Differences in the dopaminergic reward system in rats that passively and actively behave in the Porsolt test

Mechanism of GW117 antidepressant action: melatonin receptor-mediated regulation of sleep rhythm

Alterations in circadian sleep patterns constitute a salient manifestation in major depressive disorder. GW117, an emergent antidepressant, functions as an agonist for melatonin 1 and melatonin 2 (MT1/MT2) receptors, in tandem with antagonism of the serotonin (5-HT) 2C receptor. The present investigation is dedicated to elucidating the role and underlying mechanisms by which GW117 ameliorates circadian sleep disruptions. Utilizing an adapted chronic unpredictable mild stress protocol, we induced a depressive-like phenotype and perturbed circadian rhythms in rodent models. Our methodological approach integrated quantitative polymerase chain reaction (qPCR) in real-time, enzyme-linked immunosorbent assay (ELISA), and immunoblotting techniques to probe alterations in the expression of core circadian genes and homeostatic sleep markers. The impact of GW117 was assessed across various dosages (10, 20, and 40 mg/kg) on these molecular signatures. In a parallel examination, we evaluated the influence of GW117 (administered at 15, 40, and 60 mg/kg) on the sleep patterns of healthy mice. The results showed that GW117 significantly improved sleep-wake circadian rhythms, altered sleep architecture, and shortened sleep latency. Furthermore, GW117 increased the expression of several clock genes in the hypothalamus of chronic unpredictable mild stress model rats and normal mice. It also regulated circadian biomarkers, including melatonin and cortisol. Based on our findings, we propose that the beneficial effects of GW117 on sleep rhythms may be due to the melatonin system-mediated activation of the Wnt/β-catenin signaling pathway.

Depressive-like phenotype enhances relapse of nicotine seeking after forced abstinence in rats

OBJECTIVE

Comorbidity of depression and drug addiction is common, but effective treatment is missing. A rat model combining the olfactory bulbectomy (OBX) model and IV drug self-administration has provided evidence of differential reactivity of the OBX rats towards drugs of abuse. This study evaluates nicotine taking and seeking behaviour in this model.

METHODS

Adult male Wistar rats were used; in one group, the OBX was performed while the other group was sham-operated. After three weeks of nicotine self-administration (fixed ratio-1 schedule), rats underwent two weeks of forced abstinence followed by a drug-free relapse-like session. Two doses of nicotine were studied: 0.019 and 0.030 mg/kg per infusion. The locomotor test took place before the self-administration protocol and on the first day of abstinence.

RESULTS

OBX induced characteristic hyperactive locomotor phenotype. OBX rats self-administered more nicotine in the experiment using 0.019 mg/kg per infusion, but they reached lower drug intake in the study using 0.030 mg/kg per infusion. However, relapse of nicotine seeking after forced abstinence was significantly higher in the OBX groups in both cohorts.

CONCLUSION

These results are in line with previous studies showing OBX-induced dissimilarities in drug-seeking and drug-taking and represent complementary information to reports on other substances.

The effects of the recurrent social isolation stress on fear extinction and dopamine D2 receptors in the amygdala and the hippocampus

BACKGROUND

The present study assessed the influence of recurrent social isolation stress on the aversive memory extinction and dopamine D₂ receptors (D₂R) expression in the amygdala and the hippocampus subnuclei. We also analyzed the expression of epigenetic factors potentially associated with fear extinction: miRNA-128 and miRNA-142 in the amygdala.

METHODS

Male adult fear-conditioned rats had three episodes of 48 h social isolation stress before each fear extinction session in weeks intervals. Ninety minutes after the last extinction session, the D₂R expression in the nuclei of the amygdala and the hippocampus (immunocytochemical technique), and mRNA levels for D₂R in the amygdala were assessed (PCR). Moreover, we evaluated the levels of miRNA-128 and miRNA-142 in the amygdala.

RESULTS

It was found that recurrent social isolation stress decreased the fear extinction rate. The extinguished isolated rats were characterized by higher expression of D₂R in the CA1 area of the hippocampus compared to the extinguished and the control rats. In turn, the isolated group presented higher D₂R immunoreactivity in the CA1 area compared to the extinguished, the control, and the extinguished isolated animals. Moreover, the extinguished animals had higher expression of D₂R in the central amygdala than the control and the extinguished isolated rats. These changes were accompanied by the increase in miRNA-128 level in the amygdala in the extinguished isolated rats compared to the control, the extinguished, and the isolated rats. Moreover, the extinguished rats had lower expression of miRNA-128 compared to the control and the isolated animals.

CONCLUSIONS

Our results suggest that social isolation stress impairs aversive memory extinction and coexists with changes in the D₂R expression in the amygdala and hippocampus and increased expression of miRNA-128 in the amygdala.

Adolescent high-fructose corn syrup consumption leads to dysfunction in adult affective behaviors and mesolimbic proteins in male Sprague-Dawley rats

Adolescence is a critical period of development, during which the brain undergoes rapid maturation. Problematically, adolescents are the top consumers of high fructose corn syrup (HFCS) sweetened beverages and snacks, which may have neurodevelopmental consequences. While HFCS consumption has been linked to an increased likelihood of obesity and other physical health impairments, the link between HFCS and persistent behavioral changes is not yet fully established. The present study aimed to assess whether adolescent HFCS consumption could lead to alterations in adult behaviors and protein expression, following cessation. Adolescent HFCS-exposure contributed to deficits in learning and motivation on an effort-related T-Maze procedure, as well as increased immobility time in the forced swim paradigm during adulthood. Molecularly, protracted decreases in accumbal dopamine D1 and D2 receptors and protein kinase G (PKG), as well as increases in tyrosine hydroxylase and GluA2 receptor subunits, were observed following HFCS-exposure. Taken together, these data suggest that adolescent HFCS-consumption leads to protracted dysfunction in affective behaviors and alterations in accumbal proteins which persist following cessation of HFCS-consumption.

Sex- and age- dependent effect of pre-gestational chronic stress and mirtazapine treatment on neurobehavioral development of Wistar rat offspring

Hormonal fluctuations, such as the perinatal period, may increase susceptibility of women to depression, which in turn exert a negative impact on child's neurodevelopment, becoming a risk factor in development of neuropsychiatric disorders. Moreover, the use of antidepressants during this critical period presents a serious health concern for both the mother and the child, due to the consequences of treatment in terms of the reliability and safety for the proper neurodevelopment of the organism being not well known. Atypical antidepressants, such as mirtazapine, that targets both serotonergic and noradrenergic systems in the central nervous system (CNS), represent a novel focus of research due to its unique pharmacological profile. The aim of this work was to study the effects of maternal depression and/or perinatal antidepressant mirtazapine treatment on the neurobehavioral development of the offspring. Pre-gestationally chronically stressed or non-stressed Wistar rat dams were treated with either mirtazapine (10 mg/kg/day) or vehicle during pregnancy and lactation followed by analysis of offspring's behavior at juvenile and adolescent age. We found mirtazapine induced significant alterations of nursing behavior. In offspring, pregestational stress (PS) had an anxiogenic effect on adolescent males (p≤0.05) and increased their active behavior in forced swim test (p≤0.01). Interaction between pregestational stress and mirtazapine treatment variously induced anxiolytic changes of juvenile (p≤0.05) and adolescent (p≤0.05) females and impairment of spatial memory (p≤0.01) in adolescent females as well. Hippocampal density of synaptophysin, pre-synaptic protein marker, was decreased mainly by mirtazapine treatment. In conclusion, our results show mirtazapine induced significant alterations in maternal behavior and several sex- and age-dependent changes in neurobehavioral development of offspring caused by both prenatal mirtazapine treatment and/or chronic pregestational stress.

Extracellular Dopamine Levels in Nucleus Accumbens after Chronic Stress in Rats with Persistently High vs. Low 50-kHz Ultrasonic Vocalization Response

Fifty-kHz ultrasonic vocalizations (USVs) in response to an imitation of rough-and-tumble play ('tickling') have been associated with positive affective states and rewarding experience in the rat. This USV response can be used as a measure of inter-individual differences in positive affect. We have previously shown that rats with persistently low positive affectivity are more vulnerable to the effects of chronic variable stress (CVS). To examine whether these differential responses are associated with dopaminergic neurotransmission in the nucleus accumbens (NAc), juvenile male Wistar rats were categorized as of high or low positive affectivity (HC and LC, respectively), and after reaching adulthood, extracellular dopamine (DA) levels in the NAc shell were measured using in vivo microdialysis after three weeks of CVS. Baseline levels of DA were compared as well as the response to K⁺-induced depolarization and the effect of glial glutamate transporter EAAT2 inhibition by 4 mM l-trans-pyrrolidine-2,4-dicarboxylate (PDC). DA baseline levels were higher in control LC-rats, and stress significantly lowered the DA content in LC-rats. An interaction of stress and affectivity appeared in response to depolarization where stress increased the DA output in HC-rats whereas it decreased it in LC-rats. These results show that NAc-shell DA is differentially regulated in response to stress in animals with high and low positive affect.

GW117: A novel serotonin (5-HT2C ) receptor antagonist and melatonin (MT1 /MT2 ) receptor agonist with potential antidepressant-like activity in rodents

Aims

To evaluate the antidepressant‐like effect of compound GW117 in rodents using in vitro binding and uptake assays as well in vivo behavioral tests.

Methods

We investigated the target profile of GW117 using [³⁵S]‐GTPγS and [³H]PIP binding. Using the forced swimming test and chronic unpredictable stress in rats, tail suspension test in mice and rats, and learned helplessness model in mice, we further revealed the antidepressant‐like and anxiolytic‐like effects of GW117.

Results

The current study suggests that GW117 displays serotonin 2C (5‐HT_2C) receptor antagonist and melatonin type 1 and 2 (MT₁/MT₂) receptor agonist properties, as well as evident antidepressant and anxiolytic effects.

Conclusion

These data suggest that GW117 is probably a potent antidepressant.

Forced swim stressor: Trends in usage and mechanistic consideration

The acquired immobility response during the “forced swim test (FST)” is not a rodent model of depression, but the test has some validity in predicting a compound's antidepressant potential. Nevertheless, 60% of the about 600 papers that were published annually the past 2 years label the rodent's immobility response as depression‐like behaviour, but the relative contribution per country is changing. When the Editors‐in‐Chief of 5 journals publishing most FST papers were asked for their point of view on labelling immobility as depression‐like behaviour and despair, they responded that they primarily rely on the reviewers regarding scientific merit of the submission. One Editor informs authors of the recent NIMH notice (https://grants.nih.gov/grants/guide/notice‐files/NOT‐MH‐19‐053.html) which encourages investigators to use animal models “for” addressing neurobiological questions rather than as model “of” specific mental disorders. The neurobiological questions raised by use of the FST fall in two categories. First, research on the role of endocrine and metabolic factors, with roots in the 1980s, and with focus on the bottom‐up action of glucocorticoids on circuits processing salient information, executive control and memory consolidation. Second, recent findings using novel technological and computational advances that have allowed great progress in charting top‐down control in the switch from active to passive coping with the inescapable stressor executed by neuronal ensembles of the medial prefrontal cortex via the peri‐aquaductal grey. It is expected that combining neural top‐down and endocrine bottom‐up approaches will provide new insights in the role of stress‐coping and adaptation in pathogenesis of mental disorders.

Sex Differences in the Sustained Effects of Ketamine on Resilience to Chronic Stress

Exposure to stress is recognized to be a triggering factor in several mood disorders, including depression and anxiety. There is very little understanding of why female subjects have a significantly higher risk for these conditions than males. Recent findings in male rodents indicated that prophylactic ketamine can prevent the development of a stress-induced depressive-like phenotype, providing a pharmacological tool to study the mechanisms underlying stress resilience. Unfortunately, none of these studies incorporated female subjects, nor did they provide a mechanistic understanding of the effects of ketamine on stress resilience. Our previous work identified the prefrontal glutamatergic and parvalbumin (PV) systems as potential molecular mechanisms underlying sex differences in susceptibility to stress-induced emotional deregulations. To further address this point, we treated male and female mice with a single dose of ketamine before exposure to a chronic stress paradigm to determine whether stress-resilience induced by a pre-exposure to ketamine is similar in males and females and whether modulation of the prefrontal glutamatergic and PV systems by ketamine is associated with these behavioral effects. Ketamine prevented chronic stress-induced changes in behaviors in males, which was associated with a reduction in expression of PV and the NMDA receptor NR1 subunit. Ketamine did not protect females against the effects of chronic stress and did not change significantly prefrontal gene expression. Our data highlight fundamental sex differences in the sustained effects of ketamine. They also further implicate prefrontal glutamatergic transmission and PV in resilience to chronic stress.

Chemogenetic inhibition of lateral habenula projections to the dorsal raphe nucleus reduces passive coping and perseverative reward seeking in rats

The lateral habenula (LHb) processes information about aversive experiences that contributes to the symptoms of stress disorders. Previously, we found that chemogenetic inhibition of rat LHb neurons reduced immobility in the forced swim test, but the downstream target of these neurons was not known. Using an intersectional viral vector strategy, we selectively transduced three different output pathways from the LHb by injecting AAV8-DIO-hM₄Di into the LHb and CAV2-CRE (a retrograde viral vector) into one of the three target areas as follows: dorsal raphe nucleus (DRN), ventral tegmental area (VTA), or rostromedial tegmentum (RMTg). Using the forced swim test, we found that chemogenetic inhibition of DRN-projecting LHb neurons reduced passive coping (immobility), whereas inhibition of the other pathways did not. Chemogenetic activation of DRN-projecting neurons using hM3Dq in another cohort did not further exacerbate immobility. We next examined the impact of inhibiting DRN-projecting LHb neurons on reward sensitivity, perseverative behavior, and anxiety-like behavior using saccharin preference testing, reward-omission testing, and open-field testing, respectively. There was no effect of inhibiting any of these pathways on reward sensitivity, locomotion, or anxiety-like behavior, but inhibiting DRN-projecting LHb neurons reduced perseverative licking during reward-omission testing, whereas activating these neurons increased perseverative licking. These results support the idea that inhibiting LHb projections to the DRN provides animals with resilience during highly stressful or frustrating conditions but not under low-stress circumstances, and that inhibiting these neurons may promote persistence in active coping strategies.

The Effect of Short-Term Physical Activity on the Oxidative Stress in Rats with Different Stress Resistance Profiles in Cerebral Hypoperfusion

Oxidative stress associated with chronic cerebral hypoperfusion is one of the fundamental factors leading to neurodegenerative diseases. To prevent oxidative stress, physical activity is effective. Physical exercise enables development of rehabilitation techniques that can progressively increase patients' stress resistance. We determined the oxidative stress dynamics in experimental hypoperfusion and modeled rehabilitation measures, comparing sex and stress resistance levels. The experiment was performed on 240 Wistar rats of both sexes over a period of 90 days. Based on behavioral test results obtained using the open field test, the rats were divided into active animals with predicted higher stress resistance (HSR) and passive animals with predicted lower stress resistance (LSR). TBA (thiobarbituric acid) plasma concentration of the active products (malondialdehyde-MDA), blood plasma (NO-X) concentration, and L-citrulline (LC) concentration were determined spectrophotometrically at the corresponding wave length (nm). The intensity of oxidative stress was evaluated using the chemoluminscent method to determine the blood plasma antioxidant activity on the BCL-07 biochemoluminometer. This study revealed two stages of oxidative stress: a less pronounced phase covering the first days after surgery and a main one, which starts from the month after the operation to 3 months. Female sex and a high initial level of stress resistance reduced the severity of oxidative stress. Physical activity commencing a week after the surgery resulted in "reloading" the adaptive mechanisms and slowed the onset of the main stage, leading to a decrease in the free-radical process in all studied subgroups and the greater blood plasma (NO)-X decrease in the male animals. Future neuropharmacological intervention most likely will be able to determine the pathophysiology mechanism of chronic brain hypoperfusion and potentially extending adaptive responses.