,14]. The repertoire of stress mediators also enables temporal specificity in the regulation of hippocampal neurons [6,14]. Although rapid actions of glucocorticoids have been uncovered [31,32], the receptors of these stress hormones primarily act as transcriptional regulators [30], thereby modulating neuronal function within the timeframe of hours to weeks. In contrast, neurotransmitters and neuropeptides, rapidly released within the hippocampus [6,14], can impact synaptic function and spine dynamics within the timeframe of milliseconds to minutes [6,14]. Working in concert, these diverse signaling pathways provide for the modulation of hippocampal neurons both temporally and spatially, and allow for the fine-tuning of learning and memory processes in response to ever changing environmental conditions [6].

This review focuses on the increasingly recognized contributions of the neuropeptide, corticotropin-releasing hormone (CRH; also known as corticotrophin-releasing factor, CRF), to the structural and functional effects of stress on the hippocampus. The canonical role of CRH is to initiate the neurohormonal response to stress, via its release from cell bodies within the hypothalamic paraventricular nucleus [33]. CRH-expressing neurons are also found in several discrete brain regions, where they contribute to many of the neural and behavioral effects of stress [33,34]. Synthesis and release of CRH within the hippocampus itself is now established [35–37], and this review discusses the function of hippocampal CRH in mediating the effects of stresses on the function and structure of hippocampal synapses, dendritic spines and neurons throughout development and adulthood.

The architecture and operation of the hippocampal CRH system

A substantial population of CRH-producing cells exists within the pyramidal cell layer of the adult hippocampus [35,36,38,39]. Co-localization studies demonstrate that CRH-producing cells within the mature hippocampus are primarily basket- and chandelier-type interneurons, whose axons form perisomatic and axo-axonic synapses on pyramidal cells, respectively [35,36,39] (Figure 2). Although these interneurons synthesize and release the inhibitory neurotransmitter GABA, the physiological actions of CRH in the hippocampus are generally excitatory [40,41]; CRH applied to hippocampal slices increases the firing rates of pyramidal cells via a shortening of the after-hyperpolarization, and in the presence of an excitatory stimulus, essentially “amplifies” this input [41]. Notably, during development, the numbers of CRH-expressing interneurons are particularly high, and these neurons are accompanied by a transient population of CRH-expressing Cajal-Retzius-like cells, suggesting a role for the peptide in hippocampal maturation [35].

CRH exerts its effects via two receptors: CRH receptor type 1 (CRHR₁) and type 2 (CRHR₂), which belong to the superfamily of G-protein coupled receptors [42]. CRHR₁ and CRHR₂ differ in their distribution [37,43,44], as well as their role in mediating behavioral and endocrine responses to stress [45,46]. Pharmacological and physiological data indicate that CRHR₁ is primarily responsible for mediating the synaptic actions of this peptide on hippocampal pyramidal cells [47,48]. CRHR₁ expression is abundant in pyramidal cells [36,37,44,49], where CRHR₂ expression is limited [37,44]. CRHR₁ is found not only on the somata [37], but also at asymmetric (excitatory) post-synaptic densities on dendritic spines [36,50], consistent with an interaction of CRH with excitatory hippocampal neurotransmission [49]. The importance of CRH-CRHR₁ signaling in the stress-induced activation of hippocampal neurons has been demonstrated: short, physical/psychological stress activates CRHR₁-containing pyramidal cells (as indicated by increases in immediate early gene expression), and selective blockade of CRHR₁ prior to the stress prevents this activation [36,51]. Furthermore, knockout studies suggest that CRHR₁ signaling is required for hippocampal plasticity even in the absence of stress: synaptic potentiation is deficient in hippocampal slices from mice lacking CRHR₁ [47], and these mice have learning deficits [52].

Although a role for CRH-CRHR₁ signaling in the response of pyramidal neurons to stress is thus apparent, the source of the peptide is not completely resolved. Theoretically, CRH could arrive at hippocampal synapses from other brain regions [53], such as the amygdala [54], where it is released during stress. However, as mentioned above, CRH is synthesized within a substantial population of hippocampal interneurons, and electron microscopy studies have demonstrated that CRH is stored within releasable vesicle pools at axon terminals of these neurons, suggesting local release of endogenous hippocampal CRH [35,36]. In addition, pharmacological and genetic evidence support a role for endogenous hippocampal CRH in sculpting the hippocampus (Box 2).

The effects of CRH on hippocampal structure and function are dose- and time-dependent

CRH (both endogenous peptide released during stress, as well as exogenous peptide applied in vivo and in vitro), has dose- and time-dependent effects on mature hippocampus, illustrating the complex relationship between stress and hippocampal function. In particular, the duration of the stress crucially influences its consequences on learning and memory processes [6,7,12,14]. Stress (and CRH application) lasting minutes results in structural and functional consequences that differ significantly from those of stress lasting for hours, though both of these time-frames might be considered acute [6,12]. Chronic stress, lasting days and weeks, exerts still more distinctive changes in hippocampal physiology and structure [7,10,14–16] (Table 1).

During minutes-long stress, CRH release potentiates synaptic plasticity [55,56] and primes long-term potentiation (LTP) [57,58], a cellular process generally believed to underlie learning and memory [59,60]. The facilitating role of CRH on hippocampal function is further apparent from studies showing improved acquisition and retention in several hippocampus-dependent tasks upon administration of CRH into the brain[57,58,61–64]. The mechanisms by which CRH influences synaptic function and memory at the seconds-to-minutes time window are not fully clarified, but may involve increased presynaptic glutamate release [41] and/or enhanced postsynaptic excitability [49], at least in part via reduction of after-hyperpolarization [40,41], likely through suppression of slow Ca²⁺-activated K⁺ currents [65].

In contrast to the above, the functional effects of stress and CRH exposure lasting roughly an hour or longer transition into reduced synaptic function and plasticity and impairments of learning and memory [10,14]. Within hippocampus, structural changes after hours-long stress include a loss of dendritic spines within the apical dendrites of CA1 and CA3 pyramidal neurons [23,24,50]. The loss of spines within stratum radiatum depends on CRHR₁ signaling [22,50], although additional stress-induced spine loss might involve signaling by other stress mediators [23,66,67]. CRH application in nanomolar concentrations (to resemble physiological stress-levels, see Box 2) onto hippocampal organotypic slice cultures recapitulates spine loss in a pattern similar to that provoked by stressful events [22], further supporting a role of CRH-CRHR₁ signaling in mediating the cellular effects of stress in the hippocampus. This CRH-mediated selective spine loss, resulting in a net loss of excitatory synapses, is functionally important because it is associated with attenuated LTP selectively within the affected synapses, and with memory defects [50].

When stress extends over days and weeks (ie. chronic stress), it is associated with additional structural changes in hippocampal neurons, including lower total dendritic length and complexity [15–18]. CRH signaling likely contributes to these changes as well: application of presumed stress levels of the peptide chronically (1–2 weeks) onto hippocampal organotypic cultures leads to impoverished dendritic arborization and reduced total dendritic length [68]. Accordingly, preventing CRH-CRHR₁ signaling chronically, via pharmacological blockade or genetic knockout of the CRHR₁ receptor, leads to exuberant dendritic arborization [68]. These structural changes in CRH-treated or chronically stressed hippocampus, together with structural [17,18,69] and functional [28,70] effects of canonical stress mediators (including glucocorticoids) likely contribute to the well-established deficits in learning and memory [10,11,15]. Whether the effects of CRH and glucocorticoids take place independently, or if there are direct interactions among their signaling pathways within the hippocampus, remains to be determined ([6], see Box 3). However, recent studies using forebrain-specific knockout of CRHR₁ suggest that, even in the presence of a functional glucocorticoid system, the absence of CRHR₁ attenuates the detrimental effects of chronic stress on dendritic arborization and spatial memory [71].

Taken together, these lines of research illustrate the complex effects of CRH on hippocampal synaptic plasticity and memory function. Although the optimal range of CRH (both the concentration and duration) for facilitating vs. impairing cognitive function remains unclear, the available data suggest that low-level activation of CRHR₁ for short durations can enhance hippocampal function, whereas longer exposure to high levels of CRH may be deleterious. These data may reflect levels of CRH present at hippocampal synapses during acute and long-lasting stress, respectively (see Box 2).

Sculpting of the hippocampus by stress during development: a role for CRH

Sculpting the aging hippocampus: a role for CRH?

The effects of stress on the aging hippocampus are protean [29], and the role of CRH in the structural and functional changes to the aging hippocampus appears to be dose-dependent. A ‘physiological’ low-level of CRH release might be required for normal cognitive function. In the aging brain, relatively high binding-protein levels sequester the peptide and may impede CRH-CRHR₁ signaling [99]. Thus, strategies that aimed to release endogenous CRH from its binding protein, thereby increasing peptide available to bind CRHR₁, have improved cognitive outcome in aged rodents [99]. Notably, significant reduction in CRH levels, coupled with augmentation of CRHR₁, has been found in patients with Alzheimer’s disease [100,101].

In contrast, subsequent to chronic (or early-life) stress, pathologically high levels of CRH within the hippocampus might be detrimental [74,76]. Stress has been shown to augment tau hyperphosphorylation (a hallmark of neurofibrillary tangles) and β-amyloid levels in transgenic mouse models of Alzheimer’s disease [102,103]. Furthermore, CRH (at high hippocampal levels) may interact with both of these molecules to impair cognitive function [102–104]. Indeed, CRHR₁ blockade prior to the stress prevents the subsequent changes in tau phosphorylation [102] and β-amyloid levels [103], as well as the impairments in learning and memory [102]. Even in the absence of stress, high levels of hippocampal CRH are sufficient to increase both tau phosphorylation [102,104] and β-amyloid levels [103], further confirming a role for CRH in directly regulating these molecules. [102,104][103]Thus, in the aging hippocampus, there may a relative deficit in CRH levels during physiological conditions, but an enhanced sensitivity to the detrimental effects of pathological levels of CRH released during chronic or severe stress [29].

Concluding remarks

The effects of stress on the hippocampus are dynamic and complex, influenced by the duration and context of the stress, as well as by age and gender [5–7,12,13,105]. Because of the prevalence of stress, and the detrimental consequences of chronic stress, the mechanisms by which stress impacts the hippocampus have received significant attention. The intrinsic hippocampal neuro-peptide CRH contributes significantly to these mechanisms, constraining (both temporally and spatially) the neuronal and synaptic populations that are impacted by stress. CRH contributes to stress-adaptive mechanisms that sculpt the hippocampus, designed to improve hippocampal function in acute, threatening circumstances, and perhaps to promote extinction of memories of extremely adverse situations. However, prolongation or hyper-activation of these CRH-mediated mechanisms may lead to maladaptive consequences and long-term cognitive impairments [5–7]. Notably, whereas the current review highlights the important contributions of hippocampal CRH in mediating the effects of stress on the hippocampus, the plethora of structural and functional changes provoked by stress reflects the result of a broad repertoire of stress-activated mediators acting via numerous and likely integrated mechanisms. A key focus for future research is to uncover how CRH interacts with adrenal stress hormones and stress-related neurotransmitters (Box 3). The major challenge will be to examine the interactive, integrated contribution of all of these mediators to the sculpting of the hippocampus by stress.