Molecular neurobiology of drug addiction

Neuroadaptations of Cdk5 in cholinergic interneurons of the nucleus accumbens and prefrontal cortex of inbred alcohol-preferring rats following voluntary alcohol drinking

BACKGROUND

Neurobiological studies have identified brain areas and related molecular mechanisms involved in alcohol abuse and dependence. Specific cell types in these brain areas and their role in alcohol-related behaviors, however, have not yet been identified. This study examined the involvement of cholinergic cells in inbred alcohol-preferring rats following 1 month of alcohol drinking. Cyclin-dependent kinase 5 (Cdk5) immunoreactivity (IR), a marker of neuronal plasticity, was examined in cholinergic neurons of the nucleus accumbens (NuAcc) and prefrontal cortex (PFC) and other brain areas implicated in alcohol drinking, using dual immunocytochemical (ICC) procedures. Single Cdk5 IR was also examined in several brain areas implicated in alcohol drinking.

METHODS

The experimental group self-administered alcohol using a 2-bottle-choice test paradigm with unlimited access to 10% (v/v) alcohol and water for 23 h/d for 1 month. An average of 6 g/kg alcohol was consumed daily. Control animals received identical treatment, except that both bottles contained water. Rats were perfused and brain sections were processed for ICC procedures.

RESULTS

Alcohol drinking resulted in a 51% increase in Cdk5 IR cholinergic interneurons in the shell NuAcc, while in the PFC there was a 51% decrease in the percent of Cdk5 IR cholinergic interneurons in the infralimbic region and a 46% decrease in Cdk5 IR cholinergic interneurons in the prelimbic region. Additionally, single Cdk5 IR revealed a 42% increase in the central nucleus of the amygdala (CNA).

CONCLUSIONS

This study identified Cdk5 neuroadaptation in cholinergic interneurons of the NuAcc and PFC and in other neurons of the CNA following 1 month of alcohol drinking. These findings contribute to our understanding of the cellular and molecular basis of alcohol drinking and toward the development of improved region and cell-specific pharmacotherapeutic and behavioral treatment programs for alcohol abuse and alcoholism.

Neural mechanisms of addiction: the role of reward-related learning and memory

Addiction is a state of compulsive drug use; despite treatment and other attempts to control drug taking, addiction tends to persist. Clinical and laboratory observations have converged on the hypothesis that addiction represents the pathological usurpation of neural processes that normally serve reward-related learning. The major substrates of persistent compulsive drug use are hypothesized to be molecular and cellular mechanisms that underlie long-term associative memories in several forebrain circuits (involving the ventral and dorsal striatum and prefrontal cortex) that receive input from midbrain dopamine neurons. Here we review progress in identifying candidate mechanisms of addiction.

Calmodulin-Stimulated Adenylyl Cyclase Gene Deletion Affects Morphine Responses

To define the roles of the calmodulin-stimulated adenylyl cyclases (AC1 and AC8) in morphine-induced analgesia, tolerance, physical dependence, and conditioned place preference, we used mice having targeted disruptions of either the AC1 or AC8 genes or both genes [double knockout mice (DKO)]. Mice lacking either AC1 or AC8 genes or DKO did not differ from wild-type mice in short-term antinociceptive responses to morphine measured in the tail-flick analgesia assay. Morphine tolerance that developed immediately within 3 h of morphine administration (10 mg/kg s.c.) was significantly attenuated in DKO mice and AC8 single knockout mice. Tolerance induced continually by daily injections of morphine (10 mg/kg s.c.) was also reduced in DKO mice. In DKO mice continually treated with morphine, there was a significant reduction in withdrawal behaviors, including reduced wet-dog shakes and forepaw tremor after naloxone injection (10 mg/kg i.p.). Morphine produced hyperlocomotion and conditioned place preference in wild-type mice, whereas DKO mice displayed significantly less hyperlocomotion and conditioned place preference. Furthermore, the significant increase in phosphorylated cAMP-response element binding protein (CREB) staining in ventral tegmental area induced by long-term morphine treatment was not evident in DKO mice, suggesting that CREB activation by morphine requires cAMP generated by AC1 and AC8. These results support the hypothesis that calmodulin-stimulated adenylyl cyclases are important mediators of the neuronal responses to morphine.

Transcriptional control of midbrain dopaminergic neuron development

Although loss of midbrain dopaminergic neurons is associated with one of the most common human neurological disorders, Parkinson's disease, little is known about the specification of this neuronal subtype. Hence, the recent identification of major transcriptional determinants regulating the development of these neurons has brought much excitement and encouragement to this field. These new findings will help to elucidate the genetic program that promotes the generation of midbrain dopaminergic neurons. Importantly, these discoveries will also significantly advance efforts to differentiate stem cells into midbrain dopaminergic neurons that can be used for therapeutic use in treating Parkinson's disease.

Ethanol activates cAMP response element-mediated gene expression in select regions of the mouse brain

The specific brain regions that contribute to behavioral changes produced by ethanol are not clearly understood. We know that cAMP-PKA signaling has been strongly implicated in the CNS effects of ethanol. Ethanol promotes activation and translocation of the PKA catalytic subunit (Calpha) into the nucleus in cell lines and primary neuronal cultures. PKA Calpha translocation to the nucleus is followed by cAMP Response Element protein phosphorylation (pCREB) and cAMP Response Element (CRE)-mediated gene expression. Here, we use X-gal histochemistry to map CRE-mediated gene transcription in the brain of CRE-lacZ transgenic mice following ethanol injection.

RESULTS

3 h after i.p. ethanol injection (3.2 g/kg, 16% wt/vol.), the number of X-gal positive cells was increased in the nucleus accumbens (202 +/- 63 cells/field compared to 71 +/- 47 cells/field in saline injected controls, P < 0.05 by paired t-test, n = 10). Similar increases were found in other mesolimbic areas and brain regions associated with rewarding and addictive responses. These include: prefrontal cortex, lateral and medial septum, basolateral amygdala, paraventricular and anterior hypothalamus, centromedial thalamus, CA1 region of hippocampus and dentate gyrus, substantia nigra pars compacta, ventral tegmental area, geniculate nucleus and the superior colliculus.

CONCLUSION

these results confirm and extend current concepts that ethanol stimulates cAMP-PKA signaling in brain regions involved in CNS responses to ethanol.

Dehydroepiandrosterone (DHEA) attenuates cocaine-seeking behavior in the self-administration model in rats

The aim of this study was to determine the possible involvement of the neurosteroid dehydroepiandrosterone (DHEA) in cocaine-seeking behavior in a self-administration model in rats. DHEA pretreatment (continued thereafter concomitantly with cocaine self-administration) attenuated cocaine-seeking behavior and elevated the levels of dopamine and serotonin in several brain regions relevant to cocaine addiction. Chronic cocaine self-administration induced elevation in brain DHEA, its sulfate ester, DHEAS, and pregnenolone. The increased brain DHEA following cocaine self-administration may serve as a compensatory protective mechanism geared to attenuate the craving for cocaine. Such anti-craving activity is further enhanced by DHEA treatment before and during cocaine self-administration.

Virally mediated increased neurotensin 1 receptor in the nucleus accumbens decreases behavioral effects of mesolimbic system activation

Dopamine receptor agonist and NMDA receptor antagonist activation of the mesolimbic dopamine system increases locomotion and disrupts prepulse inhibition of the acoustic startle response (PPI), paradigms frequently used to study both the pharmacology of antipsychotic drugs and drugs of abuse. In rats, virally mediated overexpression of the neurotensin 1 (NT1) receptor in the nucleus accumbens antagonized d-amphetamine- and dizocilpine-induced PPI disruption, hyperlocomotion, and D-amphetamine-induced rearing. The NT receptor antagonist SR 142948A [2-[[5-(2,6-dimethoxyphenyl)-1-(4-N-(3-dimethylaminopropyl)-N-methylcarbamoyl)-2-isopropylphenyl)-1H-pyrazole-3-carbonyl]amino] adamantane-2-carboxylic acid, hydrochloride] blocked inhibition of dizocilpine-induced hyperlocomotion mediated by overexpression of the NT1 receptor. Together, these results suggest that increased nucleus accumbens NT neurotransmission, via the NT1 receptor, can decrease the effects of activation of the mesolimbic dopamine system and disruption of the glutamatergic input from limbic cortices, resembling the action of the atypical antipsychotic drug clozapine. In contrast to clozapine, virally mediated overexpression of the NT1 receptor in the nucleus accumbens had prolonged protective effects (up to 4 weeks after viral injection) without perturbing baseline PPI and locomotor behaviors. These data further confirm the NT1 receptor as the receptor mediating the antistimulant- and antipsychotic-like properties of NT and provide rationale for the development of NT1 receptor agonists as novel antipsychotic drugs. In addition, the NT1 receptor vector might be a valuable tool for understanding the mechanism of action of antipsychotic drugs and drugs of abuse and may have potential therapeutic applications.

Contribution of brainstem GABAA synaptic transmission to morphine analgesic tolerance

Chronic opioid-induced analgesic tolerance remains a major obstacle to improving clinical management of moderate to severe chronic pain. Our understanding of the underlying mechanisms for opioid tolerance is only partially understood at present. In this study, we investigated the effects of chronic morphine on GABA(A) receptor-mediated synaptic transmission, a major opioid target for pain inhibition, and the behavioral role of GABA synaptic transmission in the development of morphine tolerance. In the nucleus raphe magnus (NRM), a critical brainstem site for opioid analgesia, the GABA(A) receptor-mediated inhibitory postsynaptic current (IPSC) was significantly increased in NRM neurons kept in a morphine-tolerant state from chronic morphine-treated rats. The potency of cAMP analogs for enhancing the GABA IPSC was also enhanced. The protein kinase A (PKA) inhibitor H89 reversed the chronic morphine-induced synaptic adaptation in GABA IPSCs. Behaviorally, a low dose of GABA(A) receptor antagonist bicuculline microinjected into the NRM, ineffective alone, blocked morphine antinociception in control rats, but failed to do so in morphine-tolerant rats. With chronic treatment through daily NRM microinjections, bicuculline augmented the development of morphine tolerance, whereas the GABA(A) receptor agonist muscimol or H89 significantly attenuated the development of morphine tolerance. These results suggest that chronic morphine increases GABA synaptic activity through upregulation of the AMP system in morphine-tolerant NRM neurons and that while chronic GABA(A) receptor antagonism in the NRM augments morphine tolerance, chronic activation of NRM GABA(A) receptors or PKA inhibition reduces morphine tolerance with increased analgesic efficacy of chronic morphine.

Effects of chronic nicotine administration and its withdrawal on striatal FosB/DeltaFosB and c-Fos expression in rats and mice

DeltaFosB, a member of Fos family of transcription factors, is implicated in behavioural responses and synaptic plasticity induced by abused drugs. We studied the expressions of FosB/DeltaFosB and c-Fos immunohistochemically in two dopaminergic brain areas, nucleus accumbens (NAcc) and caudate-putamen (CPu). In mice neither 2- nor 7-week oral nicotine treatment induced expression of long-lived DeltaFosB isoforms although during the treatment in the NAcc FosB/DeltaFosB expression was increased as was c-Fos in the CPu. In rats given nicotine subcutaneously once daily for 5days FosB/DeltaFosB expression was elevated in the NAcc still after 24-h withdrawal suggesting accumulation of DeltaFosB but in the CPu neither FosB/DeltaFosB nor c-Fos expression was altered. Thus, in rats repeated nicotine administration seems mainly affect the NAcc paralleling with the evidence that nicotine stimulates preferentially mesolimbic dopamine system. Also, repeated nicotine induced behavioural sensitization in rats agreeing with suggested role of DeltaFosB in the development of psychomotor sensitization. However, in mice given nicotine via drinking fluid although striatal fosB and c-fos were activated by nicotine even after 7-week treatment no evidence of accumulation of long-lived DeltaFosB was found suggesting perhaps a species difference or more likely a role for the manner of administration.

Impaired bidirectional synaptic plasticity and procedural memory formation in striatum-specific cAMP response element-binding protein-deficient mice

The striatum has a well documented role in procedural learning and memory. However, the synaptic and molecular mechanisms of acquisition and storage of this form of memory remain poorly understood. We examined procedural memory and plasticity in transgenic mice reversibly expressing a dominant-negative cAMP response element-binding protein (CREB) mutant in the dorsal striatum. In these transgenic mice, corticostriatal long-term potentiation and depression are abolished, indicating that CREB function is essential for bidirectional long-term synaptic plasticity in this structure. Importantly, CREB-deficient animals show reversible alterations in several forms of striatum-dependent memory, including footshock avoidance learning and "response" learning in the cross maze. These findings implicate transcriptional regulation by CREB family transcription factors in striatum-dependent information processing and provide the first clear correlation between procedural learning and memory and synaptic plasticity at the corticostriatal synapse.

Parsing molecular and behavioral effects of cocaine in mitogen- and stress-activated protein kinase-1-deficient mice

Although the induction of persistent behavioral alterations by drugs of abuse requires the regulation of gene transcription, the precise intracellular signaling pathways that are involved remain mainly unknown. Extracellular signal-regulated kinase (ERK) is critical for the expression of immediate-early genes in the striatum in response to cocaine and Delta9-tetrahydrocannabinol and for the rewarding properties of these drugs. Here we show that in mice a single injection of cocaine (10 mg/kg) activates mitogen- and stress-activated protein kinase 1 (MSK1) in dorsal striatum and nucleus accumbens. Cocaine-induced phosphorylation of MSK1 threonine 581 and cAMP response element-binding protein (CREB) serine 133 (Ser133) were blocked by SL327, a drug that prevents ERK activation. Cocaine increased the acetylation of histone H4 lysine 5 and phosphorylation of histone H3 Ser10, demonstrating the existence of drug-induced chromatin remodeling in vivo. In MSK1 knock-out (KO) mice CREB and H3 phosphorylation in response to cocaine (10 mg/kg) were blocked, and induction of c-Fos and dynorphin was prevented, whereas the induction of Egr-1 (early growth response-1)/zif268/Krox24 was unaltered. MSK1-KO mice had no obvious neurological defect but displayed a contrasted behavioral phenotype in response to cocaine. Acute effects of cocaine and dopamine D1 or D2 agonists were unaltered. Sensitivity to low doses, but not high doses, of cocaine was increased in the conditioned place preference paradigm, whereas locomotor sensitization to repeated injections of cocaine was decreased markedly. Our results show that MSK1 is a major striatal kinase, downstream from ERK, responsible for the phosphorylation of CREB and H3 and is required specifically for the induction of c-Fos and dynorphin as well as for locomotor sensitization.

Chronic exposure to Delta9-tetrahydrocannabinol downregulates oxytocin and oxytocin-associated neurophysin in specific brain areas

Cannabinoids are widely abused drugs. Our goal was to identify genes modulated by Delta9-tetrahydrocannabinol (Delta9-THC) treatment. We found that chronic administration of Delta9-THC (1.5 mg/kg/day, i.p.; 7 days) to rats, downregulates the expression of oxytocin-neurophysin (OT-NP) mRNA and of OT and oxytocin-associated NP (NPOT) immunoreactivity in nucleus accumbens (NAc) and ventral tegmental area (VTA), brain areas involved in reward and addiction. Real-time PCR revealed a 60% and 53% reduction of OT-NP mRNA in NAc and VTA, respectively, under chronic treatment, while no changes were observed in NAc after 24 h. Immunohistochemistry showed a large decrease in number of OT and NPOT-stained fibers in NAc (by 59% and 52%, respectively) and VTA (by 50% and 56%, respectively). No changes in cell staining were observed in the paraventricular nucleus and supraoptic nucleus. As OT is known to inhibit development of drug tolerance and attenuate withdrawal symptoms, we suggest that OT downregulation could play a role during the establishment of the chronic effects of Delta9-THC.

FACS-array profiling of striatal projection neuron subtypes in juvenile and adult mouse brains

A major challenge in systems neuroscience is to perform precise molecular genetic analyses of a single neuronal population in the context of the complex mammalian brain. Existing technologies for profiling cell type-specific gene expression are largely limited to immature or morphologically identifiable neurons. In this study, we developed a simple method using fluorescent activated cell sorting (FACS) to purify genetically labeled neurons from juvenile and adult mouse brains for gene expression profiling. We identify and verify a new set of differentially expressed genes in the striatonigral and striatopallidal neurons, two functionally and clinically important projection neuron subtypes in the basal ganglia. We further demonstrate that Ebf1 is a lineage-specific transcription factor essential to the differentiation of striatonigral neurons. Our study provides a general approach for profiling cell type-specific gene expression in the mature mammalian brain and identifies a set of genes critical to the function and dysfunction of the striatal projection neuron circuit.

CREB-binding protein controls response to cocaine by acetylating histones at the fosB promoter in the mouse striatum

Remodeling chromatin is essential for cAMP-regulated gene expression, necessary not only for development but also for memory storage and other enduring mental states. Histone acetylation and deacetylation mediate long-lasting forms of synaptic plasticity in Aplysia as well as cognition in mice. Here, we show that histone acetylation by the cAMP-response element binding protein (CREB)-binding protein (CBP) mediates sensitivity to cocaine by regulating expression of the fosB gene and its splice variant, DeltafosB, a transcription factor previously implicated in addiction. Using the chromatin immunoprecipitation assay with antibodies against histone H4 or CBP, we find that CBP is recruited to the fosB promoter to acetylate histone H4 in response to acute exposure to cocaine. We show that mutant mice that lack one allele of the CBP gene and have normal levels of fosB expression are less sensitive to chronic (10-day) administration of cocaine than are wild-type mice. This decreased sensitivity is correlated with decreased histone acetylation and results in decreased fosB expression and diminished accumulation of DeltafosB. Thus, CBP, which forms part of the promoter complex with CREB, mediates sensitivity to cocaine by acetylating histones.

Beta-arrestin goes nuclear

Arrestins have important roles in the cytoplasm and at the plasma membrane, including the desensitization and internalization of G protein-coupled receptors (GPCRs). In this issue of Cell, Kang et al. (2005) provide evidence that beta-arrestin 1 moves to the nucleus in response to GPCR stimulation, where it regulates gene expression by facilitating histone acetylation at specific gene promoters.

Galanin attenuates cyclic AMP regulatory element-binding protein (CREB) phosphorylation induced by chronic morphine and naloxone challenge in Cath.a cells and primary striatal cultures

Repeated morphine administration leads to molecular alterations of the neural circuitry in the locus coeruleus and nucleus accumbens. These changes include increased activity of several components of the cAMP signaling pathway that are thought to be associated with psychological and somatic signs of opiate withdrawal. The neuropeptide galanin has been shown to attenuate cAMP signaling in multiple cell types. The current study demonstrates that acute galanin treatment blocks the consequences of increased cAMP signaling following chronic opiate administration and withdrawal in Cath.a cells and primary cultures of striatal neurons as measured by phosphorylation of the transcription factor cAMP regulatory element-binding protein (CREB). In addition, galanin-mediated attenuation of CREB phosphorylation is independent of galanin-induced extracellular signal-regulated kinase (ERK) 1/2 phosphorylation in Cath.a cells. These data suggest that galanin receptors may serve as an additional potential therapeutic target for the treatment of opiate withdrawal.

Dynamic regulation of fibroblast growth factor 2 (FGF-2) gene expression in the rat brain following single and repeated cocaine administration

Administration of drugs of abuse can produce long-lasting effects on brain function, which involve modifications at neurotransmitter level as well as changes in proteins important for structural alterations of selected brain regions. The contribution of trophic factors in these events has so far been underestimated. Here, we demonstrate that a single cocaine injection selectively up-regulated fibroblast growth factor 2 (FGF-2) mRNA levels in the striatum and prefrontal cortex within 2 h, an effect that vanished by 24 h. However, prolonged exposure (5 or 14 days) to cocaine treatment produced an enduring elevation of FGF-2 mRNA levels that was evident 72 h after the last injection in the prefrontal cortex and could even persist for 14 days in the striatum, raising the possibility that cocaine treatment primes the brain, resulting in longer-lasting FGF-2 up-regulation in regions that are highly innervated by dopaminergic projections. The expression of FGF-2 was also significantly increased in the midbrain following acute or 5-day injection, suggesting that modulation of FGF-2 biosynthesis in dopamine-producing cells occurs only during early stages of cocaine exposure. Our results point to important mechanistic conclusions as to how cocaine alters FGF-2 expression. Whereas cocaine-induced changes in FGF-2 gene expression following a single injection could be ascribed to increased release of transmitters (mainly dopamine), enhanced FGF-2 gene expression following repeated administration identifies the trophic factor as part of the adaptive changes set in motion by cocaine.

Persistent changes in motivation to self-administer cocaine following modulation of cyclic AMP-dependent protein kinase A (PKA) activity in the nucleus accumbens

Drug-induced neuroadaptations within the nucleus accumbens, including activation of cAMP-dependent protein kinase A (PKA), may contribute to the synaptic plasticity and behavioural changes that underlie drug addiction. As a direct test of this hypothesis, we examined the effects in rats of PKA activation (Sp-cAMPS infusions of 10 and 20 nmol/side) and inhibition (Rp-cAMPS infusions of 10 and 20 nmol/side) in the nucleus accumbens on motivation to obtain cocaine as measured by responding under the progressive-ratio schedule. Bilateral infusions of Sp-cAMPS (20 nmol/side) resulted in an increase in progressive-ratio responding for cocaine and this effect persisted for several days. In contrast, Rp-cAMPS (20 nmol/side) produced persistent decreases in progressive-ratio responding for cocaine beginning on the day of administration and lasting for several days. These data suggest that alternations in PKA activity within the nucleus accumbens as a consequence of repeated cocaine exposure may contribute to addiction by producing persistent increases in motivation to obtain cocaine.

Receptor systems: will mining the receptorome yield novel targets for pharmacotherapy?

We have recently defined the receptorome as 'that part of the proteome encoding receptors'. In this article, I provide a general overview of the members of the receptorome as well as methods used to screen the receptorome-both in silico and physically. Case histories of receptorome-based discovery efforts are then highlighted and the relevance of this approach to the discovery and validation of molecular targets for drug abuse treatment is emphasized.

Dependência de drogas

O texto discute três teorias atuais de dependência de drogas: a Teoria Comportamental da dependência como escolha de Heyman, a Teoria da Sensibilização do incentivo de Robinson e Berridge, e a Teoria Neurobiológica da dependência como escolha, de Kalivas. Todas concordam em caracterizar a dependência como resultante de processos de aprendizagem em que droga e estímulos associados a seus efeitos adquirem controle potente sobre o comportamento. Diferenciam-se quanto aos processos de aprendizagem envolvidos. A Teoria Comportamental enfatiza componentes operantes e sustenta que o consumo repetido de drogas diminui o valor reforçador de atividades concorrentes. A Teoria da Sensibilização enfatiza componentes respondentes, propondo a dependência como resultado da sensibilização da potência eliciadora de estímulos condicionados aos efeitos da droga. A Teoria Neurobiológica integra as duas primeiras, descrevendo as mudanças no circuito do reforço que acontecem no processo de dependência.

Mu-opioid receptor-mediated phosphorylation of IkappaB kinase in human neuroblastoma SH-SY5Y cells

Opioid receptors are involved in regulating neuronal survival. Here we demonstrate that activation of the mu-opioid receptor in human neuroblastoma SH-SY5Y cells led to the phosphorylations of IkappaB kinase (IKK) and p65, denoting the stimulation of the nuclear factor-kappaB (NFkappaB) transcription factor. This response was mediated through pertussis toxin-sensitive G proteins. The mu-opioid-induced IKK phosphorylation required extracellular signal-regulated protein kinase, phosphatidylinositol 3-kinase and c-Src. Moreover, c-Jun N-terminal kinase and calmodulin-dependent kinase II also participated in the IKK activation, despite the lack of involvement of phospholipase Cbeta and protein kinase C. These data suggest that the mu-opioid receptor is capable of simulating NFkappaB signaling via the phosphorylation of IKK and p65 in human neuroblastoma SH-SY5Y cells.

A Proteomics Approach to Identify Long-Term Molecular Changes in Rat Medial Prefrontal Cortex Resulting from Sucrose Self-Administration

The medial prefrontal cortex (mPFC) is involved in the processing and retrieval of reward-related information. Here, we investigated long-lasting changes in protein composition of the mPFC in rats with a history of sucrose self-administration. Protein levels were analyzed using 2-D PAGE and MALDI-TOF sequencing. From approximately 1500 spots, 28 regulated proteins were unambiguously identified and were involved in cytoskeleton organization, energy metabolism, oxidative stress, neurotransmission, and neuronal outgrowth and differentiation. For several proteins, this change was also found as a long-lasting alteration in gene expression. We show that self-administration of sucrose produces long-lasting molecular neuroadaptations in the mPFC that may be involved in reward-related information processing.

The slowpoke gene is necessary for rapid ethanol tolerance in Drosophila

BACKGROUND

Ethanol is one of the most commonly used drugs in the world. We are interested in the compensatory mechanisms used by the nervous system to counter the effects of ethanol intoxication. Recently, the slowpoke BK-type calcium-activated potassium channel gene has been shown to be involved in ethanol sensitivity in Caenorhabditis elegans and in rapid tolerance to the anesthetic benzyl alcohol in Drosophila.

METHODS

We used Drosophila mutants to investigate the role of slowpoke in rapid tolerance to sedation with ethanol vapor. Rapid tolerance was defined as a reduction in the sedative phase caused by a single previous sedation. The ethanol and water contents of flies were measured to determine if pharmacodynamic changes could account for tolerance.

RESULTS

A saturated ethanol air stream caused sedation in <20 min and resulted in rapid tolerance that was apparent 4 hr after sedation. Two independently isolated null mutations in the slowpoke gene eliminated the capacity for tolerance. In addition, a third mutation that blocked expression specifically in the nervous system also blocked rapid tolerance. Water measurements showed that both ethanol and mock sedation caused equivalent dehydration. Furthermore, a single prior exposure to ethanol did not cause a change in the ethanol clearance rate.

CONCLUSIONS

Rapid tolerance, measured as a reduction in the duration of sedation, is a pharmacokinetic response to ethanol that does not occur without slowpoke expression in the nervous system in Drosophila. The slowpoke channel must be involved in triggering or producing a homeostatic mechanism that opposes the sedative effects of ethanol.

Activation of the spinal ERK signaling pathway contributes naloxone-precipitated withdrawal in morphine-dependent rats

Extracellular signal-regulated kinase (ERK), a mitogen-activated protein kinases (MAPK), transduces a broad range of extracellular stimuli into diverse intracellular responses. Recent studies have showed that ERK activation in the supraspinal level involved in the development of drug dependence, especially in psychological dependence. In this study, we reported that the spinal ERK signaling pathway was activated by chronic morphine injection. There was a further increase in ERK activation after naloxone-precipitated withdrawal. Furthermore, attenuation of the spinal ERK phosphorylation by intrathecal a MAPK kinase (MEK) inhibitor U0126 or knockdown of the spinal ERK by antisense oligonucleotides not only decreased the scores of morphine withdrawal, but also attenuated withdrawal-induced allodynia, which were accompanied by decreased ERK phosphorylation in the spinal cord. The spinal ERK inhibition or knockdown also reduced morphine withdrawal-induced phosphorylation of cAMP response element binding protein (CREB), which is one of the important downstream substrates of ERK pathway, and Fos expression. The involvement of the spinal ERK in morphine withdrawal was supported by our finding that intrathecal N-methyl-D-aspartate receptor antagonist MK-801 or protein kinase C inhibitor chelerythrine chloride suppressed withdrawal-induced ERK activation in the spinal cord and attenuated morphine withdrawal symptoms. These findings suggest activation of the spinal ERK signaling pathway contributes naloxone-precipitated withdrawal in morphine-dependent rats.

Expression profile of addicsin/GTRAP3-18 mRNA in mouse brain

Addicsin is a murine homologue of rat glutamate-transporter-associated protein 3-18 (GTRAP3-18), a putative modulator of neural glutamate-transporter excitatory amino acid carrier 1 (EAAC1). The other molecular functions of addicsin, however, remain largely unknown. We analyzed here the regional and cellular distribution of addicsin transcript in the mature brain using in situ hybridization analysis, and examined the sequential addicsin mRNA expression levels in the developing brain using Northern blot analysis. In the mature brain, we found addicsin mRNA to be ubiquitously expressed in neuron-like cells, but not glial cells, in various brain regions including the olfactory bulbs, hippocampus and cerebral cortex. In the hippocampus, addicsin mRNA was expressed in the neuron-like cells of the CA1-CA3 pyramidal cell layer and the interneuron-like cells of the stratum oriens, stratum radiatum and stratum lacunosum-moleculare. Addicsin transcripts were also extremely abundant in trigeminal neurons such as the principal trigeminal nucleus, mesencephalic trigeminal nucleus and motor trigeminal nucleus. Our evidence suggests that addicsin mRNA is chiefly expressed in the excitatory and inhibitory neurons, and that addicsin may participate in the functional expression of the somatic sensory system by modulation of EAAC1-mediated glutamate transport. Northern blot analysis revealed that addicsin mRNA levels increased with maturation, reaching their maximum level at postnatal day 5 (P5), then significantly declined by about 50% until P14, suggesting that addicsin may contribute to embryogenesis and synaptogenesis in the developing brain. Thus, addicsin may participate in multiple physiological functions in the developing and mature brain.

The role of central dopamine D3 receptors in drug addiction: a review of pharmacological evidence

The cDNA for the dopamine D3 receptor was isolated and characterized in 1990. Subsequent studies have indicated that D3 receptors, as well as D3 receptor mRNA, are primarily localized in limbic regions in mammals. This finding led to the postulate that D3 receptors may be involved in drug dependence and addiction. However, this hypothesis has been difficult to test due to the lack of compounds with high selectivity for central D3 receptors. The interpretation of results from studies using mixed D2/D3 agonists and/or antagonists is problematic because these agents have low selectivity for D3 over D2 receptors and it is likely that their actions are primarily related to D2 receptor antagonism and possibly interaction with other neurotransmitter receptors. Currently, with the synthesis and characterization of new highly selective D3 receptor antagonists such as SB-277011-A this difficulty has been surmounted. The purpose of the present article is to review, for the first time, the effects of various putative D3 receptor selective compounds in animal models of drug dependence and addiction. The results obtained with highly selective D3 receptor antagonists such as SB-277011-A, SB-414796, and NGB-2904 indicate that central D3 receptors may play an important role in drug-induced reward, drug-taking, and cue-, drug-, and stress-induced reinstatement of drug-seeking behavior. Provided these results can be extrapolated to human drug addicts, they suggest that selective DA D3 receptor antagonists may prove effective as potential pharmacotherapeutic agents to manage drug dependence and addiction.

Alcohol tolerance in large-conductance, calcium-activated potassium channels of CNS terminals is intrinsic and includes two components: decreased ethanol potentiation and decreased channel density

Tolerance is an important element of drug addiction and provides a model for understanding neuronal plasticity. The hypothalamic-neurohypophysial system (HNS) is an established preparation in which to study the actions of alcohol. Acute application of alcohol to the rat neurohypophysis potentiates large-conductance calcium-sensitive potassium channels (BK), contributing to inhibition of hormone secretion. A cultured HNS explant from adult rat was used to explore the molecular mechanisms of BK tolerance after prolonged alcohol exposure. Ethanol tolerance was intrinsic to the HNS and consisted of: (1) decreased BK potentiation by ethanol, complete within 12 min of exposure, and (2) decreased current density, which was not complete until 24 hr after exposure, indicating that the two components of tolerance represent distinct processes. Single-channel properties were not affected by chronic exposure, suggesting that decreased current density resulted from downregulation of functional channels in the membrane. Indeed, we observed decreased immunolabeling against the BK alpha-subunit on the surface of tolerant terminals. Analysis using confocal microscopy revealed a reduction of BK channel clustering, likely associated with the internalization of the channel.

GalR1, but not GalR2 or GalR3, levels are regulated by galanin signaling in the locus coeruleus through a cyclic AMP-dependent mechanism

The galanin receptors GalR1, GalR2 and GalR3 are widely expressed throughout the mouse brain and are enriched in catecholaminergic nuclei. Here, we show that GalR1 protein levels are regulated by neuronal activity and changes in cAMP levels. GalR1, but not GalR2 or GalR3, is specifically up-regulated in the LC-like Cath.a cell line in a cAMP-dependent manner. GalR1 protein and mRNA levels are also up-regulated in the LC of galanin knockout mice, whereas GalR2 and GalR3 are not. Lack of galanin-maintained cAMP tone in the galanin knockout mouse appears to result in a loss of negative feedback resulting in increased levels of CREB phosphorylation and increased GalR1 expression. These findings suggest that changes in levels of GalR1 may play an important role in modulating signaling events and neuroplasticity underlying physiological functions of the LC.

Cocaine-induced alterations in dopamine receptor signaling: Implications for reinforcement and reinstatement

The transition from casual drug use to addiction, and the intense drug craving that accompanies it, has been postulated to result from neuroadaptations within the limbic system caused by repeated drug exposure. This review will examine the implications of cocaine-induced alterations in mesolimbic dopamine receptor signaling within the context of several widely used animal models of addiction. Extensive evidence indicates that dopaminergic mechanisms critically mediate behavioral sensitization to cocaine, cocaine-induced conditioned place preference, cocaine self-administration, and the drug prime-induced reinstatement of cocaine-seeking behavior. The propagation of the long-term neuronal changes associated with recurring cocaine use appears to occur at the level of postreceptor signal transduction. Repeated cocaine treatment causes an up-regulation of the 3',5'-cyclic adenosine monophosphate (cAMP)-signaling pathway within the nucleus accumbens, resulting in a dys-regulation of balanced D1/D2 dopamine-like receptor signaling. The intracellular events arising from enhanced D1-like postsynaptic signaling mediate both facilitatory and compensatory responses to the further reinforcing effects of cocaine.

Palmitoylation regulates plasma membrane-nuclear shuttling of R7BP, a novel membrane anchor for the RGS7 family

The RGS7 (R7) family of RGS proteins bound to the divergent Gβ subunit Gβ5 is a crucial regulator of G protein–coupled receptor (GPCR) signaling in the visual and nervous systems. Here, we identify R7BP, a novel neuronally expressed protein that binds R7–Gβ5 complexes and shuttles them between the plasma membrane and nucleus. Regional expression of R7BP, Gβ5, and R7 isoforms in brain is highly coincident. R7BP is palmitoylated near its COOH terminus, which targets the protein to the plasma membrane. Depalmitoylation of R7BP translocates R7BP–R7–Gβ5 complexes from the plasma membrane to the nucleus. Compared with nonpalmitoylated R7BP, palmitoylated R7BP greatly augments the ability of RGS7 to attenuate GPCR-mediated G protein–regulated inward rectifying potassium channel activation. Thus, by controlling plasma membrane nuclear–shuttling of R7BP–R7–Gβ5 complexes, reversible palmitoylation of R7BP provides a novel mechanism that regulates GPCR signaling and potentially transduces signals directly from the plasma membrane to the nucleus.

In vivo up-regulation of brain-derived neurotrophic factor in specific brain areas by chronic exposure to Delta9-tetrahydrocannabinol

Cannabinoids are widely abused drugs. Here we show that chronic administration of Delta(9)-tetrahydrocannabinol (Delta(9)-THC), the active psychotropic agent in marijuana and hashish, at 1.5 mg per kg per day intraperitoneally for 7 days, increases the expression, at both mRNA and protein levels, of brain-derived neurotrophic factor (BDNF), in specific rat brain areas, notably in those involved in reward and addiction. Real-time PCR revealed a 10-fold up-regulation of BDNF mRNA in the nucleus accumbens (NAc) upon chronic Delta(9)-THC treatment, but there was no change at 3 or 24 h after a single injection. Smaller increases in mRNA levels were found in the ventral tegmental area (VTA), medial prefrontal cortex and paraventricular nucleus (PVN). Immunohistochemistry showed large increases in BDNF-stained cells in the NAc (5.5-fold), posterior VTA (4-fold) and PVN (1.7-fold), but no change was observed in the anterior VTA, hippocampus or dorsal striatum. Altogether, our study indicates that chronic exposure to Delta(9)-THC up-regulates BDNF in specific brain areas involved with reward, and provides evidence for different BDNF expression in the anterior and posterior VTA. Moreover, BDNF is known to modulate synaptic plasticity and adaptive processes underlying learning and memory, leading to long-term functional and structural modification of synaptic connections. We suggest that Delta(9)-THC up-regulation of BDNF expression has an important role in inducing the neuroadaptive processes taking place upon exposure to cannabinoids.

Changes in gene expression within the nucleus accumbens and striatum following sexual experience

Sexual experience, like repeated drug use, produces long-term changes including sensitization in the nucleus accumbens and dorsal striatum. To better understand the molecular mechanisms underlying the neuroadaptations following sexual experience, we employed a DNA microarray approach to identify genes differentially expressed between sexually experienced and sexually naive female hamsters within the nucleus accumbens and dorsal striatum. For 6 weeks, a stimulus male was placed in the home cage of one-half of the hormonally primed, ovariectomized female hamsters. On the seventh week, the two experimental groups were subdivided, with one half paired with a stimulus male. In comparison with sexually naive animals, sexually experienced hamsters receiving a stimulus male on week 7 exhibited an increase in a large number of genes. Conversely, sexually experienced female hamsters not receiving a stimulus male on week 7 exhibited a reduction in the expression of many genes. For directional changes and the categories of genes regulated by the experimental conditions, data were consistent across the nucleus accumbens and dorsal striatum. However, the specific genes exhibiting changes in expression were disparate. These experiments, among the first to profile genes regulated by female sexual behavior, will provide insight into the mechanisms by which both motivated behaviors and drugs of abuse induce long-term changes in the mesolimbic and nigrostriatal dopamine pathways.

cAMP-mediated mechanisms for pain sensitization during opioid withdrawal

Chronic opioid-induced drug dependence and withdrawal syndrome after opioid cessation remain a severe obstacle in clinical treatment of chronic pain and opioid drug addiction. One of the key symptoms during opioid withdrawal is a state of sensitized pain. The most significant molecular adaptation induced by chronic opioids in the brain is upregulation of the cAMP-signaling pathway. Although the cAMP system is known to have multiple effects on central neuron functions, how its upregulation mediates behavioral opioid dependence and withdrawal-induced pain in vivo remains unclear. In this study, we demonstrate that withdrawal from chronic morphine significantly upregulates the mRNA level of adenylyl cyclase (AC) VI and VIII isoforms and immunoreactivity of ACV/VI in the nucleus raphe magnus (NRM), a brainstem site critically involved in opioid modulation of pain. In cellular studies of NRM neurons containing mu-opioid receptors, we show that morphine withdrawal significantly increases glutamate synaptic transmission via a presynaptic mechanism mediated by an upregulated cAMP pathway. Morphine withdrawal also enhances the hyperpolarization-activated current in these neurons by increased intracellular cAMP. Both of the withdrawal-induced cAMP actions increase the excitability of these mu-receptor-containing neurons, which are thought to facilitate spinal pain transmission. Furthermore, in morphine-dependent rats in vivo, blocking the cAMP pathway significantly reduces withdrawal-induced pain sensitization. These results illustrate neurobiological mechanisms for the cAMP-mediated withdrawal pain and provide potential therapeutic targets for the treatment of opioid dependence and withdrawal-related problems.

Genetic susceptibility to substance dependence

Despite what is often believed, the majority of those who experiment with substances with a dependence potential do not develop dependence. However, there is a subpopulation of users that easily becomes dependent on substances, and these individuals exhibit pre-existing comorbid traits, including novelty seeking and antisocial behavior. There appears to be a genetic basis for the susceptibility to dependence and these comorbid traits. Animal studies have identified specific genes that can alter susceptibility to dependence and response to novelty. The mechanisms underlying the genetic susceptibility to dependence and response to novelty are complex, but genetic susceptibility plays a significant role in the transition from substance use to dependence and from chronic use to addiction. We discuss two models to explain how genetic variations alter dependence susceptibility. Identification of the specific genes involved in these processes would help to identify individuals that are vulnerable to dependence/addiction and to devise novel treatment strategies.

Combining research approaches to advance our understanding of drug addiction

Drug addiction is a complex behavior, likely to be influenced by various genes, environmental factors, and gene-gene and gene-environment interactions. Various aspects of addiction are studied by different disciplines. Animal studies are increasing insight into brain regions and genes associated with addiction. Epidemiologic studies are establishing the factors increasing risk for initiation and continuation of substance use. Twin and adoption studies are increasing our understanding of the complex mechanisms involved in substance use, including comorbidity and gene environment interaction. Finally, molecular genetic studies in humans are starting to yield some converging findings. It is argued and illustrated with examples that greater awareness of progress in other disciplines can speed up our understanding of the complex processes involved in addiction. This should help our ability to identify who is at increased risk of becoming addicted and the development of prevention and intervention strategies targeted at an individual's specific needs.

Cocaine-induced expression changes of axon guidance molecules in the adult rat brain

Administration of drugs of abuse induces strong molecular adaptations and plasticity within the mesolimbic dopamine (DA) system, a pathway essential for reward-seeking behavior. Little is known about the specific targets involved in this neuroadaptation process, but there are indications that cocaine and other drugs of abuse share the ability to alter the morphology of neuronal dendrites and spines, the primary site of excitatory synapses in the brain. Axon guidance molecules, the very molecular cues that regulate the formation of axon-target connections during development, may mediate these alterations. To test this hypothesis, we investigated mRNA expression changes of 39 axon guidance molecules, including 17 Semaphorins, 12 Ephs, 8 Ephrins, and 2 neuropilins in the mesolimbic dopamine system of cocaine-treated animals under different paradigms by mean of DNA-Microarray and quantitative real-time PCR. In all cases, strong changes in gene expression are observed, yielding to up or downregulation of these axon guidance molecules. Our data suggest that cocaine treatment induces activation of a complex program of synaptic rearrangements, which may partly recapitulate the plastic changes occurring during development, and may underlie the important neuroplastic adaptations that occur in the reward- and memory-related brain centers following drug action. We conclude that in some brain regions, exposure to psychomotor-stimulant drugs produce expression changes in axon guidance molecules, which may contribute to cognitive deficits associated with drug abuse.

Unifying mechanism for toxicity and addiction by abused drugs: electron transfer and reactive oxygen species

Abused drugs are of grave concern throughout the world for a variety of reasons. Although impressive advances have been made, there are many unknown mechanistic aspects. This report presents a novel hypothesis based on a unifying theme for action of the major classes of abused drugs, in addition to commonly abused therapeutic drugs. The approach is based on electron transfer (ET), reactive oxygen species (ROS), and oxidative stress (OS). It is significant that physiologically active substances generally incorporate ET functionalities, either per se, or more usually in their metabolites. In order to achieve ET in vivo, the reduction potential must be more positive than -0.5 V, which is the case for metabolites of abused drugs, except for special cases. Since the ET process is catalytic, only small quantities of agent are needed for generation of large amounts of ROS during redox cycyling. Bioaction with cellular materials could entail ET alone or participation of ROS. In the abused category, among the main classes of ET functionalities are quinones and iminiums, with alpha-dicarbonyl and nitroxyl radical being rarer. Nicotine yields nicotine iminium, myosmine iminium, and DNA base iminium via alkylation by a metabolic nitrosamine. In the case of alcohol, diacetyl (an alpha-dicarbonyl) is formed, which can lead to conjugated imine (or iminium) by condensation with pri-amine of protein. Phencyclidine is unusual since the iminium product is non-conjugated. However, data indicate that the conformation present at the binding site can accommodate delocalization of the derived radical. For cocaine, various metabolites may play a role: iminium, nitroxyl radical, nitrosonium and formaldehyde. Dealkylation of the ether moiety of ecstasy provides a catechol function capable of redox cycling with the o-quinone partner. Amphetamine and methamphetamine also appear to function by way of the catechol route, as well as morphine and heroin. Tetrahydrocannabinol produces an epoxide, a functionality capable of DNA base alkylation accompanied by ROS. LSD undergoes oxidation to a phenol which may be a quinone precursor. Therapeutic drugs display the indicated metabolic relationships: benzodiazepines, iminium; phenytoin, quinone; phenobarbital, catechol; aspirin, catechol and hydroquinone; acetaminophen, iminoquinone. Extensive evidence exists for formation of ROS, organ injury by OS, depletion of AOs, and protection by AOs for the various drugs. There is also discussion of computational approaches, addiction mechanism and prevention, and health promotion.

Structural plasticity associated with exposure to drugs of abuse

Persistent changes in behavior and psychological function that occur as a function of experience, such those associated with learning and memory, are thought to be due to the reorganization of synaptic connections (structural plasticity) in relevant brain circuits. Some of the most compelling examples of experience-dependent changes in behavior and psychological function, changes that can last a lifetime, are those that accrue with the development of addictions. However, until recently, there has been almost no research on whether potentially addictive drugs produce forms of structural plasticity similar to those associated with other forms of experience-dependent plasticity. In this paper we summarize evidence that, indeed, exposure to amphetamine, cocaine, nicotine or morphine produces persistent changes in the structure of dendrites and dendritic spines on cells in brain regions involved in incentive motivation and reward (such as the nucleus accumbens), and judgment and the inhibitory control of behavior (such as the prefrontal cortex). It is suggested that structural plasticity associated with exposure to drugs of abuse reflects a reorganization of patterns of synaptic connectivity in these neural systems, a reorganization that alters their operation, thus contributing to some of the persistent sequela associated with drug use--including addiction.

slo K(+) channel gene regulation mediates rapid drug tolerance

Changes in neural activity caused by exposure to drugs may trigger homeostatic mechanisms that attempt to restore normal neural excitability. In Drosophila, a single sedation with the anesthetic benzyl alcohol changes the expression of the slo K(+) channel gene and induces rapid drug tolerance. We demonstrate linkage between these two phenomena by using a mutation and a transgene. A mutation that eliminates slo expression prevents tolerance, whereas expression from an inducible slo transgene mimics tolerance in naive animals. The behavioral response to benzyl alcohol can be separated into an initial phase of hyperkinesis and a subsequent phase of sedation. The hyperkinetic phase causes a drop in slo gene expression and makes animals more sensitive to benzyl alcohol. It is the sedative phase that stimulates slo gene expression and induces tolerance. We demonstrate that the expression level of slo is a predictor of drug sensitivity.

Neurochemical and behavioral consequences of widespread gene knockdown in the adult mouse brain by using nonviral RNA interference

Gene expression analysis implicates an increasing number of novel genes in the brain as potential targets for the treatment of neurological and psychiatric disorders. Frequently, these genes are ubiquitously expressed in the brain and, thus, may contribute to a pathophysiological state through actions in several brain nuclei. Current strategies employing genetically modified animals for in vivo validation of such targets are time-consuming and often limited by developmental adaptations. Somatic gene manipulation using viral-mediated RNA interference (RNAi) has emerged recently, although restricting the target validation to specific brain nuclei. We investigated whether nonviral infusion of short interfering RNA (siRNA) into the ventricular system would enable a sequence-specific gene knockdown. The temporality and extent of siRNA-induced down-regulation were analyzed by targeting a transgene, EGFP, in mice overexpressing EGFP. Extensive knockdown of EGFP was observed, especially in regions adjacent or dorsoventrally and mediolaterally distant to the infusion site (dorsal third ventricle), with lesser knockdown in more distal regions. We challenged our RNAi approach to generate a specific knockdown of an endogenous gene, encoding the dopamine transporter (DAT) in regions (ventral midbrain) far distal to the infusion site. DAT-siRNA infusion in adult mice produced a significant down-regulation of DAT mRNA and protein in the brain and also elicited a temporal hyperlocomotor response similar to that (but delayed) obtained upon infusion of GBR-12909, a pharmacologically selective DAT inhibitor. Application of this nonviral RNAi approach may accelerate target validation for neuropsychiatric disorders that involve a complex interplay of gene(s) from various brain regions.

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.

Endogenous regulator of g protein signaling proteins reduce {mu}-opioid receptor desensitization and down-regulation and adenylyl cyclase tolerance in C6 cells

Chronic exposure of cells to mu-opioid agonists leads to tolerance which can be measured by a reduced ability to activate signaling pathways in the cell. Cell signaling through inhibitory G proteins is negatively regulated by RGS (regulator of G protein signaling) proteins. Here we examine the hypothesis that the GTPase accelerating activity of RGS proteins, by altering the lifetime of Galpha and Gbetagamma, plays a role in the development of cellular tolerance to mu-opioids. C6 glioma cells were stably transfected with mu-opioid receptor and pertussis toxin (PTX)-insensitive Galpha(o) that was either sensitive or insensitive to endogenous RGS proteins. Cells were treated with PTX to uncouple endogenous Galpha proteins followed by exposure to the mu-opioid agonists [d-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO) or morphine. Receptor desensitization as measured by agonist-stimulated [(35)S]GTPgammaS binding and receptor down-regulation as measured by [(3)H]diprenorphine binding were increased in cells expressing RGS-insensitive Galpha(o). Exposure to high concentrations of morphine or the peptidic mu-opioid agonist DAMGO led to a tolerance to inhibit adenylyl cyclase activity in both cell types with a rapid (30 min) and a slower component. Using a submaximal concentration of DAMGO to induce a reduced level of tolerance, a shift in the concentration-effect curve for DAMGO to inhibit adenylyl cyclase activity was seen in the cells expressing RGS-insensitive Galpha(o), but not in the cells expressing RGS-sensitive Galpha(o), which can be partly explained by an increased supersensitization of the adenylyl cyclase response. The results show that RGS proteins endogenously expressed in C6 cells reduce agonist-induced mu-opioid receptor desensitization, down-regulation, and sensitivity to tolerance to inhibit adenylyl cyclase activity.

Neurobiologic processes in drug reward and addiction

Neurophysiologic processes underlie the uncontrolled, compulsive behaviors defining the addicted state. These"hard-wired"changes in the brain are considered critical for the transition from casual to addictive drug use. This review of preclinical and clinical (primarily neuroimaging) studies will describe how the delineation between pleasure, reward, and addiction has evolved as our understanding of the biologic mechanisms underlying these processes has progressed. Although the mesolimbic dopaminergic efflux associated with drug reward was previously considered the biologic equivalent of pleasure, dopaminergic activation occurs in the presence of unexpected and novel stimuli (either pleasurable or aversive) and appears to determine the motivational state of wanting or expectation. The persistent release of dopamine during chronic drug use progressively recruits limbic brain regions and the prefrontal cortex, embedding drug cues into the amygdala (through glutaminergic mechanisms) and involving the amygdala, anterior cingulate, orbitofrontal cortex, and dorsolateral prefrontal cortex in the obsessive craving for drugs. The abstinent, addicted brain is subsequently primed to return to drug use when triggered by a single use of drug, contextual drug cues, craving, or stress, with each process defined by a relatively distinct brain region or neural pathway. The compulsive drive toward drug use is complemented by deficits in impulse control and decision making, which are also mediated by the orbitofrontal cortex and anterior cingulate. Within this framework, future targets for pharmacologic treatment are suggested.