Distinct ventral and dorsal hippocampus AP5 anxiolytic effects revealed in the elevated plus-maze task in rats

Involvement of the basolateral amygdaloid nucleus anterior part 5-HT7 receptors in the regulation of anxiety-like behaviors in hemiparkinsonian rats

Anxiety is a common non-motor symptom of Parkinson's disease (PD), but its neurobiological mechanism is obscure. 5-hydroxytryptamine7 (5-HT7) receptor is associated with anxiety and is widely distributed in brain regions related to emotion regulation, including anterior part of basolateral amygdaloid nucleus (BLA), and monosynaptic glutamatergic BLA to ventral hippocampus (vHPC) (BLAGlu-vHPC) pathway modulates anxiety-related behaviors. Measurable pathological and pathophysiological changes within the amygdala and hippocampus have also been reported in PD patients and parkinsonian animals. Thus, we hypothesized that BLA 5-HT7 receptors might regulate PD-related anxiety through BLAGlu-vHPC pathway. In this study, we found that down-regulation of BLA 5-HT7 receptors by RNA interference produced anxiolytic effects in sham and 6-hydroxydopamine-lesioned rats. And intra-BLA injection of 5-HT7 receptor agonist AS19 and antagonist SB269970 induced anxiogenic and anxiolytic responses in the two groups of rats. Further, intra-BLA injection of AS19 and SB269970 increased and decreased the mean firing rate of BLA glutamatergic neurons and vHPC extracellular glutamate levels in sham and the lesioned rats, respectively. Compared to sham rats, the effects of AS19 and SB269970 on the anxiety-related behaviors, firing activity and transmitter levels were decreased in the lesioned rats, which are associated with decreased expression of 5-HT7 receptors on BLAGlu-vHPC pathway after substantia nigra pars compacta lesion. Collectively, these results suggest that activation and blockade of 5-HT7 receptors on the BLAGlu-vHPC pathway are involved in the regulation of PD-related anxiety, and dopaminergic lesion decreases the expression of 5-HT7 receptors on this neural pathway.

Hippocampal Synaptic Plasticity: Integrating Memory and Anxiety Impairments in the Early Stages of Alzheimer's Disease

A decline in hippocampal function has long been associated with the progression of cognitive impairments in patients with Alzheimer's disease (AD). The disruption of hippocampal synaptic plasticity [primarily the reduction of long-term potentiation LTP] by excess production of soluble beta-amyloid (Aβ) has long been accepted as the mechanism by which AD pathology impairs memory, at least during the early stages of AD pathogenesis. However, the premise that hippocampal LTP underpins the formation of associative, long-term memories has been challenged. Here, we consider evidence that this canonical view of LTP needs to be refined. Similarly, the view that the hippocampus simply supports memory ignores the wealth of data showing that the hippocampus is functionally heterogeneous along its septo-temporal axis. The ventral (but not the dorsal) hippocampus plays a major role in modulating emotional reactions to conflict. Here, we suggest that hippocampal LTP is not involved in forming long-term associative memories, but instead contributes to the disambiguation of overlapping memories in situations of conflict and associative interference. This conceptualisation of hippocampal synaptic plasticity may help explain how early-stage AD pathology may impact both memory and anxiety.

The role of NMDA receptors in fish stress response: Assessments based on physiology of the caudal neurosecretory system and defensive behavior

Stress strongly influences the physiology and behavior of animals, and leads into a pathological condition and disease. NMDA receptors (NMDARs) play a crucial role in the modulation of neural activity. To understand the role of NMDARs in fish stress response, we used NMDARs agonist aspartate to test the functional role of its input on the Dahlgren cell population in the caudal neurosecretory system (CNSS) of the olive flounder. In addition, the effect of the NMDARs antagonist D-AP5 on the expression of genes of the main secretory products of the CNSS after stress was investigated by using qPCR technology and the effect of the NMDARs antagonist D-AP5 on post-stress behavior was explored by behavioral methods. Ex vivo electrophysiological experiments showed that the NMDARs agonist aspartate enhanced the firing frequency of Dahlgren cells. Additionally, aspartate treatment increased the incidence of cells exhibiting bursting firing pattern, this result is corroborated by the observed upregulation in the expression of ion channels and major hormone genes in the CNSS. Furthermore, the excitatory influence of aspartate was effectively counteracted by NMDARs antagonist D-AP5. Interestingly, NMDARs antagonist D-AP5 treatment also significantly decreased the plasma cortisol levels and the expression of CRH, UI, and UII in CNSS after acute stress. Treatment with D-AP5 effectively attenuated the stress response, as evidenced by alterations in respiratory metabolism, sand-burying behavior, swimming distance, simulated capture, and escape response. In conclusion, modulation of Dahlgren cell excitability in the CNSS by NMDARs contributes to the regulation of the stress response, NMDARs antagonist D-AP5 can effectively suppress stress response in flounder by regulating the stress hormone expression and secretion. CLINICAL TRIAL REGISTRATION: Project code SHOU-DW-2022-032.

The effects of moderate prenatal alcohol exposure on performance in hippocampal-sensitive spatial memory and anxiety tasks by adult male and female rat offspring

Moderate prenatal alcohol exposure (mPAE) results in structural alterations to the hippocampus. Previous studies have reported impairments in hippocampal-sensitive tasks, but have not compared performance between male and female animals. In the present study, performance in hippocampal-sensitive spatial memory and anxiety behavior tests were compared across adult male and female saccharin (SACC) control mPAE Long-Evans rat offspring. Two tests of spatial memory were conducted that were aimed at assessing memory for recently acquired spatial information: A delayed spatial alternation task using an M-shaped maze and a delayed match-to-place task in the Morris water task. In both tasks, rats in SACC and mPAE groups showed similar learning and retention of a spatial location even after a 2-h interval between encoding and retention. A separate group of adult male and female SACC and mPAE rat offspring were tested for anxiety-like behaviors in the elevated plus-maze paradigm. In this test, both male and female mPAE rats exhibited a significantly greater amount of time and a greater number of head dips in the open arms, while locomotion and open arm entries did not differ between groups. The results suggest that mPAE produces a reduction in anxiety-like behaviors in both male and female rats in the elevated plus-maze.

Inactivation of the dorsal CA1 hippocampus impairs the consolidation of discriminative avoidance memory by modulating the intrinsic and extrinsic hippocampal circuitry

Despite progress in understanding the role of the dorsal hippocampus in the acquisition, consolidation and retrieval of episodic-like memory, plastic changes within the intra- and extrahippocampal circuits for aversive memory formation and anxiety-like behaviours must still be identified since both processes contribute to multiple aspects of flexible decision-making. Here, we investigated the effect of reversible inactivation induced by a muscimol microinfusion into the dorsal CA1 subfield (dCA1) either prior to acquisition or to retrieval testing of a discriminative avoidance task performed in a plus-maze apparatus (PM-DAT). Differential cAMP-response-element-binding protein 1 (CREB-1) expression in the dorsal and ventral CA1 and CA3 of the hippocampus (dCA1, dCA3, vCA1, and vCA3), dorsal dentate gyrus (dDG), and infralimbic (IL) and prelimbic (PrL) regions of the medial prefrontal cortex was also assessed to investigate the molecular changes associated with the consolidation or retrieval of episodic-like memory and anxiety. Adult male Wistar rats were assigned to two control groups, learning (no surgery/no microinfusion, n = 7) and sham-operated (sham surgery/no microinfusion, n = 6) groups, or four experimental groups, in which the vehicle (0.5 µl per side, n = 8/per group) or a GABA_A receptor agonist (0.5 µg/0.5 µl muscimol/per side) was bilaterally microinfused in the dCA1 30 min prior to training (n = 9) or prior to testing sessions (n = 6) with a 24 h intertrial interval. Memory was evaluated using the percentage of time spent in the nonaversive enclosed arms, whereas anxiety was measured by calculating the percentages of time spent and entries into open arms and the percentage of time spent self-grooming. Our findings corroborated previous data showing that the dCA1 is required for discriminative avoidance consolidation. Furthermore, additional information indicated that impaired long-term memory was associated with downregulated CREB-1 expression in the dDG and vCA3. Moreover, memory retrieval was not impaired by dCA1 inactivation prior to the testing session, which was associated with the upregulation of CREB-1 in the dCA3 and vCA1 and downregulation in the dCA1 and vCA3. Differential expression of CREB was not identified in the IL or PrL areas. These results improve our understanding of how the hippocampal circuitry mediates the acquisition and retrieval of aversive memory and anxiety.

Adolescent Δ-9-tetrahydrocannabinol exposure induces differential acute and long-term neuronal and molecular disturbances in dorsal vs. ventral hippocampal subregions

Chronic exposure to Δ-9-tetrahydrocannabinol (THC) during adolescence is associated with long-lasting cognitive impairments and enhanced susceptibility to anxiety and mood disorders. Previous evidence has revealed functional and anatomical dissociations between the posterior vs. anterior portions of the hippocampal formation, which are classified as the dorsal and ventral regions in rodents, respectively. Notably, the dorsal hippocampus is critical for cognitive and contextual processing, whereas the ventral region is critical for affective and emotional processing. While adolescent THC exposure can induce significant morphological disturbances and glutamatergic signaling abnormalities in the hippocampus, it is not currently understood how the dorsal vs. ventral hippocampal regions are affected by THC during neurodevelopment. In the present study, we used an integrative combination of behavioral, molecular, and neural assays in a neurodevelopmental rodent model of adolescent THC exposure. We report that adolescent THC exposure induces long-lasting memory deficits and anxiety like-behaviors concomitant with a wide range of differential molecular and neuronal abnormalities in dorsal vs. ventral hippocampal regions. In addition, using matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS), we show for the first time that adolescent THC exposure induces significant and enduring dysregulation of GABA and glutamate levels in dorsal vs. ventral hippocampus. Finally, adolescent THC exposure induced dissociable dysregulations of hippocampal glutamatergic signaling, characterized by differential glutamatergic receptor expression markers, profound alterations in pyramidal neuronal activity and associated oscillatory patterns in dorsal vs. ventral hippocampal subregions.

PEN Receptor GPR83 in Anxiety-Like Behaviors: Differential Regulation in Global vs Amygdalar Knockdown

Anxiety disorders are prevalent across the United States and result in a large personal and societal burden. Currently, numerous therapeutic and pharmaceutical treatment options exist. However, drugs to classical receptor targets have shown limited efficacy and often come with unpleasant side effects, highlighting the need to identify novel targets involved in the etiology and treatment of anxiety disorders. GPR83, a recently deorphanized receptor activated by the abundant neuropeptide PEN, has also been identified as a glucocorticoid regulated receptor (and named GIR) suggesting that this receptor may be involved in stress-responses that underlie anxiety. Consistent with this, GPR83 null mice have been found to be resistant to stress-induced anxiety. However, studies examining the role of GPR83 within specific brain regions or potential sex differences have been lacking. In this study, we investigate anxiety-related behaviors in male and female mice with global knockout and following local GPR83 knockdown in female mice. We find that a global knockdown of GPR83 has minimal impact on anxiety-like behaviors in female mice and a decrease in anxiety-related behaviors in male mice. In contrast, a local GPR83 knockdown in the basolateral amygdala leads to more anxiety-related behaviors in female mice. Local GPR83 knockdown in the central amygdala or nucleus accumbens (NAc) showed no significant effect on anxiety-related behaviors. Finally, dexamethasone administration leads to a significant decrease in receptor expression in the amygdala and NAc of female mice. Together, our studies uncover a significant, but divergent role for GPR83 in different brain regions in the regulation of anxiety-related behaviors, which is furthermore dependent on sex.

Effects of early noise exposure on hippocampal-dependent behaviors during adolescence in male rats: influence of different housing conditions

Central nervous system (CNS) development is a very complex process that can be altered by environmental stimuli such as noise, which can generate long-term auditory and/or extra-auditory impairments. We have previously reported that early noise exposure can induce hippocampus-related behavioral alterations in postnatal day (PND) 28 adolescent rats. Furthermore, we recently found biochemical modifications in the hippocampus (HC) of these animals that seemed to endure even in more mature animals (i.e. PND35) and that have not been studied along with behavioral correlates. Thus, the aim of this work was to reveal novel data about the effects of early noise exposure on hippocampal-dependent behaviors in more mature animals. Additionally, extended enriched environment (EE) housing was evaluated to determine its capacity to induce behavioral modifications, either by its neuroprotective ability or the greater stimulation that it generates. Male Wistar rats were exposed to different noise schemes at PND7 or PND15. Upon weaning, some animals were transferred to EE whereas others were kept in standard cages. At PND35, different hippocampal-dependent behavioral assessments were performed. Results showed noise-induced behavioral changes that differed according to the scheme and age of exposure used. In addition, housing in an EE was effective either in preventing some of these changes or in inducing the appearance of new behavioral modifications. These findings suggest that CNS development would be sensitive to the effects of different type of environmental stimuli such as noise or enriched housing, leading to maladaptive behavioral changes that last even until adolescence.

Inhibiting ventral hippocampal NMDA receptors and Arc increases energy intake in male rats

Research into the neural mechanisms that underlie higher-order cognitive control of eating behavior suggests that ventral hippocampal (vHC) neurons, which are critical for emotional memory, also inhibit energy intake. We showed previously that optogenetically inhibiting vHC glutamatergic neurons during the early postprandial period, when the memory of the meal would be undergoing consolidation, caused rats to eat their next meal sooner and to eat more during that next meal when the neurons were no longer inhibited. The present research determined whether manipulations known to interfere with synaptic plasticity and memory when given pretraining would increase energy intake when given prior to ingestion. Specifically, we tested the effects of blocking vHC glutamatergic N-methyl-D-aspartate receptors (NMDARs) and activity-regulated cytoskeleton-associated protein (Arc) on sucrose ingestion. The results showed that male rats consumed a larger sucrose meal on days when they were given vHC infusions of the NMDAR antagonist APV or Arc antisense oligodeoxynucleotides than on days when they were given control infusions. The rats did not accommodate for that increase by delaying the onset of their next sucrose meal (i.e., decreased satiety ratio) or by eating less during the next meal. These data suggest that vHC NMDARs and Arc limit meal size and inhibit meal initiation.

Anxiety and Alzheimer's disease: Behavioral analysis and neural basis in rodent models of Alzheimer's-related neuropathology

Alzheimer's disease (AD) pathology is commonly associated with cognitive decline but is also composed of neuropsychiatric symptoms including psychological distress and alterations in mood, including anxiety and depression. Emotional dysfunction in AD is frequently modeled using tests of anxiety-like behavior in transgenic rodents. These tests often include the elevated plus-maze, light/dark test and open field test. In this review, we describe prototypical behavioral paradigms used to examine emotional dysfunction in transgenic models of AD, specifically anxiety-like behavior. Next, we summarize the results of studies examining anxiety-like behavior in transgenic rodents, noting that the behavioral outcomes using these paradigms have produced inconsistent results. We suggest that future research will benefit from using a battery of tests to examine emotional behavior in transgenic AD models. We conclude by discussing putative, overlapping neurobiological mechanisms underlying AD-related neuropathology, stress and anxiety-like behavior reported in AD models.

Western Diet Consumption During Development: Setting the Stage for Neurocognitive Dysfunction

The dietary pattern in industrialized countries has changed substantially over the past century due to technological advances in agriculture, food processing, storage, marketing, and distribution practices. The availability of highly palatable, calorically dense foods that are shelf-stable has facilitated a food environment where overconsumption of foods that have a high percentage of calories derived from fat (particularly saturated fat) and sugar is extremely common in modern Westernized societies. In addition to being a predictor of obesity and metabolic dysfunction, consumption of a Western diet (WD) is related to poorer cognitive performance across the lifespan. In particular, WD consumption during critical early life stages of development has negative consequences on various cognitive abilities later in adulthood. This review highlights rodent model research identifying dietary, metabolic, and neurobiological mechanisms linking consumption of a WD during early life periods of development (gestation, lactation, juvenile and adolescence) with behavioral impairments in multiple cognitive domains, including anxiety-like behavior, learning and memory function, reward-motivated behavior, and social behavior. The literature supports a model in which early life WD consumption leads to long-lasting neurocognitive impairments that are largely dissociable from WD effects on obesity and metabolic dysfunction.

The effect of 5-HT4 serotonin receptors in the CA3 hippocampal region on D-AP5-induced anxiolytic-like effects: Isobolographic analyses

Increasing evidence shows the close relationship between hippocampal glutamatergic and serotonergic systems through the modulation of behavioral responses. This study aimed to investigate the possible involvement of 5-HT₄ receptors in the CA3 hippocampal region in anxiolytic-like effects induced by D-AP5 (a competitive antagonist of the glutamate NMDA [N-Methyl-D-aspartate] receptor). Male Wistar rats were placed in the elevated plus maze (EPM) apparatus that is used to assess anxiety-related behaviors, and the percentages of open arm time (%OAT) and open arm entries (%OAE) which are associated with anxiety-related behaviors were measured. The close arm entries (CAE) which is correlated with locomotor activity was also evaluated. The results showed that, intra-CA3 injection of D-AP5 (0.4 μg/rat), RS67333 (1.2 μg/rat; a 5-HT₄ receptor agonist), and RS23597-190 (1.2 μg/rat; a 5-HT₄ receptor antagonist) increased %OAT and %OAE, indicating the anxiolytic-like effect of these drugs. Also, only RS23597-190 (1.2 μg/rat) decreased CAE. Intra-CA3 injection of sub-threshold dose of RS67333 (0.012 μg/rat) or RS23597-190 (0.012 μg/rat), 5 min before the injection of D-AP5 (0.2 μg/rat) increased %OAT, indicating potentiating the anxiolytic-like effect of D-AP5. The isobolographic analyses also showed the additive or synergistic anxiolytic-like effect of intra-CA3 co-administration of D-AP5 with RS67333 or RS23597-190, respectively. In conclusion, CA3 5-HT₄ receptors are involved in D-AP5-induced anxiolytic-like behaviors in rats.

Synthetic structural modifications of neurosteroid pregnanolone sulfate: Assessment of neuroprotective effects in vivo

Neuroactive steroid 20-oxo-5β-pregnan-3α-yl L-glutamyl 1-ester (PA-Glu), a synthetic analogue of naturally occurring 20-oxo-5β-pregnan-3α-yl sulfate (pregnanolone sulfate, PA-S), inhibits N-methyl-D-aspartate (NMDA) receptors and possesses neuroprotective properties and minimal adverse effects. Herein, we report in vivo effects of new structural modifications of the PA-S molecule: a nonpolar modification of the steroid D-ring (5β-androstan-3α-yl L-glutamyl 1-ester, AND-Glu), attachment of a positively charged group to C3 (20-oxo-5β-pregnan-3α-yl L-argininate dihydrochloride salt, PA-Arg) and their combination (5β-androstan-3α-yl L-argininate dihydrochloride salt, AND-Arg). The first aim of this study was to determine the structure-activity relationship for neuroprotective effects in a model of excitotoxic hippocampal damage in rats, based on its behavioral correlate in Carousel maze. The second aim was to explore side effects of neuroprotective steroids on motor functions, anxiety (elevated plus maze) and locomotor activity (open field) and the effect of their high doses in mice. The neuroprotective properties of PA-Glu and AND-Glu were proven, with the effect of the latter appearing to be more pronounced. In contrast, neuroprotective efficacy failed when positively charged molecules (PA-Arg, AND-Arg) were used. AND-Glu and PA-Glu at the neuroprotective dose (1 mg/kg) did not unfavorably influence motor functions of intact mice. Moreover, anxiolytic effects of AND-Glu and PA-Glu were ascertained. These findings corroborate the value of research of steroidal inhibitors of NMDA receptors as potential neuroprotectants with slight anxiolytic effect and devoid of behavioral adverse effects. Taken together, the results suggest the benefit of the nonpolar D-ring modification, but not of the attachment of a positively charged group to C3.

The role of CA3 GABAB receptors on anxiolytic-like behaviors and avoidance memory deficit induced by D-AP5 with respect to Ca2+ ions

Glutamatergic and GABAergic systems play key roles in the hippocampus and affect the pathogenesis of anxiety- and memory-related processes. Some investigations have assessed the role of balancing the function of these two systems in different areas of the central nervous system (CNS) as an approach to manage the related disorders. We investigated the anxiety and avoidance memory states using the test-retest protocol in the elevated plus maze to understand the role of GABA_B receptors (GABA_BRs) in relation to the NMDA receptor blockade by D-AP5 (an NMDA receptor antagonist). Also, we examined the function of Ca²⁺ ions by blocking its entrance to the cell using SKF96365 (a Ca²⁺ channel blocker). The drugs were injected into the CA3 region before the test. Our data showed that D-AP5 induced anxiolytic-like behaviors and impaired the avoidance memory. Injection of baclofen (a GABA_BR agonist), but not phaclofen (a GABA_BR antagonist) induced anxiolytic-like behaviors. Neither baclofen nor phaclofen altered avoidance memory-related behaviors. When baclofen was injected before D-AP5, it potentiated the anxiolytic-like behaviors induced by D-AP5, but counteracted its effect on avoidance memory. Phaclofen pretreatment attenuated D-AP5-induced anxiolytic-like behaviors, but potentiated its effect on avoidance memory. The effect of baclofen application before D-AP5 on anxiety and phaclofen application before D-AP5 on avoidance memory at the heist doses were accompanied by a decrease in locomotion. The application of SKF96365 did not alter anxiety-like behaviors but induced avoidance memory impairment. SKF96365 application before the combination of baclofen and D-AP5 counteracted the effects produced by the combination of baclofen and D-AP5 on anxiety and memory states. Our findings showed that the CA3 GABA_BRs had a critical role in anxiolytic-like behaviors and avoidance memory deficit induced by D-AP5 and confirmed the role of Ca²⁺ ions in the observed results.

Reduced Anxiety-Like Behavior and Altered Hippocampal Morphology in Female p75NTR(exon IV-/-) Mice

The presence of the p75 neurotrophin receptor (p75NTR) in adult basal forebrain cholinergic neurons, precursor cells in the subventricular cell layer and the subgranular cell layer of the hippocampus has been linked to alterations in learning as well as anxiety- and depression- related behaviors. In contrast to previous studies performed in a p75NTR^{exon III−/−} model still expressing the short isoform of the p75NTR, we focused on locomotor and anxiety–associated behavior in p75NTR^{exon IV−/−} mice lacking both p75NTR isoforms. Comparing p75NTR^{exon IV−/−} and wildtype mice for both male and female animals showed an anxiolytic-like behavior as evidenced by increased central activities in the open field paradigm and flex field activity system as well as higher numbers of open arm entries in the elevated plus maze test in female p75NTR knockout mice. Morphometrical analyses of dorsal and ventral hippocampus revealed a reduction of width of the dentate gyrus and the granular cell layer in the dorsal but not ventral hippocampus in male and female p75NTR^{exon IV−/−} mice. We conclude that germ-line deletion of p75NTR seems to differentially affect morphometry of dorsal and ventral dentate gyrus and that p75NTR may play a role in anxiety-like behavior, specifically in female mice.

NMDA and D1 receptors are involved in one-trial tolerance to the anxiolytic-like effects of diazepam in the elevated plus maze test in rats

The elevated plus maze (EPM) test is used to examine anxiety-like behaviors in rodents. One interesting phenomenon in the EPM test is one-trial tolerance (OTT), which refers to the reduction in the anxiolytic-like effects of benzodiazepines when rodents are re-exposed to the EPM. However, the underlying mechanism of OTT is still unclear. In this study, we reported that OTT occurred when re-exposure to the EPM (trial 2) only depended on the prior experience of the EPM (trial 1) rather than diazepam treatment. This process was memory-dependent, as it was prevented by the N-methyl-D-aspartate (NMDA) receptors antagonist MK-801 1.5h before trial 2. In addition, OTT was maintained for at least one week but was partially abolished after an interval of 28 days. Furthermore, the administration of the D1-like receptors agonist SKF38393 to the bilateral dorsal hippocampus largely prevented OTT, as demonstrated by the ability of the diazepam treatment to produce significant anxiolytic-like effects in trial 2 after a one-day interval. These findings suggest that OTT to the EPM test may occur via the activation of NMDA receptors and the inactivation of D1-like receptors in certain brain regions, including the hippocampus.

The effect of chronic phenytoin administration on single prolonged stress induced extinction retention deficits and glucocorticoid upregulation in the rat medial prefrontal cortex

RATIONALE

Post-traumatic stress disorder (PTSD) is a chronic, debilitating disorder. Only two pharmacological agents are approved for PTSD treatment, and they often do not address the full range of symptoms nor are they equally effective in all cases. Animal models of PTSD are critical for understanding the neurobiology involved and for identification of novel therapeutic targets. Using the rodent PTSD model, single prolonged stress (SPS), we have implicated aberrant excitatory neural transmission and glucocorticoid receptor (GR) upregulation in the medial prefrontal cortex (mPFC) and hippocampus (HPC) in fear memory abnormalities associated with PTSD.

OBJECTIVE

The objective of this study is to examine the potential protective effect of antiepileptic phenytoin (PHE) administration on SPS-induced extinction retention deficits and GR expression.

METHODS

Forty-eight SPS-treated male Sprague Dawley rats or controls were administered PHE (40, 20 mg/kg, vehicle) for 7 days following SPS stressors; then, fear conditioning, extinction, and extinction retention were tested.

RESULTS

Fear conditioning and extinction were unaffected by SPS or PHE, but SPS impaired extinction retention, and both doses of PHE rescued this impairment. Similarly, SPS increased GR expression in the mPFC and dorsal HPC, and PHE prevented SPS-induced GR upregulation in the mPFC.

CONCLUSIONS

These data demonstrate that PHE administration can prevent the development of extinction retention deficits and upregulation of GR. PHE exerts inhibitory effects on voltage-gated sodium channels and decreases excitatory neural transmission via glutamate antagonism. If glutamate hyperactivity in the days following SPS contributes to SPS-induced deficits, then these data may suggest that the glutamatergic system constitutes a target for secondary prevention.

Activity-regulated gene expression in immature neurons in the dentate gyrus following re-exposure to a cocaine-paired environment

Intense craving for drug and relapse are observed in addicts who are exposed to environmental stimuli associated with drug-taking behavior even after long periods of abstinence. The hippocampus is a brain region known to be involved in contextual processing, taking place predominantly in the septal hippocampus, and emotional processing, taking place predominantly in the temporal hippocampus. Conditioned place preference is an animal model of context-conditioned reward. The dentate gyrus is a hippocampal sub-region particularly important for the acquisition of cocaine-induced place preference and is a site of continuous neurogenesis, which has been implicated in the vulnerability to drug-taking behavior. Therefore, these experiments explored the role of newly generated neurons in drug reward-context association by examining the activation, as determined by expression of the immediate early gene cfos, of young and mature granule cells in the septal and temporal dentate gyrus of adult rats that were re-exposed to a drug-paired environment following the development of cocaine place preference. The overall level of cfos expression was increased in both the septal and temporal dentate gyrus of animals that developed place preference and were re-exposed to the drug paired environment compared with re-exposure to a neutral environment. Overall level of neurogenesis, as detected by the S-phase marker 5'-bromo-2'-deoxyuridine (BrdU) and the immature neuron marker doublecortin (DCX), was unaltered by cocaine conditioning. However, the number of activated new neurons (DCX + cfos) was greater in the temporal dentate gyrus of cocaine-conditioned rats re-exposed to the drug-paired environment as compared to those re-exposed to a neutral environment. Further understanding of the role of dentate gyrus neurogenesis on the conditioned effects of drugs of abuse may provide new insights into the role of this process in the expression of addictive behaviors.

NMDA receptor activation antagonizes the NMDA antagonist-induced antianxiety effect in the elevated plus-maze test in mice

BACKGROUND

The purpose of this study was to determine how the activation of different regulatory domains of the NMDA complex affects the antianxiety effect of antagonists acting at its distinct binding sites.

METHODS

The anxiolytic-like activity was assessed by the elevated plus-maze test in mice.

RESULTS

The anxiolytic activity of CGP 37849 (a competitive NMDA receptor antagonist) and L-701,324 (an antagonist at glycine site) was confirmed, but effects of both were significantly reduced by N-methyl-D-aspartic acid (NMDA) or by D-serine agonists at glutamate and glycine site of the NMDA receptor complex, respectively.

CONCLUSION

The obtained data suggest that stimulation of the glutamate or glycine recognition site of the NMDA receptor complex significantly decreases the antianxiety properties of antagonists of either site.

Repeated cocaine enhances ventral hippocampal-stimulated dopamine efflux in the nucleus accumbens and alters ventral hippocampal NMDA receptor subunit expression

Dopaminergic neurotransmission in the nucleus accumbens is important for various reward-related cognitive processes including reinforcement learning. Repeated cocaine enhances hippocampal synaptic plasticity, and phasic elevations of accumbal dopamine evoked by unconditioned stimuli are dependent on impulse flow from the ventral hippocampus. Therefore, sensitized hippocampal activity may be one mechanism by which drugs of abuse enhance limbic dopaminergic activity. In this study, in vivo microdialysis in freely moving adult male Sprague-Dawley rats was used to investigate the effect of repeated cocaine on ventral hippocampus-mediated dopaminergic transmission within the medial shell of the nucleus accumbens. Following seven daily injections of saline or cocaine (20 mg/kg, ip), unilateral infusion of N-methyl-d-aspartate (NMDA, 0.5 μg) into the ventral hippocampus transiently increased both motoric activity and ipsilateral dopamine efflux in the medial shell of the nucleus accumbens, and this effect was greater in rats that received repeated cocaine compared to controls that received repeated saline. In addition, repeated cocaine altered NMDA receptor subunit expression in the ventral hippocampus, reducing the NR2A : NR2B subunit ratio. Together, these results suggest that repeated exposure to cocaine produces maladaptive ventral hippocampal-nucleus accumbens communication, in part through changes in glutamate receptor composition. A behaviorally sensitizing regimen of cocaine (20 mg/kg, ip 7 days) also sensitized ventral hippocampus (hipp)-mediated dopaminergic transmission within the nucleus accumbens (Nac) to NMDA stimulation (bolts). This was associated with reduced ventral hippocampal NR2A:NR2B subunit ratio, suggesting that repeated exposure to cocaine produces changes in hippocampal NMDA receptor composition that lead to enhanced ventral hippocampus-nucleus accumbens communication.

Involvement of NR1, NR2A different expression in brain regions in anxiety-like behavior of prenatally stressed offspring

Prenatal stress (PS) has been shown to be associated with anxiety. However, the underlying neurological mechanisms are not well understood. To determine the effects of PS on anxiety-like behavior in the adult offspring, we evaluated anxiety-like behavior using open field test (OFT) and elevated plus maze (EPM) in the 3-month offspring. Both male and female offspring showed a significant reduction of crossing counts in the center, total crossing counts, rearing counts and time spent in the center in the OFT, and only male offspring showed a decreased percentage of open-arm entries and open-arm time in open arms in the EPM. Additionally, expression of NR1 and NR2A subunit of N-methyl-D-aspartate receptor (NMDAR) in the hippocampus (HIP), prefrontal cortex (PFC) and striatum (STR) was studied. Our results showed that PS reduced NR1 and NR2A expression in the HIP, NR2A expression in the PFC and STR in the offspring. The altered NR1 and NR2A could have potential impact on anxiety-like behavior in the adult offspring exposed to PS.

NMDA-dependent phase synchronization between septal and temporal CA3 hippocampal networks

Increasing evidence suggests that synchronization between brain regions is essential for information exchange and memory processes. However, it remains incompletely known which synaptic mechanisms contribute to the process of synchronization. Here, we investigated whether NMDA receptor-mediated synaptic plasticity was an important player in synchronization between septal and temporal CA3 areas of the rat hippocampus. We found that both the septal and temporal CA3 regions intrinsically generate weakly synchronized δ frequency oscillations in the complete hippocampus in vitro. Septal and temporal oscillators differed in frequency, power, and rhythmicity, but both required GABAA and AMPA receptors. NMDA receptor activation, and most particularly the NR2B subunit, contributed considerably more to rhythm generation at the temporal than the septal region. Brief activation of NMDA receptors by application of extracellular calcium dramatically potentiated the septal-temporal coherence for long durations (>40 min), an effect blocked by the NMDA antagonist AP-5. This long-lasting NMDA-receptor-dependent increase in coherence was also associated with an elevated phase locking of spikes locally and across regions. Changes in coherence between oscillators were associated with increases in phase locking between oscillators independent of oscillator amplitude. Finally, although the septal CA3 rhythm preceded the oscillations in temporal regions in control conditions, this was reversed during the NMDA-dependent enhancement in coherence, suggesting that NMDA receptor activation can change the direction of information flow along the septotemporal CA3 axis. These data demonstrate that plastic changes in communication between septal and temporal hippocampal regions can arise from the NMDA-dependent phase locking of neural oscillators.

The ventral hippocampus NMDA receptor/nitric oxide/guanylate cyclase pathway modulates cardiovascular responses in rats

The hippocampus is a limbic structure that is involved in the expression of defensive reactions and autonomic changes in rats. The injection of L-glutamate (L-glu) into the ventral hippocampus (VH) decreases blood pressure and heart rate in anesthetized rats. Activation of NMDA receptors in the VH increases the production of nitric oxide (NO), leading to guanylate cyclase activation. The hypothesis of the present study was that a local NMDA receptor-NO-guanylate cyclase interaction mediates the cardiovascular effects of microinjection of L-glu into the VH. Microinjection of increasing doses of L-glu (30, 60 and 200 nmol/200 nL) into the VH of conscious rats caused dose-related pressor and tachycardiac responses. The cardiovascular effects of L-glu were abolished by local pretreatment with: the glutamate receptor antagonist AP-7 (0.4 nmol); the selective neuronal NO synthase (nNOS) inhibitor N(ω)-Propyl-L-arginine (0.04 nmol); the NO scavenger C-PTIO (2 nmol) or the guanylate cyclase inhibitor 1H-[1,2,4] oxadiazolol [4,3-a]quinoxalin-1-one (2 nmol). Moreover, these cardiovascular responses were blocked by intravenous pretreatment with: the ganglionic blocker mecamylamine (2mg/Kg); the nonselective β-adrenergic receptor antagonist propranolol (2mg/Kg); the β1-adrenergic receptor selective antagonist atenolol (1mg/kg). However, pretreatment with the selective α1-adrenergic receptor antagonist prazosin (0,5mg/kg) caused only a small reduction in the pressor response, without affecting the L-glu evoked tachycardia. In conclusion, our results suggest that cardiovascular responses caused by L-glu microinjection into the VH are mediated by NMDA glutamate receptors and involve local nNOS and guanylate cyclase activation. Moreover, these cardiovascular responses are mainly mediated by cardiac sympathetic nervous system activation, with a small involvement of the vascular sympathetic nervous system.

Morphine-induced anxiolytic-like effect in morphine-sensitized mice: involvement of ventral hippocampal nicotinic acetylcholine receptors

In the present study, the effects of repeated intra-ventral hippocampal (intra-VH) microinjections of nicotinic acetylcholine receptor agonist or antagonist on morphine-induced anxiolytic-like behavior were investigated in morphine-sensitized mice using elevated plus-maze. Intraperitoneal (i.p.) administration of different doses of morphine (5, 7.5 and 10mg/kg) increased the percentage of open arm time (%OAT), open arm entries (%OAE), but not locomotor activity, indicating an anxiolytic-like response to morphine. The maximum response was obtained by 7.5mg/kg of the opioid. The anxiety-like behavior which was induced by a lower dose of morphine (5mg/kg) was significantly increased in mice that had previously received once daily injections of morphine (10 and 20mg/kg, i.p.) for 3 days. It should be considered that this treatment also increased locomotor activity in morphine-sensitized mice. Furthermore, the response to an ineffective dose of morphine (5mg/kg, i.p.) in the EPM was significantly increased in the animals that had previously received nicotine for 3 days (0.1, 0.3, 0.5 and 0.7 μg/mouse; intra-VH), 5 min prior to the injections of morphine (5mg/kg/day × 3 days; i.p.). On the other hand, the increase of morphine-induced anxiolytic-like effect in animals that had previously received the 3-day morphine (20mg/kg) was dose dependently suppressed by once daily injections of mecamylamine (0.5, 1 and 2 μg/mouse/day × 3 days; intra-VH). It is important to note that repeated intra-VH administrations of the same doses of nicotine or mecamylamine alone caused no significant change in morphine (5mg/kg)-induced anxiety-like parameters in the EPM. In conclusion, it seems that morphine sensitization affects the anxiety-like behavior in the EPM and the cholinergic system in the ventral hippocampus, via nicotinic receptors, may play an important role in this effect.

Gestational and lactational exposure to bisphenol-A affects anxiety- and depression-like behaviors in mice

Bisphenol-A (BPA), an environmental endocrine disruptor, has attracted attention because of its adverse effects on the brain and behavioral development. Previous evidence indicates that perinatal exposure to low levels of BPA affects anxiety-like and cognitive behaviors in adult rodents. The present study aims to investigate the changes of anxiety- and depression-like behaviors of perinatally exposed mice in adulthood following the gestational (gestation days 7 to 20) or lactational (postnatal days 1 to 14) exposure to BPA (0.4 or 4 mg/kg/d). The results indicated that both gestational and lactational exposures to BPA increased anxiety and depression-like behavior in mice of both sexes. The females with gestational exposure exhibited an increased anxiety-like state in the four models tested, including the open field, dark-light transition task, mirrored maze, and elevated plus maze tasks. Furthermore, the females with lactational exposure and the males with gestational exposure exhibited an anxiogenic-like behavior in two models, whereas the males with lactational exposure exhibited an anxiogenic-like behavior only in the elevated plus maze test. The results of the forced swim task showed that gestational exposure markedly increased the immobile time in both sexes, and the same effect was induced by lactational exposure only with 4 mg/kg/d BPA. Furthermore, western blot analyses showed that both gestational and lactational exposures inhibited the expression of the AMPA receptor subunit GluR1 in the hippocampus and amygdala in mice of both sexes, whereas the level of the NMDA receptor subunit NR1 was increased in the amygdala following gestational exposure but was reduced in the hippocampus of the females with lactational exposure. These results suggest that both gestational and lactational exposures to BPA increased anxiety- and depression-like behaviors of adult mice of both sexes. In addition gestational exposure exhibited a stronger effect on anxiety-like state in females. The altered levels of AMPA and NMDA receptors in the hippocampus and amygdala may be associated with BPA-induced behavioral changes.

Long-term behavioral and NMDA receptor effects of young-adult corticosterone treatment in BDNF heterozygous mice

Psychiatric illnesses, such as schizophrenia, are most likely caused by an interaction between genetic predisposition and environmental factors, including stress during development. The neurotrophin, brain-derived neurotrophic factor (BDNF) has been implicated in this illness as BDNF levels are decreased in the brain of patients with schizophrenia. The aim of the present study was to assess the combined effect of reduced BDNF levels and postnatal stress, simulated by chronic young-adult treatment with the stress hormone, corticosterone. From 6 weeks of age, female and male BDNF heterozygous mice and their wild-type controls were chronically treated with corticosterone in their drinking water for 3 weeks. At 11 weeks of age, male, but not female BDNF heterozygous mice treated with corticosterone exhibited a profound memory deficit in the Y-maze. There were no differences between the groups in baseline prepulse inhibition (PPI), a measure of sensorimotor gating, or its disruption by treatment with MK-801. However, an increase in startle caused by MK-801 treatment was absent in male, but not female BDNF heterozygous mice, irrespective of corticosterone treatment. Analysis of protein levels of the NMDA receptor subunits NR1, NR2A, NR2B and NR2C, showed a marked increase of NR2B levels in the dorsal hippocampus of male BDNF heterozygous mice treated with corticosterone. In the ventral hippocampus, significantly reduced levels of NR2A, NR2B and NR2C were observed in male BDNF heterozygous mice. The NMDA receptor effects in hippocampal sub-regions could be related to the spatial memory deficits and the loss of the effect of MK-801 on startle in these mice, respectively. No significant changes in NMDA receptor subunit levels were observed in any of the female groups. Similarly, no significant changes in levels of BDNF or its receptor, TrkB, were found other than the expected reduced levels of BDNF in heterozygous mice. In conclusion, the data show differential interactive effects of reduced levels of BDNF expression and corticosterone treatment on spatial memory and startle in male and female mice, accompanied by significant, but region-specific changes in NMDA receptor subunit levels in the dorsal and ventral hippocampus. These results could be important for our understanding of the interaction of neurodevelopmental stress and BDNF deficiency in cognitive and anxiety-related symptoms of psychiatric illnesses, such as schizophrenia.

Genetic, pharmacological and lesion analyses reveal a selective role for corticohippocampal GLUN2B in a novel repeated swim stress paradigm

Glutamate and N-methyl-d-aspartate receptor (NMDAR) dysfunction is strongly implicated in the pathophysiology of mood and anxiety disorders. Treatment with NMDAR antagonists has antidepressant efficacy in treatment-resistant depressives. In preclinical rodent models, NMDAR antagonist administration reduces anxiety- and stress-related behaviors in concert with increases in prefrontal cortical (PFC) dendritic spinogenesis and synaptic proteins. While these effects have been attributed to actions at the NMDAR GluN2B subunit, the precise role of cortical GluN2B in mediating emotional behaviors and stress-responsivity is not fully understood. Here, we employed a novel mutant model in which the GluN2B subunit is postnatally deleted in principal neurons in the cortex and the dorsal CA1 subregion of the hippocampus. GluN2BKO mice were phenotyped on a battery of tests for anxiety-related (light/dark exploration, stress-induced hyperthermia) and antidepressant-sensitive (sucrose preference, novelty-induced hypophagia, single-trial forced swim) behaviors. A novel repeated inescapable forced swim paradigm (riFS) was developed to assess behavioral responses to repeated stress in the GluN2BKO mice. For comparison, non-mutant C57BL/6J mice were tested for single-trial forced swim behavior after systemic Ro 25-6981 treatment and for riFS behavior after lesions of the ventromedial prefrontal cortex. riFS-induced alterations in corticolimbic GluN2B expression were also examined in C57BL/6J mice. We found that GluN2BKO mice reduced "despair-like" behavior in the riFS procedure, as compared to GluN2BFLOX controls. By contrast, GluN2BKO mice showed minimal alterations on anxiety-like or antidepressant-sensitive assays, including the single-trial forced swim test. In C57BL/6J mice, induction of "despair-like" responses in the riFS test was attenuated by vmPFC lesions, and was associated with changes in limbic GluN2B expression. Collectively, these data suggest that cortical GluN2B plays a major role in modulating adaptive responses to stress. Current findings provide further support for GluN2B as a key mechanism underlying stress responsivity, and a novel pharmacotherapeutic target for stress-related neuropsychiatric disorders.

New perspectives in glutamate and anxiety

Anxiety and stress-related disorders, namely posttraumatic stress disorder (PTSD), generalized anxiety disorder (GAD), obsessive-compulsive disorder (ODC), social and specific phobias, and panic disorder, are a major public health issue. A growing body of evidence suggests that glutamatergic neurotransmission may be involved in the biological mechanisms underlying stress response and anxiety-related disorders. The glutamatergic system mediates the acquisition and extinction of fear-conditioning. Thus, new drugs targeting glutamatergic neurotransmission may be promising candidates for new pharmacological treatments. In particular, N-methyl-d-aspartate receptors (NMDAR) antagonists (AP5, AP7, CGP37849, CGP39551, LY235959, NPC17742, and MK-801), NMDAR partial agonists (DCS, ACPC), α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptors (AMPARs) antagonists (topiramate), and several allosteric modulators targeting metabotropic glutamate receptors (mGluRs) mGluR1, mGluR2/3, and mGluR5, have shown anxiolytic-like effects in several animal and human studies. Several studies have suggested that polyamines (agmatine, putrescine, spermidine, and spermine) may be involved in the neurobiological mechanisms underlying stress-response and anxiety-related disorders. This could mainly be attributed to their ability to modulate ionotropic glutamate receptors, especially NR2B subunits. The aim of this review is to establish that glutamate neurotransmission and polyaminergic system play a fundamental role in the onset of anxiety-related disorders. This may open the way for new drugs that may help to treat these conditions.

Neuroanatomy of anxiety

The evolutionary approach to human anxiety is based on the defensive responses that nonhuman animals show to fear-provoking stimuli. Studies performed mostly on rodents have related areas such as the medial prefrontal cortex, amygdaloid and hypothalamic nuclei, hipoccampal formation, and midbrain central gray to these responses. It is clear, however, that animals show different and sometimes opposite responses according to the threatening stimulus. These responses include immediate reactions such as freezing or flight, behavioral inhibition or avoidance, which are organized by at least partially distinct brain systems. As discussed in this chapter, several pieces of evidence indicate that these brain systems are similar in rodents and primates. In addition, recent neuroimaging studies also suggest dysfunctions in these systems are probably related to anxiety disorders in humans.

Hippocampal NMDA receptors and anxiety: at the interface between cognition and emotion

David De Wied had a fundamental interest in the brain and behaviour, with a particular interest in the interface between cognition and emotion, and how impairments at this interface could underlie human psychopathology. The NMDA subtype of glutamate receptor is an important mediator of synaptic plasticity and plays a central role in the neurobiological mechanisms of emotionality, as well as learning and memory. NMDA receptor antagonists affect various aspects of emotionality including fear, anxiety and depression, as well as impairing certain forms of learning and memory. The hippocampus is a key brain structure, implicated in both cognition and emotion. Lesion studies in animals have suggested that dorsal and ventral sub-regions of the hippocampus are differentially involved in dissociable aspects of hippocampus-dependent behaviour. Cytotoxic lesions of the dorsal hippocampus (septal pole) in rodents impair spatial learning but have no effect on anxiety, whereas ventral hippocampal lesions reduce anxiety but are without effect on spatial memory. This role for the ventral hippocampus in anxiety is distinct from the role of the amygdala in other aspects of emotional processing, such as fear conditioning. Recent studies with genetically modified mice have shown that NR1 NMDA receptor subunit deletion, specifically from the granule cells of the dentate gyrus, not only impairs short-term spatial memory but also reduces anxiety. This suggests that NMDA receptors in ventral hippocampus may be a key locus supporting the anxiolytic effects of NMDA receptor antagonists. These data support Gray's neuropsychological account of hippocampal function.

Role of ventral hippocampal nitric oxide/cGMP pathway in anxiety-related behaviors in rats submitted to the elevated T-maze

The L-arginine/nitric oxide (NO)/cGMP pathways have been implicated in the control of a variety of physiological mechanisms and are believed to participate in the modulation of anxiety in the CNS. The aim of this study was to investigate the effects of N(G)-nitro-L-arginine-methyl-ester (L-NAME), a non-selective inhibitor of NO synthase (NOS); 7-nitroindazole (7-NI), a preferential inhibitor of neuronal NOS; and sodium nitroprusside (SNP), an NO donor, administered into the ventral hippocampus (VH) of rats submitted to the elevated T-maze (ETM). The ETM, an animal model derived from the elevated plus-maze, allows the measurement of two defensive behavioral responses in the same rat: inhibitory avoidance and escape. Results showed that L-NAME and 7-NI impaired the acquisition of inhibitory avoidance and prolonged escape latency in the ETM, suggesting an anxiolytic-like and panicolytic-like effect, respectively. SNP facilitated the acquisition of inhibitory avoidance without interfering with escape performance, suggesting an anxiogenic-like effect. Treatment with methylene blue did not alter per se any of the behavioral responses measured in the ETM, but blocked the effect promoted by SNP. Thus, altogether these results suggest that NO in the VH is critically involved in the modulation of defensive behavior of rats exposed to the ETM.

Expression of alpha-synuclein is increased in the hippocampus of rats with high levels of innate anxiety

A genomic region neighboring the alpha-synuclein gene, on rat chromosome 4, has been associated with anxiety- and alcohol-related behaviors in different rat strains. In this study, we have investigated potential molecular and physiological links between alpha-synuclein and the behavioral differences observed between Lewis (LEW) and Spontaneously Hypertensive (SHR) inbred rats, a genetic model of anxiety. As expected, LEW rats appeared more fearful than SHR rats in three anxiety models: open field, elevated plus maze and light/dark box. Moreover, LEW rats displayed a higher preference for alcohol and consumed higher quantities of alcohol than SHR rats. alpha-Synuclein mRNA and protein concentrations were higher in the hippocampus, but not the hypothalamus of LEW rats. This result inversely correlated with differences in dopamine turnover in the hippocampus of LEW and SHR rats, supporting the hypothesis that alpha-synuclein is important in the downregulation of dopamine neurotransmission. A novel single nucleotide polymorphism was identified in the 3'-untranslated region (3'-UTR) of the alpha-synuclein cDNA between these two rat strains. Plasmid constructs based on the LEW 3'-UTR sequence displayed increased expression of a reporter gene in transiently transfected PC12 cells, in accordance with in-vivo findings, suggesting that this nucleotide exchange might participate in the differential expression of alpha-synuclein between LEW and SHR rats. These results are consistent with a novel role for alpha-synuclein in modulating rat anxiety-like behaviors, possibly through dopaminergic mechanisms. Since the behavioral and genetic differences between these two strains are the product of independent evolutionary histories, the possibility that polymorphisms in the alpha-synuclein gene may be associated with vulnerability to anxiety-related disorders in humans requires further investigation.

High dose of simvastatin induces hyperlocomotive and anxiolytic-like activities: The association with the up-regulation of NMDA receptor binding in the rat brain

Statins are widely being used for the treatment of a variety of conditions beyond their original indication for lowering cholesterol. We have previously reported that simvastatin affected the dopaminergic system in the rat brain. This study aims to investigate locomotor and anxiety effects along with the regional changes of N-methyl-d-aspartate (NMDA) receptors in the rat brain after 4-week administration of simvastatin. Hyperlocomotive and anxiolytic-like activities in the rat were observed after chronic administration of high dose simvastatin (10 mg/kg/day). Distributions and alterations of NMDA receptors in the post-mortem rat brain were detected by [(3)H] MK-801 binding autoradiography. Simvastatin increased [(3)H] MK-801 binding, predominantly in the prefrontal cortex (20%, p=0.003), primary motor cortex (20%, p<0.001), cingulate cortex (28%, p<0.001), hippocampus (41%, p<0.001), caudate putamen (30%, p=0.029), nucleus accumbens (27%, p=0.035) and amygdala (45%, p<0.001) compared to controls. Significant positive correlations were identified between hyperlocomotive as well as anxiolytic-like activities and the upregulation of NMDA receptors in different brain regions. Our results also provide strong evidence that chronic high dose simvastatin administration is to exhibit NMDA antagonist-like effects, which would partially explain the anxiolytic and hyperlocomotor activities. These findings contribute to a better understanding of the critical roles of simvastatin in modulating psycho-neurodegenerative disorders, via NMDA receptors.