Epac Signaling Is Required for Cocaine-Induced Change in AMPA Receptor Subunit Composition in the Ventral Tegmental Area

Epac2-mediated synaptic insertion of Ca2+-permeable AMPARs in the nucleus accumbens contributes to incubation of cocaine craving

The accumulation of GluA2-lacking Ca2+-permeable AMPARs (CP-AMPARs) in the medium spiny neurons (MSNs) of the nucleus accumbens (NAc) is required for the expression of incubation of cocaine craving. The exchange protein directly activated by cAMP (Epac) is an intracellular effector of cAMP and a guanine nucleotide exchange factor for the small GTPase Rap1. Epac2 has been implicated in the trafficking of AMPA receptors at central synapses. We tested the hypothesis that Epac2 activation contributes to the accumulation of CP-AMPARs in NAc MSNs and incubation of cocaine craving. Here we demonstrate that the selective Epac2 agonist S-220 facilitated the synaptic insertion of GluA2-lacking CP-AMPARs at excitatory synapses onto NAc MSNs. In addition, prolonged abstinence from cocaine self-administration in rats resulted in elevated Rap1-GTP levels in the NAc, implying that Epac2 is activated during incubation. Importantly, we show that AAV-mediated shRNA knockdown of Epac2 in the NAc core attenuated the accumulation of CP-AMPARs and cue-induced drug-seeking behavior after prolonged abstinence from cocaine self-administration. In contrast, acute pharmacological inhibition of Epac2 with the selective Epac2 inhibitor ESI-05 did not alter CP-AMPARs that had already accumulated during incubation, and intra-NAc application of ESI-05 did not significantly affect cue-induced drug seeking following prolonged abstinence. Taken together, these results suggest that Epac2 activation during the period of incubation, but not during cue-induced drug seeking, leads to the accumulation of CP-AMPARs in NAc MSNs, which in turn contributes to incubation of cocaine craving.

Treadmill Exercise Facilitates Synaptic Plasticity in APP/PS1 Mice by Regulating Hippocampal AMPAR Activity

Accumulating evidence underscores exercise as a straightforward and cost-effective lifestyle intervention capable of mitigating the risk and slowing the emergence and progression of Alzheimer's disease (AD). However, the intricate cellular and molecular mechanisms mediating these exercise-induced benefits in AD remain elusive. The present study delved into the impact of treadmill exercise on memory retrieval performance, hippocampal synaptic plasticity, synaptic morphology, and the expression and activity of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic receptors (AMPARs) in 6-month-old APP/PS1 mice. APP/PS1 mice (4-month-old males) were randomly assigned to either a treadmill exercise group or a sedentary group, with C57BL/6J mice (4-month-old males) as the control group (both exercise and sedentary). The exercise regimen spanned 8 weeks. Our findings revealed that 8-week treadmill exercise reversed memory retrieval impairment in step-down fear conditioning in 6-month-old APP/PS1 mice. Additionally, treadmill exercise enhanced basic synaptic strength, short-term potentiation (STP), and long-term potentiation (LTP) of the hippocampus in these mice. Moreover, treadmill exercise correlated with an augmentation in synapse numbers, refinement of synaptic structures, and heightened expression and activity of AMPARs. Our findings suggest that treadmill exercise improves behavioral performance and facilitates synaptic transmission by increasing structural synaptic plasticity and the activity of AMPARs in the hippocampus of 6-month-old APP/PS1 mice, which is involved in pre- and postsynaptic processes.

Psilocybin analog 4-OH-DiPT enhances fear extinction and GABAergic inhibition of principal neurons in the basolateral amygdala

Psychedelics such as psilocybin show great promise for the treatment of depression and PTSD, but their long duration of action poses practical limitations for patient access. 4-OH-DiPT is a fast-acting and shorter-lasting derivative of psilocybin. Here we characterized the pharmacological profile of 4-OH-DiPT and examined its impact on fear extinction learning as well as a potential mechanism of action. First, we profiled 4-OH-DiPT at all 12 human 5-HT GPCRs. 4-OH-DiPT showed strongest agonist activity at all three 5-HT2A/2B/2C receptors with near full agonist activity at 5-HT2A. Notably, 4-OH-DiPT had comparable activity at mouse and human 5-HT2A/2B/2C receptors. In a fear extinction paradigm, 4-OH-DiPT significantly reduced freezing responses to conditioned cues in a dose-dependent manner with a greater potency in female mice than male mice. Female mice that received 4-OH-DiPT before extinction training had reduced avoidance behaviors several days later in the light dark box, elevated plus maze and novelty-suppressed feeding test compared to controls, while male mice did not show significant differences. 4-OH-DiPT produced robust increases in spontaneous inhibitory postsynaptic currents (sIPSCs) in basolateral amygdala (BLA) principal neurons and action potential firing in BLA interneurons in a 5-HT2A-dependent manner. RNAscope demonstrates that Htr2a mRNA is expressed predominantly in BLA GABA interneurons, Htr2c mRNA is expressed in both GABA interneurons and principal neurons, while Htr2b mRNA is absent in the BLA. Our findings suggest that 4-OH-DiPT activates BLA interneurons via the 5-HT2A receptor to enhance GABAergic inhibition of BLA principal neurons, which provides a potential mechanism for suppressing learned fear.

Novel crosstalk mechanisms between GluA3 and Epac2 in synaptic plasticity and memory in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease which accounts for the most cases of dementia worldwide. Impaired memory, including acquisition, consolidation, and retrieval, is one of the hallmarks in AD. At the cellular level, dysregulated synaptic plasticity partly due to reduced long-term potentiation (LTP) and enhanced long-term depression (LTD) underlies the memory deficits in AD. GluA3 containing α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) are one of key receptors involved in rapid neurotransmission and synaptic plasticity. Recent studies revealed a novel form of GluA3 involved in neuronal plasticity that is dependent on cyclic adenosine monophosphate (cAMP), rather than N-methyl-d-aspartate (NMDA). However, this cAMP-dependent GluA3 pathway is specifically and significantly impaired by amyloid beta (Aβ), a pathological marker of AD. cAMP is a key second messenger that plays an important role in modulating memory and synaptic plasticity. We previously reported that exchange protein directly activated by cAMP 2 (Epac2), acting as a main cAMP effector, plays a specific and time-limited role in memory retrieval. From electrophysiological perspective, Epac2 facilities the maintenance of LTP, a cellular event closely associated with memory retrieval. Additionally, Epac2 was found to be involved in the GluA3-mediated plasticity. In this review, we comprehensively summarize current knowledge regarding the specific roles of GluA3 and Epac2 in synaptic plasticity and memory, and their potential association with AD.

ITGB1 alleviates osteoarthritis by inhibiting cartilage inflammation and apoptosis via activating cAMP pathway

OBJECTIVE

We aimed to screen novel biomarkers for osteoarthritis (OA) using bioinformatic methods and explore its regulatory mechanism in OA development.

METHODS

Differentially expressed genes were screened out from GSE98918 and GSE82107 datasets. Protein-protein interaction network and enrichment analysis were employed to search for hub gene and regulatory pathway. Hematoxylin-eosin, Safranin O-Fast green staining, and immunohistochemistry were performed to assess pathological damage. TNF-α, IL-1β, and IL-6 concentrations were determined by enzyme-linked immunosorbent assay. Real-time quantitative PCR was applied to verify expression of hub genes in OA model. The expression of key protein and pathway proteins was determined by western blot. Furthermore, Cell Counting Kit-8 and flow cytometry were conducted to explore the role of hub gene in chondrocytes.

RESULTS

We identified 6 hub genes of OA, including ITGB1, COL5A1, COL1A1, THBS2, LAMA1, and COL12A1, with high prediction value. ITGB1 was screened as a pivotal regulator of OA and cAMP pathway was selected as the key regulatory pathway. ITGB1 was down-regulated in OA model. ITGB1 overexpression attenuated pathological damage and apoptosis in OA rats with the reduced levels of TNF-α, IL-1β and IL-6. ITGB1 overexpression activated cAMP pathway in vivo and vitro models. In vitro model, ITGB1 overexpression promoted cell viability, while inhibited apoptosis. ITGB1 overexpression also caused a decrease of TNF-α, IL-1β, and IL-6 concentrations. cAMP pathway inhibitor reversed the positive effect of ITGB1 on OA cell model.

CONCLUSION

ITGB1 is a novel biomarker for OA, which inhibits OA development by activating the cAMP pathway.

The old second messenger cAMP teams up with novel cell death mechanisms: potential translational therapeutical benefit for Alzheimer's disease and Parkinson's disease

Alzheimer's disease (AD) and Parkinson's disease (PD) represent the most prevalent neurodegenerative disorders severely impacting life expectancy and quality of life of millions of people worldwide. AD and PD exhibit both a very distinct pathophysiological disease pattern. Intriguingly, recent researches, however, implicate that overlapping mechanisms may underlie AD and PD. In AD and PD, novel cell death mechanisms, encompassing parthanatos, netosis, lysosome-dependent cell death, senescence and ferroptosis, apparently rely on the production of reactive oxygen species, and seem to be modulated by the well-known, "old" second messenger cAMP. Signaling of cAMP via PKA and Epac promotes parthanatos and induces lysosomal cell death, while signaling of cAMP via PKA inhibits netosis and cellular senescence. Additionally, PKA protects against ferroptosis, whereas Epac1 promotes ferroptosis. Here we review the most recent insights into the overlapping mechanisms between AD and PD, with a special focus on cAMP signaling and the pharmacology of cAMP signaling pathways.

Treadmill Exercise Reduces Neuroinflammation, Glial Cell Activation and Improves Synaptic Transmission in the Prefrontal Cortex in 3 × Tg-AD Mice

Physical exercise improves memory and cognition in physiological aging and Alzheimer's disease (AD), but the mechanisms remain poorly understood. Here, we test the hypothesis that Aβ oligomer accumulation, neuroinflammation, and glial cell activation may lead to disruption of synaptic transmission in the prefrontal cortex of 3 × Tg-AD Mice, resulting in impairment of learning and memory. On the other hand, treadmill exercise could prevent the pathogenesis and exert neuroprotective effects. Here, we used immunohistochemistry, western blotting, enzyme-linked immunosorbent assay, and slice electrophysiology to analyze the levels of GSK3β, Aβ oligomers (Aβ dimers and trimers), pro-inflammatory cytokines (IL-1β, IL-6, and TNFα), the phosphorylation of CRMP2 at Thr514, and synaptic currents in pyramidal neurons in the prefrontal cortex. We show that 12-week treadmill exercise beginning in three-month-old mice led to the inhibition of GSK3β kinase activity, decreases in the levels of Aβ oligomers, pro-inflammatory cytokines (IL-1β, IL-6, and TNFα), and the phosphorylation of CRMP2 at Thr514, reduction of microglial and astrocyte activation, and improvement of excitatory and inhibitory synaptic transmission of pyramidal neurons in the prefrontal cortex of 3 × Tg-AD Mice. Thus, treadmill exercise reduces neuroinflammation, glial cell activation and improves synaptic transmission in the prefrontal cortex in 3 × Tg-AD mice, possibly related to the inhibition of GSK3β kinase activity.

Epac2 in midbrain dopamine neurons contributes to cocaine reinforcement via enhancement of dopamine release

Repeated exposure to drugs of abuse results in an upregulation of cAMP signaling in the mesolimbic dopamine system, a molecular adaptation thought to be critically involved in the development of drug dependence. Exchange protein directly activated by cAMP (Epac2) is a major cAMP effector abundantly expressed in the brain. However, it remains unknown whether Epac2 contributes to cocaine reinforcement. Here, we report that Epac2 in the mesolimbic dopamine system promotes cocaine reinforcement via enhancement of dopamine release. Conditional knockout of Epac2 from midbrain dopamine neurons (Epac2-cKO) and the selective Epac2 inhibitor ESI-05 decreased cocaine self-administration in mice under both fixed-ratio and progressive-ratio reinforcement schedules and across a broad range of cocaine doses. In addition, Epac2-cKO led to reduced evoked dopamine release, whereas Epac2 agonism robustly enhanced dopamine release in the nucleus accumbens in vitro. This mechanism is central to the behavioral effects of Epac2 disruption, as chemogenetic stimulation of ventral tegmental area (VTA) dopamine neurons via deschloroclozapine (DCZ)-induced activation of Gs-DREADD increased dopamine release and reversed the impairment of cocaine self-administration in Epac2-cKO mice. Conversely, chemogenetic inhibition of VTA dopamine neurons with Gi-DREADD reduced dopamine release and cocaine self-administration in wild-type mice. Epac2-mediated enhancement of dopamine release may therefore represent a novel and powerful mechanism that contributes to cocaine reinforcement.

Integrated Excitatory/Inhibitory Imbalance and Transcriptomic Analysis Reveals the Association between Dysregulated Synaptic Genes and Anesthetic-Induced Cognitive Dysfunction

Emerging evidence from human epidemiologic and animal studies has demonstrated that developmental anesthesia neurotoxicity could cause long-term cognitive deficits and behavioral problems. However, the underlying mechanisms remain largely unknown. We conducted an electrophysiological analysis of synapse activity and a transcriptomic assay of 24,881 mRNA expression on hippocampal tissues from postnatal day 60 (P60) mice receiving propofol exposure at postnatal day 7 (P7). We found that developmentally propofol-exposed P60 mouse hippocampal neurons displayed an E/I imbalance, compared with control mice as evidenced by the decreased excitation and increased inhibition. We found that propofol exposure at P7 led to the abnormal expression of 317 mRNAs in the hippocampus of P60 mice, including 23 synapse-related genes. Various bioinformatic analyses revealed that these abnormally expressed synaptic genes were associated with the function and development of synapse activity and plasticity, E/I balance, behavior, and cognitive impairment. Our findings suggest that the altered E/I balance may constitute a mechanism for propofol-induced long-term impaired learning and memory in mice. The transcriptomic and bioinformatic analysis of these dysregulated genes related to synaptic function paves the way for development of therapeutic strategies against anesthetic neurodegeneration through the restoration of E/I balance and the modification of synaptic gene expression.

CB2 Agonist GW842166x Protected against 6-OHDA-Induced Anxiogenic- and Depressive-Related Behaviors in Mice

In addition to motor dysfunction, patients with Parkinson's disease (PD) are often affected by neuropsychiatric disorders, such as anxiety and depression. In animal models, activation of the endocannabinoid (eCB) system produces anxiolytic and antidepressant-like behavioral effects. CB2 agonists have demonstrated neuroprotective effects against neurotoxin-induced dopamine neuron loss and deficits in motor function. However, it remains unknown whether CB2 agonism ameliorates anxiogenic- and depressive-like behaviors in PD models. Here, we report that the selective CB2 agonist GW842166x exerted neuroprotective effects against 6-hydroxydopamine (6-OHDA)-induced loss of dopaminergic terminals and dopamine release in the striatum, which were blocked by the CB2 antagonist AM630. We found that 6-OHDA-treated mice exhibited anxiogenic- and depressive-like behaviors in the open-field, sucrose preference, novelty-suppressed feeding, marble burying, and forced swim tests but did not show significant changes in the elevated plus-maze and light-dark box test. GW842166x treatments ameliorated 6-OHDA-induced anxiogenic- and depressive-like behaviors, but the effects were blocked by CB2 antagonism, suggesting a CB2-dependent mechanism. These results suggest that the CB2 agonist GW842166x not only reduces 6-OHDA-induced motor function deficits but also anxiogenic- and depressive-like behaviors in 6-OHDA mouse models of PD.

Ibudilast attenuates cocaine self-administration and prime- and cue-induced reinstatement of cocaine seeking in rats

Ibudilast is a non-selective phosphodiesterase (PDE) inhibitor and glial cell modulator which has shown great promise for the treatment of drug and alcohol use disorders in recent clinical studies. However, it is unknown whether and how ibudilast affects cocaine seeking behavior. Here we show that systemic administration of ibudilast dose-dependently reduced cocaine self-administration under fixed- and progressive-ratio reinforcement schedules in rats and shifted cocaine dose-response curves downward. In addition, ibudilast decreased cocaine prime- and cue-induced reinstatement of cocaine seeking. These results indicate that ibudilast was effective in reducing the reinforcing effects of cocaine and relapse to cocaine seeking. Chronic cocaine exposure induces cAMP-related neuroadaptations in the reward circuitry of the brain. To investigate potential mechanisms for ibudilast-induced attenuation of cocaine self-administration, we recorded from ventral tegmental area (VTA) dopamine neurons in ex vivo midbrain slices prepared from rats that had undergone saline and cocaine self-administration. We found cocaine self-administration led to a decrease in inhibitory postsynaptic currents (IPSCs), an increase in the AMPAR/NMDAR ratio, and an increase in the excitation to inhibition (E/I) ratio. Ibudilast pretreatments enhanced GABAergic inhibition and did not further change cocaine-induced potentiation of excitation, leading to normalization of the E/I ratio. Restoration of the balance between excitation and inhibition in VTA dopamine neurons may contribute to the attenuation of cocaine self-administration by ibudilast.

EPAC2 acts as a negative regulator in Matrigel-driven tubulogenesis of human microvascular endothelial cells

Angiogenesis is physiologically essential for embryogenesis and development and reinitiated in adult animals during tissue growth and repair. Forming new vessels from the walls of existing vessels occurs as a multistep process coordinated by sprouting, branching, and a new lumenized network formation. However, little is known regarding the molecular mechanisms that form new tubular structures, especially molecules regulating the proper network density of newly formed capillaries. This study conducted microarray analyses in human primary microvascular endothelial cells (HMVECs) plated on Matrigel. The RAPGEF4 gene that encodes exchange proteins directly activated by cAMP 2 (EPAC2) proteins was increased in Matrigel-driven tubulogenesis. Tube formation was suppressed by the overexpression of EPAC2 and enhanced by EPAC2 knockdown in endothelial cells. Endothelial cell morphology was changed to round cell morphology by EPAC2 overexpression, while EPAC2 knockdown showed an elongated cell shape with filopodia-like protrusions. Furthermore, increased EPAC2 inhibited endothelial cell migration, and ablation of EPAC2 inversely enhanced cell mobility. These results suggest that EPAC2 affects the morphology and migration of microvascular endothelial cells and is involved in the termination and proper network formation of vascular tubes.

In trans neuregulin3-Caspr3 interaction controls DA axonal bassoon cluster development

Dopamine (DA) transmission is critical to motivation, movement, and emotion. Unlike glutamatergic and GABAergic synapses, the development of DA synapses is less understood. We show that bassoon (BSN) clusters along DA axons in the core of nucleus accumbens (NAcc) were increased in neonatal stages and reduced afterward, suggesting DA synapse elimination. Remarkably, DA neuron-specific ablating neuregulin 3 (NRG3), a protein whose levels correlate with BSN clusters, increased the clusters and impaired DA release and behaviors related to DA transmission. An unbiased screen of transmembrane proteins with the extracellular domain (ECD) of NRG3 identified Caspr3 (contactin associate-like protein 3) as a binding partner. Caspr3 was enriched in striatal medium spiny neurons (MSNs). NRG3 and Caspr3 interact in trans, which was blocked by Caspr3-ECD. Caspr3 null mice displayed phenotypes similar to those in DAT-Nrg3^f/f mice in DA axonal BSN clusters and DA transmission. Finally, in vivo disruption of the NRG3-Caspr3 interaction increased BSN clusters. Together, these results demonstrate that DA synapse development is controlled by trans interaction between NRG3 in DA neurons and Caspr3 in MSNs, identifying a novel pair of cell adhesion molecules for brain circuit wiring.

Striatal Cholinergic Interneurons Are a Novel Target of Corticotropin Releasing Factor

Cholinergic interneurons (CINs) are critical regulators of striatal network activity and output. Changes in CIN activity are thought to encode salient changes in the environment and stimulus-response-outcome associations. Here we report that the stress-associated neuropeptide corticotropin releasing factor (CRF) produces a profound and reliable increase in the spontaneous firing of CINs in both dorsal striatum and nucleus accumbens (NAc) through activation of CRF type 1 receptors, production of cAMP and reduction in spike accommodation in male mice. The increase of CIN firing by CRF results in the activation muscarinic acetylcholine receptors type 5, which mediate potentiation of dopamine transmission in the striatum. This study provides critical mechanistic insight into how CRF modulates striatal activity and dopamine transmission in the NAc to likely account for CRF facilitation of appetitive behaviors.SIGNIFICANCE STATEMENT Although the presence of CRF receptors in the dorsal and ventral striatum has been acknowledged, the cellular identity and the functional consequences of receptor activation is unknown. Here we report that striatal cholinergic interneurons express CRF-R1 receptors and are acutely activated by the neuropeptide CRF that is released in response to salient environmental stimuli. Cholinergic interneurons make <1% of the cells in the striatum but are critical regulators of the striatal circuitry and its output. CRF's fast and potent activation of cholinergic interneurons could have far reaching behavioral implications across motivated behaviors controlled by the striatum.

Loss of EPAC2 alters dendritic spine morphology and inhibitory synapse density

EPAC2 is a guanine nucleotide exchange factor that regulates GTPase activity of the small GTPase Rap and Ras and is highly enriched at synapses. Activation of EPAC2 has been shown to induce dendritic spine shrinkage and increase spine motility, effects that are necessary for synaptic plasticity. These morphological effects are dysregulated by rare mutations of Epac2 associated with autism spectrum disorders. In addition, EPAC2 destabilizes synapses through the removal of synaptic GluA2/3-containing AMPA receptors. Previous work has shown that Epac2 knockout mice (Epac2^−/−) display abnormal social interactions, as well as gross disorganization of the frontal cortex and abnormal spine motility in vivo. In this study we sought to further understand the cellular consequences of knocking out Epac2 on the development of neuronal and synaptic structure and organization of cortical neurons. Using primary cortical neurons generated from Epac2^+/+ or Epac2^−/− mice, we confirm that EPAC2 is required for cAMP-dependent spine shrinkage. Neurons from Epac2^−/− mice also displayed increased synaptic expression of GluA2/3-containing AMPA receptors, as well as of the adhesion protein N-cadherin. Intriguingly, analysis of excitatory and inhibitory synaptic proteins revealed that loss of EPAC2 resulted in altered expression of vesicular GABA transporter (VGAT) but not vesicular glutamate transporter 1 (VGluT1), indicating an altered ratio of excitatory and inhibitory synapses onto neurons. Finally, examination of cortical neurons located within the anterior cingulate cortex further revealed subtle deficits in the establishment of dendritic arborization in vivo. These data provide evidence that loss of EPAC2 enhances the stability of excitatory synapses and increases the number of inhibitory inputs.

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EPAC2 is required for cAMP-dependent spine remodeling.

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Loss of EPAC2 results in over-stabilized excitatory synapses.

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Loss of EPAC2 results in an increase in inhibitory input onto neurons.

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EPAC2 is required for correct dendritic arborization and spine formation in vivo.

Intracellular cAMP Sensor EPAC: Physiology, Pathophysiology, and Therapeutics Development

This review focuses on one family of the known cAMP receptors, the exchange proteins directly activated by cAMP (EPACs), also known as the cAMP-regulated guanine nucleotide exchange factors (cAMP-GEFs). Although EPAC proteins are fairly new additions to the growing list of cAMP effectors, and relatively "young" in the cAMP discovery timeline, the significance of an EPAC presence in different cell systems is extraordinary. The study of EPACs has considerably expanded the diversity and adaptive nature of cAMP signaling associated with numerous physiological and pathophysiological responses. This review comprehensively covers EPAC protein functions at the molecular, cellular, physiological, and pathophysiological levels; and in turn, the applications of employing EPAC-based biosensors as detection tools for dissecting cAMP signaling and the implications for targeting EPAC proteins for therapeutic development are also discussed.

VTA mTOR Signaling Regulates Dopamine Dynamics, Cocaine-Induced Synaptic Alterations, and Reward

Mechanistic target of rapamycin (mTOR) regulates long-term synaptic plasticity, learning, and memory by controlling dendritic protein synthesis. The mTOR inhibitor rapamycin has been shown to attenuate the behavioral effects of drugs of abuse, including cocaine. Using viral vectors to selectively delete mTOR in the ventral tegmental area (VTA) in adult male mTOR^loxP/loxP mice, we investigated the role of mTOR in regulating neuronal morphology, basal synaptic transmission, dopamine dynamics, and cocaine-induced synaptic plasticity and rewarding effects. We find that targeted deletion of mTOR in the VTA had no significant effects on soma size and dendritic morphology of VTA neurons but significantly decreased dopamine release and reuptake in the nucleus accumbens (NAc) shell, a major target region. Western blot analysis revealed that mTOR deletion led to decreases in phosphorylated tyrosine hydroxylase (pTH-Ser40) levels in the VTA and dopamine transporter expression in the NAc. mTOR deletion had no significant effects on basal excitatory transmission in VTA dopamine neurons but caused an increase in GABAergic inhibition because of an increase in VTA GABAergic neuron firing. Furthermore, mTOR deletion attenuated conditioned place preference to cocaine and cocaine-induced potentiation of excitation and reduction of GABAergic inhibition in VTA dopamine neurons. Taken together, these results suggest that loss of mTOR in the VTA shifts the balance of excitatory and inhibitory synaptic transmission and decreases dopamine release and reuptake in the NAc. In addition, VTA mTOR signaling regulates cocaine-cue associative learning and cocaine-induced synaptic plasticity in VTA dopamine neurons.

HCN2 channels in the ventral tegmental area regulate behavioral responses to chronic stress

Dopamine neurons in the ventral tegmental area (VTA) are powerful regulators of depression-related behavior. Dopamine neuron activity is altered in chronic stress-based models of depression, but the underlying mechanisms remain incompletely understood. Here, we show that mice subject to chronic mild unpredictable stress (CMS) exhibit anxiety- and depressive-like behavior, which was associated with decreased VTA dopamine neuron firing in vivo and ex vivo. Dopamine neuron firing is governed by voltage-gated ion channels, in particular hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. Following CMS, HCN-mediated currents were decreased in nucleus accumbens-projecting VTA dopamine neurons. Furthermore, shRNA-mediated HCN2 knockdown in the VTA was sufficient to recapitulate CMS-induced depressive- and anxiety-like behavior in stress-naïve mice, whereas VTA HCN2 overexpression largely prevented CMS-induced behavioral deficits. Together, these results reveal a critical role for HCN2 in regulating VTA dopamine neuronal activity and depressive-related behaviors.

Resveratrol modulates cocaine-induced inhibitory synaptic plasticity in VTA dopamine neurons by inhibiting phosphodiesterases (PDEs)

Resveratrol is a natural phytoalexin synthesized by plants, including grapes. It displays a wide range of neuroprotective benefits associated with anti-aging. Recent studies have shown that resveratrol regulates dopaminergic transmission and behavioral effects of drugs of abuse. The goal of the present study is to investigate whether and how resveratrol alters basal inhibitory synaptic transmission and cocaine-induced inhibitory synaptic plasticity in dopamine neurons of the ventral tegmental area (VTA). We report that resveratrol elevated cAMP levels by itself and further potentiated a forskolin-induced increase in cAMP levels in midbrain slices, consistent with reported effects of inhibition of phosphodiesterases (PDEs). Resveratrol potentiated GABA_A and GABA_B-mediated inhibitory postsynaptic currents (IPSCs) in VTA dopamine neurons, and these effects were mediated by a protein kinase A (PKA)-dependent enhancement of presynaptic GABA release. In addition, we found that resveratrol blocked endocannabinoid-mediated long-term synaptic depression in VTA dopamine neurons. Resveratrol pretreatments attenuated cocaine-induced conditioned place preference and blocked the cocaine-induced reduction of GABAergic inhibition in VTA dopamine neurons. Together, these results provide evidence that resveratrol modulates basal inhibitory synaptic transmission, cocaine-induced synaptic plasticity, and drug-cue associative learning.

PDE4 Inhibition Restores the Balance Between Excitation and Inhibition in VTA Dopamine Neurons Disrupted by Repeated In Vivo Cocaine Exposure

Phosphodiesterase type 4 (PDE4) is a family of enzymes that selectively degrade intracellular cAMP. PDE4 inhibitors have been shown to regulate the rewarding and reinforcing effects of cocaine, but the underlying mechanisms remain poorly understood. Here we show that pretreatments with the PDE4 inhibitor rolipram attenuated cocaine-induced locomotor sensitization in mice. Repeated cocaine exposure in vivo caused a decrease in inhibitory postsynaptic currents (IPSCs) and an increase in the AMPAR/NMDAR ratio in ventral tegmental area (VTA) dopamine neurons in midbrain slices ex vivo. Cocaine exposure disrupted the balance between excitation and inhibition as shown by an increase in the excitation to inhibition (E/I) ratio. Rolipram pretreatments in vivo prevented cocaine-induced reductions in GABAergic inhibition but did not further increase cocaine-induced potentiation of excitation, leading to the restoration of a balance between excitation and inhibition and normalization of the E/I ratio. In support of this idea, we found that repeated cocaine exposure led to an increase in the single-unit action potential firing rate in vivo in VTA dopamine neurons, which was blocked by rolipram pretreatments. These results suggest that repeated cocaine exposure in vivo disrupts the balance between excitation and inhibition in VTA dopamine neurons, while PDE4 inhibition reestablishes the balance between excitation and inhibition through distinct mechanisms.

Exchange proteins directly activated by cAMP (EPACs): Emerging therapeutic targets

Exchange proteins directly activated by cAMP (EPACs) are critical cAMP-dependent signaling pathway mediators. The discovery of EPAC proteins has significantly facilitated understanding on cAMP-dependent signaling pathway and efforts along this line open new avenues for developing novel therapeutics for cancer, diabetes, heart failure, inflammation, infections, neurological disorders and other human diseases. Over the past decade, important progress has been made in the identification of EPAC agonists, antagonists and their biological and pharmacological applications. In this review, we briefly summarize recently reported novel functions of EPACs and the discovery of their small molecule modulators. The challenges and future perspectives are also discussed.

The Epac-Phospholipase Cε Pathway Regulates Endocannabinoid Signaling and Cocaine-Induced Disinhibition of Ventral Tegmental Area Dopamine Neurons

Exchange protein directly activated by cAMP (Epac) is a direct effector for the ubiquitous second messenger cAMP. Epac activates the phospholipase Cε (PLCε) pathway. PLCβ has been linked to the synthesis of the endocannabinoid 2-arachidonoylglycerol (2-AG). Here, we report that Epac facilitates endocannabinoid-mediated retrograde synaptic depression through activation of PLCε. Intracellular loading of a selective Epac agonist 8-CPT-2Me-cAMP into ventral tegmental area (VTA) dopamine neurons enabled previously ineffective stimuli to induce depolarization-induced suppression of inhibition (DSI) and long-term depression of IPSCs (I-LTD) in the VTA. DSI and I-LTD are mediated by 2-AG since they were blocked by a diacylglycerol lipase inhibitor. The effects of 8-CPT-2Me-cAMP on DSI and I-LTD were absent in Epac2 and PLCε knock-out mice, but remained intact in Epac1 knock-out mice. These results identify a novel mechanism for on-demand synthesis of retrograde signaling 2-AG by the Epac2-PLCε pathway. We investigated the functional significance of Epac2-PLCε-2-AG signaling in regulating inhibitory synaptic plasticity in VTA dopamine neurons induced by in vivo cocaine exposure. We showed that cocaine place conditioning led to a decrease in the frequency and amplitude of spontaneous IPSCs and an increase in action potential firing in wild-type mice, but not in Epac2 or PLCε knock-out mice. Together, these results indicate that the Epac2-PLCε-2-AG signaling cascade contributes to cocaine-induced disinhibition of VTA dopamine neurons.SIGNIFICANCE STATEMENT 2-arachidonoylglycerol (2-AG) is an endogenous cannabinoid that depresses synaptic transmission through stimulation of CB₁ receptors. Among the six isoforms of phospholipase C (PLC; PLCβ, PLCγ, PLCδ, PLCε, PLCζ, PLCη), only PLCβ has been linked to 2-AG synthesis. Here we demonstrate that 8-CPT-2Me-cAMP, a selective agonist of the cAMP sensor protein Epac, enhances 2-AG-mediated synaptic depression in ventral tegmental area (VTA) dopamine neurons via activation of PLCε. These results identify a novel mechanism for 2-AG synthesis via activation of the Epac-PLCε pathway. Furthermore, we show that cocaine-induced conditioned place preference and disinhibition of VTA dopamine neurons were impaired in mice lacking Epac or PLCε. Thus, the Epac-PLCε signaling pathway contributes to cocaine-induced disinhibition of VTA dopamine neurons and formation of drug-associated memories.