Hypocretin / Orexin Receptor 1 Knockdown in GABA or Dopamine Neurons in the Ventral Tegmental Area Differentially Impact Mesolimbic Dopamine and Motivation for Cocaine

EAAT2 Activation Regulates Glutamate Excitotoxicity and Reduces Impulsivity in a Rodent Model of Parkinson's Disease

Parkinson's disease (PD) is a systemic disease characterized by motor and nonmotor impairments. Loss of dopaminergic neurons in the substantia nigra pars compacta region in PD disrupts dopamine-glutamate homeostasis in the corticostriatal circuit, contributing to cognitive impairment. In addition, excitatory amino acid transporter-2 (EAAT2), localized predominantly to astrocytes and responsible for > 80% of synaptic glutamate clearance, is downregulated in PD, causing glutamate spillover and excitotoxicity. This altered dopamine-glutamate homeostasis and excitotoxicity may affect reward-mediated decision-making behaviors and promote impulsive behaviors in PD. In this study, we hypothesized that GTS467, a small-molecule activator of EAAT2, could effectively reduce excitotoxicity and treat cognitive impairment without promoting impulsive behavior in PD. Rats that were unilaterally lesioned with the 6-OHDA toxin to produce Parkinsonian symptoms were referred to as lesioned rats. Lesioned rats were trained to meet baseline criteria in a 5-choice serial reaction time task, and the chronic effects of GTS467 were assessed after 3 weeks of treatment. The results showed that chronic treatment with GTS467 significantly improved correct responses and reduced premature impulsive responses and omissions compared with saline treatment. This improvement in performance correlated with a reduction in glutamate levels, an increase in EAAT2 expression, and normalization of NMDA receptor subunit expression and signaling. Furthermore, transcriptomic studies on the prefrontal cortex tissue have shown the differential expression of genes involved in neuroprotection, neuroinflammation, learning, and memory. These results validate the role of glutamate excitotoxicity in promoting impulsive behaviors and suggest that GTS467 can be developed as a therapeutic agent to reduce cognitive impairment and impulsive behaviors in PD.

Dual Hypocretin Receptor Antagonism Reduces Oxycodone Seeking During Abstinence

A major barrier in the treatment of opioid use disorder is persistent drug craving during abstinence. While opioid-based medications have been used to treat opioid use disorder for decades, there is an urgent need for novel, non-opioid-based pharmacotherapies. The hypocretin/orexin (hypocretin) system is a promising target for treating opioid use disorder due to its influence on motivation for drugs of abuse through actions on dopamine transmission. We recently showed that intermittent access (IntA) to oxycodone promoted sustained oxycodone seeking and alterations in dopamine transmission during abstinence. In the current studies, we investigated to what extent suvorexant, an FDA-approved dual hypocretin receptor antagonist, reduces oxycodone seeking and restores dopamine function during abstinence. Results indicated that IntA to oxycodone produced sustained cue-induced oxycodone seeking after a 14-day abstinence period, which was associated with reduced dopamine uptake in the nucleus accumbens core as we have previously shown. Treatment with suvorexant 24 h prior to a cue-induced seeking test significantly reduced oxycodone seeking and normalized aberrant dopamine uptake. These findings suggest that targeting hypocretin receptors may be a promising strategy for reducing opioid craving and associated neuroadaptations, thus lowering the risk of relapse.

A Preliminary Report on the Effects of Daridorexant in Patients with Comorbid Insomnia and Substance Use Disorders

Background. Sleep disturbances are frequent in patients with substance use disorders (SUDs) and are associated with craving and addiction relapses, leading to increased clinical severity and detrimental outcomes. Daridorexant, a selective dual orexin receptor antagonist, has been approved for persistent insomnia disorder (ID), but specific insights on patients with SUDs are lacking. Methods. This observational, retrospective study investigated the effects of a three-month treatment with daridorexant (50 mg/day) in 41 outpatients with comorbid IDs and SUDs. Improvement in subjective sleep measures, assessed with the Insomnia Severity Index (ISI) and subjective total sleep time, was the primary outcome measure. Changes in anxiety and depression symptoms, quality of life, clinical global severity, and craving were also investigated through the following: Hamilton Anxiety and Depression Rating Scale; Five-item World Health Organization Well-Being Index; Clinical Global Impression Severity Scale; Visual Analog Scale for Craving. Results. All sleep outcomes significantly improved throughout treatment, which was generally safe and well tolerated, with mild and transient drowsiness and sluggishness reported in 21.1% of patients. Similar improvements were observed in psychopathology, quality of life, and craving, and positive correlations were found among ISI scores and anxiety/depression symptoms and craving. An abstinence rate (i.e., absence of any substance use, regardless of the amount, throughout treatment) of 65.8% was also detected at the endpoint. Conclusions. These preliminary findings suggest that daridorexant might represent a promising tool for treating insomnia in patients with SUDs. Identifying interventions effectively targeting insomnia with a good safety/tolerability profile in SUDs is crucial to achieve remission and full functional recovery.

Hypocretin Receptor 1 Blockade Early in Abstinence Prevents Incubation of Cocaine Seeking and Normalizes Dopamine Transmission

Abstinence from cocaine use has been shown to elicit a progressive intensification or incubation of cocaine craving/seeking that is posited to contribute to propensity for relapse. While the mechanisms underlying incubation of cocaine seeking remain elusive, considerable evidence suggests that abstinence from cocaine promotes mesolimbic dopamine adaptations that contribute to exaggerated cocaine seeking. Consequently, preventing these dopamine adaptations may reduce incubation of cocaine seeking and thereby reduce the likelihood of relapse. In the present studies, we first examined if incubation of cocaine seeking was associated with aberrant dopamine transmission in the nucleus accumbens after seven days of abstinence from intermittent access to cocaine. Given the extensive evidence that hypocretins/orexins regulate motivation for cocaine, we then examined to what extent hypocretin receptor 1 antagonism on the first day of abstinence prevented incubation of cocaine seeking and dopamine adaptations later in abstinence. Results indicated that abstinence from intermittent access to cocaine engendered robust incubation of cocaine seeking in both female and male rats. We also observed aberrant dopamine transmission, but only in rats that displayed incubation of cocaine seeking. Further, we showed that a single injection of the hypocretin receptor 1 antagonist, RTIOX-276, on the first day of abstinence prevented incubation of cocaine seeking and aberrant dopamine transmission. These findings suggest that hypocretin receptor 1 antagonism may serve as a viable therapeutic for reducing cocaine craving/seeking, thus reducing the likelihood of relapse.

Inactivation of ERK1/2 Signaling in Dopaminergic Neurons by Map Kinase Phosphatase MKP3 Regulates Dopamine Signaling and Motivation for Cocaine

The mesolimbic dopamine system is a crucial component of reward and reinforcement processing, including the psychotropic effects of drugs of abuse such as cocaine. Drugs of abuse can activate intracellular signaling cascades that engender long-term molecular changes to brain reward circuitry, which can promote further drug use. However, gaps remain about how the activity of these signaling pathways, such as ERK1/2 signaling, can affect cocaine-induced neurochemical plasticity and cocaine-associated behaviors specifically within dopaminergic cells. To enable specific modulation of ERK1/2 signaling in dopaminergic neurons of the ventral tegmental area, we utilize a viral construct that Cre dependently expresses Map kinase phosphatase 3 (MKP3) to reduce the activity of ERK1/2, in combination with transgenic rats that express Cre in tyrosine hydroxylase (TH)-positive cells. Following viral transfection, we found an increase in the surface expression of the dopamine transporter (DAT), a protein associated with the regulation of dopamine signaling, dopamine transmission, and cocaine-associated behavior. We found that inactivation of ERK1/2 reduced post-translational phosphorylation of the DAT, attenuated the ability of cocaine to inhibit the DAT, and decreased motivation for cocaine without affecting associative learning as tested by conditioned place preference. Together, these results indicate that ERK1/2 signaling plays a critical role in shaping the dopamine response to cocaine and may provide additional insights into the function of dopaminergic neurons. Further, these findings lay important groundwork toward the assessment of how signaling pathways and their downstream effectors influence dopamine transmission and could ultimately provide therapeutic targets for treating cocaine use disorders.