Reduction of voluntary alcohol intake in the rat by modulation of the dopaminergic mesolimbic system: transplantation of ventral mesencephalic cell suspensions

The dopamine system and alcohol dependence

Alcohol dependence is a common mental disorder that is associated with substantial disease burden. Current efforts at prevention and treatment of alcohol dependence are of very limited effectiveness. A better understanding of the biological mechanisms underlying dependence is essential to improving the outcomes of treatment and prevention initiatives. To date, most of the efforts have focused on the key role of the dopamine system in the complex etiological network of alcohol dependence. This review summarizes current research about the relationships between alcohol consumption and the dopaminergic system. We find that many of the currently available studies have contradictory results, presumably due to differences in methodology, non-linear dosage effects, use of different samples, and the possible confounding effects of other neurotransmitter systems.

Neuronal nicotinic acetylcholine receptors: common molecular substrates of nicotine and alcohol dependence

Alcohol and nicotine are often co-abused. As many as 80-95% of alcoholics are also smokers, suggesting that ethanol and nicotine, the primary addictive component of tobacco smoke, may functionally interact in the central nervous system and/or share a common mechanism of action. While nicotine initiates dependence by binding to and activating neuronal nicotinic acetylcholine receptors (nAChRs), ligand-gated cation channels normally activated by endogenous acetylcholine (ACh), ethanol is much less specific with the ability to modulate multiple gene products including those encoding voltage-gated ion channels, and excitatory/inhibitory neurotransmitter receptors. However, emerging data indicate that ethanol interacts with nAChRs, both directly and indirectly, in the mesocorticolimbic dopaminergic (DAergic) reward circuitry to affect brain reward systems. Like nicotine, ethanol activates DAergic neurons of the ventral tegmental area (VTA) which project to the nucleus accumbens (NAc). Blockade of VTA nAChRs reduces ethanol-mediated activation of DAergic neurons, NAc DA release, consumption, and operant responding for ethanol in rodents. Thus, ethanol may increase ACh release into the VTA driving activation of DAergic neurons through nAChRs. In addition, ethanol potentiates distinct nAChR subtype responses to ACh and nicotine in vitro and in DAergic neurons. The smoking cessation therapeutic and nAChR partial agonist, varenicline, reduces alcohol consumption in heavy drinking smokers and rodent models of alcohol consumption. Finally, single nucleotide polymorphisms in nAChR subunit genes are associated with alcohol dependence phenotypes and smoking behaviors in human populations. Together, results from pre-clinical, clinical, and genetic studies indicate that nAChRs may have an inherent role in the abusive properties of ethanol, as well as in nicotine and alcohol co-dependence.

Serum creatine kinase activity differentiates alcohol syndromes of dependence, withdrawal and delirium tremens

Previous reports described significant differences in serum creatine kinase (CK) activity in bipolar disorder and various forms of depression. The comorbidity of depression and alcohol syndromes was also widely described. We aim to examine potential differences in serum CK level in different alcohol-related syndromes. We assessed morning serum CK activity in 114 inpatients, diagnosed by the Structured Clinical Interview for DSM-IV: Fifty-five subjects with alcohol dependence, 28 with alcohol withdrawal and 31 with delirium tremens (DT's). We found low normal CK activity for the alcohol dependence, higher for alcohol withdrawal and the highest for DT's. Peripheral CK activity of four patients that were admitted during each of the three phases showed similar pattern. These findings may be related to enhanced dopamine activity in alcohol dependence and conversely, to a significant decrease in dopamine activity during withdrawal syndromes. We suggest a supplementary simple laboratory tool for the detection of alcohol-related states.

Differences in ethanol ingestion between cholecystokinin-A receptor deficient and -B receptor deficient mice

AIMS

Cholecystokinin (CCK) modulates dopamine release in the nucleus accumbens through the CCK-A receptor (CCK-AR). The dopaminergic neurotransmission between the ventral tegmental area and the limbic forebrain is a critical neurobiological component of alcohol and drug self-administration. Based on the evidence of interaction between CCK and dopamine, we had found previously that the CCK-AR gene -81A/G polymorphism was associated with alcohol dependence. Since the precise mechanism underlying this association has not been elucidated, the role of CCK-AR in ethanol ingestion was examined using CCK-AR gene deficient (-/-) mice and compared with those of CCK-BR(-/-) and wild-type mice.

METHODS

The two-bottle choice protocol was conducted and the righting reflex was examined in these three genotypes. Furthermore, the protein level of dopamine 2 receptor (D2R) in the nucleus accumbens was determined by western blotting.

RESULTS

CCK-AR(-/-) mice consumed more ethanol than CCK-BR(-/-) and wild-type mice, and showed no aversion to high concentrations of ethanol solution. However, the difference was actually in the total fluid consumption and alcohol preference remained unchanged, indicating that the differences were not specific to alcohol. Behavioral sensitivity to ethanol, examined using the righting reflex, did not differ significantly between the groups. D2R expression in the nucleus accumbens was significantly lower in the CCK-BR(-/-) mice and was significantly higher in CCK-AR(-/-) mice than in wild-type mice.

CONCLUSIONS

Voluntary ingestion of ethanol differed between CCK-AR(-/-) and CCK-BR(-/-) mice. The difference might be attributable in part to the different levels of D2R expression in the nucleus accumbens.

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.

Dopamine D1 and D2 receptor Co-activation generates a novel phospholipase C-mediated calcium signal

Although dopamine D1 and D2 receptors belong to distinct subfamilies of dopamine receptors, several lines of evidence indicate that they are functionally linked. However, a mechanism for this linkage has not been elucidated. In this study, we demonstrate that agonist stimulation of co-expressed D1 and D2 receptors resulted in an increase of intracellular calcium levels via a signaling pathway not activated by either receptor alone or when only one of the co-expressed receptors was activated by a selective agonist. Calcium signaling by D1-D2 receptor co-activation was abolished following treatment with a phospholipase C inhibitor but not with pertussis toxin or inhibitors of protein kinase A or protein kinase C, indicating coupling to the G(q) pathway. We also show, by co-immunoprecipitation from rat brain and from cells co-expressing the receptors, that D1 and D2 receptors are part of the same heteromeric protein complex and, by immunohistochemistry, that these receptors are co-expressed and co-localized within neurons of human and rat brain. This demonstration that D1 and D2 receptors have a novel cellular function when co-activated in the same cell represents a significant step toward elucidating the mechanism of the functional link observed between these two receptors in brain.

Nicotine-induced fos expression in the pedunculopontine mesencephalic tegmentum in the rat

The aim of this study was to assess the effects of a single dose of nicotine (NIC, 0.3 or 1.0 mg/kg, s.c.), after survival times of 30, 60 or 120 min, on immediate early gene expression in the pedunculopontine mesencephalic tegmentum (PMT), using Fos-immunocytochemistry. Either doses of NIC strongly increased Fos-immunoreactivity in both the pedunculopontine tegmental nucleus (PPTg) and the laterodorsal tegmental nucleus (LDTg), as compared to the saline controls, at 30 min and 60 min. In comparison, the effects of NIC-induced Fos expression in the caudate-putamen (CP) were not as strong as the ones observed in the PPTg and LDTg. In fact, at 30 min the 0.3 mg/kg dose of NIC did not induce Fos-expression, unlike the PPTg and LDTg. The CP response was more noticeable in the mediodorsal than in the laterodorsal region. Double-labelling studies using Fos-immunoreactivity and NADPH-diaphorase histochemistry for cholinergic cells in the PPTg and LDTg revealed that, in general, cholinergic neurons had Fos negative nuclei, although double-labelled neurons were occasionally seen in the PPTg. In conclusion, systemically administered NIC activates the neuronal population of the PPTg and the LDTg possibly by directly targeting nicotinic receptors that may be located in non-cholinergic neurons. We postulate that activation of these non-cholinergic neurons modulates the activity of cholinergic cells in the PMT, which in turn may alter dopamine release in the mesolimbic system.

The pedunculopontine tegmental nucleus and the role of cholinergic neurons in nicotine self-administration in the rat: a correlative neuroanatomical and behavioral study

The objective of this study was to determine whether the pedunculopontine tegmental nucleus plays a role in the maintenance of nicotine self-administration, and whether the ascending cholinergic projection from this nucleus to midbrain dopamine neurons in the ventral tegmental area might be involved. Studies were done with rats trained to self-administer nicotine intravenously. Self-administration was examined before and after the pedunculopontine tegmental nucleus was lesioned with the ethylcholine mustard aziridinium ion, a selective cholinergic toxin. Lesions were assessed qualitatively and quantitatively in histological sections stained for either nicotinamide adenine dinucleotide phosphate-diaphorase histochemistry to identify cholinergic neurons, or for Nissl. Self-administration was also tested after an acute manipulation in which microinfusions of the nicotinic cholinergic antagonist dihydro-beta-erythroidine were made into the pedunculopontine tegmentum. Infusions of neurotoxin into the pedunculopontine tegmentum reduced nicotine self-administration behaviour when tested weeks later. Toxin treatment reduced the number of cholinergic neurons in the tegmentum, while largely sparing the non-cholinergic population in this area. Lesions were limited to the pedunculopontine area and did not extend to the neighboring laterodorsal tegmental nucleus or to the substantia nigra. Acute manipulation of the pedunculopontine tegmental nucleus with microinfusions of dihydro-beta-erythroidine also produced an attenuation of nicotine self-administration. Collectively these data show that the pedunculopontine tegmental nucleus is part of the neuronal circuitry mediating nicotine self-administration, and that the population of cholinergic neurons is likely a critical element.

Alcohol preference and sensitivity are markedly reduced in mice lacking dopamine D2 receptors

Although dopaminergic transmission has been strongly implicated in alcohol self-administration, the involvement of specific dopamine receptor subtypes has not been well established. We studied the ethanol preference and sensitivity of D2-receptor-deficient mice to directly evaluate whether dopamine D2 receptors contribute to alcohol (ethanol) consumption. We report a marked aversion to ethanol in these mice, relative to the high preference and consumption exhibited by wild-type littermates. Sensitivity to ethanol-induced locomotor impairment was also reduced in these mutant mice, although they showed a normal locomotor depressant response to the dopamine D1 antagonist SCH-23390. These data demonstrate that dopamine signaling via D2 receptors is an essential component of the molecular pathway determining ethanol self-administration and sensitivity.

Cholecystokinergic innervation of nucleus accumbens subregions

Behavioral and pharmacological evidence has shown a different and opposite role of the neuropeptide cholecystokinin (CCK) on the dopamine (DA) function in the caudal versus rostral part of the nucleus accumbens. Previous reports have speculated that the caudal region of the nucleus accumbens would receive CCKergic innervation from dopaminergic neurons of the mesencephalic ventral tegmental area, whereas the CCKergic input to the rostral accumbens would originate in non-dopaminergic neurons from extra-mesencephalic areas of the brain. In the present study, this issue was addressed using retrograde tracing techniques in conjunction with immunocytochemistry. Retrograde tracers were injected in the three compartments of the accumbens (i.e., rostral pole, core and septal shell). In summary, our results demonstrate that 1) the main CCKergic input of the accumbens originates in the ventral mesencephalon; 2) the rostral pole is equally innervated by CCK neurons projecting from both substantia nigra pars compacta and ventral tegmental area; 3) the primary source of CCK innervation of the accumbal core is the substantia nigra pars compacta; and 4) whereas the CCKergic input to the septal shell originates primarily in the ventral tegmental area. Additionally, our results also showed that most of the CCKergic neurons projecting to any of the accumbal compartments also produce dopamine. These data constitute the first neuroanatomical evidence for the differential effects of CCK on dopamine actions in the different regions of the nucleus accumbens.

Focal ethanol elevates extracellular dopamine and serotonin concentrations in the rat ventral tegmental area

The present study describes the effects of local perfusion with ethanol on extracellular dopamine and serotonin (5-hydroxytryptamine, 5-HT) concentrations in the ventral tegmental area. Various concentrations of ethanol in artificial cerebrospinal fluid (ACSF) (0.1-10%, v/v) were administered through a microdialysis probe into the ventral tegmental area of freely moving Sprague-Dawley rats. A significant and concentration-dependent increment in dialysate output of both dopamine and serotonin was observed after local infusion of ethanol. Perfusion with Ca(2+)-free medium or tetrodotoxin (1 microM in ACSF) produced a significant reduction in basal extracellular dopamine and serotonin concentration but failed to block dopamine or serotonin release produced by infusion of 10% ethanol. Perfusion with 100 mM K+ before and after infusion of 10% ethanol revealed that the second perfusion with high K+ solution still produced an increase in dopamine and serotonin concentration, similar in magnitude to the first response, indicating that perfusion with 10% ethanol did not cause irreversible damage to either dopamine cell bodies or serotonin terminals in the ventral tegmental area. These results suggest that dopamine and serotonin release from the ventral tegmental area produced by focal application of 10% ethanol is mediated, at least in part, by a non-exocytotic mechanism. Direct stimulation of the ventral tegmental area dopamine neurons by ethanol might be involved in the reinforcing properties of the drug.