Effects of naltrexone on the accumulation of L-3, 4-dihydroxyphenylalanine and 5-hydroxy-L-tryptophan and on the firing rate induced by acute ethanol administration

Disparity between tonic and phasic ethanol-induced dopamine increases in the nucleus accumbens of rats

BACKGROUND

Dopamine concentrations in the nucleus accumbens fluctuate on phasic (subsecond) and tonic (over minutes) timescales in awake rats. Acute ethanol increases tonic concentrations of dopamine, but its effect on subsecond dopamine transients has not been fully explored.

METHODS

We measured tonic and phasic dopamine fluctuations in the nucleus accumbens of rats in response to ethanol (within-subject cumulative dosing, 0.125 to 2 g/kg, i.v.).

RESULTS

Microdialysis samples yielded significant tonic increases in dopamine concentrations at 1 to 2 g/kg ethanol in each rat, while repeated saline infusions had no effect. When monitored with fast scan cyclic voltammetry, ethanol increased the frequency of dopamine transients in 6 of 16 recording sites, in contrast to the uniform effect of ethanol as measured with microdialysis. In the remaining 10 recording sites that were unresponsive to ethanol, dopamine transients either decreased in frequency or were unaffected by cumulative ethanol infusions, patterns also observed during repeated saline infusions. The responsiveness of particular recording sites to ethanol was not correlated with either core versus shell placement of the electrodes or the basal rate of dopamine transients. Importantly, the phasic response pattern to a single dose of ethanol at a particular site was qualitatively reproduced when a second dose of ethanol was administered, suggesting that the variable between-site effects reflected specific pharmacology at that recording site.

CONCLUSIONS

These data demonstrate that the relatively uniform dopamine concentrations obtained with microdialysis can mask a dramatic heterogeneity of phasic dopamine release within the accumbens.

The novel micro-opioid receptor antagonist, [N-allyl-Dmt(1)]endomorphin-2, attenuates the enhancement of GABAergic neurotransmission by ethanol

AIMS

We investigated the effects of [N-allyl-Dmt(1)]endomorphin-2 (TL-319), a novel and highly potent micro-opioid receptor antagonist, on ethanol (EtOH)-induced enhancement of GABA(A) receptor-mediated synaptic activity in the hippocampus.

METHODS

Evoked and spontaneous inhibitory postsynaptic currents (eIPSCs and sIPSCs) were isolated from CA1 pyramidal cells from brain slices of male rats using whole-cell patch-clamp techniques.

RESULTS

TL-319 had no effect on the baseline amplitude of eIPSCs or the frequency of sIPSCs. However, it induced a dose-dependent suppression of an ethanol-induced increase of sIPSC frequency with full reversal at concentrations of 500 nM and higher. The non-specific competitive opioid receptor antagonist naltrexone also suppressed EtOH-induced increases in sIPSC frequency but only at a concentration of 60 microM.

CONCLUSION

These data indicate that blockade of micro-opioid receptors by low concentrations of [N-allyl-Dmt(1)]endomorphin-2 can reverse ethanol-induced increases in GABAergic neurotransmission and possibly alter its anxiolytic or sedative effects. This suggests the possibility that high potency opioid antagonists may emerge as possible candidate compounds for the treatment of ethanol addiction.

Gene transcription alterations associated with decrease of ethanol intake induced by naltrexone in the brain of Wistar rats

Preclinical and clinical studies suggest that the administration of the opioid antagonist naltrexone decreases the intake of ethanol. However, the neuroplastic adaptations in the brain associated to reduction of ethanol consumption remains to be elucidated. The aim of the study was to identify gene transcription alterations underlying the attenuation of voluntary ethanol intake by administration of naltrexone in rats. Increasing doses of naltrexone (0.7 mg/kg, 4 days and 1.4 mg/kg/day, 4 days) to rats with acquired high preferring ethanol consumption (>3.5 g of ethanol/kg/day) decreased voluntary ethanol intake (50%). Voluntary ethanol consumption altered mu-opioid receptor function in the cingulate cortex, caudate-putamen (CPu), nucleus accumbens core (Acb C) and shell (Acb S), the expression of tyrosine hydroxylase (TH) in the ventral tegmental area and substantia nigra, proenkephalin (PENK) in the piriform cortex, olfactory tubercle, CPu, Acb C and Acb S, ventromedial nucleus (VMN) and paraventricular nucleus (PVN) of the hypothalamus, corticotropin releasing factor (CRF) in PVN, cannabinoid CB(1) receptor (CB1-R) in the CPu, hippocampus and VMN, and serotonin transporter (5-HTT) in the dorsal and median raphe nuclei. The reduction of ethanol intake induced by naltrexone was associated with a blockade or significant reduction of the changes produced by ethanol in the expression of these genes in key regions related to drug dependence. These results point to a role for the mu-opioid receptor, TH, PENK, CRF, CB1-R, and 5-HTT genes in specific brain regions in the modulation of neuroadaptative mechanisms associated to the decrease of ethanol intake induced by naltrexone.

Chronic ethanol exposure during adolescence increases basal dopamine in the nucleus accumbens septi during adulthood

BACKGROUND

In humans, adolescent exposure to alcohol is associated with the onset of adult alcohol dependency and suggests that early use potentiates vulnerability to addiction. The aim of the present study was to address whether chronic ethanol exposure during adolescence would alter nucleus accumbens septi (NAcc) dopamine (DA) levels in the adult brain.

METHODS

Rats were injected daily from postnatal day (PND) 30 to 50 with either 0.75 g/kg/i.p. ethanol or saline followed by an ethanol-abstinent period from PND 51 to 65. Changes in extracellular DA levels in the anterior NAcc shell were measured via the no net flux (NNF) paradigm.

RESULTS

Extracellular DA levels were greater in rats chronically treated with ethanol during adolescence (6.5 nM DA) in comparison with saline-exposed controls (3.6 nM DA). There were no differences in extraction fraction (E(d)), an indirect measure of DA reuptake, between ethanol-treated (87%) and nontreated (68%) rats.

CONCLUSIONS

Together these findings suggest that changes in extracellular DA may be an underlying physiological mechanism in adolescent vulnerability to the rewarding properties of ethanol.

Genetic moderators of naltrexone's effects on alcohol cue reactivity

BACKGROUND

Naltrexone (NTX) reduces drinking and craving in alcoholic individuals in treatment and also in heavy drinkers. Polymorphisms in the D4 dopamine receptor (DRD4) gene and mu-opiate receptor gene (OPRM1) may moderate NTX's effects on craving. This study examined these candidate genes as moderators of the effects of NTX on cue-elicited urge to drink in non-treatment-seeking heavy drinkers.

METHOD

Data from the subset of 93 participants who consented for genetic testing in a larger study of medication effects were used to examine pharmacogenetic hypotheses. The non-treatment-seeking male and female heavy drinkers (62% alcohol dependent) were genotyped for the variable number of tandem repeats polymorphism in the DRD4 gene [L=7 or more (n=34), S=less than 7 (n=56)] and Asn40Asp single-nucleotide polymorphism in the OPRM1 gene [29 aspartate (Asp) carriers and 59 asparagine (Asn) homozygotes]. Ten days after randomization to NTX (50 mg) or placebo, participants completed an alcohol cue reactivity assessment.

RESULTS

Any medication effects were all accounted for by interaction with genotype. Naltrexone increased urge for alcohol in Asp carriers across alcohol and neutral beverage cue trials and had no effect on homozygous Asn carriers. Asp40 carriers on either medication had greater decreases (from resting baseline) in mean arterial blood pressure across all beverage cue trials compared with Asn carriers. For DRD4, no differential medication effects by DRD4 polymorphism were found. Alcohol dependence diagnosis did not moderate the effects of gene and medication on cue-elicited measures.

DISCUSSION

The differential responses to NTX due to variation in the OPRM1 gene may help explain conflicting results in clinical trials and suggest directions for patient-treatment matching.

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

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

Neonatal administration of monosodium glutamate prevents the development of ethanol- but not psychostimulant-induced sensitization: a putative role of the arcuate nucleus

Lesions of the arcuate nucleus by monosodium glutamate, goldthioglucose and oestradiol valerate treatments are known to prevent the acute stimulating effect of ethanol in mice. On the basis of these results, the current study analysed whether a lesion of the arcuate nucleus by monosodium glutamate was able to block ethanol-induced locomotor sensitization. To produce the arcuate nucleus lesions, pups were injected with saline or monosodium glutamate (4 mg/g body weight) subcutaneously on 5 alternate days, starting on postnatal day one. Sensitization treatments began 10 weeks after the initial lesions. Sensitization training consisted of six trials on alternate days, in which groups of mice were treated with ethanol (2 g/kg) or saline, and then tested in an open-field for the induction of locomotor activity. The present study demonstrated that animals with monosodium glutamate-induced lesions did not develop locomotor sensitization to ethanol. Different groups of mice were used to assay blood ethanol levels and to evaluate the effect of arcuate nucleus lesions on psychostimulant-induced locomotor sensitization. Sensitization to cocaine or amphetamine was spared in monosodium glutamate-pre-treated animals, although the lesion of arcuate nucleus reduced the sensitivity of mice to cocaine. Our findings therefore suggest that the arcuate nucleus may be critical for the neuroadaptations that underlie the behavioural sensitization to ethanol, in contrast to those mediating psychostimulant-induced sensitization.

Endogenous opiates: 2000

This paper is the twenty-third installment of the annual review of research concerning the opiate system. It summarizes papers published during 2000 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Terminal effects of ethanol on dopamine dynamics in rat nucleus accumbens: an in vitro voltammetric study

To assess the direct effects of acute ethanol on dopamine (DA) terminals, evoked DA release and uptake were measured in rat nucleus accumbens slices using fast-scan cyclic voltammetry. Low and moderate concentrations of ethanol (20, 45 and 100 mM) did not alter evoked DA release, while high concentrations (150 and 200 mM) significantly decreased DA release (18 and 36%, respectively) in a calcium-dependent manner. No significant difference was found between the rate of DA disappearance measured before and after the drug. These data indicate that uptake of DA through the dopamine transporter is unaffected by ethanol, even at high concentrations. Therefore, low to moderate concentrations of ethanol have no effect on DA dynamics at the level of the nerve terminal in the nucleus accumbens. This is consistent with the hypothesis that cell body regions of DA neurons are the primary target for the stimulating and reinforcing effects of ethanol. High concentrations of ethanol can locally depress DA release, and this may correlate with the sedative actions of the drug.

Regional central nervous system densities of delta-opioid receptors in alcohol-preferring P, alcohol-nonpreferring NP, and unselected Wistar rats

The densities of delta-opioid receptors in the central nervous system of alcohol-naive, adult, male, alcohol-preferring P, alcohol-nonpreferring NP, and Wistar rats were examined with the use of quantitative autoradiography. Slides with coronal 20-microm sections through the regions of interest were incubated in 5 nM [3H]-[D-Pen(2),D-Pen(5)]enkephalin (DPDPE) to label delta(1)-opioid receptor sites. Nonspecific binding was determined in the presence of 10 microM naloxone. Significant differences between the P and the NP rat lines were found in numerous cortical regions, the basolateral amygdala, and the posterior hippocampus, with 10%-20% lower [3H]-DPDPE binding found in the P line. In most regions examined, binding levels in the Wistar rats were intermediate between those of the P and the NP rats. Significantly lower [3H]-DPDPE binding levels in the P rat may indicate fewer delta(1)-opioid receptors or decreased binding affinity. The lower binding in certain limbic regions, such as the basolateral amygdala and posterior hippocampus, as well as cortical differences in the P rat may be associated with the divergent alcohol drinking behaviors found between the P and the NP lines.