Appetitive operant conditioning in mice: heritability and dissociability of training stages

Motivational disturbances in rodent models of neuropsychiatric disorders

Motivated behavior is integral to the survival of individuals, continuously directing actions toward rewards or away from punishments. The orchestration of motivated behavior depends on interactions among different brain circuits, primarily within the dopaminergic system, that subserve the analysis of factors such as the effort necessary for obtaining the reward and the desirability of the reward. Impairments in motivated behavior accompany a wide range of neuropsychiatric disorders, decreasing the patients’ quality of life. Despite its importance, motivation is often overlooked as a parameter in neuropsychiatric disorders. Here, we review motivational impairments in rodent models of schizophrenia, depression, and Parkinson’s disease, focusing on studies investigating effort-related behavior in operant conditioning tasks and on pharmacological interventions targeting the dopaminergic system. Similar motivational disturbances accompany these conditions, suggesting that treatments aimed at ameliorating motivation levels may be beneficial for various neuropsychiatric disorders.

Sex-dependent alterations in behavior, drug responses and dopamine transporter expression in heterozygous DAT-Cre mice

Heterozygous mice that express Cre-recombinase under the dopamine transporter promoter (DAT-Cre knock in mice, or KI) are widely used for targeting midbrain dopamine neurons, under the assumption that their constitutive physiology is not affected. We report here that these mice display striking sex-dependent behavioral and molecular differences in relation to wildtypes (WT). Male and female KI mice were constitutively hyperactive, and male KI mice showed attenuated hyperlocomotor responses to amphetamine. In contrast, female KIs displayed a marked reduction in locomotion ("calming" effect) in response to the same dose of amphetamine. Furthermore, male and female DAT-Cre KI mice showed opposing differences in reinforcement learning, with females showing faster conditioning and males showing slower extinction. Other behavioral variables, including working memory and novelty preference, were not changed compared to WT. These effects were paralleled by differences in striatal DAT expression that disproportionately affected female KI mice. Our findings reveal clear limitations of the DAT-Cre line that must be considered when using this model.

3D Printable Device for Automated Operant Conditioning in the Mouse

Operant conditioning (OC) is a classical paradigm and a standard technique used in experimental psychology in which animals learn to perform an action to achieve a reward. By using this paradigm, it is possible to extract learning curves and measure accurately reaction times (RTs). Both these measurements are proxy of cognitive capabilities and can be used to evaluate the effectiveness of therapeutic interventions in mouse models of disease. Here, we describe a fully 3D printable device that is able to perform OC on freely moving mice, while performing real-time tracking of the animal position. We successfully trained six mice, showing stereotyped learning curves that are highly reproducible across mice and reaching >70% of accuracy after 2 d of conditioning. Different products for OC are commercially available, though most of them do not provide customizable features and are relatively expensive. This data demonstrate that this system is a valuable alternative to available state-of-the-art commercial devices, representing a good balance between performance, cost, and versatility in its use.

Quantitative trait locus mapping and analysis of heritable variation in affiliative social behavior and co-occurring traits

Humans exhibit broad heterogeneity in affiliative social behavior. Twin and family studies show that individual differences in core dimensions of social behavior are heritable, yet there are knowledge gaps in understanding the underlying genetic and neurobiological mechanisms. Animal genetic reference panels (GRPs) provide a tractable strategy for examining the behavioral and genetic architecture of complex traits. Here, using males from 50 mouse strains from the BXD GRP, 4 domains of affiliative social behavior-social approach, social recognition, direct social interaction (DSI) (partner sniffing) and vocal communication-were examined in 2 widely used behavioral tasks-the 3-chamber and DSI tasks. There was continuous and broad variation in social and nonsocial traits, with moderate to high heritability of social approach sniff preference (0.31), ultrasonic vocalization (USV) count (0.39), partner sniffing (0.51), locomotor activity (0.54-0.66) and anxiety-like behavior (0.36). Principal component analysis shows that variation in social and nonsocial traits are attributable to 5 independent factors. Genome-wide mapping identified significant quantitative trait loci for USV count on chromosome (Chr) 18 and locomotor activity on Chr X, with suggestive loci and candidate quantitative trait genes identified for all traits with one notable exception-partner sniffing in the DSI task. The results show heritable variation in sociability, which is independent of variation in activity and anxiety-like traits. In addition, a highly heritable and ethological domain of affiliative sociability-partner sniffing-appears highly polygenic. These findings establish a basis for identifying functional natural variants, leading to a new understanding typical and atypical sociability.

Handling method alters the hedonic value of reward in laboratory mice

Mice are the most widely used model species for drug discovery and scientific research. Consequently, it is important to refine laboratory procedures and practices to ensure high standards of welfare and scientific data quality. Recent studies have identified that the standard practice of handling laboratory mice by their tails increases behaviours indicative of anxiety, which can be overcome by handling mice using a tunnel. However, despite clear negative effects on mice’s behaviour, tunnel handling has yet to be widely implemented. In this study, we provide the first evidence that tail handling also reduces mice’s responses to reward. Anhedonia is a core symptom of clinical depression, and is measured in rodents by assessing how they consume a sucrose solution: depressed mice consume less sucrose and the size of their licking bouts when drinking (their ‘lick cluster sizes’) also tend to be smaller. We found that tail handled mice showed more anhedonic responses in both measures compared to tunnel handled mice, indicative of a decreased responsiveness to reward and potentially a more depressive-like state. Our findings have significant implications for the welfare of laboratory mice as well as the design and interpretation of scientific studies, particularly those investigating or involving reward.

A GABAergic Projection from the Centromedial Nuclei of the Amygdala to Ventromedial Prefrontal Cortex Modulates Reward Behavior

The neural circuitry underlying mammalian reward behaviors involves several distinct nuclei throughout the brain. It is widely accepted that the midbrain dopamine (DA) neurons are critical for the reward-related behaviors. Recent studies have shown that the centromedial nucleus of the amygdala (CeMA) has a distinct role in regulating reward-related behaviors. However, the CeMA and ventromedial PFC (vmPFC) interaction in reward regulation remains poorly understood. Here, we identify and dissect a GABAergic projection that originates in the CeMA and terminates in the vmPFC (VGat-Cre^CeMA-vmPFC) using viral-vector-mediated, cell-type-specific optogenetic techniques in mice. Pathway-specific optogenetic activation of the VGat-Cre^CeMA-vmPFC circuit in awake, behaving animals produced a positive, reward-like phenotype in real-time place preference and increased locomotor activity in open-field testing. In sucrose operant conditioning, the photoactivation of these terminals increased nose-poking effort with no effect on licking behavior and robustly facilitated the extinction of operant behavior. However, photoactivation of these terminals did not induce self-stimulation in the absence of an external reward. The results described here suggest that the VGat-Cre^CeMA-vmPFC projection acts to modulate existing reward-related behaviors.

SIGNIFICANCE STATEMENT

Many studies have shown that the interactions between the centromedial nucleus of the amygdala (CeMA) and ventromedial PFC (vmPFC) have critical roles for emotional regulation. However, most studies have associated this circuit with fear and anxiety behaviors and emphasized top-down processing from vmPFC to CeMA. Here, we provide new evidence for bottom-up CeMA to vmPFC influence on reward-related behaviors. Although previous work implicated the CeMA in incentive salience, our results isolate the investigation to a specific CeMA GABAergic projection to the vmPFC. This long-range GABAergic interaction between amygdala and frontal cortex adds a new dimension to the complex regulation of reward-related behaviors.

Early life stress produces compulsive-like, but not impulsive, behavior in females

Adverse experiences during childhood are associated with the development of psychiatric disorders later in life. In particular, childhood abuse and neglect are risk factors for addictive disorders, such as substance misuse and pathological gambling. Impulsivity and compulsivity are key features of these disorders. Therefore, we investigated whether childhood adversity might increase vulnerability for addictive disorders through promotion of compulsive and impulsive behaviors. Rats were exposed to a brief, variable childhood or prepubertal stress protocol (Postnatal Days 25-27), and their behavior in a delay discounting task was compared with that of control animals in adulthood. Prepubertal stress produced compulsive-type behavior in females. Specifically, stressed females displayed inappropriate responses during a choice phase of the task, perseverating with nosepoke responding instead of choosing between 2 levers. Stressed females also showed learning impairments during task training. However, prepubertal stress was not associated with the development of impulsive behavior, as rates of delay discounting were not affected in either sex. Childhood adversity may contribute to the establishment and maintenance of addictive disorders by increasing perseveration in females. Perseverative behavior may therefore provide a viable therapeutic target for preventing the development of addictive disorders in individuals exposed to childhood adversity. These effects were not seen in males, highlighting sex differences in response to early life stress.

Mice in Bion-M 1 space mission: training and selection

After a 16-year hiatus, Russia has resumed its program of biomedical research in space, with the successful 30-day flight of the Bion-M 1 biosatellite (April 19-May 19, 2013). The principal species for biomedical research in this project was the mouse. This paper presents an overview of the scientific goals, the experimental design and the mouse training/selection program. The aim of mice experiments in the Bion-M 1 project was to elucidate cellular and molecular mechanisms, underlying the adaptation of key physiological systems to long-term exposure in microgravity. The studies with mice combined in vivo measurements, both in flight and post-flight (including continuous blood pressure measurement), with extensive in vitro studies carried out shortly after return of the mice and in the end of recovery study. Male C57/BL6 mice group housed in space habitats were flown aboard the Bion-M 1 biosatellite, or remained on ground in the control experiment that replicated environmental and housing conditions in the spacecraft. Vivarium control groups were used to account for housing effects and possible seasonal differences. Mice training included the co-adaptation in housing groups and mice adaptation to paste food diet. The measures taken to co-adapt aggressive male mice in housing groups and the peculiarities of "space" paste food are described. The training program for mice designated for in vivo studies was broader and included behavioral/functional test battery and continuous behavioral measurements in the home-cage. The results of the preliminary tests were used for the selection of homogenous groups. After the flight, mice were in good condition for biomedical studies and displayed signs of pronounced disadaptation to Earth's gravity. The outcomes of the training program for the mice welfare are discussed. We conclude that our training program was effective and that male mice can be successfully employed in space biomedical research.

Cocaine self-administration behavior in inbred mouse lines segregating different capacities for inhibitory control

RATIONALE

Various dimensions of impulsivity have been linked to substance abuse and dependence, both as consequences of, and as predisposing factors to addiction. With respect to the latter, they may be quantitative indicators of liability for substance use disorders (SUD) and aid in determining underlying genetic influences. We have previously determined that inhibitory control over impulsive responding, as measured by a reversal learning task, is heritable and under substantial genetic control, however their role as explaining variables for aspects of SUD have not been well explored.

OBJECTIVE

The aim of this study was to test for an association between genetically determined differences in inhibitory control and addiction-related phenotypes, such that phenotypes of poor inhibitory control would predict propensity for elevated operant drug-seeking and -taking behaviors.

METHODS

Mice from BxD strains with either good reversal learning (GRL) or poor reversal learning (PRL) ability were tested for intravenous cocaine self-administration under FR1, FR2, and FR5 reinforcement schedules. Additionally, locomotor responses to experimenter-delivered cocaine were assessed.

RESULTS

Compared to GRL strains, PRL strains acquired self-administration behavior more rapidly and administered cocaine at greater rates under all schedules of reinforcement, without any differences in discrimination index. In addition, PRL mice also exhibited increased responding during time-out periods. PRL strains also showed larger locomotor responses to 10 or 20 mg/kg injections of cocaine.

CONCLUSIONS

These studies demonstrate that heritable strain differences in inhibitory control do influence drug self-administration, thus suggest that genetically driven impulsivity of this type may predispose susceptibility to drug abuse and addiction.

Sustained attention in mice: expanding the translational utility of the SAT by incorporating the Michigan Controlled Access Response Port (MICARP)

Advances in mouse genetic technology have spurred increasing interest in the development of cognitive tasks for mice. Here, we describe and discuss the modifications necessary to adapt a task for the assessment of sustained attention performance for use in mice, including for taxing the top-down control of such performance. The validity of the Sustained Attention Task (SAT), including the distractor version (dSAT), has previously been demonstrated in rats and humans. This task requires moveable or retractable operanda; insertion of operanda into the operant chambers cues animals to respond to a prior signal or non-signal event, reporting either a hit or a miss, or a correct rejection or false alarm, respectively. Retractable levers did not support sufficiently high and stable levels of performance in mice. Given the widespread use of static nose-poke devices for testing operant performance in mice, we therefore designed and fabricated a retractable nose-poke device. As this device extends into chambers, a hole for nose-poking is slowly opened and closed again as the device retracts (termed the "Michigan Controlled Access Response Port", MICARP). Results describe the effects of variation of signal duration and event rate, trial outcome and trial type probability, effects of mice deprivation levels, and the reliability of SAT and dSAT performance. Mice perform the SAT and dSAT at levels comparable to those observed in rats. This task will be of assistance in expanding the translational usefulness of the SAT and dSAT.