Ethanol increases the firing rate of dopamine neurons of the rat ventral tegmental area in vitro

Ethanol-Induced Regulation of GABAA Subunit mRNAs in Prefrontal Fields of Cynomolgus Monkeys

BACKGROUND

Recent evidence indicates that functional impairment of the orbital and medial fields of the prefrontal cortex may underlie the deficits in executive control of behavior that characterize addictive disorders, including alcohol addiction. Moreover, previous studies have indicated that alcohol alters GABA neurotransmission and one substrate of these effects may be through the reconfiguration of the subunits constituting the GABA(A) receptor complex. Given that GABAergic transmission has an integral role in cortical processing, influencing local and interregional communication, understanding alcohol-induced alterations in GABA(A) receptors in prefrontal fields of the primate brain may provide insight into the functional impairment of these brain regions in the alcohol-addicted state and extend our understanding of the molecular consequences of long-term use in these critical brain regions.

METHODS AND RESULTS

To address this problem, the effects of chronic ethanol self-administration in male cynomolgus monkeys on GABA(A) receptor subunit mRNA expression was studied in 3 frontal cortical fields: orbitofrontal cortex (OFC; area 13), anterior cingulate cortex (ACC; area 24), and the dorsolateral prefrontal cortex (DLPFC; area 46). Quantitative polymerase chain reaction revealed significant alterations in GABA(A) subunit mRNA expression in the OFC and DLPFC but not in the ACC. Specifically, expression of the alpha2, alpha4, beta1, beta3, and gamma1 to gamma3 subunit mRNAs was significantly less in the OFC, whereas the expression of beta1, beta2, gamma1, and delta subunit mRNAs was less in the DLPFC of alcohol-treated monkeys.

CONCLUSION

These findings suggest that ethanol-induced alterations in GABA(A) function may be due to alterations in GABA(A) subunit mRNA levels and subunit-specific alterations are selective to particular cortical fields.

Characterization of ethanol-induced dopamine elevation in the rat nucleus accumbens

Ethanol-induced accumbal dopamine elevations have been linked to ethanol consumption. It is unclear, however, where along the mesolimbic dopamine system this effect is initiated and why the ethanol-induced dopamine elevations are transient, returning to pre-drug baseline before brain and blood ethanol levels decline. Using in vivo microdialysis, Experiment 1 investigated the effect of local ethanol application in the nucleus accumbens, the ventral tegmental area and the nucleus accumbens+the ventral tegmental area, on accumbal dopamine. Experiment 2 examined whether the rapid withdrawal of dopamine response to ethanol involves activation of GABA(A)-receptors, by analyzing the effect of accumbal co-perfusion of picrotoxin and ethanol. In Experiment 1, ethanol perfusion into the ventral tegmental area alone did not affect accumbal dopamine. Ethanol co-perfusion of one of the tested doses into the ventral tegmental+the nucleus accumbens produced higher dopamine levels than ethanol perfusion into the nucleus accumbens alone during 120-160 min following perfusion onset. In Experiment 2, accumbal ethanol perfusion caused a transient increase in nucleus accumbens dopamine. Co-perfusion of ethanol and picrotoxin produced a sustained dopamine elevation. These data support the hypothesis that the primary effect of ethanol on accumbal dopamine is in the nucleus accumbens, but that a secondary effect of nucleus accumbens ethanol perfusion, such as release of acetylcholine in the ventral tegmental area, enables ethanol to act as a nicotinic acetylcholine receptor co-agonist in this area. Moreover, recruitment of GABA(A)-receptor activity appears responsible for the second, declining phase with respect to dopamine levels following ethanol administration.

Glutamate-dopamine cotransmission and reward processing in addiction

While Dale's principle of "one neuron, one neurotransmitter" has undergone revisions to incorporate evidence of the corelease of atypical neurotransmitters such as neuropeptides, the corelease of classical neurotransmitters has only recently been realized. Surprisingly, numerous studies now indicate that the corelease of neurotransmitters in the mammalian central nervous system is not an obscure and rare phenomenon but is widespread and involves most classical neurotransmitters systems. However, the suggestion that glutamate can be coreleased with dopamine (DA) has remained controversial. Furthermore, glutamate-DA cotransmission has not yet been seriously considered in the context of the neurocircuitry of addiction. If glutamate is in fact coreleased with DA as some evidence now suggests, this may have significant implications for advancing our understanding of the interactive role that these 2 neurotransmitters play in cognitive and reward processes. In this commentary, we review the evidence for and against glutamate as a cotransmitter and discuss the potential role of glutamate-DA corelease in addiction. In particular, we describe a recently proposed model in which coreleased glutamate transmits a temporally precise prediction error signal of reward described by Schultz et al., whereas the function of coreleased DA is to exert prolonged modulatory influences on neuronal activity. In addition, we suggest that as alcohol consumption transitions from recreational use to addiction, there is a corresponding transition in the reward valence signal from better than predicted to worse than predicted.

Chronic ethanol consumption increases dopamine uptake in the nucleus accumbens of high alcohol drinking rats

Past research has indicated that chronic ethanol exposure enhances dopamine (DA) neurotransmission in several brain regions. The present study examined the effects of chronic ethanol drinking on dopamine transporter (DAT) function in the nucleus accumbens (Acb) of High-Alcohol-Drinking replicate line 1 (HAD-1) rats. HAD rats were given concurrent 24-h access to 15% ethanol and water or water alone for 8 weeks. Subsequently, DA uptake and the V(max) of the DAT were compared between the two groups using homogenates of the nucleus accumbens. DA uptake was measured following a 2 min incubation at 37 degrees C in the presence of 8 nM [(3)H]DA. For kinetic analyses, DA uptake was assessed in the presence of 5 concentrations of [(3)H]DA ranging from 8 nM to 500 nM. Analyses of the data revealed a significant increase in DA uptake in the ethanol group compared to water controls. Kinetic analyses revealed the change in DA uptake to be a consequence of an increase in the V(max) of transport. These findings demonstrate that chronic free-choice oral ethanol consumption in HAD-1 female rats increases DA uptake in the Acb by increasing the V(max) of the transporter. However, it is not known whether the ethanol-induced change in V(max) is caused by differences in the actual number of available transporter sites or from a difference in the velocity of operation of a similar number of transporters. Overall, the data indicate that chronic ethanol consumption by HAD-1 rats produces prolonged neuroadaptations within the mesolimbic DA system, which may be important for the understanding of the neurobiological basis of alcoholism.

No role of the dopamine transporter in acute ethanol effects on striatal dopamine dynamics

The acute effects of ethanol on dopamine (DA) release and clearance in the caudate-putamen were evaluated in wild-type and dopamine transporter (DAT) knockout (DAT-KO) mice, using microdialysis and voltammetry. Dialysate DA levels were elevated, approximately 80% above baseline levels, after administration of 2 g/kg ethanol in both wild-type and DAT-KO mice. In brain slices containing the caudate-putamen, a low (20 mM) concentration of ethanol produced no change in electrically stimulated DA release in either wild-type or DAT-KO mice. A high concentration (200 mM) of ethanol caused a similar decrease in DA release in slices from both types of mice. DA clearance was unaltered across the genotypes at low and high concentrations of ethanol. The fact that ethanol had similar effects in wild-type and DAT-KO mice, measured by in vivo microdialysis or brain slice voltammetry, supports the idea that acute ethanol does not interact with the DAT to produce its effects on the DA system.

Ethanol inhibition of m-current and ethanol-induced direct excitation of ventral tegmental area dopamine neurons

Ethanol-induced excitation of ventral tegmental area dopamine (DA VTA) neurons is thought to be critical for the reinforcing effects of ethanol. Although ligand-gated ion channels are known to be the targets of ethanol, ethanol modulation of voltage-dependent ion channels of central neurons has not been well studied. We have demonstrated that ethanol excites DA VTA neurons by the reduction of sustained K(+) currents and recently reported that M-current (I(M)) regulates action potential generation through fast and slow afterhyperpolarization phases. In the present study we thus examined whether ethanol inhibition of I(M) contributes to the excitation of DA VTA neurons using nystatin-perforated patch current- and voltage-clamp recordings. Ethanol (20-120 mM) reduced I(M) in a concentration-dependent manner and increased the spontaneous firing frequency of DA VTA neurons. Ethanol-induced increase in spontaneous firing frequency correlated positively with ethanol inhibition of I(M) with a slope value of 1.3. Specific I(M) inhibition by XE991 (0.3-10 microM) increased spontaneous firing frequency which correlated positively with I(M) inhibition with a slope value of 0.5. In the presence of 10 muM XE991, a concentration that produced maximal inhibition of I(M), ethanol still increased the spontaneous firing frequency of DA VTA neurons in a concentration-dependent manner. Thus we conclude that, although ethanol causes inhibition of I(M) and this results in some increase in the firing frequency of DA VTA neurons, another effect of ethanol is primarily responsible for the ethanol-induced increase in firing rate in these neurons.

The effects of long chain-length n-alcohols on the firing frequency of dopaminergic neurons of the ventral tegmental area

The dopaminergic neurons of the ventral tegmental area (DA VTA neurons) have been implicated in the reinforcing properties of drugs of abuse, including ethanol (ethyl alcohol). Ethanol increases the spontaneous firing frequency of DA VTA neurons in vitro, in both brain slices and acutely dissociated neurons, and also in vivo. In many systems, longer n-alkyl alcohols have a more potent effect than ethanol, and the potency is a function of the number of carbons in the alkyl chain. We studied n-alcohols of chain length 1 (methanol) to 5 (pentanol) on the firing rate of DA VTA neurons in brain slice preparations. All of the alcohols studied produced increases in the spontaneous firing frequency in DA VTA neurons; as the chain length increased, lower concentrations of the alcohols were needed to produce the same percentage increase in firing. With very high concentrations of all the alcohols except methanol, we observed apparent depolarization block of firing. In addition, trichloroethanol (TCE), the active metabolite of chloral hydrate, increased the firing frequency of DA VTA neurons, and the EC(40) (concentration to produce a 40% increase in firing rate) of TCE was below that of ethanol. These studies indicate that excitation of VTA dopamine neurons by n-alcohols is related to the chain length of the carbons. This is likely to be a characteristic of the ethanol-sensitive element of DA VTA neurons and may be useful in identifying the element of the membrane that is responsible for ethanol-induced excitation.

Neuropsychology and neuropharmacology of P3a and P3b

Perspectives on the P300 event-related brain potential (ERP) are reviewed by outlining the distinction between the P3a and P3b subcomponents. The critical factor for eliciting P3a is how target/standard discrimination difficulty rather than novelty modulates task processing. The neural loci of P3a and P3b generation are sketched and a theoretical model is developed. P3a originates from stimulus-driven disruption of frontal attention engagement during task processing. P3b originates when temporal-parietal mechanisms process the stimulus information for memory storage. The neuropharmacological implications of this view are then outlined by evaluating how acute and chronic use of ethanol, marijuana, and nicotine affect P3a and P3b. The findings suggest that the circuit underlying ERP generation is influenced in a different ways for acute intake and varies between chronic use levels across drugs. Theoretical implications are assessed.

Genetic polymorphisms of alcohol and aldehyde dehydrogenase, dopamine and serotonin transporters in familial and non-familial alcoholism

One hundred and eleven male patients with alcohol dependence and 123 nonalcoholic healthy men were tested for the genetic polymorphisms of alcohol dehydrogenase 2 (ADH2), aldehyde dehydrogenase 2 (ALDH2), serotonin transporter (5-HTT) and dopamine transporter (DAT1). There were significant differences in genotype frequencies of ADH2 C992G and A13543G SNPs between alcoholic patients with family history of alcohol dependence (familial) and alcoholic patients without family history (non-familial). Genotype and allele frequencies of ALDH2 G1951A SNP in familial or non-familial alcoholic patients differ from normal controls. Neither 5-HTTLPR L/S nor DAT1 G2319A SNP genotypes nor alleles discriminated alcoholic patients from normal controls. These findings suggest that the genetic characteristics of alcohol metabolism in non-familial alcoholics fall between non-alcoholism and familial alcoholics.

Is ethanol a pro-drug? Acetaldehyde contribution to brain ethanol effects

This article presents the proceedings of a symposium at the 2004 meeting of the International Society for Biomedical Research on Alcoholism, held in Mannheim, Germany. The symposium was organized by Etienne Quertemont and chaired by C. J. Peter Eriksson. The presentations were (1) Brain ethanol metabolism and its behavior consequences, by Sergey M. Zimatkin and P. S. Pronko; (2) Acetaldehyde increases dopaminergic neuronal activity: a possible mechanism for acetaldehyde reinforcing effects, by Marco Diana and Milena Pisano; (3) Contrasting the reinforcing actions of acetaldehyde and ethanol within the ventral tegmental area (VTA) of alcohol-preferring (P) rats, by Zachary A. Rodd and Richard R. Bell; (4) Molecular and biochemical changes associated with acetaldehyde in human alcoholism and alcohol abuse, by C. J. Peter Eriksson.

Involvement of the endogenous cannabinoid system in the effects of alcohol in the mesolimbic reward circuit: electrophysiological evidence in vivo

RATIONALE

Several lines of evidence indicate that the endogenous cannabinoid system is involved in the pharmacological and behavioural effects of alcohol. The mesolimbic dopaminergic (DA) system and the nucleus accumbens (NAc) process rewarding properties of drugs of abuse, including alcohol and cannabinoids, whereas endocannabinoids in these regions modulate synaptic function and mediate short- and long-term forms of synaptic plasticity.

OBJECTIVES

The present study was designed to investigate the contribution of the endogenous cannabinoid system in alcohol electrophysiological effects in the mesolimbic reward circuit.

METHODS

We utilized extracellular single cell recordings from ventral tegmental area (VTA) DA and NAc neurons in anesthetized rats. DA neurons were antidromically identified as projecting to the shell of NAc, whereas NAc putative medium spiny neurons were identified by their evoked responses to basolateral amygdala (BLA) stimulation.

RESULTS

Alcohol stimulated firing rate of VTA DA neurons and inhibited BLA-evoked NAc neuron spiking responses. The cannabinoid type-1 receptor (CB1) antagonist rimonabant (SR141716A) fully antagonized alcohol effect in both regions. In the NAc, either inhibition of the major catabolic enzyme of the endocannabinoid anandamide, the fatty-acid amyd hydrolase, with URB597 or a pretreatment with the CB1 receptor agonist WIN55212-2 significantly depressed alcohol-induced effects in the NAc.

CONCLUSIONS

These results corroborate the notion of the involvement of endocannabinoids and their receptors in the actions of alcohol and highlight the endocannabinoid system as a valuable target in the therapy for alcoholism.

Acute Alcohol Withdrawal is Associated with c-Fos Expression in the Basal Ganglia and Associated Circuitry: C57BL/6J and DBA/2J Inbred Mouse Strain Analyses

BACKGROUND

The DBA/2J (D2) and C57BL/6J (B6) mouse strains are the most widely studied genetic models of severe and mild acute alcohol withdrawal, respectively. Previous studies have identified quantitative trait loci and genes involved in risk for acute ethanol withdrawal using mapping populations derived from the D2 and B6 strains, but the brain region(s) and circuit(s) by which these genes and their protein products influence ethanol physiological dependence and associated withdrawal remain to be elucidated.

METHODS

B6 and D2 were administered a sedative-hypnotic dose of ethanol (4 g/kg) or saline (control) and returned to their home cages where they were left undisturbed for 7 hr, which has been shown in previous studies to correspond to peak acute ethanol withdrawal severity. The mice were then euthanized and assessed for their numbers of c-Fos immunoreactive neurons across 26 brain regions. The question addressed was whether or not ethanol-withdrawn D2 and B6 mice differed in c-Fos induction (neural activation) within circuitry that could explain the severe ethanol withdrawal of the D2 strain and the mild ethanol withdrawal in B6 strain mice.

RESULTS

At peak acute ethanol-withdrawal ethanol-withdrawn D2 and B6 mice differed in neural activation within the basal ganglia, including the subthalamic nucleus and the two major output nuclei of the basal ganglia (the medial globus pallidus and the substantia nigra pars reticulata). Genotype-dependent c-Fos induction was also apparent in associated circuitry including the lateral septum, the ventral tegmental area, the nucleus accumbens core, the dorsolateral caudate putamen, the substantia nigra pars compacta, the cingulate and entorhinal cortices, and the ventral pallidum. D2 and B6 mice showed comparable neural activation in the bed nucleus of the stria terminalis, and the nucleus accumbens shell.

CONCLUSIONS

The present studies are the first to use immediate early gene product expression to assess the pattern of neural activation associated with acute ethanol withdrawal. Our results point to the involvement of an extended basal ganglia circuit in genetically determined differences in acute ethanol withdrawal. Based on these data, we suggest that quantitative trait genes (QTGs) involved in acute ethanol withdrawal exert their effects on this phenotype via one or more of the brain regions and circuits identified. As more information becomes available that integrates neural circuit and QTG analyses, the precise mechanisms by which QTGs affect ethanol physiological dependence and associated withdrawal will become apparent.

Hyperpolarization-activated cation current (Ih) is an ethanol target in midbrain dopamine neurons of mice

Ethanol stimulates the firing activity of midbrain dopamine (DA) neurons, leading to enhanced dopaminergic transmission in the mesolimbic system. This effect is thought to underlie the behavioral reinforcement of alcohol intake. Ethanol has been shown to directly enhance the intrinsic pacemaker activity of DA neurons, yet the cellular mechanism mediating this excitation remains poorly understood. The hyperpolarization-activated cation current, Ih, is known to contribute to the pacemaker firing of DA neurons. To determine the role of Ih in ethanol excitation of DA neurons, we performed patch-clamp recordings in acutely prepared mouse midbrain slices. Superfusion of ethanol increased the spontaneous firing frequency of DA neurons in a reversible fashion. Treatment with ZD7288, a blocker of Ih, irreversibly depressed basal firing frequency and significantly attenuated the stimulatory effect of ethanol on firing. Furthermore, ethanol reversibly augmented Ih amplitude and accelerated its activation kinetics. This effect of ethanol was accompanied by a shift in the voltage dependence of Ih activation to more depolarized potentials and an increase in the maximum Ih conductance. Cyclic AMP mediated the depolarizing shift in Ih activation but not the increase in the maximum conductance. Finally, repeated ethanol treatment in vivo induced downregulation of Ih density in DA neurons and an accompanying reduction in the magnitude of ethanol stimulation of firing. These results suggest an important role of Ih in the reinforcing actions of ethanol and in the neuroadaptations underlying escalation of alcohol consumption associated with alcoholism.

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

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

Prolonged increase in the sensitivity of the posterior ventral tegmental area to the reinforcing effects of ethanol following repeated exposure to cycles of ethanol access and deprivation

The posterior ventral tegmental area (VTA) is a neuroanatomical substrate mediating the reinforcing effects of ethanol in rats. Repeated alcohol deprivations produce robust ethanol intakes of alcohol-preferring (P) rats during relapse and increase the reinforcing effects of oral alcohol self-administration. The objective of this study was to test the hypothesis that alcohol drinking and repeated alcohol deprivations will increase the reinforcing effects of ethanol within the posterior VTA of P rats. Groups of female P rats were used (alcohol-naive, continuous access, and repeatedly deprived). Each rat was implanted with a guide cannula aimed at the posterior VTA. Depression of the active lever produced the infusion of 100 nl of artificial cerebrospinal fluid (CSF) or ethanol (25-300 mg%). Each rat was given only one ethanol concentration during the 4-h sessions conducted every other day. Compared with the infusions of artificial CSF, the alcohol-naive group reliably self-infused 75 and 150 mg% ethanol, but not the lower or higher concentrations. On the other hand, the continuous access group had significantly higher self-infusions of 50, 75, 150, and 300 mg% ethanol compared with artificial CSF infusions. The repeatedly deprived group also self-infused significantly more of 50, 75, 150, and 300 mg% ethanol than artificial CSF; moreover, the number of infusions for all four concentrations was higher in the repeatedly deprived versus the continuous access group. Chronic alcohol drinking by P rats increased the reinforcing effects of ethanol within the posterior VTA, and repeated alcohol deprivations produced a further increase in these reinforcing effects of ethanol.

A conceptualization of integrated actions of ethanol contributing to its GABAmimetic profile: a commentary

Early behavioral investigations supported the contention that systemic ethanol displays a GABAmimetic profile. Microinjection of GABA agonists into brain and in vivo electrophysiological studies implicated a regionally specific action of ethanol on GABA function. While selectivity of ethanol to enhance the effect of GABA was initially attributed an effect on type-I-benzodiazepine (BZD)-GABA(A) receptors, a lack of ethanol's effect on GABA responsiveness from isolated neurons with this receptor subtype discounted this contention. Nonetheless, subsequent work identified GABA(A) receptor subtypes, with limited distribution in brain, sensitive to enhancement of GABA at relevant ethanol concentrations. In view of these data, it is hypothesized that the GABAmimetic profile for ethanol is due to activation of mechanisms associated with GABA function, distinct from a direct action on the majority of postsynaptic GABA(A) receptors. The primary action proposed to account for ethanol's regional specificity on GABA transmission is its ability to release GABA from some, but not all, presynaptic GABAergic terminals. As systemic administration of ethanol increases neuroactive steroids, which can enhance GABA responsiveness, this elevated level of neurosteroids is proposed to magnify the effect of GABA released by ethanol. Additional factors contributing to the degree to which ethanol interacts with GABA function include an involvement of GABA(B) and other receptors that influence ethanol-induced GABA release, an effect of phosphorylation on GABA responsiveness, and a regional reduction of glutamatergic tone. Thus, an integration of these consequences induced by ethanol is proposed to provide a logical basis for its in vivo GABAmimetic profile.

Voluntary ethanol intake increases extracellular acetylcholine levels in the ventral tegmental area in the rat

AIMS

Concurrent use of ethanol and nicotine (tobacco) is often seen in human beings. In previous animal experiments, we have demonstrated that nicotinic acetylcholine receptors, especially alpha-conotoxin MII and mecamylamine sensitive receptors located in the ventral tegmental area may be involved in the stimulatory, dopamine enhancing, and rewarding effects of ethanol in rodents. Ethanol may exert these effects via direct interaction with nicotinic acetylcholine receptors and/or indirectly via enhancement of extracellular acetylcholine levels in the ventral tegmental area. The present experiments investigated a possible indirect effect of ethanol in stimulating the mesoaccumbal dopamine system.

METHODS

Neurochemical effects of voluntary ethanol intake on extracellular ventral tegmental acetylcholine and accumbal dopamine levels were measured by means of in vivo microdialysis with a two-probe approach in freely moving rats.

RESULTS

Obtained data indicate that voluntary ethanol intake ( approximately 0.7 g/kg/h) leads to an increase of extracellular acetylcholine levels in the ventral tegmental area, and an almost time-locked increase of dopamine levels in the nucleus accumbens. A positive correlation between the ventral tegmental acetylcholine levels and ethanol intake as well as preference was also observed.

CONCLUSION

The present results suggest that voluntary ethanol intake enhances extracellular ventral tegmental acetylcholine that may interact with nicotinic acetylcholine receptors, possibly alpha-conotoxin MII sensitive receptors, localized in the ventral tegmental area that subsequently may stimulate dopamine overflow in the nucleus accumbens.

Chronic ethanol drinking by alcohol-preferring rats increases the sensitivity of the posterior ventral tegmental area to the reinforcing effects of ethanol

BACKGROUND

The ventral tegmental area (VTA) is involved in regulating ethanol drinking, and the posterior VTA seems to be a neuroanatomical substrate that mediates the reinforcing effects of ethanol in ethanol-naive Wistar and ethanol-naive alcohol-preferring (P) rats. The objective of this study was to test the hypothesis that chronic ethanol drinking increases the sensitivity of the posterior VTA to the reinforcing effects of ethanol.

METHODS

Two groups of female P rats (one given water as its sole source of fluid and the other given 24-hr free-choice access to 15% ethanol and water for at least 8 weeks) were stereotaxically implanted with guide cannulae aimed at the posterior VTA. One week after surgery, rats were placed in standard two-lever (active and inactive) operant chambers and connected to the microinfusion system. Depression of the active lever produced the infusion of 100 nl of artificial cerebrospinal fluid (CSF) or ethanol. The ethanol-naive and chronic ethanol-drinking groups were assigned to subgroups to receive artificial CSF or 25, 50, 75, or 125 mg/dl of ethanol (n = 6-9/dose/group) to self-infuse (FR1 schedule) during the 4-hr sessions given every other day.

RESULTS

Compared with the infusions of artificial CSF, the control group reliably (p < 0.05) self-infused 75 and 125 mg/dl of ethanol but not the lower concentrations. The ethanol-drinking group had significantly (p < 0.05) higher self-infusions of 50, 75, and 125 mg/dl of ethanol than artificial CSF during the four acquisition sessions; the number of infusions of all three doses was higher in the ethanol-drinking group than in the ethanol-naive group. Both groups decreased responding on the active lever when artificial CSF was substituted for ethanol, and both groups demonstrated robust reinstatement of responding on the active lever when ethanol was restored.

CONCLUSIONS

Chronic ethanol drinking by P rats increased the sensitivity of the posterior VTA to the reinforcing effects of ethanol.

Acute Ethanol Decreases Dopamine Transporter Velocity in Rat Striatum: In Vivo and In Vitro Electrochemical Measurements

BACKGROUND

Ethanol increases dopamine transporter (DAT) velocity when measured in cell expression systems, but its effects in vivo are mixed. The present experiments examined the effect of acute ethanol on dopamine transmission, particularly DAT velocity, in anesthetized animals as well as rat striatal suspensions.

METHODS

To determine the effect of acute ethanol on DAT function in vivo, we measured dopamine uptake in real time using fast-scan cyclic voltammetry and constant potential amperometry in the olfactory tubercle of anesthetized rats. Dopamine fibers were electrically stimulated, and the resulting transient dopamine signals were analyzed to describe the release and uptake kinetics. We also measured the effect of ethanol on DAT velocity in vitro in striatal tissue suspensions using rotating disk electrode voltammetry.

RESULTS

Ethanol (2.5 and 4 g/kg, intraperitoneally) decreased the electrically stimulated dopamine signal in the olfactory tubercle by 35-55%. The slope of the clearance curve of dopamine was 40% shallower after both doses of ethanol, indicating slower uptake. Modeling the data using Michaelis-Menten uptake kinetics showed that the slower uptake was due to a decrease in DAT V(max). These results were confirmed in vitro, because ethanol decreased the velocity of dopamine uptake by 35% in striatal tissue suspensions.

CONCLUSIONS

These results indicate that acute ethanol decreases DAT function in rat dorsal and ventral striatum in anesthetized rats and tissue suspensions, in contrast to its effects on human DAT expressed in single cells. Given the variety of molecular targets of ethanol in the brain, including the DAT itself, it is likely that several mechanisms converge to produce a net effect on DAT regulation and function that could very well be different in intact tissue versus single cells.

The tipsy terminal: presynaptic effects of ethanol

Considerable evidence suggests that the synapse is the most sensitive CNS element for ethanol effects. Although most alcohol research has focussed on the postsynaptic sites of ethanol action, especially regarding interactions with the glutamatergic and GABAergic receptors, few such studies have directly addressed the possible presynaptic loci of ethanol action, and even fewer describe effects on synaptic terminals. Nonetheless, there is burgeoning evidence that presynaptic terminals play a major role in ethanol effects. The methods used to verify such ethanol actions range from electrophysiological analysis of paired-pulse facilitation (PPF) and spontaneous and miniature synaptic potentials to direct recording of ion channel activity and transmitter/messenger release from acutely isolated synaptic terminals, and microscopic observation of vesicular release, with a focus predominantly on GABAergic, glutamatergic, and peptidergic synapses. The combined data suggest that acute ethanol administration can both increase and decrease the release of these transmitters from synaptic terminals, and more recent results suggest that prolonged or chronic ethanol treatment (CET) can also alter the function of presynaptic terminals. These new findings suggest that future analyses of synaptic effects of ethanol should attempt to ascertain the role of presynaptic terminals and their involvement in alcohol's behavioral actions. Other future directions should include an assessment of ethanol's effects on presynaptic signal transduction linkages and on the molecular machinery of transmitter release and exocytosis in general. Such studies could lead to the formulation of new treatment strategies for alcohol intoxication, alcohol abuse, and alcoholism.

Ethanol-induced conditioned place preference is expressed through a ventral tegmental area dependent mechanism

The authors examined the role of the ventral tegmental area (VTA) and nucleus accumbens (NAc) in the expression of ethanol-induced conditioned place preference (CPP). After cannulas were implanted, male DBA/2J mice underwent an unbiased Pavlovian-conditioning procedure for ethanol-induced CPP. Before preference testing, the mice were injected intra-VTA (Experiments 1 and 3) or intra-NAc (Experiment 2) with the nonselective opioid antagonist methylnaloxonium (0-ng, 375-ng, or 750-ng total infusion; Experiments 1 and 2) or the gamma aminobutyric acid (GABA(B)) agonist baclofen (0-ng, 25-ng, or 50-ng total infusion; Experiment 3). Intra-VTA methylnaloxonium or baclofen decreased ethanol-induced CPP, whereas intra-NAc methylnaloxonium had no effect. These findings indicate that the conditioned rewarding effect of ethanol is expressed through a VTA-dependent mechanism that involves both opioid and GABA(B) receptors.

The effect of baclofen alone and in combination with naltrexone on ethanol consumption in the rat

Naltrexone has been evaluated in preclinical animal models of ethanol consumption and found to be effective in most reports. In clinical use, naltrexone has not proved to be as efficacious in preventing relapse. While naltrexone targets opioid receptors, many other neurotransmitter systems are targeted by ethanol and, to a greater or lesser extent, contribute to modulating ethanol's reinforcing effects. There has been indication that drugs active at the gamma amino butyric acid B (GABAB) receptors can affect the self-administration of many drugs with abuse potential. The experiments reported here evaluated the effect of three doses of baclofen (2.5, 5.0, or 7.5 mg/kg), a GABAB agonist, administered alone or in combination with a single dose of naltrexone (1.0 mg/kg). In Experiment 1, both naltrexone and baclofen, at the two higher doses tested, significantly reduced ethanol consumption in Wistar rats using a limited access procedure on Drug Days 1 and 2. When combined on Drug Days 3 and 4, baclofen/naltrexone was significantly more effective in reducing ethanol consumption than did either drug alone. Neither drug, alone or in combination, had an effect on water consumption. In Experiment 2, both baclofen and naltrexone again significantly reduced ethanol consumption, with no evidence that chronic administration across Drug Days 3 and 4 further reduced consumption compared with Drug Days 1 and 2. The clinical use of multiple pharmacotherapeutic agents in combination may allow for the use of lower doses of individual components, thereby reducing the negative side effects that contribute to lower compliance and higher relapse.

Role of dopamine in behavioral sensitization to ethanol in DBA/2J mice

Behavioral sensitization has been proposed to play an important role in addiction. Elucidation of the neural processes mediating sensitization may therefore lead to the development of new pharmacotherapeutic treatments. A large number of studies have examined sensitization to psychostimulants and morphine. In contrast, despite the prevalence of alcoholism, the neural processes underlying sensitization to ethanol have not been identified. The aim of the present study was to examine the role of different components of the dopamine system in sensitization to the locomotor stimulant effects of ethanol in DBA/2J mice. Sensitization was induced by administering ethanol [2 g/kg intraperitoneally (ip)] before locomotor activity trials. Control groups received saline (12.5 ml/kg ip) before each activity trial. The ability of the dopamine uptake inhibitors GBR 12909 (3.33-10.0 mg/kg) and bupropion (20 and 30 mg/kg) to cross-sensitize to ethanol was then examined. In addition, the effects of SKF 82958 (0.1-1.0 mg/kg), a dopamine D(1) (D(1)) receptor agonist, quinpirole (0.05 and 0.1 mg/kg), a dopamine D(2)/D(3) (D(2)/D(3)) receptor agonist, and a combination of SKF 82958 and quinpirole were examined. Cross-sensitization was observed between the dopamine uptake inhibitor GBR 12909 and ethanol. In contrast, the less selective uptake inhibitor bupropion did not exhibit cross-sensitization. Similarly, stimulation of D(1) and D(2)/D(3) receptors did not cause cross-sensitization even when the agonists were administered together. Taken together, these data suggest that sensitization to ethanol is associated with changes in the dopaminergic system.

The Role of Acetaldehyde in the Central Effects of Ethanol

This article represents the proceedings of a symposium at the 2004 annual meeting of the Research Society on Alcoholism in Vancouver, Canada. The symposium was organized by Etienne Quertemont and chaired by Kathleen A. Grant. The presentations were (1) Behavioral stimulant effects of intracranial injections of ethanol and acetaldehyde in rats, by Mercè Correa, Maria N. Arizzi and John D. Salamone; (2) Behavioral characterization of acetaldehyde in mice, by Etienne Quertemont and Sophie Tambour; (3) Role of brain catalase and central formed acetaldehyde in ethanol's behavioral effects, by Carlos M.G. Aragon; (4) Contrasting the reinforcing actions of acetaldehyde and ethanol within the ventral tegmental area (VTA) of alcohol-preferring (P) rats, by William J. McBride, Zachary A. Rodd, Avram Goldstein, Alejandro Zaffaroni and Ting-Kai Li; and (5) Acetaldehyde increases dopaminergic transmission in the limbic system, by Milena Pisano and Marco Diana.

Differential increase in taurine levels by low-dose ethanol in the dorsal and ventral striatum revealed by microdialysis with on-line capillary electrophoresis

Ethanol increases taurine efflux in the nucleus accumbens or ventral striatum (VS), a dopaminergic terminal region involved in positive reinforcement. However, this has been found only at ethanol doses above 1 g/kg intraperitoneally, which is higher than what most rats will self-administer. We used a sensitive on-line assay of microdialysate content to test whether lower doses of ethanol selectively increase taurine efflux in VS as opposed to other dopaminergic regions not involved in reinforcement (e.g., dorsal striatum; DS). Adult male rats with microdialysis probes in VS or DS were injected with ethanol (0, 0.5, 1, and 2 g/kg intraperitoneally), and the amino acid content of the dialysate was measured every 11 sec using capillary electrophoresis and laser-induced fluorescence detection. In VS, 0.5 g/kg ethanol significantly increased taurine levels by 20% for 10 min. A similar increase was seen after 1 g/kg ethanol, which lasted for about 20 min after injection. A two-phased taurine efflux was observed with the 2.0 g/kg dose, where taurine was increased by 2-fold after 5 min but it remained elevated by 30% for at least 60 min. In contrast, DS exhibited much smaller dose-related increases in taurine. Glycine, glutamate, serine, and gamma-aminobutyric acid were not systematically affected by lower doses of ethanol; however, 2 g/kg slowly decreased these amino acids in both brain regions during the hour after injection. These data implicate a possible role of taurine in the mechanism of action of ethanol in the VS. The high sensitivity and time resolution afforded by capillary electrophoresis and laser-induced fluorescence detection will be useful for detecting subtle changes of neuronally active amino acids levels due to low doses of ethanol.

Association of the -141C Del variant of the dopamine D2 receptor (DRD2) with positive family history and suicidality in German alcoholics

Several lines of evidence indicate an involvement of the dopaminergic system in alcoholism, withdrawal, suicidality, and attention-deficit hyperactivity disorder (ADHD). The functionally relevant -141C Ins/Del polymorphism located upstream to exon 1 in the 5'-region of the dopamine D2 receptor (DRD2) gene might be an interesting candidate gene. We investigated a sample of 1,126 well-characterized, primary chronic alcoholics of German descent according to a phenotype-genotype strategy, i.e., alcoholics suffering from severe withdrawal complications such as seizure or delirium, family history positive (FH+) alcoholics, alcoholics with an antisocial personality disorder (ASPD), alcoholics with an ADHD, and type 1 or type 2 alcoholics according to Cloninger's typology. Compared to the control subjects, there was a significant excess of the -141C Del allele in alcoholics with a paternal and grandpaternal history of alcoholism and in alcoholic subgroups with suicidality or without a history of withdrawal symptoms. There were no significant differences in allele frequency between the entire group or subgroups of alcoholics and healthy controls. Therefore, the -141C Del variant of the DRD2 might be a protective factor against the development of withdrawal symptoms. However, it might also be a risk factor in a highly burdened subgroup of alcoholics with a paternal and grandpaternal history of alcoholism and it might contribute to the substantially higher likelihood of suicide in alcoholics.

Dual synaptic sites of D1-dopaminergic regulation of ethanol sensitivity of NMDA receptors in nucleus accumbens

Regulation of NMDAreceptor-mediated synaptic transmission onto accumbal medium spiny neurons (MSN) may constitute an important site in drug reward and reinforcement in mesolimbic structures. Previously, we reported that D(1)-like dopamine receptors activate a postsynaptic cAMP/PKA/DARPP-32 signaling cascade culminating in phosphorylation of SER897-NR1 subunits and a reduction in the sensitivity to ethanol of NMDA receptor-mediated synaptic transmission. Here, we use a detailed electrophysiological analysis of D(1)-like receptor regulation of the ethanol sensitivity of accumbal NMDA receptors (NMDARs) through recordings of quantal Sr(2+)-supported NMDA miniature synaptic currents (mEPSCs) in reduced Mg(2+) (0.6 mM) and report dual presynaptic and postsynaptic components of D(1)-like regulation of ethanol sensitivity of NMDARs. Ethanol inhibited NMDA mEPSC amplitude and frequency in a dose-dependent manner (25-75 mM), indicating inhibitory effects on presynaptic and postsynaptic components NMDA receptor-mediated synaptic transmission. The presynaptic inhibitory effect was corroborated by analysing the ratio of paired-pulse facilitation (PPF) of Ca(2+)-supported NMDA EPSCs. Activation of D(1) receptors with the agonist, SKF 38393 (25 microM), reversed ethanol suppression of NMDA mEPSC frequency and amplitude. Furthermore, the Mg(2+)-dependent decay off-rate of NMDA mEPSCs was substantially reduced by ethanol in a manner strongly reversed by the D(1) agonist. D(1) receptor-mediated attenuation of both the presynaptic and postsynaptic actions of ethanol was completely blocked by a D(1) selective antagonist (SCH 23390). These data suggest that D(1)-like receptors modulate both the presynaptic and postsynaptic effects of ethanol on NMDA receptor-mediated synaptic transmission in nucleus accumbens (NAc) and that these interactions may contribute to ethanol-induced neuroadaptation of the reward pathway.

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.

Long-lasting alterations of the mesolimbic dopamine system after periadolescent ethanol drinking by alcohol-preferring rats

BACKGROUND

This study tested the hypothesis that ethanol consumption by alcohol-preferring (P) rats during the periadolescent period causes persistent alterations in the mesolimbic dopamine (DA) system. After ethanol drinking during periadolescence, P rats were examined for alterations in basal locomotor activity, changes in extracellular DA levels and extraction fraction in the nucleus accumbens (NAc) by using no-net-flux (NNF) microdialysis, and changes in the response of the mesolimbic DA system to ethanol.

METHODS

Male P rat pups were given 24-hr free-choice access to 15% (v/v) ethanol from postnatal day (PD) 30 through PD 60. On PD 70, rats were assessed for locomotor activity. On PD 70 to 80, rats were implanted with bilateral guide cannulas aimed above the NAc. After at least 5 days, microdialysis probes were inserted bilaterally; on the following day, NNF microdialysis experiments were conducted. On the day after the NNF experiment, conventional microdialysis experiments were conducted to measure extracellular levels of DA in response to intraperitoneal injection of saline or ethanol 2.5 g/kg.

RESULTS

Compared with the ethanol-naive group, ethanol drinking by P rats during periadolescence did not alter basal locomotor activity, nor did it alter the basal extracellular concentration of DA. There was, however, a significant increase in the extraction fraction of DA of ethanol-drinking animals relative to the controls (57.4 +/- 2.7% and 45.8 +/- 2.3%, respectively). Additionally, compared with controls, P rats with exposure to ethanol during the periadolescent period showed a prolonged increase in the extracellular levels of DA after a challenge dose of ethanol.

CONCLUSIONS

The results of the microdialysis experiments suggest that periadolescent ethanol drinking by P rats increases basal DA neurotransmission (as indicated by higher DA clearance while maintaining the same extracellular DA concentrations) and prolongs the response of DA neurotransmission to ethanol.

Interactions between low concentrations of ethanol and nicotine on firing rate of ventral tegmental dopamine neurones

This study investigated interactions between ethanol and nicotine on dopamine-sensitive neurone firing in the ventral tegmental area (VTA), recorded in midbrain slices. No changes in spontaneous activity of the neurones were seen with nicotine at 10, 25, or 100 nM; at 250 nM there was a small significant increase in firing rate. Ethanol, applied alone, caused a significant increase in firing rate at 40 mM and at 60 nM but not at 20 mM. Combinations of 10 or 25 nM nicotine with 20 or 40 mM ethanol did not result in increased firing rates compared with either drug alone. However, nicotine 100 nM plus ethanol 60 mM significantly increased the rate of spontaneous firing compared with that after either drug alone at these concentrations. In contrast, nicotine at 250 nM plus ethanol at 60 mM did not increase firing rate, compared with each drug alone. Ketanserin, 2 microM, prevented the potentiating effect of nicotine 100 nM plus ethanol 60 mM. The results show synergism between ethanol and nicotine at specific concentrations that are likely to be present in the brain during the behavioural effects of these drugs, but the interaction is complex and may involve multiple drug actions.

Ethanol-induced Fos immunoreactivity in the extended amygdala and hypothalamus of the rat brain: focus on cholinergic interneurons of the nucleus accumbens

BACKGROUND

The primary goal of this study was to investigate the effects of varying doses of ethanol on cellular activation, as measured by Fos immunoreactivity, in brain areas that have been implicated in the reinforcing and anxiolytic effects of substance abuse and dependence, namely, the extended amygdala and hypothalamus. Specific regions examined included the central nucleus of the amygdala, bed nucleus of the stria terminalis, substantia innominata, and nucleus accumbens of the extended amygdala, as well as the paraventricular nucleus of the hypothalamus. The cholinergic interneurons of the nucleus accumbens were of particular interest, because these cells have recently been reported to play a pivotal role in substance abuse.

METHODS

Adult Sprague-Dawley rats underwent 10 days of handling and 5 days of habituation. Animals then received an injection of saline or 0.5, 1, or 2 g/kg of ethanol. Rats were perfused 2 hr after the injections, and brain sections were processed for single Fos or dual Fos/choline acetyltransferase immunolabeling procedures. The number of Fos-positive neurons was calculated from a 0.45-mm sample area from each of the brain regions examined.

RESULTS

A dose of 2 g/kg of ethanol significantly increased the number of Fos-immunoreactive neurons in the central nucleus of the amygdala by 149%, in the shell nucleus accumbens by 80%, and in the paraventricular nucleus of the hypothalamus by 321%. Additionally, 1 g/kg of ethanol significantly increased the percentage of Fos-immunoreactive cholinergic neurons in the nucleus accumbens by 59%.

CONCLUSIONS

The findings reported in this study reveal region-specific and dose-dependent changes in Fos immunoreactivity in the extended amygdala and hypothalamus and, more specifically, an increase in neuronal activation of cholinergic cells in the shell nucleus accumbens. These findings contribute to our current knowledge of the brain areas and cellular microcircuits involved in the underlying basis of substance abuse and dependence.

Dopamine activity in the nucleus accumbens during consummatory phases of oral ethanol self-administration

BACKGROUND This present study was designed to clarify the role of dopamine in the nucleus accumbens during operant ethanol self-administration by separating bar pressing (ethanol seeking) from ethanol consumption. Furthermore, we sought to define the relationship between ethanol in the brain and the accumbal dopamine response after oral self-administration of ethanol.

METHODS

Two separate groups of male Long-Evans rats were trained to bar press with 10% ethanol or water. Rats were trained to elicit an escalating number of bar presses across daily sessions before gaining access to the drinking solution for 20 min. Microdialysis was performed before (during a waiting period), during, and after bar pressing and drinking. A handling control group was included, but did not receive training.

RESULTS

A significant increase in dopamine occurred during placement of the rats into the operant chamber in trained rats and handling controls. The lever-pressing period did not produce an increase in dialysate dopamine. Accumbal dopamine was increased in the first 5 min of ethanol, but not water, consumption. Ethanol appeared in the dialysate sample following ethanol availability, and peak concentrations were reached at 10 min. Most of the ethanol and water consumption occurred within 5 min of fluid access. The probes were distributed in the core (32%), shell (32%), and core plus shell (36%) regions of the nucleus accumbens.

CONCLUSIONS

The enhancement of dopamine during transfer into the operant chamber does not depend on anticipation or operant training with ethanol or water reinforcement. Furthermore, the difference between the time course of accumbal dopamine and ethanol in dialysates suggests that the dopamine response is not solely due to pharmacological effects of ethanol. The dopamine response may be associated with the stimulus properties of ethanol presentation, which would be strongest during consumption.

Possible interaction of alcohol dehydrogenase and aldehyde dehydrogenase genes with the dopamine D2 receptor gene in anxiety-depressive alcohol dependence

BACKGROUND

The role of the dopamine D2 receptor (DRD2) gene in the development of alcohol abuse or dependence is controversial. The controversy is due in part to the disparate definitions pertaining to the control groups used and to the definitions of subtypes in alcohol dependence. In the Han Chinese population, the alcohol dehydrogenase 1B*2/*2 (ADH1B*2/*2) genotype and the aldehyde dehydrogenase 2*2 (ALDH2*2) allele have been considered as protective factors against alcohol abuse or dependence. Moreover, the ADH1B and ALDH2 genes might be involved in dopamine metabolism. We hypothesized that the ADH1B and ALDH2 genes might interact with the DRD2 gene and that the association between the DRD2 gene and alcohol dependence might be affected by different ADH1B and ALDH2 genotypes. This study examined whether the DRD2 gene is associated with specific subtypes of alcohol dependence and evaluated the relationship between the DRD2 gene and alcohol-metabolizing genes in a specific subtype of alcohol dependence.

METHODS

Of the 465 Han Chinese subjects who were recruited for the study, 71 were classified with pure alcohol dependence, 113 with both alcohol dependence and anxiety-depression (ANX/DEP ALC), and 129 with anxiety-depression but without alcohol dependence (ANX/DEP). The remaining 152 subjects were supernormal controls. All subjects were interviewed with the Chinese version of the modified Schedule of Affective Disorders and Schizophrenia-Lifetime; all alcohol dependence, anxiety, and major depressive diagnoses were made according to DSM-IV criteria.

RESULTS

The DRD2 gene was not found to be associated with pure alcohol dependence or ANX/DEP, but was found to be associated with ANX/DEP ALC. Furthermore, the association between the DRD2 gene and ANX/DEP ALC was shown to be under the control of the ALDH2*1/*1 and ADH1B*1/*2 genotypes.

CONCLUSIONS

ANX/DEP ALC is a specific subtype of alcohol dependence. Because ANX/DEP ALC was associated with the DRD2 gene only under the stratification of ADH1B*1/*2 or ALDH2*1/*1, the DRD2 gene might interact with the ADH1B gene and the ALDH2 gene, respectively, in the development of ANX/DEP ALC in the Taiwan Han Chinese population.

Learning mechanisms in addiction: synaptic plasticity in the ventral tegmental area as a result of exposure to drugs of abuse

One of the central questions in neurobiology is how experience modifies neural function, and how changes in the nervous system permit an animal to adapt its behavior to a changing environment. Learning and adaptation to a host of different environmental stimuli exemplify processes we know must alter the nervous system because the behavioral output changes after experience. Alterations in behavior after exposure to addictive drugs are a striking example of chemical alterations of nervous system function producing long-lasting changes in behavior. The alterations produced in the central nervous system (CNS) by addictive drugs are of interest because of their relationship to human substance abuse but also because these CNS alterations produce dramatic, easily observed alterations in behavior in response to discrete stimuli. Considerable study has been given to behavioral and biochemical correlates of addiction over the past 50 or more years; however, our understanding of the cellular physiological responses of affected CNS neurons is in its infancy. This review focuses on alterations in cellular and synaptic physiology in the ventral tegmental area (VTA) in response to addictive drugs.

Suppression by baclofen of the stimulation of alcohol intake induced by morphine and WIN 55,212-2 in alcohol-preferring rats

Administration of morphine and cannabinoids stimulates alcohol intake in rats. The present study investigated whether the promoting effect of morphine and of the cannabinoid receptor agonist, WIN 55,212-2 [(R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone], on alcohol intake was prevented by the gamma-aminobutyric (GABA)(B) receptor agonist, baclofen. Sardinian alcohol-preferring (sP) rats were given alcohol (10%, v/v) and water under the standard homecage two-bottle-free choice regimen with unlimited access for 24 h/day. Baclofen (0, 0.5 and 1 mg/kg; i.p.) was administered acutely 30 min before lights off. Morphine (0 and 1 mg/kg, s.c.) or WIN 55,212-2 (0 and 2 mg/kg, i.p.) was administered acutely 10 min after baclofen. Alcohol intake was recorded 60 min after lights off. As predicted, both morphine and WIN 55,212-2 produced a specific and marked increase in alcohol intake. Pretreatment with baclofen, which failed to alter alcohol intake when given alone, dose-dependently suppressed morphine- and WIN 55,212-2-induced promotion of alcohol drinking. These results suggest the involvement of the GABA(B) receptor in the neural circuitry mediating the stimulating effect of morphine and cannabinoids on alcohol consumption in sP rats.

Intracranial self-administration of ethanol within the ventral tegmental area of male Wistar rats: evidence for involvement of dopamine neurons

Previous work from our laboratory indicated that female Wistar rats will self-administer ethanol (EtOH) directly into the posterior ventral tegmental area (VTA). These results suggested that VTA dopamine (DA) neurons might be involved in mediating the reinforcing actions of EtOH within this region. The objectives of this study were to determine (1) the dose-response effects for the self-administration of EtOH into the VTA of male Wistar rats, and (2) the involvement of VTA DA neurons in the reinforcing actions of EtOH within the VTA. Adult male Wistar rats were implanted stereotaxically with guide cannulas aimed at the posterior or anterior VTA. After 1 week, rats were placed in standard two-lever (active and inactive) experimental chambers for a total of seven to eight sessions. The first experiment determined the intracranial self-administration of EtOH (0-400 mg%) into the posterior and anterior VTA. The second experiment examined the effects of coadministration of the D2/3 agonist quinpirole on the acquisition and maintenance of EtOH self-infusions into the posterior VTA. The final experiment determined the effects of a D2 antagonist (sulpiride) to reinstate self-administration behavior in rats given EtOH and quinpirole to coadminister. Male Wistar rats self-infused 100-300 mg% EtOH directly into the posterior, but not anterior, VTA. Coadministration of quinpirole prevented the acquisition and extinguished the maintenance of EtOH self-infusion into the posterior VTA, and addition of sulpiride reinstated EtOH self-administration. The results of this study indicate that EtOH is reinforcing within the posterior VTA of male Wistar rats and suggest that activation of VTA DA neurons is involved in this process.

Acetaldehyde increases dopaminergic neuronal activity in the VTA

Acetaldehyde is the first and principal metabolite of ethanol administered systemically. To its rise in blood, after administration of disulfiram, is ascribed the aversive reaction that should discourage alcoholics from drinking. In the present study, we sought to determine the effect of acetaldehyde on the electrophysiological properties of dopamine (DA)-containing neurons in the ventro tegmental area (VTA) of rats in vivo. Intravenous (i.v.) administration of acetaldehyde (5-40 mg/kg) readily and dose-dependently increased the firing rate, spikes/burst, and burst firing of VTA neurons. Ethanol (250-1000 mg/kg/i.v.) administration produced similar increments in electrophysiological parameters. In addition, a second group of rats was pretreated with the alcohol-dehydrogenase inhibitor 4-methyl-pyrazole (90 mg/kg) intraperitoneally (i.p.), and ethanol and acetaldehyde were administered i.v. at the same doses, 48 h later. In this group, ethanol effects were drastically reduced and the firing rate, spikes/burst, and burst firing were not significantly altered. In contrast, acetaldehyde fully retained its capacity to stimulate electrophysiological indices. The results indicate that acetaldehyde produces electrophysiological actions on VTA neurons in vivo, similar to those produced by ethanol, and significantly participate in ethanol-induced increment in DA neuronal activity. These results also suggest that acetaldehyde, by increasing DA neuronal activity in the VTA, may significantly contribute to the centrally mediated positive motivational properties of ethanol, which would oppose the well-known peripherally originating aversive properties.

Alcohol effects on human risk taking

RATIONALE

Despite a well-established relationship between alcohol and risky behavior in the natural environment, laboratory investigations have not reliably shown acute alcohol effects on human risk-taking.

OBJECTIVES

The present study was designed to demonstrate a dose-response relationship between acute alcohol administration and human risk taking. Further, this investigation sought to delineate behavioral mechanisms that may be involved in alcohol-induced changes in the probability of risky behavior.

METHODS

Using a laboratory measure of risk taking designed to address acute drug effects, 16 adults were administered placebo, 0.2, 0.4, and 0.8 g/kg alcohol in a within-subject repeated measures experimental design. The risk-taking task presented subjects with a choice between two response options operationally defined as risky and non-risky. Data analyses examined: breath alcohol level (BAL), subjective effects, response rates, distribution of choices between the risky and non-risky option, and trial-by-trial probabilities of making losing and winning risky responses.

RESULTS

The alcohol administration produced the expected changes in BAL, subjective effects, and response rate. Alcohol dose-dependently increased selection of the risky response option, and at the 0.8 g/kg dose, increased the probability of making consecutive losing risky responses following a gain on the risky response option.

CONCLUSIONS

Acute alcohol administration can produce measurable changes in human risk-taking under laboratory conditions. Shifts in trial-by-trial response probabilities suggest insensitivity to past rewards and more recent losses when intoxicated, an outcome consistent with previous studies. This shift in sensitivity to consequences is a possible mechanism in alcohol-induced changes in risk taking.

Serotonin 5-HT2 Receptors and Alcohol: Reward, Withdrawal and Discrimination

This article represents the proceedings of a symposium at the 2003 Research Society on Alcoholism meeting in Fort Lauderdale, Florida. The organizer was Karl J. Buck, and the chairperson was Mark S. Brodie. The presentations were (1) The Multiple PDZ Domain Protein May Mediate Genetic Differences in Ethanol Withdrawal Severity Via Interaction With 5-HT2 Receptors, by Matthew T. Reilly and Kari J. Buck; (2) The Ionic Mechanism of Serotonin Potentiation of Ethanol Excitation of Ventral Tegmental Area Neurons, by Mark S. Brodie; and (3) 5-HT(2C) Receptor Agonists in the Discriminative Stimulus Effects of Ethanol, by Laura M. Rogers, Ken Szeliga, and Kathleen Grant.

Pharmacology of drugs of abuse

The pharmacology of most addictive substances is being studied extensively, not just for their acute effects but also the mechanisms that lead to drug seeking and addiction. The understanding of how these drugs alter their effects at the molecular level with continuing use gives promise toward investigation of novel substances that may be used for treatment. Genetic predisposition and gender differences are also some of the areas where more research is needed. Women who are addicted are likely to continue drug use during pregnancy, which can have an impact on the next generation. Prevention measures at the population level are as important. Programs need to address risks, social issues, and environmental factors that promote drug use and addiction.

Ethanol Drinking and Deprivation Alter Dopaminergic and Serotonergic Function in the Nucleus Accumbens of Alcohol-Preferring Rats

The alcohol deprivation effect is a temporary increase in the intake of, or preference for, ethanol after a period of deprivation that may result from persistent changes in key limbic regions thought to regulate alcohol drinking, such as the nucleus accumbens. The present study tested the hypothesis that chronic alcohol drinking under continuous 24-h free-choice conditions alters dopamine and serotonin neurotransmission in the nucleus accumbens and that these alterations persist in the absence of alcohol. Using the no-net-flux microdialysis method, the steady-state extracellular concentration (point of no-net-flux) for dopamine was approximately 25% higher in the adult female alcohol-preferring P rats given prior access to 10% ethanol, even after 2 weeks of ethanol abstinence, compared with the P rats gives access only to water. However, the extracellular concentration of serotonin was approximately 35% lower in animals given 8 weeks of continuous access to ethanol compared with water controls and animals deprived of ethanol for 2 weeks. The effect of local perfusion with 100 microM sulpiride (D(2) receptor antagonist) and 35 microM 1-(m-chlorophenyl)-biguanide (5-hydroxytryptamine(3) receptor agonist) on dopamine overflow were reduced approximately 33% in both groups of ethanol-exposed P rats compared with water controls. Free-choice alcohol drinking by P rats alters dopamine and serotonin neurotransmission in the nucleus accumbens, and many of these effects persist for at least 2 weeks in the absence of ethanol, suggesting that these underlying persistent changes may be in part responsible for increased ethanol drinking observed in the alcohol-deprivation effect.

Ethanol withdrawal reduces the number of spontaneously active ventral tegmental area dopamine neurons in conscious animals

Withdrawal from chronic ethanol treatment leads to a reduction in the electrical activity in dopamine (DA) neurons in the ventral tegmental area (VTA). However, there is disagreement on how the electrical activity is reduced (i.e., in the number of spontaneously active DA neurons or their firing rates and burst firing activity) and on the underlying mechanisms. The use of general anesthesia has been suggested to cause this discrepancy. In the present study, we demonstrate that ethanol withdrawal, in conscious animals, causes a reduction in the number of spontaneously active VTA DA neurons, but not in their firing rate or burst firing activity. Similar results were obtained in a previous study using anesthetized preparation, showing that general anesthesia does not cause this difference. Ethanol withdrawal-induced reduction in a number of spontaneously active VTA DA neurons could be mediated by depolarization inactivation because this effect was reversed by systemic administration of amphetamine, which inhibits VTA DA neurons by hyperpolarization. In addition, the withdrawal effect was normalized by acute ethanol administration, suggesting that the decrease in the number of spontaneously active VTA DA neurons represents an adaptational change to chronic ethanol treatment. Because the electrical activity of DA neurons controls the release of DA, it is possible that the decreased DA release during ethanol withdrawal observed in previous studies is caused by the reduction in the electrical activity of VTA DA neurons.

Involvement of non-exocytotic mechanisms in ethanol-induced in vivo dopamine release: comparisons with cocaine

In order to determine whether a non-exocytotic mechanism was involved in ethanol-induced in vivo dopamine release in the nucleus accumbens, extracellular dopamine concentrations were measured via intracerebral microdialysis in freely moving Sprague-Dawley rats. Effects of ethanol on dopamine release in the nucleus accumbens were compared with those by cocaine, a drug that increases synaptic dopamine by a mechanism, which depends on neuronal activity and involves an exocytotic process. Administration of ethanol (80 mM) or cocaine (10 microM) via a dialysis probe increased extracellular dopamine concentrations in the nucleus accumbens. Pretreatments with tetrodotoxin (2 microM) or Ca2+ withdrawal did not block the ability of ethanol to increase nucleus accumbens dopamine. The blockade of dopamine autoreceptors by local infusion of sulpiride did not significantly alter the effect of ethanol on nucleus accumbens dopamine either. As opposed to ethanol, however, cocaine-induced increases in nucleus accumbens dopamine were sensitive to tetrodotoxin or Ca2+ omission. In addition, pretreatments with sulpiride significantly potentiated the effect of cocaine on extracellular dopamine concentrations. These differences in responses to tetrodotoxin, Ca2+ withdrawal and inhibition of dopamine autoreceptors suggest that a non-exocytotic mechanism may be involved in dopamine release in the nucleus accumbens evoked by focally applied ethanol.

Ethanol consumption in mice with a targeted disruption of the dopamine-3 receptor gene

Considerable evidence suggests that the mesolimbic dopaminergic system is an important substrate for the rewarding effects of ethanol consumption. Previous data have demonstrated that pharmacological agents that alter dopamine signaling also influence the self-administration of ethanol. The present experiments were designed to assess the role of the dopamine-3 receptor (D3-R) on voluntary ethanol consumption in C57BL/6 mice. Mice with targeted disruption of the D3-R gene (D3-R - /-) were compared to wild-type controls in an ethanol intake paradigm. In Experiment 1, mice had 24-hour access to ethanol each day in a two-bottle choice paradigm for a period of 7 days per concentration. The concentrations tested were 3, 6, 10 and 15%. In Experiment 2, mice had I hour of access to ethanol each day in a two-bottle paradigm for a period of 7 days per concentration. The same concentrations in Experiment I were compared in Experiment 2. In Experiment 3 we sought to test the development of a conditioned taste aversion (CTA) after receiving an intraperitoneal (ip.) injection of 2.0 g/kg ethanol. In Experiment 4, blood ethanol levels where assessed following a 2.0 g/kg ip.injection of ethanol. Experiment 5 assessed taste preference for saccharine and quinine in wild-type and D3-R -/- mice. Contrary to our predictions, both D3-R -/- and wild-types on a CS57BL/6 background had similar intakes of ethanol, at all concentrations tested, in the 24-hour and 1-hour intake paradigms. Wild-type and D3-R -/- mice respond to injected ethanol similarly by developing a conditioned taste aversion. Metabolic analysis revealed mutant mice are slower in metabolizing a bolus injection of ethanol. Lastly, wild-type and D3-R -/- mice showed similar consumption to increasing concentration of both sweet and bitter tastes. These data suggest that deletion of the D3-R gene does not increase ethanol consumption above that found on the C57BL/6 genetic background. Furthermore, the D3-R -/- mice adequately learn a CTA to ethanol and do not ham differing taste reactivity to saccharin or quinine. However, D3-R -/- mice do appear to have a slower rate of ethanol metabolism.