Reelin, the HPA Axis, and Chronic Stress: Implications for the Neurobiology of Depression

Abstract

Stress and associated dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis is central to major depression etiology, as HPA axis hyperactivity and hypercortisolism are common occurrences in individuals with depression. Reelin, an extracellular matrix glycoprotein that plays numerous roles in brain development and function, is downregulated in several neuropsychiatric disorders. Reelin and its receptors are present throughout the HPA axis, and research has found that reelin levels are decreased in animal models of chronic stress in the hypothalamus. There may therefore be roles for reelin in regulating aspects of healthy HPA axis function and for reelin supplementation in restoring various facets of HPA axis function in chronic stress conditions, particularly those associated with HPA axis inhibition and stimulation. This review aims to summarize proposed roles for reelin in HPA axis activity, with a focus on the direct and indirect roles of reelin in restoring HPA axis homeostasis in chronic stress and depression. This review will explore possible interactions between reelin and the HPA axis in chronic stress conditions in order to expand our understanding of the mechanisms underlying the neurobiology of depression.

Introduction

In recent years, there has been a stronger focus on the role of the neuro-endocrine and neuro-immune systems in contributing to the development and progression of depression. The hypothalamic-pituitary-adrenal (HPA) axis regulates the stress response to modify physiological and behavioral reactions to environmental and internal challenges. Under healthy conditions, the stress response is a self-limiting process that is regulated by negative feedback loops in response to elevated levels of glucocorticoids (GCs). ¹ However, under conditions of intense, frequent, or prolonged stress, negative feedback can be disrupted causing sustained elevations in circulating GCs. ¹ Chronic elevation of GCs has been proposed to cause widespread effects on the brain and body, notably via immune activation, that contribute to the development of depressive symptoms. ²

The Reeler mouse was first identified in 1951 following observations of physical and behavioral abnormalities that included tremors, ataxia, and impaired motor coordination. ³ These characteristics are the results of an autosomal recessive mutation in the reln gene which encodes the reelin protein that was later identified in 1995.^3,4 In embryonic development, reelin is secreted by Cajal-Retzius cells in the marginal zone of the cortex and hippocampus and controls the migration of newborn neurons to ensure proper layering of neurons in the cortex, hippocampus, cerebellum, and spinal cord.^5–7 Accordingly, Reeler mice lack reelin and have aberrant lamination of layered structures of the brain, leading to the aforementioned abnormalities. ⁴

Reelin serves a variety of functions in the adult brain, notably the regulation of synaptic plasticity, dendritogenesis, and neurotransmission.^8,9 Reelin binding induces clustering of very low-density lipoprotein receptor (VLDLR) and apolipoprotein E receptor 2 (ApoER2), and phosphorylation of the intracellular adaptor protein Disabled-1 (Dab1) by Src and Fyn, two Src family kinases.^10,11 Phosphorylation of Dab1 activates downstream signaling cascades through the mechanistic target of rapamycin complex 1 (mTORC1) that promote long-term potentiation (LTP) by driving the membrane insertion and retention of synapse-related proteins and protein complexes such as N-methyl-D-aspartate (NMDA) receptors and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors.^12,13 Reelin binding to ApoER2 modulates synaptic plasticity pre-synaptically by triggering an influx of Ca²⁺ ions which enhances spontaneous neurotransmission without affecting evoked neurotransmission. ¹⁴ Reelin facilitates dendritogenesis and synaptogenesis through N-cofilin-, Rap1-, and Glycogen synthase kinase 3 beta (GSK3β)-mediated modulation of cytoskeletal dynamics to influence the extension and arborization of dendritic processes and dendritic spine maturation.^10,15 In the subgranular zone of the dentate gyrus, reelin promotes the maturation of newborn neurons, as well as their integration into existing circuits. ¹⁶

Hippocampal reelin and peripheral reelin levels are reduced in individuals with depression and in other neuropsychiatric disorders (eg schizophrenia, bipolar disorder, Alzheimer's disease, autism).^17–21 Decreases to reelin-immunoreactive (IR) cell populations and reelin mRNA have been shown in the hippocampus and hypothalamus in preclinical rodent models of chronic stress, and homozygous (reln−/−) and heterozygous (reln+/−) Reeler mice emulate symptoms of a range of neuropsychiatric conditions.^22–25 As depression and chronic stress are commonly associated with disrupted HPA axis function, and evidence suggests that reelin signaling may impact HPA axis activity, there is a possibility that alterations in reelin and reelin signaling pathways are implicated in dysregulated HPA axis function in the context of chronic stress and/or depression.

This narrative review provides a comprehensive overview of our present day understanding of reelin in the HPA axis, highlighting potential mechanisms through which reelin and the HPA axis interact in conditions of chronic stress. However, further research is necessary before a complete understanding can be developed of the numerous mechanisms through which reelin and the HPA axis interact. The goal of this review is to guide future research seeking to expand our current knowledge of reelin, the HPA axis, and the neurobiology of depression.

Neurobiological Progressions of Chronic Stress and Hypothalamic-Pituitary-Adrenal Axis Involvement

The HPA axis is a precisely regulated system that rapidly generates GCs in response to stress which promotes energy mobilization, suppresses immune responses, and modulates emotion, mood, cognition, and learning.^26–29 Parvocellular neurons in the hypothalamic paraventricular nucleus (PVN) release corticotropin-releasing hormone (CRH) through the hypophyseal portal system which directly innervates the anterior pituitary. ³⁰ In response, corticotroph cells of the anterior pituitary gland enhance release of pro-opiomelanocortin, which is cleaved into adrenocorticotropic hormone (ACTH) and various byproducts and released into circulation. ³¹ ACTH stimulates the secretion of GCs (primarily cortisol in humans and corticosterone (CORT) in rodents) from endocrine cells in the zona fasciculata of the adrenal cortex. ³⁰ GCs diffuse across the cell membrane of target cells and bind to cytosolic glucocorticoid receptors (GR), triggering GR trafficking to the nucleus to affect gene transcription. ³² Virtually all tissues of the body express GRs. ³³ As mentioned above, under healthy conditions the HPA axis is tightly regulated through a GC-dependent negative feedback on the hypothalamus and anterior pituitary that dampens the production of both CRH and ACTH. ¹ Under conditions of prolonged stress, however, chronic exposure to GCs results in blunted sensitivity of the HPA axis to inhibition by GCs.^1,34 This blunted sensitivity is partly the result of a downregulation of GRs that impairs homeostatic suppression of the HPA axis, resulting in prolonged elevation of circulating GCs.^35,36

Under healthy conditions, GCs act to suppress immune responses primarily through genomic mechanisms including through the repression of activator protein 1 and nuclear factor-κB, which reduces lymphocyte proliferation and activity and impedes the expression of cytokines, chemokines, and adhesion molecules by innate immune cells. ³⁷ Similar to the HPA axis, immune cells become resistant to the inhibitory effects of GCs under conditions of chronic stress in part through stress-induced disruptions to GR signaling pathways, leading to increased production of pro-inflammatory cytokines.^38,39 Evidence suggests that in the brain, pro-inflammatory cytokines decrease monoamine signaling directly through the inhibition of vesicular monoamine transporter-dependent packaging into secretory vesicles. ⁴⁰ Similarly, these cytokines may decrease signaling indirectly through the promotion of microglia-mediated production of reactive oxygen species and reactive nitrogen species. ⁴⁰ Disruptions to monoamine synthesis and signaling induced by pro-inflammatory cytokines contributes to the mood disturbances observed in chronic stress and depression. Additionally, pro-inflammatory cytokines such as interleukin-6, tumor necrosis factor-α, and interferon-γ bias tryptophan/kynurenine synthesis towards kynurenine, decreasing serotonin synthesis and promoting the inhibition of the excitatory amino acid transporter, leading to excitotoxicity and neuron cell death at glutamatergic terminals. ⁴¹ Pro-inflammatory cytokines also inhibit brain-derived neurotrophic factor (BDNF), a modulator of synaptic function, at glutamatergic terminals.^42,43 In the context of synaptic plasticity, GCs inhibit signaling through the mechanistic target of rapamycin (mTOR), a key component for the transcription of proteins necessary for glutamatergic synapse maturation and LTP. ⁴⁴ As glutamate signaling is necessary for LTP, a reduction or loss of glutamate release, receptors, or clearance experienced in unhealthy or chronically stressed conditions can be detrimental to processes of learning and memory and can contribute to the cognitive symptoms characteristic of depression. ⁴⁴

In addition to immune-related effects, high levels of GCs affect neuron structure and survival. Preclinical animal studies have shown reduced hippocampal volume in response to chronic stress-induced elevation of GCs. ⁴⁵ GCs also suppress cell proliferation and survival of progenitor cells in the dentate gyrus through the inhibition of cyclic adenosine monophosphate response element binding protein (CREB) and BDNF signaling, resulting in reduced neurogenesis. ⁴⁶ Chronic stress induces retraction of apical dendrites in CA3 hippocampal neurons, impairing CA3-CA1 communication. ⁴⁷ In addition to effects in the hippocampus, GCs have been shown to impair frontal lobe activity and induce cortical atrophy.^48–51 In consideration of the primary functions of the hippocampus in memory and the frontal cortex in executive function, these findings suggest that increased activity of the HPA axis elevates GC levels which in turn disrupts neuronal survival and maturation, leading to difficulties with learning, trouble concentrating, and impaired problem-solving abilities. ⁵²

Reelin Within the Hypothalamic-Pituitary-Adrenal Axis

Reelin is expressed in all the three primary components of the HPA axis: the hypothalamus, pituitary gland, and adrenal glands.^53–55 The presence of reelin and its two primary receptors throughout the HPA axis therefore puts forth a possible role for reelin signaling in HPA axis activity.^55,56

Reelin is Expressed throughout the HPA Axis

Many studies of reelin expression in the hypothalamus have focused on reelin in development, although recent research has begun to explore hypothalamic reelin expression in adulthood. A steady increase in reelin expression in several hypothalamic structures has been observed in developing mice embryos. ⁵³ The PVN of the hypothalamus, central to the initiation of the body's response to stressors, is an area of particularly high reelin expression in utero, however, reelin mRNA is also present in the lateral, anterior, and supraoptic divisions of the hypothalamus during this period. Reelin mRNA was detected in the hypothalamic arcuate nucleus (ARC), dorsomedial nucleus (DMH), and PVN of adult mice aged 12–24 weeks, providing support for the emerging concept of reelin expression in the adult hypothalamus. ⁵⁴ Similarly, another study of adult rats identified a large number of reelin-IR cells in the PVN in both males and females, providing further support for this idea (see Table 1). ⁵⁷ This same study took a deeper look at reelin co-localization in the PVN, determining that seventeen to thirty percent of reelin-IR cells co-localized with parvocellular oxytocin-positive neurons and suggesting that reelin may be expressed adjacent to or within oxytocin-positive neurons in the PVN. ⁵⁷ Additional study of reelin in PVN neurons found some co-localization of reelin-IR cells with parvocellular CRH-positive neurons and magnocellular arginine vasopressin (AVP)-positive neurons, although this co-localization was minimal (Figure 1). ⁵⁷

Table 1.

Summary of the Literature Discussed in the Text Describing the Location of Reelin and its Receptors VLDLR and ApoER2 Throughout the HPA Axis.

Molecule	Organ	Region	Reference
Reelin protein	Hypothalamus	Paraventricular nucleus, arcuate nucleus, lateral nucleus, anterior nucleus, dorsomedial nucleus	Alcántara et al 53 , Roberts et al 54 , Sánchez-Lafuente et al 57
	Pituitary gland	Pars intermedia	Smalheiser et al 55
	Adrenal gland	Adrenal medulla	Smalheiser et al 55
VLDLR	Hypothalamus	Paraventricular nucleus, arcuate nucleus, suprachiasmatic nucleus, ventromedial nucleus	Roberts et al 54
	Pituitary gland	Unknown
	Adrenal gland	Zona glomerulosa	Xing et al 56
ApoER2	Hypothalamus	Paraventricular nucleus, arcuate nucleus, suprachiasmatic nucleus, dorsomedial nucleus, ventromedial nucleus	Roberts et al 54
	Pituitary gland	Unknown
	Adrenal gland	Unknown

Figure 1.

Summary of the regions of the endocrine organs of the HPA axis in which the reelin protein has been identified (A). Summary of the HPA axis, including the hormones implicated in HPA axis function (B). Summary of the PVN functions in which reelin is likely implicated (C). Proposed roles for reelin are denoted by *. Created in BioRender. Thom, S.A. (2025). https://BioRender.com/uy4gmzq.

The two primary reelin receptors VLDLR and ApoER2 have also been detected within the adult hypothalamus. As with reelin mRNA, VLDLR mRNA was detected in the PVN and ARC as well as in the ventromedial nucleus (VMH) and suprachiasmatic nucleus (SCN). ⁵⁴ ApoER2 mRNA was found to be abundant in all of these regions in addition to the DMH.

Reelin immunoreactivity has also been detected in certain cells of the pituitary pars intermedia that co-express α-melanocyte-stimulating hormone, a neuropeptide with homeostatic and anti-inflammatory functions capable of stimulating the HPA axis (see Table 1).^55,58,59 Reelin immunoreactivity has also been identified in chromaffin cells of the adrenal medulla in female rats (Figure 1). ⁵⁵ These cells primarily produce the catecholamines epinephrine and norepinephrine, both of which are necessary for the activation of the sympathetic nervous system in response to acute stress.^60,61 VLDLR mRNA has also been detected in the zona glomerulosa of the adrenal gland, where it may be implicated in the VLDL-stimulated production of aldosterone. ⁵⁶ Adrenal production of aldosterone is similarly stimulated by pituitary-derived ACTH (see Table 1). ⁶²

Reelin in Healthy HPA Axis Function

The activation of CRH transcription in the PVN requires the phosphorylation of CREB. ⁶³ Reelin binding to VLDLR and ApoER2 initiates a signaling cascade that promotes CREB phosphorylation via increased Ca²⁺ influx through NMDARs, and reelin supplementation rapidly induces transient CREB phosphorylation in the hippocampus.^9,64 Although there is no specific evidence for reelin-induced CREB phosphorylation in the PVN (to the author's knowledge), reelin may contribute to the initiation of the stress response through CREB phosphorylation in the PVN in a similar manner to hippocampal processes, as reelin is present in the PVN and cells in the PVN express reelin receptors.^53,54 In line with this, homozygous (reln−/−) or heterozygous (reln+/−) Reeler mice have significantly less circulating ACTH, indicating a potential role for reelin in controlling the production of CRH and ACTH. ⁶⁵

It has been proposed that reelin mediates oxytocin expression in the PVN and may dampen HPA axis activity through oxytocin-induced GABAergic transmission. Oxytocin release in the PVN inhibits CRH production through the stimulation of GABAergic interneurons to increase GABA transmission through GABA-A receptors on CRH-producing neurons.^66,67 Oxytocin additionally inhibits the secretion of ACTH from the pituitary and cortisol from the adrenal cortex, providing comprehensive suppression of HPA axis activity. ⁶⁸ A significant positive correlation between reelin-IR cell density and oxytocin-IR cell density has been shown within the PVN of both male and female rats. ⁵⁷ Chronic stress in the form of three weeks of daily injections of CORT significantly decreases oxytocin-IR cell density in the PVN, and a single intravenous injection of reelin (3 µg) rescues these CORT-induced changes. ²³ Furthermore, reelin's function in strengthening and stabilizing synapses by rescuing deficits in LTP, enhancing NMDA receptor activity, and recruiting AMPA receptors may extend to oxytocinergic neurons in the PVN to further support oxytocinergic control over the HPA axis.^13,69 Reelin has been implicated in both the stimulation and inhibition of the HPA axis, suggesting that in healthy conditions, reelin contributes to homeostatic signaling through the HPA axis. Future research should elucidate additional mechanisms through which reelin controls HPA axis signaling, and if reelin is a necessary factor for proper functioning of the HPA axis.

Sex Differences in HPA Axis Function and Sexually Dimorphic Expression of Reelin in the HPA Axis

Women are about twice as likely to be diagnosed with depression than men. ⁷⁰ Similarly, the lifetime prevalence of stress-related anxiety disorders including generalized anxiety disorder and post-traumatic stress disorder is thought to be higher in women than in men. ⁷¹ It has been proposed that sex differences in HPA axis function contribute to the higher incidence of stress-related disorders in females. ⁷² Sex differences in HPA axis function have also been observed in animal models of chronic stress. Compared to males, female rats show a more robust HPA axis-based response to acute restraint stress and exhibit slower resolution of stress-related biochemical changes following a stressor, as evidenced by a delayed return to baseline for ACTH and cortisol/CORT levels.^73–76 Female rats exhibit greater plasma ACTH and CORT levels as well as CRH mRNA expression compared to males following acute stress. ⁷⁷ In addition, female rats have been found to have a lower number of hypothalamic CORT cytosolic binding sites and lesser affinity for CORT compared to males. ⁷⁸ GR knockout studies have shown differential results depending on region and sex including exaggerated stress responses and depressive-like behavior in males but not females with GR deletion from either the forebrain or the PVN as well as heightened responses in females but not males following GR deletion from GABAergic neurons.^79–81 Interestingly, clinical studies show that women produce stronger responses to stress in low-estrogen phases of the menstrual cycle, suggesting that gonadal hormones may influence the magnitude of a HPA axis response.^82,83 Together, these findings underscore that sex-based distinctions in HPA axis function may help explain the greater vulnerability of women to chronic stress.

Reelin may be implicated in the sex-specific response to stress, as its expression has been shown to vary by sex in basal and chronic stress conditions. The basal density of reelin-IR cells in the hypothalamus was found to be significantly lower in the PVN and medial preoptic area in female rats when compared to males. ⁵⁷ Additionally, basal levels of the N-terminal reelin fragment were lower in the hypothalamus of female rats relative to males. ⁸⁴ Following long-term exposure to CORT, females did not experience a significant change in reelin-IR cells in the PVN nor did they experience any alterations to N-terminal fragment levels relative to the vehicle-treated controls.^57,84 Similarly, female rats exposed to chronic CORT did not exhibit any significant changes in their levels of DNMT3a, an enzyme that catalyzes DNA methylation of the reelin promoter, compared to control rats. ⁸⁴ Researchers also failed to observe any significant difference in MeCP2, a protein implicated in the regulation of gene transcription, between the control and CORT-treated females. ⁸⁵ Studies of illnesses in which reelin is downregulated have uncovered hypermethylated reln promoters to which MeCP2 binds in order to suppress transcription. An increase in hypermethylated reln promoters correlates with a decrease in reelin levels. ⁸⁶

In contrast with the female rats, male rats exposed to chronic CORT experienced a significant decrease in reelin-IR cells and N-terminal fragment levels in the PVN relative to control males.^57,84 CORT males also expressed significantly higher levels of MeCP2 and less DNMT3a than the male controls. ⁸⁴

Altogether, this highlights the sex-specific nature of not only basal reelin expression in the hypothalamus, but also the sexually dimorphic impact of chronic stress on reelin-IR cells, reelin protein fragments, and epigenetic regulators of reelin expression. This suggests that the proposed roles for reelin in regulating HPA axis activity may be more pronounced in males compared to females, despite the greater vulnerability of women to chronic stress-related conditions such as depression. Sex differences in both basal and CORT-influenced reelin expression in the hypothalamus may indicate that reelin is implicated in males’ resilience to stress.

Roles for Reelin in Modulating a Dysfunctional Hypothalamic-Pituitary-Adrenal Axis

Targeting HPA Axis Innervations to Restore HPA Axis Reactivity

The hippocampus and prefrontal cortex (PFC) are heavily involved in regulating HPA axis activity, in part through the inhibition of GC release via PFC and hippocampal output to the bed nucleus of the stria terminalis in the limbic forebrain.^87,88 This output in turn inhibits activity of the PVN to hinder initiation of the body's response to stressors.^87,88 It has been proposed that HPA axis hyperactivity can initiate hippocampal atrophy, which exacerbates HPA axis hyperactivity due to a loss of hippocampal inhibitory feedback over the HPA axis.^89,90 Altered PVN excitability following chronic stress has been reported in various studies.^91,92 Rats exposed to a 3-week chronic unpredictable stress paradigm displayed reduced frequency of miniature inhibitory postsynaptic currents in parvocellular neurons in the PVN, revealing a connection between chronic stress and a reduced number of functional GABAergic synaptic contacts. ⁹² In addition, rats undergoing 1 week of chronic variable stress exhibited an increased number of glutamatergic boutons in parvocellular neurons of the PVN. ⁹³ Loss of GCs due to adrenalectomy has also been shown to increase the number of GABAergic synapses terminating on PVN CRH-synthesizing neurons. ⁹⁴ Decreased levels of GCs associated with adrenalectomy enhances inhibitory signaling of CRH-synthesizing cells, which supports a feedback loop in which GCs regulate HPA axis network excitability (Figure 1). Altogether, these studies highlight inhibitory hippocampal and excitatory GABAergic inputs to the HPA axis as promising targets for the restoration of healthy HPA axis function.

Oxytocin exerts an inhibitory effect on HPA axis activity through projections to GABAergic neurons that inhibit CRH-producing neurons.^66,95 It is released by magnocellular and parvocellular neurons in the PVN, and through projections to the median eminence of the hypothalamus, oxytocin inhibits ACTH-producing cells in the anterior pituitary (Figure 1). ⁶⁷ In contrast, AVP, which is synthesized in the PVN, acts synergistically with CRH to stimulate the production of ACTH.^96,97 Parvocellular neurons in the PVN co-expressing AVP and CRH project to the median eminence, where AVP is released and acts upon VP-1b receptors in the anterior pituitary, triggering a signaling pathway culminating in the release of ACTH (Figure 1). ⁹⁷ Due to their roles in inhibiting and stimulating CRH and ACTH production, oxytocin, AVP, and CRH-positive neurons present additional targets for reelin in the regulation of HPA axis activity.

Reelin Administration Influences HPA Axis Function

Hypercortisolism is a common occurrence in individuals with depression and is associated with a hyperactive HPA axis producing excess GCs.^98–100 Although studies linking hypercortisolism and reelin in humans are lacking, pre-clinical studies have found that chronically elevated CORT leads to a significant downregulation of reelin-IR cell counts in the hippocampus of adult rats and PVN of male rats and that the administration of reelin rescues these deficits.^22,23,57,69 (Figure 1). Similarly, it is currently understood that reelin administration is capable of rescuing chronic stress-induced decreases to hippocampal LTP, although it is not yet known whether reelin has the same effects on hypothalamic LTP. ⁶⁹ Studies of heterozygous (reln+/−) Reeler mice have revealed that mice deficient in reelin experience disruptions to the maturation of GABAergic synaptic transmission in the medial PFC, highlighting a connection between reelin and synaptic plasticity in inhibitory GABAergic neurons. ¹⁰¹ In addition, a study of reelin knockout mice identified reduced GABA receptor expression on the plasma membrane of glutamatergic neurons in the neocortex. ¹⁰² Since reelin administration restores hippocampal LTP, it may indirectly strengthen hippocampal inhibitory control over hypothalamic activity, highlighting a potential role for reelin in dampening HPA axis hyperactivity in chronic stress and depression.

Research shows that adult hippocampus neurogenesis (AHN) modifies HPA axis activation.^103,104 Chronic unpredictable mild stress reduces AHN and impairs negative feedback regulation, and AHN knockout mice show a heightened HPA axis response to acute stress, characterized by elevated circulating CORT and prolonged recovery to baseline levels of CORT.^103,105,106 Increasing AHN throughout chronic stress via deletion of the pro-apoptotic gene Bax from neural progenitor cells works in an opposing manner, alleviating anhedonia-like behaviors and partially attenuating HPA axis reactivity to acute stress. ¹⁰⁷ Reelin administration in a rodent model of chronic stress has been shown to rescue CORT-induced deficits in hippocampal neuron dendritic complexity, maturation, and survival. ¹⁰⁸ Reelin's involvement in developing dendritic complexity and neuronal maturation, processes important for neurogenesis, underscores the role of reelin administration in rescuing chronic stress-induced deficits in AHN. ¹⁰⁹

As discussed previously, reelin co-localizes with oxytocin-positive parvocellular neurons in the PVN, and further study of reelin has uncovered a positive correlation between reelin-IR and oxytocin-positive cell counts following the administration of a single dose of recombinant reelin.^23,57 Rats receiving chronic CORT have exhibited a decrease in oxytocin-positive cells compared to controls, and administration of reelin following CORT exposure has been shown to partially restore the number of oxytocin-positive cells in the PVN. ²³ Although the exact mechanisms through which reelin and oxytocin interact are unknown, this research demonstrates that, in a rodent model of chronic stress, chronically elevated GCs are capable of decreasing oxytocin expression in the PVN and that reelin administration is capable of partially rescuing this decrease. Reelin administration may therefore be beneficial in rescuing inhibition of the HPA axis via interactions with oxytocin-positive neurons. Further research is required, however, to gain an understanding of the functional links between reelin and OXT. Researchers might consider experimenting with PVN OXT-positive neuron readouts in rats exposed to chronic CORT administration, although this study would have to account for sex differences in reelin expression in the PVN following chronic CORT. ⁵⁷ Alternately, CNQX might be used to block AMPA receptors in the hypothalamus, as previous research has discovered that CNQX blocks reelin's effects on certain behavioral tests as well as neuronal survival and maturation. ¹⁰⁸

Four to seven percent of reelin, CRH, and AVP-positive cells were found to co-localize with one another in a rodent study of chronic stress. ⁵⁷ A study of postmortem tissue of depressed patients identified elevated AVP receptor 1a expression in the PVN, while another found elevated plasma AVP and cortisol levels in humans experiencing psychological stress.^110,111 The co-localization of reelin, CRH, and AVP, although minute, highlights a possible relationship between reelin, CRH, and AVP expression, although it is not known whether reelin administration is capable of altering CRH- or AVP-positive cell counts. Further research is required to determine whether reelin can exert a stimulatory effect on HPA axis activity via CRH- and AVP-positive neurons.

Much of the evidence highlighting the possible roles for reelin in influencing dysfunctional HPA axis activity in conditions of chronic stress supports reelin's involvement in inhibitory functions. The presence of some evidence favoring reelin's role in HPA axis stimulation does, however, suggest that both excessive and insufficient reelin levels could play a role in dysregulating HPA axis activity. This idea is supported by research done by our lab which demonstrates that reelin levels are downregulated in a chronic CORT model of depression and that reelin acts in a dose-dependent manner with high and low doses rescuing CORT-induced behavioral and neurochemical changes to a lesser degree than what was determined to be the optimal dose (3 μg). ²² Researchers in our lab explored the effects of 0.5, 1, 3, 5, 7, and 9 μg of recombinant reelin in rescuing various behavioral and neurochemical changes associated with chronic CORT administration. ²² Dose effects followed a U-shaped profile, and 3 μg was found to rescue CORT-induced changes in the forced swim test and hippocampal reelin-positive cell counts to a greater extent than the other doses tested. 3 μg was also shown to significantly rescue CORT-induced changes in serotonin transporter (SERT) cluster size and GluA1+ cell density.

As previously discussed, preclinical animal studies have explored the effects of reelin administration on various morphological, neurochemical, and behavioral changes associated with a chronic stress phenotype in which hippocampal reelin levels are significantly reduced.^{22,23,69,108,112,113} Results from these studies suggest that increasing endogenous reelin in conditions where reelin is initially downregulated may restore some functions disrupted by chronic stress, such as synaptic plasticity and dendritogenesis, in certain animal models.

Conversely, anti-reelin therapies are undergoing preclinical experimentation as a potential treatment option for certain autoimmune diseases such as multiple sclerosis and rheumatoid arthritis.^114,115 Reelin is currently understood to play a role in inflammation via the modulation of leukocyte-endothelial cell adhesion, and as such it has been theorized that plasma reelin depletion may be effective in ameliorating certain chronic inflammatory illnesses. ¹¹⁶ This idea that excessive reelin might be harmful has been supported by animal studies of reelin overexpression in the CNS, which have shown that excess reelin can lead to abnormalities in the organization of adult-generated neurons, synaptic connectivity, and LTP induction. ⁸

Altogether, preclinical studies have demonstrated that reelin is less effective in carrying out its functions and is potentially harmful at deficient and excessive levels. The concept of restoring reelin homeostasis rather than administering or depleting reelin may prove valuable in the development of novel reelin-informed therapeutics that regulate dysfunctional HPA axis activity in chronic stress and depression.

Reelin-Informed Considerations for the Development of Novel Therapeutics Targeting the HPA Axis

Known antidepressants such as vortioxetine have been shown to normalize plasma reelin levels in individuals with depression as well as hippocampal reelin levels in rats exposed to chronic stress.^20,117,118 Likewise, ketamine, a NMDA receptor agonist with antidepressant-like effects, has been theorized to act via pathways similar to reelin. ⁶⁹ Many of ketamine's antidepressant-like actions are mediated by AMPA receptors. ¹¹⁹ Administration of the AMPA receptor antagonist CNQX has been shown to block the effects of reelin administration on a chronic stress phenotype. ¹⁰⁸ Similarly, disrupting reelin, ApoER2, and downstream SFKs was shown to block some of the antidepressant-like effects of ketamine. ¹²⁰ Altogether this highlights that reelin is potentially more important to existing pathways than is presently understood, and targeting the reelin pathway directly may have important implications for the development of novel therapeutics. Given the involvement of reelin in several cancers and atherosclerosis, however, interventions modifying reelin must be thoroughly tested for safety efficacy.^116,121,122

Another important consideration for the development of therapeutics targeting the reelin pathway is whether reelin can cross the blood brain barrier, as many of reelin's effects in a chronic stress phenotype are observed in the central nervous system. Reelin immunolabeling has been detected in the caveolae of endothelial cells in some small capillaries around layer I of the cortex and the stratum lacunosum moleculare of the hippocampus in adult male rats, suggesting that reelin or reelin peptides may be capable of crossing the BBB. ¹²³ Additionally, both intrahippocampal infusion and intravenous injection of exogenous recombinant reelin in adult rats has been shown to increase endogenous reelin in the hippocampus and hypothalamus in a chronic stress phenotype.^22,108 This suggests that reelin administration outside of the central nervous system may lead to effects similar to reelin infusion directly into the brain. Whether the mechanisms through which exogenous reelin influences endogenous reelin levels are direct or indirect is still unknown (to the author's knowledge).

The development of reelin-informed therapeutics must also take into consideration possible biomarkers that can be used to assess therapeutic efficacy. Several blood-based biomarkers can be evaluated in order to assess the effects of reelin administration and the restoration of reelin homeostasis on a dysregulated HPA axis. Cortisol is one hormone that can be assessed. As previously discussed, chronic stress can result in HPA axis hyperactivity and lead to a prolonged elevation of cortisol levels. ¹ Cortisol levels can be assessed via plasma, saliva, or urine, but collections must take into account the time in which the sample is collected, as cortisol levels naturally fluctuate throughout the day.^124–126 Similarly, ACTH is another hormone directly involved in HPA axis function that can be measured to evaluate the effects of reelin administration. As with cortisol, HPA axis hyperactivity induced by chronic stress and related disorders results in elevated ACTH levels. ¹²⁷ ACTH levels can be assessed via blood test (see Table 2). ¹²⁷

Table 2.

Summary of the Proposed Biomarkers for the Assessment of HPA Axis Function Following the Restoration of Reelin Homeostasis.

Biomarker	Rationale	Reference
Cortisol	Chronic stress can result in HPA axis hyperactivity, which leads to a prolonged elevation of cortisol levels. Cortisol levels can be assessed via plasma, saliva, or urine	Sheng et al 1 , Fang et al 124 , Raff et al 125 , Flack et al 126
ACTH	HPA axis hyperactivity induced by chronic stress results in elevated levels of ACTH. ACTH levels can be assessed via blood test	Kunugi et al 127
FKBP5	High levels of FKBP5 are associated with chronic stress and depression. FKBP5 regulates GR sensitivity by reducing their binding affinity for cortisol and impairs the negative feedback loop, making the HPA axis more prone to hyperactivity. FKBP5 mRNA expression can be assessed via blood test	Ising et al 128 , Binder 129
BDNF	High levels of cortisol have been linked to reduced plasma BDNF levels in an animal model and have been rescued by antidepressant treatment in depressed patients. BDNF levels can be assessed via blood test	Smith et al 130 , Murakami et al 131 , Brunoni et al 132
Pro-Inflammatory Cytokines	Chronic stress has been shown to diminish the ability of cortisol to suppress the immune system, resulting in a prolonged elevation of pro-inflammatory cytokines such as interleukin-6 and interleukin-1. A blood sample can be taken to assess pro-inflammatory cytokine levels	Sorrells et al 133 , Howren et al 134 , Yoshimura et al 135

High levels of the protein FKBP5 are associated with HPA axis hyperactivity and have been observed in individuals with depression. ¹²⁸ FKBP5 regulates GR sensitivity by reducing their binding affinity for cortisol and at high levels can impair the negative feedback loop regulating the HPA axis, making it more prone to hyperactivity. ¹²⁹ Animal studies have demonstrated that high levels of CORT resulting from chronic stress are linked to reduced neuronal BDNF levels.^130,131 Studies of individuals with depression have also observed an increase in plasma BDNF following antidepressant treatment. ¹³² A blood test can be used to quantify both FKBP5 mRNA expression and BDNF levels in individuals experiencing chronic stress and depression-related HPA axis dysfunction (see Table 2).^128,132

Prolonged elevation of cortisol levels as a result of HPA axis hyperactivity can also suppress the immune system. ¹³³ This results in elevated levels of pro-inflammatory cytokines such as interleukin-6 and interleukin-1, as have been observed in depressed patients.^133–135 A blood sample can be taken to assess pro-inflammatory cytokine levels in patients experiencing HPA axis hyperactivity as a result of depression and other stress-related disorders (see Table 2). ¹³⁵

Conclusions

In summary, this review discusses the extensive presence of reelin and its receptors throughout the HPA axis, particularly within the various nuclei of the hypothalamus. This review proposes several roles for reelin administration in various aspects of HPA axis inhibition and stimulation. It ultimately highlights an important function for reelin homeostasis in potentially restoring disrupted HPA axis activity in chronic stress conditions. Altogether, these conclusions call attention to future avenues for research uncovering the mechanisms through which reelin may act within the HPA axis. Further research will expand current understanding of the roles of reelin in chronic stress, providing valuable insight into the neurobiology of depression.

Abbreviations

ACTH

adrenocorticotropic hormone

AHN

adult hippocampal neurogenesis

AMPA

α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid

ApoER2

Apolipoprotein E receptor 2

ARC

arcuate nucleus

AVP

arginine vasopressin

BDNF

Brain-derived neurotrophic factor

CORT

corticosterone

CREB

Cyclic adenosine monophosphate response element-binding protein

CRH

corticotropin-releasing hormone

Dab1

Disabled-1

DMH

dorsomedial nucleus

GABA

γ-Aminobutyric acid

GC

glucocorticoid

GR

glucocorticoid receptor

GSK3β

Glycogen synthase kinase 3 beta

HPA

hypothalamic-pituitary-adrenal

IR

immunoreactive

LTP

long-term potentiation

mTOR

Mechanistic target of rapamycin

mTORC1

Mammalian target of rapamycin complex 1

NMDA

N-methyl-D-aspartate

PI3K

Phosphoinositide-3′ kinase

PFC

prefrontal cortex

PVN

paraventricular nucleus

SCN

suprachiasmatic nucleus

SERT

Serotonin transporter

SFK

Src family kinase

VMH

ventromedial nucleus

VLDLR

Very low density lipoprotein receptor