Overexpression of UCP4 in astrocytic mitochondria prevents multilevel dysfunctions in a mouse model of Alzheimer's disease.

Alzheimer's disease (AD) is becoming increasingly prevalent worldwide. It represents one of the greatest medical challenges as no pharmacologic treatments are available to prevent disease progression. Astrocytes play crucial functions within neuronal circuits by providing metabolic and functional support, regulating interstitial solute composition, and modulating synaptic transmission. In addition to these physiological functions, growing evidence points to an essential role of astrocytes in neurodegenerative diseases like AD. Early-stage AD is associated with hypometabolism and oxidative stress. Contrary to neurons that are vulnerable to oxidative stress, astrocytes are particularly resistant to mitochondrial dysfunction and are therefore more resilient cells. In our study, we leveraged astrocytic mitochondrial uncoupling and examined neuronal function in the 3xTg AD mouse model. We overexpressed the mitochondrial uncoupling protein 4 (UCP4), which has been shown to improve neuronal survival in vitro. We found that this treatment efficiently prevented alterations of hippocampal metabolite levels observed in AD mice, along with hippocampal atrophy and reduction of basal dendrite arborization of subicular neurons. This approach also averted aberrant neuronal excitability observed in AD subicular neurons and preserved episodic-like memory in AD mice assessed in a spatial recognition task. These findings show that targeting astrocytes and their mitochondria is an effective strategy to prevent the decline of neurons facing AD-related stress at the early stages of the disease.

Activation of lactate receptor HCAR1 down-modulates neuronal activity in rodent and human brain tissue.

Lactate can be used by neurons as an energy substrate to support their activity. Evidence suggests that lactate also acts on a metabotropic receptor called HCAR1, first described in the adipose tissue. Whether HCAR1 also modulates neuronal circuits remains unclear. In this study, using qRT-PCR, we show that HCAR1 is present in the human brain of epileptic patients who underwent resective surgery. In brain slices from these patients, pharmacological HCAR1 activation using a non-metabolized agonist decreased the frequency of both spontaneous neuronal Ca2+ spiking and excitatory post-synaptic currents (sEPSCs). In mouse brains, we found HCAR1 expression in different regions using a fluorescent reporter mouse line and in situ hybridization. In the dentate gyrus, HCAR1 is mainly present in mossy cells, key players in the hippocampal excitatory circuitry and known to be involved in temporal lobe epilepsy. By using whole-cell patch clamp recordings in mouse and rat slices, we found that HCAR1 activation causes a decrease in excitability, sEPSCs, and miniature EPSCs frequency of granule cells, the main output of mossy cells. Overall, we propose that lactate can be considered a neuromodulator decreasing synaptic activity in human and rodent brains, which makes HCAR1 an attractive target for the treatment of epilepsy.

The Lactate Receptor HCAR1 Modulates Neuronal Network Activity through the Activation of Gα and Gßγ Subunits.

The discovery of a G-protein-coupled receptor for lactate named hydroxycarboxylic acid receptor 1 (HCAR1) in neurons has pointed to additional nonmetabolic effects of lactate for regulating neuronal network activity. In this study, we characterized the intracellular pathways engaged by HCAR1 activation, using mouse primary cortical neurons from wild-type (WT) and HCAR1 knock-out (KO) mice from both sexes. Using whole-cell patch clamp, we found that the activation of HCAR1 with 3-chloro-5-hydroxybenzoic acid (3Cl-HBA) decreased miniature EPSC frequency, increased paired-pulse ratio, decreased firing frequency, and modulated membrane intrinsic properties. Using fast calcium imaging, we show that HCAR1 agonists 3,5-dihydroxybenzoic acid, 3Cl-HBA, and lactate decreased by 40% spontaneous calcium spiking activity of primary cortical neurons from WT but not from HCAR1 KO mice. Notably, in neurons lacking HCAR1, the basal activity was increased compared with WT. HCAR1 mediates its effect in neurons through a Giα-protein. We observed that the adenylyl cyclase-cAMP-protein kinase A axis is involved in HCAR1 downmodulation of neuronal activity. We found that HCAR1 interacts with adenosine A1, GABAB, and α2A-adrenergic receptors, through a mechanism involving both its Giα and Gißγ subunits, resulting in a complex modulation of neuronal network activity. We conclude that HCAR1 activation in neurons causes a downmodulation of neuronal activity through presynaptic mechanisms and by reducing neuronal excitability. HCAR1 activation engages both Giα and Gißγ intracellular pathways to functionally interact with other Gi-coupled receptors for the fine tuning of neuronal activity.SIGNIFICANCE STATEMENT Expression of the lactate receptor hydroxycarboxylic acid receptor 1 (HCAR1) was recently described in neurons. Here, we describe the physiological role of this G-protein-coupled receptor (GPCR) and its activation in neurons, providing information on its expression and mechanism of action. We dissected out the intracellular pathway through which HCAR1 activation tunes down neuronal network activity. For the first time, we provide evidence for the functional cross talk of HCAR1 with other GPCRs, such as GABAB, adenosine A1- and α2A-adrenergic receptors. These results set HCAR1 as a new player for the regulation of neuronal network activity acting in concert with other established receptors. Thus, HCAR1 represents a novel therapeutic target for pathologies characterized by network hyperexcitability dysfunction, such as epilepsy.

Cell-Type-Specific Gene Expression Profiling in Adult Mouse Brain Reveals Normal and Disease-State Signatures.

The role of brain cell-type-specific functions and profiles in pathological and non-pathological contexts is still poorly defined. Such cell-type-specific gene expression profiles in solid, adult tissues would benefit from approaches that avoid cellular stress during isolation. Here, we developed such an approach and identified highly selective transcriptomic signatures in adult mouse striatal direct and indirect spiny projection neurons, astrocytes, and microglia. Integrating transcriptomic and epigenetic data, we obtained a comprehensive model for cell-type-specific regulation of gene expression in the mouse striatum. A cross-analysis with transcriptomic and epigenomic data generated from mouse and human Huntington's disease (HD) brains shows that opposite epigenetic mechanisms govern the transcriptional regulation of striatal neurons and glial cells and may contribute to pathogenic and compensatory mechanisms. Overall, these data validate this less stressful method for the investigation of cellular specificity in the adult mouse brain and demonstrate the potential of integrative studies using multiple databases.

Control of Glutamate Transport by Extracellular Potassium: Basis for a Negative Feedback on Synaptic Transmission.

Glutamate and K+, both released during neuronal firing, need to be tightly regulated to ensure accurate synaptic transmission. Extracellular glutamate and K+ ([K+]o) are rapidly taken up by glutamate transporters and K+-transporters or channels, respectively. Glutamate transport includes the exchange of one glutamate, 3 Na+, and one proton, in exchange for one K+. This K+ efflux allows the glutamate binding site to reorient in the outwardly facing position and start a new transport cycle. Here, we demonstrate the sensitivity of the transport process to [K+]o changes. Increasing [K+]o over the physiological range had an immediate and reversible inhibitory action on glutamate transporters. This K+-dependent transporter inhibition was revealed using microspectrofluorimetry in primary astrocytes, and whole-cell patch-clamp in acute brain slices and HEK293 cells expressing glutamate transporters. Previous studies demonstrated that pharmacological inhibition of glutamate transporters decreases neuronal transmission via extrasynaptic glutamate spillover and subsequent activation of metabotropic glutamate receptors (mGluRs). Here, we demonstrate that increasing [K+]o also causes a decrease in neuronal mEPSC frequency, which is prevented by group II mGluR inhibition. These findings highlight a novel, previously unreported physiological negative feedback mechanism in which [K+]o elevations inhibit glutamate transporters, unveiling a new mechanism for activity-dependent modulation of synaptic activity.

Synaptic scaling up in medium spiny neurons of aged BACHD mice: A slow-progression model of Huntington's disease.

Huntington's disease (HD) is an autosomal dominant disease that develops in midlife (~ 40 years-old at onset) and then progresses slowly. It is still unclear how striatal medium spiny neurons (MSNs), the most vulnerable neurons in HD, maintain their function for decades despite the chronic expression of mutant huntingtin (mHTT). In this study, we used aged BACHD mice, a HD model expressing the full-length human mHTT gene, to investigate the molecular, morphological and functional properties of striatal MSNs. We report that the density of dendritic spines in MSNs is substantially lower in aged BACHD mice than in wild-type (WT) mice, in the absence of major dendritic changes and neuronal loss. This spine loss is accompanied by changes in transcription, resulting in a low expression of the striatum-specific G protein-coupled receptor 88 (Gpr88) as well as a reorganization of the composition of AMPAR subunits (high Gria1/Gria2 mRNA ratio). We also detected functional changes in BACHD MSNs. Notably, BACHD MSNs were hyperexcitable and the amplitude of AMPAR-mediated synaptic currents was higher than in WT MSNs. Altogether, these data show that both the intrinsic properties and the strength of the remaining synapses are modified in MSNs with low dendritic spine density in aged BACHD mice. These homeostatic mechanisms may compensate for the substantial loss of synaptic inputs and thus alleviate the deleterious effects of mHTT expression on the activity of MSNs and also possibly on the motor phenotype in aged BACHD.

Electrophysiological changes precede morphological changes to frontal cortical pyramidal neurons in the rTg4510 mouse model of progressive tauopathy.

Whole-cell patch-clamp recordings and high-resolution morphometry were used to assess functional and structural properties of layer 3 pyramidal neurons in early (<4 months) and advanced (>8 months) stages of tauopathy in frontal cortical slices prepared from rTg4510 tau mutant (P301L) mice. In early tauopathy, dendritic architecture is preserved. In advanced tauopathy, neurons can be categorized as either "atrophic" (58 %)-exhibiting marked atrophy of the apical tuft, or "intact" (42 %)-with normal apical tufts and, in some instances, proliferative sprouting of oblique branches of the apical trunk. Approximately equal numbers of atrophic and intact neurons contain neurofibrillary tangles (NFTs) or are tangle-free, lending further support to the idea that NFTs per se are not toxic. Spine density is decreased due to a specific reduction in mushroom spines, but filopodia are increased in both atrophic and intact neurons. By contrast to these morphological changes, which are robust only in the advanced stage, significant electrophysiological changes are present in the early stage and persist in the advanced stage in both atrophic and intact neurons. The most marked of these changes are: a depolarized resting membrane potential, an increased depolarizing sag potential and increased action potential firing rates-all indicative of hyperexcitability. Spontaneous excitatory postsynaptic currents are not reduced in frequency or amplitude in either stage. The difference in the time course of functionally important electrophysiological changes versus regressive morphological changes implies differences in pathogenic mechanisms underlying functional and structural changes to neurons during progressive tauopathy.

Homeostatic responses by surviving cortical pyramidal cells in neurodegenerative tauopathy.

Cortical neuron death is prevalent by 9 months in rTg(tau(P301L))4510 tau mutant mice (TG) and surviving pyramidal cells exhibit dendritic regression and spine loss. We used whole-cell patch-clamp recordings to investigate the impact of these marked structural changes on spontaneous excitatory and inhibitory postsynaptic currents (sEPSCs and sIPSCs) of layer 3 pyramidal cells in frontal cortical slices from behaviorally characterized TG and non-transgenic (NT) mice at this age. Frontal lobe function of TG mice was intact following a short delay interval but impaired following a long delay interval in an object recognition test, and cortical atrophy and cell loss were pronounced. Surviving TG cells had significantly reduced dendritic diameters, total spine density, and mushroom spines, yet sEPSCs were increased and sIPSCs were unchanged in frequency. Thus, despite significant regressive structural changes, synaptic responses were not reduced in TG cells, indicating that homeostatic compensatory mechanisms occur during progressive tauopathy. Consistent with this idea, surviving TG cells were more intrinsically excitable than NT cells, and exhibited sprouting of filopodia and axonal boutons. Moreover, the neuropil in TG mice showed an increased density of asymmetric synapses, although their mean size was reduced. Taken together, these data indicate that during progressive tauopathy, cortical pyramidal cells compensate for loss of afferent input by increased excitability and establishment of new synapses. These compensatory homeostatic mechanisms may play an important role in slowing the progression of neuronal network dysfunction during neurodegenerative tauopathies.

Dendritic vulnerability in neurodegenerative disease: insights from analyses of cortical pyramidal neurons in transgenic mouse models.

In neurodegenerative disorders, such as Alzheimer's disease, neuronal dendrites and dendritic spines undergo significant pathological changes. Because of the determinant role of these highly dynamic structures in signaling by individual neurons and ultimately in the functionality of neuronal networks that mediate cognitive functions, a detailed understanding of these changes is of paramount importance. Mutant murine models, such as the Tg2576 APP mutant mouse and the rTg4510 tau mutant mouse have been developed to provide insight into pathogenesis involving the abnormal production and aggregation of amyloid and tau proteins, because of the key role that these proteins play in neurodegenerative disease. This review showcases the multidimensional approach taken by our collaborative group to increase understanding of pathological mechanisms in neurodegenerative disease using these mouse models. This approach includes analyses of empirical 3D morphological and electrophysiological data acquired from frontal cortical pyramidal neurons using confocal laser scanning microscopy and whole-cell patch-clamp recording techniques, combined with computational modeling methodologies. These collaborative studies are designed to shed insight on the repercussions of dystrophic changes in neocortical neurons, define the cellular phenotype of differential neuronal vulnerability in relevant models of neurodegenerative disease, and provide a basis upon which to develop meaningful therapeutic strategies aimed at preventing, reversing, or compensating for neurodegenerative changes in dementia.

Age-related vascular pathology in transgenic mice expressing presenilin 1-associated familial Alzheimer's disease mutations.

Mutations in the presenilin 1 (PS1) gene are the most commonly recognized cause of familial Alzheimer's disease (FAD). Besides senile plaques, neurofibrillary tangles, and neuronal loss, Alzheimer's disease (AD) is also accompanied by vascular pathology. Here we describe an age-related vascular pathology in two lines of PS1 FAD-mutant transgenic mice that mimics many features of the vascular pathology seen in AD. The pathology was especially prominent in the microvasculature whose vessels became thinned and irregular with the appearance of many abnormally looped vessels as well as string vessels. Stereologic assessments revealed a reduction of the microvasculature in the hippocampus that was accompanied by hippocampal atrophy. The vascular changes were not congophilic. Yet, despite the lack of congophilia, penetrating vessels at the cortical surface were often abnormal morphologically and microhemorrhages sometimes occurred. Altered immunostaining of blood vessels with basement membrane-associated antigens was an early feature of the microangiopathy and was associated with thickening of the vascular basal laminae and endothelial cell alterations that were visible ultrastructurally. Interestingly, although the FAD-mutant transgene was expressed in neurons in both lines of mice, there was no detectable expression in vascular endothelial cells or glial cells. These studies thus have implications for the role of neuronal to vascular signaling in the pathogenesis of the vascular pathology associated with AD.

Protein kinase C activity is associated with prefrontal cortical decline in aging.

Aging is associated with deficiencies in the prefrontal cortex, including working memory impairment and compromised integrity of neuronal dendrites. Although protein kinase C (PKC) is implicated in structural plasticity, and overactivation of PKC results in working memory impairments in young animals, the role of PKC in prefrontal cortical impairments in the aged has not been examined. This study provides the first evidence that PKC activity is associated with prefrontal cortical dysfunction in aging. Pharmacological inhibition of PKC with chelerythrine rescued working memory impairments in aged rats and enhanced working memory in aged rhesus monkeys. Improvement correlated with age, with older monkeys demonstrating a greater degree of improvement following PKC inhibition. Furthermore, PKC activity within the prefrontal cortex was inversely correlated with the length of basal dendrites of prefrontal cortical neurons, as well as with working memory performance in aged rats. Together these findings indicate that PKC is dysregulated in aged animals and that PKC inhibitors may be useful in the treatment of cognitive deficits in the elderly.

Significant structural but not physiological changes in cortical neurons of 12-month-old Tg2576 mice.

Amyloid-beta (Abeta) plays a key role in the etiology of Alzheimer's disease, and pyramidal cell dendrites exposed to Abeta exhibit dramatic structural alterations, including reduced dendritic spine densities. To determine whether such structural alterations lead to electrophysiological changes, whole-cell patch clamp recordings with biocytin filling were used to assess both the electrophysiological and morphological properties of layer 3 pyramidal cells in frontal cortical slices prepared from 12-month-old Tg2576 amyloid precursor protein (APP) mutant vs. wild-type (Wt) mice. Tg2576 cells exhibited significantly increased dendritic lengths and volumes and decreased spine densities, while the total number of spines was not different from Wt. Tg2576 and Wt cells did not differ with regard to passive membrane, action potential firing or glutamatergic spontaneous excitatory postsynaptic current properties. Thus, overexpression of mutated APP in young Tg2576 mice leads to significant changes in neuronal morphological properties which do not have readily apparent functional consequences.

The IRG mouse: a two-color fluorescent reporter for assessing Cre-mediated recombination and imaging complex cellular relationships in situ.

The Cre-loxP system is widely used for making conditional alterations to the mouse genome. Cre-mediated recombination is frequently monitored using reporter lines in which Cre expression activates a reporter gene driven by a ubiquitous promoter. Given the distinct advantages of fluorescent reporters, we developed a transgenic reporter line, termed IRG, in which DsRed-Express, a red fluorescent protein (RFP) is expressed ubiquitously prior to Cre-mediated recombination and an enhanced green fluorescent protein (EGFP) following recombination. Besides their utility for monitoring Cre-mediated recombination, we show that in IRG mice red and green native fluorescence can be imaged simultaneously in thick tissue sections by confocal microscopy allowing for complex reconstructions to be created that are suitable for analysis of neuronal morphologies as well as neurovascular interactions in brain. IRG mice should provide a versatile tool for analyzing complex cellular relationships in both neural and nonneural tissues.

Stereologic estimates of total spinophilin-immunoreactive spine number in area 9 and the CA1 field: relationship with the progression of Alzheimer's disease.

The loss of presynaptic markers is thought to represent a strong pathologic correlate of cognitive decline in Alzheimer's disease (AD). Spinophilin is a postsynaptic marker mainly located to the heads of dendritic spines. We assessed total numbers of spinophilin-immunoreactive puncta in the CA1 and CA3 fields of hippocampus and area 9 in 18 elderly individuals with various degrees of cognitive decline. The decrease in spinophilin-immunoreactivity was significantly related to both Braak neurofibrillary tangle (NFT) staging and clinical severity but not A beta deposition staging. The total number of spinophilin-immunoreactive puncta in CA1 field and area 9 were significantly related to MMSE scores and predicted 23.5 and 61.9% of its variability. The relationship between total number of spinophilin-immunoreactive puncta in CA1 field and MMSE scores did not persist when adjusting for Braak NFT staging. In contrast, the total number of spinophilin-immunoreactive puncta in area 9 was still significantly related to the cognitive outcome explaining an extra 9.6% of MMSE and 25.6% of the Clinical Dementia Rating scores variability. Our data suggest that neocortical dendritic spine loss is an independent parameter to consider in AD clinicopathologic correlations.

Repeated stress alters dendritic spine morphology in the rat medial prefrontal cortex.

Anatomical alterations in the medial prefrontal cortex (mPFC) are associated with hypothalamopituitary adrenal (HPA) axis dysregulation, altered stress hormone levels, and psychiatric symptoms of stress-related mental illnesses. Functional imaging studies reveal impairment and shrinkage of the mPFC in such conditions, and these findings are paralleled by experimental studies showing dendritic retraction and spine loss following repeated stress in rodents. Here we extend this characterization to how repeated stress affects dendritic spine morphology in mPFC through the utilization of an automated approach that rapidly digitizes, reconstructs three dimensionally, and calculates geometric features of neurons. Rats were perfused after being subjected to 3 weeks of daily restraint stress (6 hours/day), and intracellular injections of Lucifer Yellow were made in layer II/III pyramidal neurons in the dorsal mPFC. To reveal spines in all angles of orientation, deconvolved high-resolution confocal laser scanning microscopy image stacks of dendritic segments were reconstructed and analyzed for spine volume, surface area, and length using a Rayburst-based automated approach (8,091 and 8,987 spines for control and stress, respectively). We found that repeated stress results in an overall decrease in mean dendritic spine volume and surface area, which was most pronounced in the distal portion of apical dendritic fields. Moreover, we observed an overall shift in the population of spines, manifested by a reduction in large spines and an increase in small spines. These results suggest a failure of spines to mature and stabilize following repeated stress and are likely to have major repercussions on function, receptor expression, and synaptic efficacy.

Interactions of primary neuroepithelial progenitor and brain endothelial cells: distinct effect on neural progenitor maintenance and differentiation by soluble factors and direct contact.

Neurovascular interactions are crucial for the normal development of the central nervous system. To study such interactions in primary cultures, we developed a procedure to simultaneously isolate neural progenitor and endothelial cell fractions from embryonic mouse brains. Depending on the culture conditions endothelial cells were found to favor maintenance of the neuroprogenitor phenotype through the production of soluble factors, or to promote neuronal differentiation of neural progenitors through direct contact. These apparently opposing effects could reflect differential cellular interactions needed for the proper development of the brain.

Changes in the structural complexity of the aged brain.

Structural changes of neurons in the brain during aging are complex and not well understood. Neurons have significant homeostatic control of essential brain functions, including synaptic excitability, gene expression, and metabolic regulation. Any deviations from the norm can have severe consequences as seen in aging and injury. In this review, we present some of the structural adaptations that neurons undergo throughout normal and pathological aging and discuss their effects on electrophysiological properties and cognition. During aging, it is evident that neurons undergo morphological changes such as a reduction in the complexity of dendrite arborization and dendritic length. Spine numbers are also decreased, and because spines are the major sites for excitatory synapses, changes in their numbers could reflect a change in synaptic densities. This idea has been supported by studies that demonstrate a decrease in the overall frequency of spontaneous glutamate receptor-mediated excitatory responses, as well as a decrease in the levels of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and N-methyl-d-aspartate receptor expression. Other properties such as gamma-aminobutyric acid A receptor-mediated inhibitory responses and action potential firing rates are both significantly increased with age. These findings suggest that age-related neuronal dysfunction, which must underlie observed decline in cognitive function, probably involves a host of other subtle changes within the cortex that could include alterations in receptors, loss of dendrites, and spines and myelin dystrophy, as well as the alterations in synaptic transmission. Together these multiple alterations in the brain may constitute the substrate for age-related loss of cognitive function.

Pepsin pretreatment allows collagen IV immunostaining of blood vessels in adult mouse brain.

While the brain vasculature can be imaged with many methods, immunohistochemistry has distinct advantages due to its simplicity and applicability to archival tissue. However, immunohistochemical staining of the murine brain vasculature in aldehyde fixed tissue has proven elusive and inconsistent using current protocols. Here we investigated whether antigen retrieval methods could improve vascular staining in the adult mouse brain. We found that pepsin digestion prior to immunostaining unmasked widespread collagen IV staining of the cerebrovasculature in the adult mouse brain. Pepsin treatment also unmasked widespread vascular staining with laminin, but only marginally improved isolectin B4 staining and did not enhance vascular staining with fibronectin, perlecan or CD146. Collagen IV immunoperoxidase staining was easily combined with cresyl violet counterstaining making it suitable for stereological analyses of both vascular and neuronal parameters in the same tissue section. This method should be widely applicable for labeling the brain vasculature of the mouse in aldehyde fixed tissue from both normal and pathological states.

Stress-induced alterations in prefrontal cortical dendritic morphology predict selective impairments in perceptual attentional set-shifting.

Stressful life events have been implicated clinically in the pathogenesis of mental illness, but the neural substrates that may account for this observation remain poorly understood. Attentional impairments symptomatic of these psychiatric conditions are associated with structural and functional abnormalities in a network of prefrontal cortical structures. Here, we examine whether chronic stress-induced dendritic alterations in the medial prefrontal cortex (mPFC) and orbital frontal cortex (OFC) underlie impairments in the behaviors that they subserve. After 21 d of repeated restraint stress, rats were tested on a perceptual attentional set-shifting task, which yields dissociable measures of reversal learning and attentional set-shifting, functions that are mediated by the OFC and mPFC, respectively. Intracellular iontophoretic injections of Lucifer yellow were performed in a subset of these rats to examine dendritic morphology in layer II/III pyramidal cells of the mPFC and lateral OFC. Chronic stress induced a selective impairment in attentional set-shifting and a corresponding retraction (20%) of apical dendritic arbors in the mPFC. In stressed rats, but not in controls, decreased dendritic arborization in the mPFC predicted impaired attentional set-shifting performance. In contrast, stress was not found to adversely affect reversal learning or dendritic morphology in the lateral OFC. Instead, apical dendritic arborization in the OFC was increased by 43%. This study provides the first direct evidence that dendritic remodeling in the prefrontal cortex may underlie the functional deficits in attentional control that are symptomatic of stress-related mental illnesses.

Repeated stress induces dendritic spine loss in the rat medial prefrontal cortex.

The prefrontal cortex (PFC) plays an important role in higher cognitive processes, and in the regulation of stress-induced hypothalamic-pituitary-adrenal (HPA) activity. Here we examined the effect of repeated restraint stress on dendritic spine number in the medial PFC. Rats were perfused after receiving 21 days of daily restraint stress, and intracellular iontophoretic injections of Lucifer Yellow were carried out in layer II/III pyramidal neurons in the anterior cingulate and prelimbic cortices. We found that stress results in a significant (16%) decrease in apical dendritic spine density in medial PFC pyramidal neurons, and confirmed a previous observation that total apical dendritic length is reduced by 20% in the same neurons. We estimate that nearly one-third of all axospinous synapses on apical dendrites of pyramidal neurons in medial PFC are lost following repeated stress. A decrease in medial PFC dendritic spines may not only be indicative of a decrease in the total population of axospinous synapses, but may impair these neurons' capacity for biochemical compartmentalization and plasticity in which dendritic spines play a major role. Dendritic atrophy and spine loss may be important cellular features of stress-related psychiatric disorders where the PFC is functionally impaired.