DARPP-32 phosphorylation opposes the behavioral effects of nicotine

Exploring the Role of DARPP-32 in Addiction: A Review of the Current Limitations of Addiction Treatment Pathways and the Role of DARPP-32 to Improve Them

We are amidst a global addiction crisis, yet stigmas surrounding addiction counterintuitively prevail. Understanding and appreciating the neurobiology of addiction is essential to dissolve this stigma and for the development of new pharmacological agents to improve upon currently narrow therapeutic options. This review highlights this and evaluates dopamine-and-cAMP-regulated phosphoprotein, Mr 32 kDa (DARPP-32) as a potential target to treat various forms of substance abuse. Despite the proven involvement of DARPP-32 in addiction pathophysiology, no robust investigations into compounds that could pharmacologically modulate it have been carried out. Agents capable of altering DARPP-32 signalling in this way could prevent or reverse drug abuse and improve upon currently substandard treatment options.

Epistatic evidence for gender-dependant slow neurotransmission signalling in substance use disorders: PPP1R12B versus PPP1R1B

BACKGROUND

Slow neurotransmission including DARPP-32 signalling is implicated in substance use disorders (SUDs) by experimental systems but not yet in the human aetiology. PPP1R12B, encoding another protein in the DARPP-32 family, hasn't been studied in the brain.

METHODS

Brain-regional gene activity was assessed in three different animal models of SUDs for mRNA level alterations. Genetic associations were assessed by meta-analysis of pre-existing dbGaP GWAS datasets for main effects and epistasis with known genetic risks, followed by cell type-specific pathway delineation. Parkinson's disease (PD) was included as a dopamine-related disease control for SUDs.

FINDINGS

In animal models of SUDs, environmentally-altered PPP1R12B expression sex-dependently involves motivation-related brain regions. In humans with polysubstance abuse, meta-analysis of pre-existing datasets revealed that PPP1R12B and PPP1R1B, although expressed in dopamine vs. dopamine-recipient neurons, exerted similar interactions with known genetic risks such as ACTR1B and DRD2 in men but with ADH1B, HGFAC and DRD3 in women. These interactions reached genome-wide significances (P_meta<10^-20) for SUDs but not for PD (disease selectivity: P = 4.8 × 10^-142, OR = 6.7 for PPP1R12B; P = 8.0 × 10^-8, OR = 2.1 for PPP1R1B). CADM2 was the common risk in the molecular signalling regardless of gender and cell type.

INTERPRETATION

Gender-dependant slow neurotransmission may convey both genetic and environmental vulnerabilities selectively to SUDs.

FUNDING

Grants from National Institute on Drug Abuse (NIDA) and National Institute on Alcohol Abuse and Alcoholism (NIAAA) of U.S.A. and National Natural Science Foundation of China (NSFC).

Effects of nicotine on DARPP-32 and CaMKII signaling relevant to addiction

Paul Greengard brought to neuroscience the idea of, and evidence for, the role of second messenger systems in neuronal signaling. The fundamental nature of his contributions is evident in the far reach of his work, relevant to various subfields and topics in neuroscience. In this review, we discuss some of Greengard's work from the perspective of nicotinic acetylcholine receptors and their relevance to nicotine addiction. Specifically, we review the roles of dopamine- and cAMP-regulated phospho-protein of 32kDa (DARPP-32) and Ca2+/calmodulin-dependent kinase II (CaMKII) in nicotine-dependent behaviors. For each protein, we discuss the historical context of their discovery and initial characterization, focusing on the extensive biochemical and immunohistochemical work conducted by Greengard and colleagues. We then briefly summarize contemporary understanding of each protein in key intracellular signaling cascades and evidence for the role of each protein with respect to systems and behaviors relevant to nicotine addiction.

Association studies of up to 1.2 million individuals yield new insights into the genetic etiology of tobacco and alcohol use

Tobacco and alcohol use are leading causes of mortality that influence risk for many complex diseases and disorders1. They are heritable2,3 and etiologically related4,5 behaviors that have been resistant to gene discovery efforts6-11. In sample sizes up to 1.2 million individuals, we discovered 566 genetic variants in 406 loci associated with multiple stages of tobacco use (initiation, cessation, and heaviness) as well as alcohol use, with 150 loci evidencing pleiotropic association. Smoking phenotypes were positively genetically correlated with many health conditions, whereas alcohol use was negatively correlated with these conditions, such that increased genetic risk for alcohol use is associated with lower disease risk. We report evidence for the involvement of many systems in tobacco and alcohol use, including genes involved in nicotinic, dopaminergic, and glutamatergic neurotransmission. The results provide a solid starting point to evaluate the effects of these loci in model organisms and more precise substance use measures.

A novel animal model for neuroinflammation and white matter degeneration

Small interference RNA has been widely used to suppress gene expression. Three different short hairpin RNAs (shRNAs) against dopamine D1 receptor (Drd1), driven by mouse U6 promoter in self-complementary AAV8 vector (scAAV8), were used to silence mouse striatal Drd1 expression. Transduction of mouse striatum with all three scAAV8-D1shRNA viruses, but not the control scAAV8 virus, causes extensive neuroinflammation, demyelination, and axon degeneration. RNA interference is known to be coupled to the innate immune system as a host cell defense against virus infection. Activation of the innate immune system may play a causal role in the development of neuroinflammation and white matter degeneration, providing a novel animal model for multiple sclerosis (MS) and other neuroinflammatory diseases.

Down-regulation of Decapping Protein 2 mediates chronic nicotine exposure-induced locomotor hyperactivity in Drosophila

Long-term tobacco use causes nicotine dependence via the regulation of a wide range of genes and is accompanied by various health problems. Studies in mammalian systems have revealed some key factors involved in the effects of nicotine, including nicotinic acetylcholine receptors (nAChRs), dopamine and other neurotransmitters. Nevertheless, the signaling pathways that link nicotine-induced molecular and behavioral modifications remain elusive. Utilizing a chronic nicotine administration paradigm, we found that adult male fruit flies exhibited locomotor hyperactivity after three consecutive days of nicotine exposure, while nicotine-naive flies did not. Strikingly, this chronic nicotine-induced locomotor hyperactivity (cNILH) was abolished in Decapping Protein 2 or 1 (Dcp2 or Dcp1) -deficient flies, while only Dcp2-deficient flies exhibited higher basal levels of locomotor activity than controls. These results indicate that Dcp2 plays a critical role in the response to chronic nicotine exposure. Moreover, the messenger RNA (mRNA) level of Dcp2 in the fly head was suppressed by chronic nicotine treatment, and up-regulation of Dcp2 expression in the nervous system blocked cNILH. These results indicate that down-regulation of Dcp2 mediates chronic nicotine-exposure-induced locomotor hyperactivity in Drosophila. The decapping proteins play a major role in mRNA degradation; however, their function in the nervous system has rarely been investigated. Our findings reveal a significant role for the mRNA decapping pathway in developing locomotor hyperactivity in response to chronic nicotine exposure and identify Dcp2 as a potential candidate for future research on nicotine dependence.

Environmental enrichment alters nicotine-mediated locomotor sensitization and phosphorylation of DARPP-32 and CREB in rat prefrontal cortex

Exposure within an environmental enrichment paradigm results in neurobiological adaptations and decreases the baseline of locomotor activity. The current study determined activation of DARPP-32 (dopamine- and cAMP-regulated phosphoprotein-32) and CREB (cAMP response element binding protein), and locomotor activity in rats raised in enriched (EC), impoverished (IC), and standard (SC) conditions following repeated administration of nicotine or saline. In the saline-control group, the basal phosphorylation state of DARPP-32 at Threonine-34 site (pDARPP-32 Thr34) in the prefrontal cortex (PFC) was lower in EC compared to IC and SC rats, which was positively correlated with their respective baseline activities. While nicotine (0.35 mg/kg, freebase) produced locomotor sensitization across all housing conditions when the nicotine-mediated locomotor activity was expressed as a percent change from their respective saline control, EC rats displayed greater sensitization to nicotine than IC and SC rats. Consistent with the behavioral findings, repeated nicotine injection increased pDARPP-32 Thr34 in PFC of EC and IC rats and in nucleus accumbens of EC rats; however, the magnitude of change from saline control in nicotine-induced enhancement of pDARPP-32 Thr34 in PFC was strikingly increased in EC rats relative to IC rats. Moreover, EC rats had lower basal phosphorylation levels of CREB at serine 133 in PFC and nucleus accumbens compared to IC and SC rats, whereas the nicotine-induced increase in phosphorylated CREB-Ser133 was more pronounced in PFC of EC rats relative to IC and SC rats. Collectively, these findings suggest innovative insights into advancing our understanding of the molecular mechanisms of enrichment-induced changes in the motivational effects of nicotine, and aiding in the identification of new therapeutic strategies for tobacco smokers.

α6β2* and α4β2* nicotinic acetylcholine receptors as drug targets for Parkinson's disease

Parkinson's disease is a debilitating movement disorder characterized by a generalized dysfunction of the nervous system, with a particularly prominent decline in the nigrostriatal dopaminergic pathway. Although there is currently no cure, drugs targeting the dopaminergic system provide major symptomatic relief. As well, agents directed to other neurotransmitter systems are of therapeutic benefit. Such drugs may act by directly improving functional deficits in these other systems, or they may restore aberrant motor activity that arises as a result of a dopaminergic imbalance. Recent research attention has focused on a role for drugs targeting the nicotinic cholinergic systems. The rationale for such work stems from basic research findings that there is an extensive overlap in the organization and function of the nicotinic cholinergic and dopaminergic systems in the basal ganglia. In addition, nicotinic acetylcholine receptor (nAChR) drugs could have clinical potential for Parkinson's disease. Evidence for this proposition stems from studies with experimental animal models showing that nicotine protects against neurotoxin-induced nigrostriatal damage and improves motor complications associated with l-DOPA, the "gold standard" for Parkinson's disease treatment. Nicotine interacts with multiple central nervous system receptors to generate therapeutic responses but also produces side effects. It is important therefore to identify the nAChR subtypes most beneficial for treating Parkinson's disease. Here we review nAChRs with particular emphasis on the subtypes that contribute to basal ganglia function. Accumulating evidence suggests that drugs targeting α6β2* and α4β2* nAChR may prove useful in the management of Parkinson's disease.

Adult mice voluntarily progress to nicotine dependence in an oral self-selection assay

Nicotine has both rewarding and aversive properties in rodents, as shown by intravenous self-administration, intracranial self-stimulation, and conditioned place preference experiments. However, high throughput models of nicotine reward have not been developed in mice. In previous two-bottle studies, mice often chose to drink less from the nicotine bottle than from the water bottle, which raises the question whether these paradigms provide a model of the reinforcing properties of oral nicotine. We hypothesized that previous two-bottle choice paradigms included factors (such as the brief duration of trials, the addition of flavorings to both bottles, water bottles located relatively close to each other, etc.) that may have obstructed the formation of a learned association between the taste of nicotine and its delayed pharmacological effects. Here we show that a paradigm designed to simplify the acquisition of a learned association resulted in nicotine consumption by various strains and sexes that diverged progressively over a period of seven weeks. The strain and sex with the highest nicotine consumption (C57BL/6J females) showed steady and statistically significant increases in nicotine consumption throughout this period. C57BL/6J females were clearly responding to the reinforcing properties of nicotine because they chose to drink over 70% of their fluids from the nicotine bottle. Moreover, they became nicotine dependent, as shown by highly significant nicotine withdrawal symptoms after the nicotine bottle was removed. The strain and sex with the lowest consumption (A/J males) showed a significant decrease in nicotine consumption, and by the end of the experiment were drinking only 24% of their fluids from the nicotine bottle.

DARPP-32, Jack of All Trades… Master of Which?

DARPP-32 (PPP1R1B) was discovered as a substrate of cAMP-dependent protein kinase (PKA) enriched in dopamine-innervated brain areas. It is one of three related, PKA-regulated inhibitors of protein phosphatase-1 (PP1). These inhibitors seem to have appeared in early vertebrate ancestors, possibly Gnathostomes. DARPP-32 has additional important biochemical properties including inhibition of PKA when phosphorylated by Cdk5 and regulation by casein kinases 1 and 2. It is highly enriched in specific neuronal populations, especially striatal medium-size spiny neurons. As PP1 inhibitor DARPP-32 amplifies and/or mediates many actions of PKA at the plasma membrane and in the cytoplasm, with a broad spectrum of potential targets and functions. DARPP-32 also undergoes a continuous and tightly regulated cytonuclear shuttling. This trafficking is controlled by phosphorylation of Ser-97, which is necessary for nuclear export. When phosphorylated on Thr-34 and dephosphorylated on Ser-97, DARPP-32 can inhibit PP1 in the nucleus and modulate signaling pathways involved in the regulation of chromatin response. Recent work with multiple transgenic and knockout mutant mice has allowed the dissection of DARPP-32 function in striato-nigral and striato-pallidal neurons. It is implicated in the action of therapeutic and abused psychoactive drugs, in prefrontal cortex function, and in sexual behavior. However, the contribution of DARPP-32 in human behavior remains poorly understood. Post-mortem studies in humans suggest possible alterations of DARPP-32 levels in schizophrenia and bipolar disorder. Genetic studies have revealed a polymorphism with possible association with psychological and psychopathological traits. In addition, a short isoform of DARPP-32, t-DARPP, plays a role in cancer, indicating additional signaling properties. Thus, DARPP-32 is a non-essential but tightly regulated signaling hub molecule which may improve the general performance of the neuronal circuits in which it is expressed.

Sex differences in response to nicotine in C57Bl/6:129SvEv mice

INTRODUCTION

Human studies suggest that smoking behavior in men may depend more on the pharmacological effects of nicotine, whereas in women, this behavior may rely more on nonpharmacological factors associated with smoking. Investigation of these parameters in mice may also reveal sex differences.

METHODS

Male and female C57Bl/6:129SvEv hybrid mice were exposed to increasing concentrations of nicotine in a voluntary oral nicotine consumption paradigm in which we measured fluid consumption over time and dose. Separate cohorts of mice were exposed to nicotine in a place-conditioning paradigm, and preference was determined. These behavioral models were used to examine sex differences in mice as they rely on pharmacological as well as nonpharmacological factors. Mice exposed to nicotine or saline also were tested for sex differences in locomotor-activating effects of nicotine and in analgesia using standard activity monitoring and hot-plate tests.

RESULTS

Females responded more to the conditioned rewarding effects of nicotine compared with males. Males, however, seemed more responsive to the pharmacological aspects of nicotine by reducing nicotine drinking at higher concentrations; females maintained consistent levels of intake over several doses of nicotine. Male and female mice demonstrated similar locomotor and antinociceptive responses to nicotine.

DISCUSSION

These data suggest that place-conditioning and two-bottle choice paradigms may be more sensitive measures of sexual dimorphism in the C57BL/6:129SvEv hybrid mouse than are locomotor or nociception assays.

Chronic nicotine exposure induces a long-lasting and pathway-specific facilitation of LTP in the amygdala

Nicotine, in the form of tobacco, is the most commonly used drug of abuse. In addition to its rewarding properties, nicotine also affects many cognitive and emotional processes that involve several brain regions, including hippocampus and amygdala. Long-term changes in synaptic strength in these brain regions after drug exposure may be importantly correlated with behavioral changes induced by nicotine. Here, we study the effect of chronic oral administration of nicotine on the long-term synaptic potentiation in the amygdala, a key structure for emotional memory. We find that oral administration of nicotine for 7 d produces a significant enhancement of LTP in the amygdala. This facilitation is pathway specific: Nicotine selectively facilitates LTP in the cortical-lateral amygdala pathway, but not the thalamic-lateral and the lateral-basolateral synaptic pathway. The synaptic facilitation induced by a 7-d exposure to nicotine is long-lasting, it persists for 72 h after cessation of nicotine but decays 8 d after its cessation. In contrast, a shorter exposure of nicotine (24 h) induces only a short-lasting facilitation of synaptic plasticity that dissipates 24 and 72 h after cessation of nicotine. The facilitation of LTP in the amygdala after exposure to nicotine is mediated by removal of GABAergic inhibition, is dependent on the activation NMDA receptors, and can be prevented by blocking either alpha7 or beta2 nACh receptors. Our results indicate that chronic exposure to nicotine can promote the induction of long-lasting modifications of synapses in a specific pathway in the amygdala. These changes in synaptic plasticity may contribute to the complex neural adaptations and behaviors caused by nicotine.

Nicotine reduces levodopa-induced dyskinesias in lesioned monkeys

OBJECTIVE

Levodopa, the gold standard for Parkinson's disease treatment, is associated with debilitating abnormal involuntary movements or dyskinesias, for which few treatments are currently available. Studies have implicated numerous neurotransmitters in the development of levodopa-induced dyskinesias. However, the cholinergic system has received little attention despite an extensive overlap between dopaminergic terminals and cholinergic interneurons in the striatum and the well-known ability of nicotine to stimulate striatal dopamine release. Our objective, therefore, was to determine whether nicotine treatment reduced levodopa-induced dyskinesias.

METHODS

The effect of nicotine (provided in the drinking water) was determined on dyskinesias induced by levodopa (5 mg/kg twice daily by oral gavage) in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys.

RESULTS

Nicotine pretreatment reduced peak and total levodopa-induced dyskinesias in levodopa-naive monkeys over an 8-week period, with a decrease in total dyskinesias of about 50%. A crossover study was then done in which levodopa-treated monkeys originally receiving vehicle were administered nicotine, whereas the levodopa-treated animals initially receiving nicotine were placed on vehicle. Nicotine treatment to levodopa-primed monkeys led to an approximately 35% reduction in total dyskinesias that lasted for at least 8 weeks, at which time the study was ended. In contrast, a significant increase in levodopa-induced dyskinesias was observed in the group of animals that had previously received nicotine and were then switched to vehicle. Nicotine treatment did not appear to affect the antiparkinsonian action of levodopa.

INTERPRETATION

These data suggest that nicotine or selective nicotinic agonists may represent a useful treatment strategy to reduce levodopa-induced dyskinesias.

Pleiotropic impact of constitutive fosB inactivation on nicotine-induced behavioral alterations and stress-related traits in mice

Multiple genes are thought to influence both susceptibility to nicotine dependence and its comorbid behavioral traits in humans. However, which specific genes contribute to this pleiotropic effect is poorly understood. Previous rodent studies have shown that many addictive substances and stressful stimuli increase the expression of the transcription factor FosB in limbic and associated regions and that the protein products of fosB contribute to certain behavioral effects of cocaine and morphine. However, the role of this gene in nicotine-regulated behaviors and dependence-related behavioral traits is unknown. We tested the hypothesis that a constitutive level of FosB affects nicotine-regulated behaviors and comorbid behavioral traits using constitutive fosB knockout (KO) mice. Following repeated or prolonged nicotine administration, but not a single acute administration, KO mice were impaired in conditioned place preference, oral nicotine intake and motor suppression. In wild-type mice, repeated nicotine injections, but not a single acute injection, increased the expression of FosB and its truncated variant DeltaFosB in the targets but not at the origins of the mesolimbic and nigrostriatal dopamine pathways; no detectable level of FosB/DeltaFosB was found in KO mice. In tasks designed to assess behavioral traits, KO mice exhibited more pronounced behavioral abnormalities when stress levels were high than when they were minimized. Our results suggest that the constitutive absence of fosB has a pleiotropic influence on the behavioral effects of repeated or prolonged nicotine administration and on stress-related behavioral traits in mice.

Role of calcineurin in nicotine-mediated locomotor sensitization

Calcineurin is a serine/threonine phosphatase that contributes to the effects of nicotine on calcium signaling in cultured cortical neurons; however, the role of calcineurin in behavioral responses to nicotine in vivo has not been examined. We therefore determined whether calcineurin blockade could alter nicotine-mediated locomotor sensitization in Sprague Dawley rats using systemic or brain region-specific administration of the calcineurin inhibitors cyclosporine or FK506. Systemic cyclosporine administration decreased calcineurin activity in the brain, attenuated nicotine-mediated locomotor sensitization, and blocked the effects of nicotine on DARPP32 (dopamine- and cAMP-regulated phosphoprotein-32) activation in the striatum. Direct infusion of calcineurin inhibitors cyclosporine or FK506 into the ventral tegmental area (VTA) also attenuated nicotine-mediated locomotor sensitization, whereas infusion of rapamycin, which binds to FK-binding protein but does not inhibit calcineurin, did not affect sensitization. Together, the data suggest that activation of calcineurin, particularly in the VTA, is a novel signaling event important for nicotine-mediated behavior and intracellular signaling.

Monoamine oxidase A knockout mice exhibit impaired nicotine preference but normal responses to novel stimuli

Nicotine is thought to act on brain monoamine systems that normally mediate diverse motivational behaviors. How monoamine-related genes contribute to behavioral traits (e.g. responses to novel stimuli) comorbid with the susceptibility to nicotine addiction is still poorly understood. We examined the impact of constitutive monoamine oxidase A (MAOA) deficiency in mice on nicotine reward and responses to novel stimuli. Age-matched, male Maoa-knockout (KO) mice and wild-type (WT) littermates were tested for nicotine-induced conditioned place preference (CPP); voluntary oral nicotine preference/intake; spontaneous locomotor activity in a novel, inescapable open field; and novelty place preference. Nicotine preference in WT mice was reduced in Maoa-KO mice in the CPP and oral preference/intake tests. Control experiments showed that these phenotypes were not due to abnormalities in nicotine metabolism, fluid intake or response to taste. In contrast, Maoa-KO mice were normal in their behavioral response to a novel, inescapable open field and in their preference for a novel place. The observed phenotypes suggest that a constitutive deficiency of MAOA reduces the rewarding effects of nicotine without altering behavioral responses to novel stimuli in mice. Constitutive MAOA activity levels are likely to contribute to the vulnerability or resiliency to nicotine addiction by altering the rewarding effects of nicotine.