Locomotor- and Reward-Enhancing Effects of Cocaine Are Differentially Regulated by Chemogenetic Stimulation of Gi-Signaling in Dopaminergic Neurons

Chemogenetic Signaling in Space and Time: Considerations for Designing Neuroscience Experiments Using DREADDs

The use of designer receptors exclusively activated by designer drugs (DREADDs) has led to significant advances in our understanding of the neural circuits that govern behavior. By allowing selective control over cellular activity and signaling, DREADDs have become an integral tool for defining the pathways and cellular phenotypes that regulate sleep, pain, motor activity, goal-directed behaviors, and a variety of other processes. In this review, we provide a brief overview of DREADDs and discuss notable discoveries in the neurosciences with an emphasis on circuit mechanisms. We then highlight methodological approaches to achieve pathway specific activation of DREADDs. Finally, we discuss spatial and temporal constraints of DREADDs signaling and how these features can be incorporated into experimental designs to precisely dissect circuits of interest.

Adolescent THC impacts on mPFC dopamine-mediated cognitive processes in male and female rats

Rationale

Adolescent cannabis use is linked to later-life changes in cognition, learning, and memory. Rodent experimental studies suggest Δ9-tetrahydrocannabinol (THC) influences development of circuits underlying these processes, especially in the prefrontal cortex, which matures during adolescence.

Objective

We determined how 14 daily THC injections (5mg/kg) during adolescence persistently impacts medial prefrontal cortex (mPFC) dopamine-dependent cognition.

Methods

In adult Long Evans rats treated as adolescents with THC (AdoTHC), we quantify performance on two mPFC dopamine-dependent reward-based tasks-strategy set shifting and probabilistic discounting. We also determined how acute dopamine augmentation with amphetamine (0, 0.25, 0.5 mg/kg), or specific chemogenetic stimulation of ventral tegmental area (VTA) dopamine neurons and their projections to mPFC impacts probabilistic discounting.

Results

AdoTHC sex-dependently impacts acquisition of cue-guided instrumental reward seeking, but has minimal effects on set-shifting or probabilistic discounting in either sex. When we challenged dopamine circuits acutely with amphetamine during probabilistic discounting, we found reduced discounting of improbable reward options, with AdoTHC rats being more sensitive to these effects than controls. In contrast, neither acute chemogenetic stimulation of VTA dopamine neurons nor pathway-specific chemogenetic stimulation of their projection to mPFC impacted probabilistic discounting in control rats, although stimulation of this cortical dopamine projection slightly disrupted choices in AdoTHC rats.

Conclusions

These studies confirm a marked specificity in the cognitive processes impacted by AdoTHC exposure. They also suggest that some persistent AdoTHC effects may alter amphetamine-induced cognitive changes in a manner independent of VTA dopamine projections to mPFC, or via alterations of non-VTA dopamine neurons.

Spinal projecting neurons in rostral ventromedial medulla co-regulate motor and sympathetic tone

Many behaviors require the coordinated actions of somatic and autonomic functions. However, the underlying mechanisms remain elusive. By opto-stimulating different populations of descending spinal projecting neurons (SPNs) in anesthetized mice, we show that stimulation of excitatory SPNs in the rostral ventromedial medulla (rVMM) resulted in a simultaneous increase in somatomotor and sympathetic activities. Conversely, opto-stimulation of rVMM inhibitory SPNs decreased both activities. Anatomically, these SPNs innervate both sympathetic preganglionic neurons and motor-related regions in the spinal cord. Fiber-photometry recording indicated that the activities of rVMM SPNs correlate with different levels of muscle and sympathetic tone during distinct arousal states. Inhibiting rVMM excitatory SPNs reduced basal muscle and sympathetic tone, impairing locomotion initiation and high-speed performance. In contrast, silencing the inhibitory population abolished muscle atonia and sympathetic hypoactivity during rapid eye movement (REM) sleep. Together, these results identify rVMM SPNs as descending spinal projecting pathways controlling the tone of both the somatomotor and sympathetic systems.

Role of dopamine neurons in familiarity

Dopamine neurons signal the salience of environmental stimuli and influence learning, although it is less clear if these neurons also determine the salience of memories. Ventral tegmental area (VTA) dopamine neurons increase their firing in the presence of new objects and reduce it upon repeated, inconsequential exposures, marking the shift from novelty to familiarity. This study investigates how dopamine neuron activity during repeated familiar object exposure affects an animal's preference for new objects in a subsequent novel object recognition (NOR) test. We hypothesize that a single familiarization session will not sufficiently lower dopamine activity, such that the memory of a familiar object remains salient, leading to equal exploration of familiar and novel objects and weaker NOR discrimination. In contrast, multiple familiarization sessions likely suppress dopamine activity more effectively, reducing the salience of the familiar object and enhancing subsequent novelty discrimination. Our experiments in mice indicated that multiple familiarization sessions reduce VTA dopamine neuron activation, as measured by c-Fos expression, and enhance novelty discrimination compared with a single familiarization session. Dopamine neurons that show responsiveness to novelty were primarily located in the paranigral nucleus of the VTA and expressed vesicular glutamate transporter 2 transcripts, marking them as dopamine-glutamate neurons. Chemogenetic inhibition of dopamine neurons during a single session paralleled the effects of multiple sessions, improving NOR. These findings suggest that a critical role of dopamine neurons during the transition from novelty to familiarity is to modulate the salience of an object's memory.

Somatostatin interneurons control the timing of developmental desynchronization in cortical networks

Synchronous neuronal activity is a hallmark of the developing brain. In the mouse cerebral cortex, activity decorrelates during the second week of postnatal development, progressively acquiring the characteristic sparse pattern underlying the integration of sensory information. The maturation of inhibition seems critical for this process, but the interneurons involved in this crucial transition of network activity in the developing cortex remain unknown. Using in vivo longitudinal two-photon calcium imaging during the period that precedes the change from highly synchronous to decorrelated activity, we identify somatostatin-expressing (SST+) interneurons as critical modulators of this switch in mice. Modulation of the activity of SST+ cells accelerates or delays the decorrelation of cortical network activity, a process that involves regulating the maturation of parvalbumin-expressing (PV+) interneurons. SST+ cells critically link sensory inputs with local circuits, controlling the neural dynamics in the developing cortex while modulating the integration of other interneurons into nascent cortical circuits.

Bar press durations as a reliable and robust measure of frustration-related operant behavior: Sensitivity to incentive downshift and dose-response paradigms

In humans, frustrating experiences are known to trigger relapse events and individuals with higher frustration intolerance show increased risk of developing substance use disorders (SUDs). Despite this clear relationship, frustration-related behavior is seldom studied concurrently with self-administration behavior in rodent models. A major obstacle has been the lack of robust, quantitative assays of frustration-related operant behavior thus far. In previous work, we identified increased bar press (BP) durations in response to frustrating conditions in rats self-administering natural or drug rewards. Here, to propose BP durations as a measure of frustration-related behavior, we conducted an operant successive negative contrast (oSNC) study and found that increases in BP durations are observed in the absence of increased effort, providing evidence that this is a psychological phenomenon. Moreover, we assess the viability of widespread use of BP duration measurements as a behavioral tool by quantifying performance as it pertains to sensitivity, robustness, replicability, and sex differences. We conclude that increases in BP durations are a highly sensitive psychological response to frustrating conditions and that this measure is robust, replicable, and applicable to both sexes.

Role of Dopamine Neurons in Familiarity

Dopamine neurons signal the salience of environmental stimuli, influencing learning and motivation. However, research has not yet identified whether dopamine neurons also modulate the salience of memory content. Dopamine neuron activity in the ventral tegmental area (VTA) increases in response to novel objects and diminishes as objects become familiar through repeated presentations. We proposed that the declined rate of dopamine neuron activity during familiarization affects the salience of a familiar object's memory. This, in turn, influences the degree to which an animal distinguishes between familiar and novel objects in a subsequent novel object recognition (NOR) test. As such, a single familiarization session may not sufficiently reduce dopamine activity, allowing the memory of a familiar object to maintain its salience and potentially attenuating NOR. In contrast, multiple familiarization sessions could lead to more pronounced dopamine activity suppression, strengthening NOR. Our data in mice reveals that, compared to a single session, multiple sessions result in decreased VTA dopamine neuron activation, as indicated by c-Fos measurements, and enhanced novelty discrimination. Critically, when VTA dopamine neurons are chemogenetically inhibited during a single familiarization session, NOR improves, mirroring the effects of multiple familiarization sessions. In summary, our findings highlight the pivotal function of dopamine neurons in familiarity and suggest a role in modulating the salience of memory content.

Comparison of three DREADD agonists acting on Gq-DREADDs in the ventral tegmental area to alter locomotor activity in tyrosine hydroxylase:Cre male and female rats

RATIONALE

Despite the increasingly pervasive use of chemogenetic tools in preclinical neuroscience research, the in vivo pharmacology of DREADD agonists remains poorly understood. The pharmacological effects of any ligand acting at receptors, engineered or endogenous, are influenced by numerous factors including potency, time course, and receptor selectivity. Thus, rigorous comparison of the potency and time course of available DREADD ligands may provide an empirical foundation for ligand selection.

OBJECTIVES

Compare the behavioral pharmacology of three different DREADD ligands clozapine-N-oxide (CNO), compound 21 (C21), and deschloroclozapine (DCZ) in a locomotor activity assay in tyrosine hydroxylase:cre recombinase (TH:Cre) male and female rats.

METHODS

Locomotor activity in nine adult TH:Cre Sprague-Dawley rats (5 female, 4 male) was monitored for two hours following administration of d-amphetamine (vehicle, 0.1-3.2 mg/kg, IP), DCZ (vehicle, 0.32-320 µg/kg, IP), CNO (vehicle, 0.32-10 mg/kg), and C21 (vehicle, 0.1-3.2 mg/kg, IP). Behavioral sessions were conducted twice per week prior to and starting three weeks after bilateral intra-VTA hM3Dq DREADD virus injection.

RESULTS

d-Amphetamine significantly increased locomotor activity pre- and post-DREADD virus injection. DCZ, CNO, and C21 did not alter locomotor activity pre-DREADD virus injection. There was no significant effect of DCZ, CNO, and C21 on locomotor activity post-DREADD virus injection; however, large individual differences in both behavioral response and receptor expression were observed.

CONCLUSIONS

Large individual variability was observed in both DREADD agonist behavioral effects and receptor expression. These results suggest further basic research would facilitate the utility of these chemogenetic tools for behavioral neuroscience research.

Dopamine Neuron Activity and Stress Signaling as Links Between Social Hierarchy and Psychopathology Vulnerability

BACKGROUND

Social status in humans, generally reflected by socioeconomic status, has been associated, when constrained, with heightened vulnerability to pathologies including psychiatric diseases. Social hierarchy in mice translates into individual and interdependent behavioral strategies of animals within a group. The rules leading to the emergence of a social organization are elusive, and detangling the contribution of social status from other factors, whether environmental or genetic, to normal and pathological behaviors remains challenging.

METHODS

We investigated the mechanisms shaping the emergence of a social hierarchy in isogenic C57BL/6 mice raised in groups of 4 using conditional mutant mouse models and chemogenetic manipulation of dopamine midbrain neuronal activity. We further studied the evolution of behavioral traits and the vulnerability to psychopathological-like phenotypes according to the social status of the animals.

RESULTS

Higher sociability predetermined higher social hierarchy in the colony. Upon hierarchy establishment, higher-ranked mice showed increased anxiety and better cognitive abilities in a working memory task. Strikingly, the higher-ranked mice displayed a reduced activity of dopaminergic neurons within the ventral tegmental area, paired with a decreased behavioral response to cocaine and a decreased vulnerability to depressive-like behaviors following repeated social defeats. The pharmacogenetic inhibition of this neuronal population and the genetic inactivation of glucocorticoid receptor signaling in dopamine-sensing brain areas that resulted in decreased dopaminergic activity promoted accession to higher social ranks.

CONCLUSIONS

Dopamine activity and its modulation by the stress response shapes social organization in mice, potentially linking interindividual and social status differences in vulnerability to psychopathologies.

Chronic corticosterone improves perseverative behavior in mice during sequential reversal learning

BACKGROUND

Stressful life events can both trigger development of psychiatric disorders and promote positive behavioral changes in response to adversities. The relationship between stress and cognitive flexibility is complex, and conflicting effects of stress manifest in both humans and laboratory animals.

OBJECTIVE

To mirror the clinical situation where stressful life events impair mental health or promote behavioral change, we examined the post-exposure effects of stress on cognitive flexibility in mice.

METHODS

We tested female C57BL/6JOlaHsd mice in the touchscreen-based sequential reversal learning test. Corticosterone (CORT) was used as a model of stress and was administered in the drinking water for two weeks before reversal learning. Control animals received drinking water without CORT. Behaviors in supplementary tests were included to exclude non-specific confounding effects of CORT and improve interpretation of the results.

RESULTS

CORT-treated mice were similar to controls on all touchscreen parameters before reversal. During the low accuracy phase of reversal learning, CORT reduced perseveration index, a measure of perseverative responding, but did not affect acquisition of the new reward contingency. This effect was not related to non-specific deficits in chamber activity. CORT increased anxiety-like behavior in the elevated zero maze test and repetitive digging in the marble burying test, reduced locomotor activity, but did not affect spontaneous alternation behavior.

CONCLUSION

CORT improved cognitive flexibility in the reversal learning test by extinguishing prepotent responses that were no longer rewarded, an effect possibly related to a stress-mediated increase in sensitivity to negative feedback that should be confirmed in a larger study.

The DREADDful Hurdles and Opportunities of the Chronic Chemogenetic Toolbox

The chronic character of chemogenetics has been put forward as one of the assets of the technique, particularly in comparison to optogenetics. Yet, the vast majority of chemogenetic studies have focused on acute applications, while repeated, long-term neuromodulation has only been booming in the past few years. Unfortunately, together with the rising number of studies, various hurdles have also been uncovered, especially in relation to its chronic application. It becomes increasingly clear that chronic neuromodulation warrants caution and that the effects of acute neuromodulation cannot be extrapolated towards chronic experiments. Deciphering the underlying cellular and molecular causes of these discrepancies could truly unlock the chronic chemogenetic toolbox and possibly even pave the way for chemogenetics towards clinical application. Indeed, we are only scratching the surface of what is possible with chemogenetic research. For example, most investigations are concentrated on behavioral read-outs, whereas dissecting the underlying molecular signature after (chronic) neuromodulation could reveal novel insights in terms of basic neuroscience and deregulated neural circuits. In this review, we highlight the hurdles associated with the use of chemogenetic experiments, as well as the unexplored research questions for which chemogenetics offers the ideal research platform, with a particular focus on its long-term application.

Divergent brainstem opioidergic pathways that coordinate breathing with pain and emotions

Breathing can be heavily influenced by pain or internal emotional states, but the neural circuitry underlying this tight coordination is unknown. Here we report that Oprm1 (μ-opioid receptor)-expressing neurons in the lateral parabrachial nucleus (PBL) are crucial for coordinating breathing with affective pain in mice. Individual PBL^Oprm1 neuronal activity synchronizes with breathing rhythm and responds to noxious stimuli. Manipulating PBL^Oprm1 activity directly changes breathing rate, affective pain perception, and anxiety. Furthermore, PBL^Oprm1 neurons constitute two distinct subpopulations in a "core-shell" configuration that divergently projects to the forebrain and hindbrain. Through non-overlapping projections to the central amygdala and pre-Bötzinger complex, these two subpopulations differentially regulate breathing, affective pain, and negative emotions. Moreover, these subsets form recurrent excitatory networks through reciprocal glutamatergic projections. Together, our data define the divergent parabrachial opioidergic circuits as a common neural substrate that coordinates breathing with various sensations and behaviors such as pain and emotional processing.

Chemogenetic inhibition of trigeminal ganglion neurons attenuates behavioural and neural pain responses in a model of trigeminal neuropathic pain

BACKGROUND

Nerve injury can lead to ectopic activation of injured nociceptorsand central sensitization characterized by allodynia and hyperalgesia. Reduction in the activity of primary afferent neurons has been shown to be sufficient in alleviating peripherally generated pain. The cell bodies of such trigeminal nociceptors are located in the trigeminal ganglia (TG) with central processes that terminate in the brainstem trigeminal nucleus caudalis (TNC). The TG is therefore a strategic locus where afferent input can be manipulated. We hypothesized that chemogenetic inhibition of TG would suppress TNC neuronal activity and attenuate pain behaviour in a rat model of painful traumatic trigeminal neuropathy (PTTN).

METHODS

Trigeminal neuropathic pain was induced in adult male Sprague-Dawley rats (n = 24) via chronic constriction injury to the infraorbital nerve (ION-CCI). Naïve and sham rats were used as controls (n = 20/group). Rats within each group received TG-directed microinjections of AAV virus containing either the inhibitory hM4Di-DREADD construct or EGFP.

RESULTS

In the ION-CCI group, systemic administration of the DREADD agonist clozapine N-oxide (CNO) reversed the hypersensitivity phenotype in animals expressing hM4Di but not EGFP. CNO-mediated activation of hM4Di DREADD in ION-CCI animals was also associated with reduced Fos expression in the TNC elicited by repeated mechanical stimulation of the dermatome ipsilateral to the injury. There was no effect of CNO on pain behaviour or TNC Fos expression in eGFP animals.

CONCLUSION

Our results indicate that DREADDs may offer an effective therapeutic approach for treatment of trigeminal neuropathic pain.

SIGNIFICANCE

Trigeminal neuropathic pain is highly resistant to therapy and we are in dire need of novel approaches. This study provides further evidence for the successful application of DREADDs as an effective tool for modulating central nervous system function. CNO mediated activation of hM4Di-DREADDs in the trigeminal ganglion (TG) attenuates nerve injury induced neuropathic pain by acting on hyperactive TG cells. It also establishes the TG as an effective target to manage pain in the face and head. Accessing the TG in clinical populations is a relatively simple and safe procedure, making this approach highly significant. Moreover, the methodology described here has applications in trigeminal neuropathic pain from traumatic other etiologies and in spinal neuropathic pain. Chronic pain syndromes are characterized by a progressive failure of brain centers to adequately inhibit pain and as these are identified, we may be able to target them for therapy. Therefore, our findings might have wide application in chronic pain syndromes.

VTA MC3R neurons control feeding in an activity- and sex-dependent manner in mice

Increasing evidence indicates that the melanocortin and mesolimbic dopamine (DA) systems interact to regulate feeding and body weight. Because melanocortin-3 receptors (MC3R) are highly expressed in the ventral tegmental area (VTA), we tested whether VTA neurons expressing these receptors (VTA MC3R neurons) control feeding and body weight in vivo. We also tested whether there were sex differences in the ability of VTA MC3R neurons to control feeding, as MC3R -/- mice show sex-dependent alterations in reward feeding and DA levels, and there are clear sex differences in multiple DA-dependent behaviors and disorders. Designer receptors exclusively activated by designer drugs (DREADD) were used to acutely activate and inhibit VTA MC3R neurons and changes in food intake and body weight were measured. Acutely altering the activity of VTA MC3R neurons decreased feeding in an activity- and sex-dependent manner, with acute activation decreasing feeding, but only in females, and acute inhibition decreasing feeding, but only in males. These differences did not appear to be due to sex differences in the number of VTA MC3R neurons, the ability of hM3Dq to activate VTA MC3R neurons, or the proportion of VTA MC3R neurons expressing tyrosine hydroxylase (TH). These studies demonstrate an important role for VTA MC3R neurons in the control of feeding and reveal important sex differences in behavior, whereby opposing changes in neuronal activity in male and female mice cause similar changes in behavior.

Differential Impact of Inhibitory G-Protein Signaling Pathways in Ventral Tegmental Area Dopamine Neurons on Behavioral Sensitivity to Cocaine and Morphine

Drugs of abuse engage overlapping but distinct molecular and cellular mechanisms to enhance dopamine (DA) signaling in the mesocorticolimbic circuitry. DA neurons of the ventral tegmental area (VTA) are key substrates of drugs of abuse and have been implicated in addiction-related behaviors. Enhanced VTA DA neurotransmission evoked by drugs of abuse can engage inhibitory G-protein-dependent feedback pathways, mediated by GABA_B receptors (GABA_BRs) and D₂ DA receptors (D₂Rs). Chemogenetic inhibition of VTA DA neurons potently suppressed baseline motor activity, as well as the motor-stimulatory effect of cocaine and morphine, confirming the critical influence of VTA DA neurons and inhibitory G-protein signaling in these neurons on this addiction-related behavior. To resolve the relative influence of GABA_BR-dependent and D₂R-dependent signaling pathways in VTA DA neurons on behavioral sensitivity to drugs of abuse, we developed a neuron-specific viral CRISPR/Cas9 approach to ablate D₂R and GABA_BR in VTA DA neurons. Ablation of GABA_BR or D₂R did not impact baseline physiological properties or excitability of VTA DA neurons, but it did preclude the direct somatodendritic inhibitory influence of GABA_BR or D₂R activation. D₂R ablation potentiated the motor-stimulatory effect of cocaine in male and female mice, whereas GABA_BR ablation selectively potentiated cocaine-induced activity in male subjects only. Neither D₂R nor GABA_BR ablation impacted morphine-induced motor activity. Collectively, our data show that cocaine and morphine differ in the extent to which they engage inhibitory G-protein-dependent feedback pathways in VTA DA neurons and highlight key sex differences that may impact susceptibility to various facets of addiction.

The Basolateral Amygdala to Nucleus Accumbens Core Circuit Mediates the Conditioned Reinforcing Effects of Cocaine-Paired Cues on Cocaine Seeking

BACKGROUND

Individuals addicted to cocaine spend much of their time foraging for the drug. Pavlovian drug-associated conditioned stimuli exert a major influence on the initiation and maintenance of drug seeking often long into abstinence, especially when presented response-contingently, acting as conditioned reinforcers that bridge delays to drug use. The acquisition of cue-controlled cocaine seeking has been shown to depend on functional interactions between the basolateral amygdala (BLA) and the nucleus accumbens core (NAcC). However, the precise neuronal circuits underlying the acquisition of cue-controlled cocaine-seeking behavior have not been elucidated.

METHODS

Here, we used a projection-specific Cre-dependent DREADD (designer receptor exclusively activated by designer drugs)-mediated causal approach to test the hypothesis that the direct projections from the BLA to the NAcC are required for the acquisition of cue-controlled cocaine-seeking behavior.

RESULTS

In Sprague Dawley rats with Cre-mediated expression of the inhibitory DREADD hM4D(Gi) in the NAcC-projecting BLA neurons, treatment with clozapine N-oxide, but not vehicle, selectively prevented the impact of cocaine-associated conditioned reinforcers on cocaine seeking under a second-order schedule of reinforcement. This effect was attributable to the chemogenetic inhibition of the NAcC-projecting BLA neurons, as it was reversible, and it was absent in clozapine N-oxide-treated rats expressing an empty control virus. In contrast, chemogenetic inhibition of the anterior insula, which receives collateral projections from NAcC-projecting BLA neurons, was without effect.

CONCLUSIONS

These data demonstrate that the acquisition of cue-controlled cocaine seeking that depends on the conditioned reinforcing effects of cocaine cues requires activity in the direct projections from the BLA to the NAcC.

Impact of Acute and Persistent Excitation of Prelimbic Pyramidal Neurons on Motor Activity and Trace Fear Learning

Drug-induced neuroadaptations in the mPFC have been implicated in addictive behaviors. Repeated cocaine exposure has been shown to increase pyramidal neuron excitability in the prelimbic (PL) region of the mouse mPFC, an adaptation attributable to a suppression of G protein-gated inwardly rectifying K⁺ (GIRK) channel activity. After establishing that this neuroadaptation is not seen in adjacent GABA neurons, we used viral GIRK channel ablation and complementary chemogenetic approaches to selectively enhance PL pyramidal neuron excitability in adult mice, to evaluate the impact of this form of plasticity on PL-dependent behaviors. GIRK channel ablation decreased somatodendritic GABA_B receptor-dependent signaling and rheobase in PL pyramidal neurons. This manipulation also enhanced the motor-stimulatory effect of cocaine but did not impact baseline activity or trace fear learning. In contrast, selective chemogenetic excitation of PL pyramidal neurons, or chemogenetic inhibition of PL GABA neurons, increased baseline and cocaine-induced activity and disrupted trace fear learning. These effects were mirrored in male mice by selective excitation of PL pyramidal neurons projecting to the VTA, but not NAc or BLA. Collectively, these data show that manipulations enhancing the excitability of PL pyramidal neurons, and specifically those projecting to the VTA, recapitulate behavioral hallmarks of repeated cocaine exposure in mice.SIGNIFICANCE STATEMENT Prolonged exposure to drugs of abuse triggers neuroadaptations that promote core features of addiction. Understanding these neuroadaptations and their implications may suggest interventions capable of preventing or treating addiction. While previous work showed that repeated cocaine exposure increased the excitability of pyramidal neurons in the prelimbic cortex (PL), the behavioral implications of this neuroadaptation remained unclear. Here, we used neuron-specific manipulations to evaluate the impact of increased PL pyramidal neuron excitability on PL-dependent behaviors. Acute or persistent excitation of PL pyramidal neurons potentiated cocaine-induced motor activity and disrupted trace fear conditioning, effects replicated by selective excitation of the PL projection to the VTA. Our work suggests that hyperexcitability of this projection drives key behavioral hallmarks of addiction.

Sequential reversal learning: a new touchscreen schedule for assessing cognitive flexibility in mice

RATIONALE

The widespread deficits in cognitive flexibility observed across psychiatric disorders call for improved rodent tests to understand the biology of cognitive flexibility and development of better psychotherapeutics. Current reversal learning paradigms have a forced-choice setup that challenges the interpretation of results.

OBJECTIVES

We aimed at developing a free-choice reversal learning test, where images are presented sequentially and animals are free to move, to enable investigation of the cognitive sub-processes that occur during reversal.

METHODS

Behavior in female C57BL/6JOlaHsd mice was characterized using chronic fluoxetine as a reference compound. Additional tests were included to support the interpretation of results and exclude confounding pharmacological effects. Behaviors in vehicle-treated mice were furthermore analyzed for relatedness to deepen the understanding of parameters measured.

RESULTS

We found that exploitation of the previously rewarded image was independent of exploration and acquisition of the new reward contingency and could be differentially modulated by fluoxetine, supporting recent theories that these processes are not mutually exclusive. Specifically, fluoxetine reduced mistake rate, premature and perseverative responses, and promoted conservative strategies during reversal without affecting hit rate. These effects appeared to be most prominent during the late stage of reversal learning, where accuracy was above chance level. Analysis of behaviors in vehicle-treated mice suggested that exploitation was related to an impulsive-like deficit in response inhibition, while exploration was more related to motivation.

CONCLUSIONS

This new schedule was feasible, easy to implement, and can provide a deeper understanding of the cognitive sub-processes during reversal.

Kisspeptin-8 Induces Anxiety-Like Behavior and Hypolocomotion by Activating the HPA Axis and Increasing GABA Release in the Nucleus Accumbens in Rats

Kisspeptins (Kp) are RF-amide neuropeptide regulators of the reproductive axis that also influence anxiety, locomotion, and metabolism. We aimed to investigate the effects of intracerebroventricular Kp-8 (an N-terminally truncated octapeptide) treatment in Wistar rats. Elevated plus maze (EPM), computerized open field (OF), and marble burying (MB) tests were performed for the assessment of behavior. Serum LH and corticosterone levels were determined to assess kisspeptin1 receptor (Kiss1r) activation and hypothalamic-pituitary-adrenal axis (HPA) stimulation, respectively. GABA release from the nucleus accumbens (NAc) and dopamine release from the ventral tegmental area (VTA) and NAc were measured via ex vivo superfusion. Kp-8 decreased open arm time and entries in EPM, and also raised corticosterone concentration, pointing to an anxiogenic effect. Moreover, the decrease in arm entries in EPM, the delayed increase in immobility accompanied by reduced ambulatory activity in OF, and the reduction in interactions with marbles show that Kp-8 suppressed exploratory and spontaneous locomotion. The increase in GABA release from the NAc might be in the background of hypolocomotion by inhibiting the VTA-NAc dopaminergic circuitry. As Kp-8 raised LH concentration, it could activate Kiss1r and stimulate the reproductive axis. As Kiss1r is associated with hyperlocomotion, it is more likely that neuropeptide FF receptor activation is involved in the suppression of locomotor activity.

Altered baseline and amphetamine-mediated behavioral profiles in dopamine transporter Cre (DAT-Ires-Cre) mice compared to tyrosine hydroxylase Cre (TH-Cre) mice

RATIONALE

Transgenic mouse lines expressing Cre-recombinase under the regulation of either dopamine transporter (DAT) or tyrosine hydroxylase (TH) promoters are commonly used to study the dopamine (DA) system. While use of the TH promoter appears to have less liability to changes in native gene expression, transgene insertion in the DAT locus results in reduced DAT expression and function. This confound is sometimes overlooked in genetically targeted behavioral experiments.

OBJECTIVES

We sought to evaluate the suitability of DAT-Ires-Cre and TH-Cre transgenic lines for behavioral pharmacology experiments with DA agonists. We hypothesized that DAT-Ires-Cre expression would impact DAT-mediated behaviors, but no impact of TH-Cre expression would be observed.

METHODS

DAT-Ires-Cre and TH-Cre mice bred on mixed 129S6/C57BL/6 and pure C57BL/6 backgrounds were evaluated for novelty-induced, baseline, and amphetamine (AMPH)-induced locomotion, and for AMPH and D1 agonist (SKF-38393)-induced preservative behaviors.

RESULTS

DAT-Ires-Cre mice on both mixed 129S6/C57BL/6 and pure C57BL/6 backgrounds displayed increased novelty-induced activity and decreased AMPH-induced locomotion, with mixed results for AMPH-induced stereotypy. TH-Cre mice on both backgrounds showed typical baseline activity and AMPH-induced stereotypy, with a difference in AMPH-induced locomotion observed only on the mixed background. Both transgenic lines displayed unaltered SKF-38393-induced grooming behavior.

CONCLUSIONS

Our findings indicate that the DAT-Ires-Cre transgenic line may lead to confounds for experiments that are dependent on DAT expression. The TH-Cre transgenic line studied here may be a more useful option, depending on background strain, because of its lack of baseline and drug-induced phenotypes. These data highlight the importance of appropriate controls in studies employing transgenic mice.

Effects of brief inhibition of the ventral tegmental area dopamine neurons on the cocaine seeking during abstinence

Preclinical studies strongly suggest that cocaine seeking depends on the neuronal activity of the ventral tegmental area (VTA) and phasic dopaminergic (DA) signaling. Notably, VTA pharmacological inactivation or dopamine receptor blockade in the forebrain may induce behavioral inhibition in general and acute aversive states in particular, thus reducing cocaine seeking indirectly. Such artifacts hinder successful translation of these findings in clinical studies and practice. Here, we aimed to evaluate if dynamic VTA manipulations effectively reduce cocaine seeking. We used male tyrosine hydroxylase (TH) IRES-Cre⁺ rats along with optogenetic tools to inhibit directly and briefly VTA DA neurons during conditioned stimulus (CS)-induced cocaine seeking under extinction conditions. The behavioral effects of optogenetic inhibition were also assessed in the real-time dynamic place aversion, conditioned place aversion, and CS-induced food-seeking tests. We found that brief and nondysphoric/nonsedative pulses of VTA photo-inhibition (1 s every 9 s, ie, for 10% of time) attenuated CS-induced cocaine seeking under extinction conditions in rats expressing archaerhodopsin selectively on the TH⁺ neurons. Furthermore, direct inhibition of the VTA DA activity reduced CS-induced cocaine seeking 24 hours after photo-modulation. Importantly, such effect appears to be selective for cocaine seeking as similar inhibition of the VTA DA activity had no effect on CS-induced food seeking. Thus, briefly inhibiting VTA DA activity during CS-induced cocaine seeking drastically and selectively reduces seeking without behavioral artifacts such as sedation or dysphoria. Our results point to the therapeutic possibilities of coupling nonpharmacologic treatments with extinction training in reducing cocaine addiction.

Chemogenetic Manipulation of Dopamine Neurons Dictates Cocaine Potency at Distal Dopamine Transporters

The reinforcing efficacy of cocaine is largely determined by its capacity to inhibit the dopamine transporter (DAT), and emerging evidence suggests that differences in cocaine potency are linked to several symptoms of cocaine use disorder. Despite this evidence, the neural processes that govern cocaine potency in vivo remain unclear. In male rats, we used chemogenetics with intra-VTA microinfusions of the agonist clozapine-n-oxide to bidirectionally modulate dopamine neurons. Using ex vivo fast scan cyclic voltammetry, pharmacological probes of the DAT, biochemical assessments of DAT membrane availability and phosphorylation, and cocaine self-administration, we tested the effects of chemogenetic manipulations on cocaine potency at distal DATs in the nucleus accumbens as well as the behavioral economics of cocaine self-administration. We discovered that chemogenetic manipulation of dopamine neurons produced rapid, bidirectional modulation of cocaine potency at DATs in the nucleus accumbens. We then provided evidence that changes in cocaine potency are associated with alterations in DAT affinity for cocaine and demonstrated that this change in affinity coincides with DAT conformation biases and changes in DAT phosphorylation state. Finally, we showed that chemogenetic manipulation of dopamine neurons alters cocaine consumption in a manner consistent with changes in cocaine potency at distal DATs. Based on the spatial and temporal constraints inherent to our experimental design, we posit that changes in cocaine potency are driven by alterations in dopamine neuron activity. When considered together, these observations provide a novel mechanism through which GPCRs regulate cocaine's pharmacological and behavioral effects.SIGNIFICANCE STATEMENT Differences in the pharmacological effects of cocaine are believed to influence the development and progression of cocaine use disorder. However, the biological and physiological processes that determine sensitivity to cocaine remain unclear. In this work, we use a combination of chemogenetics, fast scan cyclic voltammetry, pharmacology, biochemistry, and cocaine self-administration with economic demand analysis to demonstrate a novel mechanism by which cocaine potency is determined in vivo These studies identify a novel process by which the pharmacodynamics of cocaine are derived in vivo, and thus this work has widespread implications for understanding the mechanisms that regulate cocaine consumption across stages of addiction.

Neuroadaptations in the dorsal hippocampus underlie cocaine seeking during prolonged abstinence

Relapse vulnerability in substance use disorder is attributed to persistent cue-induced drug seeking that intensifies (or "incubates") during drug abstinence. Incubated cocaine seeking has been observed in both humans with cocaine use disorder and in preclinical relapse models. This persistent relapse vulnerability is mediated by neuroadaptations in brain regions involved in reward and motivation. The dorsal hippocampus (DH) is involved in context-induced reinstatement of cocaine seeking but the role of the DH in cocaine seeking during prolonged abstinence has not been investigated. Here we found that transforming growth factor-β (TGF-β) superfamily member activin A is increased in the DH on abstinence day (AD) 30 but not AD1 following extended-access cocaine self-administration compared to saline controls. Moreover, activin A does not affect cocaine seeking on AD1 but regulates cocaine seeking on AD30 in a bidirectional manner. Next, we found that activin A regulates phosphorylation of NMDA receptor (NMDAR) subunit GluN2B and that GluN2B-containing NMDARs also regulate expression of cocaine seeking on AD30. Activin A and GluN2B-containing NMDARs have both previously been implicated in hippocampal synaptic plasticity. Therefore, we examined synaptic strength in the DH during prolonged abstinence and observed an increase in moderate long-term potentiation (LTP) in cocaine-treated rats compared to saline controls. Lastly, we examined the role of DH projections to the lateral septum (LS), a brain region implicated in cocaine seeking and found that DH projections to the LS govern cocaine seeking on AD30. Taken together, this study demonstrates a role for the DH in relapse behavior following prolonged abstinence from cocaine self-administration.

Prenatal and postnatal alcohol exposure increases vulnerability to cocaine addiction in adult mice

BACKGROUND AND PURPOSE

Alcohol exposure in utero may lead to a wide range of long-lasting morphological and behavioural deficiencies known as fetal alcohol spectrum disorders (FASD), associated with a higher risk of later developing neuropsychiatric disorders. However, little is known about the long-term consequences of cocaine use and abuse in individuals with FASD. This study aimed to investigate the effects of maternal binge alcohol drinking during prenatal and postnatal periods on cocaine reward-related behaviours in adult offspring.

EXPERIMENTAL APPROACH

Pregnant C57BL/6 female mice were exposed to an experimental protocol of binge alcohol consumption (drinking-in-the-dark test) from gestation to weaning. Male offspring were subsequently left undisturbed until reaching adulthood and were tested for cocaine-induced motivational responses (conditioned place preference, behavioural sensitization and operant self-administration). Protein expression of dopamine- and glutamate-related molecules was assessed following cocaine-induced reinstatement.

KEY RESULTS

The results show that prenatal and postnatal alcohol exposure enhanced the preference for the cocaine-paired chamber in the conditioned place preference test. Furthermore, early alcohol-exposed mice displayed attenuated cocaine-induced behavioural sensitization but also higher cocaine self-administration. Furthermore, alterations in glutamatergic excitability (GluA1/GluA2 ratio) and ΔFosB expression were found in the prefrontal cortex and the striatum of alcohol-exposed mice after cocaine-primed reinstatement.

CONCLUSION AND IMPLICATIONS

Our findings demonstrate that maternal binge-like alcohol consumption during gestation and lactation alters sensitivity to the reinforcing effects of cocaine in adult offspring mice. Together, such data suggest that prenatal and postnatal alcohol exposure may underlie an enhanced susceptibility of alcohol-exposed offspring to develop drug addiction later in adulthood.

Newly Developed Dopamine D3 Receptor Antagonists, R-VK4-40 and R-VK4-116, Do Not Potentiate Cardiovascular Effects of Cocaine or Oxycodone in Rats

Opioid and cocaine abuse are major public health burdens. Existing medications for opioid use disorder are limited by abuse liability and side effects, whereas no treatments are currently approved in the United States for cocaine use disorder. Dopamine D₃ receptor (D₃R) antagonists have shown promise in attenuating opioid and cocaine reward and mitigating relapse in preclinical models. However, translation of D₃R antagonists to the clinic has been hampered by reports that the D₃R antagonists GSK598,809 (5-(5-((3-((1S,5R)-1-(2-fluoro-4-(trifluoromethyl)phenyl)-3-azabicyclo[3.1.0]hexan-3-yl)propyl)thio)-4-methyl-4H-1,2,4-triazol-3-yl)-4-methyloxazole) and SB-277,011A (2-(2-((1r,4r)-4-(2-oxo-2-(quinolin-4-yl)ethyl)cyclohexyl)ethyl)-1,2,3,4-tetrahydroisoquinoline-6-carbonitrile) have adverse cardiovascular effects in the presence of cocaine. Recently, we developed two structurally novel D₃R antagonists, R-VK4-40 and R-VK4-116, which are highly selective for D₃R and display translational potential for treatment of opioid use disorder. Here, we tested whether R-VK4-40 ((R)-N-(4-(4-(2-Chloro-3-ethylphenyl)piperazin-1-yl)-3-hydroxybutyl)-1H-indole-2-carboxamide) and R-VK4-116 ((R)-N-(4-(4-(3-Chloro-5-ethyl-2-methoxyphenyl)piperazin-1-yl)-3-hydroxybutyl)-1H-indole-2-carboxamide) have unwanted cardiovascular effects in the presence of oxycodone, a prescription opioid, or cocaine in freely moving rats fitted with surgically implanted telemetry transmitters. We also examined cardiovascular effects of the D₃R antagonist, SB-277,011A, and L-741,626 (1-((1H-indol-3-yl)methyl)-4-(4-chlorophenyl)piperidin-4-ol), a dopamine D₂ receptor-selective antagonist, for comparison. Consistent with prior reports, SB-277,011A increased blood pressure, heart rate, and locomotor activity alone and in the presence of cocaine. L-741,626 increased blood pressure and heart rate. In contrast, R-VK4-40 alone dose-dependently reduced blood pressure and heart rate and attenuated oxycodone-induced increases in blood pressure and oxycodone or cocaine-induced increases in heart rate. Similarly, R-VK4-116 alone dose-dependently reduced cocaine-induced increases in blood pressure and heart rate. These results highlight the safety of new D₃R antagonists and support the continued development of R-VK4-40 and R-VK4-116 for the treatment of opioid and cocaine use disorders. SIGNIFICANCE STATEMENT: Opioid and cocaine abuse are major public health challenges and new treatments that do not adversely impact the cardiovascular system are needed. Here, we show that two structurally novel dopamine D₃ receptor antagonists, R-VK4-40 and R-VK4-116, do not potentiate, and may even protect against, oxycodone- or cocaine-induced changes in blood pressure and heart rate, supporting their further development for the treatment of opioid and/or cocaine use disorders.

Deletion of the Creatine Transporter (Slc6a8) in Dopaminergic Neurons Leads to Hyperactivity in Mice

The lack of cerebral creatine (Cr) causes intellectual disability and epilepsy. In addition, a significant portion of individuals with Cr transporter (Crt) deficiency (CTD), the leading cause of cerebral Cr deficiency syndromes (CCDS), are diagnosed with attention-deficit hyperactivity disorder. While the neurological effects of CTD are clear, the mechanisms that underlie these deficits are unknown. Part of this is due to the heterogenous nature of the brain and the unique metabolic demands of specific neuronal systems. Of particular interest related to Cr physiology are dopaminergic neurons, as many CCDS patients have ADHD and Cr has been implicated in dopamine-associated neurodegenerative disorders, such as Parkinson's and Huntington's diseases. The purpose of this study was to examine the effect of a loss of the Slc6a8 (Crt) gene in dopamine transporter (Slc6a3; DAT) expressing cells on locomotor activity and motor function as the mice age. Floxed Slc6a8 (Slc6a8^flox) mice were mated to DAT^IREScre expressing mice to generate DAT-specific Slc6a8 knockouts (dCrt^-/y). Locomotor activity, spontaneous activity, and performance in the challenging beam test were evaluated monthly in dCrt^-/y and control (Slc6a8^flox) mice from 3 to 12 months of age. dCrt^-/y mice were hyperactive compared with controls throughout testing. In addition, dCrt^-/y mice showed increased rearing and hindlimb steps in the spontaneous activity test. Latency to cross the narrow bridge was increased in dCrt^-/y mice while foot slips were unchanged. Taken together, these data suggest that the lack of Cr in dopaminergic neurons causes hyperactivity while sparing motor function.

G protein-coupled receptor signaling in VTA dopaminergic neurons bidirectionally regulates the acute locomotor response to amphetamine but does not affect behavioral sensitization

Amphetamine (AMPH) acts as a substrate of the dopamine transporter (DAT) and causes a dramatic increase in extracellular dopamine (DA). Upon entering DA neurons, AMPH promotes DA efflux via DAT through a mechanism implicating depletion of DA from vesicular stores, activation of kinase pathways and transporter phosphorylation. Despite the role of intracellular signaling for AMPH action, it remains elusive how the response to AMPH is affected in vivo by metabotropic regulation via G protein coupled receptor signaling pathways. Here, we show by employment of Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) that the acute hyperlocomotor response to AMPH is bidirectionally regulated by metabotropic input to VTA DA neurons with a markedly enhanced response upon activation of a Gs-coupled pathway and a markedly decreased locomotor response upon activation of a Gi-coupled pathway. The unique mechanism of action for AMPH was underlined by the absence of an effect of Gs activation on the locomotor response to the DAT inhibitor cocaine. Regardless of the profound effect on the acute AMPH response, repeated Gs activation or Gi activation did not affect development of AMPH sensitization. Furthermore, activation of a Gs-pathway or activation of a Gi-pathway in DA neurons did not have any effect on the AMPH-induced locomotor response in the AMPH sensitized mice. This suggests induction of alterations in DA neuronal functions that overrule the stimulatory or inhibitory effect of metabotropic input seen in drug-naïve mice. The data thereby underline the remarkable strength of maladaptive changes that occur upon intake of strong psychostimulants. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

Modulating Dopamine Signaling and Behavior with Chemogenetics: Concepts, Progress, and Challenges

For more than 60 years, dopamine (DA) has been known as a critical modulatory neurotransmitter regulating locomotion, reward-based motivation, and endocrine functions. Disturbances in DA signaling have been linked to an array of different neurologic and psychiatric disorders, including Parkinson's disease, schizophrenia, and addiction, but the underlying pathologic mechanisms have never been fully elucidated. One major obstacle limiting interpretation of standard pharmacological and transgenic interventions is the complexity of the DA system, which only appears to widen as research progresses. Nonetheless, development of new genetic tools, such as chemogenetics, has led to an entirely new era for functional studies of neuronal signaling. By exploiting receptors that are engineered to respond selectively to an otherwise inert ligand, so-called Designer Receptors Exclusively Activated by Designer Drugs (DREADDs), chemogenetics enables pharmacological remote control of neuronal activity. Here we review the recent, extensive application of this technique to the DA field and how its use has advanced the study of the DA system and contributed to our general understanding of DA signaling and related behaviors. Moreover, we discuss the challenges and pitfalls associated with the chemogenetic technology, such as the metabolism of the DREADD ligand clozapine N-oxide (CNO) to the D2 receptor antagonist clozapine. We conclude that despite the recent concerns regarding CNO, the chemogenetic toolbox provides an exceptional approach to study neuronal function. The huge potential should promote continued investigations and additional refinements to further expound key mechanisms of DA signaling and circuitries in normal as well as maladaptive behaviors.

An Intersectional Approach to Target Neural Circuits With Cell- and Projection-Type Specificity: Validation in the Mesolimbic Dopamine System

Development of tools to manipulate activity of specific neurons is important for dissecting the function of neural circuits. Viral vectors and conditional transgenic animal lines that target recombinases to specific cells facilitate the successful manipulation and recording of specific subsets of neurons. So far, it has been possible to target neuronal subtypes within a certain brain region based on transcriptional control regions from a gene selectively expressed in those cells or based upon its projections. Nevertheless, there are only a few tools available that combine this and target a neuronal subtype within a projection. We tested a viral vector system, consisting of a canine adenovirus type 2 expressing a Cre-dependent Flp recombinase (CavFlexFlp) and an adeno-associated viral (AAV) vector expressing a Flp-dependent cDNA, which targets neurons in a subtype- and projection-specific manner. As proof of principle we targeted expression of a Designer Receptor Exclusively Activated by Designer Drugs (DREADD) to the dopamine neurons of the mesolimbic projection, which allows the transient activation of neurons by the ligand Clozapine-N-Oxide (CNO). We validated that the system specifically targets dopamine neurons and that chemogenetic activation of these neurons induces an increase in locomotor activity. We thus validated a valuable tool that allows in vivo neuronal activation in a projection- and subtype-specific manner.

Differential effects of chemogenetic inhibition of dopamine and norepinephrine neurons in the mouse 5-choice serial reaction time task

Attention-deficit/hyperactivity disorder (ADHD) is a psychiatric disorder characterized by inattention, aberrant impulsivity, and hyperactivity. Although the underlying pathophysiology of ADHD remains unclear, dopamine and norepinephrine signaling originating from the ventral tegmental area (VTA) and locus coeruleus (LC) is thought to be critically involved. In this study, we employ Designer Receptor Exclusively Activated by Designer Drugs (DREADDs) together with the mouse 5-Choice Serial Reaction Time Task (5-CSRTT) to investigate the necessary roles of these catecholamines in ADHD-related behaviors, including attention, impulsivity, and motivation. By selective inhibition of tyrosine hydroxylase (TH)-positive VTA dopamine neurons expressing the Gi-coupled DREADD (hM4Di), we observed a marked impairment of effort-based motivation and subsequently speed and overall vigor of responding. At the highest clozapine N-oxide (CNO) dose tested (i.e. 2 mg/kg) to activate hM4Di, we detected a reduction in locomotor activity. DREADD-mediated inhibition of LC norepinephrine neurons reduced attentional performance in a variable stimulus duration test designed to increase task difficulty, specifically by increasing trials omissions, reducing mean score, and visual processing speed. These findings show that VTA dopamine and LC norepinephrine neurons differentially affect attention, impulsive and motivational control. In addition, this study highlights how molecular genetic probing of selective catecholamine circuits can provide valuable insights into the mechanisms underlying ADHD-relevant behaviors.

Chemogenetic Targeting of Dorsomedial Direct-pathway Striatal Projection Neurons Selectively Elicits Rotational Behavior in Mice

The striatum of the basal ganglia is pivotal for voluntary movements and is implicated in debilitating movement disorders such as Parkinsonism and dystonia. Striatum projects to downstream nuclei through direct (dSPN) and indirect (iSPN) pathway projection neurons thought to exert opposite effects on movement. In rodent models of striatal function, unilateral dopamine deprivation induces ipsiversive rotational behavior. The dSPNs of the dorsal striatum are believed to engage distinct motor programs but underlying mechanisms remain unclear. Here, we show by employing chemogenetics [Designer Receptors Exclusively Activated by Designer Drugs (DREADDs)] that unilateral inhibition of dorsomedial dSPNs is sufficient to selectively impair contraversive movement and elicit ipsiversive rotational behavior in mice. Adeno-associated virus (AAV) encoding Cre-dependent G_i-coupled DREADD was injected unilaterally into the dorsomedial striatum of Drd1-Cre mice, resulting in expression of the modified human M4 muscarinic receptor (hM4Di) in ∼20% of dorsostriatal dSPNs. Upon hM4Di activation, a striking positive linear correlation was found between turn ratio and viral expression, which corroborates a relationship between unilateral inhibition of dorsomedial dSPNs and rotational behavior. Bursts of ipsiversive rotations were interspersed with normal ambulation. However, partial unilateral inhibition of ∼20% of dorsostriatal dSPNs did not affect horizontal and vertical locomotion or forelimb use preference. Overall, our results substantiate a unique role of dSPNs in promoting response bias in rotational behavior and show this to be a highly sensitive measure of dSPN performance.