Variation in the form of Pavlovian conditioned approach behavior among outbred male Sprague-Dawley rats from different vendors and colonies: sign-tracking vs. goal-tracking

Incentive-salience attribution is attenuated in spontaneously hypertensive rats, an animal model of ADHD

Spontaneously Hypertensive Rats (SHR) have been extensively studied as an animal model of Attention Deficit Hyperactivity Disorder (ADHD) because they show some of the defining features of that disorder, like some forms of impulsivity and hyperactivity. However, other characteristics of the disorder, like a deficit in motivation, have been scarcely studied in the SHR strain. In the present report, we studied in 45 SHR and 45 Wistar rats as a comparison group, the capacity of attribution of incentive salience to a stimulus predictor of reinforcement, which has become a central concept in the study of motivation. We employed the Pavlovian Conditioned-Approach (PCA) task, in which a lever is presented 8 s before a pellet is delivered. The attribution of incentive salience is indicated by responses to the lever, in contrast to the absence of attribution of incentive salience, which is indicated by entrances to the pellet receptacle. For quantifying the attribution of incentive salience, we employed the PCA index, which integrates three related variables for each type of response, lever presses and entrances to the feeder: 1) the number of responses, 2) the latency to the first response, and 3) the probability that at least one response occurred during the presence of the lever. SHR showed lower levels of PCA, suggesting a deficit in the attribution of incentive salience to the lever. This finding replicates the results reported by previous research that compared SHR's performance in the PCA task against that of Sprague-Dawley rats.

Acute MK-801 increases measures of both sign-tracking and goal-tracking in male Sprague-Dawley rats

Sign-tracking is a Pavlovian conditioned approach behavior thought to be important in understanding cue-driven relapse to drug use, and strategies for reducing sign-tracking may have some benefit in preventing relapse. A previous study successfully employed the NMDA receptor antagonist MK-801 in preventing the development of sign-tracking (but not goal-tracking) in a conditioned approach task. In this study, we focused on whether MK-801 would have similar effects on previously established sign-tracking behavior. MK-801 was administered after training in a standard sign-/goal-tracking task using a retractable lever as a conditioned stimulus and a sucrose pellet as unconditioned stimulus. It was found that MK-801 increased measures of both sign- and goal-tracking in subjects who had previously learned the task. The NMDA receptor appears to play a complex role in governing behavior related to sign-tracking.

The effect of corticosterone on the acquisition of Pavlovian conditioned approach behavior is dependent on sex and vendor

Cues in the environment become predictors of biologically relevant stimuli, such as food, through associative learning. These cues can not only act as predictors but can also be attributed with incentive motivational value and gain control over behavior. When a cue is imbued with incentive salience, it attains the ability to elicit maladaptive behaviors characteristic of psychopathology. We can capture the propensity to attribute incentive salience to a reward cue in rats using a Pavlovian conditioned approach paradigm, in which the presentation of a discrete lever-cue is followed by the delivery of a food reward. Upon learning the cue-reward relationship, some rats, termed sign-trackers, develop a conditioned response directed towards the lever-cue; whereas others, termed goal-trackers, approach the food cup upon lever-cue presentation. Here, we assessed the effects of systemic corticosterone (CORT) on the acquisition and expression of sign- and goal-tracking behaviors in male and female rats, while examining the role of the vendor (Charles River or Taconic) from which the rats originated in these effects. Male and female rats from Charles River had a greater tendency to sign-track than those from Taconic. Administration of CORT enhanced the acquisition of sign-tracking behavior in males from Charles River and females from both vendors. Conversely, administration of CORT had no effect on the expression of the conditioned response. These findings demonstrate a role for CORT in cue-reward learning and suggest that inherent tendencies towards sign- or goal-tracking may interact with this physiological mediator of motivated behavior.

Highlights

Male and female rats from Charles River exhibit more sign-tracking relative to those from Taconic.Corticosterone increases the acquisition of sign-tracking in male rats from Charles River.Corticosterone increases the acquisition of sign-tracking in female rats, regardless of vendor.There is no effect of corticosterone on the expression of sign-tracking behavior in either male or female rats.

Environmental enrichment promotes adaptive responding during tests of behavioral regulation in male heterogeneous stock rats

Organisms must regulate their behavior flexibly in the face of environmental challenges. Failure can lead to a host of maladaptive behavioral traits associated with a range of neuropsychiatric disorders, including attention deficit hyperactivity disorder, autism, and substance use disorders. This maladaptive dysregulation of behavior is influenced by genetic and environmental factors. For example, environmental enrichment produces beneficial neurobehavioral effects in animal models of such disorders. The present study determined the effects of environmental enrichment on a range of measures related to behavioral regulation using a large cohort of male, outbred heterogeneous stock (HS) rats as subjects. Subjects were reared from late adolescence onwards either in pairs in standard housing with minimal enrichment (n = 200) or in groups of 16 in a highly enriched environment consisting of a large multi-level cage filled with toys, running wheels, and shelters (n = 64). Rats were subjected to a battery of tests, including: (i) locomotor response to novelty, (ii) light reinforcement, (iii) social reinforcement, (iv) reaction time, (v) a patch-depletion foraging test, (vi) Pavlovian conditioned approach, (vii) conditioned reinforcement, and (viii) cocaine conditioned cue preference. Results indicated that rats housed in the enriched environment were able to filter out irrelevant stimuli more effectively and thereby regulate their behavior more efficiently than standard-housing rats. The dramatic impact of environmental enrichment suggests that behavioral studies using standard housing conditions may not generalize to more complex environments that may be more ethologically relevant.

Inhibition of Dopamine Neurons Prevents Incentive Value Encoding of a Reward Cue: With Revelations from Deep Phenotyping

The survival of an organism is dependent on its ability to respond to cues in the environment. Such cues can attain control over behavior as a function of the value ascribed to them. Some individuals have an inherent tendency to attribute reward-paired cues with incentive motivational value, or incentive salience. For these individuals, termed sign-trackers, a discrete cue that precedes reward delivery becomes attractive and desirable in its own right. Prior work suggests that the behavior of sign-trackers is dopamine-dependent, and cue-elicited dopamine in the NAc is believed to encode the incentive value of reward cues. Here we exploited the temporal resolution of optogenetics to determine whether selective inhibition of ventral tegmental area (VTA) dopamine neurons during cue presentation attenuates the propensity to sign-track. Using male tyrosine hydroxylase (TH)-Cre Long Evans rats, it was found that, under baseline conditions, ∼84% of TH-Cre rats tend to sign-track. Laser-induced inhibition of VTA dopamine neurons during cue presentation prevented the development of sign-tracking behavior, without affecting goal-tracking behavior. When laser inhibition was terminated, these same rats developed a sign-tracking response. Video analysis using DeepLabCutTM revealed that, relative to rats that received laser inhibition, rats in the control group spent more time near the location of the reward cue even when it was not present and were more likely to orient toward and approach the cue during its presentation. These findings demonstrate that cue-elicited dopamine release is critical for the attribution of incentive salience to reward cues.SIGNIFICANCE STATEMENT Activity of dopamine neurons in the ventral tegmental area (VTA) during cue presentation is necessary for the development of a sign-tracking, but not a goal-tracking, conditioned response in a Pavlovian task. We capitalized on the temporal precision of optogenetics to pair cue presentation with inhibition of VTA dopamine neurons. A detailed behavioral analysis with DeepLabCutTM revealed that cue-directed behaviors do not emerge without dopamine neuron activity in the VTA. Importantly, however, when optogenetic inhibition is lifted, cue-directed behaviors increase, and a sign-tracking response develops. These findings confirm the necessity of dopamine neuron activity in the VTA during cue presentation to encode the incentive value of reward cues.

The role of sex on sign-tracking acquisition and outcome devaluation sensitivity in Long Evans rats

Cues that predict rewards can trigger reward-seeking behaviors but also can, in some cases, become targets of motivation themselves. One behavioral phenomenon that captures this idea is sign-tracking in which animals, including humans, interact with reward-predictive cues even though it is not necessary to do so. Sign-tracking in rats has been studied in the domain of motivation and in how motivated behaviors can or cannot become excessive and habit-like over time. Many prior studies look at sign-tracking examine this behavior in male subjects, but there are few papers that look at this behavior in female subjects. Moreover, it is unknown where there might be sex-related variation in how flexible sign-tracking is when faced with changing reward values. Therefore, we asked if there were sex differences in the acquisition of sign-tracking behavior and if there were any sex differences in how sensitive animals were in their sign-tracking following reward devaluation. In contrast to previous reports, we found that males and females show no differences in how they acquire sign-tracking and in ultimate sign-tracking levels following training. Additionally, we found no difference in how quickly males and females learned to devalue the food reward, and we found no differences in sign-tracking levels by sex following outcome devaluation. We believe that this is primarily due to our experiment being performed in the Long Evans strain but also believe that there are many other factors contributing to differences between our study and previous work.

Effects of predictive and incentive value manipulation on sign- and goal-tracking behavior

When a neutral stimulus is repeatedly paired with an appetitive reward, two different types of conditioned approach responses may develop: a sign-tracking response directed toward the neutral cue, or a goal-tracking response directed toward the location of impending reward delivery. Sign-tracking responses have been postulated to result from attribution of incentive value to conditioned cues, while goal-tracking reflects the assignment of only predictive value to the cue. We therefore hypothesized that sign-tracking rats would be more sensitive to manipulations of incentive value, while goal-tracking rats would be more responsive to changes in the predictive value of the cue. We tested sign- and goal-tracking before and after devaluation of a food reward using lithium chloride, and tested whether either response could be learned under negative contingency conditions that precluded any serendipitous reinforcement of the behavior that might support instrumental learning. We also tested the effects of blocking the predictive value of a cue using simultaneous presentation of a pre-conditioned cue. We found that sign-tracking was sensitive to outcome devaluation, while goal-tracking was not. We also confirmed that both responses are Pavlovian because they can be learned under negative contingency conditions. Goal-tracking was almost completely blocked by a pre-conditioned cue, while sign-tracking was much less sensitive to such interference. These results indicate that sign- and goal-tracking may follow different rules of reinforcement learning and suggest a need to revise current models of associative learning to account for these differences.

Sprague Dawley rats from different vendors vary in the modulation of prepulse inhibition of startle (PPI) by dopamine, acetylcholine, and glutamate drugs

RATIONALE

Rodent vendors are often utilized interchangeably, assuming that the phenotype of a given strain remains standardized between colonies. Several studies, however, have found significant behavioral and physiological differences between Sprague Dawley (SD) rats from separate vendors. Prepulse inhibition of startle (PPI), a form of sensorimotor gating in which a low-intensity leading stimulus reduces the startle response to a subsequent stimulus, may also vary by vendor. Differences in PPI between rat strains are well known, but divergence between colonies within the SD strain lacks thorough examination.

OBJECTIVES

We explored intrastrain variation in PPI by testing SD rats from two vendors: Envigo and Charles River (CR).

METHODS

We selected drugs acting on four major neurotransmitter systems that have been repeatedly shown to modulate PPI: dopamine (apomorphine; 0.5, 1.5, 3.0 mg/kg), acetylcholine (scopolamine; 0.1, 0.5, 1.0 mg/kg), glutamate (dizocilpine; 0.5, 1.5, 2.5 mg/kg), and serotonin (2,5-Dimethoxy-4-iodoamphetamine, DOI; 0.25, 0.5, 1.0 mg/kg). We determined PPI and startle amplitude for each drug in male and female Envigo and CR SD rats.

RESULTS

SD rats from Envigo showed dose-dependent decreases in PPI after apomorphine, scopolamine, or dizocilpine administration, without significant effects on startle amplitude. SD rats from CR were less sensitive to modulation of PPI and/or more sensitive to modulation of startle amplitude, across the three drugs.

CONCLUSIONS

SD rats showed vendor differences in sensitivity to pharmacological modulation of PPI and startle. We encourage researchers to sample rats from separate vendors before experimentation to identify the most suited source of subjects for their specific endpoints.

Environmental enrichment promotes adaptive responding during tests of behavioral regulation in male heterogeneous stock rats

Organisms must regulate their behavior flexibly in the face of environmental challenges. Failure can lead to a host of maladaptive behavioral traits associated with a range of neuropsychiatric disorders, including attention deficit hyperactivity disorder, autism, and substance use disorders. This maladaptive dysregulation of behavior is influenced by genetic and environmental factors. For example, environmental enrichment produces beneficial neurobehavioral effects in animal models of such disorders. The present study determined the effects of environmental enrichment on a range of measures related to behavioral regulation using a large cohort of male, outbred heterogeneous stock (HS) rats as subjects to mimic the genetic variability found in the human population. Subjects were reared from late adolescence onwards either in pairs in standard housing with minimal enrichment (n=200) or in groups of 16 in a highly enriched environment consisting of a large multi-level cage filled with toys, running wheels, and shelters (n=64). Rats were subjected to a battery of tests, including: (i) locomotor response to novelty, (iI) light reinforcement, (iii) social reinforcement, (iv) reaction time, (v) a patch-depletion foraging test, (vi) Pavlovian conditioned approach, (vii) conditioned reinforcement, and (viii) cocaine conditioned cue preference. Results indicated that rats housed in the enriched environment were able to filter out irrelevant stimuli more effectively and thereby regulate their behavior more efficiently than standard-housing rats. The dramatic impact of environmental enrichment suggests that behavioral studies using standard housing conditions may not generalize to more complex environments that may be more ethologically relevant.

Divergent risky decision-making and impulsivity behaviors in Lewis rat substrains with low genetic difference

Substance use disorder (SUD) is associated with a cluster of cognitive disturbances that engender vulnerability to ongoing drug seeking and relapse. Two of these endophenotypes-risky decision-making and impulsivity-are amplified in individuals with SUD and are augmented by repeated exposure to illicit drugs. Identifying genetic factors underlying variability in these behavioral patterns is critical for early identification, prevention, and treatment of SUD-vulnerable individuals. Here, we compared risky decision-making and different facets of impulsivity between two fully inbred substrains of Lewis rats-LEW/NCrl and LEW/NHsd. We performed whole genome sequencing of both substrains to identify almost all relevant variants. We observed substantial differences in risky decision-making and impulsive behaviors. Relative to LEW/NHsd, the LEW/NCrl substrain accepts higher risk options in a decision-making task and higher rates of premature responses in the differential reinforcement of low rates of responding task. These phenotypic differences were more pronounced in females than males. We defined a total of ∼9,000 polymorphisms between these substrains at 40× whole genome short-read coverage. Roughly half of variants are located within a single 1.5 Mb region of Chromosome 8, but none impact protein-coding regions. In contrast, other variants are widely distributed, and of these, 38 are predicted to cause protein-coding variants. In conclusion, Lewis rat substrains differ significantly in risk-taking and impulsivity and only a small number of easily mapped variants are likely to be causal. Sequencing combined with a reduced complexity cross should enable identification of one or more variants underlying multiple complex addiction-relevant behaviors. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

Conditioned approach behavior of SHR and SD rats during Pavlovian conditioning

Individual differences in reward-related learning are relevant to many behavioral disorders. Sensory cues that predict reward can become incentive stimuli that adaptively support behavior, or alternatively, cause maladaptive behaviors. The spontaneously hypertensive rat (SHR) expresses a genetically determined elevated sensitivity to delay of reward, and has been extensively studied as a behavioral model for attention deficit hyperactivity disorder (ADHD). We investigated reward-related learning in the SHR, comparing them to Sprague-Dawley (SD) rats as a reference strain. A standard Pavlovian conditioned approach task was used, in which a lever cue was followed by reward. Lever presses could occur while the lever was extended, but had no effect on reward delivery. The behavior of both the SHRs and the SD rats showed that they learnt that the lever cue predicted reward. However, the pattern of behavior differed between the strains. During lever cue presentation, SD rats pressed the lever more often and made fewer magazine entries than SHRs. When lever contacts that did not result in lever presses were analyzed, there was no significant difference between SHRs and SDs. These results suggest that the SHRs attributed less incentive value to the conditioned stimulus than the SD rats. During the presentation of the conditioned cue, cue directed responses are called sign tracking responses, whereas responses directed towards the food magazine are called goal tracking responses. Analysis of behavior using a standard Pavlovian conditioned approach index to quantify sign and goal tracking tendencies showed that both strains had a tendency towards goal tracking in this task. However, the SHRs showed a significantly greater goal tracking tendency than the SD rats. Taken together, these findings suggest that attribution of incentive value to reward predicting cues is attenuated in SHRs, which might explain their elevated sensitivity to delay of reward.

Neuro-Immune Modulation of Cholinergic Signaling in an Addiction Vulnerability Trait

Sign-tracking (ST) describes the propensity to approach and contact a Pavlovian reward cue. By contrast, goal-trackers (GTs) respond to such a cue by retrieving the reward. These behaviors index the presence of opponent cognitive-motivational traits, with STs exhibiting attentional control deficits, behavior dominated by incentive motivational processes, and vulnerability for addictive drug taking. Attentional control deficits in STs were previously attributed to attenuated cholinergic signaling, resulting from deficient translocation of intracellular choline transporters (CHTs) into synaptosomal plasma membrane. Here, we investigated a posttranslational modification of CHTs, poly-ubiquitination, and tested the hypothesis that elevated cytokine signaling in STs contributes to CHT modification. We demonstrated that intracellular CHTs, but not plasma membrane CHTs, are highly ubiquitinated in male and female sign-tracking rats when compared with GTs. Moreover, levels of cytokines measured in cortex and striatum, but not spleen, were higher in STs than in GTs. Activation of the innate immune system by systemic administration of the bacterial endotoxin lipopolysaccharide (LPS) elevated ubiquitinated CHT levels in cortex and striatum of GTs only, suggesting ceiling effects in STs. In spleen, LPS increased levels of most cytokines in both phenotypes. In cortex, LPS particularly robustly increased levels of the chemokines CCL2 and CXCL10. Phenotype-specific increases were restricted to GTs, again suggesting ceiling effects in STs. These results indicate that interactions between elevated brain immune modulator signaling and CHT regulation are essential components of the neuronal underpinnings of the addiction vulnerability trait indexed by sign-tracking.

Reward-Mediated, Model-Free Reinforcement-Learning Mechanisms in Pavlovian and Instrumental Tasks Are Related

Model-free and model-based computations are argued to distinctly update action values that guide decision-making processes. It is not known, however, if these model-free and model-based reinforcement learning mechanisms recruited in operationally based instrumental tasks parallel those engaged by pavlovian-based behavioral procedures. Recently, computational work has suggested that individual differences in the attribution of incentive salience to reward predictive cues, that is, sign- and goal-tracking behaviors, are also governed by variations in model-free and model-based value representations that guide behavior. Moreover, it is not appreciated if these systems that are characterized computationally using model-free and model-based algorithms are conserved across tasks for individual animals. In the current study, we used a within-subject design to assess sign-tracking and goal-tracking behaviors using a pavlovian conditioned approach task and then characterized behavior using an instrumental multistage decision-making (MSDM) task in male rats. We hypothesized that both pavlovian and instrumental learning processes may be driven by common reinforcement-learning mechanisms. Our data confirm that sign-tracking behavior was associated with greater reward-mediated, model-free reinforcement learning and that it was also linked to model-free reinforcement learning in the MSDM task. Computational analyses revealed that pavlovian model-free updating was correlated with model-free reinforcement learning in the MSDM task. These data provide key insights into the computational mechanisms mediating associative learning that could have important implications for normal and abnormal states.SIGNIFICANCE STATEMENT Model-free and model-based computations that guide instrumental decision-making processes may also be recruited in pavlovian-based behavioral procedures. Here, we used a within-subject design to test the hypothesis that both pavlovian and instrumental learning processes were driven by common reinforcement-learning mechanisms. Sign-tracking and goal-tracking behaviors were assessed in rats using a pavlovian conditioned approach task, and then instrumental behavior was characterized using an MSDM task. We report that sign-tracking behavior was associated with greater model-free, but not model-based, learning in the MSDM task. These data suggest that pavlovian and instrumental behaviors may be driven by conserved reinforcement-learning mechanisms.

A mechanical task for measuring sign- and goal-tracking in humans: A proof-of-concept study

Cue-based associative learning (i.e., Pavlovian conditioning) is a foundational component of behavior in almost all forms of animal life and may provide insight into individual differences in addiction liability. Cues can take on incentive-motivational properties (i.e., incentive salience) through Pavlovian learning. Extensive testing with non-human animals (primarily rats) has demonstrated significant variation among individuals in the behaviors this type of learning evokes. So-named "sign-trackers" and "goal-trackers" have been examined in many studies of non-human animals, but this work in humans is still a nascent area of research. In the present proof-of-concept study, we used a Pavlovian conditioned approach task to investigate human sign- and goal-tracking in emerging adults. Conditioned behaviors that developed over the course of the task were directed toward the reward-cue and toward the reward location. Participants' eye-gaze and behavior during the task were submitted to a latent profile analysis, which revealed three groups defined as sign-trackers (n = 10), goal-trackers (n = 4), and intermediate responders (n = 36). Impulsivity was a significant predictor of the sign-tracking group relative to the goal-tracking group. The present study provides preliminary evidence that a simple procedure can produce learned Pavlovian conditioned approach behavior in humans. Though further investigation is required, findings provide a promising step toward the long-term goal of translating important insights gleaned from basic research into treatment strategies that can be applied to clinical populations.

Basal Forebrain Chemogenetic Inhibition Converts the Attentional Control Mode of Goal-Trackers to That of Sign-Trackers

Sign tracking versus goal tracking in rats indicate vulnerability and resistance, respectively, to Pavlovian cue-evoked addictive drug taking and relapse. Here, we tested hypotheses predicting that the opponent cognitive-behavioral styles indexed by sign tracking versus goal tracking include variations in attentional performance which differentially depend on basal forebrain projection systems. Pavlovian Conditioned Approach (PCA) testing was used to identify male and female sign-trackers (STs) and goal-trackers (GTs), as well as rats with an intermediate phenotype (INTs). Upon reaching asymptotic performance in an operant task requiring the detection of visual signals (hits) as well as the reporting of signal absence for 40 min per session, GTs scored more hits than STs, and hit rates across all phenotypes correlated with PCA scores. STs missed relatively more signals than GTs specifically during the last 15 min of a session. Chemogenetic inhibition of the basal forebrain decreased hit rates in GTs but was without effect in STs. Moreover, the decrease in hits in GTs manifested solely during the last 15 min of a session. Transfection efficacy in the horizontal limb of the diagonal band (HDB), but not substantia innominate (SI) or nucleus basalis of Meynert (nbM), predicted the behavioral efficacy of chemogenetic inhibition in GTs. Furthermore, the total subregional transfection space, not transfection of just cholinergic neurons, correlated with performance effects. These results indicate that the cognitive-behavioral phenotype indexed by goal tracking, but not sign tracking, depends on activation of the basal forebrain-frontal cortical projection system and associated biases toward top-down or model-based performance.

Biochemical Neuroadaptations in the Rat Striatal Dopaminergic System after Prolonged Exposure to Methamphetamine Self-Administration

Perturbations in striatal dopamine (DA) homeostasis might underlie the behavioral and pathobiological consequences of METH use disorder in humans. To identify potential consequences of long-term METH exposure, we modeled the adverse consequence DSM criterion of substance use disorders by giving footshocks to rats that had escalated their intake of METH during a drug self-administration procedure. Next, DA D1 receptor antagonist, SCH23390 was injected. Thereafter, rats were euthanized to measure several indices of the striatal dopaminergic system. Footshocks split the METH rats into two phenotypes: (i) shock-sensitive that decreased their METH-intake and (ii) shock-resistant that continued their METH intake. SCH23390 caused substantial dose-dependent reduction of METH taking in both groups. Stopping SCH23390 caused re-emergence of compulsive METH taking in shock-resistant rats. Compulsive METH takers also exhibited greater incubation of METH seeking than non-compulsive rats during withdrawal from METH SA. Analyses of DA metabolism revealed non-significant decreases (about 35%) in DA levels in resistant and sensitive rats. However, striatal contents of the deaminated metabolites, DOPAL and DOPAC, were significantly increased in sensitive rats. VMAT2 and DAT protein levels were decreased in both phenotypes. Moreover, protein expression levels of the D1-like DA receptor, D5R, and D2-like DA receptors, D3R and D4R, were significantly decreased in the compulsive METH takers. Our results parallel findings in post-mortem striatal tissues of human METH users who develop Parkinsonism after long-term METH intake and support the use of this model to investigate potential therapeutic interventions for METH use disorder.

Appetitive 50 kHz calls in a pavlovian conditioned approach task in Cacna1c haploinsufficient rats

We have previously shown that rats emit high-frequency 50 kHz ultrasonic vocalizations (USV) during sign- and goal-tracking in a common Pavlovian conditioned approach task. Such 50 kHz calls are probably related to positive affect and are associated with meso-limbic dopamine function. In humans, the CACNA1C gene, encoding for the α_1C subunit of the L-type voltage-gated calcium channel Ca_V1.2, is implicated in several mental disorders, including mood disorders associated with altered dopamine signaling. In the present study, we investigated sign- and goal-tracking behavior and the emission of 50 kHz USV in Cacna1c haploinsufficent rats in a task where food pellet delivery is signaled by an appearance of an otherwise inoperable lever. Over the course of this Pavlovian training, these rats not only increased their approach to the reward site, but also their rates of pressing the inoperable lever. During subsequent extinction tests, where reward delivery was omitted, extinction patterns differed between reward site (i.e. magazine entries) and lever, since magazine entries quickly declined whereas behavior towards the lever transiently increased. Based on established criteria to define sign- or goal-tracking individuals, no CACNA1C rat met a sign-tracking criterion, since around 42% of rats tested where goal-trackers and the other 58% fell into an intermediate range. Regarding USV, we found that the CACNA1C rats emitted 50 kHz calls with a clear subject-dependent pattern; also, most of them were of a flat subtype and occurred mainly during initial habituation phases without cues or rewards. Compared, to previously published wildtype controls, Cacna1c haploinsufficent rats displayed reduced numbers of appetitive 50 kHz calls. Moreover, similar to wildtype littermate controls, 50 kHz call emission in Cacna1c haploinsufficent rats was intra-individually stable over training days and was negatively associated with goal-tracking. Together, these findings provide evidence in support of 50 kHz calls as trait marker. The finding that Cacna1c haploinsufficent rats show reductions of 50 kHz calls accompanied with more goal-tracking, is consistent with the assumption of altered dopamine signaling in these rats, a finding which supports their applicability in models of mental disorders.

Genetic characterization of outbred Sprague Dawley rats and utility for genome-wide association studies

Sprague Dawley (SD) rats are among the most widely used outbred laboratory rat populations. Despite this, the genetic characteristics of SD rats have not been clearly described, and SD rats are rarely used for experiments aimed at exploring genotype-phenotype relationships. In order to use SD rats to perform a genome-wide association study (GWAS), we collected behavioral data from 4,625 SD rats that were predominantly obtained from two commercial vendors, Charles River Laboratories and Harlan Sprague Dawley Inc. Using double-digest genotyping-by-sequencing (ddGBS), we obtained dense, high-quality genotypes at 291,438 SNPs across 4,061 rats. This genetic data allowed us to characterize the variation present in Charles River vs. Harlan SD rats. We found that the two populations are highly diverged (F_ST > 0.4). Furthermore, even for rats obtained from the same vendor, there was strong population structure across breeding facilities and even between rooms at the same facility. We performed multiple separate GWAS by fitting a linear mixed model that accounted for population structure and using meta-analysis to jointly analyze all cohorts. Our study examined Pavlovian conditioned approach (PavCA) behavior, which assesses the propensity for rats to attribute incentive salience to reward-associated cues. We identified 46 significant associations for the various metrics used to define PavCA. The surprising degree of population structure among SD rats from different sources has important implications for their use in both genetic and non-genetic studies.

Outbred Sprague Dawley rats are among the most commonly used rats for neuroscience, physiology and pharmacological research; in the year 2020, 4,188 publications contained the keyword “Sprague Dawley”. Rats identified as “Sprague Dawley” are sold by several commercial vendors, including Charles River Laboratories and Harlan Sprague Dawley Inc. (now Envigo). Despite their widespread use, little is known about the genetic diversity of SD. We genotyped more than 4,000 SD rats, which we used for a genome-wide association study (GWAS) and to characterize genetic differences between SD rats from Charles River Laboratories and Harlan. Our analysis revealed extensive population structure both between and within vendors. The GWAS for Pavlovian conditioned approach (PavCA) identified a number of genome-wide significant loci for that complex behavioral trait. Our results demonstrate that, despite sharing an identical name, SD rats that are obtained from different vendors are very different. Future studies should carefully define the exact source of SD rats being used and may exploit their genetic diversity for genetic studies of complex traits.

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

RATIONALE

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.

OBJECTIVE

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.

RESULTS

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.

CONCLUSIONS

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.

Quantifying the instrumental and noninstrumental underpinnings of Pavlovian responding with the Price equation

The Price equation is a mathematical expression of selectionist and non-selectionist pressures on biological, cultural, and behavioral change. We use it here to specify instrumental and noninstrumental behaviors as they arise within the context of the Pavlovian autoshaping procedure, for rats trained under reward certainty and reward uncertainty. The point of departure for this endeavor is that some portion of autoshaped behavior referred to as goal-tracking appears instrumental-a function of resource attainment (the individual approaches the location where the unconditioned stimulus is to be delivered). By contrast, some other portion of autoshaped behavior referred to as sign-tracking is noninstrumental-irrelevant to making contact with the to-be-delivered unconditioned stimulus. A Price equation model is proposed that unifies our understanding of Pavlovian autoshaping behavior by isolating operant and respondent influences on goal-tracking (instrumental) and sign-tracking (noninstrumental) behavior.

An operant ethanol self-administration paradigm that discriminates between appetitive and consummatory behaviors reveals distinct behavioral phenotypes in commonly used rat strains

Alcohol use disorder (AUD) constitutes a major burden to global health. Recently, the translational success of animal models of AUD has come under increased scrutiny. Efforts to refine models to gain a more precise understanding of the neurobiology of addiction are warranted. Appetitive responding for ethanol (seeking) and its consumption (taking) are governed by distinct neurobiological mechanisms. However, consumption is often inferred from appetitive responding in operant ethanol self-administration paradigms, preventing identification of distinct experimental effects on seeking and taking. In the present study, male Long-Evans, Wistar, and Sprague-Dawley rats were trained to lever press for ethanol using a lickometer-equipped system that precisely measures both appetitive and consummatory behavior. Three distinct operant phenotypes emerged during training: 1) Drinkers, who lever press and consume ethanol; 2) Responders, who lever press but consume little to no ethanol; and 3) Non-responders, who do not lever press. While the prevalence of each phenotype differed across strains, appetitive and consummatory behavior was similar across strains within each phenotype. Appetitive and consummatory behaviors were significantly correlated in Drinkers, but not Responders. Analysis of drinking microstructure showed that greater consumption in Drinkers relative to Responders is due to increased incentive for ethanol rather than increased palatability. Importantly, withdrawal from chronic ethanol exposure resulted in a significant increase in appetitive responding in both Drinkers and Responders, but only Drinkers exhibited a concomitant increase in ethanol consumption. Together, these data reveal important strain differences in appetitive and consummatory responding for ethanol and uncover the presence of distinct operant phenotypes.

Nicotine Enhances Goal-Tracking in Ethanol and Food Pavlovian Conditioned Approach Paradigms

Rationale

Nicotine promotes alcohol intake through pharmacological and behavioral interactions. As an example of the latter, nicotine can facilitate approach toward food- and alcohol-associated stimuli ("sign-tracking") in lever-Pavlovian conditioned approach (PavCA) paradigms. However, we recently reported that nicotine can also enhance approach toward locations of reward delivery ("goal-tracking") triggered by ethanol-predictive stimuli when the location of ethanol delivery is non-static (i.e., a retractable sipper bottle).

Objective

To determine whether the non-static nature of the reward location could have biased the development of goal-tracking in our previous study (Loney et al., 2019); we assessed the effect of nicotine in a lever-PavCA paradigm wherein the location of ethanol delivery was static (i.e., a stationary liquid receptacle). Then, to determine whether nicotine's enhancement of goal-tracking is unique to ethanol-predictive stimuli, we assessed the effect of systemic nicotine on approach triggered by food-predictive stimuli in a lever-PavCA paradigm.

Methods

Long-Evans rats were used in two PavCA experiments wherein a lever predicted the receipt of ethanol (15% vol/vol; experiment 1) or food (experiment 2) into a stationary receptacle. Prior to testing, rats were administered nicotine (0.4 mg/kg subcutaneously) or saline systemically.

Results

In both experiments, nicotine increased measures of goal-tracking, but not sign-tracking.

Conclusion

Nicotine can facilitate approach to reward locations without facilitating approach to reward-predictive stimuli. As such, conceptualization of the mechanisms by which nicotine affects behavior must be expanded to explain an enhancement of goal-tracking by nicotine.

Sign tracking predicts suboptimal behavior in a rodent gambling task

RATIONALE

Reward-associated cues can promote maladaptive behavior, including risky decision-making in a gambling setting. A propensity for sign tracking over goal tracking-i.e., interaction with a reward-predictive cue rather than the site of reward-demonstrates an individual's tendency to transfer motivational value to a cue. However, the relationship of sign tracking to risky decision-making remains unclear.

OBJECTIVES

To determine whether sign tracking predicts risky choice, we used a Pavlovian conditioned approach task to evaluate the tendency of male rats to sign track to a lever cue and then trained rats on a rodent gambling task (rGT) with win-associated cues. We also tested the effects of D-amphetamine, quinpirole (a D₂/D₃ receptor agonist), and PD128907 (a D₃ receptor agonist) on gambling behavior in sign tracker and goal tracker individuals.

RESULTS

Increased sign tracking relative to goal tracking was associated with suboptimal performance on the rGT, including decreased selection of the optimal choice, increased selection of a high-risk/high-reward option, and increased impulsive premature choices. Amphetamine increased choices of a low-risk/low-reward option at the expense of optimal and high-risk choices, whereas quinpirole and PD128907 had little effect on choice allocation, but reduced impulsivity. Drug effects were similar across sign tracker and goal tracker individuals.

CONCLUSIONS

Cue reactivity, as measured by sign tracking, is predictive and may be an important driver of risky and impulsive choices in a gambling setting laden with salient audiovisual cues. Evaluating an individual's sign tracking behavior may be an avenue to predict vulnerability to pathological gambling and the efficacy of treatments.

Sign- and goal-tracking score does not correlate with addiction-like behavior following prolonged cocaine self-administration

RATIONALE

In classical conditioning, sign-tracking reflects behavior directed toward a conditioned stimulus (CS) in expectation of a reward (unconditioned stimulus, US); in contrast, goal-tracking describes behavior directed toward the location of delivery of a US. As cues previously paired with drugs of abuse promote drug-seeking and drug-taking behavior in both animals and humans and thus contribute to the severity of substance abuse, sign-tracking may represent a maladaptive cue-focused behavior that may increase addiction vulnerability as compared to goal-tracking. Recent studies do, in fact, support this possibility. Previous work in this area has focused primarily on paradigms using relatively limited exposure to drug rather than extended drug intake.

OBJECTIVES

Here, we used the DSM-IV-based 3-criteria (3-CRIT) model and examined whether a relationship exists between sign- or goal-tracking phenotypes and the prevalence of criteria associated with addiction-like behavior following extended cocaine self-administration as measured in this model.

METHODS

Forty-six male Sprague Dawley rats underwent a Pavlovian conditioned approach (PCA) procedure and were characterized along a continuum as goal-trackers (GTs), intermediates (INTs), or sign-trackers (STs). The animals were subsequently trained to intravenous self-administer cocaine during 45 self-administration (SA) sessions and characterized for the 3 criteria outlined in the model: persistence of drug-seeking, motivation for cocaine-taking, and resistance to punishment.

RESULTS

We performed correlational analyses on the traits measured, finding no relationships between PCA score and addiction-like characteristics measured using the 3-CRIT model of addiction. However, STs showed significantly greater resistance to punishment than GTs.

CONCLUSIONS

Phenotyping along a continuum of PCA scores may not be a valid predictor for identifying vulnerability to the addiction-like behaviors examined using the 3-CRIT model. However, PCA phenotype may predict a single feature of the 3-CRIT model, resistance to punishment, among those rats classified as either STs or GTs.

Bisphenol F Exposure in Adolescent Heterogeneous Stock Rats Affects Growth and Adiposity

Bisphenol F (BPF) is increasingly substituting bisphenol A in manufacturing polycarbonates and consumer products. The cardiometabolic effects of BPF in either humans or model organisms are not clear, and no studies to date have investigated the role of genetic background on susceptibility to BPF-induced cardiometabolic traits. The primary goal of this project was to determine if BPF exposure influences growth and adiposity in male N:NIH heterogeneous stock (HS) rats, a genetically heterogeneous population. Littermate pairs of male HS rats were randomly exposed to either vehicle (0.1% ethanol) or 1.125 µg/ml BPF in 0.1% ethanol for 5 weeks in drinking water starting at 3 weeks-of-age. Water consumption and body weight was measured weekly, body composition was determined using nuclear magnetic resonance, urine and feces were collected in metabolic cages, and blood and tissues were collected at the end of the study. BPF-exposed rats showed significantly increased body growth and abdominal adiposity, risk factors for cardiometabolic disease. Urine output was increased in BPF-exposed rats, driving a trend in increased creatinine clearance. We also report the first relationship between a bisphenol metabolizing enzyme and a bisphenol-induced phenotype. Preliminary heritability estimates of significant phenotypes suggest that BPF exposure may alter trait variation. These findings support BPF exposure as a cardiometabolic disease risk factor and indicate that the HS rat will be a useful model for dissecting gene by BPF interactions on metabolic health.

Systematic Review and Methodological Considerations for the Use of Single Prolonged Stress and Fear Extinction Retention in Rodents

Posttraumatic stress disorder (PTSD) is a mental health condition triggered by experiencing or witnessing a terrifying event that can lead to lifelong burden that increases mortality and adverse health outcomes. Yet, no new treatments have reached the market in two decades. Thus, screening potential interventions for PTSD is of high priority. Animal models often serve as a critical translational tool to bring new therapeutics from bench to bedside. However, the lack of concordance of some human clinical trial outcomes with preclinical animal efficacy findings has led to a questioning of the methods of how animal studies are conducted and translational validity established. Thus, we conducted a systematic review to determine methodological variability in studies that applied a prominent animal model of trauma-like stress, single prolonged stress (SPS). The SPS model has been utilized to evaluate a myriad of PTSD-relevant outcomes including extinction retention. Rodents exposed to SPS express an extinction retention deficit, a phenotype identified in humans with PTSD, in which fear memory is aberrantly retained after fear memory extinction. The current systematic review examines methodological variation across all phases of the SPS paradigm, as well as strategies for behavioral coding, data processing, statistical approach, and the depiction of data. Solutions for key challenges and sources of variation within these domains are discussed. In response to methodological variation in SPS studies, an expert panel was convened to generate methodological considerations to guide researchers in the application of SPS and the evaluation of extinction retention as a test for a PTSD-like phenotype. Many of these guidelines are applicable to all rodent paradigms developed to model trauma effects or learned fear processes relevant to PTSD, and not limited to SPS. Efforts toward optimizing preclinical model application are essential for enhancing the reproducibility and translational validity of preclinical findings, and should be conducted for all preclinical psychiatric research models.

Integration of evidence across human and model organism studies: A meeting report

The National Institute on Drug Abuse and Joint Institute for Biological Sciences at the Oak Ridge National Laboratory hosted a meeting attended by a diverse group of scientists with expertise in substance use disorders (SUDs), computational biology, and FAIR (Findability, Accessibility, Interoperability, and Reusability) data sharing. The meeting's objective was to discuss and evaluate better strategies to integrate genetic, epigenetic, and 'omics data across human and model organisms to achieve deeper mechanistic insight into SUDs. Specific topics were to (a) evaluate the current state of substance use genetics and genomics research and fundamental gaps, (b) identify opportunities and challenges of integration and sharing across species and data types, (c) identify current tools and resources for integration of genetic, epigenetic, and phenotypic data, (d) discuss steps and impediment related to data integration, and (e) outline future steps to support more effective collaboration-particularly between animal model research communities and human genetics and clinical research teams. This review summarizes key facets of this catalytic discussion with a focus on new opportunities and gaps in resources and knowledge on SUDs.

Sensitivity to food and cocaine cues are independent traits in a large sample of heterogeneous stock rats

Sensitivity to cocaine and its associated stimuli ("cues") are important factors in the development and maintenance of addiction. Rodent studies suggest that this sensitivity is related, in part, to the propensity to attribute incentive salience to food cues, which, in turn, contributes to the maintenance of cocaine self-administration, and cue-induced relapse of drug-seeking. Whereas each of these traits has established links to drug use, the relatedness between the individual traits themselves has not been well characterized in preclinical models. To this end, the propensity to attribute incentive salience to a food cue was first assessed in two distinct cohorts of 2716 outbred heterogeneous stock rats (HS; formerly N:NIH). We then determined whether each cohort was associated with performance in one of two paradigms (cocaine conditioned cue preference and cocaine contextual conditioning). These measure the unconditioned locomotor effects of cocaine, as well as conditioned approach and the locomotor response to a cocaine-paired floor or context. There was large individual variability and sex differences among all traits, but they were largely independent of one another in both males and females. These findings suggest that these traits may contribute to drug-use via independent underlying neuropsychological processes.

The synthetically produced predator odor 2,5-dihydro-2,4,5-trimethylthiazoline increases alcohol self-administration and alters basolateral amygdala response to alcohol in rats

Post-traumatic stress disorder (PTSD) is a psychiatric illness that can increase the risk for developing an alcohol use disorder (AUD). While clinical data has been useful in identifying similarities in the neurobiological bases of these disorders, preclinical models are essential for understanding the mechanism(s) by which stressors increase the risk for escalated alcohol consumption. The purpose of these studies was to examine if exposure of male Long-Evans rats to the synthetically derived predator odor 2,5-dihydro-2,4,5-trimethylthiazoline (TMT; a component of fox feces) would increase sweetened alcohol self-administration, potentially by facilitating transfer of salience towards cues, and alter neuronal response to alcohol as measured by the immediate early gene c-Fos. In experiment 1, rats exposed to repeated (4×) TMT showed reductions in port entries in Pavlovian conditioned approach and increases in sweetened alcohol self-administration. In experiment 2, rats exposed to repeated TMT showed blunted basolateral amygdala c-Fos response to alcohol. In experiment 3, rats exposed to single, but not repeated TMT, showed increases in sweetened alcohol self-administration, and no change in anxiety-like behavior or hyperarousal. In experiment 4, rats continued to show a significant corticosterone response to TMT after repeated exposures. In summary, exposure of male rats to TMT can cause escalations in sweetened alcohol self-administration and reduction in BLA response to alcohol. These studies outline and utilize a novel preclinical model that can be used to further neurobiological understanding of the emergence of escalated alcohol consumption following stress exposure.

Vendor differences in anxiety-like behaviors in female and male Sprague Dawley rats

Although Sprague Dawley outbred rats are commonly used in behavioral, physiological, and pharmacological studies, dramatic differences in responses may emerge from rats obtained from different suppliers even when sex, age, and environmental conditions are maintained constant. In the present study, we compared behavioral responses on three tests related to anxiety of Sprague Dawley female and male rats obtained from three different vendors in the United States: Charles River, Envigo, and Taconic. All rats were tested in the open field, light-dark box, and elevated zero maze. We found reduced time spent in the center area of the open field and decreased light compartment duration in the light-dark box test in female and male rats from Taconic compared to Charles River and Envigo rats, suggesting anxiety-like behaviors differ between the three vendors. No vendor differences were found on performance in the elevated zero maze. Furthermore, the contribution of stress hormones to vendor differences was examined by measuring serum corticosterone levels in rats 30 min after exposure to the elevated zero maze. There were no vendor differences in corticosterone levels, suggesting that endogenous levels of stress hormones most likely did not contribute to vendor differences in anxiety-like behaviors. Collectively, these findings highlight the importance of vendor selection of the Sprague Dawley stock for research involving behavioral tests related to anxiety.

Sign tracking predicts cue-induced but not drug-primed reinstatement to methamphetamine seeking in rats: Effects of oxytocin treatment

BACKGROUND

The incentive sensitisation theory of addiction posits that drug-associated stimuli become imbued with incentive motivational properties, driving pathological drug seeking. However, pre-existing variability in the incentive salience to non-drug reward cues ('sign trackers' (STs); 'goal trackers' (GTs)) is also predictive of the desire for and relapse to cocaine and opioids. Here, we asked whether variation in propensity to attribute incentive salience to a food cue is predictive of reinstatement to the highly addictive psychostimulant methamphetamine (METH), and whether treatment with the promising anti-addiction therapy oxytocin differentially reduces METH behaviour between STs and GTs.

METHODS

Rats were trained to associate a Pavlovian cue with delivery of a sucrose pellet over 8 days. They then received jugular vein catheters for intravenous METH self-administration, followed by behavioural extinction, and cue-induced and METH-primed reinstatement to METH-seeking behaviours. Oxytocin was administered prior to self-administration and reinstatement tests.

RESULTS

Despite the self-administration of similar amounts of METH, STs reinstated more to METH cues than did GTs, yet METH-priming reinstated STs and GTs similarly. Furthermore, oxytocin attenuated cue-induced reinstatement more so in STs than in GTs, and reduced METH-primed reinstatement to a greater extent in the top quartile of reinstaters, indicating that oxytocin treatment may be most effective for those at highest risk of addiction.

CONCLUSIONS

This pre-existing bias towards reward cues presents a possible tool to screen for METH addiction susceptibility and may be useful for understanding the neurobiology of addiction and for pharmacotherapeutic discovery.

Individual variation in the vigor and form of Pavlovian conditioned responses: Analysis of a model system

Pavlovian conditioning results in individual variation in the vigor and form of acquired behaviors. Here, we describe a general-process model of associative learning (HeiDI; How excitation and inhibition determine ideo-motion) that provides an analysis for such variation together with a range of other important group-level phenomena. The model takes as its starting point the idea that pairings of a conditioned stimulus (CS) and an unconditioned stimulus (US) result in the formation of reciprocal associations between their central representations. The asymptotic values of these associations and the rate at which these are reached are held to be influenced by the perceived salience of the CS (αCS) and US (βUS). Importantly, whether this associative knowledge is exhibited in behavior that reflects the properties of the CS (e.g., sign-tracking) or US (e.g., goal-tracking) is also influenced by the relative values of αCS and βUS. In this way, HeiDI provides an analysis for both quantitative and qualitative individual differences generated by Pavlovian conditioning procedures.

Ultrasonic vocalizations and individual differences in rats performing a Pavlovian conditioned approach task

Rats emit distinct types of ultrasonic vocalizations (USV), including high-frequency 50-kHz USV, which occur in appetitive situations. Such 50-kHz USV are thought to reflect positive affective states, for example in case of reward anticipation, and are linked to dopamine signaling. The present study was conducted to investigate whether rats emit 50-kHz USV during a Pavlovian conditioned approach task and whether trait-like differences in 50-kHz USV emission are associated with sign- versus goal-tracking. We hypothesize that individuals engaging more with a cue predicting a food reward will also elicit more 50-kHz USV. In order to test this, we investigated 34 female rats and gauged USV while they underwent a Pavlovian conditioned approach training and extinction paradigm. For one, we found a high subject-dependent variability in the emission of 50-kHz calls. These were not largely affected by state differences, since these 50-kHz USV were observed throughout the task. During task progress and in most subjects, there was a rather complete shift toward goal-tracking, but subjects engaging more with the cue predicting a reward also emitted higher numbers of appetitive 50-kHz calls. This supports the hypothesis that sign-tracking is positively associated with the emission of 50-kHz USV. The high subject-dependent variability in the emission of 50-kHz calls warrants special attention in future appetitive studies.

Adapting Genotyping-by-Sequencing and Variant Calling for Heterogeneous Stock Rats

The heterogeneous stock (HS) is an outbred rat population derived from eight inbred rat strains. HS rats are ideally suited for genome wide association studies; however, only a few genotyping microarrays have ever been designed for rats and none of them are currently in production. To address the need for an efficient and cost effective method of genotyping HS rats, we have adapted genotype-by-sequencing (GBS) to obtain genotype information at large numbers of single nucleotide polymorphisms (SNPs). In this paper, we have outlined the laboratory and computational steps we took to optimize double digest genotype-by-sequencing (ddGBS) for use in rats. We evaluated multiple existing computational tools and explain the workflow we have used to call and impute over 3.7 million SNPs. We have also compared various rat genetic maps, which are necessary for imputation, including a recently developed map specific to the HS. Using our approach, we obtained concordance rates of 99% with data obtained using data from a genotyping array. The principles and computational pipeline that we describe could easily be adapted for use in other species for which reliable reference genome sets are available.

Unpredictability as a modulator of drug self-administration: Relevance for substance-use disorders

Drug self-administration has been regarded as a gold-standard preclinical model of addiction and substance-use disorder (SUD). However, investigators are becoming increasingly aware, that certain aspects of addiction or SUDs experienced by humans are not accurately captured in our preclinical self-administration models. The current review will focus on two such aspects of current preclinical drug self-administration models: 1) Predictable vs. unpredictable drug access in terms of the time and effort put into obtaining drugs (i.e., response requirement) and drug quality (i.e., amount) and 2) rich vs. lean access to drugs. Some behavioral and neurobiological mechanisms that could contribute to excessive allocation of behavior toward drug-seeking and drug-taking at the expense of engaging in nondrug-related activities are discussed, and some directions for future research are identified. Based on the experiments reviewed, lean and unpredictable drug access could worsen drug-seeking and drug-taking behavior in individuals with SUDs. Once more fully explored, this area of research will help determine whether and how unpredictable and lean cost requirements affect drug self-administration in preclinical laboratory studies with nonhuman subjects and will help determine whether incorporating these conditions in current self-administration models will increase their predictive validity.

Mapping sign-tracking and goal-tracking onto human behaviors

As evidenced through classic Pavlovian learning mechanisms, environmental cues can become incentivized and influence behavior. These stimulus-outcome associations are relevant in everyday life but may be particularly important for the development of impulse control disorders including addiction. Rodent studies have elucidated specific learning profiles termed 'sign-tracking' and 'goal-tracking' which map onto individual differences in impulsivity and other behaviors associated with impulse control disorders' etiology, course, and relapse. Whereas goal-trackers are biased toward the outcome, sign-trackers fixate on features that are associated with but not necessary for achieving an outcome; a pattern of behavior that often leads to escalation of reward-seeking that can be maladaptive. The vast majority of the sign- and goal-tracking research has been conducted using rodent models and very few have bridged this concept into the domain of human behavior. In this review, we discuss the attributes of sign- and goal-tracking profiles, how these are manifested neurobiologically, and how these distinct learning styles could be an important tool for clinical interventions in human addiction.

Sign-tracking behavior is sensitive to outcome devaluation in a devaluation context-dependent manner: implications for analyzing habitual behavior

Motivationally attractive cues can draw in behavior in a phenomenon termed incentive salience. Incentive cue attraction is an important model for animal models of drug seeking and relapse. One question of interest is the extent to which the pursuit of motivationally attractive cues is related to the value of the paired outcome or can become unrelated and habitual. We studied this question using a sign-tracking (ST) paradigm in rats, in which a lever stimulus preceding food reward comes to elicit conditioned lever-interaction behavior. We asked whether reinforcer devaluation by means of conditioned taste aversion, a classic test of habitual behavior, can modify ST to incentive cues, and whether this depends upon the manner in which reinforcer devaluation takes place. In contrast to several recent reports, we conclude that ST is indeed sensitive to reinforcer devaluation. However, this effect depends critically upon the congruence between the context in which taste aversion is learned and the context in which it is tested. When the taste aversion successfully transfers to the testing context, outcome value strongly influences ST behavior, both when the outcome is withheld (in extinction) and when animals can learn from outcome feedback (reacquisition). When taste aversion does not transfer to the testing context, ST remains high. In total, the extent to which ST persists after outcome devaluation is closely related to the extent to which that outcome is truly devalued in the task context. We believe this effect of context on devaluation can reconcile contradictory findings about the flexibility/inflexibility of ST. We discuss this literature and relate our findings to the study of habits generally.

Genome-Wide Association Study in Two Cohorts from a Multi-generational Mouse Advanced Intercross Line Highlights the Difficulty of Replication Due to Study-Specific Heterogeneity

There has been extensive discussion of the "Replication Crisis" in many fields, including genome-wide association studies (GWAS). We explored replication in a mouse model using an advanced intercross line (AIL), which is a multigenerational intercross between two inbred strains. We re-genotyped a previously published cohort of LG/J x SM/J AIL mice (F34; n = 428) using a denser marker set and genotyped a new cohort of AIL mice (F39-43; n = 600) for the first time. We identified 36 novel genome-wide significant loci in the F34 and 25 novel loci in the F39-43 cohort. The subset of traits that were measured in both cohorts (locomotor activity, body weight, and coat color) showed high genetic correlations, although the SNP heritabilities were slightly lower in the F39-43 cohort. For this subset of traits, we attempted to replicate loci identified in either F34 or F39-43 in the other cohort. Coat color was robustly replicated; locomotor activity and body weight were only partially replicated, which was inconsistent with our power simulations. We used a random effects model to show that the partial replications could not be explained by Winner's Curse but could be explained by study-specific heterogeneity. Despite this heterogeneity, we performed a mega-analysis by combining F34 and F39-43 cohorts (n = 1,028), which identified four novel loci associated with locomotor activity and body weight. These results illustrate that even with the high degree of genetic and environmental control possible in our experimental system, replication was hindered by study-specific heterogeneity, which has broad implications for ongoing concerns about reproducibility.

Individual variation in the attribution of incentive salience to social cues

Research on the attribution of incentive salience to drug cues has furthered our understanding of drug self-administration in animals and addiction in humans. The influence of social cues on drug-seeking behavior has garnered attention recently, but few studies have investigated how social cues gain incentive-motivational value. In the present study, a Pavlovian conditioned approach (PCA) procedure was used to identify rats that are more (sign-trackers; STs) or less (goal-trackers; GTs) prone to attribute incentive salience to food reward cues. In Experiment 1, a novel procedure employed social ‘peers’ to compare the tendency of STs and GTs to attribute incentive salience to social reward cues as well as form a social-conditioned place preference. In Experiment 2, social behavior of STs and GTs was compared using social interaction and choice tests. Finally, in Experiment 3, levels of plasma oxytocin were measured in STs and GTs seven days after the last PCA training session, because oxytocin is known to modulate the mesolimbic reward system and social behavior. Compared to GTs, STs attributed more incentive salience to social-related cues and exhibited prosocial behaviors (e.g., social-conditioned place preference, increased social interaction, and social novelty-seeking). No group differences were observed in plasma oxytocin levels. Taken together, these experiments demonstrate individual variation in the attribution of incentive salience to both food- and social-related cues, which has important implications for the pathophysiology of addiction.

Revisiting the validity of the mouse tail suspension test: Systematic review and meta-analysis of the effects of prototypic antidepressants

Animal models in neuropsychiatric research need validation. One way to address external validity is systematic reviews and meta-analyses. The present study presents a meta-analysis of the effects of antidepressants in the mouse tail suspension test (TST). A PubMed search identified studies that examined imipramine and fluoxetine effects in the TST. Inclusion criteria were testing in the light phase; trial duration was six minutes; immobility time scored 6 or (last) 4 min; adult mice; acute intraperitoneal (IP) administration. Effect sizes (ES) were estimated using Cohen's d, heterogeneity of ES with Cochran's Q test, correlations between dose and ES with Pearson's correlation and differences between strains with Analysis of variance. Results show that antidepressants decrease immobility time in the TST and a correlation between drug dose and ES but no effects of strain. We suggest that the TST is a valid tool to quantitatively, consistently and reproducibly capture the immobility-reducing aspects of fluoxetine and imipramine and that the lack of strain effects is due to small number of experiments in many of the strains.

Nesting material enrichment reduces severity of overgrooming-related self-injury in individually housed rats

Individual or singly-housing laboratory rats is common in many animal facilities, but has an adverse impact on the welfare of this social species. It has previously been shown that a small proportion of individually housed mice (∼5%) engage in pathological overgrooming behaviour, but this has not been assessed in rats. We performed an observational study to determine the prevalence of overgrooming-related self-injury and whether providing nesting material enrichment throughout an animal's life would affect the prevalence or severity of overgrooming-related self-injury. Due to protocol differences between projects in our behavioural neuroscience lab, unenriched rats received a nylabone and a shelter (n = 167), while baseline-enriched rats received a nylabone, shelter and shredded paper nesting material throughout experiments (n = 238). Unenriched rats received nesting material enrichment after the onset of overgrooming-related self-injury. Over 18 months, rats were monitored by their experimenters on a daily basis (5-7 days/week over 2-3 months/project) and any cases of overgrooming-related self-injury were recorded. Replicating the findings of previous studies in mice, we observed 20 cases of overgrooming-related self-injury (∼5%) with no difference in prevalence between rats on the basis of supplier, cage position, experimental procedure (behavioural only or involving surgical procedures), reinforcer (ethanol or sugar) or level of baseline-enrichment. While there was no difference in onset severity between rats that were unenriched at baseline and baseline-enriched rats, baseline-enriched rats had lower self-injury severity scores at one-, two- and four-week follow-ups. These results suggest that nesting material enrichment provided throughout an animal's life may reduce overgrooming-related self-injury.

Insulin Treatment Attenuates Small Nerve Fiber Damage in Rat Model of Type 2 Diabetes

INTRODUCTION

Current clinical guidelines for management of diabetic peripheral neuropathy (DPN) emphasize good glycemic control. However, this has limited effect on prevention of DPN in type 2 diabetic (T2D) patients. This study investigates the effect of insulin treatment on development of DPN in a rat model of T2D to assess the underlying causes leading to DPN.

METHODS

Twelve-week-old male Sprague-Dawley rats were allocated to a normal chow diet or a 45% kcal high-fat diet. After eight weeks, the high-fat fed animals received a mild dose of streptozotocin to induce hyperglycemia. Four weeks after diabetes induction, the diabetic animals were allocated into three treatment groups receiving either no insulin or insulin-releasing implants in a high or low dose. During the 12-week treatment period, blood glucose and body weight were monitored weekly, whereas Hargreaves' test was performed four, eight, and 12 weeks after treatment initiation. At study termination, several blood parameters, body composition, and neuropathy endpoints were assessed.

RESULTS

Insulin treatment lowered blood glucose in a dose-dependent manner. In addition, both doses of insulin lowered lipids and increased body fat percentage. High-dose insulin treatment attenuated small nerve fiber damage assessed by Hargreaves' test and intraepidermal nerve fiber density compared to untreated diabetes and low-dose insulin; however, neuropathy was not completely prevented by tight glycemic control. Linear regression analysis revealed that glycemic status, circulating lipids, and sciatic nerve sorbitol level were all negatively associated with the small nerve fiber damage observed.

CONCLUSION

In summary, our data suggest that high-dose insulin treatment attenuates small nerve fiber damage. Furthermore, data also indicate that both poor glycemic control and dyslipidemia are associated with disease progression. Consequently, this rat model of T2D seems to fit well with progression of DPN in humans and could be a relevant preclinical model to use in relation to research investigating treatment opportunities for DPN.

Rodent ketamine depression-related research: Finding patterns in a literature of variability

Discovering that the anesthetic drug ketamine has rapidly acting antidepressant effects in many individuals with major depression is one of the most important findings in clinical psychopharmacology in recent decades. The initial report of these effects in human subjects was based on a foundation of rodent preclinical studies carried out in the 1990s, and subsequent investigation has included both further studies in individuals with depression, as well as reverse translational experiments in animal models, especially rodents. While there is general agreement in the rodent literature that ketamine has rapidly-acting, and generally sustained, antidepressant-like properties, there are also points of contention across studies, including the precise mechanism of action of this drug. In this review, we briefly summarize prominent yet variable findings regarding the mechanism of action. We also discuss a combination of similarities and variances in the rodent literature in the antidepressant-like effects of ketamine as a function of dose, species and strain, test, stressor, and presumably sex of the experimenter. We then present previously unpublished mouse strain comparison data suggesting that subanesthetic ketamine does not have robust antidepressant-like properties in unstressed animals, and may actually promote depression-like behavior, in contrast to widely reported findings. We conclude that the data best support the notion of ketamine action principally via NMDA receptor antagonism, transiently boosting glutamatergic (and possibly other) signaling in diverse brain circuits. We also suggest that future studies should address in greater detail the extent to which antidepressant-like properties of this drug are stress-sensitive, in an effort to better model major depression present in humans.

Neural circuits linking sleep and addiction: Animal models to understand why select individuals are more vulnerable to substance use disorders after sleep deprivation

Individuals differ widely in their drug-craving behaviors. One reason for these differences involves sleep. Sleep disturbances lead to an increased risk of substance use disorders and relapse in only some individuals. While animal studies have examined the impact of sleep on reward circuitry, few have addressed the role of individual differences in the effects of altered sleep. There does, however, exist a rodent model of individual differences in reward-seeking behavior: the sign/goal-tracker model of Pavlovian conditioned approach. In this model, only some rats show the key behavioral traits associated with addiction, including impulsivity and poor attentional control, making this an ideal model system to examine individually distinct sleep-reward interactions. Here, we describe how the limbic neural circuits responsible for individual differences in incentive motivation overlap with those involved in sleep-wake regulation, and how this model can elucidate the common underlying mechanisms. Consideration of individual differences in preclinical models would improve our understanding of how sleep interacts with motivational systems, and why sleep deprivation contributes to addiction in only select individuals.

Using Heterogeneous Stocks for Fine-Mapping Genetically Complex Traits

In this chapter we will review both the rationale and experimental design for using Heterogeneous Stock (HS) populations for fine-mapping of complex traits in mice and rats. We define an HS as an outbred population derived from an intercross between two or more inbred strains. HS have been used to perform genome-wide association studies (GWAS) for multiple behavioral, physiological, and gene expression traits. GWAS using HS require four key steps, which we review: selection of an appropriate HS population, phenotyping, genotyping, and statistical analysis. We provide advice on the selection of an HS, comment on key issues related to phenotyping, discuss genotyping methods relevant to these populations, and describe statistical genetic analyses that are applicable to genetic analyses that use HS.

High-fructose diet initiated during adolescence does not affect basolateral amygdala excitability or affective-like behavior in Sprague Dawley rats

Patients with type-2 diabetes, obesity, and metabolic syndrome have a significantly increased risk of developing depression. Dysregulated metabolism may contribute to the etiology of depression by affecting neuronal activity in key limbic areas. The basolateral amygdala (BLA) acts as a critical emotional valence detector in the brain's limbic circuit, and shows hyperactivity and abnormal glucose metabolism in depressed patients. Furthermore, administering a periadolescent high-fructose diet (HFrD; a model of metabolic syndrome) to male Wistar rats increases anxiety- and depressive-like behavior. Repeated shock stress in Sprague Dawley rats similarly increases anxiety-like behavior and increases BLA excitability. We therefore investigated whether a metabolic stressor (HFrD) would have similar effects as shock stress on BLA excitability in Sprague Dawley rats. We found that a HFrD did not affect the intrinsic excitability of BLA neurons. Fructose-fed Sprague Dawley rats had elevated body fat mass, but did not show increases in metabolic efficiency and fasting blood glucose relative to control. Finally unlike Wistar rats, fructose-fed Sprague Dawley rats did not show increased anxiety- and depressive-like behavior. These results suggest that genetic differences between rat strains may affect susceptibility to a metabolic insult. Collectively, these data show that a periadolescent HFrD disrupts metabolism, but does not change affective behavior or BLA excitability in Sprague Dawley rats.

Incentive salience attribution, "sensation-seeking" and "novelty-seeking" are independent traits in a large sample of male and female heterogeneous stock rats

There are a number of traits that are thought to increase susceptibility to addiction, and some of these are modeled in preclinical studies. For example, "sensation-seeking" is predictive of the initial propensity to take drugs; whereas "novelty-seeking" predicts compulsive drug-seeking behavior. In addition, the propensity to attribute incentive salience to reward cues can predict the propensity to approach drug cues, and reinstatement or relapse, even after relatively brief periods of drug exposure. The question addressed here is the extent to which these three 'vulnerability factors' are related; that is, predictive of one another. Some relationships have been reported in small samples, but here a large sample of 1,598 outbred male and female heterogeneous stock rats were screened for Pavlovian conditioned approach behavior (to obtain an index of incentive salience attribution; 'sign-tracking'), and subsequently tested for sensation-seeking and novelty-seeking. Despite the large N there were no significant correlations between these traits, in either males or females. There were, however, novel relationships between multiple measures of incentive salience attribution and, based on these findings, we generated a new metric that captures "incentive value". Furthermore, there were sex differences on measures of incentive salience attribution and sensation-seeking behavior that were not previously apparent.