Dopamine responsiveness to drugs of abuse: A shell-core investigation in the nucleus accumbens of the mouse

Menthol facilitates dopamine-releasing effect of nicotine in rat nucleus accumbens

Menthol is a significant flavoring additive in tobacco products. Accumulating clinical evidence suggests that menthol may promote tobacco smoking and nicotine dependence. Our previous studies demonstrated that menthol enhanced nicotine reinforcement in rats. However, it is unclear whether menthol interacts with nicotine at the neurochemical level. The present study used intracranial microdialysis to examine whether and the ways in which menthol affects nicotine-induced dopamine release in rats in the nucleus accumbens core (NAc), a terminal field of brain reward circuitry. To make comparisons with our previous work that showed an enhancing effect of menthol on nicotine self-administration behavior, male Sprague-Dawley rats were first trained in 20 daily 1-h sessions to press a lever for intravenous nicotine self-administration (15 μg/kg/infusion). Dopamine levels were then measured in the right NAc using intracranial microdialysis coupled with high-performance liquid chromatography. Five minutes before microdialysis, the rats received an intraperitoneal injection of menthol (0, 1, 2.5, and 5 mg/kg), a subcutaneous injection of nicotine (0.2 mg/kg or its vehicle), or both. Menthol alone did not affect dopamine levels in dialysates, whereas nicotine alone elevated dopamine levels. Combined nicotine and menthol administration significantly increased dopamine levels compared with nicotine alone. These data indicate a facilitating effect of menthol on nicotine-induced dopamine release in the NAc. These findings shed light on our understanding of the neurobiological mechanisms that underlie the menthol-induced enhancement of nicotine reinforcement.

Cyclic nucleotide phosphodiesterases: potential therapeutic targets for alcohol use disorder

Alcohol use disorder (AUD), which combines the criteria of both alcohol abuse and dependence, contributes as an important causal factor to multiple health and social problems. Given the limitation of current treatments, novel medications for AUD are needed to better control alcohol consumption and maintain abstinence. It has been well established that the intracellular signal transduction mediated by the second messengers cyclic AMP (cAMP) and cyclic GMP (cGMP) crucially underlies the genetic predisposition, rewarding properties, relapsing features, and systemic toxicity of compulsive alcohol consumption. On this basis, the upstream modulators phosphodiesterases (PDEs), which critically control intracellular levels of cyclic nucleotides by catalyzing their degradation, are proposed to play a role in modulating alcohol abuse and dependent process. Here, we highlight existing evidence that correlates cAMP and cGMP signal cascades with the regulation of alcohol-drinking behavior and discuss the possibility that PDEs may become a novel class of therapeutic targets for AUD.

Targeting Phosphodiesterases in Pharmacotherapy for Substance Dependence

Substance dependence is a chronic relapsing brain disorder associated with adaptational changes in synaptic plasticity and neuronal functions. The high levels of substance consumption and relapse rate suggest more reliable medications are in need to better address the underlying causes of this disease. It has been well established that the intracellular second messengers cyclic AMP (cAMP) and cyclic GMP (cGMP) and their signaling systems play an important role in the molecular mechanisms of substance taking behaviors. On this basis, the phosphodiesterase (PDE) superfamily, which crucially controls cyclic nucleotide levels by catalyzing their hydrolysis, has been proposed as a novel class of therapeutic targets for substance use disorders. This chapter reviews the expression patterns of PDEs in the brain with regard to neural structures underlying the dependent process and highlights available evidence for a modulatory role of PDEs in substance dependence.

Role of nucleus accumbens μ opioid receptors in the effects of morphine on ERK1/2 phosphorylation

RATIONALE

Despite the critical role attributed to phosphorylated extracellular signal regulated kinase (pERK1/2) in the nucleus accumbens (Acb) in the actions of addictive drugs, the effects of morphine on ERK1/2 phosphorylation in this area are still controversial.

OBJECTIVES

In order to investigate further this issue, we studied (1) the ability of morphine to affect ERK1/2 phosphorylation in the shell (AcbSh) and core (AcbC) of Sprague-Dawley and Wistar rats and of CD-1 and C57BL/6J mice and (2) the role of dopamine D1 and μ-opioid receptors in Sprague-Dawley rats and CD-1 mice.

METHODS

The pERK1/2 expression was assessed by immunohistochemistry.

RESULTS

In rats, morphine decreased AcbSh and AcbC pERK1/2 expression, whereas in mice, increased it preferentially in the AcbSh compared with the AcbC. Systemic SCH 39166 decreased pERK1/2 expression on its own in the AcbSh and AcbC of Sprague-Dawley rats and CD-1 mice; furthermore, in rats, SCH 39166 disclosed the ability of morphine to stimulate pERK1/2 expression. Systemic (rats and mice) and intra-Acb (rats) naltrexone prevented both decreases, in rats, and increases, in mice.

CONCLUSIONS

These findings confirm the differential effects of morphine in rats and mice Acb and that D1 receptors exert a facilitatory role on ERK1/2 phosphorylation; furthermore, they indicate that, in rats, removal of the D1-dependent pERK1/2 expression discloses the stimulatory influence of morphine on ERK1/2 phosphorylation and that the morphine's ability to decrease pERK1/2 expression is mediated by Acb μ-opioid receptors. Future experiments may disentangle the psychopharmacological significance of the effects of morphine on pERK1/2 in the Acb.

Cocaine-induced endocannabinoid release modulates behavioral and neurochemical sensitization in mice

The endocannabinoid system has been implicated in the development of synaptic plasticity induced by several drugs abused by humans, including cocaine. However, there remains some debate about the involvement of cannabinoid receptors/ligands in cocaine-induced plasticity and corresponding behavioral actions. Here, we show that a single cocaine injection in Swiss-Webster mice produces behavioral and neurochemical alterations that are under the control of the endocannabinoid system. This plasticity may be the initial basis for changes in brain processes leading from recreational use of cocaine to its abuse and ultimately to dependence. Locomotor activity was monitored with photobeam cell detectors, and accumbens shell/core microdialysate dopamine levels were monitored by high-performance liquid chromatography with electrochemical detection. Development of single-trial cocaine-induced behavioral sensitization, measured as increased distance traveled in sensitized mice compared to control mice, was paralleled by a larger stimulation of extracellular dopamine levels in the core but not the shell of the nucleus accumbens. Both the behavioral and neurochemical effects were reversed by CB1 receptor blockade produced by rimonabant pre-treatments. Further, both behavioral and neurochemical cocaine sensitization were facilitated by pharmacological blockade of endocannabinoid metabolism, achieved by inhibiting the fatty acid amide hydrolase enzyme. In conclusion, our results suggest that a single unconditioned exposure to cocaine produces sensitization through neuronal alterations that require regionally specific release of endocannabinoids. Further, the present results suggest that endocannabinoids play a primary role from the earliest stage of cocaine use, mediating the inception of long-term brain-adaptive responses, shaping central pathways and likely increasing vulnerability to stimulant abuse disorders.

Dopamine receptor blockade modulates the rewarding and aversive properties of nicotine via dissociable neuronal activity patterns in the nucleus accumbens

The mesolimbic pathway comprising the ventral tegmental area (VTA) and projection terminals in the nucleus accumbens (NAc) has been identified as a critical neural system involved in processing both the rewarding and aversive behavioral effects of nicotine. Transmission through dopamine (DA) receptors functionally modulates these effects directly within the NAc. Nevertheless, the neuronal mechanisms within the NAc responsible for these bivalent behavioral effects are presently not known. Using an unbiased conditioned place preference procedure combined with in vivo neuronal recordings, we examined the effects of nicotine reward and aversion conditioning on intra-NAc neuronal sub-population activity patterns. We report that intra-VTA doses of nicotine that differentially produce rewarding or aversive behavioral effects produce opposite effects on sub-populations of fast-spiking interneurons (FSIs) or medium spiny neurons (MSNs) within the shell region of the NAc (NAshell). Thus, while the rewarding effects of intra-VTA nicotine were associated with inhibition of FSI and activation of MSNs, the aversive effects of nicotine produced the opposite pattern of NAshell neuronal population activity. Blockade of DA transmission with a broad-spectrum DA receptor antagonist, α-flupenthixol, strongly inhibited the spontaneous activity of NAshell FSIs, and reversed the conditioning properties of intra-VTA nicotine, switching nicotine-conditioned responses from aversive to rewarding. Remarkably, DA receptor blockade switched intra-NAshell neuronal population activity from an aversion to a reward pattern, concomitant with the observed switch in behavioral conditioning effects.

Overexpression of hyperpolarization-activated cyclic nucleotide-gated channels into the ventral tegmental area increases the rewarding effects of ethanol in UChB drinking rats

BACKGROUND

A number of studies have shown that ethanol (EtOH) activates dopamine neurocircuitries and is self-administered into the ventral tegmental area (VTA) of the rat brain. In vitro and in silico studies have showed that hyperpolarization-activated cyclic nucleotide-gated (HCN) ionic channels on VTA dopamine neurons may constitute a molecular target of EtOH; however, there is no in vivo evidence supporting this assumption.

METHODS

Wistar-derived University of Chile Drinking (UChB) rats were microinjected into the VTA with a lentiviral vector coding for rat HCN-2 ionic channel or a control vector. Four days after vector administration, daily voluntary EtOH intake was assessed for 30 days under a free-access paradigm to 5% EtOH and water. After EtOH consumption studies, the effect of HCN-2 overexpression was also assessed on EtOH-induced conditioned place preference (CPP); EtOH-induced locomotion, and EtOH-induced dopamine release in the nucleus accumbens (NAcc).

RESULTS

Rats microinjected with the HCN-2 coding vector into the VTA showed (i) a ~2-fold increase in their voluntary EtOH intake compared to control animals, (ii) lentiviral-HCN-2-treated animals also showed an increased CPP to EtOH (~3-fold), (iii) a significant higher locomotor activity (~2-fold), and (iv) increased dopamine release in NAcc upon systemic administration of EtOH (~2-fold).

CONCLUSIONS

Overexpression of HCN-2 ionic channel in the VTA of rats results in an increase in voluntary EtOH intake, EtOH-induced CPP, locomotor activity, and dopamine release in NAcc, suggesting that HCN levels in the VTA are relevant for the rewarding properties of EtOH.

Knockdown of tropomyosin-related kinase B receptor expression in the nucleus accumbens shell prevents intermittent social defeat stress-induced cross-sensitization to amphetamine in rats

The nucleus accumbens (NAc) is a critical brain region for the rewarding effects of drugs of abuse. Brain-derived neurotrophic factor (BDNF) can facilitate stress- and drug-induced neuroadaptation in the mesocorticolimbic system. BDNF-containing projections to the NAc originate from the ventral tegmental area (VTA) and the prefrontal cortex, and BDNF release activates tropomyosin-related kinase B (TrkB). In this study, we examined the necessity for BDNF-TrkB signaling in the NAc shell during social defeat stress-induced cross-sensitization to amphetamine. Adeno-associated virus expressing short hairpin RNA directed against TrkB (AAV-shTrkB) was infused bilaterally into the NAc shell to knock down TrkB, whereas AAV-GFP (green fluorescent protein) was used as the control virus. Rats were exposed to intermittent social defeat stress or handling procedures; amphetamine challenge was given at 10 days after the last defeat and locomotor activity was measured. Stressed rats that received the control virus showed cross-sensitization to amphetamine compared with the handled rats. In contrast, NAc TrkB knockdown prevented social defeat stress-induced cross-sensitization. TrkB knockdown in the NAc was found to reduce the level of phospho-extracellular signal-regulated kinase 1 in this region. NAc TrkB knockdown also prevented stress-induced elevation of BDNF and the glutamate receptor type 1 (GluA1) subunit of AMPA receptor in the VTA, as well as ΔFosB expression in the NAc. These findings indicated that BDNF-TrkB signaling in the NAc shell was required for social defeat stress-induced cross-sensitization. NAc TrkB-BDNF signaling also appeared to be involved in the regulation of GluA1 in the VTA, as well as in the NAc ΔFosB accumulation that could trigger cross-sensitization after social defeat stress.

Metabotropic glutamate receptor 5 within nucleus accumbens shell modulates environment-elicited cocaine conditioning expression

The metabotropic glutamate receptors 5 (mGluRs5) within the Nucleus Accumbens (NAc) have been implicated in the modulation of psychostimulant reward. We hypothesized that blockade of mGluR5 within the NAc shell would impair cocaine conditioning in rats. For this study, animals were implanted with cannulae within the NAc shell, and separate groups were exposed to a multimodal environment within activity chambers that signaled cocaine (cocaine-paired) or saline (controls, cocaine-unpaired) injections. Prior to placing the animals in the chambers, rats received systemic intraperitoneal injections of saline or cocaine for 10 consecutive sessions. In the test session (D12), animals were exposed to the multimodal environment without any cocaine or saline pre-treatment. Before placing the rats in the chambers, separate groups of animals were infused within the NAc shell with 2.5, 12 or 25 nmol/0.5 μl/side of 2-methyl-6-(phenylethynyl) pyridine (MPEP), an antagonist of mGluR5 or with vehicle. Blockade of the mGluR5 subtype at a 2.5 nmol dose showed no significant difference in either the ambulatory distance (AD) or the vertical plane move time (VPT). In contrast, mGluR5 blockade at 12 nmol and 25 nmol decreased conditioned locomotion in the cocaine-paired groups. An association of the environmental cues with the effects of cocaine implies the involvement of memory process during the conditioning response. Our results suggest that mGluR5 within the NAc shell could be modulating the expression of memory related to the association of environmental cues with the effects of cocaine. We suggest that mGluR5 could be taking into account to further studies related with cocaine exposure and cocaine addiction treatments.

Presynaptic dopamine modulation by stimulant self-administration

The mesolimbic dopamine system is an essential participant in the initiation and modulation of various forms of goal-directed behavior, including drug reinforcement and addiction processes. Dopamine neurotransmission is increased by acute administration of all drugs of abuse, including the stimulants cocaine and amphetamine. Chronic exposure to these drugs via voluntary self-administration provides a model of stimulant abuse that is useful in evaluating potential behavioral and neurochemical adaptations that occur during addiction. This review describes commonly used methodologies to measure dopamine and baseline parameters of presynaptic dopamine regulation, including exocytotic release and reuptake through the dopamine transporter in the nucleus accumbens core, as well as dramatic adaptations in dopamine neurotransmission and drug sensitivity that occur with acute non-contingent and chronic, contingent self-administration of cocaine and amphetamine.

Methylphenidate amplifies the potency and reinforcing effects of amphetamines by increasing dopamine transporter expression

Methylphenidate (MPH) is commonly diverted for recreational use, but the neurobiological consequences of exposure to MPH at high, abused doses are not well defined. Here we show that MPH self-administration in rats increases dopamine transporter (DAT) levels and enhances the potency of MPH and amphetamine on dopamine responses and drug-seeking behaviours, without altering cocaine effects. Genetic overexpression of the DAT in mice mimics these effects, confirming that MPH self-administration-induced increases in DAT levels are sufficient to induce the changes. Further, this work outlines a basic mechanism by which increases in DAT levels, regardless of how they occur, are capable of increasing the rewarding and reinforcing effects of select psychostimulant drugs, and suggests that individuals with elevated DAT levels, such as ADHD sufferers, may be more susceptible to the addictive effects of amphetamine-like drugs.

Alcohol-preferring (P) rats are more sensitive than Wistar rats to the reinforcing effects of cocaine self-administered directly into the nucleus accumbens shell

Wistar rats will self-administer cocaine directly into the nucleus accumbens shell (AcbSh), but not into the nucleus accumbens core. In human and animal literature, there is a genetic association between alcoholism and cocaine dependency. The current experiment examined whether selective breeding for high alcohol preference is also associated with greater sensitivity of the AcbSh to the reinforcing properties of cocaine. P and Wistar rats were given cocaine (0, 100, 200, 400, or 800 pmol/100 nl) to self-infuse into the AcbSh. Rats were given cocaine for the first 4 sessions (acquisition), artificial CSF for sessions 5 and 6 (extinction), and cocaine again in session 7 (reinstatement). During acquisition, P rats self-infused 200-800 pmol cocaine (59 infusions/session), whereas Wistar rats only reliably self-infused 800 pmol cocaine (38 infusions/session). Furthermore, P rats received a greater number of cocaine infusions in the 200, 400 and 800 pmol cocaine groups compared to respective Wistar groups during acquisition. Both P and Wistar rats reduced responding on the active lever when aCSF was substituted for cocaine, and reinstated responding in session 7 when cocaine was restored. However, P rats had significantly greater infusions during session 7 compared to session 4 at all concentrations of cocaine tested, whereas Wistar rats only displayed greater infusions during session 7 compared to session 4 at the 400 and 800 pmol cocaine concentrations. The present results suggest that, compared to Wistar rats, the AcbSh of P rats was more sensitive to the reinforcing effects of cocaine. The reinstatement data suggest that the AcbSh of P rats may have become sensitized to the reinforcing effects of cocaine. Overall, the findings from this study support a genetic association between high alcohol preference and greater sensitivity to the reinforcing effects of cocaine.

An investigation of bupropion substitution for the interoceptive stimulus effects of nicotine

Although the exact mechanism that makes bupropion hydrochloride (Zyban) effective as a smoking cessation aid has not been fully elucidated, studies have found that bupropion and nicotine share behavioural and neurophysiological properties suggesting that bupropion might serve as a substitute for nicotine. In fact, bupropion prompts nicotine-appropriate responding in operant and Pavlovian drug discrimination studies with rats. A majority of the literature examining this substitution pattern has been done with an operant paradigm. The present research extended this literature by further characterising the behavioural and neuropharmacological properties underlying the substitution for a nicotine conditioned stimulus (CS). Examination of the dose-effect function and temporal dynamics of this substitution pattern showed that bupropion (20 mg/kg) produced conditioned responding similar to nicotine (0.4 mg base/kg) (ED(50) = 9.9 mg/kg) at 15 and 30 min after injection and partially substituted 5 and 60 min post-injection. Bupropion produced a pattern of conditioned responding similar to nicotine during a 60-min extinction test. Additionally, it has been hypothesised that bupropion and nicotine have an overlapping dopaminergic mechanism. We tested the effects of bupropion pretreatment, the nicotine dose-effect function and the ability of dopamine antagonist to block the substitution of bupropion for nicotine. Pretreatment with doses of bupropion that did not substitute for the nicotine stimulus (5 and 10 mg/kg) did not affect nicotine-conditioned responding; pretreatment with 20 mg/kg attenuated nicotine-evoked responding. Pretreatment with the dopamine antagonists SCH-23390 and eticlopride blocked the substitution. Finally, S,S-hydroxybupropion, the major metabolite of bupropion in humans, did not substitute for the nicotine CS.

A functional fast scan cyclic voltammetry assay to characterize dopamine D2 and D3 autoreceptors in the mouse striatum

Dopamine D2 and D3 autoreceptors are located on pre-synaptic terminals and are known to control the release and synthesis of dopamine. Dopamine D3 receptors have a fairly restricted pattern of expression in the mammalian brain. Their localization in the nucleus accumbens core and shell is of particular interest because of their association with the rewarding properties of drugs of abuse. Using background subtracted fast scan cyclic voltammetry, we investigated the effects of dopamine D2 and D3 agonists on electrically stimulated dopamine release and uptake rates in the mouse caudate-putamen and nucleus accumbens core and shell. The dopamine D2 agonists (-)-quinpirole hydrochloride and 5,6,7,8-Tetrahydro-6-(2-propen-1-yl)-4H-thiazolo[4,5-d]azepin-2-amine dihydrochloride (B-HT 920) had the same dopamine release inhibition effects on caudate-putamen and nucleus accumbens (core and shell) based on their EC(50) and efficacies. This suggests that the dopamine D2 autoreceptor functionality is comparable in all three striatal regions investigated. The dopamine D3 agonists (4aR,10bR)-3,4a,4,10b-Tetrahydro-4-propyl-2H,5H-[1]benzopyrano-[4,3-b]-1,4-oxazin-9-ol hydrochloride ((+)-PD 128907) and (+/-)-7-Hydroxy-2-dipropylaminotetralin hydrobromide (7-OH-DPAT) had a significantly greater effect on dopamine release inhibition in the nucleus accumbens shell than in caudate-putamen. This study confirms that, the dopamine D3 autoreceptor functionality is greater in the nucleus accumbens shell followed by the nucleus accumbens core, with the caudate-putamen having the least. Neither dopamine D2 nor D3 agonists affected the uptake rates in nucleus accumbens but concentrations greater than 0.3 muM lowered the uptake rate in caudate-putamen. To validate our method of evaluating dopamine D2 and D3 autoreceptors, sulpiride (D2 antagonist) and nafadotride (D3 antagonist) were used to reverse the effects of the dopamine agonists to approximately 100% of the pre-agonist dopamine release concentration. Finally, these results demonstrate a functional voltammetric assay that characterizes dopamine D2-like agonist as either D2- or D3-preferring agonists by taking advantage of the unique receptor density within the striatum.

Mecamylamine - a nicotinic acetylcholine receptor antagonist with potential for the treatment of neuropsychiatric disorders

Mecamylamine (Inversine), the first orally available antihypertensive agent launched in the 1950s, is rarely used today for hypertension because of its widespread ganglionic side effects at antihypertensive doses (25 - 90 mg/day). However, more recent clinical studies suggest that mecamylamine is effective at much lower doses for blocking the central and peripheral effects of nicotine. Pharmacologically, mecamylamine has been well characterized as a nonselective and noncompetitive antagonist of nicotinic acetylcholine receptors (nAChRs). Because mecamylamine easily crosses the blood - brain barrier at relatively low doses (2.5 - 10 mg), it has been used by several research groups over the past two decades investigating the role of central nAChRs in the etiology and treatment of various neuropsychiatric disorders, including addiction disorders, Tourette's syndrome, schizophrenia and various cognitive and mood disorders. Two independent Phase II clinical trials recently confirmed mecamylamine's hypothesized antidepressant activity and suggest that it may be effective as an augmentation pharmacotherapy for SSRI treatment resistant major depression. These areas of investigation for mecamylamine are reviewed and recommendations for future research directions are proposed.

Ventral and dorsal striatal dopamine efflux and behavior in rats with simple vs. co-morbid histories of cocaine sensitization and neonatal ventral hippocampal lesions

RATIONAL

Exposing animal models of mental illness to addictive drugs provides an approach to understanding the neural etiology of dual diagnosis disorders. Previous studies have shown that neonatal ventral hippocampal lesions (NVHL) in rats produce features of both schizophrenia and addiction vulnerability.

OBJECTIVE

This study investigated ventral and dorsal striatal dopamine (DA) efflux in NVHL rats combined with behavioral sensitization to cocaine.

METHODS

Adult NVHL vs. SHAM-operated rats underwent a 5-day injection series of cocaine (15 mg/kg/day) vs. saline. One week later, rats were cannulated in nucleus accumbens SHELL, CORE, or caudate-putamen. Another week later, in vivo microdialysis sampled DA during locomotor testing in which a single cocaine injection (15 mg/kg) was delivered.

RESULTS

NVHLs and cocaine history significantly increased behavioral activation approximately 2-fold over SHAM-saline history rats. DA efflux curves corresponded time dependently with the cocaine injection and locomotor curves and varied significantly by striatal region: Baseline DA levels increased 5-fold while cocaine-stimulated DA efflux decreased by half across a ventral to dorsal striatal gradient. However, NVHLs, prior cocaine history, and individual differences in behavior were not underpinned by differential DA efflux overall or within any striatal region.

CONCLUSION

Differences in ventral/dorsal striatal DA efflux are not present in and are not required for producing differential levels of acute cocaine-induced behavioral activation in NVHLs with and without a behaviorally sensitizing cocaine history. These findings suggest other neurotransmitter systems, and alterations in striatal network function post-synaptic to DA transmission are more important to understanding the interactive effects of addictive drugs and mental illness.

Prodynorphin gene disruption increases the sensitivity to nicotine self-administration in mice

The endogenous opioid system has been reported to participate in nicotine behavioural responses. The aim of the study was to determine the contribution of the endogenous peptides derived from prodynorphin in acute and chronic nicotine responses, mainly those related to its addictive properties. Locomotion and nociception were evaluated after acute nicotine administration in prodynorphin knockout mice. In addition, nicotine rewarding properties were investigated in the place-conditioning and the intravenous self-administration paradigms. The somatic signs of nicotine withdrawal were also analysed after the injection of the nicotinic antagonist mecamylamine in nicotine-dependent mice. The hypolocomotor and antinociceptive effects induced by acute nicotine administration were not modified in knockout (KO) animals. Nicotine also produced similar conditioned place preference in both genotypes. However, a shift to the left in the percentage of acquisition of intravenous nicotine-self administration was observed in prodynorphin KO mice. Indeed, a significant increase in the number of KO mice acquiring this operant behaviour was revealed when low doses of nicotine were used. Nicotine physical dependence was similar in wild-type and KO animals. These findings reveal a specific role of endogenous peptides derived from prodynorphin in nicotine self-administration, probably through the modulation of its aversive effects.

Strain differences between Lewis and Fischer 344 rats in the modulation of dopaminergic receptors after morphine self-administration and during extinction

The Lewis (LEW) and Fischer 344 (F344) rat strains have been used as a model to study genetic vulnerability to drug addiction and they differ in their dopaminergic systems. We have studied the variation in the D1-like and D2-like receptors in distinct brain regions of LEW and F344 rats that self-administered morphine (1 mg/kg) for 15 days and also after different extinction periods (3, 7 and 15 days). Under basal conditions, binding to D1-like receptors in the olfactory tubercle and substantia nigra, and to D2-like receptors in the Pyriform cortex and hippocampal-CA1 was lower in LEW rats than in F344 rats. Conversely, the LEW rats exhibited stronger D2-like binding in the caudate-putamen. In most brain regions there was a decrease in D1-like binding in LEW rats after self-administration while the F344 animals displayed an increment. Additionally, D2 receptors of LEW rats were down-regulated after self-administration in the caudate-putamen and in the nucleus accumbens (shell and core divisions). Binding to D1-like receptors increased in both strains in the early phases of extinction, while in the later stages a differential regulation was observed between both strains. During the early phases of extinction only F344 rats showed alterations in D2-like receptor binding, however in the latter phases a specific modulation occurred in both strains. These differences in basal D1-like and D2-like receptor binding, and their differential modulation after self-administration and during extinction, may be reflected in the greater vulnerability to opiate addiction shown by LEW strain.

Accumbal dopamine and serotonin activity throughout acquisition and expression of place conditioning: correlative relationships with preference and aversion

The ability of addictive drugs to induce adaptations in mesolimbic dopamine (DA) activity offers an attractive neurobiological explanation for enhanced incentive motivation toward drug-associated stimuli in addiction. However, direct evidence supporting this is sparse. By tracking neurochemical activity within the mouse nucleus accumbens via microdialysis during repeated pairing of morphine with environmental stimuli, we reveal a predictive relationship between enhanced DA responses to morphine and subsequent preference towards a morphine-paired stimulus. A similar relationship for serotonin (5-HT) was observed, suggesting that these neuromodulatory systems work in concert. During expression of preference towards a morphine-paired stimulus, extracellular DA was not enhanced but was negatively associated with this behavior on a subject-by-subject basis. In contrast, avoidance of an aversively-paired stimulus (the opiate antagonist naloxone) was associated with enhanced extracellular DA levels, and also the balance between DA and 5-HT responses. These findings reveal a tangible predictive relationship between drug-induced neural adaptations and conditioned behavior, and emphasize that DA activity is not generalized to all subcomponents of behavior conditioned by addictive drugs. They further provide evidence for an active role of DA-5-HT interactions in the expression of learned behavior.

Induction of brain CYP2E1 changes the effects of ethanol on dopamine release in nucleus accumbens shell

CYP2E1 is an important enzyme involved in the brain metabolism of ethanol that can be induced by chronic consumption of alcohol. Recent works have highlighted the importance of this system in the context of the behavioural effects of ethanol. Unfortunately, the underlying neurochemical events for these behavioural changes, has not been yet explored. In this work, we have started this exploration by analyzing the possible changes in the neurochemical response of the mesolimbic system to ethanol after pharmacological induction of brain CYP2E1. We have used the dopamine extracellular levels in nucleus accumbens (NAc) core and shell, measured by means of microdialysis in vivo, as an index of the effects of ethanol. Acetone 1% in the tap water was used to induce brain CYP2E1. Efficacy of the induction protocol was assessed by immunoblotting. Intravenous administration of 1.5 g/kg of ethanol in control rats provoked a significant increase of the dopamine levels in both the core (up to 127% of baseline) and the shell (up to 122% of baseline) of the NAc. However, the same dose of ethanol in acetone-treated rats only increased the dopamine extracellular levels in the core (up to 142% of baseline) whereas dopamine levels in the shell subregion remain unaltered relative to baseline. The results of this study indicate that induction of CYP2E1 changes the response of the mesolimbic system to ethanol in a region-dependent manner. Two hypotheses are postulated to explain the observed effects.

A data mining approach to in vivo classification of psychopharmacological drugs

Data mining is a powerful bioinformatics strategy that has been successfully applied in vitro to screen for gene-expression profiles predicting toxicological or carcinogenic response ('class predictors'). In this report we used a data mining algorithm named Pattern Array (PA) in vivo to analyze mouse open-field behavior and characterize the psychopharmacological effects of three drug classes--psychomotor stimulant, opioid, and psychotomimetic. PA represents rodent movement with approximately 100,000 complex patterns, defined as multiple combinations of several ethologically relevant variables, and mines them for those that maximize any effect of interest, such as the difference between drug classes. We show that PA can discover behavioral predictors of all three drug classes, thus developing a reliable drug-classification scheme in small group sizes. The discovered predictors showed orderly dose dependency despite being explicitly mined only for class differences, with the high doses scoring 4-10 standard deviations from the vehicle group. Furthermore, these predictors correctly classified in a dose-dependent manner four 'unknown' drugs (ie that were not used in the training process), and scored a mixture of a psychomotor stimulant and an opioid as being intermediate between these two classes. The isolated behaviors were highly heritable (h(2)>50%) and replicable as determined in 10 inbred strains across three laboratories. PA can in principle be applied for mining behaviors predicting additional properties, such as within-class differences between drugs and within-drug dose-response, all of which can be measured automatically in a single session per animal in an open-field arena, suggesting a high potential as a tool in psychotherapeutic drug discovery.

Modelling human drug abuse and addiction with dedicated small animal positron emission tomography

Drug addiction is a chronically relapsing brain disorder, which causes substantial harm to the addicted individual and society as a whole. Despite considerable research we still do not understand why some people appear particularly disposed to drug abuse and addiction, nor do we understand how frequently co-morbid brain disorders such as depression and attention-deficit hyperactivity disorder (ADHD) contribute causally to the emergence of addiction-like behaviour. In recent years positron emission tomography (PET) has come of age as a translational neuroimaging technique in the study of drug addiction, ADHD and other psychopathological states in humans. PET provides unparalleled quantitative assessment of the spatial distribution of radiolabelled molecules in the brain and because it is non-invasive permits longitudinal assessment of physiological parameters such as binding potential in the same subject over extended periods of time. However, whilst there are a burgeoning number of human PET experiments in ADHD and drug addiction there is presently a paucity of PET imaging studies in animals despite enormous advances in our understanding of the neurobiology of these disorders based on sophisticated animal models. This article highlights recent examples of successful cross-species convergence of findings from PET studies in the context of drug addiction and ADHD and identifies how small animal PET can more effectively be used to model complex psychiatric disorders involving at their core impaired behavioural self-control.

The shell of the nucleus accumbens has a higher dopamine response compared with the core after non-contingent intravenous ethanol administration

Dopamine increases in the nucleus accumbens after ethanol administration in rats, but the contributions of the core and shell subregions to this response are unclear. The goal of this study was to determine the effect of various doses of i.v. ethanol infusions on dopamine in these two subregions of the nucleus accumbens. Male Long-Evans rats were infused with either acute i.v. ethanol (0.5, 1.0, 1.5 g/kg), repeated i.v. ethanol (four 1.0 g/kg infusions resulting in a cumulative dose of 4.0 g/kg), or saline as a control for each condition. Dopamine and ethanol were measured in dialysate samples from each experiment. The in vivo extraction fraction for ethanol of probes was determined using i.v. 4-methylpyrazole, and was used to estimate peak brain ethanol concentrations after the infusions. The peak brain ethanol concentrations after the 0.5, 1.0 and 1.5 g/kg ethanol infusions were estimated to be 20, 49 and 57 mM, respectively. A significant dopamine increase was observed for the 0.5 g/kg ethanol group when collapsed across subregions. However, both the 1.0 g/kg and 1.5 g/kg ethanol infusions produced significant increases in dopamine levels in the shell that were significantly higher than those in the core. An ethanol dose-response effect on dopamine in the shell was observed when saline controls, 0.5, 1.0, and 1.5 g/kg groups were compared. For the cumulative-dosing study, the first, second, and fourth infusions resulted in significant increases in dopamine in the shell. However, these responses were not significantly different from one another. The results of this study show that the shell has a stronger response than the core to i.v. ethanol, that dopamine in the shell increases in a dose-dependent manner between 0.5-1.0 g/kg doses, but that the response to higher ethanol doses reaches a plateau.

Morphine enhances purine nucleotide catabolism in vivo and in vitro

AIM

To investigate the effect and mechanism of morphine on purine nucleotide catabolism.

METHODS

The rat model of morphine dependence and withdrawal and rat C6 glioma cells in culture were used. Concentrations of uric acid in the plasma were measured by the uricase-rap method, adenosine deaminase (ADA) and xanthine oxidase (XO) in the plasma and tissues were measured by the ADA and XO test kit. RT-PCR and RT-PCR-Southern blotting were used to examine the relative amount of ADA and XO gene transcripts in tissues and C6 cells.

RESULTS

(i) the concentration of plasma uric acid in the morphine-administered group was significantly higher (P<0.05) than the control group; (ii) during morphine administration and withdrawal periods, the ADA and XO concentrations in the plasma increased significantly (P<0.05); (iii) the amount of ADA and XO in the parietal lobe, liver, small intestine, and skeletal muscles of the morphine-administered groups increased, while the level of ADA and XO in those tissues of the withdrawal groups decreased; (iv) the transcripts of the ADA and XO genes in the parietal lobe, liver, small intestine, and skeletal muscles were higher in the morphine-administered group. The expression of the ADA and XO genes in those tissues returned to the control level during morphine withdrawal, with the exception of the skeletal muscles; and (v) the upregulation of the expression of the ADA and XO genes induced by morphine treatment could be reversed by naloxone.

CONCLUSION

The effects of morphine on purine nucleotide metabolism might be an important, new biochemical pharmacological mechanism of morphine action.

Amphetamine effects in microtine rodents: a comparative study using monogamous and promiscuous vole species

We compared amphetamine-induced dopamine release in the nucleus accumbens of vole species that exhibit differing mating systems to examine potential interactions between social organization and substance abuse. We found no species or regional differences in basal extracellular dopamine, however, monogamous voles had greater and longer-lasting increases in extracellular dopamine after amphetamine treatment than did promiscuous voles. We then examined whether amphetamine-induced increase in extracellular dopamine could induce pair bonds in monogamous voles. We found that, despite increasing dopamine in the nucleus accumbens, amphetamine administration did not induce pair-bonds in male prairie voles unless the animals were pretreated to preclude D1 receptor activation, which is known to inhibit pair-bond formation. These results support suggestions that social attachment and substance abuse share a common neural substrate.

There and back again: a tale of norepinephrine and drug addiction

Fueled by anatomical, electrophysiological, and pharmacological analyses of endogenous brain reward systems, norepinephrine (NE) was identified as a key mediator of both natural and drug-induced reward in the late 1960s and early 1970s. However, reward experiments from the mid-1970s that could distinguish between the noradrenergic and dopaminergic systems resulted in the prevailing view that dopamine (DA) was the primary 'reward transmitter' (a belief holding some sway still today), thereby pushing NE into the background. Most damaging to the NE hypothesis of reward were studies demonstrating that NE receptor antagonists and NE reuptake inhibitors failed to impact drug self-administration. In recent years new tools, such as genetically engineered mice, and new experimental paradigms, such as reinstatement of drug seeking following withdrawal, have propelled NE back into the awareness of addiction researchers. Of particular interest is disulfiram, an inhibitor of the NE biosynthetic enzyme dopamine beta-hydroxylase, which has demonstrated promising efficacy in the treatment of cocaine dependence in preliminary clinical trials. The purpose of this review is to synthesize the new data linking NE to critical aspects of DA signaling and drug addiction, with a focus on psychostimulants (eg, cocaine), opiates (eg, morphine), and alcohol.

Catecholamine mapping within nucleus accumbens: differences in basal and amphetamine-stimulated efflux of norepinephrine and dopamine in shell and core

The nucleus accumbens is believed to play a critical role in mediating the behavioral responses to rewarding stimuli. Although most studies of the accumbens focus on dopamine, it receives afferents from many other nuclei, including noradrenergic cell groups in the brainstem. We used in vivo microdialysis to measure extracellular levels of both norepinephrine and dopamine in the accumbens shell and core. Regional analysis of shell and core and border regions demonstrated that norepinephrine was high in shell and decreased from medial shell to lateral core, where baseline levels were low or undetectable. Conversely, extracellular dopamine in core was twice the level seen in shell. Both catecholamines increased following a single injection of amphetamine (2 mg/kg, i.p.). The norepinephrine response was greater and long-lasting in shell compared with core. The maximal dopamine response was higher in core than in shell, but the duration of the effect was comparable in both regions. The distinct neurochemical characteristics of shell and core are likely to contribute to the functional heterogeneity of the two subregions. Furthermore, norepinephrine may be involved in many of the functions generally attributed to the accumbens, either directly or indirectly via modulation of extracellular dopamine.

Selective dopaminergic lesions of the ventral tegmental area impair preference for sucrose but not for male sexual pheromones in female mice

The role of the meso-accumbens dopaminergic pathway in reward-related behaviours is the subject of intense investigation. In this regard, here we analyse the effects of specific lesions of dopaminergic cells of the ventral tegmental area (VTA) of female mice on two goal-directed behaviours, namely sucrose preference (intake of sucrose solution vs. water) and preference for male sexual pheromones (exploration of male-soiled vs. clean bedding). The results indicate that partial lesions of the VTA that impair neither locomotion nor general exploratory behaviour reduce the preference for sucrose (over a 48-h period) but do not alter the innate attraction that females display for male sexual pheromones (in 5-min tests). This differential effect of the lesions can be interpreted as demonstrating the existence of separate neural mechanisms and circuits for signalling the reward of different natural reinforcers (e.g. sweet taste of sucrose and sexual pheromones). Alternatively, VTA lesions may result in an impaired attribution of incentive salience (which depends on the dopaminergic tegmento-striatal system) of sucrose-predicting cues, thus leading to a long-term decrease in sucrose consumption. By contrast, the same lesions do not affect the unconditioned attraction to male-derived pheromones, which may depend on amygdalo-striatal pathways.

Attenuation of nicotine-induced rewarding effects in A2A knockout mice

The non-selective A2A antagonist caffeine has been reported to modify nicotine-induced locomotor and reinforcing effects. In the present study, we have investigated the specific role of A2A adenosine receptors in the behavioural responses induced by nicotine by using genetically modified mice lacking A2A adenosine receptors. Acute nicotine administration induced a similar decrease of locomotor activity in A2A knockout mice and wild-type littermates. Acute antinociceptive responses elicited by nicotine in the tail-immersion and hot-plate tests were unaffected in these mutant mice. The rewarding properties of nicotine were then investigated using the place-conditioning paradigm. Nicotine-induced conditioned place preference was suppressed in A2A knockout mice. Accordingly, in vivo microdialysis studies revealed that the extracellular levels of dopamine in the nucleus accumbens were not increased after nicotine administration in mutant mice. Wild-type and A2A knockout mice were trained in conditioned taste aversion procedure in which drinking a saccharin or saline solution was paired with nicotine or saline injections. A similar reduction in the intake of nicotine-paired solution in this paradigm was obtained in both genotypes. Finally, the administration of the nicotinic antagonist mecamylamine in nicotine-dependent mice precipitated a similar withdrawal syndrome in both genotypes. Together, the present results identify A2A adenosine receptors as an important factor that contributes to the rewarding properties of nicotine.

Preexposure to nicotine alters the subsequent locomotor stimulant effects of bupropion in rats

Little is known about the interaction between nicotine and bupropion (Zyban), but many studies suggest they have neurological and behavioral similarities. One feature of drugs with common profiles is the ability to cross-sensitize possibly through neurological changes in the reward pathway. Activation of this pathway might explain the effectiveness of bupropion as a smoking cessation aid. The present research investigated whether repeated nicotine administration altered the subsequent locomotor effects of bupropion. In experiment 1, rats were preexposed to nicotine (0.4 mg/kg subcutaneously) or saline on eight separate occasions in the home cage and then tested with bupropion (0, 20, or 30 mg/kg) in locomotor chambers. The acute stimulant effect of 30 mg/kg of bupropion was potentiated by nicotine preexposure. In experiment 2, rats received nicotine repeatedly paired with the locomotor chambers or home cages. An additive effect was observed between acute bupropion and nicotine-conditioned hyperactivity in the chamber-paired group. This enhancement of the acute locomotor effects of bupropion might reflect alterations in common dopaminergic processes.

The decreased cyclic-AMP dependent-protein kinase A function in the nucleus accumbens: a role in alcohol drinking but not in anxiety-like behaviors in rats

The nucleus accumbens (NAc) brain structures have been implicated in the reward and reinforcing properties of ethanol. The present study investigated the role of nucleus accumbal cyclic AMP (cAMP)-dependent protein kinase A (PKA) signaling in alcohol drinking and anxiety-like behaviors of rats. It was found that infusion of PKA inhibitor (Rp-cAMP) into the NAc shell significantly increased the alcohol but not the sucrose intake, without modulating the anxiety-like behaviors, as measured by elevated plus maze test in rats. PKA inhibitor infusion into the NAc shell significantly decreased the protein levels of alpha-catalytic subunit of PKA (PKA-Calpha) and phosphorylated cAMP response element-binding protein (p-CREB) as well as decreased the protein levels of neuropeptide Y (NPY) in the shell but not in the NAc core of rats. On the other hand, infusion of PKA activator (Sp-cAMP) or NPY alone into the NAc shell did not produce any changes in alcohol intake; however, when these agents were coinfused with PKA inhibitor, they significantly attenuated the increases in alcohol preference induced by pharmacological inhibition of PKA. Interestingly, PKA activator coinfusion with PKA inhibitor into the NAc shell significantly normalized the PKA inhibitor-induced decreases in the protein levels of PKA-Calpha and p-CREB as well as of NPY in the NAc shell of rats. Taken together, these results provide the first evidence that decreased PKA function in the NAc shell is involved in alcohol drinking but not in anxiety-like behaviors of rats. Furthermore, decreased function of PKA may regulate alcohol drinking behaviors via CREB-mediated decreased expression of NPY in the NAc shell of rats.

DRD2 Gene Transfer Into the Nucleus Accumbens Core of the Alcohol Preferring and Nonpreferring Rats Attenuates Alcohol Drinking

BACKGROUND

Transient overexpression of the dopamine D2 receptor (DRD2) gene in the nucleus accumbens (NAc) using an adenoviral vector has been associated with a significant decrease in alcohol intake in Sprague Dawley rats. This overexpression of DRD2 reduced alcohol consumption in a two-bottle-choice paradigm and supported the view that high levels of DRD2 may be protective against alcohol abuse.

METHODS

Using a limited access (1 hr) two-bottle-choice (water versus 10% ethanol) drinking paradigm, we examined the effects of the DRD2 vector in alcohol intake in the genetically inbred alcohol-preferring (P) and -nonpreferring (NP) rats. In addition, micro-positron emission tomography imaging was used at the completion of the study to assess in vivo the chronic (7 weeks) effects of ethanol exposure on DRD2 levels between the two groups.

RESULTS

P rats that were treated with the DRD2 vector (in the NAc) significantly attenuated their alcohol preference (37% decrease) and intake (48% decrease), and these measures returned to pretreatment levels by day 20. A similar pattern of behavior (attenuation of ethanol drinking) was observed in NP rats. Analysis of the [C]raclopride micro-positron emission tomography data after chronic (7 weeks) exposure to ethanol revealed clear DRD2 binding differences between the P and NP rats. P rats showed 16% lower [C]raclopride specific binding in striatum than the NP rats.

CONCLUSIONS

These findings further support our hypothesis that high levels of DRD2 are causally associated with a reduction in alcohol consumption and may serve as a protective factor against alcoholism. That this effect was seen in P rats, which are predisposed to alcohol intake, suggests that they are protective even in those who are genetically predisposed to high alcohol intake. It is noteworthy that increasing DRD2 significantly decreased alcohol intake but did not abolish it, suggesting that high DRD2 levels may specifically interfere with the administration of large quantities of alcohol. The significantly higher DRD2 concentration in NP than P rats after 7 weeks of ethanol therefore could account for low alcohol intake.

Long-term effects of a single adult methamphetamine challenge: minor impact on dopamine fibre density in limbic brain areas of gerbils

BACKGROUND

The aim of the study was to test long-term effects of (+)-methamphetamine (MA) on the dopamine (DA) innervation in limbo-cortical regions of adult gerbils, in order to understand better the repair and neuroplasticity in disturbed limbic networks.

METHODS

Male gerbils received a single high dose of either MA (25 mg/kg i.p.) or saline on postnatal day 180. On postnatal day 340 the density of immunoreactive DA fibres and calbindin and parvalbumin cells was quantified in the right hemisphere.

RESULTS

No effects were found in the prefrontal cortex, olfactory tubercle and amygdala, whereas the pharmacological impact induced a slight but significant DA hyperinnervation in the nucleus accumbens. The cell densities of calbindin (CB) and parvalbumin (PV) positive neurons were additionally tested in the nucleus accumbens, but no significant effects were found. The present results contrast with the previously published long-term effects of early postnatal MA treatment that lead to a restraint of the maturation of DA fibres in the nucleus accumbens and prefrontal cortex and a concomitant overshoot innervation in the amygdala.

CONCLUSION

We conclude that the morphogenetic properties of MA change during maturation and aging of gerbils, which may be due to physiological alterations of maturing vs. mature DA neurons innervating subcortical and cortical limbic areas. Our findings, together with results from other long-term studies, suggest that immature limbic structures are more vulnerable to persistent effects of a single MA intoxication; this might be relevant for the assessment of drug experience in adults vs. adolescents, and drug prevention programs.

Dopamine D3 receptor modulation of dopamine efflux in the rat nucleus accumbens

The effect of antipsychotics on electrically evoked dopamine efflux in the rat nucleus accumbens core and shell was investigated, using in vitro fast cyclic voltammetry. In the nucleus accumbens core, the dopamine D2/D3 receptor agonist, (+/-)7-OH-DPAT ((+/-)-2-dipropylamino-7-hydroxy-1,2,3,4-tetrahydronaphthalene), inhibited dopamine efflux with a pEC50 of 8.1. Clozapine, haloperidol, sulpiride and the selective dopamine D3 receptor antagonist, SB-277011-A, had no effect on dopamine efflux per se but all attenuated the (+/-)7-OH-DPAT-induced-inhibition of dopamine efflux, with pA2 values of 6.6, 7.9, 7.0 and 7.6, respectively. In the nucleus accumbens shell, (+/-)7-OH-DPAT inhibited dopamine efflux with a pEC50 of 8.3. Clozapine and SB-277011-A had no effect on dopamine efflux. In contrast, haloperidol and sulpiride significantly increased dopamine efflux through a D2 receptor-mediated mechanism. Clozapine, haloperidol, sulpiride and SB-277011-A attenuated the (+/-)7-OH-DPAT-induced inhibition with pA2 values of 7.3, 8.6, 7.6 and 8.2, respectively. These data demonstrate that dopamine efflux is modulated by both dopamine D2 and D3 receptors in the rat nucleus accumbens.

Effects of environmental and pharmacological stressors on c-fos and corticotropin-releasing factor mRNA in rat brain: Relationship to the reinstatement of alcohol seeking

We have observed marked heterogeneity among different stressors in their ability to reinstate alcohol seeking in rats. Of the stressors we have tested, only the environmental stressor footshock and the pharmacological stressor yohimbine induce reinstatement. The reasons for such differences among stressors are not known. The purpose of the experiments presented here is to determine the neuroanatomical substrates that underlie these behavioral differences. To this end, we assessed whether stressors effective in inducing reinstatement of alcohol seeking activate a different set of neuronal pathways than do those that are ineffective, using the technique of in situ hybridization of the mRNAs for c-fos, a marker of neuronal activation, and corticotropin-releasing factor (CRF), a stress-related peptide we have shown to be critical to footshock-induced reinstatement of alcohol seeking. Exposure of rats to the environmental stressors footshock, restraint or social defeat, or the pharmacological stressors yohimbine or FG-7142 increased levels of the mRNAs for c-fos and CRF in the brain in a number of areas previously shown to be responsive to stressors. We found regionally specific effects of the stressors on c-fos and CRF mRNA in brain regions associated with the rewarding effects of alcohol and other abused drugs. The two stressors we have previously shown to be effective in inducing reinstatement of alcohol seeking, footshock and yohimbine, induced c-fos mRNA in the shell of the nucleus accumbens, and the basolateral and central amygdalar nuclei. These two stressors also induced CRF mRNA in the dorsal region of the bed nucleus of the stria terminalis. Taken together, these results provide evidence that activity in these regions may be involved in the reinstatement of alcohol seeking induced by these stressors. These results are also in keeping with the previously demonstrated role of CRF neurons in the dorsal bed nucleus of the stria terminalis in the reinstatement of alcohol seeking induced by stress.

Local effects of acute ethanol on dopamine neurotransmission in the ventral striatum in C57BL/6 mice

In this study, fast-scan cyclic voltammetry in brain slices was used to evaluate the effects of acute ethanol on dopamine terminal release and uptake in the nucleus accumbens of C57BL/6 mice. We found that pharmacologically relevant concentrations of ethanol (20 and 100 mM) did not alter electrically evoked dopamine release, while the highest concentration (200 mM) significantly decreased release (approximately 45%). No significant changes were observed in the rate of dopamine uptake after ethanol (20, 100 or 200 mM). In addition, it was established that a moderate dose (2 g/kg, i.p.) of ethanol did not alter the rate of dopamine synthesis, measured as L-dihydroxyphenylalanine (L-DOPA) accumulation. However, a high dose (5 g/kg, i.p.) of ethanol significantly increased the levels of L-DOPA to 60% above the control value. These data are consistent with earlier findings obtained in brain slices from rats; dopamine release, but not clearance, is affected by acute ethanol.

Nicotine-induced antinociception, rewarding effects, and physical dependence are decreased in mice lacking the preproenkephalin gene

It has been shown previously that the endogenous opioid system may be involved in the behavioral effects of nicotine. In the present study, the participation of endogenous enkephalins on nicotine responses has been investigated by using preproenkephalin knock-out mice. Acute nicotine-induced hypolocomotion remained unaffected in these mice. In contrast, antinociception elicited in the tail-immersion and hot-plate tests by acute nicotine administration was reduced in mutant animals. The rewarding properties of nicotine were then investigated using the place-conditioning paradigm. Nicotine induced a conditioned place preference in wild-type animals, but this effect was absent in knock-out mice. Accordingly, in vivo microdialysis studies revealed that the enhancement in dopamine extracellular levels in the nucleus accumbens induced by nicotine was also reduced in preproenkephalin-deficient mice. Finally, the somatic expression of the nicotine withdrawal syndrome precipitated in nicotine-dependent mice by mecamylamine was significantly attenuated in mutant animals. In summary, the present results indicate that endogenous opioid peptides derived from preproenkephalin are involved in the antinociceptive and rewarding properties of nicotine and participate in the expression of physical nicotine dependence.

Mesolimbic dopamine super-sensitivity in melanin-concentrating hormone-1 receptor-deficient mice

Melanin-concentrating hormone (MCH) neurons and MCH-1 receptors (MCH1r) densely populate mesolimbic dopaminergic brain regions such as the nucleus accumbens (NAc). The regulation of dopamine by MCH1r was suggested to be an important mechanism underlying the hyperactive phenotype of MCH1r knock-out (ko) mice. However, MCH1r modulation of monoamine neurotransmission has yet to be examined. We tested whether dopamine, norepinephrine, and serotonin function is dysregulated in MCH1r ko and wild-type (wt) mice. MCH1r ko mice exhibited robust hyperactivity in a novel or familiar environment and were super-sensitive to the locomotor activating effects of d-amphetamine and the D1 agonist 2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benazepine HCl. The D2 agonist, quinpirole, decreased locomotion similarly in both ko and wt mice. Tissue contents of dopamine within the NAc and caudate-putamen were not significantly different in ko compared with wt mice. Basal and amphetamine-evoked NAc dopamine, norepinephrine, and serotonin efflux, as measured using in vivo microdialysis, were not significantly different between genotypes. In contrast, D1-like and D2-like receptor binding were significantly higher within the olfactory tubercle, ventral tegmental area, and NAc core and shell of ko mice. Norepinephrine transporter (NET) binding was significantly elevated within the NAc shell and globus pallidus of ko mice, whereas serotonin transporter binding was decreased in the NAc shell. Thus, deletion of MCH1r results in an upregulation of mesolimbic dopamine receptors and NET, indicating that MCH1r may negatively modulate mesolimbic monoamine function. MCH1r may be an important therapeutic target for neuropsychiatric disorders involving dysregulation of limbic monoamine systems.

The mesolimbic dopamine system: the final common pathway for the reinforcing effect of drugs of abuse?

In this review we will critically assess the hypothesis that the reinforcing effect of virtually all drugs of abuse is primarily dependent on activation of the mesolimbic dopamine system. The focus is on five classes of abused drugs: psychostimulants, opiates, ethanol, cannabinoids and nicotine. For each of these drug classes, the pharmacological and physiological mechanisms underlying the direct or indirect influence on mesolimbic dopamine transmission will be reviewed. Next, we evaluate behavioral pharmacological experiments that specifically assess the influence of activation of the mesolimbic dopamine system on drug reinforcement, with particular emphasis on animal experiments using drug self-administration paradigms. There is overwhelming evidence that all five classes of abused drugs increase dopamine transmission in limbic regions of the brain through interactions with a variety of transporters, ionotropic receptors and metabotropic receptors. Behavioral pharmacological experiments indicate that increased dopamine transmission is clearly both necessary and sufficient to promote psychostimulant reinforcement. For the other four classes of abused substances, self-administration experiments suggest that although increasing mesolimbic dopamine transmission plays an important role in the reinforcing effects of opiates, ethanol, cannabinoids and nicotine, there are also dopamine-independent processes that contribute significantly to the reinforcing effects of these compounds.

The role of mesolimbic dopamine in the development and maintenance of ethanol reinforcement

The neurobiological processes by which ethanol seeking and consumption are established and maintained are thought to involve areas of the brain that mediate motivated behavior, such as the mesolimbic dopamine system. The mesolimbic dopamine system is comprised of cells that originate in the ventral tegmental area (VTA) and project to several forebrain regions, including a prominent terminal area, the nucleus accumbens (NAcc). The NAcc has been subdivided into core and shell subregions. Both areas receive converging excitatory input from the cortex and amygdala and dopamine input from the VTA, with the accumbal medium spiny neuron situated to integrate the signals. Although forced ethanol administration enhances dopamine activity in the NAcc, conclusions regarding the role of mesolimbic dopamine in ethanol reinforcement cannot be made from these experiments. Behavioral experiments consistently show that pharmacological manipulations of the dopamine transmission in the NAcc alter responding for ethanol, although ethanol reinforcement is maintained after lesions of the accumbal dopamine system. Additionally, extracellular dopamine increases in the NAcc during operant self-administration of ethanol, which is consistent with a role of dopamine in ethanol reinforcement. Behavioral studies that distinguish appetitive responding from ethanol consumption show that dopamine is important in ethanol-seeking behavior, whereas neurochemical studies suggest that accumbal dopamine is also important during ethanol consumption before pharmacological effects occur. Cellular studies suggest that ethanol alters synaptic plasticity in the mesolimbic system, possibly through dopaminergic mechanisms, and this may underlie the development of ethanol reinforcement. Thus, anatomical, pharmacological, neurochemical, cellular, and behavioral studies are more clearly defining the role of mesolimbic dopamine in ethanol reinforcement.

Nicotine-induced changes in neurotransmitter levels in brain areas associated with cognitive function

Nicotine, one of the most widespread drugs of abuse, has long been shown to impact areas of the brain involved in addiction and reward. Recent research, however, has begun to explore the positive effects that nicotine may have on learning and memory. The mechanisms by which nicotine interacts with areas of cognitive function are relatively unknown. Therefore, this paper is part of an ongoing study to evaluate regional effects of nicotine enhancement of cognitive function. Nicotine-induced changes in the levels of three neurotransmitters, dopamine (DA), serotonin (5-HT), norepinepherine (NE), their metabolites, homovanillic acid (HVA), dihydroxyphenylacetic acid (DOPAC), 5-hydroxyindoleacetic acid (5-HIAA), and their precursor, L-DOPA, were evaluated in the ventral and dorsal hippocampus (VH and DH), prefrontal and medial temporal cortex (PFC and MTC), and the ventral tegmental area (VTA) using in vivo microdialysis in awake, freely moving, male Sprague-Dawley rats. The animals were treated with acute nicotine (0.5 mg/kg, s.c.) halfway through the 300-min experimental period. The reuptake blockers, desipramine (100 microM) and fluoxetine (30 microM), were given to increase the levels of NE and 5-HT so that they could be detected. Overall, a nicotine-induced DA increase was found in some areas, and this increase was potentiated by desipramine and fluoxetine. The two DA metabolites, HVA and DOPAC, increased in all the areas throughout the experiments, both with and without the inhibitors, indicating a rapid metabolism of the released DA. The increase in these metabolites was greater than the increase in DA. 5-HT was increased in the DH, MTC, and VTA in the presence of fluoxetine; its metabolite, 5-HIAA, was increased in the presence and absence of fluoxetine. Except in the VTA, NE levels increased to a similar extent with desipramine and fluoxetine. Overall, nicotine appeared to increase the release and turnover of these three neurotransmitters, which was indicated by significant increases in their metabolites. Furthermore, DA, and especially HVA and DOPAC, increased for the 150 min following nicotine administration; 5-HT and NE changes were shorter in duration. As gas chromatography experiments showed that nicotine levels in the brain decreased by 75% after 150 min, this may indicate that DA is more susceptible to lower levels of nicotine than 5-HT or NE. In conclusion, acute nicotine administration caused alterations in the levels of DA, 5-HT, and NE, and in the metabolism of DA and 5-HT, in brain areas that are involved in cognitive processes.

Endogenous opiates and behavior: 2003

This paper is the 26th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2003 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

The gene transcription factor cyclic AMP-responsive element binding protein: role in positive and negative affective states of alcohol addiction

The gene transcription factor cyclic adenosine monophosphate (cAMP)-responsive element binding (CREB) protein is a nuclear protein that regulates synaptic plasticity via modulating the expression of several (cAMP)-inducible genes. Alcohol addiction is a complex psychiatric disorder and is characterized by a compulsive and uncontrolled pattern of alcohol drinking by an individual in spite of the adverse consequences of its abuse. Ethanol produces both euphoric (reward and reinforcing) and dysphoric (negative withdrawal reactions) effects and these are most likely involved in the initiation and maintenance of alcohol use and abuse. Several neurotransmitter systems in the brain might be involved in the effects of alcohol but the exact molecular mechanisms of both the positive and negative affective states of alcohol abuse are still unclear. Recent research in molecular neurosciences using animal models have identified the role of extended amygdaloid (shell structures of nucleus accumbens [NAc] and central and medial amygdaloid nuclei) CREB signaling in positive and negative affective states of alcohol drinking behaviors. This review article highlights the current findings on the role of nucleus accumbal and amygdaloid CREB signaling in behavioral consequences of alcohol use and abuse.

Sugar-dependent rats show enhanced intake of unsweetened ethanol

Rats show signs of dependence on sugar when it is available intermittently, including bingeing, withdrawal, and cross-sensitization with amphetamine. In the current study, we sought to determine whether sugar-dependent rats would show increased intake of unsweetened ethanol and, conversely, whether intermittent access to ethanol would augment sugar consumption. In Experiment 1, with intermittent versus ad libitum access to ethanol, Sprague-Dawley rats were given escalating concentrations of ethanol (1%, 2%, 4%, 7%, and 9%) over the course of 20 days. Rats in the intermittent ethanol access group, with 12-h daily access, consumed more 4%, 7%, and 9% ethanol during the first hour of access, and more 9% ethanol daily, than did rats in the ad libitum ethanol access group. In Experiment 2, with ethanol as a gateway to sugar intake, the rats from Experiment 1 were switched to 10% sucrose with 12-h daily access for 1 week. Rats in the intermittent ethanol access group consumed significantly more sugar than was consumed by rats in a control group with no prior ethanol experience. In Experiment 3, with sugar as a gateway to ethanol to determine whether sugar dependence leads to increased ethanol intake, four groups were maintained for 21 days according to the following designations: intermittent access to sugar and chow, ad libitum access to sugar and chow, intermittent access to chow, or ad libitum access to chow. Four days later, all groups were switched to intermittent ethanol access, as described in Experiment 1. The group with intermittent access to sugar and chow consumed the most 9% ethanol, supporting the suggestion that sugar dependence alters a rat's proclivity to drink ethanol. These results may relate to the co-morbidity between binge-eating disorders and alcohol intake and the tendency of people abstaining from alcohol to consume excessive amounts of sugar. In conclusion, bingeing on either ethanol or sugar fosters intake of the other.