Low Responder Rats Research Articles

Premotor projections from the locus coeruleus and periaqueductal grey are altered in two rat models with inborn differences in emotional behavior

Emotionally motivated behaviors rely on the coordinated activity of descending neural circuits involved in motor and autonomic functions. Using a pseudorabies (PRV) tract-tracing approach in typically behaving rats, our group previously identified descending premotor, presympathetic, and dual-labeled premotor-presympathetic populations throughout the central rostral-caudal axis. The premotor-presympathetic populations are thought to integrate somatomotor and sympathetic activity. To determine whether these circuits are dysregulated in subjects with altered emotional regulation, subsequent neuroanatomical analyses were performed in male subjects of two distinct genetic models relevant to clinical depression and anxiety: the Wistar Kyoto (WKY) rat and selectively bred Low Novelty Responder (bLR) rat. The present study explored alterations in premotor efferents from locus coeruleus (LC) and subdivisions of the periaqueductal grey (PAG), two areas involved in emotionally motivated behaviors. Compared to Sprague Dawley rats, WKY rats had significantly fewer premotor projections to hindlimb skeletal muscle from the LC and from the dorsomedial (DMPAG), lateral (LPAG), and ventrolateral (VLPAG) subdivisions of PAG. Relative to selectively bred High Novelty Responder (bHR) rats, bLR rats had significantly fewer premotor efferents from LC and dorsolateral PAG (DLPAG). Cumulatively, these results demonstrate that somatomotor circuitry in several brain areas involved in responses to stress and emotional stimuli are altered in rat models with depression-relevant phenotypes. These somatomotor circuit differences could be implicated in motor-related impairments in clinically depressed patients.

3,4-Methylenedioxypyrovalerone High-Responder Phenotype as a Tool to Evaluate Candidate Medications for Stimulant Use Disorder.

Despite decades of research, there are no medications approved by the United States Food and Drug Administration to treat stimulant use disorders. Self-administration procedures are widely used to screen candidate medications for stimulant use disorder, although preclinical reductions in stimulant self-administration have not translated to meaningful reductions in stimulant use in humans. One possible reason for this discordance is that most preclinical studies evaluate candidate medications under conditions that promote predictable, and well-regulated patterns of drug-taking rather than the dysregulated and/or compulsive patterns of drug-taking characteristic of a stimulant use disorder. A subset of rats ("high-responders") that self-administer 3,4-methelyendioxypyrovalerone (MDPV), a monoamine uptake inhibitor, develop high levels of dysregulated drug-taking consistent with behaviors related to stimulant use disorders. Because MDPV acts on dopamine, serotonin (5-HT), and sigma receptor systems, the current studies compared the potency and effectiveness of a dopamine D3 receptor partial agonist (VK4-40) or antagonist (VK4-116), a sigma receptor antagonist (BD1063), a dopamine D2/D3/sigma receptor antagonist (haloperidol), and a 5-HT2C receptor agonist (CP-809,101) to reduce MDPV (0.0032-0.1 mg/kg/infusion) self-administration in high- and low-responding rats as well as rats self-administering cocaine (0.032-1 mg/kg/infusion). VK4-40, VK4-116, haloperidol, and CP-809,101 were equipotent and effective at reducing drug-taking in all three groups of rats, including the high-responders; however, VK4-116 and CP-809,101 were less potent at reducing drug-taking in female compared with male rats. Together, these studies suggest that drugs targeting dopamine D3 or 5-HT2C receptors can effectively reduce dysregulated patterns of stimulant use, highlighting their potential utility for treating stimulant use disorders. SIGNIFICANCE STATEMENT: There are no United States Food and Drug Administration-approved treatments for stimulant use disorder, perhaps in part because candidate medications are most often evaluated in preclinical models using male subjects with well-regulated drug-taking. In an attempt to better model aberrant drug taking, this study found compounds acting at dopamine D3 or 5-HT2C receptors can attenuate drug-taking in male and female rats that self-administered two different stimulants and exhibited either a high or low substance use disorder-like phenotype.

Adolescent cocaine differentially impacts psychomotor sensitization and epigenetic profiles in adult male rats with divergent affective phenotypes.

Adolescent drug use reliably predicts increased addiction liability in adulthood, but not all individuals are equally impacted. To explore the biological bases of this differential reactivity to early life drug experience, we used a genetic rat model of temperament and evaluated the impact of adolescent cocaine exposure on adult psychomotor sensitization. Relative to adult bred low-responder (bLR) rats, bred high-responders (bHR) are more sensitive to the psychomotor-activating effects of cocaine and reinstate drug-seeking behavior more readily following prolonged cocaine exposure and/or abstinence. We found that a 7-day sensitizing cocaine regimen (15 mg/kg/day) during either adolescence or adulthood produced psychomotor sensitization in bHRs only, while a dual cocaine exposure prevented further sensitization, suggesting limits on neuroplasticity. By contrast, adolescent cocaine in bLRs shifted their resilient phenotype, rendering them more responsive to cocaine in adulthood following adolescent cocaine. To begin to explore the neural correlates of these behavioral phenotypes, we assessed two functionally opposite epigenetic chromatin modifications implicated in addiction liability, permissive acetylation (ac) and repressive tri-methylation (me3) on Histone 3 Lysine 9 (H3K9), in four striatal sub-regions. In bHRs, decreased H3K9me3 and increased acH3K9 in the nucleus accumbens (NAc) core associated with cocaine sensitization. In bLRs, the combination of cocaine exposure in adolescence and adulthood, which lead to an increased response to a cocaine challenge, also increased acH3K9 in the core. Thus, adolescent cocaine experience interacts with genetic background to elicit different behavioral profiles relevant to addiction in adulthood, with concurrent modifications in the epigenetic histone profiles in the NAc that associate with cocaine sensitization and with metaplasticity.

Selective Optogenetic Dopamine Terminal Stimulation Reveals Role of GABAergic Signaling in Individual Differences in Dopamine Release

An estimated 70.5% of Americans aged 12 or older report having used an illicit substance at least once, while about 10‐20% of those individuals ultimately develop a substance use disorder (SUD). These data highlight substantial individual variability in the risk of developing SUD following initial drug use. Identifying markers of increased risk provides a significant opportunity to identify at‐risk populations and to aid in the prevention of the development of SUD. In preclinical rodent models, locomotor response to a novel environment predicts drug use vulnerability. Rodents that demonstrate higher locomotor response to an inescapable novel environment (high responders; HR) acquire self‐administration (SA) of drugs more rapidly and at lower doses compared to low responders (LR). Striatal dopamine (DA) signaling is critical for the acquisition of drug SA and tonic/phasic firing patterns of DA neurons encode information about salient environmental stimuli and rewards. HR rats show increased phasic DA signaling to reward‐predictive cues compared to LR rats. Dopamine release dynamics are tightly controlled by local modulation within the nucleus accumbens (NAc), including by nicotinic acetylcholine receptors (nAChRs), but the mechanisms by which this local modulation may contribute to differential DA release is not fully understood. Our lab has previously shown that locomotor response to a novel environment predicts nAChR modulation of phasic DA signals in the NAc. Specifically, desensitization or blockade of α6β2‐containing (α6β2*) nAChRs within the NAc was found to augment phasic DA signals in brain slices of HR rats, and reduce phasic DA signals in LR rats. However, these studies utilized non‐specific electrical stimulation to record DA release. In the present study, we used ex vivo fast‐scan cyclic voltammetry with an optogenetic approach to selectively stimulate DA neuron terminals or cholinergic interneurons (CINs) within the NAc to examine the specific contributions of acetylcholine (ACh) to the overall DA response in HR and LR rats. We find that, contrary to previous results with electrical stimulation, selective DA phasic stimulation results in lower DA release in HRs compared to LRs, while blockade of α6β2* nAChRs using light stimulation results in higher DA release in HRs. This suggests that additional circuitry is likely involved in differential DA signaling within the NAc. Given the tonic activity of CINs and presence of nAChRs on GABA interneurons, we next used pharmacological manipulations to examine the possibility of GABA signaling contributing to individual differences in DA release. Here, we find that antagonism of GABA receptors blocks the previously seen differential effects of α6β2* nAChR antagonism on DA release, suggesting that GABA interneurons do play a critical role in differential DA signaling in HRs versus LRs. In sum, these data help form a clearer understanding of the mechanisms underlying individual differences in DA signaling, which may be a primary driver of differences in sensitivity to the reinforcing effects of drugs of abuse. Investigating these mechanisms allows us to better identify behavioral and neurochemical markers of SUD vulnerability in humans and to ultimately stimulate development of individualized and effective treatments.

Differences in microglia morphological profiles reflect divergent emotional temperaments: insights from a selective breeding model

Microglia play critical roles in healthy brain development and function, as well as the neuropathology underlying a range of brain diseases. Despite evidence for a role of microglia in affective regulation and mood disorders, little is known regarding how variation in microglia status relates to individual differences in emotionality. Using a selective breeding model, we have generated rat lines with unique temperamental phenotypes that reflect broad emotional traits: bred low responder rats (bLRs) are novelty-averse and model a passive coping style, whereas bred high responder rats (bHRs) are highly exploratory and model an active coping style. To identify a functional role of microglia in these phenotypes, we administered minocycline, an antibiotic with potent microglia inhibiting properties and observed shifts in forced swim, sucrose preference, and social interaction behaviors in bLRs. Using detailed anatomical analyses, we compared hippocampal microglia profiles of bHRs and bLRs and found that although the lines had similar numbers of microglia, selective breeding was associated with a shift in the morphological features of these cells. Specifically, microglia from bLRs were characterized by a hyper-ramified morphology, with longer processes and more complicated branching patterns than microglia from bHRs. This morphology is thought to reflect an early stage of microglia activation and suggests that bLR microglia are in a reactive state even when animals are not overtly challenged. Taken together, our results provide novel evidence linking variation in inborn temperament with differences in the baseline status of microglia and implicate a role for microglia in shaping enduring emotional characteristics.

Investigating individual differences in opioid-taking and opioid-seeking behavior in male rats.

Understanding the behavioral and neurobiological factors that render some individuals more susceptible than others to opioid addiction will be critical in combatting the opioid crisis. The purpose of the current study was to determine if behavioral traits associated with an increased likelihood to take and seek cocaine are the same traits that render one more susceptible to opioid-taking and opioid-seeking behavior. Individual differences in the acquisition of remifentanil self-administration and subsequent cue-induced reinstatement of remifentanil-seeking behavior were investigated using two animal models: the high-responder (HR)/low-responder (LR) and sign-tracker (ST)/goal-tracker (GT) models. Relative to LR rats, HR rats show increased novelty-induced locomotion or "sensation-seeking" behavior, and are more likely to acquire cocaine-taking behavior and do so at a faster rate. Relative to GT rats, ST rats attribute greater incentive motivational value to reward cues and are more likely to exhibit reinstatement of cocaine-seeking behavior. In contrast to previous work using cocaine, we did not observe individual differences with respect to the acquisition of remifentanil self-administration- or cue-induced reinstatement of remifentanil-seeking behavior within the context of either the HR/LR or ST/GT model. Thus, neither the sensation-seeking trait nor the propensity to attribute incentive motivational value to reward cues predicts remifentanil-taking or remifentanil-seeking behavior. These findings suggest that different traits may confer the initiation of opioid- vs. cocaine-taking behavior, and the propensity to relapse to opioid- vs. cocaine-seeking. Additional studies are needed to identify which neurobehavioral constructs confer liability to opioid use and relapse.

Response to Novelty Predicts α7 Nicotinic Receptor and Voltage‐Gated Calcium Channel Modulation of Dopamine Release

An estimated 70.5% of Americans aged 12 or older report having used an illicit substance at least once, while about 10-20% of those individuals ultimately develop a substance use disorder (SUD). These data highlight substantial individual variability in the risk of developing SUD following initial drug use. Identifying markers of increased risk provides a significant opportunity to identify at-risk populations and to aid in the prevention of the development of SUD. In preclinical rodent models, locomotor response to a novel environment predicts drug use vulnerability. Rodents that demonstrate higher locomotor response to an inescapable novel environment (high responders; HR) acquire self-administration (SA) of drugs more rapidly and stably compared to low responders (LR). Striatal dopamine (DA) signaling is critical for the acquisition of drug SA and tonic/phasic firing patterns of DA neurons encode information about salient environmental stimuli and rewards. HR rats show increased phasic DA signaling to reward-predictive cues compared to LR rats. Dopamine release dynamics are tightly controlled by local modulation within the nucleus accumbens (NAc), including through glutamatergic signaling and modulation by nicotinic acetylcholine receptors (nAChRs), but the mechanisms by which this local modulation may contribute to differential DA release is not fully understood. Our lab has previously shown that locomotor response to a novel environment predicts nAChR modulation of phasic DA signals in the NAc. Specifically, desensitization or blockade of α6β2-containing nAChRs within the NAc was found to augment phasic DA signals in brain slices of HR rats, and reduce phasic DA signals in LR rats. However, other nAChR subtypes may also contribute to individual differences in DA signaling, including α7 nAChRs localized to striatal glutamate terminals. In the present study, we used ex vivo fast-scan cyclic voltammetry to examine how antagonism of α7 nAChRs impacts DA release across a range of stimulation frequencies in the NAc of HR and LR animals and examine what mechanisms may be mediating this effect. We find that modulation of DA release by α7 nAChRs can be predicted by response to novelty at phasic-like stimulations. Given the role that nAChRs play in calcium (Ca2+) entry into the cell, we additionally tested the possibility that differential Ca2+ utilization is one mechanism underlying differential DA modulation by nAChRs and utilized pharmacological manipulations to test possible voltage-gated Ca2+ channel (VGCC) subtype-specific effects. Here, we demonstrate that lowered Ca2+ concentration unmasks a diverging DA release profile between HRs and LRs at phasic-like stimulation frequencies and find that P/Q-type VGCCs appear to drive these individual differences in Ca2+ utilization. In sum, these data help form a more coherent understanding of the mechanisms underlying individual differences in vulnerability. Investigating these mechanisms allows us to better identify behavioral and neurochemical markers of substance abuse risk in humans and to ultimately stimulate development of more individualized and effective treatments.

The sensation seeking trait confers a dormant susceptibility to addiction that is revealed by intermittent cocaine self-administration in rats

Heightened sensation seeking is associated with an increased risk of substance use disorder in clinical populations. In rats, sensation seeking is often examined by measuring locomotor reactivity to a novel environment. So-called high responders (HR) acquire self-administration of psychostimulants more quickly and consume higher amounts of drug compared to low responder (LR) rats, indicating that the HR trait might confer a stronger addiction propensity. However, studies of addiction-like behaviors in HR vs LR rats have typically utilized self-administration paradigms that do not dissociate individual differences in the hedonic/reinforcing and motivational properties of a drug. Moreover, little attention has been given to whether HR rats are more susceptible to drug-access conditions that promote a state-dependent addiction phenotype. We report that on a behavioral economics task, HR rats have higher preferred brain-cocaine levels compared to LR rats but do not differ with respect to their demand elasticity for cocaine. In contrast, when tested on an intermittent access schedule of cocaine self-administration, which has been shown to promote several addiction-related endophenotypes, HR rats exhibit greater escalation of intake and more drastic reductions in cocaine demand elasticity. Together, these data indicate that the HR trait does not confer higher extant addiction behavior, but rather that this phenotype is associated with a propensity for addiction that remains dormant until it is actuated by intermittent drug intake. These findings reveal a ‘trait’ (HR) by ‘state’ (intermittent drug intake) interaction that produces a strong addiction-like phenotype.This article is part of the special issue on ‘Vulnerabilities to Substance Abuse'.

Modeling heritability of temperamental differences, stress reactivity, and risk for anxiety and depression: Relevance to research domain criteria (RDoC).

Animal models provide important tools to study biological and environmental factors that shape brain function and behavior. These models can be effectively leveraged by drawing on concepts from the National Institute of Mental Health Research Domain Criteria (RDoC) Initiative, which aims to delineate molecular pathways and neural circuits that underpin behavioral anomalies that transcend psychiatric conditions. To study factors that contribute to individual differences in emotionality and stress reactivity, our laboratory utilized Sprague-Dawley rats that were selectively bred for differences in novelty exploration. Selective breeding for low versus high locomotor response to novelty produced rat lines that differ in behavioral domains relevant to anxiety and depression, particularly the RDoC Negative Valence domains, including acute threat, potential threat, and loss. Bred Low Novelty Responder (LR) rats, relative to their High Responder (HR) counterparts, display high levels of behavioral inhibition, conditioned and unconditioned fear, avoidance, passive stress coping, anhedonia, and psychomotor retardation. The HR/LR traits are heritable, emerge in the first weeks of life, and appear to be driven by alterations in the developing amygdala and hippocampus. Epigenomic and transcriptomic profiling in the developing and adult HR/LR brain suggest that DNA methylation and microRNAs, as well as differences in monoaminergic transmission (dopamine and serotonin in particular), contribute to their distinct behavioral phenotypes. This work exemplifies ways that animal models such as the HR/LR rats can be effectively used to study neural and molecular factors driving emotional behavior, which may pave the way toward improved understanding the neurobiological mechanisms involved in emotional disorders.

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.

Chronic stress in adolescence differentially affects cocaine vulnerability in adulthood in a selectively bred rat model of individual differences: role of accumbal dopamine signaling

Stress during adolescence has profound effects on the onset and severity of substance use later in life. However, not everyone with adverse experiences during this period will go on to develop a substance use disorder in adulthood, and the factors that alter susceptibility to substance use remain unknown. Here, we investigated individual differences in response to stress and drugs of abuse using our selectively bred high-responder (bHR) and low-responder (bLR) rats. These animals model extremes of temperamental tendencies and differ dramatically in both stress responsiveness and addiction-related traits. The present study investigated how environmental interventions in the form of a chronic variable stress (CVS) regimen in early adolescence interact with the bHR/bLR phenotype to alter behavioral sensitization to cocaine in adulthood. We also determined whether accumbal dopamine signaling is involved in the interaction of stress history and cocaine by assessing the mRNA levels of dopamine D1 (D1R) and D2 (D2R) receptors. Our results showed that CVS history alone had enduring and phenotype-specific effects on accumbal dopamine signaling. Importantly, adolescent stress had opposing effects in the two lines- decreasing the locomotor response to cocaine challenge in bHRs but increasing this measure in bLRs. Moreover, these opposing effects on cocaine sensitivity following adolescent CVS were accompanied by parallel effects in the accumbal dopamine system, with prior stress and cocaine exposure interacting to decrease D2R mRNA in bHRs but increase it in bLRs. Overall, these findings indicate that environmental challenges encountered in adolescence interact with genetic background to alter vulnerability to cocaine later in life. Lay Summary Stress experienced during adolescence affects the onset and severity of drug dependence later in life. However, not everyone with adverse experiences during this period will go on to develop SUD in adulthood. Using a rat model of innate differences in emotional reactivity, this study shows that the interplay between individual temperament and previous experience of adolescent stress/trauma determines whether an individual will be vulnerable or resilient to develop SUDs later in life. In addition, the present study shows that the dopamine D2 receptor in the brain’s reward center, nucleus accumbens, may be implicated in this interplay.

Genetic Liability for Internalizing Versus Externalizing Behavior Manifests in the Developing and Adult Hippocampus: Insight From a Meta-analysis of Transcriptional Profiling Studies in a Selectively Bred Rat Model

BackgroundFor more than 16 years, we have selectively bred rats for either high or low levels of exploratory activity within a novel environment. These bred high-responder (bHR) and bred low-responder (bLR) rats model temperamental extremes, exhibiting large differences in internalizing and externalizing behaviors relevant to mood and substance use disorders. MethodsWe characterized persistent differences in gene expression related to bHR/bLR phenotype across development and adulthood in the hippocampus, a region critical for emotional regulation, by meta-analyzing 8 transcriptional profiling datasets (microarray and RNA sequencing) spanning 43 generations of selective breeding (postnatal day 7: n = 22; postnatal day 14: n = 49; postnatal day 21: n = 21; adult: n = 46; all male). We cross-referenced expression differences with exome sequencing within our colony to pinpoint candidates likely to mediate the effect of selective breeding on behavioral phenotype. The results were compared with hippocampal profiling from other bred rat models. ResultsGenetic and transcriptional profiling results converged to implicate multiple candidate genes, including two previously associated with metabolism and mood: Trhr and Ucp2. Results also highlighted bHR/bLR functional differences in the hippocampus, including a network essential for neurodevelopmental programming, proliferation, and differentiation, centering on Bmp4 and Mki67. Finally, we observed differential expression related to microglial activation, which is important for synaptic pruning, including 2 genes within implicated chromosomal regions: C1qa and Mfge8. ConclusionsThese candidate genes and functional pathways may direct bHR/bLR rats along divergent developmental trajectories and promote a widely different reactivity to the environment.

Individual differences in modulation of dopamine release in a rodent model of substance abuse vulnerability

Despite significant advances in understanding the neurobiology underlying substance use disorders (SUDs), about 20.3 million Americans still suffered from SUD in 2018. This is due, in part, to limited effectiveness of current treatment strategies. Development of more effective treatments may be advanced by the ability to identify markers of increased risk of developing SUD following initial drug use and to target treatments to this vulnerable subpopulation. This need to better understand individual differences in SUD vulnerability is demonstrated in that only 10–20% of individuals that experiment with drugs of abuse ultimately develop SUD. Some behavioral traits such as sensation seeking are predictive of increased vulnerability, but the neurobiology underlying these individual differences remains unclear. To investigate this neurobiology, sensation seeking can be modeled in rodents by examining locomotor response to a novel environment. Rodents that demonstrate higher locomotor response to an inescapable novel environment (high responders; HR) acquire self‐administration (SA) of drugs more rapidly and stably compared to low responders (LR). Phasic firing of mesolimbic dopamine (DA) neurons plays an integral role in encoding reward‐associations and occurs in response to unexpected rewards and reward‐predictive stimuli. HR rats show increased phasic DA signaling to reward‐predictive cues compared to LR rats. Nicotinic acetylcholine receptors (nAChRs) on DA terminals in the nucleus accumbens (NAc) modulate DA release in an action potential‐independent manner through acetylcholine signaling. Our lab has previously shown that locomotor response to a novel environment can predict nAChR modulation of phasic DA signals in the NAc. Specifically, desensitization or blockade of α6β2‐containing nAChRs within the NAc was found to augment phasic DA signal in brain slices of HR but not LR animals. In the present study, we used ex vivo fast‐scan cyclic voltammetry (FSCV) to further investigate the role of NAc nAChRs in modulating DA release in HRs versus LRs. We find that modulation of DA release by α7 nAChRs can be predicted by locomotor response to novelty at phasic‐like stimulations. Given the role that nAChRs play in Ca2+ entry into the cell, we additionally tested the possibility that differential Ca2+ utilization is one mechanism underlying differential DA modulation by nAChRs and utilized pharmacological manipulations to test possible voltage‐gated Ca2+ channel (VGCC) subtype‐specific effects. Here, we demonstrate that lowered Ca2+ concentration unmasks a diverging DA release profile between HRs and LRs at stimulation frequencies modeling phasic firing and find that P/Q‐type VGCCs appear to drive these individual differences in Ca2+ utilization. In sum, these data help form a more coherent understanding of the mechanisms underlying individual differences in vulnerability. Investigating these mechanisms allows us to better identify behavioral and neurochemical markers of substance abuse risk in humans and to ultimately stimulate development of more individualized and effective treatments.Support or Funding InformationK99/R00 DA 311791, P50 DA006634, P50 AA026117, T32 DA041349‐03, Peter F. McManus Charitable Trust

Decreased Density of Perineuronal Net in Prelimbic Cortex Is Linked to Depressive-Like Behavior in Young-Aged Rats.

Perineuronal nets (PNNs) are condensed extracellular matrix (ECM) structures regulating developmental plasticity and protecting neurons against oxidative stress. PNN abnormalities have been observed in various psychiatric disorders such as schizophrenia and bipolar disorder, but the relationship between PNN density and depression still remains unclear. In the present study, we examined the density and components of PNNs including aggrecan, neurocan and Tenascin-R in the prelimbic cortex (PrL) after chronic unpredictable mild stress (CUMS). We found that depressive-like behaviors were induced after 30 days of CUMS accompanied by decreases in PNN+ cell density and aggrecan expression in the PrL. In addition, rats subjected to 20 days of CUMS were separated into vulnerable and resilient subpopulations that differ along several behavioral domains. Consistently, the density of PNNs and the expression level of neurocan in the vulnerable group were decreased compared to control and resilient groups. Finally, we examined individual differences based on locomotion in a novel context and classified rats as high responding (HR) and low responding (LR) phenotypes. The density of PNNs and the expression level of neurocan in the LR group were lower than the HR group. Moreover, the LR rats were more susceptible to depressive-like behaviors compared with HR rats. Altogether, these results suggest that the density of PNNs in the PrL is associated with depressive-like behaviors in young-aged rats, and it may serve as a potential endophenotype or therapeutic target for depression.

Individual Differences in Ethanol Drinking and Seeking Behaviors in Rats Exposed to Chronic Intermittent Ethanol Vapor Exposure is Associated with Altered CaMKII Autophosphorylation in the Nucleus Accumbens Shell.

Chronic intermittent ethanol vapor exposure (CIE) in rodents produces reliable and high blood ethanol concentration and behavioral symptoms associated with moderate to severe alcohol use disorder (AUD)—for example, escalation of operant ethanol self-administration, a feature suggestive of transition from recreational to addictive use, is a widely replicated behavior in rats that experience CIE. Herein, we present evidence from a subset of rats that do not demonstrate escalation of ethanol self-administration following seven weeks of CIE. These low responders (LR) maintain low ethanol self-administration during CIE, demonstrate lower relapse to drinking during abstinence and reduced reinstatement of ethanol seeking triggered by ethanol cues when compared with high responders (HR). We examined the blood ethanol levels in LR and HR rats during CIE and show higher levels in LR compared with HR. We also examined peak corticosterone levels during CIE and show that LR rats have higher levels compared with HR rats. Lastly, we evaluated the levels of Ca2+/calmodulin-dependent protein kinase II (CaMKII) in the nucleus accumbens shell and reveal that the activity of CaMKII, which is autophosphorylated at site Tyr-286, is significantly reduced in HR rats compared with LR rats. These findings demonstrate that dysregulation of the hypothalamic–pituitary–adrenal axis activity and plasticity-related proteins regulating molecular memory in the nucleus accumbens shell are associated with higher ethanol-drinking and -seeking in HR rats. Future mechanistic studies should evaluate CaMKII autophosphorylation-dependent remodeling of glutamatergic synapses in the ventral striatum as a plausible mechanism for the CIE-induced enhanced ethanol drinking and seeking behaviors.

Oxidative metabolism alterations in the emotional brain of anxiety-prone rats

Mood disorders such as anxiety and depression are heterogeneous disorders with many sufferers unresponsive to current pharmacological treatments. Individual differences in temperament represent one factor that may underlie symptom heterogeneity, so understanding its biological underpinnings can help pave the way to personalized therapies and improved patient outcomes. The present study uses a rodent model of temperamental differences to examine whether individual differences in emotional behavior phenotypes correspond to altered limbic brain cellular metabolism, an indicator of neuronal activity. The model uses two selectively bred rat lines – high novelty responder rats (HRs) that show highly exploratory behavior in a novel environment, active coping style and resilience to chronic mild stress compared to low novelty responder rats (LRs), which are inhibited in novel environments, display passive coping style, and are susceptible to chronic stress. Utilizing transcriptome data from a prior study in adult HR/LR rats, we first show that a preponderance of genes differing in the HR vs. LR hippocampus and amygdala are involved in cellular metabolism. This led us to then ask if oxygen consumption was altered in isolated mitochondria of the hippocampus and amygdala of HR/LR rats; here we found increased oxygen consumption reserve capacity in LR amygdala. Our last experiment examined activity of cytochrome c oxidase (COX), an enzyme responsible for ATP production and correlate of metabolic activity, in several brain regions of HR/LR rats. We found that LRs displayed higher COX activity in the dentate gyrus, prefrontal cortex, and dorsal raphe compared to HRs, with no significant HR/LR difference in nuclei of the amygdala. Correlational analyses of COX activity across brain regions suggested divergent connectivity between the prefrontal cortex, amygdala, hippocampus, and dorsal raphe of HR vs. LR rats. Together these studies point to altered cellular metabolism in the limbic brain of LR/HR animals, which may reflect altered neural circuitry that drives their divergent behavioral profiles.

Exploratory locomotion, a predictor of addiction vulnerability, is oligogenic in rats selected for this phenotype

Artificially selected model organisms can reveal hidden features of the genetic architecture of the complex disorders that they model. Addictions are disease phenotypes caused by different intermediate phenotypes and pathways and thereby are potentially highly polygenic. High responder (bHR) and low responder (bLR) rat lines have been selectively bred (b) for exploratory locomotion (EL), a behavioral phenotype correlated with novelty-seeking, impulsive response to reward, and vulnerability to addiction, and is inversely correlated with spontaneous anxiety and depression-like behaviors. The rapid response to selection indicates loci of large effect for EL. Using exome sequencing of HR and LR rats, we identified alleles in gene-coding regions that segregate between the two lines. Quantitative trait locus (QTL) analysis in F2 rats derived from a bHR × bLR intercross confirmed that these regions harbored genes affecting EL. The combined effects of the seven genome-wide significant QTLs accounted for approximately one-third of the total variance in EL, and two-thirds of the variance attributable to genetic factors, consistent with an oligogenic architecture of EL estimated both from the phenotypic distribution of F2 animals and rapid response to selection. Genetic association in humans linked APBA2, the ortholog of the gene at the center of the strongest QTL, with substance use disorders and related behavioral phenotypes. Our finding is also convergent with molecular and animal behavioral studies implicating Apba2 in locomotion. These results provide multilevel evidence for genes/loci influencing EL. They shed light on the genetic architecture of oligogenicity in animals artificially selected for a phenotype modeling a more complex disorder in humans.

Physiological adaptations to resistance training in rats selectively bred for low and high response to aerobic exercise training.

What is the central question of this study? Can phenotypic traits associated with low response to one mode of training be extrapolated to other exercise-inducible phenotypes? The present study investigated whether rats that are low responders to endurance training are also low responders to resistance training. What is the main finding and its importance? After resistance training, rats that are high responders to aerobic exercise training improved more in maximal strength compared with low-responder rats. However, the greater gain in strength in high-responder rats was not accompanied by muscle hypertrophy, suggesting that the responses observed could be mainly neural in origin. The purpose of this study was to determine whether rats selectively bred for low and high response to aerobic exercise training co-segregate for differences in muscle adaptations to ladder-climbing resistance training. Five high-responder (HRT) and five low-responder (LRT) rats completed the resistance training, while six HRT and six LRT rats served as sedentary control animals. Before and after the 6week intervention, body composition was determined by dual energy X-ray absorptiometry. Before tissue harvesting, the right triceps surae muscles were loaded by electrical stimulation. Muscle fibre cross-sectional areas, nuclei per cell, phosphorylation status of selected signalling proteins of mTOR and Smad pathways, and muscle protein, DNA and RNA concentrations were determined for the right gastrocnemius muscle. The daily protein synthesis rate was determined by the deuterium oxide method from the left quadriceps femoris muscle. Tissue weights of fore- and hindlimb muscles were measured. In response to resistance training, maximal carrying capacity was greater in HRT (∼3.3 times body mass) than LRT (∼2.5 times body mass), indicating greater improvements of strength in HRT. However, muscle hypertrophy that could be related to greater strength gains in HRT was not observed. Furthermore, noteworthy changes within the experimental groups or differences between groups were not observed in the present measures. The lack of hypertrophic muscular adaptations despite considerable increases in muscular strength suggest that adaptations to the present ladder-climbing training in HRT and LRT rats were largely induced by neural adaptations.

Modulation of striatal dopamine release by nicotinic receptors in adolescent and adult rats

Adolescence is a developmental period characterized by increased vulnerability to substance use and dependence. However, mechanisms mediating this vulnerability are not well understood. Previous work has shown that locomotor responses in a novel environment predict acquisition of nicotine and cocaine self‐administration in adult rats, with high‐responders (HR) acquiring faster and at lower doses than low‐responders (LR). Additionally, adult rats with higher vulnerability have more sensitive nicotine acetylcholine receptor (nAChR) modulation of striatal dopamine release to phasic firing‐like stimulation, which is known to be important in acquisition of self‐administration. Here we examined how antagonism of nAChRs altered dopamine release in the nucleus accumbens core in adolescent and adult rats using fast scan cyclic voltammetry. We found that non‐selective antagonism of nAChRs, by mecamylamine (2 uM) or a desensitizing dose of nicotine (500 nM), increased dopamine release in HRs to stimulation that models phasic dopamine release, but suppressed dopamine release in LR rats. Further, we report a‐conotoxin PIA (100 nM), an a6‐selective antagonist, produced similar effects as the non‐selective antagonists, indicating that facilitation of dopamine in the HR rats is mediated by a6‐containing nAChR. When dopamine release in adolescent rats was tested under the same parameters, nAChR modulation of dopamine release was similar to HR adult rats, with non‐selective and a6‐selective antagonists facilitating dopamine release following phasic firing‐like stimulation. This suggests a role for a6‐containing nAChR modulation of striatal dopamine release in mediating vulnerability to drug abuse.Support or Funding InformationThis work is supported by NIDA grant R00 DA031791 awarded to MJFThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

The in Vivo Neurochemical Profile of Selectively Bred High-Responder and Low-Responder Rats Reveals Baseline, Cocaine-Evoked, and Novelty-Evoked Differences in Monoaminergic Systems.

Relative to bred low-responder (bLR) rats, bred high-responder (bHR) rats have an exaggerated locomotor response to a novel environment, take more risks,are more impulsive, and more likely to exhibit compulsive drug-seeking behaviors. These phenotypic differences in addiction-related behaviors and temperament have previously been associated with differences in neurotransmitter signaling, including the mesolimbic dopamine system. In this study, we applied advanced in vivo microdialysis sampling in the nucleus accumbens of bHRs and bLRs to assess differences in basal and stimulated neurochemical efflux more broadly. We used liquid chromatography-mass spectrometry measurements of dialysate samples to quantify a panel of 17 neurochemicals, including dopamine, norepinephrine, serotonin, histamine, glutamate, GABA, acetylcholine, adenosine, DOPAC, 3-MT, HVA, 5-HIAA, normetanephrine, taurine, serine, aspartate, and glycine. We also applied a stable isotope labeling technique to assess absolute baseline concentrations of dopamine and norepinephrine in the nucleus accumbens. Finally, we investigated the role of norepinephrine tone in the nucleus accumbens on the bHR phenotype. Our findings show that bHRs have elevated basal and cocaine-evoked dopamine and norepinephrine levels in the nucleus accumbens compared to those of bLRs. Furthermore, norepinephrine signaling in the nucleus accumbens appeared to be an important contributor to the bHR phenotype because bilateral perfusion of the α1 adrenergic receptor antagonist terazosin (10 μM) into the nucleus accumbens abolished the response of bHRs to novelty. These findings are the first to demonstrate a role for norepinephrine in the bHR phenotype. They reveal a positive relationship between dopamine and norepinephrine signaling in the nucleus accumbens in mediating the exaggerated response to novelty and point to norepinephrine signaling as a potential target in the treatment of impulse control disorders.