Selective inhibition of phosphodiesterase 7 enzymes reduces motivation for nicotine use through modulation of mesolimbic dopaminergic transmission.

Background: About 5 million people die from diseases related to nicotine addiction and tobacco use each year. Nicotine-induced increase of corticomesolimbic dopaminergic (DAergic) transmission and hypodopaminergic conditions occurring during abstinence are important for maintaining drug-use habits. Methods: We examined the notion of re-equilibrating DAergic transmission by inhibiting phosphodiesterase 7 (PDE7), an intracellular enzyme highly expressed in the corticomesolimbic circuitry and responsible for the degradation of cyclic adenosine monophosphate (cAMP), the main second messenger modulated by DA receptor activation. Results: Using selective PDE7 inhibitors, we demonstrated in male rats that systemic PDE7 enzyme inhibition reduced nicotine self-administration and prevented reinstatement to nicotine seeking evoked by cues or by the pharmacological stressor yohimbine. The effect was also observed by direct application of the PDE7 inhibitors into the nucleus accumbens (NAc) shell but not into the core. Inhibition of PDE7 resulted in increased DA- and cAMP-regulated neuronal phosphoprotein (DARPP-32) and cAMP response element-binding protein (CREB) and their phosphorylated forms in the NAc. It also enhanced the DA D1 receptor agonism-mediated effects, indicating potentiation of protein kinase A (PKA)-dependent transmission downstream of D1 receptor activation. In electrophysiological recordings from DA neurons in the lateral posterior ventral tegmental area (VTA), the PDE7 inhibitors attenuated the spontaneous activity of DA neurons. This effect was exerted through the potentiation of D1 receptor signaling and the subsequent facilitation of γ-aminobutyric acid (GABA) transmission. The PDE7 inhibitors did not elicit conditioned place preference and did not induce intravenous self-administration, indicating lack of reinforcing properties. Conclusions: PDE7 inhibitors have the potential to treat nicotine abuse.SIGNIFICANCE STATEMENTThe World Health Organization (WHO) estimates that there are 1.25 billion smokers worldwide, representing one third of the global population over the age of 15. Nicotine-induced increase of corticomesolimbic dopaminergic (DAergic) transmission and hypodopaminergic conditions occurring during abstinence are critical for maintaining drug-use habits. Here we demonstrate that nicotine consumption and relapse to nicotine seeking are attenuated by re-equilibrating DAergic transmission through inhibition of phosphodiesterase 7 (PDE7), an intracellular enzyme responsible for the degradation of cyclic adenosine monophosphate (cAMP), the main second messenger modulated by DA receptor activation. PDE7 inhibition may represent a novel treatment approach to aid smoking cessation.

Role of genetic background in the effects of adolescent nicotine exposure on mesolimbic dopamine transmission.

Smoking during adolescence may increase the likelihood to develop nicotine dependence and to abuse other drugs such as cocaine. Despite great efforts to understand underlying neurobiological mechanisms of this progression, less attention has been paid to the role of genetic factors. Here, we investigated the influence of both genetic background and age at first nicotine exposure in the long-lasting effects on mesolimbic dopamine transmission including the increased cocaine-rewarding effect. Mid-adolescent and adult rats of inbred strains Lewis (addiction prone) and Fischer 344 (addiction resistant) were administered nicotine (0.4 mg/kg) or vehicle once daily for 5 days. Changes in dopamine transmission were investigated by in vivo microdialysis and electrophysiology after 30 days of withdrawal, whereas changes in cocaine-rewarding effect were assessed via conditioned place preference paradigm. Nicotine pre-exposure differentially changed mesolimbic dopamine transmission depending on strain and age of pre-exposure. A potentiation of dopamine response to nicotine was observed in nucleus accumbens (NAc) core of both strains and age groups, whereas dopamine response in NAc shell was enhanced exclusively in Lewis rats exposed to nicotine during adolescence. A similar response was observed following cocaine challenge at adulthood. Changes in VTA dopamine cell population and activity were observed only in adolescent nicotine-pretreated Lewis rats, which also showed an increased cocaine-rewarding effect at adulthood. These results highlight the influence of genetic background in the long-lasting effects of nicotine exposure and suggest that exposure during adolescence might increase nicotine and cocaine-rewarding properties in genetically vulnerable individuals, thereby facilitating progression toward dependence.

PPARα modulation of mesolimbic dopamine transmission rescues depression-related behaviors.

Depressive disorders cause a substantial burden for the individual and the society. Key depressive symptoms can be modeled in animals and enable the development of novel therapeutic interventions. Chronic unavoidable stress disrupts rats' competence to escape noxious stimuli and self-administer sucrose, configuring a depression model characterized by escape deficit and motivational anhedonia associated to impaired dopaminergic responses to sucrose in the nucleus accumbens shell (NAcS). Repeated treatments that restore these responses also relieve behavioral symptoms. Ventral tegmental area (VTA) dopamine neurons encode reward and motivation and are implicated in the neuropathology of depressive-like behaviors. Peroxisome proliferator-activated receptors type-α (PPARα) acutely regulate VTA dopamine neuron firing via ß2 subunit-containing nicotinic acetylcholine receptors (ß2*nAChRs) through phosphorylation and this effect is predictive of antidepressant-like effects. Here, by combining behavioral, electrophysiological and biochemical techniques, we studied the effects of repeated PPARα stimulation by fenofibrate on mesolimbic dopamine system. We found decreased ß2*nAChRs phosphorylation levels and a switch from tonic to phasic activity of dopamine cells in the VTA, and increased phosphorylation of dopamine and cAMP-regulated phosphoprotein Mr 32,000 (DARPP-32) in the NAcS. We then investigated whether long-term fenofibrate administration to stressed rats reinstated the decreased DARPP-32 response to sucrose and whether this effect translated into antidepressant-like properties. Fenofibrate restored dopaminergic responses to appetitive stimuli, reactivity to aversive stimuli and motivation to self-administer sucrose. Overall, this study suggests PPARα as new targets for antidepressant therapies endowed with motivational anti-anhedonic properties, further supporting the role of an unbalanced mesolimbic dopamine system in pathophysiology of depressive disorders.

Interactions between the endocannabinoid and nicotinic cholinergic systems: preclinical evidence and therapeutic perspectives.

RATIONALE: Several lines of evidence suggest that endocannabinoid and nicotinic cholinergic systems are implicated in the regulation of different physiological processes, including reward, and in the neuropathological mechanisms of psychiatric diseases, such as addiction. A crosstalk between these two systems is substantiated by the overlapping distribution of cannabinoid and nicotinic acetylcholine receptors in many brain structures. OBJECTIVE: We will review recent preclinical data showing how the endocannabinoid and nicotinic cholinergic systems interact bidirectionally at the level of the brain reward pathways, and how this interaction plays a key role in modulating nicotine and cannabinoid intake and dependence. RESULTS: Many behavioral and neurochemical effects of nicotine that are related to its addictive potential are reduced by pharmacological blockade or genetic deletion of type-1 cannabinoid receptors, inhibition of endocannabinoid uptake or metabolic degradation, and activation of peroxisome proliferator-activated-receptor-α. On the other hand, cholinergic antagonists at α7 nicotinic acetylcholine receptors as well as endogenous negative allosteric modulators of these receptors are effective in blocking dependence-related effects of cannabinoids. CONCLUSIONS: Pharmacological manipulation of the endocannabinoid system and endocannabinoid-like neuromodulators shows promise in the treatment of nicotine dependence and in relapse prevention. Likewise, drugs acting at nicotinic acetylcholine receptors might prove useful in the therapy of cannabinoid dependence. Research by Steven R. Goldberg has significantly contributed to the progress in this research field.

Adolescent exposure to THC in female rats disrupts developmental changes in the prefrontal cortex.

Current concepts suggest that exposure to THC during adolescence may act as a risk factor for the development of psychiatric disorders later in life. However, the molecular underpinnings of this vulnerability are still poorly understood. To analyze this, we investigated whether and how THC exposure in female rats interferes with different maturational events occurring in the prefrontal cortex during adolescence through biochemical, pharmacological and electrophysiological means. We found that the endocannabinoid system undergoes maturational processes during adolescence and that THC exposure disrupts them, leading to impairment of both endocannabinoid signaling and endocannabinoid-mediated LTD in the adult prefrontal cortex. THC also altered the maturational fluctuations of NMDA subunits, leading to larger amounts of gluN2B at adulthood. Adult animals exposed to THC during adolescence also showed increased AMPA gluA1 with no changes in gluA2 subunits. Finally, adolescent THC exposure altered cognition at adulthood. All these effects seem to be triggered by the disruption of the physiological role played by the endocannabinoid system during adolescence. Indeed, blockade of CB1 receptors from early to late adolescence seems to prevent the occurrence of pruning at glutamatergic synapses. These results suggest that vulnerability of adolescent female rats to long-lasting THC adverse effects might partly reside in disruption of the pivotal role played by the endocannabinoid system in the prefrontal cortex maturation.

Targeting the interaction between fatty acid ethanolamides and nicotinic receptors: therapeutic perspectives.

Nicotine is one of the drugs of abuse that frequently causes addiction and relapse during abstinence. Nicotine's strong addicting properties reside in its ability to enhance dopamine transmission, and to induce specific changes in synaptic plasticity. Currently, approved therapies for smoking cessation increase the chances of remaining abstinent, but lack high levels of efficacy and are associated with significant adverse side effects. As a result, there is an urgent need for more effective antismoking medications. Studies have revealed that drugs targeting the peroxisome proliferator-activated-receptor-α (PPARα) show promise for the treatment of nicotine addiction. These drugs include synthetic PPARα ligands, such as the clinically available hypolipidemic fibrates, and drugs that increase levels of endogenous endocannabinoid-like fatty acid ethanolamides (FAEs) that act as PPARα agonists. In this review, we will discuss the specific interaction between PPARα and nicotine, and the molecular mechanisms whereby these intracellular receptors regulate nicotinic acetylcholine receptor functions in neurons. Modulation of neurophysiological, neurochemical and behavioral effects of nicotine by PPARα will be also reviewed. Indeed, a picture is emerging where FAEs are endogenous regulators of acetylcholine transmission. Notably, the implications of this specific cross talk extend beyond nicotine addiction, and might bear relevance for psychiatric disorders and epilepsy.

PPAR-alpha agonists as novel antiepileptic drugs: preclinical findings.

Nicotinic acetylcholine receptors (nAChRs) are involved in seizure mechanisms. Hence, nocturnal frontal lobe epilepsy was the first idiopathic epilepsy linked with specific mutations in α4 or ß2 nAChR subunit genes. These mutations confer gain of function to nAChRs by increasing sensitivity toward acetylcholine. Consistently, nicotine elicits seizures through nAChRs and mimics the excessive nAChR activation observed in animal models of the disease. Treatments aimed at reducing nicotinic inputs are sought as therapies for epilepsies where these receptors contribute to neuronal excitation and synchronization. Previous studies demonstrated that peroxisome proliferator-activated receptors-α (PPARα), nuclear receptor transcription factors, suppress nicotine-induced behavioral and electrophysiological effects by modulating nAChRs containing ß2 subunits. On these bases, we tested whether PPARα agonists were protective against nicotine-induced seizures. To this aim we utilized behavioral and electroencephalographic (EEG) experiments in C57BL/J6 mice and in vitro patch clamp recordings from mice and rats. Convulsive doses of nicotine evoked severe seizures and bursts of spike-waves discharges in ∼100% of mice. A single dose of the synthetic PPARα agonist WY14643 (WY, 80 mg/kg, i.p.) or chronic administration of fenofibrate, clinically available for lipid metabolism disorders, in the diet (0.2%) for 14 days significantly reduced or abolished behavioral and EEG expressions of nicotine-induced seizures. Acute WY effects were reverted by the PPARα antagonist MK886 (3 mg/kg, i.p.). Since neocortical networks are crucial in the generation of ictal activity and synchrony, we performed patch clamp recordings of spontaneous inhibitory postsynaptic currents (sIPSCs) from frontal cortex layer II/III pyramidal neurons. We found that both acute and chronic treatment with PPARα agonists abolished nicotine-induced sIPSC increases. PPARα within the CNS are key regulators of neuronal activity through modulation of nAChRs. These effects might be therapeutically exploited for idiopathic or genetically determined forms of epilepsy where nAChRs play a major role.

Hub and switches: endocannabinoid signalling in midbrain dopamine neurons.

The last decade has provided a wealth of experimental data on the role played by lipids belonging to the endocannabinoid family in several facets of physiopathology of dopamine neurons. We currently suggest that these molecules, being intimately connected with diverse metabolic and signalling pathways, might differently affect various functions of dopamine neurons through activation not only of surface receptors, but also of nuclear receptors. It is now emerging how dopamine neurons can regulate their constituent biomolecules to compensate for changes in either internal functions or external conditions. Consequently, dopamine neurons use these lipid molecules as metabolic and homeostatic signal detectors, which can dynamically impact cell function and fitness. Because dysfunctions of the dopamine system underlie diverse neuropsychiatric disorders, including schizophrenia and drug addiction, the importance of better understanding the correlation between an unbalanced endocannabinoid signal and the dopamine system is even greater. Particularly, because dopamine neurons are critical in controlling incentive-motivated behaviours, the involvement of endocannabinoid molecules in fine-tuning dopamine cell activity opened new avenues in both understanding and treating drug addiction. Here, we review recent advances that have shed new light on the understanding of differential roles of endocannabinoids and their cognate molecules in the regulation of the reward circuit, and discuss their anti-addicting properties, particularly with a focus on their potential engagement in the prevention of relapse.

Inhibitory inputs from rostromedial tegmental neurons regulate spontaneous activity of midbrain dopamine cells and their responses to drugs of abuse.

The rostromedial tegmental nucleus (RMTg), a structure located just posterior to the ventral tegmental area (VTA), is an important site involved in aversion processes. The RMTg contains γ-aminobutyric acid neurons responding to noxious stimuli, densely innervated by the lateral habenula and providing a major inhibitory projection to reward-encoding dopamine (DA) neurons in the VTA. Here, we studied how RMTg neurons regulate both spontaneous firing of DA cells and their response to the cannabinoid agonist WIN55212-2 (WIN), morphine, cocaine, and nicotine. We utilized single-unit extracellular recordings in anesthetized rats and whole-cell patch clamp recordings in brain slices to study RMTg-induced inhibition of DA cells and inhibitory postsynaptic currents (IPSCs) evoked by stimulation of caudal afferents, respectively. The electrical stimulation of the RMTg elicited a complete suppression of spontaneous activity in approximately half of the DA neurons examined. RMTg-induced inhibition correlated with firing rate and pattern of DA neurons and with their response to a noxious stimulus, highlighting that inhibitory inputs from the RMTg strongly control spontaneous activity of DA cells. Both morphine and WIN depressed RMTg-induced inhibition of DA neurons in vivo and IPSCs evoked by RMTg stimulation in brain slices with presynaptic mechanisms. Conversely, neither cocaine nor nicotine modulated DA neuron responses to RMTg stimulation. Our results further support the role of the RMTg as one of the main inhibitory afferents to DA cells and suggest that cannabinoids and opioids might disinhibit DA neurons by profoundly influencing synaptic responses evoked by RMTg activation.

Endogenous fatty acid ethanolamides suppress nicotine-induced activation of mesolimbic dopamine neurons through nuclear receptors.

Nicotine stimulates the activity of mesolimbic dopamine neurons, which is believed to mediate the rewarding and addictive properties of tobacco use. Accumulating evidence suggests that the endocannabinoid system might play a major role in neuronal mechanisms underlying the rewarding properties of drugs of abuse, including nicotine. Here, we investigated the modulation of nicotine effects by the endocannabinoid system on dopamine neurons in the ventral tegmental area with electrophysiological techniques in vivo and in vitro. We discovered that pharmacological inhibition of fatty acid amide hydrolase (FAAH), the enzyme that catabolizes fatty acid ethanolamides, among which the endocannabinoid anandamide (AEA) is the best known, suppressed nicotine-induced excitation of dopamine cells. Importantly, this effect was mimicked by the administration of the FAAH substrates oleoylethanolamide (OEA) and palmitoylethanolamide (PEA), but not methanandamide, the hydrolysis resistant analog of AEA. OEA and PEA are naturally occurring lipid signaling molecules structurally related to AEA, but devoid of affinity for cannabinoid receptors. They blocked the effects of nicotine by activation of the peroxisome proliferator-activated receptor-alpha (PPAR-alpha), a nuclear receptor transcription factor involved in several aspects of lipid metabolism and energy balance. Activation of PPAR-alpha triggered a nongenomic stimulation of tyrosine kinases, which might lead to phosphorylation and negative regulation of neuronal nicotinic acetylcholine receptors. These data indicate for the first time that the anorexic lipids OEA and PEA possess neuromodulatory properties as endogenous ligands of PPAR-alpha in the brain and provide a potential new target for the treatment of nicotine addiction.

Long-lasting potentiation of GABAergic synapses in dopamine neurons after a single in vivo ethanol exposure.

The mesolimbic dopamine (DA) system originating in the ventral tegmental area (VTA) is involved in many drug-related behaviors, including ethanol self-administration. In particular, VTA activity regulating ethanol consummatory behavior appears to be modulated through GABA(A) receptors. Previous exposure to ethanol enhances ethanol self-administration, but the mechanisms underlying this phenomenon are not well understood. In this study, we examined changes occurring at GABA synapses onto VTA DA neurons after a single in vivo exposure to ethanol. We observed that evoked GABA(A) IPSCs in DA neurons of ethanol-treated animals exhibited paired-pulse depression (PPD) compared with saline-treated animals, which exhibited paired-pulse facilitation (PPF). Furthermore, PPD was still present 1 week after the single exposure to ethanol. An increase in frequency of spontaneous miniature GABA(A) IPSCs (mIPSCs) was also observed in the ethanol-treated animals. Additionally, the GABA(B) receptor antagonist (3-aminopropyl)(diethoxymethyl) phosphinic acid shifted PPD to PPF, indicating that presynaptic GABA(B) receptor activation, likely attributable to GABA spillover, might play a role in mediating PPD in the ethanol-treated mice. The activation of adenylyl cyclase by forskolin increased the amplitude of GABA(A) IPSCs and the frequency of mIPSCs in the saline- but not in the ethanol-treated animals. Conversely, the protein kinase A (PKA) inhibitor N-[z-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide significantly decreased both the frequency of spontaneous mIPSCs and the amplitude of GABA(A) IPSCs in the ethanol-treated mice but not in the saline controls. The present results indicate that potentiation of GABAergic synapses, via a PKA-dependent mechanism, occurs in the VTA after a single in vivo exposure to ethanol, and such potentiation might be a key synaptic modification underlying increased ethanol intake.