Phosphodiesterase 4 inhibitors and drugs of abuse: current knowledge and therapeutic opportunities

Circular RNAs in the human brain are tailored to neuron identity and neuropsychiatric disease

Little is known about circular RNAs (circRNAs) in specific brain cells and human neuropsychiatric disease. Here, we systematically identify over 11,039 circRNAs expressed in vulnerable dopamine and pyramidal neurons laser-captured from 190 human brains and non-neuronal cells using ultra-deep, total RNA sequencing. 1526 and 3308 circRNAs are custom-tailored to the cell identity of dopamine and pyramidal neurons and enriched in synapse pathways. 29% of Parkinson's and 12% of Alzheimer's disease-associated genes produced validated circRNAs. circDNAJC6, which is transcribed from a juvenile-onset Parkinson's gene, is already dysregulated during prodromal, onset stages of common Parkinson's disease neuropathology. Globally, addiction-associated genes preferentially produce circRNAs in dopamine neurons, autism-associated genes in pyramidal neurons, and cancers in non-neuronal cells. This study shows that circular RNAs in the human brain are tailored to neuron identity and implicate circRNA-regulated synaptic specialization in neuropsychiatric diseases.

Selective PDE4B and PDE4D inhibitors produce distinct behavioral responses to ethanol and GABAergic drugs in mice

Apremilast is a phosphodiesterase (PDE) type 4 inhibitor that is nonselective at subtypes PDE4A-D. It modulates ethanol and GABAergic responses via protein kinase A (PKA) phosphorylation of specific GABAA receptor subunits and has opposite effects on ethanol-induced ataxia in wild-type and GABAA β3-S408/409A knock-in mice. We hypothesized that these different effects are due to preferential actions at different PDE4 subtypes. To test this hypothesis, we compared effects of selective PDE4 inhibitors on responses to ethanol and GABAergic drugs in male and female C57BL/6J mice. The PDE4B inhibitor A33 accelerated recovery from ataxia induced by ethanol and diazepam but did not alter ataxia induced by propofol. The PDE4D inhibitor D159687 accelerated recovery from diazepam-induced ataxia but prolonged recovery from ethanol- and propofol-induced ataxia. A33 shortened, while D159687 prolonged, the sedative-hypnotic effects of ethanol. Both drugs shortened diazepam's sedative-hypnotic effects. The modulatory effects of A33 and D159687 were completely prevented by the PKA inhibitor H89. Only D159687 prevented development of acute functional tolerance to ethanol-induced ataxia. D159687 transiently reduced two-bottle choice drinking in male and female mice that had consumed ethanol for 3 weeks and transiently reduced two-bottle choice, every-other-day drinking in male mice. A33 did not alter ethanol drinking in either procedure. Neither drug altered binge-like ethanol consumption or blood ethanol clearance. Thus, D159687 produced behavioral effects similar to apremilast, although it produced a more transient and smaller reduction in drinking. These results indicate that PDE4D inhibition contributes to apremilast's ability to reduce ethanol drinking, whereas PDE4B inhibition is not involved.

Circular RNAs in the human brain are tailored to neuron identity and neuropsychiatric disease

Little is known about circular RNAs (circRNAs) in specific brain cells and human neuropsychiatric disease. Here, we systematically identified over 11,039 circRNAs expressed in vulnerable dopamine and pyramidal neurons laser-captured from 190 human brains and non-neuronal cells using ultra-deep, total RNA sequencing. 1,526 and 3,308 circRNAs were custom-tailored to the cell identity of dopamine and pyramidal neurons and enriched in synapse pathways. 88% of Parkinson's and 80% of Alzheimer's disease-associated genes produced circRNAs. circDNAJC6, produced from a juvenile-onset Parkinson's gene, was already dysregulated during prodromal, onset stages of common Parkinson's disease neuropathology. Globally, addiction-associated genes preferentially produced circRNAs in dopamine neurons, autism-associated genes in pyramidal neurons, and cancers in non-neuronal cells. This study shows that circular RNAs in the human brain are tailored to neuron identity and implicate circRNA- regulated synaptic specialization in neuropsychiatric diseases.

Multivariate genome-wide association meta-analysis of over 1 million subjects identifies loci underlying multiple substance use disorders

Genetic liability to substance use disorders can be parsed into loci that confer general or substance-specific addiction risk. We report a multivariate genome-wide association meta-analysis that disaggregates general and substance-specific loci for published summary statistics of problematic alcohol use, problematic tobacco use, cannabis use disorder, and opioid use disorder in a sample of 1,025,550 individuals of European descent and 92,630 individuals of African descent. Nineteen independent SNPs were genome-wide significant (P < 5e-8) for the general addiction risk factor (addiction-rf), which showed high polygenicity. Across ancestries, PDE4B was significant (among other genes), suggesting dopamine regulation as a cross-substance vulnerability. An addiction-rf polygenic risk score was associated with substance use disorders, psychopathologies, somatic conditions, and environments associated with the onset of addictions. Substance-specific loci (9 for alcohol, 32 for tobacco, 5 for cannabis, 1 for opioids) included metabolic and receptor genes. These findings provide insight into genetic risk loci for substance use disorders that could be leveraged as treatment targets.

Sceletium tortuosum: A review on its phytochemistry, pharmacokinetics, biological, pre-clinical and clinical activities

ETHNOPHARMACOLOGICAL RELEVANCE

Sceletium tortuosum (L.) N.E.Br., the most sought after and widely researched species in the genus Sceletium is a succulent forb endemic to South Africa. Traditionally, this medicinal plant is mainly masticated or smoked and used for the relief of toothache, abdominal pain, as a mood-elevator, analgesic, hypnotic, anxiolytic, thirst and hunger suppressant, and for its intoxicating/euphoric effects. Sceletium tortuosum is currently of widespread scientific interest due to its clinical potential in treating anxiety and depression, relieving stress in healthy individuals, and enhancing cognitive functions. These pharmacological actions are attributed to its phytochemical constituents referred to as mesembrine-type alkaloids.

AIM OF THE REVIEW

The aim of this review was to comprehensively summarize and critically evaluate recent research advances on the phytochemistry, pharmacokinetics, biological, pre-clinical and clinical activities of the medicinal plant S. tortuosum. Additionally, current ongoing research and future perspectives are also discussed.

METHODS

All relevant scientific articles, books, MSc and Ph.D. dissertations on botany, behavioral pharmacology, traditional uses, and phytochemistry of S. tortuosum were retrieved from different databases (including Science Direct, PubMed, Google Scholar, Scopus and Web of Science). For pharmacokinetics and pharmacological effects of S. tortuosum, the focus fell on relevant publications published between 2009 and 2021.

RESULTS

Twenty-five alkaloids belonging to four structural classes viz: mesembrine, Sceletium A4, joubertiamine, and tortuosamine, have been identified from S. tortuosum, of which the mesembrine class is predominant. The crude extracts and commercially available standardized extracts of S. tortuosum have displayed a wide spectrum of biological activities (e.g. antimalarial, anti-oxidant, neuromodulatory, immunomodulatory, anti-HIV, neuroprotection) in in vitro or in vivo studies. While the plant has been studied in clinical populations, this has only been in healthy subjects, so that further study in pathological states remains to be done. Nevertheless, the aforementioned studies have demonstrated that S. tortuosum has potential for enhancing cognitive function and managing anxiety and depression.

CONCLUSION

As an important South African medicinal plant, S. tortuosum has garnered many research advances on its phytochemistry and biological activities over the last decade. These scientific studies have shown that S. tortuosum has various bioactivities. The findings have further established the link between the phytochemistry and pharmacological application, and support the traditional use of S. tortuosum in the indigenous medicine of South Africa.

Roflumilast treatment during forced abstinence reduces relapse to methamphetamine seeking and taking

Methamphetamine (METH) is a psychostimulant with high abuse potential. Currently, there are no pharmacological treatments specific for METH abuse or stimulant use disorder generally. Although phosphodiesterase inhibitors have shown some promise, current animal models have not examined their use in abstinence from stimulant abuse. We employed a METH self-administration model in the rat followed by a forced abstinence period during which roflumilast, a phosphodiesterase 4 inhibitor, was administered. A detailed behavioral analysis of chronic treatment with roflumilast during 7 days of forced abstinence showed that roflumilast reduced METH seeking and METH taking upon subsequent relapse test. Roflumilast treatment during 7 days of forced abstinence did not affect sucrose seeking and sucrose taking behaviors. These data suggest that roflumilast may be a treatment for METH use disorder that is effective when administered only during abstinence.

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.

Ibrutinib as a potential therapeutic for cocaine use disorder

Cocaine use presents a worldwide public health problem with high socioeconomic cost. No current pharmacologic treatments are available for cocaine use disorder (CUD) or cocaine toxicity. To explore pharmaceutical treatments for tthis disorder and its sequelae we analyzed gene expression data from post-mortem brain tissue of individuals with CUD who died from cocaine-related causes with matched cocaine-free controls (n = 71, Mage = 39.9, 100% male, 49% with CUD, 3 samples/brain regions). To match molecular signatures from brain pathology with potential therapeutics, we leveraged the L1000 database honing in on neuronal mRNA profiles of 825 repurposable compounds (e.g., FDA approved). We identified 16 compounds that were negatively associated with CUD gene expression patterns across all brain regions (padj < 0.05), all of which outperformed current targets undergoing clinical trials for CUD (all padj > 0.05). An additional 43 compounds were positively associated with CUD expression. We performed an in silico follow-up potential therapeutics using independent transcriptome-wide in vitro (neuronal cocaine exposure; n = 18) and in vivo (mouse cocaine self-administration; n = 12-15) datasets to prioritize candidates for experimental validation. Among these medications, ibrutinib was consistently linked with the molecular profiles of both neuronal cocaine exposure and mouse cocaine self-administration. We assessed the therapeutic efficacy of ibrutinib using the Drosophila melanogaster model. Ibrutinib reduced cocaine-induced startle response and cocaine-induced seizures (n = 61-142 per group; sex: 51% female), despite increasing cocaine consumption. Our results suggest that ibrutinib could be used for the treatment of cocaine use disorder.

Sceletium tortuosum: A review on its phytochemistry, pharmacokinetics, biological and clinical activities

ETHNOPHARMACOLOGICAL RELEVANCE

Sceletium tortuosum (L.) N.E.Br, the most sought after and widely researched species in the genus Sceletium is a succulent forb endemic to South Africa. Traditionally, this medicinal plant is mainly masticated or smoked and used for the relief of toothache, abdominal pain, and as a mood-elevator, analgesic, hypnotic, anxiolytic, thirst and hunger suppressant, and for its intoxicating/euphoric effects. Sceletium tortuosum is currently of widespread scientific interest due to its clinical potential in treating anxiety and depression, relieving stress in healthy individuals, and enhancing cognitive functions. These pharmacological actions are attributed to its phytochemical constituents referred to as mesembrine-type alkaloids.

AIM OF THE REVIEW

The aim of this review was to comprehensively summarize and critically evaluate recent research advances on the phytochemistry, pharmacokinetics, biological and clinical activities of the medicinal plant S. tortuosum. Additionally, current ongoing research and future perspectives are also discussed.

METHODS

All relevant scientific articles, books, MSc and Ph.D. dissertations on botany, behavioral pharmacology, traditional uses, and phytochemistry of S. tortuosum were retrieved from different databases (including Science Direct, PubMed, Google Scholar, Scopus and Web of Science). For pharmacokinetics and pharmacological effects of S. tortuosum, the focus fell on relevant publications published between 2009 and 2021.

RESULTS

Twenty-five alkaloids belonging to four structural classes viz: mesembrine, Sceletium A4, joubertiamine, and tortuosamine, have been identified from S. tortuosum, of which the mesembrine class is predominant. The crude extracts and commercially available standardized extracts of S. tortuosum have displayed a wide spectrum of biological activities (e.g. antimalarial, anti-oxidant, immunomodulatory, anti-HIV, neuroprotection, enhancement of cognitive function) in in vitro or in vivo studies. This plant has not yet been studied in a clinical population, but has potential for enhancing cognitive function, and managing anxiety and depression.

CONCLUSION

As an important South African medicinal plant, S. tortuosum has garnered many research advances on its phytochemistry and biological activities over the last decade. These scientific studies have shown that S. tortuosum has various bioactivities. The findings have further established the link between the phytochemistry and pharmacological application, and support the traditional use of S. tortuosum in the indigenous medicine of South Africa.

Genetic Modulation of Initial Sensitivity to Δ9-Tetrahydrocannabinol (THC) Among the BXD Family of Mice

Cannabinoid receptor 1 activation by the major psychoactive component in cannabis, Δ9-tetrahydrocannabinol (THC), produces motor impairments, hypothermia, and analgesia upon acute exposure. In previous work, we demonstrated significant sex and strain differences in acute responses to THC following administration of a single dose (10 mg/kg, i.p.) in C57BL/6J (B6) and DBA/2J (D2) inbred mice. To determine the extent to which these differences are heritable, we quantified acute responses to a single dose of THC (10 mg/kg, i.p.) in males and females from 20 members of the BXD family of inbred strains derived by crossing and inbreeding B6 and D2 mice. Acute THC responses (initial sensitivity) were quantified as changes from baseline for: 1. spontaneous activity in the open field (mobility), 2. body temperature (hypothermia), and 3. tail withdrawal latency to a thermal stimulus (antinociception). Initial sensitivity to the immobilizing, hypothermic, and antinociceptive effects of THC varied substantially across the BXD family. Heritability was highest for mobility and hypothermia traits, indicating that segregating genetic variants modulate initial sensitivity to THC. We identified genomic loci and candidate genes, including Ndufs2, Scp2, Rps6kb1 or P70S6K, Pde4d, and Pten, that may control variation in THC initial sensitivity. We also detected strong correlations between initial responses to THC and legacy phenotypes related to intake or response to other drugs of abuse (cocaine, ethanol, and morphine). Our study demonstrates the feasibility of mapping genes and variants modulating THC responses in the BXDs to systematically define biological processes and liabilities associated with drug use and abuse.

Synthesis and SAR Studies of 1H-Pyrrolo[2,3-b]pyridine-2-carboxamides as Phosphodiesterase 4B (PDE4B) Inhibitors

Herein we report the synthesis, SAR, and biological evaluation of a series of 1H-pyrrolo[2,3-b]pyridine-2-carboxamide derivatives as selective and potent PDE4B inhibitors. Compound 11h is a PDE4B preferring inhibitor and exhibited acceptable in vitro ADME and significantly inhibited TNF-α release from macrophages exposed to pro-inflammatory stimuli (i.e., lipopolysaccharide and the synthetic bacterial lipopeptide Pam3Cys). In addition, 11h was selective against a panel of CNS receptors and represents an excellent lead for further optimization and preclinical testing in the setting of CNS diseases.

Glucosamine Enhancement of BDNF Expression and Animal Cognitive Function

Brain-derived neurotrophic factor (BDNF) is an important factor for memory consolidation and cognitive function. Protein kinase A (PKA) signaling interacts significantly with BDNF-provoked downstream signaling. Glucosamine (GLN), a common dietary supplement, has been demonstrated to perform a variety of beneficial physiological functions. In the current study, an in vivo model of 7-week-old C57BL/6 mice receiving daily intraperitoneal injection of GLN (0, 3, 10 and 30 mg/animal) was subjected to the novel object recognition test in order to determine cognitive performance. GLN significantly increased cognitive function. In the hippocampus GLN elevated tissue cAMP concentrations and CREB phosphorylation, and upregulated the expression of BDNF, CREB5 and the BDNF receptor TrkB, but it reduced PDE4B expression. With the in vitro model in the HT22 hippocampal cell line, GLN exposure significantly increased protein and mRNA levels of BDNF and CREB5 and induced cAMP responsive element (CRE) reporter activity; the GLN-mediated BDNF expression and CRE reporter induction were suppressed by PKA inhibitor H89. Our current findings suggest that GLN can exert a cognition-enhancing function and this may act at least in part by upregulating the BDNF levels via a cAMP/PKA/CREB-dependent pathway.

KVA-D-88, a Novel Preferable Phosphodiesterase 4B Inhibitor, Decreases Cocaine-Mediated Reward Properties in Vivo

Cocaine addiction remains a major public concern throughout the world especially in developed countries. In the last three decades, significant achievements have led to a greater understanding of the signaling pathways involved in the development of cocaine addiction; however, there are no FDA-approved treatments available to reverse or block this brain disease due to either the unsatisfactory therapeutic efficacy or severe side effects. Previous studies have demonstrated that chronic exposure to cocaine elevates levels of cyclic AMP (cAMP) as a neuroadaptative response in reward-related brain regions. Phosphodiesterase 4 (PDE4) inhibitors, which elevate cAMP levels, have been shown to block cocaine-mediated behavioral changes related to psychoactive and reinforcing properties. Unfortunately, previously studied PDE4 inhibitors induce severe side-effects, which limit their clinical usage. In this study, we identified a novel PDE4B inhibitor, KVA-D-88, with an improved selectivity profile compared to previous compounds (e.g., rolipram). Pharmacokinetic studies have shown that this compound is brain penetrant and preferably acts on PDE4B compared to PDE4D in vitro, alluding to less unwanted side effects with KVA-D-88 in vivo. Interestingly, pretreatment with KVA-D-88 significantly inhibited cocaine-induced hyperlocomotor activity. In cocaine self-administering mice with differential schedules, KVA-D-88 strikingly decreased the number of active nose-pokes and cocaine infusions and reduced the break point. Taken together, our findings demonstrate that this novel PDE4 inhibitor, KVA-D-88, could inhibit cocaine-mediated rewarding effects implying its potential clinical usage for cocaine addiction.

The Functions of Mitochondrial 2',3'-Cyclic Nucleotide-3'-Phosphodiesterase and Prospects for Its Future

2′,3′-cyclic nucleotide-3′-phosphodiesterase (CNPase) is a myelin-associated enzyme that catalyzes the phosphodiester hydrolysis of 2’,3’-cyclic nucleotides to 2’-nucleotides. However, its presence is also found in unmyelinated cells and other cellular structures. Understanding of its specific physiological functions, particularly in unmyelinated cells, is still incomplete. This review concentrates on the role of mitochondrial CNPase (mtCNPase), independent of myelin. mtCNPase is able to regulate the functioning of the mitochondrial permeability transition pore (mPTP), and thus is involved in the mechanisms of cell death, both apoptosis and necrosis. Its participation in the development of various diseases and pathological conditions, such as aging, heart disease and alcohol dependence, is also reviewed. As such, mtCNPase can be considered as a potential target for the development of therapeutic strategies in the treatment of mitochondria-related diseases.

Unraveling the Mechanisms of Behaviors Associated With AUDs Using Flies and Worms

Alcohol use disorders (AUDs) are very common worldwide and negatively affect both individuals and societies. To understand how normal behavior turns into uncontrollable use of alcohol, several approaches have been utilized in the last decades. However, we still do not completely understand how AUDs evolve or how they are maintained in the brains of affected individuals. In addition, efficient and effective treatment is still in need of development. This review focuses on alternative approaches developed over the last 20 years using Drosophila melanogaster (Drosophila) and Caenorhabditis elegans (C. elegans) as genetic model systems to determine the mechanisms underlying the action of ethanol (EtOH) and behaviors associated with AUDs. All the results and insights of studies over the last 20 years cannot be comprehensively summarized. Thus, a few prominent examples are provided highlighting the principles of the genes and mechanisms that have been uncovered and are involved in the action of EtOH at the cellular level. In addition, examples are provided of the genes and mechanisms that regulate behaviors relevant to acquiring and maintaining excessive alcohol intake, such as decision making, reward and withdrawal, and/or relapse regulation. How the insight gained from the results of Drosophila and C. elegans models can be translated to higher organisms, such as rodents and/or humans, is discussed, as well as whether these insights have any relevance or impact on our understanding of the mechanisms underlying AUDs in humans. Finally, future directions are presented that might facilitate the identification of drugs to treat AUDs.

Therapeutic targeting of 3',5'-cyclic nucleotide phosphodiesterases: inhibition and beyond

Phosphodiesterases (PDEs), enzymes that degrade 3',5'-cyclic nucleotides, are being pursued as therapeutic targets for several diseases, including those affecting the nervous system, the cardiovascular system, fertility, immunity, cancer and metabolism. Clinical development programmes have focused exclusively on catalytic inhibition, which continues to be a strong focus of ongoing drug discovery efforts. However, emerging evidence supports novel strategies to therapeutically target PDE function, including enhancing catalytic activity, normalizing altered compartmentalization and modulating post-translational modifications, as well as the potential use of PDEs as disease biomarkers. Importantly, a more refined appreciation of the intramolecular mechanisms regulating PDE function and trafficking is emerging, making these pioneering drug discovery efforts tractable.

Transcriptional Regulators as Targets for Alcohol Pharmacotherapies

Alcohol use disorder (AUD) is a chronic relapsing brain disease that currently afflicts over 15 million adults in the United States. Despite its prevalence, there are only three FDA-approved medications for AUD treatment, all of which show limited efficacy. Because of their ability to alter expression of a large number of genes, often with great cell-type and brain-region specificity, transcription factors and epigenetic modifiers serve as promising new targets for the development of AUD treatments aimed at the neural circuitry that underlies chronic alcohol abuse. In this chapter, we will discuss transcriptional regulators that can be targeted pharmacologically and have shown some efficacy in attenuating alcohol consumption when targeted. Specifically, the transcription factors cyclic AMP-responsive element binding protein (CREB), peroxisome proliferator-activated receptors (PPARs), nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), and glucocorticoid receptor (GR), as well as the epigenetic enzymes, the DNA methyltransferases (DNMTs) and histone deacetylases (HDACs), will be discussed.

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.