L. Lumeng

Alcohol stimulates the release of dopamine and serotonin in the nucleus accumbens

The effects of acute IP administration (0.5, 1.0 or 2.0 g/kg) and local perfusion (25, 50 or 100 mM) of ethanol on the extracellular concentrations of dopamine (DA), serotonin (5-HT) and their metabolites in the nucleus accumbens (ACC) of the rat were studied with in vivo microdialysis coupled with a small-bore HPLC electrochemical detection procedure. The IP administration of 1.0 and 2.0 g/kg ethanol significantly (p less than 0.05) increased the extracellular levels of DA and 5-HT in the ACC whereas the 0.5 g/kg dose caused no change. In general, the extracellular levels of the 3 monoamine metabolites were not altered by IP ethanol except for a slight increase in the levels of homovanillic acid following the 2.0 g/kg dose. Local perfusion of 50 and 100 mM ethanol (but not 25 mM) through the microdialysis probe markedly increased (170-200% of control) the extracellular levels of DA in the ACC. Only the 100 mM concentration of ethanol altered the extracellular levels of 5-HT (2-fold increase), 3,4-dihydroxyphenylacetic acid and 5-hydroxyindoleacetic acid. Addition of 100 microM ICS 205-930 (a 5-HT3 antagonist) to the perfusate markedly reduced the 100 mM ethanol-stimulated release of DA and 5-HT. Overall, the data suggest that ethanol can stimulate the release of both DA and 5-HT in the ACC and that the action of ethanol within the ACC may be mediated in part by 5-HT3 receptors.

Naloxone attenuates voluntary ethanol intake in rats selectively bred for high ethanol preference

The effect of naloxone on voluntary ethanol intake was examined in rats which were selectively bred for oral ethanol preference (High Alcohol Drinking or HAD line). Rats of the HAD line were treated with naloxone in doses of 0.05-18.0 mg/kg b.wt. before access to water alone or to a free-choice between a 10% (v/v) ethanol solution and water. Naloxone suppressed water intake when water was presented as the sole source of fluid. In contrast, naloxone produced a dose-dependent decrease in ethanol consumption, without altering water intake, when rats were given a free-choice between the ethanol solution and water. Selective suppression of ethanol consumption by naloxone was not attributable to changes in blood ethanol concentrations or ethanol elimination rates following naloxone treatment. It appears that although naloxone may attenuate the positively reinforcing properties of both ethanol and water, ethanol drinking is a subset of consummatory behaviors that is particularly sensitive to opioid receptor blockade. The results suggest that activation of the endogenous opioid system may be an important mechanism which serves to maintain continued ethanol drinking.

Serotonin, dopamine and GABA involvement in alcohol drinking of selectively bred rats

Neurochemical and neuropharmacological studies were undertaken to assess the involvement of CNS serotonin (5-HT), dopamine (DA) and GABA systems in regulating the alcohol-drinking behavior of two lines of rats selectively bred for their high alcohol-seeking behavior, namely the alcohol-preferring P line and the high alcohol-drinking HAD line of rats. Neurochemical data indicate that high alcohol-seeking behavior (when compared with data from rats with low alcohol-seeking characteristics) is associated with: a) lower (10-20%; p less than 0.05) contents of 5-HT in certain limbic regions (e.g., nucleus accumbens, frontal cortex, hypothalamus and hippocampus); b) a lower (10-15%; p less than 0.05) content of DA in the nucleus accumbens; c) higher (20-35%; p less than 0.05) densities of 5-HT1A binding sites in some limbic regions (e.g., medial nucleus accumbens, medial prefrontal cortex and ventral hippocampus); and d) a greater (20-50%) density of GABA axon terminals in the nucleus accumbens. Furthermore, the acute administration of high doses of ethanol appears to increase the activity of the 5-HT and DA projections to the nucleus accumbens of the P line of rats (as indicated by the 20-30% elevated tissue levels of 5-HT and DA metabolites following IP ethanol administration); neuronal tolerance to alcohol appears to develop in both these monoamine pathways, as suggested by an attenuated effect on metabolite levels by a challenge dose of ethanol given to P rats that had been chronically drinking alcohol.(ABSTRACT TRUNCATED AT 250 WORDS)

Contents of monoamines in forebrain regions of alcohol-preferring (P) and -nonpreferring (NP) lines of rats.

The contents of monoamine neurotransmitters and metabolites were assayed in the frontal cortex, nucleus accumbens and anterior striatum of rats from the selectively bred alcohol-preferring P and nonpreferring NP lines. Lower levels of serotonin (20-30%) in all three brain regions of P as compared with NP rats lends support to the hypothesis that a decreased metabolic activity and/or innervation by serotonin neurons is associated with the abnormally high volitional intake of ethanol. Of additional interest, however, were the approximately 25% lower contents of dopamine and its major metabolites in the nucleus accumbens of the P rats. This observation may indicate that P rats have a specific deficiency in the dopaminergic projections from the ventral tegmental area to the nucleus accumbens and, since the accumbens is an important structure in brain reward circuitry, it might also be an important determinant of the excessive volitional intake of alcohol by P rats.

Regional brain levels of monoamines in alcohol-preferring and -nonpreferring lines of rats

Regional brain levels of serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), dopamine (DA) and norepinephrine (NE) were determined in alcohol-naive rats from lines selectively bred for alcohol preference (P) and alcohol aversion (nonpreference, NP). Based on comparison by a standard t-test, the P rats had 12% lower NE in the pons-medulla, 20% higher NE and 16% lower DA content in the cerebral cortex (CX) than did the NP rats. However, the predominant finding was that the levels of 5-HT and 5-HIAA were 12-26% lower for the P than for the NP rats in the CX, hippocampus (HIP), corpus striatum (STR), thalamus (TH) and hypothalamus (HY). Regional CNS monoamines from a group of independently bred, stock Wistar rats were also compared with the NP and P groups to determine if the selectively bred rats differed widely from an unselected population. In most instances, the NP and P rats fell within the range of the stock group. When the stock group was included in an analysis of variance of the data, post-hoc differences between the NP and P groups that remained significant were the lower levels of 5-HT (and in some cases 5-HIAA) in the CX, HIP, STR, TH and HY of the P group. In the HY and HIP, the 5-HT levels of the P and NP animals diverged significantly in opposite directions from those of the stock group, possibly suggesting an involvement of these regional serotonergic systems in alcohol preference.

Comparison of Alcohol-Preferring (P) and Nonpreferring (NP) Rats on Tests of Anxiety and for the Anxiolytic Effects of Ethanol

Rats of the selectively bred alcohol-preferring P and alcohol-nonpreferring NP lines were evaluated using three different behavioral measures of anxiety. Compared with NP rats, P rats (1) showed greater footshock-induced suppression of operant responding in an approach-avoidance conflict test; (2) spent less time in the open arms of an elevated plus maze; and (3) took longer in a passive avoidance test to step down from a platform to a grid floor where footshock was received 24 hours earlier. These findings indicate a greater degree of anxiety in the P than in the NP line of rats in these situations. Pretreatment with intraperitoneal (IP) ethanol (0.5-1.0 g/kg) injections produced anticonflict or anxiolytic effects in P but not in NP rats. However, the anticonflict effects of ethanol were small relative to those produced by chlordiazepoxide (CDP, 7.5 mg/kg) in both lines. The results demonstrate that selective breeding for divergent oral ethanol preference has produced associated differences between the P and NP lines of rats in behavioral tests of anxiety and in the anxiolytic effects of ethanol.

Importance of delta opioid receptors in maintaining high alcohol drinking.

We have previously reported that naloxone, a nonspecific opioid receptor antagonist, suppresses alcohol but not water consumption by male rats that have been genetically selected for high voluntary alcohol drinking. However, the identity of the specific opioid receptor subtype that may mediate alcohol drinking is not known. This paper reports that a selective delta opioid receptor antagonist is as effective as naloxone in suppressing alcohol consumption and that an enkephalinase inhibitor, which potentiates the action of endogenous enkephalins, increases alcohol intake. These results suggest that alcohol-induced activation of the endogenous enkephalinergic system, and occupation of delta opioid receptors, are involved in the maintenance of continued alcohol drinking.

Effect of low dose ethanol on spontaneous motor activity in alcohol-preferring and -nonpreferring lines of rats ☆

To determine if behavioral arousal may be associated with ethanol preference, the effects of low to moderate doses of ethanol on spontaneous motor activity (SMA) were studied in the selectively bred alcohol-preferring (P) and -nonpreferring (NP) lines of rats as well as in the Maudsley Reactive (MR/N) and Nonreactive (MNR/N) strains. Alcohol-naive rats had food and water available ad lib, but food was removed 24 hr before and during activity testing. After an intraperitoneal injection of saline (5 ml) or ethanol (0.12 to 1.5 g/kg), SMA was monitored every three min for 30 min in an electronic activity monitor. The P and MR/N rats exhibited increased SMA after doses of 0.12 and 0.25 g/kg. Both the NP and MNR/N rats failed to show increased SMA at any ethanol dose. Moderate doses of ethanol, 1.0 and 1.5 g/kg, consistently depressed SMA in all lines/strains. In 24 hr-fasted rats, increased SMA occurred within 6-12 min after injection, but free-fed rats exhibited increased SMA 12-24 min after an ethanol dose of 0.25 g/kg. Free-choice drinking scores (10% ethanol (v/v) versus water) for the P, MR/N, MNR/N and NP rats were 6.6 +/- 0.5, 4.9 +/- 0.8, 2.2 +/- 0.7 and 1.4 +/- 0.3 g ethanol/kg body wt/day (mean +/- SEM), respectively. The data indicate a positive relationship between ethanol preference and ethanol-induced motor stimulation and suggest that hyperactivity may be an expression of the positive reinforcing effect of ethanol for alcohol-preferring rats.

Induction of dependence on ethanol by free-choice drinking in alcohol-preferring rats

Studies were performed to examine whether chronic voluntary consumption of ethanol by the selectively-bred, alcohol-preferring P-rats produces physical dependence. Body weight reduction, food restriction and flavoring the 10% ethanol solution increased ethanol consumption from 7 to 14 g ethanol/kg body weight/day when water was freely available. Under similar conditions, consumption by selectively-bred, alcohol-nonpreferring NP-rats increased from 1 to 12 g/kg/day. Removal of ethanol after eight weeks induced physical signs of withdrawal in both lines of animals. In two subsequent studies, P-rats were given food, water and unflavored 10% ethanol ad lib for 15 and 20 weeks; ethanol consumption was 7.2 and 5.6 g/kg/day, respectively. Upon removal of ethanol, manifestations of withdrawal, scored blind in one experiment, developed in 85% of the animals and persisted for 72 hours. Importantly, none in the control groups of P and NP rats given water only exhibited these signs. The ethanol withdrawn groups were hyperactive in both the open-field and the head-poke apparatus. These results indicate that sufficient ethanol was voluntarily consumed by the selectively-bred alcohol-preferring P-rats under free-feeding conditions to produce physical dependence.

Effects of scheduled access on ethanol intake by the alcohol-preferring (P) line of rats.

The effects of scheduling the availability of ethanol on its voluntary consumption by the selectively bred alcohol-preferring P rats were examined under three conditions: unrestricted 24 hr/day access (Condition A), access limited to a continuous 4 hr/day (Condition B), and access limited to 1 hr every 3 hr, 4 times/day (Condition C). Food and water were always available. Daily alcohol intakes (mean +/- SEM) with Conditions A, B and C were 6.9 +/- 0.2, 2.1 +/- 0.2 and 4.4 +/- 0.2 g/kg, respectively, while the intake per hour of availability increased from 0.3 +/- 0.03 under Condition A to 1.1 +/- 0.4 g/kg under condition C. The amount of ethanol consumed per drinking episode under Conditions A, B and C were 1.1 +/- 0.1, 2.1 +/- 0.2 and 1.1 +/- 0.03 g/kg, respectively. Mean blood alcohol concentrations (BACs), determined periodically during the dark cycle of Condition A and five minutes after drinking episodes under Conditions B and C, were 59 +/- 10, 61 +/- 7 and 62 +/- 7 mg%, respectively. When unlimited access was reinstated after Condition C, daily alcohol consumption returned to a level similar to that under the initial Condition A (7.2 +/- 0.5 g/kg). When the ethanol concentration was increased from 5 to 20% (v/v) under Condition C, the amount of ethanol consumed per episode at 5% was significantly less than at the 10, 15 and 20% concentrations, and the volume consumed was significantly lower at the 20% concentration than at the 5, 10 and 15% concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)