Effects of chronic antidepressants in an operant conflict procedure of anxiety in the rat

ANXIOLYTICS: Origins, drug discovery, and mechanisms

Anxiety is a part of the human condition and has been managed by psychoactive substances for centuries. The current medical need and societal demand for anxiolytic medicines has not abated. The present overview provides a brief historical introduction to the discovery of modern age anxiolytics that include the benzodiazepines together with a discussion of the continuing medical need for new antianxiety medications. The paper also discusses the use and impact of behavioral pharmacology in the preclinical development of anxiolytics. The review then highlights the diversity of mechanisms for creating a new generation of anxioltyics through mechanisms beyond the potentiation of GABAA receptors and the blockade of monoamine uptake. A discussion then follows on the behavioral specificity of action of anxiolytics that includes the concept of creating an anxioselective drug, one that targets anxiety without producing untoward effects that include sedation and dependence. The use of anxiolytics in the treatment of other conditions such as substance use disorder is also briefly reviewed. Finally, a brief summary of the current status of anxiolytic drug development is provided. The review concludes with the idea that despite a host of anxiolytic drugs, the lack of efficacy in some patients and the side-effects and safety issues associated with some of these medications demands alternative medicines. Current preclinical and clinical research is ongoing with the goal of identifying such compounds.

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.

Electric foot shock stress: a useful tool in neuropsychiatric studies

Electric foot shock is a complex stressor with both physical and emotional components. It has been employed as an important tool to develop diverse animal models in the field of psychopharmacology. The electric foot shock paradigm includes acute or chronic exposures of shocks of varying intensity and duration on an electrified grid floor in an electric foot shock apparatus. Research evidence reveals that foot shocks of varying intensity produce behavioral and neurochemical changes reflecting depression, anxiety, and post-traumatic stress disorder (PTSD) in humans. Animals generally do not habituate to foot shocks in comparison to other stressors, including loud noise, bright light, and hot and cold temperatures. Additionally, it offers an experimental advantage of control over intensity and duration; therefore, by varying its application parameters, different disorder models have been created. Electric foot shock fear conditioning-induced ultrasonic vocalization and fear-potentiated startle have been explored to develop models of anxiety and panic. Similarly, fear conditioning in the form of foot shock exposure followed by situational reminders has been used to develop a model of PTSD. Electric foot shock-induced conflict has been explored to develop operant conflict models (Geller-Seifter and Vogel tests), which in turn are pharmacologically validated to screen potential anti-anxiety agents. Inescapable electric shock-induced ‘learned helplessness’ mimics the symptomology of depression, and this phenomenon has been employed to develop the model of depression. The present review describes the pharmacologically validated models of anxiety, depression, and PTSD involving electric foot shock as an aversive stimulus.

Drug-, dose- and sex-dependent effects of chronic fluoxetine, reboxetine and venlafaxine on open-field behavior and spatial memory in rats

In an effort to address the need to include both sexes in studies of effects of the SSRI fluoxetine, the NRI reboxetine and the SNRI venlafaxine on anxiety-related behavior and memory along with the use of chronic drug administration, male and female PVG/c rats were fed diets containing two doses of each drug for 21 days. The rats' anxiety level was then assessed in an open field. Short-term spatial memory for a brightness change in a Y maze was also measured. While there was little evidence of anxiolytic effects of any of the drugs, both fluoxetine and, to a lesser extent, venlafaxine appeared to be mainly anxiogenic in their action depending on both dose and sex. Reboxetine was relatively ineffective in this respect. Ability to locate the Y-maze arm that had changed (from white to black) seemed to be impaired for male (but not female) rats by both fluoxetine and venlafaxine and, to a much lesser extent, by reboxetine. Given the relative ineffectiveness of reboxetine in either test, it is possible that the effects of the other two drugs on both anxiety and memory were mainly due to their serotonin reuptake inhibiting properties. The differences that occurred between males and females in responsiveness to all three drugs supported the long-held view that both sexes should be investigated in studies of this sort, especially in view of reports of sex differences in effects of clinically prescribed antidepressants.

Long-term anxiolytic and antidepressant-like behavioural effects of tiagabine, a selective GABA transporter-1 (GAT-1) inhibitor, coincide with a decrease in HPA system activity in C57BL/6 mice

Gamma-aminobutyric acid (GABA) system plays a pivotal role in the pathophysiology of anxiety and mood disorders. This study was aimed to assess the anxiolytic and antidepressant-like properties of tiagabine, an inhibitor of the GABA transporter-1 (GAT-1), after acute and chronic administration in C57BL/6JOlaHsD mice with paroxetine as a positive control. In first experiments, the acute administration of tiagabine (7.5 mg/kg, orally [PO]) and paroxetine (10 mg/kg PO) induced anxiolytic effects in the elevated plus maze test and the modified hole board test and an antidepressant-like effect in the forced swim test. Chronic application of tiagabine (7.5 mg/kg PO) and paroxetine (10 mg/kg PO) for 22 days revealed an anxiolytic and antidepressant-like efficacy of tiagabine only. In a further experiment, we analysed the impact of chronic tiagabine versus paroxetine treatment on the hypothalamic-pituitary-adrenocortical (HPA) system regulation. GAT-1 blockade induced a setpoint-shift of the stress hormone system toward lower levels as indicated by decreased plasma corticosterone concentrations and attenuated gene expression levels of corticotropin-releasing factor in the paraventricular nucleus of the hypothalamus and of hippocampal steroid receptors. This data indicate that both acute and long-term anxiolytic and antidepressant-like properties of brain GAT-1 inhibition coincide with a reduction in HPA system activity in mice.

Repeated social defeat-induced depression-like behavioral and biological alterations in rats: involvement of cholecystokinin

Cholecystokinin (CCK) involvement in depression-like disorders is poorly documented. Here, we investigated whether CCKergic neurotransmission is relevant to depressive-like symptoms and antidepressant therapy using a novel preclinical model based on repeated social defeat over 4 weeks in rats. Repeated social defeat triggers changes that could be considered as behavioral and biological correlates of depressive symptoms in humans, such as a hyperactivity of hypothalamic-pituitary-adrenal (HPA) axis (increase of serum corticosterone levels and of adrenal gland weight), increased immobility time in the forced swimming test (FST), decrease of body weight and of sweet water consumption and reduction of hippocampal volume associated with a decreased cell proliferation in the dentate gyrus. In addition, in vivo microdialysis showed that cortical CCK release was tonically increased in defeated rats. Chronic imipramine treatment (16 mg kg(-1) per day for 25 days) prevented both the repeated social defeat-induced alterations of biological and behavioral parameters and the associated increase of cortical CCK release. Chronic blockade of CCK2 receptors by the specific antagonist CI-988 (1 mg kg(-1) per day for 25 days) also normalized immobility time in the FST and prevented HPA axis hyperactivity, reduction of hippocampal volume and cell proliferation and decreased sweet water intake normally evoked by repeated social defeat. These data showed that the repeated social-defeat paradigm can be considered as a suitable model of 'depression' in rats. The causal link between social defeat-evoked (1) increase in cortical CCKergic neurotransmission and (2) depression-like symptoms that we highlighted here strongly suggests that CCKergic systems may be a relevant target for novel antidepressant therapy.

A panic experimental model: Validation of a complex operant behavioral method in undernourished rats, with desipramine to provide a template effect profile

INTRODUCTION

Clinical studies have shown that some antidepressants may be more efficient than benzodiazepines to alleviate anxiety associated with panic disorders; however, operant conflict procedures in rats developed so far seem not particularly able to model human anxiety sensitive to antidepressant treatments. Previous panic models with learned responses did not statistically subtract the effect of confounding factors from the variable of interest.

METHODS

Undernourished rats were selected due to their behavioral and neurobiological resemblance to human patients suffering from panic disorder. The Geller-Seifter paradigm represented the stressful environmental condition in adult life. Desipramine (10 mg/kg/day) or saline were administered IP during 7 days under a cross over design (N=10). Five daily 15 min-operant sessions were carried out on each experiment. Unpunished, unrewarded and punished operant behavioral periods were identical both in their duration and in their reward system (the FR1 schedule) in order to measure response suppression, which has not been considered in previous studies with the Geller-Seifter paradigm. The dependent variable was the difference between comparable unpunished and punished periods.

RESULTS

A significant Diet x Drug interaction was observed in the dependent variable, which represented the level of "suppression/suppression release" induced by treatments.

DISCUSSION

Compared to control rats, deprived rats showed a significant and selective anticonflict effect of desipramine on the stressful and complex operant performance. The animal model of perinatally protein-deprived rats along with the Geller-Seifter's operant behavioral paradigm may represent a more sensitive approach to model human anxiety sensitive to antidepressant treatments by considering the combined impact of both early biological trauma and adult learned experiences under the same design.

Evaluation of two genetic animal models in behavioral tests of anxiety and depression

Anxiety- and depression-related disorders often appear associated and may be affected by common genetic factors. The inbred rat strains Lewis (LEW) and spontaneously hypertensive rats (SHR) and the outbred rat lines Floripa H and L, which were selectively bred for high and low locomotion in the central area of the open field (OF) test, respectively, have been proposed as experimental tools to study anxiety. The main goal of the present study was to characterize the behavior of these animals in two models of anxiety, elevated plus-maze (EPM) and OF, in two models of depression, forced swim test (FST) and tail suspension test (TST) and in their home-cages. Emotionality-related differences between LEW and SHR rats and between Floripa H and L rats were found in the EPM, OF and FST. Those lines showing low anxiety-like profiles in the EPM and OF (SHR and Floripa H) also showed low immobility in the FST. The TST failed to unveil any line differences. Factor analysis involving all tests revealed three independent factors with one of them associating anxiety-related measures from the OF and EPM to immobility in the FST. When observed in their home-cages, LEW and SHR rats showed no differences in general activity, but when acutely treated with imipramine (15mg/kg), only LEW rats were sensitive to its antidepressant effects. These results suggest the existence of a genetic link between two tests used in the screening of anxiolytic drugs and one test of antidepressant activity. Moreover, the LEW and SHR rat strains were shown to be an interesting model to study the comorbidity between anxiety- and depression-related disorders.

Age-dependent interactive changes in serotonergic and noradrenergic cortical axon terminals in F344 rats

In the frontal cortex of aging rats, we found an increase in sprouting of the noradrenergic (NA) axons originated from the locus coeruleus (LC). The serotonergic (5-HT) axons originating from the dorsal raphe (DR) share the same cortical area and their age-dependent changes and interactions with NA axons were still unclear. To compare quantitatively the extent of axonal sprouting of DR and LC neurons in the frontal cortex, we extracellularly recorded from both DR and LC neurons in the same animals and antidromically stimulated 32 cortical sites (a pair of stimulating electrodes was moved at 100-mum intervals from 500 to 2000 microm in depth). In addition, to examine the effects of degeneration of 5-HT axons on NA axons, and vice versa, we used specific neurotoxins for 5-HT (PCA) or NA (DSP-4) axons. We also used noradrenaline uptake inhibitor (maprotiline) to verify the effects of NA on degeneration of 5-HT axons. Results suggested that 5-HT axons sprouted between 15 and 17 months of age and noradrenaline accelerated the age-dependent change of 5-HT axons.

Reduction of hippocampal Na+, K+-ATPase activity in rats subjected to an experimental model of depression

The effect of a model of depression using female rats on Na+, K+-ATPase activity in hippocampal synaptic plasma membranes was studied. In addition, the effect of further chronic treatment with fluoxetine on this enzyme activity was verified. Sweet food consumption was measured to evaluate the efficacy of this model in inducing a state of reduced response to rewarding stimili. After 40 days of mild stress, a reduction in sweet food ingestion was observed. Reduction of hippocampal Na+, K+-ATPase activity was also observed. Treatment with fluoxetine increased this enzyme activity and reversed the effect of stress. Chronic fluoxetine decreased the ingestion of sweet food in both groups. This result is in agreement with suggestions that reduction of Na+, K+-ATPase activity is a caracteristic of depressive disorders. Fluoxetine reversed this effect. Therefore it is possible that altered Na+, K+-ATPase activity may be involved in the pathophysiology of depression in patients.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

Behavioral pharmacology of AR-A000002, a novel, selective 5-hydroxytryptamine(1B) antagonist

The present review summarizes the behavioral pharmacology conducted to profile the anxiolytic and antidepressant potential of the selective 5-hydroxytryptamine (HT)(1B) antagonist (R)-N-[5-methyl-8-(4-methylpiperazin-1-yl)-1,2,3,4-tetrahydro-2-naphthyl]-4-morpholinobenzamide (AR-A000002). AR-A000002 functions as a 5-HT(1B) antagonist in vivo, which was shown by the antagonism of the discriminative stimulus effects in the guinea pig of the 5-HT(1B) agonist 3-(N-methylpyrrolidin-2R-ylmethyl)-5-(3-nitropyrid-2-ylamino)-lH-indole (CP135,807). Anxiolytic activity of AR-A000002 was demonstrated in the separation-induced vocalization paradigm in guinea pig pups, and in a suppressed responding procedure in pigeons and guinea pigs, but only a weak trend was noted in a suppressed responding procedure in squirrel monkeys. Antidepressant efficacy was shown in a number of paradigms. In pigeons and guinea pigs responding under a differential reinforcement of low rates schedule of reinforcement (DRL), AR-A000002 increased the number of reinforcers earned without altering the number of responses made. In guinea pigs trained under a response duration differentiation paradigm, AR-A000002 increased mean lever-press duration. Finally, AR-A000002 was shown to block escape failures in guinea pigs submitted to a learned helplessness paradigm. Taken together, these data suggest utility for 5-HT(1B) antagonists in the treatment of both anxiety and affective disorders.

Prior exposure to the elevated plus-maze sensitizes mice to the acute behavioral effects of fluoxetine and phenelzine

A single undrugged experience of the elevated plus-maze modifies future drug responses in the test. The present study investigated the effects of maze-experience on the acute behavioral effects of the monoamine oxidase inhibitor phenelzine and the serotonin reuptake inhibitor fluoxetine. Phenelzine (2.5-12.5 mg/kg) had no clear effect on plus-maze behavior in test-naive Swiss Webster mice, but dose-dependently increased anxiety-like behavior in maze-experienced subjects. Similarly, fluoxetine (5-20 mg/kg) produced non-significant trends for increased anxiety-like behavior in maze-naive mice, but significantly and dose-dependently increased anxiety-like behavior and suppressed locomotor activity in maze-experienced mice. The anxiogenic effects of the benzodiazepine receptor inverse agonist N-methyl-beta-carboline-3-carboxamide (FG 7142) (20 mg/kg) was abolished by prior test experience, suggesting an alteration in gamma-aminobutyric acid (GABA)/benzodiazepine receptor function with maze-experience. However, the benzodiazepine receptor antagonist flumazenil (5-20 mg/kg) produced a silent profile regardless of maze-experience. Present findings provide further evidence demonstrating that prior test history is a critical consideration in mouse studies of anxiety-related behavior.

Balance control and posture in anxious mice improved by SSRI treatment

A task requiring dynamic postural stabilisation during locomotion in a conflicting visual vestibular environment (rotating beam), has been devised to assess anxiety-related balance impairments and postural changes in mice. The model, already validated with acutely administered diazepam, was used to assess the action of two chronically administered selective serotonin reuptake inhibitors (SSRIs), fluoxetine and paroxetine. On three behavioural measures (imbalance, elevation of trunk and angle of tail), observed in anxious BALB/cByJ mice, both compounds had the same diazepam-like effects: reduction in number of imbalances, higher elevation of trunk and increase in tail angle. These data suggest, for the first time, that SSRIs should be useful in the treatment of anxiety-induced balance impairments.