Effects of Highly Palatable Diet on motivation for food and resistance to punishment in rats: Role of sex and age of exposure.

Exposure to highly palatable food is believed to induce behavioral and neurobiological changes that may produce addiction-like behavior and increase the risks of obesity and overweight. Studies in rodents have led to conflicting results suggesting that several factors such as sex and age of exposure contribute to the development of maladaptive behaviors towards food. In addition, it is not clear whether effects of exposure to highly palatable diets (HPD) persist after their discontinuation, which would indicate long-term risks to develop addiction-like behavior. In this study, we investigated the persistent effects of an intermittent 8-week exposure to HPD in male and female rats as a function of age of exposure (adult and adolescent). We found that intermittent exposure to HPD did not alter body weight, but it affected consumption of standard food during the time of exposure in all groups. In addition, in adults, HPD produced a decrease in the initial baseline responding in FR1 schedules, an effect that persisted for 4 weeks in males but not in female rats. However, we found that exposure to HPD did not affect resistance to punishment measured by progressive shock strength break points or motivation for food as measured by progressive-ratio break points regardless of sex or age of exposure. Altogether, these results do not provide support for the hypothesis that intermittent exposure to HPD produce persistent increases in the vulnerability to develop addiction-like behaviors towards palatable food.

Dopamine and addiction: what have we learned from 40 years of research.

Among the neurotransmitters involved in addiction, dopamine (DA) is clearly the best known. The critical role of DA in addiction is supported by converging evidence that has been accumulated in the last 40 years. In the present review, first we describe the dopaminergic system in terms of connectivity, functioning and involvement in reward processes. Second, we describe the functional, structural, and molecular changes induced by drugs within the DA system in terms of neuronal activity, synaptic plasticity and transcriptional and molecular adaptations. Third, we describe how genetic mouse models have helped characterizing the role of DA in addiction. Fourth, we describe the involvement of the DA system in the vulnerability to addiction and the interesting case of addiction DA replacement therapy in Parkinson's disease. Finally, we describe how the DA system has been targeted to treat patients suffering from addiction and the result obtained in clinical settings and we discuss how these different lines of evidence have been instrumental in shaping our understanding of the physiopathology of drug addiction.

Reduction of Cocaine-Induced Locomotor Effects by Enriched Environment Is Associated with Cell-Specific Accumulation of ΔFosB in Striatal and Cortical Subregions.

Background: Early exposure to enriched environments has been shown to decrease the locomotor effects induced by repeated injections of cocaine and modify basal and cocaine-induced total protein levels of the transcription factor ΔFosB in the whole striatum of mice. In this study, we aimed at characterizing whether the profile of ΔFosB accumulation induced by enriched environments and cocaine would be similar or different in terms of brain areas and cell type. Methods: We used mice expressing the eGFP protein in D1 receptor positive (D1R(+)) neurons to determine whether Δ FosB induced by enriched environment or cocaine injections (5×15 mg/kg) would occur in selective subpopulations of neurons in several subregions of the striatum and prefrontal cortex. Results: We found that: (1) exposure to enriched environment reduces cocaine-induced locomotor activation, confirming our previous findings; (2) exposure to enriched environment by itself increases the accumulation of Δ FosB mostly in D1R(-) cells in the shell part of the nucleus accumbens and dorsal striatum, whereas in the nucleus accumbens core, Δ FosB accumulates in both D1R(+) and D1R(-) neurons; (3) in standard environment mice, cocaine induces accumulation of Δ FosB selectively in D1R(+) cells in the nucleus accumbens, dorsal striatum, and infralimbic cortex; and (4) the effects of enriched environments and cocaine on accumulation of Δ FosB were reciprocally blocked by their combination. Conclusions: Altogether, these results suggest that the enriched environment-induced reduction in behavioral effects of cocaine might result from 2 distinct effects on ΔFosB in striatal medium-sized spiny neurons belonging to the direct and indirect pathways.

Behavioral flexibility predicts increased ability to resist excessive methamphetamine self-administration.

Drug addiction is often associated with cognitive deficits and behavioral inflexibility that may contribute to the development and maintenance of addictive behaviors by reducing addicts' ability to control their behavior toward the drug. In this study, we investigated the relationships between pre-drug levels of behavioral flexibility and the risk to develop uncontrolled methamphetamine (METH) self-administration. First, we measured individual performance in an inter-dimensional set-shifting procedure in which animals have to switch between an external visual rule and an internal side rule in order to obtain food pellets. Then we allowed rats to self-administer METH for twenty long 14-hour sessions, and we investigated the relationships between behavioral flexibility and measures of control over drug intake. Rats rapidly acquired to self-administer high levels of METH which resulted in moderate weight loss. After several sessions of self-administration, whereas some rats progressively increased their METH intake, other rats showed very long voluntary pauses between drug injections and showed no escalation in METH self-administration. Interestingly, we found that behavioral flexibility is correlated with METH self-administration and that more flexible rats take less METH and do not escalate drug taking. These results suggest that traits of behavioral flexibility may protect against the development of excessive and dysregulated drug taking. Conversely, the inability to adapt behavioral responses as a function of the environmental contingencies may contribute to the risks to develop addiction to METH.

Functional interaction between Lypd6 and nicotinic acetylcholine receptors.

Nicotinic acetylcholine receptors (nAChRs) affect multiple physiological functions in the brain and their functions are modulated by regulatory proteins of the Lynx family. Here, we report for the first time a direct interaction of the Lynx protein LY6/PLAUR domain-containing 6 (Lypd6) with nAChRs in human brain extracts, identifying Lypd6 as a novel regulator of nAChR function. Using protein cross-linking and affinity purification from human temporal cortical extracts, we demonstrate that Lypd6 is a synaptically enriched membrane-bound protein that binds to multiple nAChR subtypes in the human brain. Additionally, soluble recombinant Lypd6 protein attenuates nicotine-induced hippocampal inward currents in rat brain slices and decreases nicotine-induced extracellular signal-regulated kinase phosphorylation in PC12 cells, suggesting that binding of Lypd6 is sufficient to inhibit nAChR-mediated intracellular signaling. We further show that perinatal nicotine exposure in rats (4 mg/kg/day through minipumps to dams from embryonic day 7 to post-natal day 21) significantly increases Lypd6 protein levels in the hippocampus in adulthood, which did not occur after exposure to nicotine in adulthood only. Our findings suggest that Lypd6 is a versatile inhibitor of cholinergic signaling in the brain, and that Lypd6 is dysregulated by nicotine exposure during early development. Regulatory proteins of the Lynx family modulate the function of nicotinic receptors (nAChRs). We report for the first time that the Lynx protein Lypd6 binds to nAChRs in human brain extracts, and that recombinant Lypd6 decreases nicotine-induced ERK phosphorylation and attenuates nicotine-induced hippocampal inward currents. Our findings suggest that Lypd6 is a versatile inhibitor of cholinergic signaling in the brain.

Chronic stimulation of the tone of endogenous anandamide reduces cue- and stress-induced relapse in rats.

BACKGROUND: The endogenous cannabinoid system plays an important role in motivation, stress, and drug abuse. Pharmacologically, the endocannabinoid system can be stimulated by either agonists of CB1 receptors or inhibition of metabolic degradation of endogenous cannabinoids and consequent increases in their brain levels. METHODS: Here, we investigated whether chronic administration during a period of withdrawal of the fatty acid amide hydrolase inhibitor URB597, which increases anandamide levels, would decrease the risks of relapse to cocaine seeking. Rats were allowed to self-administer cocaine and then they underwent forced withdrawal for 28 days, during which they were treated with URB597 or vehicle. One day after the last injection, we investigated cocaine seeking in one 6h extinction session and relapse triggered by re-exposure to drug-associated cues or a pharmacological stressor. RESULTS: We found that administration of URB597 significantly decreases cocaine-seeking behavior and cue- and stress-induced relapse. CONCLUSION: These results suggest that stimulation of the endocannabinoid system could be helpful to prevent relapse to cocaine addiction.

Delay of punishment highlights differential vulnerability to developing addiction-like behavior toward sweet food.

Resistance to punishment is commonly used to measure the difficulty in refraining from rewarding activities when negative consequences ensue, which is a hallmark of addictive behavior. We recently developed a progressive shock strength (PSS) procedure in which individual rats can titrate the amount of punishment that they are willing to tolerate to obtain food rewards. Here, we investigated the effects of a range of delays (0-12 s) on resistance to punishment measured by PSS break points. As expected from delay discounting principles, we found that delayed shock was less effective as a punisher, as revealed by higher PSS breakpoints. However, this discounting effect was not equally distributed in the population of rats, and the introduction of a delay highlighted the existence of two populations: rats that were sensitive to immediate punishment were also sensitive to delayed shock, whereas rats that were resistant to immediate punishment showed strong temporal discounting of delayed punishment. Importantly, shock-sensitive rats suppressed responding even in subsequent non-punishment sessions, and they differed from shock-resistant rats in anxiety-like behavior, but not in sensitivity to pain. These results show that manipulation of temporal contingencies of punishment in the PSS procedure provides a valuable tool to identify individuals with a double vulnerability to addiction: low sensitivity to aversion and excessive discounting of negative future consequences. Conversely, the shock-sensitive population may provide a model of humans who are vulnerable to opportunity loss due to excessive anxiety.

A self-adjusting, progressive shock strength procedure to investigate resistance to punishment: Characterization in male and female rats.

Indifference to harmful consequences is one of the main characteristics of compulsive behaviors and addiction. Animal models that provide a rapid and effective measure of resistance to punishment could be critical for the investigation of mechanisms underlying these maladaptive behaviors. Here, analogous to the progressive ratio (PR) procedure widely used to evaluate appetitive motivation as the response requirement is increased, we developed a self-adjusting, progressive shock strength (PSS) procedure. The PSS provides, within a single session, a break point that quantifies the propensity to work for a reward in spite of receiving electric footshock that progressively increases in duration. In both male and female rats, the PSS break point was sensitive to 1) hunger; and 2) changes in the qualitative, but not quantitative, incentive value of the reward. In systematic comparisons between PSS and PR procedures in the same rats, we found that both measures are sensitive to manipulations of motivational states, but they are not intercorrelated, suggesting that they measure overlapping but partially distinct processes. Importantly, the PSS procedure represents a refinement in the 3Rs principles of animal research because animals can control the strength of shock that they are willing to tolerate. This self-adjusting PSS procedure may represent a useful tool to investigate mechanisms underlying maladaptive behavior that persists in certain individuals despite harmful consequences.

Prevention of relapse to methamphetamine self-administration by environmental enrichment: involvement of glucocorticoid receptors.

RATIONALE: In rodents, environmental enrichment (EE) produces both preventive and curative effects on drug addiction, and this effect is believed to depend at least in part on EE's actions on the stress system. OBJECTIVES: This study investigated whether exposure to EE during abstinence reduces methamphetamine seeking after extended self-administration. In addition, we investigated whether these effects are associated with alterations in the levels of glucocorticoid receptors (GR) in the brain and whether administration of GR antagonists blocks methamphetamine relapse. METHODS: We allowed rats to self-administer methamphetamine for twenty 14-h sessions. After 3 weeks of abstinence either in standard (SE) or EE conditions, we measured methamphetamine seeking in a single 3-h session. Then, we used western blot techniques to measure GR levels in several brain areas. Finally, in an independent group of rats, after methamphetamine self-administration and abstinence in SE, we administered the GR antagonist mifepristone, and we investigated methamphetamine seeking. RESULTS: Exposure to EE reduced methamphetamine seeking and reversed methamphetamine-induced increases in GR levels in the ventral and dorsal hippocampus. In addition, EE decreased GR levels in the amygdala in drug-naive animals, but this effect was prevented by previous exposure to methamphetamine. Administration of mifepristone significantly decreased methamphetamine seeking. CONCLUSIONS: The anti-craving effects of EE are paralleled by restoration of methamphetamine-induced dysregulation of GR in the hippocampus. These results provide support for the hypothesis that the effect of EE on methamphetamine relapse is at least in part mediated by EE's action on the brain stress system.

NPY promotes chemokinesis and neurogenesis in the rat subventricular zone.

The subventricular zone (SVZ) is a major reservoir for stem cells in the adult mammalian brain. Neural stem cells supply the olfactory bulb with new interneurons and provide cells that migrate towards lesioned brain areas. Neuropeptide Y (NPY), one of the most abundant neuropeptides in the brain, was previously shown to induce neuroproliferation on mice SVZ cells. In the present study, performed in rats, we demonstrate the endogenous synthesis of NPY by cells in the SVZ that suggests that NPY could act as an autocrine/paracrine factor within the SVZ area. We observed that NPY promotes SVZ cell proliferation as previously reported in mice, but does not affect self-renewal of SVZ stem cells. Additionally, this study provides the first direct evidence of a chemokinetic activity of NPY on SVZ cells. Using pharmacological approaches, we demonstrate that both the mitogenic and chemokinetic properties of NPY involve Y1 receptor-mediated activation of the ERK1/2 MAP kinase pathway. Altogether, our data establish that NPY through Y1 receptors activation controls chemokinetic activity and, as for mice, is a major neuroproliferative regulator of rat SVZ cells.

Reversal of cocaine addiction by environmental enrichment.

Environmental conditions can dramatically influence the behavioral and neurochemical effects of drugs of abuse. For example, stress increases the reinforcing effects of drugs and plays an important role in determining the vulnerability to develop drug addiction. On the other hand, positive conditions, such as environmental enrichment, can reduce the reinforcing effects of psychostimulants and may provide protection against the development of drug addiction. However, whether environmental enrichment can be used to "treat" drug addiction has not been investigated. In this study, we first exposed mice to drugs and induced addiction-related behaviors and only afterward exposed them to enriched environments. We found that 30 days of environmental enrichment completely eliminates behavioral sensitization and conditioned place preference to cocaine. In addition, housing mice in enriched environments after the development of conditioned place preference prevents cocaine-induced reinstatement of conditioned place preference and reduces activation of the brain circuitry involved in cocaine-induced reinstatement. Altogether, these results demonstrate that environmental enrichment can eliminate already established addiction-related behaviors in mice and suggest that environmental stimulation may be a fundamental factor in facilitating abstinence and preventing relapse to cocaine addiction.

Environmental enrichment-inspired pharmacological tools for the treatment of addiction.

Environmental enrichment (EE) has been shown to produce powerful beneficial effects in animal models of addiction. In particular, the ability of EE to promote abstinence and prevent relapse may allow for the identification of brain mechanisms responsible for the recovery from addiction. Indeed, the effects of EE on specific brain mechanisms could be mimicked by old or new molecules, which may become novel medications, called enviromimetics. Here, we review the best known enviromimetics for the treatment of addiction and suggest that, whereas these compounds may be relatively ineffective by themselves, they may be useful complements for existing therapeutic approaches to manage addiction which includes behavioural, environmental and pharmacological interventions.

Endogenous hepatocyte growth factor is a niche signal for subventricular zone neural stem cell amplification and self-renewal.

Neural stem cells persist in the adult mammalian brain, within the subventricular zone (SVZ). The endogenous mechanisms underpinning SVZ neural stem cell proliferation, self-renewal, and differentiation are not fully elucidated. In the present report, we describe a growth-stimulatory activity of liver explant-conditioned media on SVZ cell cultures and identify hepatocyte growth factor (HGF) as a major player in this effect. HGF exhibited a mitogenic activity on SVZ cell cultures in a mitogen-activated protein kinase (MAPK) (ERK1/2)-dependent manner as U0126, a specific MAPK inhibitor, blocked it. Combining a functional neurosphere forming assay with immunostaining for c-Met, along with markers of SVZ cells subtypes, demonstrated that HGF promotes the expansion of neural stem-like cells that form neurospheres and self-renew. Immunostaining, HGF enzyme-linked immunosorbent assay and Madin-Darby canine kidney cell scattering assay indicated that SVZ cell cultures produce and release HGF. SVZ cell-conditioned media induced proliferation on SVZ cell cultures, which was blocked by HGF-neutralizing antibodies, hence implying that endogenously produced HGF accounts for a major part in SVZ mitogenic activity. Brain sections immunostaining revealed that HGF is produced by nestin-expressing cells and c-Met is expressed within the SVZ by immature cells. HGF intracerebroventricular injection promoted SVZ cell proliferation and increased the ability of these cells exposed in vivo to HGF to form neurospheres in vitro, whereas intracerebroventricular injection of HGF-neutralizing antibodies decreased SVZ cell proliferation. The present study unravels a major role, both in vitro and in vivo, for endogenous HGF in SVZ neural stem cell growth and self-renewal.

Environmental enrichment during adolescence regulates gene expression in the striatum of mice.

We have previously shown that environmental enrichment decreases the activating and rewarding effects of the psychostimulant cocaine and increases resistance to the neurotoxic effect of the Parkinson-inducing drug MPTP. These effects were accompanied by an increase in the striatal expression of the neurotrophin BDNF, an increase in the striatal levels of delta-Fos B and by a decrease in striatal levels of the dopamine transporter, the main molecular target for cocaine and MPTP. Here, we used cDNA arrays to investigate the effects of rearing mice in enriched environments from weaning to adulthood on the profile of expression of genes in the striatum focusing on genes involved in intracellular signalling and functioning. We found that mice reared in an enriched environment show several alterations in the levels of mRNA coding for proteins involved in cell proliferation, cell differentiation, signal transduction, transcription and translation, cell structure and metabolism. Several of these findings were further confirmed by real-time quantitative PCR and, in the case of protein kinase C lambda, also by western blot. These findings are the first description of alterations in striatal gene expression by an enriched environment. The striatal gene expression regulation by environment that we report here may play a role in the resistance to the effects of drugs of abuse and dopaminergic neurotoxins previously reported.

Persistent Neuroadaptations in the Expression of Genes Involved in Cholesterol Homeostasis Induced by Chronic, Voluntary Alcohol Intake in Rats.

Alcohol use disorder (AUD) is associated with persistent adaptations in the brain that are believed to participate in the long-lasting vulnerability to relapse after abstinence. Cholesterol, the major sterol compound found in the central nervous system (CNS), plays a major role in maintenance of neuronal morphology, synaptogenesis and synaptic communication and may be involved in alcohol-induced neuroadaptations. In this study, we investigated whether alcohol consumption in a two-bottle choice paradigm followed by 3 weeks of abstinence could alter the expression of genes encoding proteins involved in cholesterol homeostasis in brain regions involved in addiction and relapse, namely the prefrontal cortex (PFC), the nucleus accumbens (NAc), the mesencephalon and the amygdala. We found that voluntary alcohol intake followed by 3 weeks of forced abstinence produces changes in the transcription of several genes encoding proteins directly involved in cholesterol synthesis such as 3-hydroxyl-3-methylglutaryl-coenzyme A (HMGCoA) reductase, farnesyl-diphosphate farnesyltransferase 1 (FDFT1) and farnesyl diphosphate synthase (FDPS) and in its regulation such as sterol regulatory element-binding factor-2 (SREBF2), in cholesterol transport such as ATP-binding cassette subfamily A member 1 (ABCA1) and in cholesterol degradation such as CYP46A1. Interestingly, these changes appeared to be region-specific and suggest that previous chronic exposure to alcohol might durably increase cholesterol metabolism in the PFC, the NAc and the mesencephalon and decrease cholesterol metabolism in the amygdala. Altogether, these results suggest that alcohol consumption leads to durable deregulations in cholesterol metabolism in key areas involved in loss of control over drug use and addiction. These long-term neuroadaptations may participate in the changes in brain structure and functioning that are responsible for the long-lasting risks of relapse to alcohol.

Generalization of effects of environmental enrichment on seeking for different classes of drugs of abuse.

BACKGROUND: Addiction is a chronic disease characterized by persistent vulnerability to relapse during abstinence. In animal models of addiction, accumulating evidence suggests that exposure to environmental enrichment (EE) during periods of abstinence can have curative effects on addiction and reduce the risks of relapse. However, until present most studies have mainly focused on cocaine. In this study, we investigated whether EE could have beneficial effects on cue-induced seeking for several psychoactive drugs belonging to different pharmacological classes such as methamphetamine (METH), heroin (HER) and nicotine (NIC). METHODS: After self-administration training of METH, HER and NIC, rats were housed in enriched (EE) or standard environments (SE) for 21-28 days of forced abstinence and then drug-seeking behavior was assessed in the absence of the drug. RESULTS: We found that, compared to SE housing, exposure to EE reduced drug seeking behavior for all drugs tested. CONCLUSIONS: These findings suggest that the anti-craving effects of EE are general for a wide variety of drugs and support the hypothesis that environmental stimulation may be a general intervention for attenuating relapse in humans.

Neuropeptide Y protects against methamphetamine-induced neuronal apoptosis in the mouse striatum.

Methamphetamine (METH) is an illicit drug that causes neuronal apoptosis in the mouse striatum, in a manner similar to the neuronal loss observed in neurodegenerative diseases. In the present study, injections of METH to mice were found to cause the death of enkephalin-positive projection neurons but not the death of neuropeptide Y (NPY)/nitric oxide synthase-positive striatal interneurons. In addition, these METH injections were associated with increased expression of neuropeptide Y mRNA and changes in the expression of the NPY receptors Y1 and Y2. Administration of NPY in the cerebral ventricles blocked METH-induced apoptosis, an effect that was mediated mainly by stimulation of NPY Y2 receptors and, to a lesser extent, of NPY Y1 receptors. Finally, we also found that neuropeptide Y knock-out mice were more sensitive than wild-type mice to METH-induced neuronal apoptosis of both enkephalin- and nitric oxide synthase-containing neurons, suggesting that NPY plays a general neuroprotective role within the striatum. Together, our results demonstrate that neuropeptide Y belongs to the class of factors that maintain neuronal integrity during cellular stresses. Given the similarity between the cell death patterns induced by METH and by disorders such as Huntington's disease, our results suggest that NPY analogs might be useful therapeutic agents against some neurodegenerative processes.

Statins Reduce the Risks of Relapse to Addiction in Rats.

Statins are drugs that have been used for decades in humans for the treatment of hypercholesterolemia. More recently, several lines of evidence demonstrate that statins, in addition to their peripheral effects, produce a wide variety of effects in the brain and may be beneficial in neurological and psychiatric conditions. In this study, we allowed rats to self-administer cocaine for several weeks and, at the end of self-administration training, we treated them with low doses of statins daily for a 21-day period of abstinence. Chronic administration of brain-penetrating statins, simvastatin (1 mg/kg) and atorvastatin (1 mg/kg), reduced cocaine seeking compared with vehicle, whereas administration of pravastatin (2 mg/kg), a statin with low brain penetrability, did not. Importantly, the effects of brain-penetrating statins persisted even after discontinuation of the treatment and were specific for drug seeking because drug taking was not altered by simvastatin treatment. Finally, the effects of simvastatin were found to generalize to another drug of abuse such as nicotine, but not to food reward, and to reinstatement of cocaine seeking induced by stress. These results demonstrate that brain-penetrating statins can reduce risks of relapse to addiction. Given their well-known safety profile in humans, statins could be a novel effective treatment for relapse to cocaine and nicotine addiction and their use could be implemented in clinical settings without major health risks.

Analysis of ecstasy (MDMA)-induced transcriptional responses in the rat cortex.

3,4-methylenedioxymethamphetamine (MDMA, ecstasy) is a popular drug of abuse. MDMA is pharmacologically classified as an entactogen because of its affinities to classical hallucinogens and stimulants. Oral ingestion of a single dose of the drug is associated with euphoria, elevated self-confidence, and heightened sensory awareness in humans. Evidence for neurotoxicity in the human serotonin (5-HT) system has been provided. In rats, a single injection of MDMA induces hyperthermia and formation of reactive oxygen species. These effects may cause MDMA-associated, long-term 5-HT depletion, with the cortex being quite sensitive to the biochemical effects of MDMA. It has been suggested that these MDMA effects may be associated with molecular changes in this brain region. To test these ideas, we have made use of the cDNA array analysis, which can provide a more global view of the molecular changes secondary to MDMA injections. Our results show that the genes regulated by MDMA encode proteins that belong to signaling pathways, transcription regulators, or xenobiotic metabolism. Our observations indicate that cortical cells respond to the acute administration of MDMA by modulating transcription of several genes that might lead to long-term changes in the brain.

The nitric oxide releasing agent sodium nitroprusside modulates cocaine-induced immediate early gene expression in rat brain.

The nitric oxide (NO)/cGMP pathway triggers key events in synaptic phenomena involved in learning and memory. Using in situ hybridization, the present report demonstrates that NO released by sodium nitroprusside regulates egr-1, c-fos, and junB immediate early gene expression in rat forebrain. These genes, which are rapidly and transiently induced in response to diverse extracellular stimulation, coordinate alterations in gene expression underlying neuronal plasticity. Intracerebroventricular injection of sodium nitroprusside induced immediate early gene expression, which was highest in the nucleus accumbens. On the other hand, sodium nitroprusside abolished the cocaine-induced early gene expression in the dopaminergic projection fields nucleus accumbens, caudate-putamen, and frontal cortex. Further studies are warranted to explore the potential of the NO/cGMP/cGMP-dependent protein kinase pathway to modify cocaine-related behavioral effects.