Density Of Thin Spines Research Articles

Sexual experience induces a preponderance of mushroom spines in the medial prefrontal cortex and nucleus accumbens of male rats

Sexual experience improves copulatory performance in male rats. Copulatory performance has been associated with dendritic spines density in the medial prefrontal cortex (mPFC) and nucleus accumbens (NAcc), structures involved in the processing of sexual stimuli and the manifestation of sexual behavior. Dendritic spines modulate excitatory synaptic contacts, and their morphology is associated with the ability to learn from experience. This study was designed to determine the effect of sexual experience on the density of different types or shapes of dendritic spines in the mPFC and NAcc of male rats. A total of 16 male rats were used, half of them were sexually experienced while the other half were sexually inexperienced. After three sessions of sexual interaction to ejaculation, the sexually-experienced males presented shorter mount, intromission, and ejaculation latencies. Those rats presented a higher total dendritic density in the mPFC, and a higher numerical density of thin, mushroom, stubby, and wide spines. Sexual experience also increased the numerical density of mushroom spines in the NAcc. In both the mPFC and NAcc of the sexually experienced rats, there was a lower proportional density of thin spines and a higher proportional density of mushroom spines. Results show that the improvement in copulatory efficiency resulting from prior sexual experience in male rats is associated with changes in the proportional density of thin and mushroom dendritic spines in the mPFC and NAcc. This could represent the consolidation of afferent synaptic information in these brain regions, derived from the stimulus-sexual reward association.

The effect of single-cell knockout of Fragile X Messenger Ribonucleoprotein on synaptic structural plasticity.

Fragile X Syndrome (FXS) is the best-known form of inherited intellectual disability caused by the loss-of-function mutation in a single gene. The FMR1 gene mutation abolishes the expression of Fragile X Messenger Ribonucleoprotein (FMRP), which regulates the expression of many synaptic proteins. Cortical pyramidal neurons in postmortem FXS patient brains show abnormally high density and immature morphology of dendritic spines; this phenotype is replicated in the Fmr1 knockout (KO) mouse. While FMRP is well-positioned in the dendrite to regulate synaptic plasticity, intriguing in vitro and in vivo data show that wild type neurons embedded in a network of Fmr1 KO neurons or glia exhibit spine abnormalities just as neurons in Fmr1 global KO mice. This raises the question: does FMRP regulate synaptic morphology and dynamics in a cell-autonomous manner, or do the synaptic phenotypes arise from abnormal pre-synaptic inputs? To address this question, we combined viral and mouse genetic approaches to delete FMRP from a very sparse subset of cortical layer 5 pyramidal neurons (L5 PyrNs) either during early postnatal development or in adulthood. We then followed the structural dynamics of dendritic spines on these Fmr1 KO neurons by in vivo two-photon microscopy. We found that, while L5 PyrNs in adult Fmr1 global KO mice have abnormally high density of thin spines, single-cell Fmr1 KO in adulthood does not affect spine density, morphology, or dynamics. On the contrary, neurons with neonatal FMRP deletion have normal spine density but elevated spine formation at 1 month of age, replicating the phenotype in Fmr1 global KO mice. Interestingly, these neurons exhibit elevated thin spine density, but normal total spine density, by adulthood. Together, our data reveal cell-autonomous FMRP regulation of cortical synaptic dynamics during adolescence, but spine defects in adulthood also implicate non-cell-autonomous factors.

Protective Effects of Resveratrol on Adolescent Social Isolation-Induced Anxiety-Like Behaviors via Modulating Nucleus Accumbens Spine Plasticity and Mitochondrial Function in Female Rats.

Social isolation (SI) is a major risk factor for mood disorders in adolescents. The nucleus accumbens (NAc) is an important reward center implicated in psychiatric disorders. Resveratrol (RSV) is one of the most effective natural polyphenols with anti-anxiety and depression effects. However, little is known about the therapeutic effects and mechanisms of RSV on behavioral abnormality of adolescent social stress. Therefore, this study aimed to investigate the underlying mechanism of RSV on the amelioration of SI-induced behavioral abnormality. We found that SI induced anxiety-like behavior and social dysfunction in isolated female rats. Moreover, SI reduced mitochondrial number and ATP levels and increased thin spine density in the NAc. RNA sequencing results showed that SI changed the transcription pattern in the NAc, including 519 upregulated genes and 610 downregulated genes, especially those related to mitochondrial function. Importantly, RSV ameliorated behavioral and spine abnormalities induced by SI and increased NAc ATP levels and mitochondria number. Furthermore, RSV increased the activity of cytochrome C oxidase (COX) and upregulated mRNA levels of Cox5a, Cox6a1 and Cox7c. These results demonstrate that the modulation of spine plasticity and mitochondrial function in the NAc by RSV has a therapeutic effect on mood disorders induced by social isolation.

Disruption of amphetamine sensitization by alteration of dendritic thin spines in the nucleus accumbens core.

Repeated injections of psychomotor stimulants like amphetamine (AMPH) to rodents can induce behavioral sensitization, which represents a long-lasting craving that is usually observed in human addicts. Behavioral sensitization is characteristically maintained for a long duration, accompanied by structural plasticity in some brain areas involved in reward circuitry. For example, it increased dendritic spine densities in the nucleus accumbens (NAcc), which is considered to reflect neurophysiological changes at this site, leading to addictive behaviors. The ezrin, radixin, and moesin (ERM) proteins regulate spine maturity by modifying their phosphorylation at the C-terminal region. We previously showed that ERM phosphorylation is reduced by AMPH in the NAcc core, suggesting that ERM-mediated spine changes at this site might be associated with AMPH sensitization. To test this hypothesis, we administered AMPH to rats according to a sensitization development schedule, with lentivirus encoding a phosphomimetic pseudo-active mutant of radixin (Rdx T564D) in the NAcc core, and examined dendritic spines at this site. We found that compared to acute AMPH, AMPH sensitization increased thin spine density with a similar ratio of filopodia-like to mature thin spines. However, with Rdx T564D, the density of thin spines increased, with augmented filopodia-like thin spines, resulting in no AMPH sensitization. These results indicate that Rdx T564D forces thin spines to immaturity and thereby inhibits AMPH sensitization, for which an increase in mature thin spines is normally necessary. These findings provide significant clues to our understanding of the role of dendritic spines in mediating the development of psychomotor stimulant addiction.

Conditional deletion of ROCK2 induces anxiety-like behaviors and alters dendritic spine density and morphology on CA1 pyramidal neurons

Rho-associated kinase isoform 2 (ROCK2) is an attractive drug target for several neurologic disorders. A critical barrier to ROCK2-based research and therapeutics is the lack of a mouse model that enables investigation of ROCK2 with spatial and temporal control of gene expression. To overcome this, we generated ROCK2fl/fl mice. Mice expressing Cre recombinase in forebrain excitatory neurons (CaMKII-Cre) were crossed with ROCK2fl/fl mice (Cre/ROCK2fl/fl), and the contribution of ROCK2 in behavior as well as dendritic spine morphology in the hippocampus, medial prefrontal cortex (mPFC), and basolateral amygdala (BLA) was examined. Cre/ROCK2fl/fl mice spent reduced time in the open arms of the elevated plus maze and increased time in the dark of the light–dark box test compared to littermate controls. These results indicated that Cre/ROCK2fl/fl mice exhibited anxiety-like behaviors. To examine dendritic spine morphology, individual pyramidal neurons in CA1 hippocampus, mPFC, and the BLA were targeted for iontophoretic microinjection of fluorescent dye, followed by high-resolution confocal microscopy and neuronal 3D reconstructions for morphometry analysis. In dorsal CA1, Cre/ROCK2fl/fl mice displayed significantly increased thin spine density on basal dendrites and reduced mean spine head volume across all spine types on apical dendrites. In ventral CA1, Cre/ROCK2fl/fl mice exhibited significantly increased spine length on apical dendrites. Spine density and morphology were comparable in the mPFC and BLA between both genotypes. These findings suggest that neuronal ROCK2 mediates spine density and morphology in a compartmentalized manner among CA1 pyramidal cells, and that in the absence of ROCK2 these mechanisms may contribute to anxiety-like behaviors.

PINK1 Silencing Modifies Dendritic Spine Dynamics of Mouse Hippocampal Neurons.

PTEN-induced kinase 1 (PINK1) mutations can cause early-onset Parkinson's disease and patients are likely to develop cognitive decline, depression, and dementia. Several neurophysiological studies have demonstrated PINK1 deficiency impairs striatal and hippocampal presynaptic plasticity. Dendritic spine postsynaptic abnormalities are common in neurological diseases; however, whether PINK1 silencing modifies dendritic spine dynamics of hippocampal neurons is unclear. To address this question, confocal images of mouse cultured hippocampal neurons transfected with plasmids to silence PINK1 were analyzed. These studies revealed that PINK1 silencing increased density of thin spines and reduced head size of stubby spines. Immunoblotting analysis uncovered that PINK1 silencing decreased expression of postsynaptic density proteins (PSD95 and Shank) and glutamate receptors (NR2B and mGluR5). We also found PINK1 silencing regulated dendritic spine morphology by actin regulatory proteins (RhoGAP29 and ROCK2) and regulated neuronal survival by decreased Akt activation. These results suggest PINK1 may regulate postsynaptic plasticity in hippocampal neurons generating presymptomatic alterations in dendritic spines that eventually could lead to the neurodegeneration and cognitive decline often seen in Parkinson's disease.

Social instability in adolescence differentially alters dendritic morphology in the medial prefrontal cortex and its response to stress in adult male and female rats.

Adolescence is an important period for HPA axis development and synapse maturation and reorganization in the prefrontal cortex (PFC). Thus, stress during adolescence could alter stress-sensitive brain regions such as the PFC and may alter the impact of future stressors on these brain regions. Given that women are more susceptible to many stress-linked psychological disorders in which dysfunction of PFC is implicated, and that this increased vulnerability emerges in adolescence, stress during this time could have sex-dependent effects. Therefore, we investigated the effects of adolescent social instability stress (SIS) on dendritic morphology of Golgi-stained pyramidal cells in the medial PFC of adult male and female rats. We then examined dendritic reorganization following chronic restraint stress (CRS) with and without a rest period in adult rats that had been stressed in adolescence. Adolescent SIS conferred long-term alterations in prelimbic of males and females, whereby females show reduced apical length and basilar thin spine density and males show reduced basilar length. CRS in adulthood failed to produce immediate dendritic remodeling in SIS rats. However, CRS followed by a rest period reduced apical dendritic length and increases mushroom spine density in adolescently stressed adult males. Conversely, CRS followed by rest produced apical outgrowth and decreased mushroom spine density in adolescently stressed adult females. These results suggest that stress during adolescence alters development of the PFC and modulates stress-induced dendritic changes in adulthood.

Different roles of Rac1 in the acquisition and extinction of methamphetamine-associated contextual memory in the nucleus accumbens.

Rationale: Repeated methamphetamine (METH) exposure induces long-term cognitive deficits and pathological drug-associated memory that can be disrupted by manipulating memory reconsolidation and extinction. The nucleus accumbens (NAc) is the key region of the brain reward system and predominantly consists of two subtypes of medium spiny neurons (MSNs) based on the expression of D1 or D2 dopamine receptors (D1-MSNs or D2-MSNs). Spine structural plasticity in the NAc is critical for the acquisition, reconsolidation and extinction of drug-associated memory. However, the molecular mechanisms underlying METH-associated memory and spine remodelling in each type of MSNs in the NAc remain unknown. Here, we explored whether Rac1 in the NAc mediates METH-associated contextual memory and spine remodelling.Methods: Pharmacological and genetic manipulations of Rac1 were used to investigate its role during the acquisition, reconsolidation and extinction of METH-associated contextual memory. Recombinant adeno-associated viruses expressing mCherry under the control of the dopamine D1 receptor gene promoter (Drd1-mCherry) or dopamine D2 receptor gene promoter (Drd2-mCherry) were used to specifically label D1-MSNs or D2-MSNs.Results: Using viral-mediated gene transfer, we demonstrated that decreased Rac1 activity was required for the acquisition of METH-associated contextual memory and the METH-induced increase in thin spine density, whereas increased Rac1 signalling was important for the extinction of METH-associated contextual memory and the related elimination of thin spines. Moreover, the increase of dendritic spines was both found in D1-MSNs and D2-MSNs during the acquisition process, but extinction training selectively decreased the spine density in D1-MSNs. Interestingly, Rac1 was responsible for METH-induced spine plasticity in D1-MSNs but not in D2-MSNs. Additionally, we found that microinjection of a Rac1 inhibitor or activator into the NAc was not sufficient to disrupt reconsolidation, and the pharmacological activation of Rac1 in the NAc facilitated the extinction of METH-associated contextual memory. Regarding cognitive memory, decreased Rac1 activity improved the METH-induced impairment in object recognition memory.Conclusion: Our findings indicate that Rac1 plays opposing roles in the acquisition and extinction of METH-associated contextual memory and reveal the cell-specific role of Rac1 in METH-associated spine remodelling, suggesting that Rac1 is a potential therapeutic target for reducing relapse in METH addiction and remediating METH-induced recognition memory impairment.

Sex-Dependent Ketamine Addiction-Like Behavior Profile Following Exposure to Chronic Mild Stress

Background Ketamine has rapid antidepressant effects and shows great promise as a novel treatment for depression, but its limitations including its abuse potential are poorly understood. Given that the prevalence of depression is twice as high in women as in men and that depression and substance use disorders are highly comorbid, we hypothesized that a sex-specific responsivity to behavioral assays that characterize addiction-like behavior may arise in rats with prior exposure to chronic stress and therapeutically relevant ketamine. Methods Male and female rats that underwent chronic mild stress were treated with four 1.47 mg/kg intravenous ketamine infusions once every fourth day and underwent operant self-administration of 0.5 mg/kg/infusion ketamine. Measures of anhedonia (or lack of pleasure, a signature feature of depression), anxiety-induced neophagia, motivation to obtain ketamine, and craving were assessed using the sucrose intake test, novelty-suppressed feeding test, progressive ratio schedule of reinforcement, and incubation of craving following abstinence, respectively. Finally, dendritic spine density in the nucleus accumbens core was measured. Results Ketamine infusions reduced anxiety-induced neophagia in both male rats and female rats but had no effect on measures of anhedonia. Female rats with prior exposure to chronic mild stress had greater motivation to obtain ketamine compared to nonstressed female rats, an effect not observed in male rats. Additionally, female rats who received antidepressant ketamine infusions had a higher threshold for displaying ketamine addiction-like behavior than saline-treated female rats as well as increased thin spine density in the nucleus accumbens core. These effects were not observed in male rats. Conclusion This study shows that repeated low-dose ketamine does not increase abuse potential of subsequent ketamine. It also highlights an important female-specific effect of stress to increase ketamine addiction-like behavior, which requires further investigation for clinical populations.

Selective Loss of Thin Spines in Area 7a of the Primate Intraparietal Sulcus Predicts Age-Related Working Memory Impairment.

Brodmann area 7a of the parietal cortex is active during working memory tasks in humans and nonhuman primates, but the composition and density of dendritic spines in area 7a and their relevance both to working memory and cognitive aging remain unexplored. Aged monkeys have impaired working memory, and we have previously shown that this age-induced cognitive impairment is partially mediated by a loss of thin spines in prefrontal cortex area 46, a critical area for working memory. Because area 46 is reciprocally connected with area 7a of the parietal cortex and 7a mediates visual attention integration, we hypothesized that thin spine density in area 7a would correlate with working memory performance as well. To investigate the synaptic profile of area 7a and its relevance to working memory and cognitive aging, we investigated differences in spine type and density in layer III pyramidal cells of area 7a in young and aged, male and female rhesus macaques (Macaca mulatta) that were cognitively assessed using the delayed response test of working memory. Area 7a shows age-related loss of thin spines, and thin spine density positively correlates with delayed response performance in aged monkeys. In contrast, these cells show no age-related changes in dendritic length or branching. These changes mirror age-related changes in area 46 but are distinct from other neocortical regions, such as V1. These findings support our hypothesis that cognitive aging is driven primarily by synaptic changes, and more specifically by changes in thin spines, in key association areas.SIGNIFICANCE STATEMENT This study advances our understanding of cognitive aging by demonstrating the relevance of area 7a thin spines to working memory performance. This study is the first to look at cognitive aging in the intraparietal sulcus, and also the first to report spine or dendritic measures for area 7a in either young adult or aged nonhuman primates. These results contribute to the hypothesis that thin spines support working memory performance and confirm our prior observation that cognitive aging is driven by synaptic changes rather than changes in dendritic morphology or neuron death. Importantly, these data show that age-related working memory changes are not limited to disruptions of the prefrontal cortex but also include an association region heavily interconnected with prefrontal cortex.

Atomoxetine prevents working memory loss in hyperactive rats, mediating plastic changes in prefrontal cortex pyramidal neurons

Attention Deficit Hyperactivity Disorder (ADHD) causes impaired visuospatial working memory (VWM), which primarily maps to the prefrontal cortex. However, little is known about the synaptic processes underlying cognitive loss in ADHD, or those ultimately involved in the preventive effect observed through the clinical use of Atomoxetine (ATX). To investigate the plasticity underlying ADHD related cognitive loss, and that potentially involved in the preventive action of Atomoxetine, allocentric VWM was assessed, as well as the dendritic spine number and proportional density on pyramidal neurons in the prefrontal cerebral cortex layer III of neonatal 6-hydroxydopamine-lesioned rats. The effect of acute ATX treatment was also assessed at 28 days of age. 6-OHDA induced lesions produced increased motor activity and a loss of VWM, concomitant with a reduction in thin spine density. ATX administration reversed cognitive loss, in conjunction with a decrease in thin spines and an increase in mushroom spines. A reduction in the proportion of spines involved in learning in hyperactive animals could account for the loss in cognitive function observed. Considering thin spine density was also reduced after ATX administration, we hypothesized that the restoration in cognitive function recorded could be brought about by an increase in memory related mushroom spines.

High-Salt Exposure During Perinatal Development Enhances Stress Sensitivity.

Excess consumption of dietary sodium during pregnancy has been shown to impair offspring cardiovascular function and enhance salt preference in adulthood, but little is known regarding the long-term impact of this nutritional surplus on offspring brain morphology and behavior. Using a combination of cellular and behavioral approaches, we examined the impact of maternal salt intake during the perinatal period on structural plasticity in the prefrontal cortex (PFC) and nucleus accumbens (NAc) in weanling and adult offspring as well as reward- and stress-driven behaviors in adult offspring. We found that weanling rats born to 4% NaCl-fed dams exhibited an increase and decrease in thin spine density in the infralimbic PFC (IL-PFC) and prelimbic PFC (PL-PFC), respectively, as well as an increase in mushroom spine density in the NAc shell, compared to 1% NaCl-fed controls. Structural changes in the IL-PFC and NAc shell persisted into adulthood, the latter of which is a phenotype that has been observed in rats exposed to early life stress. There was no effect of maternal salt intake on reward-driven behaviors, including sucrose preference, conditioned place preference (CPP) for cocaine, and forced swim stress (FSS)-induced reinstatement of cocaine-induced CPP. However, rats born to high-salt fed dams spent less time swimming in the FSS and displayed heightened plasma CORT levels in response to the FSS compared to controls, suggesting that early salt exposure increases stress sensitivity. Overall, our results suggest that perinatal salt exposure evokes lasting impacts on offspring physiology and behavior.

ASD-Associated De Novo Mutations in Five Actin Regulators Show Both Shared and Distinct Defects in Dendritic Spines and Inhibitory Synapses in Cultured Hippocampal Neurons.

Many actin cytoskeleton-regulating proteins control dendritic spine morphology and density, which are cellular features often altered in autism spectrum disorder (ASD). Recent studies using animal models show that autism-related behavior can be rescued by either manipulating actin regulators or by reversing dendritic spine density or morphology. Based on these studies, the actin cytoskeleton is a potential target pathway for developing new ASD treatments. Thus, it is important to understand how different ASD-associated actin regulators contribute to the regulation of dendritic spines and how ASD-associated mutations modulate this regulation. For this study, we selected five genes encoding different actin-regulating proteins and induced ASD-associated de novo missense mutations in these proteins. We assessed the functionality of the wild-type and mutated proteins by analyzing their subcellular localization, and by analyzing the dendritic spine phenotypes induced by the expression of these proteins. As the imbalance between excitation and inhibition has been suggested to have a central role in ASD, we additionally evaluated the density, size and subcellular localization of inhibitory synapses. Common for all the proteins studied was the enrichment in dendritic spines. ASD-associated mutations induced changes in the localization of α-actinin-4, which localized less to dendritic spines, and for SWAP-70 and SrGAP3, which localized more to dendritic spines. Among the wild-type proteins studied, only α-actinin-4 expression caused a significant change in dendritic spine morphology by increasing the mushroom spine density and decreasing thin spine density. We hypothesized that mutations associated with ASD shift dendritic spine morphology from mushroom to thin spines. An M554V mutation in α-actinin-4 (ACTN4) resulted in the expected shift in dendritic spine morphology by increasing the density of thin spines. In addition, we observed a trend toward higher thin spine density with mutations in myosin IXb and SWAP-70. Myosin IIb and myosin IXb expression increased the proportion of inhibitory synapses in spines. The expression of mutated myosin IIb (Y265C), SrGAP3 (E469K), and SWAP-70 (L544F) induced variable changes in inhibitory synapses.

Loss of thin spines and small synapses contributes to defective hippocampal function in aged mice.

Aging is a normal physiological process associated with impairments in cognitive function, including learning and memory. Here, the underlying synaptic mechanisms by which aging leads to the decline of spatial learning and memory function were investigated in 25-month-old aged mice versus 2-month-old young mice. Deficits of spatial learning and memory, as well as selective loss of thin spines, but not mushroom-type spines on apical dendrites of CA1 pyramidal cells were found in aged mice. Specifically, loss of thin spines in aged mice with memory deficits was primarily found on dendritic segments located in the Schaffer pathway, and the density of thin spines significantly correlated with spatial memory performance. The loss of thin spines was evidenced by a decrease in small synapses that express diminutive amounts of postsynaptic density protein-95 and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunit GluR1. Furthermore, mushroom-type spines and GluR1-expressed large synapses were not affected in aged mice with impaired memory. Taken together, these data suggest that the selective loss of those highly plastic thin spines with sparse postsynaptic density protein-95 and GluR1 receptors may significantly contribute to cognitive deficits in aged individuals.

Running-induced memory enhancement correlates with the preservation of thin spines in the hippocampal area CA1 of old C57BL/6 mice

The effects of prolonged physical training on memory performance and underlying synaptic mechanisms were investigated in old C57BL/6 mice. Training via voluntary running wheels was initiated at 16 months of age and continued for 5 months (1 hour per day and 5 days per week), followed by learning and memory test and spine/synapse analysis. Trained old mice were compared with their age-matched sedentary controls and aged adult controls. This training improved hippocampal-dependent spatial learning and memory function in old mice, and enhanced cognition was accompanied by increased density of spines on CA1 pyramidal cells in the hippocampus. Particularly, the training selectively affected thin spines that carry small synapses in the stratum radiatum, the target area of CA3 Schaffer pathway. Furthermore, increased density of thin spines positively correlates with improved memory performance. Finally, the training prevented age-related loss of postsynaptic density protein-95 in the Schaffer pathway. These data suggest that the preservation of thin spines carrying small synapses in specific hippocampal region contributes critically to running-related improvement of learning and memory function.

Dynamic Alterations of Rat Nucleus Accumbens Dendritic Spines over 2 Months of Abstinence from Extended-Access Cocaine Self-Administration.

Chronic cocaine exposure influences the density and morphology of dendritic spines on medium spiny neurons (MSNs) in the nucleus accumbens (NAc), a critical brain region for cocaine craving. However, the relationship between spine plasticity and craving remains unclear. To study this relationship, we trained rats to self-administer cocaine using an extended-access regimen (6 h per day, 10 days); controls self-administered saline. Previously, a time-dependent intensification (incubation) of cue-induced cocaine craving has been demonstrated after withdrawal from this regimen; furthermore, Ca2+-permeable AMPA receptors (CP-AMPARs) increase in the NAc core after ~1 month of withdrawal and thereafter mediate the expression of incubated craving. Although neither craving nor CP-AMPAR levels were measured in the present study, we killed rats at four withdrawal day (WD) time-points (WD14, WD25, WD36, or WD60) selected to span the rising phase of incubation and the transition from low to high CP-AMPAR levels. MSNs were iontophoretically filled with Lucifer yellow and spines were analyzed with NeuronStudio software. Compared with saline controls, cocaine rats showed no changes in spine density or morphology in the NAc core on WD14 or WD25. On WD36, approximately the withdrawal time when stable elevation of CP-AMPAR levels is detected, the cocaine group exhibited increased density of thin spines in the NAc core. By WD60, however, this effect had reversed: the density of thin spines was lower in cocaine rats compared with saline rats. In contrast, craving and CP-AMPAR levels remain high on WD60. We also assessed spine density on WD36 in the dorsolateral striatum, a region that is not implicated in incubation of cocaine craving and does not undergo CP-AMPAR plasticity. Here, the cocaine group exhibited a small leftward shift in the distribution of spine densities plotted as a cumulative distribution, opposite to the effect found in the NAc core. Overall, our results demonstrate changes in NAc core spines over 2 months of withdrawal but no simple relationship between the time dependency of these spine changes and the previously demonstrated time course of incubation of cocaine craving. However, they raise the possibility that CP-AMPAR accumulation in the NAc core occurs in a population of thin spines that emerges after ~1 month of withdrawal.

Sex-specific effects of early life stress on social interaction and prefrontal cortex dendritic morphology in young rats

Early life stress has been linked to depression, anxiety, and behavior disorders in adolescence and adulthood. The medial prefrontal cortex (mPFC) is implicated in stress-related psychopathology, is a target for stress hormones, and mediates social behavior. The present study investigated sex differences in early-life stress effects on juvenile social interaction and adolescent mPFC dendritic morphology in rats using a maternal separation (MS) paradigm. Half of the rat pups of each sex were separated from their mother for 4h a day between postnatal days 2 and 21, while the other half remained with their mother in the animal facilities and were exposed to minimal handling. At postnatal day 25 (P25; juvenility), rats underwent a social interaction test with an age and sex matched conspecific. Distance from conspecific, approach and avoidance behaviors, nose-to-nose contacts, and general locomotion were measured. Rats were euthanized at postnatal day 40 (P40; adolescence), and randomly selected infralimbic pyramidal neurons were filled with Lucifer yellow using iontophoretic microinjections, imaged in 3D, and then analyzed for dendritic arborization, spine density, and spine morphology. Early-life stress increased the latency to make nose-to-nose contact at P25 in females but not males. At P40, early-life stress increased infralimbic apical dendritic branch number and length and decreased thin spine density in stressed female rats. These results indicate that MS during the postnatal period influenced juvenile social behavior and mPFC dendritic arborization in a sex-specific manner.

Exposure to a diet high in fat attenuates dendritic spine density in the medial prefrontal cortex.

A key factor in the development of obesity is the overconsumption of food calorically high in fat. Overconsumption of food high in fat not only promotes weight gain but elicits changes in reward processing. No studies to date have examined whether consumption of a high-fat (HF) diet alters structural plasticity in brain areas critical for reward processing, which may account for persistent changes in behavior and psychological function by reorganizing synaptic connectivity. To test whether dietary fat may induce structural plasticity we placed rats on one of three dietary conditions: ad libitum standard chow (SC), ad libitum 60% HF (HF-AL), or calorically matched 60% HF (HF-CM) for 3weeks and then quantified dendritic spine density and type on basal and apical dendrites of pyramidal cells in layer V of the medial prefrontal cortex (mPFC) and medium spiny neurons (MSNs) of the nucleus accumbens. Our results demonstrate a significant reduction in the density of thin spines on the apical and basal segments of dendrites within the infralimbic, but not prelimbic, mPFC.

Aβ-mediated spine changes in the hippocampus are microtubule-dependent and can be reversed by a subnanomolar concentration of the microtubule-stabilizing agent epothilone D

Dendritic spines represent the major postsynaptic input of excitatory synapses. Loss of spines and changes in their morphology correlate with cognitive impairment in Alzheimer's disease (AD) and are thought to occur early during pathology. Therapeutic intervention at a preclinical stage of AD to modify spine changes might thus be warranted. To follow the development and to potentially interfere with spine changes over time, we established a long term ex vivo model from organotypic cultures of the hippocampus from APP transgenic and control mice. The cultures exhibit spine loss in principal hippocampal neurons, which closely resembles the changes occurring in vivo, and spine morphology progressively changes from mushroom-shaped to stubby. We demonstrate that spine changes are completely reversed within few days after blocking amyloid-β (Aβ) production with the gamma-secretase inhibitor DAPT. We show that the microtubule disrupting drug nocodazole leads to spine loss similar to Aβ expressing cultures and suppresses DAPT-mediated spine recovery in slices from APP transgenic mice. Finally, we report that epothilone D (EpoD) at a subnanomolar concentration, which slightly stabilizes microtubules in model neurons, completely reverses Aβ-induced spine loss and increases thin spine density. Taken together the data indicate that Aβ causes spine changes by microtubule destabilization and that spine recovery requires microtubule polymerization. Moreover, our results suggest that a low, subtoxic concentration of EpoD is sufficient to reduce spine loss during the preclinical stage of AD.

Unmasking Proteolytic Activity for Adult Visual Cortex Plasticity by the Removal of Lynx1.

Experience-dependent proteolytic activity of tissue plasminogen activator (tPA) becomes restricted in the adult brain in correlation with the decline in cortical plasticity when mice are raised in standard cages. We demonstrated that removal of Lynx1, one of negative regulators of plasticity, unmasks experience-dependent tPA elevation in visual cortex of adult mice reared in standard cages. This proteolytic elevation facilitated dendritic spine reduction and ocular dominance plasticity in adult visual cortex. This is the first demonstration of adult brain to retain the intrinsic capacity to elevate tPA in an experience-dependent manner but is effectively limited by Lynx1. tPA-Lynx1 may potentially be a new candidate mechanism for interventions that were shown to activate plasticity in adult brain.