Optogenetic stimulation of VTA dopamine neurons reveals that tonic but not phasic patterns of dopamine transmission reduce ethanol self-administration

Mesolimbic Neural Response Dynamics Predict Future Individual Alcohol Drinking in Mice

BACKGROUND

Individual variability in response to rewarding stimuli is a striking but understudied phenomenon. The mesolimbic dopamine system is critical in encoding the reinforcing properties of both natural reward and alcohol; however, how innate or baseline differences in the response dynamics of this circuit define individual behavior and shape future vulnerability to alcohol remain unknown.

METHODS

Using naturalistic behavioral assays, a voluntary alcohol drinking paradigm, in vivo fiber photometry, in vivo electrophysiology, and chemogenetics, we investigated how differences in mesolimbic neural circuit activity contribute to the individual variability seen in reward processing and, by proxy, alcohol drinking.

RESULTS

We first characterized heterogeneous behavioral and neural responses to natural reward and defined how these baseline responses predicted future individual alcohol-drinking phenotypes in male mice. We then determined spontaneous ventral tegmental area dopamine neuron firing profiles associated with responses to natural reward that predicted alcohol drinking. Using a dual chemogenetic approach, we mimicked specific mesolimbic dopamine neuron firing activity before or during voluntary alcohol drinking to link unique neurophysiological profiles to individual phenotype. We show that hyperdopaminergic individuals exhibit a lower neuronal response to both natural reward and alcohol that predicts lower levels of alcohol consumption in the future.

CONCLUSIONS

These findings reveal unique, circuit-specific neural signatures that predict future individual vulnerability or resistance to alcohol and expand the current knowledge base on how some individuals are able to titrate their alcohol consumption whereas others go on to engage in unhealthy alcohol-drinking behaviors.

Neurovascular effects of cocaine: relevance to addiction

Cocaine is a highly addictive drug, and its use is associated with adverse medical consequences such as cerebrovascular accidents that result in debilitating neurological complications. Indeed, brain imaging studies have reported severe reductions in cerebral blood flow (CBF) in cocaine misusers when compared to the brains of healthy non-drug using controls. Such CBF deficits are likely to disrupt neuro-vascular interaction and contribute to changes in brain function. This review aims to provide an overview of cocaine-induced CBF changes and its implication to brain function and to cocaine addiction, including its effects on tissue metabolism and neuronal activity. Finally, we discuss implications for future research, including targeted pharmacological interventions and neuromodulation to limit cocaine use and mitigate the negative impacts.

Dopamine release in the nucleus accumbens promotes REM sleep and cataplexy

Patients with the sleep disorder narcolepsy suffer from excessive daytime sleepiness, disrupted nighttime sleep, and cataplexy-the abrupt loss of postural muscle tone during wakefulness, often triggered by strong emotion. The dopamine (DA) system is implicated in both sleep-wake states and cataplexy, but little is known about the function of DA release in the striatum and sleep disorders. Recording DA release in the ventral striatum revealed orexin-independent changes across sleep-wake states as well as striking increases in DA release in the ventral, but not dorsal, striatum prior to cataplexy onset. Tonic low-frequency stimulation of ventral tegmental efferents in the ventral striatum suppressed both cataplexy and rapid eye movement (REM) sleep, while phasic high-frequency stimulation increased cataplexy propensity and decreased the latency to REM sleep. Together, our findings demonstrate a functional role of DA release in the striatum in regulating cataplexy and REM sleep.

Oxycodone-induced dopaminergic and respiratory effects are modulated by deep brain stimulation

Introduction: Opioids are the leading cause of overdose death in the United States, accounting for almost 70,000 deaths in 2020. Deep brain stimulation (DBS) is a promising new treatment for substance use disorders. Here, we hypothesized that VTA DBS would modulate both the dopaminergic and respiratory effect of oxycodone. Methods: Multiple-cyclic square wave voltammetry (M-CSWV) was used to investigate how deep brain stimulation (130 Hz, 0.2 ms, and 0.2 mA) of the rodent ventral segmental area (VTA), which contains abundant dopaminergic neurons, modulates the acute effects of oxycodone administration (2.5 mg/kg, i.v.) on nucleus accumbens core (NAcc) tonic extracellular dopamine levels and respiratory rate in urethane-anesthetized rats (1.5 g/kg, i.p.). Results: I.V. administration of oxycodone resulted in an increase in NAcc tonic dopamine levels (296.9 ± 37.0 nM) compared to baseline (150.7 ± 15.5 nM) and saline administration (152.0 ± 16.1 nM) (296.9 ± 37.0 vs. 150.7 ± 15.5 vs. 152.0 ± 16.1, respectively, p = 0.022, n = 5). This robust oxycodone-induced increase in NAcc dopamine concentration was associated with a sharp reduction in respiratory rate (111.7 ± 2.6 min-1 vs. 67.9 ± 8.3 min-1; pre- vs. post-oxycodone; p < 0.001). Continuous DBS targeted at the VTA (n = 5) reduced baseline dopamine levels, attenuated the oxycodone-induced increase in dopamine levels to (+39.0% vs. +95%), and respiratory depression (121.5 ± 6.7 min-1 vs. 105.2 ± 4.1 min-1; pre- vs. post-oxycodone; p = 0.072). Discussion: Here we demonstrated VTA DBS alleviates oxycodone-induced increases in NAcc dopamine levels and reverses respiratory suppression. These results support the possibility of using neuromodulation technology for treatment of drug addiction.

Striatal dopamine regulates sleep states and narcolepsy-cataplexy

Disruptions to sleep can be debilitating and have a severe effect on daily life. Patients with the sleep disorder narcolepsy suffer from excessive daytime sleepiness, disrupted nighttime sleep, and cataplexy - the abrupt loss of postural muscle tone (atonia) during wakefulness, often triggered by strong emotion. The dopamine (DA) system is implicated in both sleep-wake states and cataplexy, but little is known about the function of DA release in the striatum - a major output region of midbrain DA neurons - and sleep disorders. To better characterize the function and pattern of DA release in sleepiness and cataplexy, we combined optogenetics, fiber photometry, and sleep recordings in a murine model of narcolepsy (orexin-/-; OX KO) and in wildtype mice. Recording DA release in the ventral striatum revealed OX-independent changes across sleep-wake states as well as striking increases in DA release in the ventral, but not dorsal, striatum prior to cataplexy onset. Tonic low frequency stimulation of ventral tegmental efferents in the ventral striatum suppressed both cataplexy and REM sleep, while phasic high frequency stimulation increased cataplexy propensity and decreased the latency to rapid eye movement (REM) sleep. Together, our findings demonstrate a functional role of DA release in the striatum in regulating cataplexy and REM sleep.

Aging in nucleus accumbens and its impact on alcohol use disorders

Ethanol is one of the most widely consumed drugs in the world and prolonged excessive ethanol intake might lead to alcohol use disorders (AUDs), which are characterized by neuroadaptations in different brain regions, such as in the reward circuitry. In addition, the global population is aging, and it appears that they are increasing their ethanol consumption. Although research involving the effects of alcohol in aging subjects is limited, differential effects have been described. For example, studies in human subjects show that older adults perform worse in tests assessing working memory, attention, and cognition as compared to younger adults. Interestingly, in the field of the neurobiological basis of ethanol actions, there is a significant dichotomy between what we know about the effects of ethanol on neurochemical targets in young animals and how it might affect them in the aging brain. To be able to understand the distinct effects of ethanol in the aging brain, the following questions need to be answered: (1) How does physiological aging impact the function of an ethanol-relevant region (e.g., the nucleus accumbens)? and (2) How does ethanol affect these neurobiological systems in the aged brain? This review discusses the available data to try to understand how aging affects the nucleus accumbens (nAc) and its neurochemical response to alcohol. The data show that there is little information on the effects of ethanol in aged mice and rats, and that many studies had considered 2-3-month-old mice as adults, which needs to be reconsidered since more recent literature defines 6 months as young adults and >18 months as an older mouse. Considering the actual relevance of an aged worldwide population and that this segment is drinking more frequently, it appears at least reasonable to explore how ethanol affects the brain in adult and aged models.

High frequency deep brain stimulation can mitigate the acute effects of cocaine administration on tonic dopamine levels in the rat nucleus accumbens

Cocaine's addictive properties stem from its capacity to increase tonic extracellular dopamine levels in the nucleus accumbens (NAc). The ventral tegmental area (VTA) is a principal source of NAc dopamine. To investigate how high frequency stimulation (HFS) of the rodent VTA or nucleus accumbens core (NAcc) modulates the acute effects of cocaine administration on NAcc tonic dopamine levels multiple-cyclic square wave voltammetry (M-CSWV) was used. VTA HFS alone decreased NAcc tonic dopamine levels by 42%. NAcc HFS alone resulted in an initial decrease in tonic dopamine levels followed by a return to baseline. VTA or NAcc HFS following cocaine administration prevented the cocaine-induced increase in NAcc tonic dopamine. The present results suggest a possible underlying mechanism of NAc deep brain stimulation (DBS) in the treatment of substance use disorders (SUDs) and the possibility of treating SUD by abolishing dopamine release elicited by cocaine and other drugs of abuse by DBS in VTA, although further studies with chronic addiction models are required to confirm that. Furthermore, we demonstrated the use of M-CSWV can reliably measure tonic dopamine levels in vivo with both drug administration and DBS with minimal artifacts.

Ketamine and Zinc: Treatment of Anorexia Nervosa Via Dual NMDA Receptor Modulation

Anorexia nervosa is a disorder associated with serious adverse health outcomes, for which there is currently considerable treatment ineffectiveness. Characterised by restrictive eating behaviours, distorted body image perceptions and excessive physical activity, there is growing recognition anorexia nervosa is associated with underlying dysfunction in excitatory and inhibitory neurometabolite metabolism and signalling. This narrative review critically explores the role of N-methyl-D-aspartate receptor-mediated excitatory and inhibitory neurometabolite dysfunction in anorexia nervosa and its associated biomarkers. The existing magnetic resonance spectroscopy literature in anorexia nervosa is reviewed and we outline the brain region-specific neurometabolite changes that have been reported and their connection to anorexia nervosa psychopathology. Considering the proposed role of dysfunctional neurotransmission in anorexia nervosa, the potential utility of zinc supplementation and sub-anaesthetic doses of ketamine in normalising this is discussed with reference to previous research in anorexia nervosa and other neuropsychiatric conditions. The rationale for future research to investigate the combined use of low-dose ketamine and zinc supplementation to potentially extend the therapeutic benefits in anorexia nervosa is subsequently explored and promising biological markers for assessing and potentially predicting treatment response are outlined.

Short-Term Consequences of Single Social Defeat on Accumbal Dopamine and Behaviors in Rats

The present study aimed to explore the consequences of a single exposure to a social defeat on dopamine release in the rat nucleus accumbens measured with a fast-scan cyclic voltammetry. We found that 24 h after a social defeat, accumbal dopamine responses, evoked by a high frequency electrical stimulation of the ventral tegmental area, were more profound in socially defeated rats in comparison with non-defeated control animals. The enhanced dopamine release was associated with the prolonged immobility time in the forced swim test. The use of the dopamine depletion protocol revealed no alteration in the reduction and recovery of the amplitude of dopamine release following social defeat stress. However, administration of dopamine D2 receptor antagonist, raclopride (2 mg/kg, i.p.), resulted in significant increase of the electrically evoked dopamine release in both groups of animals, nevertheless exhibiting less manifested effect in the defeated rats comparing to control animals. Taken together, our data demonstrated profound alterations in the dopamine transmission in the association with depressive-like behavior following a single exposure to stressful environment. These voltammetric findings pointed to a promising path for the identification of neurobiological mechanisms underlying stress-promoted behavioral abnormalities.

Reactivating hippocampal-mediated memories during reconsolidation to disrupt fear

Memories are stored in the brain as cellular ensembles activated during learning and reactivated during retrieval. Using the Tet-tag system in mice, we label dorsal dentate gyrus neurons activated by positive, neutral or negative experiences with channelrhodopsin-2. Following fear-conditioning, these cells are artificially reactivated during fear memory recall. Optical stimulation of a competing positive memory is sufficient to update the memory during reconsolidation, thereby reducing conditioned fear acutely and enduringly. Moreover, mice demonstrate operant responding for reactivation of a positive memory, confirming its rewarding properties. These results show that interference from a rewarding experience can counteract negative affective states. While memory-updating, induced by memory reactivation, involves a relatively small set of neurons, we also find that activating a large population of randomly labeled dorsal dentate gyrus neurons is effective in promoting reconsolidation. Importantly, memory-updating is specific to the fear memory. These findings implicate the dorsal dentate gyrus as a potential therapeutic node for modulating memories to suppress fear.

Dopamine Receptors: Is It Possible to Become a Therapeutic Target for Depression?

Dopamine and its receptors are currently recognized targets for the treatment of several neuropsychiatric disorders, including Parkinson’s disease, schizophrenia, some drug use addictions, as well as depression. Dopamine receptors are widely distributed in various regions of the brain, but their role and exact contribution to neuropsychiatric diseases has not yet been thoroughly studied. Based on the types of dopamine receptors and their distribution in different brain regions, this paper reviews the current research status of the molecular, cellular and circuit mechanisms of dopamine and its receptors involved in depression. Multiple lines of investigation of these mechanisms provide a new future direction for understanding the etiology and treatment of depression and potential new targets for antidepressant treatments.

Cocaine's effects on the reactivity of the medial prefrontal cortex to ventral tegmental area stimulation: optical imaging study in mice

BACKGROUND AND AIMS

The prefrontal cortex (PFC) is modulated by dopaminergic and glutamatergic neurons that project from the ventral tegmental area (VTA) and disruption of this modulation might facilitate impulsive behaviors during cocaine intoxication. Here, we assessed the effects of acute cocaine (30 mg/kg, i.p.) on the reactivity of the PFC to VTA stimulation.

METHODS

Using a genetically encoded calcium indicator (GCaMP6f), we optically imaged the neuronal Ca2+ reactance in medial PFC (mPFC) in response to 'tonic-like' (5 Hz) and 'phasic-like' (50 Hz) electrical VTA stimulation. The high temporal and spatial resolutions of our optical system allowed us to capture single Ca2+ neuronal transients from individual stimuli with 'tonic-like' stimulation and to visualize single neuronal activation evoked by 'phasic-like' VTA stimulation.

RESULTS

'Tonic-like' VTA stimulation induced a rapid increase in mean neuronal Ca2+ in mPFC followed by a plateau and recovery upon termination of stimulation. After cocaine, the mPFC sensitivity to 'tonic-like' VTA stimulation was attenuated, with a 50.4% reduction (P = 0.03) in the number of Ca2+ transients corresponding to single electrical stimuli but the recovery time was lengthened (4.30 ± 0.25 sec to 5.41 ± 0.24 sec, P = 0.03). 'Phasic-like' stimulation evoked a rapid Ca2+ fluorescence increase in mPFC with an immediate decay process, and while cocaine did not affect the peak response (7.17 ± 1.07% versus 7.13 ± 0.96%, P = 0.98) it shortened the recovery time to baseline (3.27 ± 0.11 sec versus 2.38 ± 0.23 sec, P = 0.005).

CONCLUSIONS

Acute cocaine impairs reactivity of medial prefrontal cortex (mPFC) to ventral tegmental area stimulation, decreasing its sensitivity to 'tonic-like' stimulation and lengthening the recovery time to return to baseline while shortening it for phasic stimulation. These changes in mPFC might contribute to cocaine binging during intoxication.

Rev-erbα Knockout Reduces Ethanol Consumption and Preference in Male and Female Mice

Alcohol use is a contributor in the premature deaths of approximately 3 million people annually. Among the risk factors for alcohol misuse is circadian rhythm disruption; however, this connection remains poorly understood. Inhibition of the circadian nuclear receptor REV-ERBα is known to disrupt molecular feedback loops integral to daily oscillations, and impact diurnal fluctuations in the expression of proteins required for reward-related neurotransmission. However, the role of REV-ERBα in alcohol and substance use-related phenotypes is unknown. Herein, we used a Rev-erbα knockout mouse line and ethanol two-bottle choice preference testing to show that disruption of Rev-erbα reduces ethanol preference in male and female mice. Rev-erbα null mice showed the lowest ethanol preference in a two-bottle choice test across all genotypes, whereas there were no ethanol preference differences between heterozygotes and wildtypes. In a separate experiment, alcohol-consuming wildtype C57Bl/6N mice were administered the REV-ERBα/β inhibitor SR8278 (25 mg/kg or 50 mg/kg) for 7 days and alcohol preference was evaluated daily. No differences in alcohol preference were observed between the treatment and vehicle groups. Our data provides evidence that genetic variation in REV-ERBα may contribute to differences in alcohol drinking.

The incentive amplifying effects of nicotine: Roles in alcohol seeking and consumption

Nicotine has a unique profile among drugs of abuse. To the noninitiated user, nicotine has powerful aversive effects and its relatively weak euphorigenic effects undergo rapid tolerance. Despite this, nicotine is commonly abused despite negative heath consequences, and nicotine users have enormous difficulty quitting. Further, nicotine is one of the most commonly co-abused substances, in that it is often taken in combination with other drugs. One explanation of this polydrug use is that nicotine has multiple appetitive and consummatory conditioning effects. For example, nicotine is a reinforcement enhancer in that it can potently increase the incentive value of other stimuli, including those surrounding drugs of abuse such as alcohol. In addition, nicotine also has a unique profile of neurobiological effects that alter regulation of alcohol intake and interoception. This review discusses the psychological and biological mechanisms surrounding nicotine's appetitive conditioning and consummatory effects, particularly its interactions with alcohol.

Applying a Fast-Scan Cyclic Voltammetry to Explore Dopamine Dynamics in Animal Models of Neuropsychiatric Disorders

Progress in the development of technologies for the real-time monitoring of neurotransmitter dynamics has provided researchers with effective tools for the exploration of etiology and molecular mechanisms of neuropsychiatric disorders. One of these powerful tools is fast-scan cyclic voltammetry (FSCV), a technique which has progressively been used in animal models of diverse pathological conditions associated with alterations in dopamine transmission. Indeed, for several decades FSCV studies have provided substantial insights into our understanding of the role of abnormal dopaminergic transmission in pathogenetic mechanisms of drug and alcohol addiction, Parkinson’s disease, schizophrenia, etc. Here we review the applications of FSCV to research neuropsychiatric disorders with particular attention to recent technological advances.

Phasic firing of dopaminergic neurons in the ventral tegmental area triggers peripheral immune responses

Dopaminergic neurons in the ventral tegmental area (VTA) play a crucial role in the processing of reward-related information. Recent studies with pharmacological manipulations of VTA neuronal activity demonstrated a VTA-induced immunoenhancement in peripheral organs. Here, to examine the detailed physiological dynamics, we took an optogenetic approach in which VTA dopaminergic neurons were selectively activated with millisecond precision. Optogenetic phasic, rather than tonic, stimulation of VTA dopaminergic neurons increased serum cytokine levels, such as IL-2, IL-4 and TNF-α. These results provide direct evidence to link dopaminergic neuronal phasic firing to peripheral immunity. Next, we tested whether cytokine induction in male mice was boosted by female encounters, a natural condition that induces increased active VTA neurons and gamma power. Female encounters increased serum IL-2 levels, which were abolished by pharmacological inhibition of VTA neuronal activity. Taken together, our results highlight the importance of the brain reward system in the treatment and management of immune-related disorders.

Distinct limbic dopamine regulation across olfactory‐tubercle subregions through integration of in vivo fast‐scan cyclic voltammetry and optogenetics

The olfactory tubercle (OT), an important component of the ventral striatum and limbic system, is involved in multi-sensory integration of reward-related information in the brain. However, its functional roles are often overshadowed by the neighboring nucleus accumbens. Increasing evidence has highlighted that dense dopamine (DA) innervation of the OT from the ventral tegmental area (VTA) is implicated in encoding reward, natural reinforcers, and motivated behaviors. Recent studies have further suggested that OT subregions may have distinct roles in these processes due to their heterogeneous DA transmission. Currently, very little is known about regulation (release and clearance) of extracellular DA across OT subregions due to its limited anatomical accessibility and proximity to other DA-rich brain regions, making it difficult to isolate VTA-DA signaling in the OT with conventional methods. Herein, we characterized heterogeneous VTA-DA regulation in the medial (m) and lateral (l) OT in "wild-type," urethane-anesthetized rats by integrating in vivo fast-scan cyclic voltammetry with cell-type specific optogenetics to stimulate VTA-DA neurons. Channelrhodopsin-2 was selectively expressed in the VTA-DA neurons of wild-type rats and optical stimulating parameters were optimized to determine VTA-DA transmission across the OT. Our anatomical, neurochemical, and pharmacological results show that VTA-DA regulation in the mOT is less dependent on DA transporters and has greater DA transmission than the lOT. These findings establish the OT as a unique, compartmentalized structure and will aid in future behavioral characterization of the roles of VTA-DA signaling in the OT subregions in reward, drug addiction, and encoding behavioral outputs necessary for survival.

Linking Ethanol-Addictive Behaviors With Brain Catecholamines: Release Pattern Matters

Using a variety of animal models that simulate key features of the alcohol use disorder (AUD), remarkable progress has been made in identifying neurochemical targets that may contribute to the development of alcohol addiction. In this search, the dopamine (DA) and norepinephrine (NE) systems have been long thought to play a leading role in comparison with other brain systems. However, just recent development and application of optogenetic approaches into the alcohol research field provided opportunity to identify neuronal circuits and specific patterns of neurotransmission that govern the key components of ethanol-addictive behaviors. This critical review summarizes earlier findings, which initially disclosed catecholamine substrates of ethanol actions in the brain and shows how the latest methodologies help us to reveal the significance of DA and NE release changes. Specifically, we focused on recent optogenetic investigations aimed to reveal cause-effect relationships between ethanol-drinking (seeking and taking) behaviors and catecholamine dynamics in distinct brain pathways. These studies gain the knowledge that is needed for the better understanding addiction mechanisms and, therefore, for development of more effective AUD treatments. Based on the reviewed findings, new messages for researches were indicated, which may have broad applications beyond the field of alcohol addiction.

Food reward depends on TLR4 activation in dopaminergic neurons

The rising prevalence of obesity and being overweight is a worldwide health concern. Food reward dysregulation is the basic factor for the development of obesity. Dopamine (DA) neurons in the ventral tegmental area (VTA) play a vital role in food reward. Toll-like receptor 4 (TLR4) is a transmembrane pattern recognition receptor that can be activated by saturated fatty acids. Here, we show that the deletion of TLR4 specifically in DA neurons increases body weight, increases food intake, and decreases food reward. Conditional deletion of TLR4 also decreased the activity of DA neurons while suppressing the expression of tyrosine hydroxylase (TH) in the VTA, which regulates the concentration of DA in the nucleus accumbens (NAc) to affect food reward. Meanwhile, AAV-Cre-GFP mediated VTA-specific TLR4-deficient mice recapitulates food reward of DAT-TLR4-KO mice. Food reward could be rescued by re-expressing TLR4 in VTA DA neurons. Moreover, effects of intra-VTA infusion of lauric acid (a saturated fatty acid with 12 carbon) on food reward were abolished in mice lacking TLR4 in DA neurons. Our study demonstrates the critical role of TLR4 signaling in regulating the activity of VTA DA neurons and the normal function of the mesolimbic DA system that may contribute to food reward.

Activation of VTA GABA neurons disrupts reward seeking by altering temporal processing

The role of ventral tegmental area (VTA) dopamine in reward, cue processing, and interval timing is well characterized. Using a combinatorial viral approach to target activating DREADDs (Designer Receptors Exclusively Activated by Designer Drugs, hM3D) to GABAergic neurons in the VTA of male rats, we previously showed that activation disrupts responding to reward-predictive cues. Here we explored how VTA GABA neurons influence the perception of time in two fixed interval (FI) tasks, one where the reward or interval is not paired with predictive cues (Non-Cued FI), and another where the start of the FI is signaled by a constant tone that continues until the rewarded response is emitted (Cued FI). Under vehicle conditions in both tasks, responding was characterized by "scalloping" over the 30 s FI, in which responding increased towards the end of the FI. However, when VTA GABA neurons were activated in the Non-Cued FI, the time between the end of the 30 s interval and when the rats made a reinforced response increased. Additionally, post-reinforcement pauses and overall session length increased. In the Cued FI task, VTA GABA activation produced erratic responding, with a decrease in earned rewards. Thus, while both tasks were disrupted by VTA GABA activation, responding that is constrained by a cue was more sensitive to this manipulation, possibly due to convergent effects on timing and cue processing. Together these results demonstrate that VTA GABA activity disrupts the perception of interval timing, particularly when the timing is set by cues.

The late consolidation of an aversive memory is promoted by VTA dopamine release in the dorsal hippocampus

The hippocampus has been implicated in the processing and storage of aversive memories but the precise mechanisms by which these memories persist in time remain elusive. We have demonstrated that dopaminergic neurotransmission in the dorsal hippocampus regulates the long-term storage of both appetitive and aversive memories at a critical time point known as "late consolidation" (12 hr after the learning experience). This modulation appears to have opposite effects depending on the valence of the stimuli, with hippocampal dopamine release peaking immediately and 13-17 hr after a rewarding experience. Here, we determined the release pattern of hippocampal dopamine following an aversive experience, in order to better understand this opposite modulation process. We observed significant increases in dopamine levels at several times (6-8, 11-12, and 15 hr) after subjecting rats to a conditioned place aversion (CPA) task with the aversive agent lithium chloride (LiCl). Early pharmacological blockade of hippocampal DA receptors impaired CPA memory consolidation. In addition and consistent with previous findings showing that late post-training infusions of dopaminergic agents into the hippocampus modulate the long-term storage of aversive memories, we found that the photostimulation of dopaminergic VTA fibers in the dorsal hippocampus 11-12 hr after CPA training was enough to transform a short-lasting long-term memory into a long-lasting one. The fact that the persistence of an aversive memory can still be affected several hours after the learning experience opens new avenues to develop behavioral and pharmacological strategies for the treatment of a variety of mental disorders.

Deep Brain Stimulation of the Lateral Hypothalamus Facilitates Extinction and Prevents Reinstatement of Morphine Place Preference in Rats

OBJECTIVES

We have previously shown that high-frequency (HF) deep brain stimulation (DBS) of the lateral hypothalamus (LH) during the acquisition phase of morphine-induced conditioned place preference (CPP) abolished the development of morphine reward. In the present study, we investigated the effect of DBS in the LH during the extinction phase of morphine CPP.

MATERIALS AND METHODS

Rats were implanted with electrodes in the LH and went through conditioning trials for morphine CPP (40 min each, for three days), followed by extinction trials (20 min, for nine days). DBS-like stimulation (square pulses at 13 or 130 Hz, 200 μA, 100 μsec) was applied during the extinction trials.

RESULTS

Rats that received HF-DBS (130 Hz) accomplished extinction of morphine place preference by day 5 of the phase, whereas those in sham-stimulation or low-frequency-DBS (LF-DBS, 13 Hz) groups reached the criterion for extinction at day 8. One day later, rats received a priming injection of morphine (2 mg/kg) to reinstate the extinguished preference. While rats in the sham-DBS and LF-DBS relapsed into the state of preferring morphine-associated context, those in the HF-DBS group did not show such preference. Rats were then proceeded into an additional phase of extinction training (20 min, once daily, three to five days) with DBS, followed by restraint stress-induced reinstatement test. Again, sham-DBS and LF-DBS had no effect on relapse to the morphine place preferring state, but HF-DBS completely prevented the relapse.

CONCLUSION

HF-DBS facilitated extinction of morphine place preference and disrupted drug priming- and stress-induced renewal of morphine place preference.

Acute and chronic bupropion treatment does not prevent morphine-induced conditioned place preference in mice

A substantial barrier to the treatment of Opioid Use Disorder (OUD) is the elevated relapse rates in affected patients, and a significant contributor to these events of relapse is exposure to cues and contexts that are intensely associated with prior drug abuse. The neurotransmitter dopamine plays a key role in reward-related behaviors, and previous studies have illustrated that dopamine hypofunction in periods of abstinence serves to prompt drug craving and seeking. We hypothesized that restoration of dopaminergic signaling could attenuate drug-seeking behaviors. Therefore, we investigated whether use of an FDA-approved drug, bupropion, an inhibitor of the dopamine transporter (DAT), or a dopamine uptake inhibitor with high affinity for DAT, JHW 007, was able to decrease preference for a drug-paired context. In these experiments, mice underwent 5 days of non-contingent morphine (10 mg/kg) exposure in a conditioned place preference (CPP) paradigm. We found that systemic injection of bupropion (20 mg/kg, i. p.) or intracranial injection of JHW 007 into the nucleus accumbens shell did not prevent the expression of morphine CPP. We then investigated whether chronic bupropion treatment (via implanted osmotic pumps) would influence morphine CPP. We observed that chronic bupropion treatment for 21 days following morphine conditioning did not attenuate the prolonged preference for morphine-paired contexts. Overall, with our dose and paradigm, neither acute nor chronic bupropion diminishes morphine CPP. Continued studies should address FDA-approved medications and their potential for recovery in OUD patients.

Different adaptations of dopamine release in Nucleus Accumbens shell and core of individual alcohol drinking groups of mice

Alcohol use disorder (AUD) places a tremendous burden on society, with approximately two billion alcohol users in the world. While most people drink alcohol recreationally, a subpopulation (3-5%) engages in reckless and compulsive drinking, leading to the development of AUD and alcohol dependence. The Ventral Tegmental Area (VTA)-Nucleus Accumbens (NAc) circuit has been shown to encode rewarding stimuli and drive individual alcohol drinking behavior. Our previous work successfully separated C57BL/6J isogenic mice into high or low alcohol drinking subgroups after a 12-day, two-bottle choice voluntary alcohol access paradigm. Electrophysiological studies revealed that low alcohol drinking mice exhibited elevated spontaneous and burst firing properties of their VTA dopamine (DA) neurons and specifically mimicking this pattern of activity in VTA-NAc neurons in high alcohol drinking mice using optogenetics decreased their alcohol preference. It is also known that VTA DA neurons encode the salience and rewarding properties of external stimuli while also regulating downstream dopamine concentrations. Here, as a follow-up to this study, we utilized Fast Scan Cyclic Voltammetry (FSCV) to examine dopamine release in the NAc shell and core between alcohol drinking groups. We observed dynamic changes of dopamine release in the core of high drinking mice, but failed to see widely significant differences of dopamine release in the shell of both groups, when compared with ethanol-naive controls. Overall, the present data suggest subregion-specific differences of evoked dopamine release in the NAc of low and high alcohol drinking mice, and may provide an anatomical substrate for individual alcohol drinking behavior. This article is part of the special issue on Stress, Addiction and Plasticity.

Leveraging Neural Networks in Preclinical Alcohol Research

Alcohol use disorder is a pervasive healthcare issue with significant socioeconomic consequences. There is a plethora of neural imaging techniques available at the clinical and preclinical level, including magnetic resonance imaging and three-dimensional (3D) tissue imaging techniques. Network-based approaches can be applied to imaging data to create neural networks that model the functional and structural connectivity of the brain. These networks can be used to changes to brain-wide neural signaling caused by brain states associated with alcohol use. Neural networks can be further used to identify key brain regions or neural "hubs" involved in alcohol drinking. Here, we briefly review the current imaging and neurocircuit manipulation methods. Then, we discuss clinical and preclinical studies using network-based approaches related to substance use disorders and alcohol drinking. Finally, we discuss how preclinical 3D imaging in combination with network approaches can be applied alone and in combination with other approaches to better understand alcohol drinking.

Behavioral, neurobiological, and neurochemical mechanisms of ethanol self-administration: A translational review

Alcohol use disorder has multiple characteristics including excessive ethanol consumption, impaired control over drinking behaviors, craving and withdrawal symptoms, compulsive seeking behaviors, and is considered a chronic condition. Relapse is common. Determining the neurobiological targets of ethanol and the adaptations induced by chronic ethanol exposure is critical to understanding the clinical manifestation of alcohol use disorders, the mechanisms underlying the various features of the disorder, and for informing medication development. In the present review, we discuss ethanol's interactions with a variety of neurotransmitter systems, summarizing findings from preclinical and translational studies to highlight recent progress in the field. We then describe animal models of ethanol self-administration, emphasizing the value, limitations, and validity of commonly used models. Lastly, we summarize the behavioral changes induced by chronic ethanol self-administration, with an emphasis on cue-elicited behavior, the role of ethanol-related memories, and the emergence of habitual ethanol seeking behavior.

Bidirectional Control of Alcohol-drinking Behaviors Through Locus Coeruleus Optoactivation

The relationship between stress and alcohol-drinking behaviors has been intensively explored; however, neuronal substrates and neurotransmitter dynamics responsible for a causal link between these conditions are still unclear. Here, we optogenetically manipulated locus coeruleus (LC) norepinephrine (NE) activity by applying distinct stimulation protocols in order to explore how phasic and tonic NE release dynamics control alcohol-drinking behaviors. Our results clearly demonstrate contrasting behavioral consequences of LC-NE circuitry activation during low and high frequency stimulation. Specifically, applying tonic stimulation during a standard operant drinking session resulted in increased intake, while phasic stimulation decreased this measure. Furthermore, stimulation during extinction probe trials, when the lever press response was not reinforced, did not significantly alter alcohol-seeking behavior if a tonic pattern was applied. However, phasic stimulation substantially suppressed the number of lever presses, indicating decreased alcohol seeking under the same experimental condition. Given the well-established correlative link between stress and increased alcohol consumption, here we provide the first evidence that tonic LC-NE activity plays a causal role in stress-associated increases in drinking.

Uncoupling nNOS-PSD-95 in mPFC inhibits morphine priming-induced reinstatement after extinction training

Extremely high relapse rate is the dramatic challenge of drug abuse at present. Environmental cues play an important role in relapse of drug abuse. However, the specific mechanism underlying relapse remains unclear. Using morphine conditioned place preference (CPP) model, we show that association of neuronal nitric oxide synthase (nNOS) with postsynaptic density-95 (PSD-95) plays a significant role in morphine priming-induced reinstatement. The nNOS-PSD-95 coupling and c-Fos expression in the medial prefrontal cortex (mPFC) was significantly increased after extinction of morphine CPP. Dissociation of nNOS-PSD-95 in the mPFC by ZL006 inhibited the reinstatement of morphine CPP induced by a priming dose of morphine. Significantly reduced phosphorylation of cAMP-response element binding protein (CREB) in the mPFC was observed in the mice exposed to morphine after the extinction training. Uncoupling nNOS-PSD-95 reversed the morphine-induced CREB dysfunction. Moreover, effects of ZL006 on the reinstatement of morphine CPP and CREB activation depended on nNOS-PSD-95 target. Together, our findings suggest that nNOS-PSD-95 in the mPFC contributes to reinstatement of morphine CPP, possibly through CREB dysfunction, offering a potential target to prevent relapse of drug abuse.

Frequency- and State-Dependent Network Effects of Electrical Stimulation Targeting the Ventral Tegmental Area in Macaques

The ventral tegmental area (VTA) is a midbrain structure at the heart of the dopaminergic system underlying adaptive behavior. Endogenous firing rates of dopamine cells in the VTA vary from fast phasic bursts to slow tonic activity. Artificial perturbations of the VTA, through electrical or optogenetic stimulation methods, generate different and sometimes even contrasting behavioral outcomes depending on stimulation parameters such as frequency, amplitude, and pulse width. Here, we investigate the global functional effects of electrical stimulation frequency (10, 20, 50, and 100 Hz) of the VTA in rhesus monkeys. We stimulated 2 animals with chronic electrodes, either awake or anesthetized, while concurrently acquiring whole-brain functional magnetic resonance imaging (fMRI) signals. In the awake state, activity as a function of stimulation frequency followed an inverted U-shape in many cortical and subcortical structures, with highest activity observed at 20 and 50 Hz and lower activity at 10 and 100 Hz. Under anesthesia, the hemodynamic responses in connected brain areas were slightly positive at 10 Hz stimulation, but decreased linearly as a function of higher stimulation frequencies. A speculative explanation for the remarkable frequency dependence of stimulation-induced fMRI activity is that the VTA makes use of different frequency channels to communicate with different postsynaptic sites.

Opposite Consequences of Tonic and Phasic Increases in Accumbal Dopamine on Alcohol-Seeking Behavior

Despite many years of work on dopaminergic mechanisms of alcohol addiction, much of the evidence remains mostly correlative in nature. Fortunately, recent technological advances have provided the opportunity to explore the causal role of alterations in neurotransmission within circuits involved in addictive behaviors. Here, we address this critical gap in our knowledge by integrating an optogenetic approach and an operant alcohol self-administration paradigm to assess directly how accumbal dopamine (DA) release dynamics influences the appetitive (seeking) component of alcohol-drinking behavior. We show that appetitive reward-seeking behavior in rats trained to self-administer alcohol can be shaped causally by ventral tegmental area-nucleus accumbens (VTA-NAc) DA neurotransmission. Our findings reveal that phasic patterns of DA release within this circuit enhance a discrete measure of alcohol seeking, whereas tonic patterns of stimulation inhibit this behavior. Moreover, we provide mechanistic evidence that tonic-phasic interplay within the VTA-NAc DA circuit underlies these seemingly paradoxical effects.

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VTA-NAc DA transmission can bidirectionally modulate motivated behavior

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Optogenetic increases in phasic DA release in the NAc enhance alcohol seeking

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Optogenetic increases in tonic DA release in the NAc inhibit alcohol seeking

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Phasic DA release can be decreased by the concurrent tonic activation

The Neuroscience of Drug Reward and Addiction

Drug consumption is driven by a drug's pharmacological effects, which are experienced as rewarding, and is influenced by genetic, developmental, and psychosocial factors that mediate drug accessibility, norms, and social support systems or lack thereof. The reinforcing effects of drugs mostly depend on dopamine signaling in the nucleus accumbens, and chronic drug exposure triggers glutamatergic-mediated neuroadaptations in dopamine striato-thalamo-cortical (predominantly in prefrontal cortical regions including orbitofrontal cortex and anterior cingulate cortex) and limbic pathways (amygdala and hippocampus) that, in vulnerable individuals, can result in addiction. In parallel, changes in the extended amygdala result in negative emotional states that perpetuate drug taking as an attempt to temporarily alleviate them. Counterintuitively, in the addicted person, the actual drug consumption is associated with an attenuated dopamine increase in brain reward regions, which might contribute to drug-taking behavior to compensate for the difference between the magnitude of the expected reward triggered by the conditioning to drug cues and the actual experience of it. Combined, these effects result in an enhanced motivation to "seek the drug" (energized by dopamine increases triggered by drug cues) and an impaired prefrontal top-down self-regulation that favors compulsive drug-taking against the backdrop of negative emotionality and an enhanced interoceptive awareness of "drug hunger." Treatment interventions intended to reverse these neuroadaptations show promise as therapeutic approaches for addiction.

Real-Time Accumbal Dopamine Response to Negative Stimuli: Effects of Ethanol

Activity in the mesolimbic dopamine (DA) pathway is known to have a role in reward processing and related behaviors. The mesolimbic DA response to reward has been well-examined, while the response to aversive or negative stimuli has been studied to a lesser extent and produced inconclusive results. However, a brief increase in the DA concentration in terminals during nociceptive activation has become an established but not well-characterized phenomenon. Consequently, the interpretation of the significance of this neurochemical response is still elusive. The present study was designed to further explore these increases in subsecond DA dynamics triggered by negative stimuli using voltammetry in anesthetized rats. Our experiments revealed that repeated exposure to a tail pinch resulted in more efficacious DA release in rat nucleus accumbens. This fact may suggest a protective nature of immediate DA efflux. Furthermore, a sensitized DA response to a neutral stimulus, such as a touch, was discovered following several noxious pinches, while a touch applied before these pinches did not trigger DA release. Finally, it was found that the pinch-evoked DA efflux was significantly decreased by ethanol acutely administrated at an analgesic dose. Taken together, these results support the hypothesis that subsecond DA release in the nucleus accumbens may serve as an endogenous antinociceptive signal.

Latency to Reward Predicts Social Dominance in Rats: A Causal Role for the Dopaminergic Mesolimbic System

Reward signals encoded in the mesolimbic dopaminergic system guide approach/seeking behaviors to all varieties of life-supporting stimuli (rewards). Differences in dopamine (DA) levels have been found between dominant and submissive animals. However, it is still unclear whether these differences arise as a consequence of the rewarding nature of the acquisition of a dominant rank, or whether they preexist and favor dominance by promoting reward-seeking behavior. Given that acquisition of a social rank determines animals' priority access to resources, we hypothesized that differences in reward-seeking behavior might affect hierarchy establishment and that modulation of the dopaminergic system could affect the outcome of a social competition. We characterized reward-seeking behaviors based on rats' latency to get a palatable-reward when given temporary access to it. Subsequently, rats exhibiting short (SL) and long (LL) latency to get the rewards cohabitated for more than 2 weeks, in order to establish a stable hierarchy. We found that SL animals exhibited dominant behavior consistently in social competition tests [for palatable-rewards and two water competition tests (WCTs)] after hierarchy was established, indicating that individual latency to rewards predicted dominance. Moreover, because SL animals showed higher mesolimbic levels of DA than LL rats, we tested whether stimulation of mesolimbic DA neurons could affect the outcome of a social competition. Indeed, a combination of optical stimulation of mesolimbic DA neurons during individual training and during a social competition test for palatable rewards resulted in improved performance on this test.

Alterations and adaptation of ventral tegmental area dopaminergic neurons in animal models of depression

Depression is one of the most prevalent psychiatric diseases, affecting the quality of life of millions of people. Ventral tegmental area (VTA) dopaminergic (DA) neurons are notably involved in evaluating the emotional and motivational value of a stimulus, in detecting reward prediction errors, in motivated learning, or in the propensity to initiate or withhold an action. DA neurons are thus involved in psychopathologies associated with perturbations of emotional and motivational states, such as depression. In this review, we focus on adaptations/alterations of the VTA, particularly of the VTA DA neurons, in the three most frequently used animal models of depression: learned helplessness, chronic mild stress and chronic social defeat.

Optogenetic investigation of neural mechanisms for alcohol-use disorder

Optogenetic techniques have been widely used in the study of neuropsychiatric diseases such as anxiety, depression, and drug addiction. Cell-type specific targeting of optogenetic tools to neurons has contributed to a tremendous understanding of the function of neural circuits for future treatment of neuropsychiatric disorders. Though optogenetics has been widely used in many research areas, the use of optogenetic tools to uncover and elucidate neural circuit mechanisms of alcohol's actions in the brain are still developing. Here in this review article, we will provide a basic introduction to optogenetics and discuss how these optogenetic experimental approaches can be used in alcohol studies to reveal neural circuit mechanisms of alcohol's actions in regions implicated in the development of alcohol addiction.

Potential Application of Optogenetic Stimulation in the Treatment of Pain and Migraine Headache: A Perspective from Animal Studies

Optogenetic manipulation is uniquely useful in unraveling the functional organization of neuronal circuits in the central nervous system by enabling reversible gain- or loss-of-function of discrete populations of neurons within restricted brain regions. This state-of-the-art technology can produce circuit-specific neuromodulation by overexpressing light-sensitive proteins (opsins) in particular cell types of interest. Here, we discuss the principle of optogenetic manipulation and its application in pain research using animal models, and we also discuss how to potentially use optogenetic stimulation in the treatment of migraine headache in the future.

The nucleus accumbens and alcoholism: a target for deep brain stimulation

Alcohol use disorder (AUD) is a difficult to treat condition with a significant global public health and cost burden. The nucleus accumbens (NAc) has been implicated in AUD and identified as an ideal target for deep brain stimulation (DBS). There are promising preclinical animal studies of DBS for alcohol consumption as well as some initial human clinical studies that have shown some promise at reducing alcohol-related cravings and, in some instances, achieving long-term abstinence. In this review, the authors discuss the evidence and concepts supporting the role of the NAc in AUD, summarize the findings from published NAc DBS studies in animal models and humans, and consider the challenges and propose future directions for neuromodulation of the NAc for the treatment of AUD.

Identification and validation of midbrain Kcnq4 regulation of heavy alcohol consumption in rodents

Currently available pharmacotherapies for treating alcohol use disorder (AUD) suffer from deleterious side effects and are not efficacious in diverse populations. Clinical and preclinical studies provide evidence that the Kcnq family of genes that encode K_V7 channels influence alcohol intake and dependence. K_V7 channels are a class of slowly activating voltage-dependent K⁺ channels that regulate neuronal excitability. Studies indicate that the K_V7 channel positive modulator retigabine can decrease dopaminergic neuron firing, alter dopamine (DA) release, and reduce alcohol intake in heavy drinking rodents. Given the critical nature of ventral tegmental area (VTA) DA to the addiction process and predominant expression of Kcnq4 in DA neurons, we investigated the role of midbrain Kcnq genes and K_V7 channels in the VTA of genetically diverse mice and long-term heavy drinking rats, respectively. Integrative bioinformatics analysis identified negative correlations between midbrain Kcnq4 expression and alcohol intake and seeking behaviors. Kcnq4 expression levels were also correlated with dopaminergic-related phenotypes in BXD strains, and Kcnq4 was present in support intervals for alcohol sensitivity and alcohol withdrawal severity QTLs in rodents. Pharmacological validation studies revealed that VTA K_V7 channels regulate excessive alcohol intake in rats with a high-drinking phenotype. Administration of a novel and selective K_V7.2/4 channel positive modulator also reduced alcohol drinking in rats. Together, these findings indicate that midbrain Kcnq4 expression regulates alcohol-related behaviors in genetically diverse mice and provide evidence that K_V7.4 channels are a critical mediator of excessive alcohol drinking.

Microfabricated Probes for Studying Brain Chemistry: A Review

Probe techniques for monitoring in vivo chemistry (e.g., electrochemical sensors and microdialysis sampling probes) have significantly contributed to a better understanding of neurotransmission in correlation to behaviors and neurological disorders. Microfabrication allows construction of neural probes with high reproducibility, scalability, design flexibility, and multiplexed features. This technology has translated well into fabricating miniaturized neurochemical probes for electrochemical detection and sampling. Microfabricated electrochemical probes provide a better control of spatial resolution with multisite detection on a single compact platform. This development allows the observation of heterogeneity of neurochemical activity precisely within the brain region. Microfabricated sampling probes are starting to emerge that enable chemical measurements at high spatial resolution and potential for reducing tissue damage. Recent advancement in analytical methods also facilitates neurochemical monitoring at high temporal resolution. Furthermore, a positive feature of microfabricated probes is that they can be feasibly built with other sensing and stimulating platforms including optogenetics. Such integrated probes will empower researchers to precisely elucidate brain function and develop novel treatments for neurological disorders.

Ventral tegmental area D2 receptor knockdown enhances choice impulsivity in a delay-discounting task in rats

Impulsivity associated with abnormal dopamine (DA) function has been observed in several disorders, including addiction. Choice impulsivity is the preference for small, immediate rewards over larger rewards after a delay, caused by excessive discounting of future rewards. Addicts have abnormally high discount rates and prefer the smaller rewards sooner. While impulsivity has been inversely correlated with DA D2 receptor (D2R) availability in the midbrain and striatum, it is difficult to mechanistically link the two, due to the diverse neuroanatomical localization of D2Rs, which are found throughout the brain, in many types of neurons and neuronal subcompartments. To determine if ventral tegmental area (VTA) D2R hypofunction is linked to impulsivity, we knocked down D2 receptors from the VTA, using an adeno-associated viral (AAV) vector that delivers short hairpin RNAs (shRNA) targeted against the D2R. The D2R knockdown is restricted to neurons whose cell bodies reside in the VTA, leaving postsynaptic D2Rs intact in the striatum, prefrontal cortex, and other mesocorticolimbic structures. Rats were trained in a delay-discounting task to assess impulsive choice until a stable discounting curve was obtained, and then received bilateral VTA infusions of the D2R shRNA or a scrambled control virus. Over the next six weeks, the discounting curve of the VTA D2R knockdown rats shifted to the left, indicating a preference for the smaller, immediate reward, whereas the curve for control rats remained stable and unchanged. Together these results demonstrate that a decrease in VTA D2Rs enhances choice impulsivity.

Effects of drugs of abuse on channelrhodopsin-2 function

Channelrhodopsins are light activated ion channels used extensively over the past decade to probe the function of genetically defined neuronal populations and distinct neural circuits with high temporal and spatial precision. The widely used Channelrhodopsin-2 variant (ChR2) is an excitatory opsin that undergoes conformational changes in response to blue light, allowing non-selective passage of protons and cations across the plasma membrane thus leading to depolarization. In the addiction neuroscience field, opsins such as ChR2 provide a means to disambiguate the overlapping circuitry involved in mediating the reinforcing and aversive effects of drugs of abuse as well as to determine the plasticity that can occur in these circuits during the development of dependence. Although ChR2 has been widely used in animal models of drug and alcohol self-administration, direct effects of drugs of abuse on ChR2 function may confound its use and lead to misinterpretation of data. As a variety of neuronal ion channels are primary targets of various drugs of abuse, it is critical to determine whether ChR2-mediated currents are modulated by these drugs. In this study, we performed whole-cell electrophysiological recordings in HEK293 cells expressing the commonly used ChR2(H134R) variant and examined the effects of various drugs of abuse and other commonly used agents on light-induced currents. We found no differences in ChR2-mediated currents in the presence of 30 μM nicotine, 30 μM cocaine, 100 μM methamphetamine or 3 mM toluene. Similarly, ChR2 currents were insensitive to 30 mM ethanol but higher concentrations (100-300 mM) produced significant effects on the desensitization and amplitude of light-evoked currents. Tetrahydrocannabinol (1-10 μM) and morphine (30-100 μM) significantly inhibited ChR2 currents while the cannabinoid receptor antagonist AM-251 had no effect. The sodium channel blocker tetrodotoxin (5 μM) and the generic channel blocker/contrast agent gadolinium chloride (10 mM) also reduced ChR2 currents while the divalent ion magnesium (10 mM) had no effect. Together, the results from this study highlight the importance of conducting appropriate control experiments when testing new compounds in combination with optogenetic approaches.

Exploring the consequences of social defeat stress and intermittent ethanol drinking on dopamine dynamics in the rat nucleus accumbens

The current study aimed to explore how presynaptic dopamine (DA) function is altered following brief stress episodes and chronic ethanol self-administration and whether these neuroadaptations modify the acute effects of ethanol on DA dynamics. We used fast-scan cyclic voltammetry to evaluate changes in DA release and uptake parameters in rat nucleus accumbens brain slices by analyzing DA transients evoked through single pulse electrical stimulation. Adult male rats were divided into four groups: ethanol-naïve or ethanol drinking (six week intermittent two-bottle choice) and stressed (mild social defeat) or nonstressed. Results revealed that the mild stress significantly increased DA release and uptake in ethanol-naïve subjects, compared to nonstressed controls. Chronic ethanol self-administration increased the DA uptake rate and occluded the effects of stress on DA release dynamics. Bath-applied ethanol decreased stimulated DA efflux in a concentration-dependent manner in all groups; however, the magnitude of this effect was blunted by either stress or chronic ethanol, or by a combination of both procedures. Together, these findings suggest that stress and ethanol drinking may promote similar adaptive changes in accumbal presynaptic DA release measures and that these changes may contribute to the escalation in ethanol intake that occurs during the development of alcohol use disorder.

Midbrain circuit regulation of individual alcohol drinking behaviors in mice

Alcohol-use disorder (AUD) is the most prevalent substance-use disorder worldwide. There is substantial individual variability in alcohol drinking behaviors in the population, the neural circuit mechanisms of which remain elusive. Utilizing in vivo electrophysiological techniques, we find that low alcohol drinking (LAD) mice have dramatically higher ventral tegmental area (VTA) dopamine neuron firing and burst activity. Unexpectedly, VTA dopamine neuron activity in high alcohol drinking (HAD) mice does not differ from alcohol naive mice. Optogenetically enhancing VTA dopamine neuron burst activity in HAD mice decreases alcohol drinking behaviors. Circuit-specific recordings reveal that spontaneous activity of nucleus accumbens-projecting VTA (VTA-NAc) neurons is selectively higher in LAD mice. Specifically activating this projection is sufficient to reduce alcohol consumption in HAD mice. Furthermore, we uncover ionic and cellular mechanisms that suggest unique neuroadaptations between the alcohol drinking groups. Together, these data identify a neural circuit responsible for individual alcohol drinking behaviors.

Mice exposed to a two-bottle alcohol choice paradigm can be divided into high and low drinking groups. Here, the authors show that stimulating VTA neurons to induce higher phasic activity patterns that are observed in low alcohol drinking mice, suppresses alcohol drinking in mice that are high alcohol drinking.

Optogenetic activation of amygdala projections to nucleus accumbens can arrest conditioned and unconditioned alcohol consummatory behavior

Following a Pavlovian pairing procedure, alcohol-paired cues come to elicit behavioral responses that lead to alcohol consumption. Here we used an optogenetic approach to activate basolateral amygdala (BLA) axonal terminals targeting the shell of nucleus accumbens (AcbSh) and investigated a possible influence over cue-conditioned alcohol seeking and alcohol drinking, based on the demonstrated roles of these areas in behavioral responding to Pavlovian cues and in feeding behavior. Rats were trained to anticipate alcohol or sucrose following the onset of a discrete conditioned stimulus (CS). Channelrhodopsin-mediated activation of the BLA-to-AcbSh pathway concurrent with each CS disrupted cued alcohol seeking. Activation of the same pathway caused rapid cessation of alcohol drinking from a sipper tube. Neither effect was accompanied by an overall change in locomotion. Finally, the suppressive effect of photoactivation on cued-triggered seeking was also evidenced in animals trained with sucrose. Together these findings suggest that photoactivation of BLA terminals in the AcbSh can override the conditioned motivational properties of reward-predictive cues as well as unconditioned consummatory responses necessary for alcohol drinking. The findings provide evidence for a limbic-striatal influence over motivated behavior for orally consumed rewards, including alcohol.

Left frontal pole theta burst stimulation decreases orbitofrontal and insula activity in cocaine users and alcohol users

BACKGROUND

Preclinical research has demonstrated a causal relationship between medial prefrontal cortex activity and cocaine self-administration. As a step towards translating those data to a neural circuit-based intervention for patients, this study sought to determine if continuous theta burst stimulation (cTBS) to the left frontal pole (FP), would attenuate frontal-striatal activity in two substance-dependent populations.

METHODS

Forty-nine substance dependent individuals (25 cocaine, 24 alcohol) completed a single-blind, sham-controlled, crossover study wherein they received 6 trains of real or sham cTBS (110% resting motor threshold, FP1) each visit. Baseline evoked BOLD signal was measured immediately before and after real and sham cTBS (interleaved TMS/BOLD imaging: single pulses to left FP; scalp-to-cortex distance covariate, FWE correction p<0.05) RESULTS: Among cocaine users, real cTBS significantly decreased evoked BOLD signal in the caudate, accumbens, anterior cingulate, orbitofrontal (OFC) and parietal cortex relative to sham cTBS. Among alcohol users, real cTBS significantly decreased evoked BOLD signal in left OFC, insula, and lateral sensorimotor cortex. There was no significant difference between the groups.

CONCLUSIONS

These data suggest that 6 trains of left FP cTBS delivered in a single day decreases TMS-evoked BOLD signal in the OFC and several cortical nodes which regulate salience and are typically activated by drug cues. The reliability of this pattern across cocaine- and alcohol-dependent individuals suggests that cTBS may be an effective tool to dampen neural circuits typically engaged by salient drug cues. Multiday studies are required to determine it this has a sustainable effect on the brain or drug use behavior.

Contrasting Regulation of Catecholamine Neurotransmission in the Behaving Brain: Pharmacological Insights from an Electrochemical Perspective

Catecholamine neurotransmission plays a key role in regulating a variety of behavioral and physiologic processes, and its dysregulation is implicated in both neurodegenerative and neuropsychiatric disorders. Over the last four decades, in vivo electrochemistry has enabled the discovery of contrasting catecholamine regulation in the brain. These rapid and spatially resolved measurements have been conducted in brain slices, and in anesthetized and freely behaving animals. In this review, we describe the methods enabling in vivo measurements of dopamine and norepinephrine, and subsequent findings regarding their release and regulation in intact animals. We thereafter discuss key studies in awake animals, demonstrating that these catecholamines are not only differentially regulated, but are released in opposition of each other during appetitive and aversive stimuli.

Altered sucrose self-administration following injection of melanocortin receptor agonists and antagonists into the ventral tegmental area

RATIONALE AND OBJECTIVES

Alpha-melanocyte stimulating hormone (αMSH) and agouti-related protein (AgRP) are antagonistic neuropeptides that play an important role in the control of feeding and body weight through their central actions on the melanocortin-3 and melanocortin-4 receptors. Increasing evidence indicates that αMSH and AgRP can interact with the mesolimbic dopamine system to regulate feeding as well as other behaviors. For example, we have shown previously that injection of melanocortin receptor agonists and antagonists into the ventral tegmental area (VTA) alters both normal home-cage feeding and the intake of sucrose solutions, but it remains unknown whether αMSH and AgRP can also act in the VTA to affect reward-related feeding.

METHODS

We tested whether injection of the melanocortin receptor agonist, MTII, or the melanocortin receptor antagonist, SHU9119, directly into the VTA affected operant responding maintained by sucrose pellets in self-administration assays.

RESULTS

Injection of MTII into the VTA decreased operant responding maintained by sucrose pellets on both fixed ratio and progressive ratio schedules of reinforcement, whereas SHU9119 increased operant responding under fixed ratio, but not progressive ratio schedules. MTII also increased and SHU9119 decreased 24-h home-cage food intake.

CONCLUSIONS

This study demonstrates that αMSH and AgRP act in the VTA to affect sucrose self-administration. Thus, it adds critical information to the growing literature showing that in addition to their well-characterized role in controlling "need-based" feeding, αMSH and AgRP can also act on the mesolimbic dopamine system to control reward-related behavior.

Recombinant Adeno-Associated Virus-mediated rescue of function in a mouse model of Dopamine Transporter Deficiency Syndrome

Dopamine Transporter Deficiency Syndrome (DTDS) is a rare autosomal recessive disorder caused by loss-of-function mutations in dopamine transporter (DAT) gene, leading to severe neurological disabilities in children and adults. DAT-Knockout (DAT-KO) mouse is currently the best animal model for this syndrome, displaying functional hyperdopaminergia and neurodegenerative phenotype leading to premature death in ~36% of the population. We used DAT-KO mouse as model for DTDS to explore the potential utility of a novel combinatorial adeno-associated viral (AAV) gene therapy by expressing DAT selectively in DA neurons and terminals, resulting in the rescue of aberrant striatal DA dynamics, reversal of characteristic phenotypic and behavioral abnormalities, and prevention of premature death. These data indicate the efficacy of a new combinatorial gene therapy aimed at rescuing DA function and related phenotype in a mouse model that best approximates DAT deficiency found in DTDS.

Unexpected global impact of VTA dopamine neuron activation as measured by opto-fMRI

Dopamine neurons in the ventral tegmental area (VTA) are strongly implicated in cognitive and affective processing as well as in psychiatric disorders, including schizophrenia, depression, attention-deficit hyperactivity disorder and substance abuse disorders. In human studies, dopamine-related functions are routinely assessed using functional magnetic resonance imaging (fMRI) measures of blood oxygenation-level-dependent (BOLD) signals during the performance of dopamine-dependent tasks. There is, however, a critical void in our knowledge about whether and how activation of VTA dopamine neurons specifically influences regional or global fMRI signals. Here, we used optogenetics in Th::Cre rats to selectively stimulate VTA dopamine neurons while simultaneously measuring global hemodynamic changes using BOLD and cerebral blood volume-weighted (CBVw) fMRI. Phasic activation of VTA dopamine neurons increased BOLD and CBVw fMRI signals in VTA-innervated limbic regions, including the ventral striatum (nucleus accumbens). Surprisingly, basal ganglia regions that receive sparse or no VTA dopaminergic innervation, including the dorsal striatum and the globus pallidus, were also activated. In fact, the most prominent fMRI signal increase in the forebrain was observed in the dorsal striatum that is not traditionally associated with VTA dopamine neurotransmission. These data establish causation between phasic activation of VTA dopamine neurons and global fMRI signals. They further suggest that mesolimbic and non-limbic basal ganglia dopamine circuits are functionally connected and thus provide a potential novel framework for understanding dopamine-dependent functions and interpreting data obtained from human fMRI studies.