The microinjection of AMPA receptor antagonist into the accumbens shell, but not into the accumbens core, induces anxiolysis in an animal model of anxiety

The imidazodiazepine, KRM-II-81: An example of a newly emerging generation of GABAkines for neurological and psychiatric disorders

GABAkines, or positive allosteric modulators of γ-aminobutyric acid-A (GABA_A) receptors, are used for the treatment of anxiety, epilepsy, sleep, and other disorders. The search for improved GABAkines, with reduced safety liabilities (e.g., dependence) or side-effect profiles (e.g., sedation) constituted multiple discovery and development campaigns that involved a multitude of strategies over the past century. Due to the general lack of success in the development of new GABAkines, there had been a decades-long draught in bringing new GABAkines to market. Recently, however, there has been a resurgence of efforts to bring GABAkines to patients, the FDA approval of the neuroactive steroid brexanolone for post-partum depression in 2019 being the first. Other neuroactive steroids are in various stages of clinical development (ganaxolone, zuranolone, LYT-300, Sage-324, PRAX 114, and ETX-155). These GABAkines and non-steroid compounds (GRX-917, a TSPO binding site ligand), darigabat (CVL-865), an α2/3/5-preferring GABAkine, SAN711, an α3-preferring GABAkine, and the α2/3-preferring GABAkine, KRM-II-81, bring new therapeutic promise to this highly utilized medicinal target in neurology and psychiatry. Herein, we also discuss possible conditions that have enabled the transition to a new age of GABAkines. We highlight the pharmacology of KRM-II-81 that has the most preclinical data reported. KRM-II-81 is the lead compound in a new series of orally bioavailable imidazodiazepines entering IND-enabling safety studies. KRM-II-81 has a preclinical profile predicting efficacy against pharmacoresistant epilepsies, traumatic brain injury, and neuropathic pain. KRM-II-81 also produces anxiolytic- and antidepressant-like effects in rodent models. Other key features of the pharmacology of this compound are its low sedation rate, lack of tolerance development, and the ability to prevent the development of seizure sensitization.

Orexin 2 receptor in the nucleus accumbens is critical for the modulation of acute stress-induced anxiety

Orexin is a neuropeptide mainly synthesized in the lateral hypothalamus/perifornical area and has been traditionally implicated in feeding, sleep-wake cycles, and reward. Intriguingly, patients with anxiety have increased levels of orexin in the cerebrospinal fluid. Pharmacological or genetic manipulation of orexin receptors affects anxiety-like behaviors in rodents, suggesting an involvement of the orexin signaling in the regulation of anxiety. Yet, the neural substrates involved remain largely unknown. The nucleus accumbens (NAc) shell holds a key position in the modulation of anxiety-related behaviors. Therefore, in the present study, by using neuropharmacology, molecular approaches and behavioral tests in rats, the role of orexin/orexin receptors in the NAc shell on the anxiety-like behaviors was investigated. We found that microinjection of orexin-A into the NAc shell induced an anxiogenic-like effect. Quantitative real-time PCR and immunofluorescence showed that the orexin 2 receptor (OX2R) is expressed and distributed in the NAc shell neurons. Activation of OX2R mimicked the anxiogenic effect of orexin-A. Moreover, infusion of an OX2R antagonist had no effect on anxiety-like behaviors in normal rats, but reversed anxiogenic effect induced by acute restraint stress. Finally, we found that downregulation of OX2R in the NAc shell caused an anxiolytic-like effect in acute restraint stressed rats, which was consistent with the pharmacological results. Together, this study suggests that OX2R in the NAc shell is involved in the regulation of acute stress-induced anxiety, and raises the possibility that OX2R antagonist may serve as an effective mean to treat anxiety disorders.

The Paraventricular Nucleus of the Thalamus as an Integrating and Relay Node in the Brain Anxiety Network

The brain anxiety network is composed of a number of interconnected cortical regions that detect threats and execute appropriate defensive responses via projections to the shell of the nucleus accumbens (NAcSh), dorsolateral region of the bed nucleus of the stria terminalis (BSTDL) and lateral region of the central nucleus of the amygdala (CeL). The paraventricular nucleus of the thalamus (PVT) is anatomically positioned to integrate threat- and arousal-related signals from cortex and hypothalamus and then relay these signals to neural circuits in the NAcSh, BSTDL, and CeL that mediate defensive responses. This review describes the anatomical connections of the PVT that support the view that the PVT may be a critical node in the brain anxiety network. Experimental findings are reviewed showing that the arousal peptides orexins (hypocretins) act at the PVT to promote avoidance of potential threats especially following exposure of rats to a single episode of footshocks. Recent anatomical and experimental findings are discussed which show that neurons in the PVT provide divergent projections to subcortical regions that mediate defensive behaviors and that the projection to the NAcSh is critical for the enhanced social avoidance displayed in rats exposed to footshocks. A theoretical model is proposed for how the PVT integrates cortical and hypothalamic signals to modulate the behavioral responses associated with anxiety and other challenging situations.

Dopamine D1 receptor in the NAc shell is involved in delayed emergence from isoflurane anesthesia in aged mice

BACKGROUND

Delayed emergence after general anesthesia tends to occur in the elderly population, but the mechanism remains unclear. Apart from age-related pharmacokinetic changes, the aging-induced structural and functional alterations in the arousal-promoting neural substrates should be considered. The nucleus accumbens (NAc) is a crucial arousal-related nucleus, in which activating medium spiny neurons (MSNs) expressing dopamine D1 receptor (D1R) could facilitate the arousal from natural sleep. Meanwhile, the dopaminergic systems decline with aging in multiple brain regions. However, whether the age-related decline in D1R in the NAc shell attenuates its arousal-promoting capacity from general anesthesia remains to be elucidated.

METHODS

We first verified the delayed emergence from isoflurane anesthesia and examined the corresponding changes of electroencephalogram (EEG) power in aged mice. In turn, the arousal-modulating capacity of D1R was characterized in the young and aged cohorts by microinjection of D1R agonist/antagonist into the NAc shell. Furthermore, to address the possible mechanism responsible for the attenuated arousal-modulating capacity of the aged NAc, the expression of D1R in the NAc shell was measured and compared between young and aged mice.

RESULTS

Our data indicated that compared with young mice, the emergence time in aged mice was notably longer, while EEG power in δ band (1-4Hz) was significantly higher and power in β band (12-25Hz) was lower. Activating or inhibiting D1R in the NAc shell by microinjection D1R agonist/antagonist promoted or delayed the emergence process in young mice. Nevertheless, this modulation capacity of D1R in the NAc shell declined in aged mice, respectively. Meanwhile, downregulation of D1R expression in the NAc shell was detected in the aged brain.

CONCLUSION

Together, these results suggest that aging attenuates the arousal-modulating capacity of D1R in the NAc shell probably through downregulation of D1R expression therein, which may provide a potential explanation and a therapeutic target for increased sensitivity to anesthetics in the elderly patients.

Glycine-Binding Site Stimulants of NMDA Receptors Alleviate Extrapyramidal Motor Disorders by Activating the Nigrostriatal Dopaminergic Pathway

Dysfunction of the N-methyl-d-aspartate (NMDA) receptor has been implicated in the pathogenesis of schizophrenia. Although agonists for the glycine-binding sites of NMDA receptors have potential as new medication for schizophrenia, their modulation of antipsychotic-induced extrapyramidal side effects (EPS) has not yet been clarified. We herein evaluated the effects of glycine-binding site stimulants of NMDA receptors on antipsychotic-induced EPS in mice and rats. d-cycloserine (DCS) and d-serine significantly improved haloperidol (HAL)-induced bradykinesia in mice, whereas glycine showed no effects. Sodium benzoate, a d-amino acid oxidase inhibitor, also attenuated HAL-induced bradykinesia. Improvements in HAL-induced bradykinesia by DCS were antagonized by the NMDA antagonist dizocilpine or nitric oxide synthase inhibitor L-N^G-Nitro-l-arginine methyl ester. In addition, DCS significantly reduced HAL-induced Fos expression in the dorsolateral striatum without affecting that in the nucleus accumbens. Furthermore, a microinjection of DCS into the substantia nigra pars compacta significantly inhibited HAL-induced EPS concomitant with elevations in dopamine release in the striatum. The present results demonstrated for the first time that stimulating the glycine-binding sites of NMDA receptors alleviates antipsychotic-induced EPS by activating the nigrostriatal dopaminergic pathway, suggesting that glycine-binding site stimulants are beneficial not only for efficacy, but also for side-effect management.

Highly palatable food access during adolescence increased anxiety-/depression-like behaviors in male, but not in female, rats

OBJECTIVES

This study was conducted to examine the sexual dimorphic effects of highly palatable food (HPF) access during adolescence on the neurochemistry and depression-/anxiety-like behaviors of rats.

METHODS

Male and female Sprague-Dawley pups had free access to chocolate cookie rich in fat (HPF) from postnatal day 28 in addition to ad libitum chow, and the control groups received only chow. The food conditions were continued throughout the entire experimental period, and the neurochemical and behavioral measurements were performed during young adulthood. Rats were subjected to the ambulatory activity, elevated plus maze, and forced swim tests. Corticosterone levels during 2 h of restraint stress were analyzed with radioimmunoassay, and ΔFosB and brain-derived neurotrophic factor (BDNF) expression in the nucleus accumbens (NAc) with Western blot analysis.

RESULTS

Cookie access did not affect body weight gain and total caloric intake in both sexes; however, it increased retroperitoneal fat depot only in males. The time spent in open arms during elevated plus maze test was decreased and immobility during forced swim test was increased in cookie-fed males, but not in cookie-fed females. Main effect of food condition on the stress-induced corticosterone increase was observed in males, but not in females, and cookie access increased BDNF expression in the NAc only in males.

CONCLUSIONS

Increased BDNF expression in the NAc and fat depot, in addition to the stress axis dysfunction, may play roles in the pathophysiology of depression- and/or anxiety-like behaviors induced by cookie access.

Beneficial Effects of Highly Palatable Food on the Behavioral and Neural Adversities induced by Early Life Stress Experience in Female Rats

This study examined the effects of highly palatable food during adolescence on the psycho-emotional and neural disturbances caused by early life stress experience in female rats. Female Sprague-Dawley pups were separated from dam for 3 h daily during the first two weeks of birth (MS) or left undisturbed (NH). Half of MS females received free access to chocolate cookies in addition to ad libitum chow from postnatal day 28. Pups were subjected to the behavioral tests during young adulthood. The plasma corticosterone response to acute stress, ΔFosB and brain-derived neurotrophic factor (BDNF) levels in the brain regions were analyzed. Total caloric intake and body weight gain during the whole experimental period did not differ among the experimental groups. Cookie access during adolescence and youth improved anxiety-/depression-like behaviors by MS experience. ΔFosB expression was decreased, but BDNF was increased in the nucleus accumbens of MS females, and ΔFosB expression was normalized and BDNF was further increased following cookie access. Corticosterone response to acute stress was blunted by MS experience and cookie access did not improve it. Results suggest that cookie access during adolescence improves the psycho-emotional disturbances of MS females, and ΔFosB and/or BDNF expression in the nucleus accumbens may play a role in its underlying neural mechanisms.

The microinjection of a cannabinoid agonist into the accumbens shell induces anxiogenesis in the elevated plus-maze

This study investigated the effect of a cannabinoid agonist injected into the shell region of the nucleus accumbens (nAcb shell) on anxiety-related behaviors. The animals (male Wistar rats) were unilaterally microinjected with either ACEA (arachidonyl-2'-chloroethylamide a CB1 receptor agonist) at doses of 0.005, 0.05 or 0.5 pmol, or vehicle (ethanol 0.04% in saline 0.9%) and submitted to the elevated plus-maze (EPM), a pre-clinical test of anxiety. The data showed that rats microinjected with ACEA (0.05 pmol/0.2 μl) into the nAcb shell exhibited decreased % open arm time and open arm entries in comparison with the control group, which is compatible with an anxiogenic-like effect. To rule out the hypothesis that spread of the drug into the ventricle was responsible for the observed anxiogenic effect, 0.05 pmol ACEA was injected into the lateral ventricle and shown not to alter the responses representative of fear/anxiety and locomotion. The locomotor activity was not changed at the dose of 0.05 pmol ACEA microinjected into the nAcb shell. The present data suggest that activation of cannabinoid receptors in the nAcb shell may modulate fear/anxiety in the EPM.

Cortical activation of accumbens hyperpolarization-active NMDARs mediates aversion-resistant alcohol intake

Compulsive drinking despite serious adverse medical, social and economic consequences is a characteristic of alcohol use disorders in humans. Although frontal cortical areas have been implicated in alcohol use disorders, little is known about the molecular mechanisms and pathways that sustain aversion-resistant intake. Here, we show that nucleus accumbens core (NAcore) NMDA-type glutamate receptors and medial prefrontal (mPFC) and insula glutamatergic inputs to the NAcore are necessary for aversion-resistant alcohol consumption in rats. Aversion-resistant intake was associated with a new type of NMDA receptor adaptation, in which hyperpolarization-active NMDA receptors were present at mPFC and insula but not amygdalar inputs in the NAcore. Accordingly, inhibition of Grin2c NMDA receptor subunits in the NAcore reduced aversion-resistant alcohol intake. None of these manipulations altered intake when alcohol was not paired with an aversive consequence. Our results identify a mechanism by which hyperpolarization-active NMDA receptors under mPFC- and insula-to-NAcore inputs sustain aversion-resistant alcohol intake.

Identification of neurons specifically activated after recall of context fear conditioning

The learning of new information and recall of that information presumably involves modification of and access to shared circuitry in the brain. However, learning and recall may involve the activation of distinct parts of that circuitry, according to the quite distinct functional differences between these two processes. Previously we examined neuronal activation following learning of context fear conditioning. Using the Fos-Tau-LacZ (FTL) transgenic mouse to label activated neurons, we identified a number of distinct populations of neurons in amygdala and hypothalamus which showed learning specific activation. These populations of neurons showed much less activation following recall. Here we ask what populations of neurons might be specifically activated following recall. We trained mice in context fear conditioning, and then looked at FTL activation following recall of context fear. We identified a number of populations of neurons which showed recall specific activation in nucleus accumbens shell, the anterio-medial bed nucleus of stria terminalis, the anterior commissural nucleus and the periventricular hypothalamic nucleus. These were all different populations of neurons compared with those activated following context fear learning. These different functional activation patterns occurring between learning and recall may reflect the different brain functions occurring between these two memory related processes.

Involvement of β3-adrenergic receptors in the control of food intake in rats

This study examined the food intake changes evoked by intracerebroventricular (icv) injection of a selective agonist (BRL37344, 2 and 20 nmol) or antagonist (SR59230A, 10 and 50 nmol) of β3-adrenergic receptors in 24-h fasted rats (adult male Wistar rats, 200-350 g, N = 6/treatment). The animals were also pretreated with saline icv (SAL) or SR59230A (50 nmol) followed by BRL37344 (20 nmol) or SAL in order to determine the selectivity of the effects evoked by BRL37344 on food intake or the selectivity of the effects evoked by SR59230A on risk assessment (RA) behavior. The highest dose of BRL37344 (N = 7) decreased food intake 1 h after the treatment (6.4 ± 0.5 g in SAL-treated vs 4.2 ± 0.8 g in drug-treated rats). While both doses of SR59230A failed to affect food intake (5.1 ± 1.1 g for 10 nmol and 6.0 ± 1.8 g for 50 nmol), this treatment reduced the RA frequency (number/30 min) (4 ± 2 for SAL-treated vs 1 ± 1 for 10 nmol and 0.5 ± 1 for 50 nmol SR59230A-treated rats), an ethological parameter related to anxiety. While pretreatment with SR59230A (7.0 ± 0.5 g) abolished the hypophagia induced by BRL37344 (3.6 ± 0.9 g), BRL37344 suppressed the reduction in RA frequency caused by SR59230A. These results show that the hypophagia caused by BRL37344 is selectively mediated by β3-adrenergic receptors within the central nervous system. Moreover, they suggest the involvement of these receptors in the control of anxiety.

Transient inactivation of the medial prefrontal cortex affects both anxiety and decision-making in male wistar rats

In both humans and rats high levels of anxiety impair decision-making in the Iowa gambling task (IGT) in male subjects. Expression of the immediate early gene c-fos as marker of neural activity in rat studies indicated a role of the medial prefrontal cortex (prelimbic and infralimbic region; mPFC) in mediating the relationship between anxiety and decision-making. To delineate this relationship further and assess the underlying neurobiology in more detail, we inactivated in the present study the mPFC in male rats using a mixture of the GABA-receptor agonists muscimol and baclofen. Rats were exposed to the elevated plus maze (EPM) to measure effects on anxiety and to the rodent version of the IGT (r-IGT). Inactivation led to increased levels of anxiety on the EPM, while not affecting general activity. The effect in the r-IGT (trials 61–120) was dependent on levels of performance prior to inactivation (trial 41–60): inactivation of the mPFC hampered task performance in rats, which already showed a preference for the advantageous option, but not in rats which were still choosing in a random manner. These data suggest that the mPFC becomes more strongly involved as rats have learned task-contingencies, i.e., choose for the best long-term option. Furthermore they suggest, along with the data of our earlier study, that both anxiety and decision-making in rats are mediated through a neural circuitry including at least the mPFC. The data are discussed in relation to recent data of rodent studies on the neural circuitry underlying decision-making.

Modulation of fear/anxiety responses, but not food intake, following α-adrenoceptor agonist microinjections in the nucleus accumbens shell of free-feeding rats

This study investigated the effect of α-adrenoceptor agonists microinjected into the shell region of the accumbens nucleus (AcbSh) on feeding and anxiety-related behaviors in free-feeding rats. Male Wistar rats with a chronically implanted cannula into the AcbSh were unilaterally microinjected with either clonidine (CLON, α(2)-adrenoceptor agonist) or phenylephrine (PHEN, α(1)-adrenoceptor agonist) at the doses of 6 and 20 nmol and submitted to the elevated plus-maze (EPM), a pre-clinical test of anxiety. Immediately after the EPM test, the animals underwent food intake evaluation for 30 min. The data showed that rats microinjected with CLON (20 nmol/0.2 μl) into the AcbSh exhibited increased %Open arm time, which is compatible with an anxiolytic-like effect. The CLON-induced anxiolysis was corroborated by increased head-dipping and decreased stretched-attend posture, two ethologically derived behaviors which are fear/anxiety-motivated. The animal's locomotor activity was not changed by 20 nmol CLON microinjection into the AcbSh. However, neither dose of PHEN microinjected into the AcbSh was able to alter either the spatial-temporal or ethological variables representative of fear/anxiety and locomotion. Food intake was not altered by any dose of CLON and PHEN microinjected into the AcbSh, but the 20 nmol CLON microinjection induced increased motor activity in the feeding test. The data suggests that noradrenergic projections to the AcbSh may underlie fear/anxiety modulation through α(2)-adrenoceptor in the AcbSh, while feeding behavior was unaffected by noradrenergic modulation in the AcbSh of free-feeding rats. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Decision-making performance is related to levels of anxiety and differential recruitment of frontostriatal areas in male rats

In humans, high levels of anxiety are associated with poor performance in the Iowa Gambling Task (IGT). The IGT measures decision-making under conditions of uncertainty. In this study, we investigated the association between anxiety and decision-making in rats. Rats were screened for anxiety on the elevated plus maze (EPM) and subsequently tested in a rat analogue of the IGT (r-IGT). We explored the role of frontostriatal areas related to r-IGT performance using c-fos immunohistochemistry following the last training-session. High levels of anxiety were associated with poor r-IGT performance: high anxious rats made fewer choices for the advantageous option and collected fewer sucrose pellets in the r-IGT than low anxious rats. Analysis of win-stay/lose-shift behaviour of choices for the advantageous option revealed that good performing-low anxious subjects showed an increase in win-stays and a decrease in lose-shifts across trial blocks while poor performing-high anxious subjects did not. Furthermore, decision-making performance and, indirectly, anxiety levels were related to neural activity in parts of the medial prefrontal cortex, that is prelimbic and infralimbic cortex, and in parts of the striatum, that is nucleus accumbens shell and core. These data suggest a similar frontostriatal circuitry underlying affective decision-making in humans and rats.

Does gene deletion of AMPA GluA1 phenocopy features of schizoaffective disorder?

Glutamatergic dysfunction is strongly implicated in schizophrenia and mood disorders. GluA1 knockout (KO) mice display schizophrenia- and depression-related abnormalities. Here, we asked whether GluA1 KO show mania-related abnormalities. KO were tested for behavior in approach/avoid conflict tests, responses to repeated forced swim exposure, and locomotor responses under stress and after psychostimulant treatment. The effects of rapid dopamine depletion and treatment with lithium or GSK-3β inhibitor on KO locomotor hyperactivity were tested. Results showed that KO exhibited novelty- and stress-induced locomotor hyperactivity, reduced forced swim immobility and alterations in approach/avoid conflict tests. Psychostimulant treatment and dopamine depletion exacerbated KO locomotor hyperactivity. Lithium, but not GSK-3β inhibitor, treatment normalized KO anxiety-related behavior and partially reversed hyperlocomotor behavior, and also reversed elevated prefrontal cortex levels of phospho-MARCKS and phospho-neuromodulin. Collectively, these findings demonstrate mania-related abnormalities in GluA1 KO and, combined with previous findings, suggest this mutant may provide a novel model of features of schizoaffective disorder.

Effect of chronic psychosocial stress-induced by subordinate colony (CSC) housing on brain neuronal activity patterns in mice

Chronic subordinate colony (CSC) housing has been recently validated as a murine model of chronic psychosocial stress which induces alterations of stress-related parameters including decreased body-weight gain and an increased level of anxiety in comparison with single housed control (SHC) mice. By using immunohistochemical immediate early gene (IEG) mapping we investigated whether CSC housing causes alterations in neuronal activation patterns in limbic areas including the amygdala, hippocampus, septum and the periaqueductal gray (PAG) and hypothalamic paraventricular nucleus (PVN). While CSC housing increased basal Zif-268 expression in the nucleus accumbens shell compared to SHC, IEG responses to subsequent open arm (OA) exposure were attenuated in the ventral and intermediate sub-regions of the lateral septum, parvocellular PVN and the dorsal CA3 region of the hippocampus of CSC compared with SHC mice. In contrast, a potentiated c-Fos response in CSC mice was observed in the dorsomedial PAG after OA exposure. Confirming previous findings obtained on the elevated plus-maze, an enhanced anxiety-related behavior in CSC compared with SHC mice was also observed during OA exposure. In order to investigate the appropriate control conditions for CSC housing, group housed control (GHC) mice were additionally included in the behavioral testing. Interestingly, GHC as well as CSC mice showed significantly less risk assessment/exploratory behavior during OA exposure compared with SHC mice indicating that group housing itself is stressful for mice and not an adequate control for the CSC paradigm. Overall, CSC housing is an ethologically relevant chronic psychosocial stressor which results in an elevated sensitivity to a subsequent novel, aversive challenge. However, the CSC-induced increase in anxiety-related behavior was accompanied by differences in neuronal activation, compared with SHC, in defined sub-regions of brain areas known to be involved in the processing of emotionality and stress responses.

The microinjection of AMPA receptor antagonist into the accumbens shell failed to change food intake, but reduced fear-motivated behaviour in free-feeding female rats

This study investigated the effect of the AMPA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 2.5 and 5.0 nmol/side) microinjected into the core and shell sub-regions of the accumbens (Acb) nucleus, on food intake and the level of anxiety in female rats. Bilateral microinjections of CNQX (5.0 nmol/side) into the Acb shell (AP, +1.08 to +2.04), but not into the Acb core, induced an anxiolytic-like effect in relation to rats microinjected with vehicle, since the animals exhibited low level of SAP in the feeding test. The anxiolytic-like effect induced by 5.0 nmol CNQX microinjection into the Acb shell may not be ascribed to changes in the motor activity of the animals, because the frequency of locomotion, rearing and grooming remained unchanged after the drug microinjection. However, neither Acb shell nor Acb core CNQX microinjections were able to change the animals food intake along 1h feeding behaviour evaluation. Food intake remained unchanged 24h after the drug microinjections either into the Acb shell or into the Acb core. The data suggest that AMPA receptor blockade in the Acb nucleus may differentially change the ingestive and defensive behaviours in female rats.