Astrocytic equilibrative nucleoside transporter type 1 upregulations in the dorsomedial and dorsolateral striatum distinctly coordinate goal-directed and habitual ethanol-seeking behaviours in mice

Adenosinergic system and nucleoside transporters in attention deficit hyperactivity disorder: Current findings

Attention deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder with high heterogeneity that can affect individuals of any age. It is characterized by three main symptoms: inattention, hyperactivity, and impulsivity. These neurobehavioral alterations and neurochemical and pharmacological findings are mainly attributed to unbalanced catecholaminergic signaling, especially involving dopaminergic pathways within prefrontal and striatal areas. Dopamine receptors and transporters are not solely implicated in this imbalance, as evidence indicates that the dopaminergic signaling is modulated by adenosine activity. To this extent, alterations in adenosinergic signaling are probably involved in ADHD. Here, we review the current knowledge about adenosine's role in the modulation of chemical, behavioral and cognitive parameters of ADHD, especially regarding dopaminergic signaling. Current literature usually links adenosine receptors signaling to the dopaminergic imbalance found in ADHD, but there is evidence that equilibrative nucleoside transporters (ENTs) could also be implicated as players in dopaminergic signaling alterations seen in ADHD, since their involvement in other neurobehavioral impairments.

External globus pallidus input to the dorsal striatum regulates habitual seeking behavior in male mice

The external globus pallidus (GPe) coordinates action-selection through GABAergic projections throughout the basal ganglia. GPe arkypallidal (arky) neurons project exclusively to the dorsal striatum, which regulates goal-directed and habitual seeking. However, the role of GPe arky neurons in reward-seeking remains unknown. Here, we identified that a majority of arky neurons target the dorsolateral striatum (DLS). Using fiber photometry, we found that arky activities were higher during random interval (RI; habit) compared to random ratio (RR; goal) operant conditioning. Support vector machine analysis demonstrated that arky neuron activities have sufficient information to distinguish between RR and RI behavior. Genetic ablation of this arkyGPe→DLS circuit facilitated a shift from goal-directed to habitual behavior. Conversely, chemogenetic activation globally reduced seeking behaviors, which was blocked by systemic D1R agonism. Our findings reveal a role of this arkyGPe→DLS circuit in constraining habitual seeking in male mice, which is relevant to addictive behaviors and other compulsive disorders.

Astrocytes: the neglected stars in the central nervous system and drug addiction

With the advent of improved tools to examine the astrocytes, which have been believed to play a supportive role in the central nervous system (CNS) for years, their participation in the operation of the CNS and drug addiction was unveiled. Assisting the formation and function of the CNS, astrocytes are involved in physiological and pathological brain activities. Drug addiction is a pervasive psychiatric disorder, characterized by compulsive drug-taking behavior and high rate of relapse, impacting individual health and society stability and safety. When exposed to drugs of abuse, astrocytes go through a series of alterations, contributing to the development of addiction. Here we review how astrocytes contribute to the CNS and drug addiction. We hope that understanding the interaction between addictive drugs and astrocytes may help discover new mechanisms underlying the addiction and produce novel therapeutic treatments.

Dopamine Release in Nucleus Accumbens Is under Tonic Inhibition by Adenosine A1 Receptors Regulated by Astrocytic ENT1 and Dysregulated by Ethanol

Striatal adenosine A₁ receptor (A₁R) activation can inhibit dopamine release. A₁Rs on other striatal neurons are activated by an adenosine tone that is limited by equilibrative nucleoside transporter 1 (ENT1) that is enriched on astrocytes and is ethanol sensitive. We explored whether dopamine release in nucleus accumbens core is under tonic inhibition by A₁Rs, and is regulated by astrocytic ENT1 and ethanol. In ex vivo striatal slices from male and female mice, A₁R agonists inhibited dopamine release evoked electrically or optogenetically and detected using fast-scan cyclic voltammetry, most strongly for lower stimulation frequencies and pulse numbers, thereby enhancing the activity-dependent contrast of dopamine release. Conversely, A₁R antagonists reduced activity-dependent contrast but enhanced evoked dopamine release levels, even for single optogenetic pulses indicating an underlying tonic inhibition. The ENT1 inhibitor nitrobenzylthioinosine reduced dopamine release and promoted A₁R-mediated inhibition, and, conversely, virally mediated astrocytic overexpression of ENT1 enhanced dopamine release and relieved A₁R-mediated inhibition. By imaging the genetically encoded fluorescent adenosine sensor [GPCR-activation based (GRAB)-Ado], we identified a striatal extracellular adenosine tone that was elevated by the ENT1 inhibitor and sensitive to gliotoxin fluorocitrate. Finally, we identified that ethanol (50 mm) promoted A₁R-mediated inhibition of dopamine release, through diminishing adenosine uptake via ENT1. Together, these data reveal that dopamine output dynamics are gated by a striatal adenosine tone, limiting amplitude but promoting contrast, regulated by ENT1, and promoted by ethanol. These data add to the diverse mechanisms through which ethanol modulates striatal dopamine, and to emerging datasets supporting astrocytic transporters as important regulators of striatal function.SIGNIFICANCE STATEMENT Dopamine axons in the mammalian striatum are emerging as strategic sites where neuromodulators can powerfully influence dopamine output in health and disease. We found that ambient levels of the neuromodulator adenosine tonically inhibit dopamine release in nucleus accumbens core via adenosine A₁ receptors (A₁Rs), to a variable level that promotes the contrast in dopamine signals released by different frequencies of activity. We reveal that the equilibrative nucleoside transporter 1 (ENT1) on astrocytes limits this tonic inhibition, and that ethanol promotes it by diminishing adenosine uptake via ENT1. These findings support the hypotheses that A₁Rs on dopamine axons inhibit dopamine release and, furthermore, that astrocytes perform important roles in setting the level of striatal dopamine output, in health and disease.

Astrocyte-neuron interaction in the dorsal striatum-pallidal circuits and alcohol-seeking behaviors

In the striatum, two main types of GABAergic medium spiny neurons (MSNs), denoted striatonigral (or direct-pathway MSNs, dMSNs) and striatopallidal neurons (indirect-pathway MSNs, iMSNs), form circuits with distinct pallidal nuclei, which sends "GO" or "NO-GO" signals through the thalamus. These striatopallidal circuits evaluate and execute reward-seeking and taking behaviors. Especially, the dorsal striatum can be further divided into the dorsomedial striatum (DMS, equivalent to caudate in primates and humans) and dorsolateral striatum (DLS, equivalent to putamen), which orchestrates goal-directed and habitual reward-seeking and taking behaviors, respectively. Using optogenetics, chemogenetics and in vivo calcium imaging technologies combined with electrophysiology and digitalized behavior phenotyping, recent studies have revealed cell-, circuit- and context-specific functions of these microcircuits in addictive behaviors. Also, region-specific astrocytes regulate the homeostatic activities of the dMSNs and iMSNs as well as the downstream circuits, which determine the net balance of cortico-striato-pallidal activities to the thalamic neurons. This review will summarize the recent progress of striatopallidal circuits focusing on astrocyte-neuron interaction and, reward- and alcohol-seeking behaviors. Our review will also discuss the translational and clinical implications of these microcircuit studies. This article is part of the special Issue on "Neurocircuitry Modulating Drug and Alcohol Abuse".