Bidirectional changes in striatal D1-dopamine receptor density during chronic ethanol intake

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

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

Involvement of nucleus accumbens dopamine D1 receptors in ethanol drinking, ethanol-induced conditioned place preference, and ethanol-induced psychomotor sensitization in mice

RATIONALE

Dopamine D1 receptor (D1R) signaling has been associated to ethanol consumption and reward in laboratory animals.

OBJECTIVES

Here, we hypothesize that this receptor, which is located within the nucleus accumbens (NAc) neurons, modulates alcohol reward mechanisms.

METHODS

To test this hypothesis, we measured alcohol consumption and ethanol-induced psychomotor sensitization and conditioned place preference (CPP) in mice that received bilateral microinjections of small interference RNA (siRNA)-expressing lentiviral vectors (LV-siD1R) producing D1R knock-down. The other group received control (LV-Mock) viral vectors into the NAc.

RESULTS

There were no differences in the total fluid consumed and also no differences in the amount of ethanol consumed between groups prior to surgery. However, after surgery, the LV-siD1R group consumed less ethanol than the control group. This difference was not associated to taste neophobia. In addition, results have shown that down-regulation of endogenous D1R using viral-mediated siRNA in the NAc significantly decreased ethanol-induced behavioral sensitization as well as acquisition, but not expression, of ethanol-induced place preference.

CONCLUSIONS

We conclude that decreased D1R expression into the NAc led to reduced ethanol rewarding properties, thereby leading to lower voluntary ethanol consumption. Together, these findings demonstrate that the D1 receptor pathway within the NAc controls ethanol reward and intake.

Dopamine and alcoholism: neurobiological basis of ethanol abuse

The role of the dopamine (DA) system in brain reward mechanisms and the development of substance abuse has been well established. We review earlier animal and human studies on DA and alcoholism with some relevant issues relating to those studies. The present animal and human data suggest several alterations in the DA system in the context of alcoholism. Receptor studies imply that DA D(2) receptor density and function are lower at least among type 1 alcoholics, which suggests that they could benefit from drugs that enhance DAergic activity, such as partial DA agonists. These drugs could help to restore suboptimal levels of DAergic activity by reducing both the craving for alcohol in abstinence and the euphoria subsequent to alcohol's release of DA in the nucleus accumbens (NAC), thus providing negative reinforcement for relapse.

Effect of one-week ethanol treatment on monoamine levels and dopaminergic receptors in rat striatum

We studied the effects of ethanol on the levels of norepinephrine, dopamine, serotonin (5-HT) and their metabolites as well as on D1- and D2-like receptors in the rat striatum. Ethanol (2 or 4 g/kg, po) was administered daily by gavage to male Wistar rats and on the 7th day, 30 min or 48 h after drug administration, the striatum was dissected for biochemical assays. Monoamine and metabolite concentrations were measured by HPLC and D1- and D2-like receptor densities were determined by binding assays. Scatchard analyses showed decreases of 30 and 43%, respectively, in D1- and D2-like receptor densities and no change in dissociation constants (Kd) 48 h after the withdrawal of the dose of 4 g/kg. Ethanol, 2 g/kg, was effective only on the density of D2-like receptors but not on Kd of either receptor. Thirty minutes after the last ethanol injection (4 g/kg), decreases of D2 receptor density (45%) as well as of Kd values (34%) were detected. However, there was no significant effect on D1-like receptor density and a 46% decrease was observed in Kd. An increase in dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC), a decrease in norepinephrine, and no alteration in 5-HT levels were demonstrated after 48-h withdrawal of 4 g/kg ethanol. Similar effects were observed in dopamine and DOPAC levels 30 min after drug administration. No alteration in norepinephrine concentration and a decrease in 5-HT levels were seen 30 min after ethanol (4 g/kg) administration. Our findings indicate the involvement of the monoaminergic system in the responses to ethanol.

The effect of chronic ethanol consumption and withdrawal on mu-opioid and dopamine D(1) and D(2) receptor density in Fawn-Hooded rat brain

Previous studies have implicated the dopamine and opioid systems in the induction and maintenance of ethanol consumption. This study investigated, in alcohol-preferring Fawn-Hooded (FH) rats, whether chronic free-choice ethanol consumption and subsequent withdrawal cause alterations in central mu-opioid, dopamine D(1), and D(2) receptor density using autoradiography. FH rats were given a free choice between a 5% ethanol solution and tap water (n = 25) and displayed a mean ethanol consumption of 5.6 g/kg/day. A parallel group of FH rats (n = 5) only had access to tap water. Rats were then withdrawn from ethanol for 0, 1, 2, 5, or 10 days and killed by cervical dislocation and decapitation. Increases in mu-opioid receptor density were observed in the nucleus accumbens and ventral tegmental area upon withdrawal compared with the ethanol naive group. In the lateral amygdala, binding in all withdrawal groups was significantly different from the ethanol naive FH rats, and also from the chronic ethanol rats. An increase in dopamine D(1) receptor density was observed in the substantia nigra, pars reticulata in the 5- and 10-day withdrawal groups compared with ethanol naive. Accumbal dopamine D(2) receptor density (+25-30%) increased in the 10-day withdrawal group compared with both naive and chronic ethanol groups. These findings demonstrate that the opioid and dopamine systems are susceptible to modulation by chronic ethanol consumption and withdrawal in the FH rat. Furthermore, although acute ethanol withdrawal results in modulation of mu-opioid receptors, effects on dopamine receptors are delayed and only become evident 5 to 10 days after withdrawal.

Differential propensity to ethanol sensitization is not associated with altered binding to D1 receptors or dopamine transporters in mouse brain

Behavioral sensitization to ethanol's stimulant effect has been proposed as a marker for individual abuse liability. In previous work we have demonstrated that mice showing an increased propensity to EtOH sensitization had higher levels of dopamine (DA) D2 receptor binding in localized brain areas compared to mice showing less sensitization. In the present study we examined whether altered binding to D1 or the DA transporter (DAT) might also be associated with differential propensity to develop EtOH sensitization. Male Swiss mice received 2.4 g/kg EtOH or saline intraperitoneally (i.p.) daily for 21 days, were tested weekly for locomotor activity, and then sacrificed. D1 and DAT binding were assessed by quantitative autoradiography using [(3)H]SCH-23390 and [(3)H]WIN 35,428, respectively. EtOH-treated mice were subdivided into sensitized and non-sensitized subgroups according to their locomotor activity during treatment. Analyses of brain D1 (19 regions) and DAT (12 regions) binding densities revealed no significant differences among EtOH-sensitized, -non-sensitized or saline groups in any of the regions measured (all p values > 0.32 for D1 and > 0.16 for DAT). These results suggest that brain D1 and DAT binding, unlike the recently reported changes in D2 binding, do not differentiate mice that develop behavioral sensitization to ethanol from those that do not.

Ethanol potentiates the function of the human dopamine transporter expressed in Xenopus oocytes

Ethanol alters a variety of properties of brain dopaminergic neurons including firing rate, synthesis, release, and metabolism. Recent studies suggest that ethanol's action on central dopamine systems may also involve modulation of dopamine transporter (DAT) activity. The human DAT was expressed in Xenopus oocytes to examine directly the effects of ethanol on transporter function. [3H]Dopamine (100 nM) accumulation into DAT-expressing oocytes increased significantly in response to ethanol (10 min; 10-100 mM). In two-electrode voltage-clamp experiments, DAT-mediated currents were also enhanced significantly by ethanol (10-100 mM). The magnitude of the ethanol-induced potentiation of DAT function depended on ethanol exposure time and substrate concentration. Cell surface DAT binding ([3H]WIN 35,428; 4 nM) also increased as a function of ethanol exposure time. Thus, the increase in dopamine uptake was associated with a parallel increase in the number of DAT molecules expressed at the cell surface. These experiments demonstrate that DAT-mediated substrate translocation and substrate-associated ionic conductances are sensitive to intoxicating concentrations of ethanol and suggest that DAT may represent an important site of action for ethanol's effects on central dopaminergic transmission. A potential mechanism by which ethanol acts to enhance DAT function may involve regulation of DAT expression on the cell surface.

Dopamine receptor gene expression in an animal model of 'behavioral dependence' on ethanol

The steady-state levels of messenger RNA (mRNA) of five cloned dopamine (D) receptors were measured in five brain regions in rats in a recently developed animal model of 'behavioral dependence' on ethanol. One group of rats was given the choice between ethanol and water over a 9-month period and developed 'behavioral dependence' on ethanol (group a). This group was compared with a group given the choice between ethanol and water for only 2 months (not yet behaviorally dependent, group b), a group forced to consume ethanol as sole fluid over a 9-month period (not behaviorally dependent, group c) and ethanol-naive control rats. All groups were sacrificed 1 month after ethanol withdrawal. The concentrations of mRNA of D3-receptors in the limbic forebrain (which included the nucleus accumbens) were significantly lowered in groups a and b, but unchanged in group c. D3 mRNA levels were reduced in the hippocampus of group b and unchanged in the cortex, amygdala and striatum. No significant changes in the mRNA concentrations of D1-, D2-, D4- or D5-receptors were seen in the five brain regions in any group. In conclusion, chronic consumption of ethanol under the 'free-choice condition', which may best induce the drug-rewarding effect, leads to specific changes in the D3-receptor gene expression which were not seen after forced ethanol administration. Changes in D3 mRNA levels were, however, not a specific correlate of 'behavioral dependence', as they were also detected in rats not yet 'behaviorally dependent' (group b).

Effects of chronic ethanol treatment and ethanol withdrawal on [3H]SCH23390 binding to rat striatal membranes

The effects of chronic ethanol administration and ethanol withdrawal on the kinetic and pharmacological properties of [3H]SCH23390 binding sites and the labelling of central dopamine D-1 receptors were studied in the striatum of the rat. Chronic 40 day ethanol treatment produced a statistically significant decrease (p < 0.05) in maximum binding (Bmax) on striatal dopamine D-1 receptors of the rat, KD remaining unaltered. The withdrawal of ethanol did not affect the kinetic binding parameters. The rank order of potency in displacing the specific [3H]SCH23390 binding of several dopamine antagonists, agonists and serotonin-related drugs was consistent with the pharmacological profile of dopamine D-1 receptors. Chronic ethanol treatment led to a statistically significant increase in receptor affinity (lower Ki than controls) for (+)-butaclamol (p < 0.05). Ethanol withdrawal for 24 hr increased the affinity of [3H]SCH23390-labeled binding sites for norepinephrine. The addition of 0.03-0.68 M ethanol in vitro had no significant effects on [3H]SCH23390 binding in striatal preparations taken from both control and ethanol-treated rats. The results show that rat striatal [3H]SCH23390-labelled binding sites are affected by different conditions of ethanol exposure, possibly suggesting the medication of striatal dopamine pathways in the responses to ethanol intoxication.

Effects of a range of dopamine receptor agonists and antagonists on ethanol intake in the rat

The aim of this study was to assess the effects of a range of dopaminergic agents on consumption of an ethanol solution (10% ethanol, 3% glucose) in rats. A two-bottle, free-choice paradigm was used following induction of ethanol consumption and preference in standard laboratory rats. The model used provides a robust and reliable level of ethanol oral administration in normal laboratory rats. Both ethanol intake and preference were reduced by a dopamine D1 receptor partial agonist, SFK 38393 ((+/-)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol hydrochloride), in a dose-dependent manner. The dopamine D2/D3 receptor agonist 7-OH-DPAT ((+/-)-7-hydroxy-N,N-(di-n-propyl-2-aminotetralin)) at the lowest dose of 0.01 mg/kg increased both ethanol intake and preference. At higher doses (0.03-0.1 mg/kg) no significant effects were found. The dopamine D1 receptor antagonist SCH 23390 (R-(+)-7-chloro-2,3,4,5-tetrahydro-3-methyl-1-phenyl-1H-3-benzazepine-8- ol), dopamine D2/D3 receptor antagonist raclopride and 5-HT2/D2 receptor antagonist risperidone did not affect ethanol consumption, although all at high doses induced a significant decrease in water intake, indicating a non-specific decrease in consummatory behavior with these compounds. These results suggest the involvement of the dopaminergic system in ethanol intake and ethanol reinforcement with dopamine D1 and D2/D3 receptors playing opposing roles. Blockade of dopamine D2 receptors had no selective effect on ethanol consumption and ethanol preference.

Effects of chronic alcohol consumption and aging on dopamine D2 receptors in Fischer 344 rats

Aging and chronic alcohol consumption are each accompanied by significant changes in dopamine and dopamine receptors. This study extended previous work by investigating the combined effects of chronic alcoholism and aging on total dopamine D2 receptors in brain areas associated with the nigrostriatal and mesocorticolimbic systems. In addition, the effects of chronic alcohol consumption and aging on the high-affinity state of D2 receptors and their conversion to the low-affinity form is included. Quantitative autoradiography was used to assess [3H]spiperone-labeled D2 receptors in tissue sections from 5- to 14- and 24-month Fischer 344 rats that were pair-fed a control or 6.6% (v/v) ethanol-containing liquid diet for 6 weeks. In addition, D2 receptors were determined in rats given the control liquid diet ad libitum. The results of these experiments demonstrated age-related changes in the nigrostriatal system. There was an age-related loss of total dopamine D2 receptors in the rostral and caudal striatum (approximately 25% decrease in Bmax). This decline in D2 receptors may be associated with changes in motor function. Despite the age-related decline in D2 receptors, there were no significant differences in the proportion of striatal receptors in the high-affinity form or in their conversion to the low-affinity state. Both aging and chronic alcohol consumption produced significant changes in the concentration of D2 receptors in brain areas associated with the mesocorticolimbic system. That is, the specific binding of [3H]spiperone was decreased in the frontal cortex of aged rats. In addition, chronic alcoholism was associated with a significant increase (approximately 20%) in the Bmax for D2 receptors in the nucleus accumbens. Nonetheless, neither age nor chronic alcohol consumption altered the proportion of high-affinity D2 receptors in the nucleus accumbens or their conversion to the lower affinity state. The observed changes in D2 receptors in the frontal cortex and nucleus accumbens are of interest because of the involvement of the mesocorticolimbic dopamine areas in the rewarding properties of alcohol and other drugs of abuse. Although aging and chronic alcoholism both produced significant changes in dopamine D2 receptor concentrations, alcohol did not accentuate the age-related loss of D2 receptors. We cannot eliminate the possibility that a more prolonged exposure of higher ethanol dose may potentiate age-related changes in the dopaminergic system.

Effects of chronic alcohol consumption and aging on dopamine D1 receptors in Fischer 344 rats

The present study examined the hypothesis that chronic alcoholism augments the age-related loss of dopamine D1 receptors. This hypothesis was investigated because previous studies reported that both aging and chronic alcoholism produce significant changes in dopaminergic systems, and because chronic alcoholism potentiates some age-related CNS losses. In addition, this study investigated the effects of aging on D1 receptors in animals 1 and 7 days after withdrawal from chronic ethanol. Quantitative autoradiography was used to measure [3H]SCH 23390 binding to D1 receptors in brain areas associated with both the nigrostriatal and mesocorticolimbic dopamine systems. Receptors were assessed in 5-, 14-, and 24-month-old male Fischer 344 rats that were pair-fed a control or 6.6% (v/v) ethanol-containing liquid diet for 6 weeks. The results of these studies demonstrated that aging is associated with a significant decline in D1 receptors in the rostral and caudal striatum, and substantia nigra of both control and ethanol-fed rats. These receptor changes in the nigrostriatal system may be associated with motor abnormalities. In addition, there was an age-related decline in D1 receptors in two brain areas of the mesocorticolimbic system: the nucleus accumbens and frontal cortex. The latter findings may be important because of the involvement of this system with the rewarding properties of ethanol and other drugs of abuse. There were no age-related differences in the response of D1 receptors to ethanol withdrawal in the caudal and rostral striatum, substantia nigra, and nucleus accumbens.(ABSTRACT TRUNCATED AT 250 WORDS)