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.

miR-98 reduces endothelial dysfunction by protecting blood-brain barrier (BBB) and improves neurological outcomes in mouse ischemia/reperfusion stroke model

Most neurological diseases, including stroke, lead to some degree of blood-brain barrier (BBB) dysfunction. A significant portion of BBB injury is caused by inflammation, due to pro-inflammatory factors produced in the brain, and by leukocyte engagement of the brain endothelium. Recently, microRNAs (miRNAs) have appeared as major regulators of inflammation-induced changes to gene expression in the microvascular endothelial cells (BMVEC) that comprise the BBB. However, miRNAs' role during cerebral ischemia/reperfusion is still underexplored. Endothelial levels of miR-98 were significantly altered following ischemia/reperfusion insults, both in vivo and in vitro, transient middle cerebral artery occlusion (tMCAO), and oxygen-glucose deprivation (OGD), respectively. Overexpression of miR-98 reduced the mouse's infarct size after tMCAO. Further, miR-98 lessened infiltration of proinflammatory Ly6C^HI leukocytes into the brain following stroke and diminished the prevalence of M1 (activated) microglia within the impacted area. miR-98 attenuated BBB permeability, as demonstrated by changes to fluorescently-labeled dextran penetration in vivo and improved transendothelial electrical resistance (TEER) in vitro. Treatment with miR-98 improved significantly the locomotor impairment. Our study provides identification and functional assessment of miRNAs in brain endothelium and lays the groundwork for improving therapeutic approaches for patients suffering from ischemic attacks.

Let-7g* and miR-98 Reduce Stroke-Induced Production of Proinflammatory Cytokines in Mouse Brain

Stroke is a debilitating illness facing healthcare today, affecting over 800,000 people and causing over 140,000 deaths each year in the United States. Despite being the third-leading cause of death, very few treatments currently exist for stroke. Often, during an ischemic attack, the blood-brain barrier (BBB) is significantly damaged, which can lead to altered interactions with the immune system, and greatly worsen the damage from a stroke. The impaired, BBB promotes the infiltration of peripheral inflammatory cells into the brain, secreting deleterious mediators (cytokines/chemokines) and resulting in permanent barrier injury. let-7 microRNAs (miRs) are critical for regulating immune responses within the BBB, particularly after ischemic stroke. We have previously shown how transient stroke decreases expression of multiple let-7 miRs, and that restoration of expression confers significant neuroprotection, reduction in brain infiltration by neutrophils, monocytes and T cells. However, the specific mechanisms of action of let-7 miRs remain unexplored, though emerging evidence implicates a range of impacts on cytokines. In the current study, we evaluate the impacts of miR-98 and let-7g* on targeting of cytokine mRNAs, cytokine release following ischemic stroke, and cell-specific changes to the neurovascular space. We determined that miR-98 specifically targets IP-10, while let-7g* specifically aims IL-8, and attenuates their levels. Both produce strong impacts on CCL2 and CCL5. Further, let-7g* strongly improves neurovascular perfusion following ischemic stroke. Together, the results of the study indicate that let-7 miRs are critical for mediating endothelial-immune reactions and improving recovery following ischemic stroke.

let-7g counteracts endothelial dysfunction and ameliorating neurological functions in mouse ischemia/reperfusion stroke model

Stroke is a debilitating disease, accounting for almost 20% of all hospital visits, and 8% of all fatalities in the United States in 2017. Following an ischemic attack, inflammatory processes originating from endothelial cells within the brain microvasculature can induce many toxic effects into the impacted area, from both sides of the blood brain barrier (BBB). In addition to increased BBB permeability, impacted brain microvascular endothelial cells can recruit macrophages and other immune cells from the periphery and can also trigger the activation of microglia and astrocytes within the brain. We have identified a key microRNA, let-7g, which levels were drastically diminished as consequence of transient middle cerebral artery occlusion (tMCAO) in vivo and oxygen-glucose deprivation (OGD) in vitro ischemia/reperfusion conditions, respectively. We have observed that let-7g* liposome-based delivery is capable of attenuating inflammation after stroke, reducing BBB permeability, limiting brain infiltration by CD3+CD4+ T-cells and Ly6G+ neutrophils, lessening microglia activation and neuronal death. These effects consequently improved clinical outcomes, shown by mitigating post-stroke gait asymmetry and extremity motor function. Due to the role of the endothelium in propagating the effects of stroke and other inflammation, treatments which can reduce endothelial inflammation and limit ischemic damage and improving recovery after a stroke are required. Our findings demonstrate a critical link between the CNS inflammation and the immune system reaction and lay important groundwork for future stroke pharmacotherapies.

Methylenedioxypyrovalerone (MDPV) impairs working memory and alters patterns of dopamine signaling in mesocorticolimbic substrates

Knowledge remains limited about how chronic cathinone exposure impacts dopamine systems in brain reward circuits. In the present study, a binge-like MDPV exposure that impaired novel object recognition (NOR) dysregulated dopamine markers in mesocorticolimbic substrates of rats, with especially profound effects on D1 and D2 receptor's and VMAT gene expression. Our data suggested that dopamine receptivity was reduced in the NAc but increased in the PFC and dopamine-producing VTA. The MDPV-induced impairment of NOR was prevented by a D1 receptor antagonist, suggesting that chronic MDPV exposure produces site-specific dysregulation of dopamine markers in the mesocorticolimbic circuit and memory deficits in the NOR test that are influenced by D1 receptors.

Hypocretin receptor 1 knockdown in the ventral tegmental area attenuates mesolimbic dopamine signaling and reduces motivation for cocaine

The hypocretin receptor 1 (HCRTr1) is a critical participant in the regulation of motivated behavior. Previous observations demonstrate that acute pharmacological blockade of HCRTr1 disrupts dopamine (DA) signaling and the motivation for cocaine when delivered systemically or directly into the ventral tegmental area (VTA). To further examine the involvement of HCRTr1 in regulating reward and reinforcement processing, we employed an adeno-associated virus to express a short hairpin RNA designed to knock down HCRTr1. We injected virus into the VTA and examined the effects of HCRTr1 knockdown on cocaine self-administration and DA signaling in the nucleus accumbens (NAc) core. We determined that the viral approach was effective at reducing HCRTr1 expression without affecting the expression of hypocretin receptor 2 or DA-related mRNAs. We next examined the effects of HCRTr1 knockdown on cocaine self-administration, observing delayed acquisition under a fixed-ratio schedule and reduced motivation for cocaine under a progressive ratio schedule. These effects did not appear to be associated with alterations in sleep/wake activity. Using fast-scan cyclic voltammetry, we then examined whether HCRTr1 knockdown alters DA signaling dynamics in the NAc core. We observed reduced DA release and slower uptake rate as well as attenuated cocaine-induced DA uptake inhibition in rats with knockdown of HCRTr1. These observations indicate that HCRTr1 within the VTA influence the motivation for cocaine, likely via alterations in DA signaling in the NAc.

Hypocretin receptor 1 blockade preferentially reduces high effort responding for cocaine without promoting sleep

Recent evidence suggests that blockade of the hypocretin receptor 1 may act as a useful pharmacotherapy for cocaine abuse. Here we investigated the extent to which various doses of a hypocretin receptor 1 antagonist, SB-334867, affect cocaine self-administration at varying doses of cocaine and across a range of effort requirements, and tested if these SB-334867 doses produce sedative effects. First, we trained animals to self-administer one of three doses of cocaine on a progressive ratio schedule, and then tested the effects of three doses of SB-334867. Responding for cocaine was then analyzed to segregate features of relatively high and low effort requirements across the progressive ratio session. In another set of experiments, we tested potential sleep-promoting effects of the same doses of SB-334867. Our data indicate that blockade of hypocretin receptor 1 preferentially reduces high effort responding for cocaine at levels that do not promote sedation.

Endocannabinoids promote cocaine-induced impulsivity and its rapid dopaminergic correlates

BACKGROUND

Impaired decision making, a hallmark of addiction, is hypothesized to arise from maladaptive plasticity in the mesolimbic dopamine pathway. The endocannabinoid system modulates dopamine activity through activation of cannabinoid type 1 receptors (CB1Rs). Here, we investigated whether impulsive behavior observed following cocaine exposure requires CB1R activation.

METHODS

We trained rats in a delay-discounting task. Following acquisition of stable performance, rats were exposed to cocaine (10 mg/kg, intraperitoneal) every other day for 14 days and locomotor activity was measured. Two days later, delay-discounting performance was re-evaluated. To assess reversal of impulsivity, injections of a CB1R antagonist (1.5 mg/kg, intraperitoneal) or vehicle were given 30 minutes before the task. During the second experiment, aimed at preventing impulsivity rather than reversing it, CB1Rs were antagonized before each cocaine injection. In this experiment, subsecond dopamine release was measured in the nucleus accumbens during delay-discounting sessions before and after cocaine treatment.

RESULTS

Blockade of CB1Rs reversed and prevented cocaine-induced impulsivity. Electrochemical results showed that during baseline and following disruption of endocannabinoid signaling, there was a robust increase in dopamine for immediate large rewards compared with immediate small rewards, but this effect reversed when the delay for the large reward was 10 seconds. In contrast, dopamine release always increased for one-pellet options at minimal or moderate delays in vehicle-treated rats.

CONCLUSIONS

Endocannabinoids play a critical role in changes associated with cocaine exposure. Cannabinoid type 1 receptor blockade may thus counteract maladaptive alterations in afferents to dopamine neurons, thereby preventing changes in dopaminergic activity underlying a loss of self-control.