Examining the Role of Microbiota in Emotional Behavior: Antibiotic Treatment Exacerbates Anxiety in High Anxiety-Prone Male Rats.

Intestinal microbiota are essential for healthy gastrointestinal function and also broadly influence brain function and behavior, in part, through changes in immune function. Gastrointestinal disorders are highly comorbid with psychiatric disorders, although biological mechanisms linking these disorders are poorly understood. The present study utilized rats bred for distinct emotional behavior phenotypes to examine relationships between emotionality, the microbiome, and immune markers. Prior work showed that Low Novelty Responder (LR) rats exhibit high levels of anxiety- and depression-related behaviors as well as myriad neurobiological differences compared to High Novelty Responders (HRs). Here, we hypothesized that the divergent HR/LR phenotypes are accompanied by changes in fecal microbiome composition. We used next-generation sequencing to assess the HR/LR microbiomes and then treated adult HR/LR males with an antibiotic cocktail to test whether it altered behavior. Given known connections between the microbiome and immune system, we also analyzed circulating cytokines and metabolic factors to determine relationships between peripheral immune markers, gut microbiome components, and behavioral measures. There were no baseline HR/LR microbiome differences, and antibiotic treatment disrupted the microbiome in both HR and LR rats. Antibiotic treatment exacerbated aspects of HR/LR behavior, increasing LRs' already high levels of anxiety-like behavior while reducing passive stress coping in both strains. Our results highlight the importance of an individual's phenotype to their response to antibiotics, contributing to the understanding of the complex interplay between gut microbes, immune function, and an individual's emotional phenotype.

Early-life exposure to the SSRI paroxetine exacerbates depression-like behavior in anxiety/depression-prone rats.

Selective serotonin reuptake inhibitor (SSRI) antidepressants are the mainstay treatment for the 10-20% of pregnant and postpartum women who suffer major depression, but the effects of SSRIs on their children's developing brain and later emotional health are poorly understood. SSRI use during pregnancy can elicit antidepressant withdrawal in newborns and increase toddlers' anxiety and social avoidance. In rodents, perinatal SSRI exposure increases adult depression- and anxiety-like behavior, although certain individuals are more vulnerable to these effects than others. Our study establishes a rodent model of individual differences in susceptibility to perinatal SSRI exposure, utilizing selectively bred Low Responder (bLR) and High Responder (bHR) rats that were previously bred for high versus low behavioral response to novelty. Pregnant bHR/bLR females were chronically treated with the SSRI paroxetine (10 mg/kg/day p.o.) to examine its effects on offspring's emotional behavior and gene expression in the developing brain. Paroxetine treatment had minimal effect on bHR/bLR dams' pregnancy outcomes or maternal behavior. We found that bLR offspring, naturally prone to an inhibited/anxious temperament, were susceptible to behavioral abnormalities associated with perinatal SSRI exposure (which exacerbated their Forced Swim Test immobility), while high risk-taking bHR offspring were resistant. Microarray studies revealed robust perinatal SSRI-induced gene expression changes in the developing bLR hippocampus and amygdala (postnatal days 7-21), including transcripts involved in neurogenesis, synaptic vesicle components, and energy metabolism. These results highlight the bLR/bHR model as a useful tool to explore the neurobiology of individual differences in susceptibility to perinatal SSRI exposure.

The melanin-concentrating hormone (MCH) system in an animal model of depression-like behavior.

Selective breeding for divergence in locomotion in a novel environment (bHR, bred High-Responder; bLR, bred Low-Responder) correlates with stress-reactivity, spontaneous anxiety-like behaviors and predicts vulnerability in a rodent model of depression. Identifying genetic factors that may account for such vulnerability are key determinants not only for the illness outcome but also for the development of better-tailored treatment options. Melanin-concentrating hormone (MCH) is a neuropeptide that exhibits some of the hallmarks of a regulator of affective states. The aim of this study was to ascertain the role of the MCH system in depression-like behaviors in bHR vs. bLR rats. bLR rats showed a 44% increase in hypothalamic pMCH mRNA and a 14% decrease in hippocampal CA1 MCH1R mRNA when compared to bHR rats. Interestingly, the amount of time that rats spent immobile in the FST (depressive-like behavior) correlated positively with the amount of hypothalamic pMCH mRNA and negatively with that of hippocampal CA1 MCH1R. The results indicate that the bLR-bHR is a useful rat model to investigate individual basal genetic differences that participate in the monitoring of emotional responsiveness (i.e., depression- and anxiety-like behaviors). They also point to the MCH system (i.e., chronically higher pMCH expression and consequently receptor down-regulation) as a candidate biomarker for the severity of depressive-like behavior. The data indicate that MCH1R participates in the modulation of depression-like behavior through a process that involves the CA1 region of the hippocampus, supporting the possible use of MCH1R antagonists in the treatment of depression.

Impact of cocaine on adult hippocampal neurogenesis in an animal model of differential propensity to drug abuse.

Hippocampal plasticity (e.g. neurogenesis) likely plays an important role in maintaining addictive behavior and/or relapse. This study assessed whether rats with differential propensity to drug-seeking behavior, bred Low-Responders (bLR) and bred High-Responders (bHR) to novelty, show differential neurogenesis regulation after cocaine exposure. Using specific immunological markers, we labeled distinct populations of adult stem cells in the dentate gyrus at different time-points of the cocaine sensitization process; Ki-67 for newly born cells, NeuroD for cells born partway, and 5-bromo-2'-deoxyuridine for older cells born prior to sensitization. Results show that: (i) bHRs exhibited greater psychomotor response to cocaine than bLRs; (ii) acute cocaine did not alter cell proliferation in bLR/bHR rats; (iii) chronic cocaine decreased cell proliferation in bLRs only, which became amplified through the course of abstinence; (iv) neither chronic cocaine nor cocaine abstinence affected the survival of immature neurons in either phenotype; (v) cocaine abstinence decreased survival of mature neurons in bHRs only, an effect that paralleled the greater psychomotor response to cocaine; and (vi) cocaine treatment did not affect the ratio of neurons to glia in bLR/bHR rats as most cells differentiated into neurons in both lines. Thus, cocaine exerts distinct effects on neurogenesis in bLR vs. bHR rats, with a decrease in the birth of new progenitor cells in bLRs and a suppression of the survival of new neurons in bHRs, which likely leads to an earlier decrease in formation of new connections. This latter effect in bHRs could contribute to their enhanced degree of cocaine-induced psychomotor behavioral sensitization.

Desipramine attenuates working memory impairments induced by partial loss of catecholamines in the rat medial prefrontal cortex.

RATIONALE: The density of tyrosine hydroxylase-immunoreactive (TH-IR) axons in the prefrontal cortex of schizophrenic subjects may be reduced by as much as 50% in the deep cortical layers (Am J Psychiatry 156:1580-1589, 1999). Previously, we demonstrated that approximately 60% loss of TH-IR axons in the rat medial prefrontal cortex (mPFC) decreases local basal and stress-evoked extracellular dopamine (DA) concentrations, suggesting that moderate loss of DA axons in the mPFC is sufficient to alter the neurochemical activity of the remaining DA neurons (Neuroscience 93:497-505, 1999). OBJECTIVES: To further assess the functional consequences of partial mPFC DA depletion, we examined the effects of 6-hydroxydopamine lesions of the rat mPFC on behavior in a T-maze delayed-response task. We also assessed whether chronic administration of the norepinephrine (NE) uptake inhibitor, desipramine (DMI), attenuates lesion-induced deficits in T-maze performance. Previous research indicates that inhibition of NE transport in the mPFC results in a concomitant increase in extracellular DA and NE. RESULTS: Moderate loss of mPFC DA and NE (approximately 50 and 10% loss, respectively) was sufficient to impair delayed-response behavior, in part due to an increase in perseverative responding. Chronic DMI treatment (3 mg/kg delivered via osmotic pumps) impaired performance of control rats but attenuated the deficits in delayed-response behavior in rats previously sustaining loss of mPFC DA and NE (approximately 75 and 35% loss, respectively). CONCLUSION: These data suggest that moderate loss of DA and NE in the prefrontal cortex is sufficient to impair cognitive function, and these behavioral effects are attenuated by inhibition of the NE transporter.