Circadian drinking rhythms and blood alcohol levels in two rat lines developed for their alcohol consumption

Rat animal models for screening medications to treat alcohol use disorders

The purpose of this review is to present animal research models that can be used to screen and/or repurpose medications for the treatment of alcohol abuse and dependence. The focus will be on rats and in particular selectively bred rats. Brief introductions discuss various aspects of the clinical picture, which provide characteristics of individuals with alcohol use disorders (AUDs) to model in animals. Following this, multiple selectively bred rat lines will be described and evaluated in the context of animal models used to screen medications to treat AUDs. Next, common behavioral tests for drug efficacy will be discussed particularly as they relate to stages in the addiction cycle. Tables highlighting studies that have tested the effects of compounds using the respective techniques are included. Wherever possible the Tables are organized chronologically in ascending order to describe changes in the focus of research on AUDs over time. In general, high ethanol-consuming selectively bred rats have been used to test a wide range of compounds. Older studies usually followed neurobiological findings in the selected lines that supported an association with a propensity for high ethanol intake. Most of these tests evaluated the compound's effects on the maintenance of ethanol drinking. Very few compounds have been tested during ethanol-seeking and/or relapse and fewer still have assessed their effects during the acquisition of AUDs. Overall, while a substantial number of neurotransmitter and neuromodulatory system targets have been assessed; the roles of sex- and age-of-animal, as well as the acquisition of AUDs, ethanol-seeking and relapse continue to be factors and behaviors needing further study. This article is part of the Special Issue entitled "Alcoholism".

Circadian activity rhythms and voluntary ethanol intake in male and female ethanol-preferring rats: effects of long-term ethanol access

Chronic alcohol (ethanol) intake alters fundamental properties of the circadian clock. While previous studies have reported significant alterations in free-running circadian period during chronic ethanol access, these effects are typically subtle and appear to require high levels of intake. In the present study we examined the effects of long-term voluntary ethanol intake on ethanol consumption and free-running circadian period in male and female, selectively bred ethanol-preferring P and HAD2 rats. In light of previous reports that intermittent access can result in escalated ethanol intake, an initial 2-week water-only baseline was followed by either continuous or intermittent ethanol access (i.e., alternating 15-day epochs of ethanol access and ethanol deprivation) in separate groups of rats. Thus, animals were exposed to either 135 days of continuous ethanol access or to five 15-day access periods alternating with four 15-day periods of ethanol deprivation. Animals were maintained individually in running-wheel cages under continuous darkness throughout the experiment to allow monitoring of free-running activity and drinking rhythms, and 10% (v/v) ethanol and plain water were available continuously via separate drinking tubes during ethanol access. While there were no initial sex differences in ethanol drinking, ethanol preference increased progressively in male P and HAD2 rats under both continuous and intermittent-access conditions, and eventually exceeded that seen in females. Free-running period shortened during the initial ethanol-access epoch in all groups, but the persistence of this effect showed complex dependence on sex, breeding line, and ethanol-access schedule. Finally, while females of both breeding lines displayed higher levels of locomotor activity than males, there was little evidence for modulation of activity level by ethanol access. These results are consistent with previous findings that chronic ethanol intake alters free-running circadian period, and show further that the development of chronobiological tolerance to ethanol may vary by sex and genotype.

Scheduled access alcohol drinking by alcohol-preferring (P) and high-alcohol-drinking (HAD) rats: modeling adolescent and adult binge-like drinking

Binge alcohol drinking continues to be a public health concern among today's youth and young adults. Moreover, an early onset of alcohol use, which usually takes the form of binge drinking, is associated with a greater risk for developing alcohol use disorders. Given this, it is important to examine this behavior in rat models of alcohol abuse and dependence. Toward that end, the objective of this article is to review findings on binge-like drinking by selectively bred alcohol-preferring (P) and high-alcohol-drinking (HAD) lines of rats. As reviewed elsewhere in this special issue, the P line meets all, and the HAD line meets most, of the proposed criteria for an animal model of alcoholism. One model of binge drinking is scheduled ethanol access during the dark cycle, which has been used by our laboratory for over 20 years. Our laboratory has also adopted a protocol involving the concurrent presentation of multiple ethanol concentrations. When this protocol is combined with limited access, ethanol intake is maximized yielding blood ethanol levels (BELs) in excess, sometimes greatly in excess, of 80 mg%. By extending these procedures to include multiple scheduled ethanol access sessions during the dark cycle for 5 consecutive days/week, P and HAD rats consume in 3 or 4 h as much as, if not more than, the amount usually consumed in a 24 h period. Under certain conditions, using the multiple scheduled access procedure, BELs exceeding 200 mg% can be achieved on a daily basis. An overview of findings from studies with other selectively bred, inbred, and outbred rats places these findings in the context of the existing literature. Overall, the findings support the use of P and HAD rats as animal models to study binge-like alcohol drinking and reveal that scheduled access procedures will significantly increase ethanol intake by other rat lines and strains as well.

Early ethanol and water consumption: accumulating experience differentially regulates drinking pattern and bout parameters in male alcohol preferring (P) vs. Wistar and Sprague Dawley rats

Alcohol-preferring (P) rats develop high ethanol intake over several weeks of water/10% ethanol (10E) choice drinking. However, it is not yet clear precisely what components of drinking behavior undergo modification to achieve higher intake. Our concurrent report compared precisely measured daily intake in P vs. non-selected Wistar and Sprague Dawley (SD) rats. Here we analyze their drinking patterns and bouts to clarify microbehavioral components that are common to rats of different genetic backgrounds, vs. features that are unique to each. Under sole-fluid conditions P, Wistar and SD rats all consumed water at a high initial rate followed by a slow maintenance phase, but 10E - in a distinctly different step-like pattern of evenly distributed bouts. During choice period, 10E vs. water patterns for P rat appeared as an overlap of sole-fluid patterns. The SD rat choice patterns resembled sole-fluid patterns but were less regular. Choice patterns in Wistar differed from both P and SD rats, by consisting of intermixed small frequent episodes of drinking both 10E and water. Wistar and SD rats increased choice ethanol intake by elevating the number of bouts. A key finding was that P rat increased choice ethanol intake through a gradual increase of the bout size and duration, but kept bout number constant. This supports the hypothesis that genetic selection modifies microbehavioral machinery controlling drinking bout initiation, duration, and other pattern features. Precision analysis of drinking patterns and bouts allows differentiation between genetic lines, and provides a venue for study of localized circuit and transmitter influences mediating mesolimbic control over ethanol consumption.

Pharmacologically relevant intake during chronic, free-choice drinking rhythms in selectively bred high alcohol-preferring mice

Multiple lines of high alcohol-preferring (HAP) mice were selectively bred for their intake of 10% ethanol (v/v) during 24-hour daily access over a 4-week period, with the highest drinking lines exhibiting intakes in excess of 20 g/kg/day. We observed circadian drinking patterns and resulting blood ethanol concentrations (BECs) in the HAP lines. We also compared the drinking rhythms and corresponding BECs of the highest drinking HAP lines to those of the C57BL/6J (B6) inbred strain. Adult male and female crossed HAP (cHAP), HAP replicate lines 1, 2, 3 and B6 mice had free-choice access to 10% ethanol and water for 3 weeks prior to bi-hourly assessments of intake throughout the dark portion of the light-dark cycle. All HAP lines reached and maintained a rate of alcohol intake above the rate at which HAP1 mice metabolize alcohol, and BECs were consistent with this finding. Further, cHAP and HAP1 mice maintained an excessive level of intake throughout the dark portion of the cycle, accumulating mean BEC levels of 261.5 ± 18.09 and 217.9 ± 25.02 mg/dl, respectively. B6 mice drank comparatively modestly, and did not accumulate high BEC levels (53.63 + 8.15 mg/dl). Free-choice drinking demonstrated by the HAP1 and cHAP lines may provide a unique opportunity for modeling the excessive intake that often occurs in alcohol-dependent individuals, and allow for exploration of predisposing factors for excessive consumption, as well as the development of physiological, behavioral and toxicological outcomes following alcohol exposure.

Effects of Lifelong Ethanol Consumption on Brain Monoamine Transmitters in Alcohol-Preferring Alko Alcohol (AA) Rats

The purpose of the present study was to examine the combined effects of aging and lifelong ethanol exposure on the levels of monoamine neurotransmitters in different regions of the brain. This work is part of a project addressing interactions of aging and lifelong ethanol consumption in alcohol-preferring AA (Alko Alcohol) line of rats, selected for high voluntary consumption of ethanol. Intake of ethanol on the level of 4.5–5 g/kg/day for about 20 months induced only limited changes in the neurotransmitter levels; the concentration of noradrenaline was significantly reduced in the frontal cortex. There was also a trend towards lower levels of dopamine and 5-hydroxytryptamine (5-HT) in the frontal cortex, and towards a lower noradrenaline level in the dorsal cortex. Aging was associated with a decreased concentration of dopamine in the dorsal cortex and with a declining trend in the striatum. The levels of 5-HT in the limbic forebrain were higher in the aged than in the young animals, and in the striatum, there was a trend towards higher levels in older animals. The data suggest that a continuous intake of moderate amounts of ethanol does not enhance the age-related alterations in brain monoamine neurotransmission, while the decline in the brain level of dopamine associated with aging may be a factor contributing to age-related neurological disorders.

Animal models for medications development targeting alcohol abuse using selectively bred rat lines: neurobiological and pharmacological validity

The purpose of this review paper is to present evidence that rat animal models of alcoholism provide an ideal platform for developing and screening medications that target alcohol abuse and dependence. The focus is on the 5 oldest international rat lines that have been selectively bred for a high alcohol-consumption phenotype. The behavioral and neurochemical phenotypes of these rat lines are reviewed and placed in the context of the clinical literature. The paper presents behavioral models for assessing the efficacy of pharmaceuticals for the treatment of alcohol abuse and dependence in rodents, with particular emphasis on rats. Drugs that have been tested for their effectiveness in reducing alcohol/ethanol consumption and/or self-administration by these rat lines and their putative site of action are summarized. The paper also presents some current and future directions for developing pharmacological treatments targeting alcohol abuse and dependence.

Chronic free-choice drinking in crossed high alcohol preferring mice leads to sustained blood ethanol levels and metabolic tolerance without evidence of liver damage

BACKGROUND

Crossed high alcohol preferring (cHAP) mice were selectively bred from a cross of the HAP1 × HAP2 replicate lines, and we demonstrate blood ethanol concentrations (BECs) during free-choice drinking that are reminiscent of those observed in alcohol-dependent humans. Therefore, this line may provide an unprecedented opportunity to learn about the consequences of excessive voluntary ethanol (EtOH) consumption, including metabolic tolerance and liver pathology. Cytochrome p450 2E1 (CYP2E1) induction plays a prominent role in driving both metabolic tolerance and EtOH-induced liver injury. In this report, we sought to characterize cHAP drinking by assessing whether pharmacologically relevant BEC levels are sustained throughout the active portion of the light-dark cycle. Given that cHAP intakes and BECs are similar to those observed in mice given an EtOH liquid diet, we assessed whether free-choice exposure results in metabolic tolerance, hepatic enzyme induction, and hepatic steatosis.

METHODS

In experiment 1, blood samples were taken across the dark portion of a 12:12 light-dark cycle to examine the pattern of EtOH accumulation in these mice. In experiments 1 and 2, mice were injected with EtOH following 3 to 4 weeks of access to water or 10% EtOH and water, and blood samples were taken to assess metabolic tolerance. In experiment 3, 24 mice had 4 weeks of access to 10% EtOH and water or water alone, followed by necropsy and hepatological assessment.

RESULTS

In experiment 1, cHAP mice mean BEC values exceeded 80 mg/dl at all sampling points and approached 200 mg/dl during the middle of the dark cycle. In experiments 1 and 2, EtOH-exposed mice metabolized EtOH faster than EtOH-naïve mice, demonstrating metabolic tolerance (p < 0.05). In experiment 3, EtOH-drinking mice showed greater expression of hepatic CYP2E1 than water controls, consistent with the development of metabolic tolerance (p < 0.05). EtOH access altered neither hepatic histology nor levels of alcohol dehydrogenase and aldehyde dehydrogenase.

CONCLUSIONS

These results demonstrate that excessive intake by cHAP mice results in sustained BECs throughout the active period, leading to the development of metabolic tolerance and evidence of CYP2E1 induction. Together, these results provide additional support for the cHAP mice as a highly translational rodent model of alcoholism.

Prairie voles as a novel model of socially facilitated excessive drinking

Social relationships strongly affect alcohol drinking in humans. Traditional laboratory rodents do not exhibit social affiliations with specific peers, and cannot adequately model how such relationships impact drinking. The prairie vole is a socially monogamous rodent used to study social bonds. The present study tested the prairie vole as a potential model for the effects of social affiliations on alcohol drinking. Same-sex adult sibling prairie voles were paired for five days, and then either separated into individual cages, or housed in pairs. Starting at the time of separation, the voles received unlimited access to alcohol in a two-bottle choice test versus water. Pair-housed siblings exhibited higher preference for alcohol, but not saccharin, than singly housed voles. There was a significant correlation between the amount of alcohol consumed by each member of a pair when they were housed together (r = 0.79), but not when housed apart (r = 0.20). Following automated analysis of circadian patterns of fluid consumption indicating peak fluid intake before and after the dark phase, a limited access two-hour two-bottle choice procedure was established. Drinking in this procedure resulted in physiologically relevant blood ethanol concentrations and increased Fos immunoreactivity in perioculomotor urocortin containing neurons (but not in nucleus accumbens or central nucleus of the amygdala). The high ethanol preference and sensitivity to social manipulation indicate that prairie voles can serve to model social influences on excessive drinking.

Repeated light-dark phase shifts modulate voluntary ethanol intake in male and female high alcohol-drinking (HAD1) rats

BACKGROUND

Chronic disruption of sleep and other circadian biological rhythms, such as occurs in shift work or in frequent transmeridian travel, appears to represent a significant source of allostatic load, leading to the emergence of stress-related physical and psychological illness. Recent animal experiments have shown that these negative health effects may be effectively modeled by exposure to repeated phase shifts of the daily light-dark (LD) cycle. As chronobiological disturbances are thought to promote relapse in abstinent alcoholics, and may also be associated with increased risk of subsequent alcohol abuse in nonalcoholic populations, the present experiment was designed to examine the effects of repeated LD phase shifts on voluntary ethanol intake in rats. A selectively bred, high alcohol-drinking (HAD1) rat line was utilized to increase the likelihood of excessive alcoholic-like drinking.

METHODS

Male and female rats of the selectively bred HAD1 rat line were maintained individually under a LD 12:12 cycle with both ethanol (10% v/v) and water available continuously. Animals in the experimental group were subjected to repeated 6-hour LD phase advances at 3 to 4 week intervals, while control rats were maintained under a stable LD cycle throughout the study. Contact-sensing drinkometers were used to monitor circadian lick patterns, and ethanol and water intakes were recorded weekly.

RESULTS

Control males showed progressively increasing ethanol intake and ethanol preference over the course of the study, but males exposed to chronic LD phase shifts exhibited gradual decreases in ethanol drinking. In contrast, control females displayed decreasing ethanol intake and ethanol preference over the course of the experiment, while females exposed to experimental LD phase shifts exhibited a slight increase in ethanol drinking.

CONCLUSIONS

Chronic circadian desynchrony induced by repeated LD phase shifts resulted in sex-specific modulation of voluntary ethanol intake, reducing ethanol intake in males while slightly increasing intake in females. While partially contrary to initial predictions, these results are consistent with extensive prior research showing that chronic stress may either increase or decrease ethanol intake, depending on strain, sex, stressor type, and experimental history. Thus, repeated LD phase shifts may provide a novel chronobiological model for the analysis of stress effects on alcohol intake.

Chronic ethanol intake modulates photic and non-photic circadian phase responses in the Syrian hamster

Chronic alcohol intake disrupts sleep and other circadian biological rhythms in both human alcoholics and in experimental animals. Recent studies from our laboratory indicate that these effects may be due, in part, to ethanol-induced alterations in fundamental properties of the circadian pacemaker. The present study explored the effects of chronic voluntary ethanol intake (25% v/v) on circadian phase responses to both photic and non-photic stimuli in Syrian hamsters. Hamsters were used in these experiments because they are a popular model organism in behavioral chronobiology research, and are characterized by unusually high levels of voluntary ethanol intake. Relative to controls, ethanol-exposed animals showed attenuation of circadian phase responses and wheel running activity following acute administration of the benzodiazepine, triazolam, a non-photic phase-shifting stimulus. In addition, ethanol-exposed animals displayed reduced phase advances, but normal phase delays, in response to brief light pulses. While the mechanisms underlying these effects remain to be elucidated, we hypothesize that ionotropic GABA and glutamate receptors may be involved, since these proteins serve as important targets for the neurobiological effects of ethanol, and are also known to be critically involved in the modulation of photic and non-photic circadian phase responses.

The alcohol-preferring AA and alcohol-avoiding ANA rats: neurobiology of the regulation of alcohol drinking

The AA (alko, alcohol) and ANA (alko, non-alcohol) rat lines were among the earliest rodent lines produced by bidirectional selection for ethanol preference. The purpose of this review is to highlight the strategies for understanding the neurobiological factors underlying differential alcohol-drinking behavior in these lines. Most early work evaluated functioning of the major neurotransmitter systems implicated in drug reward in the lines. No consistent line differences were found in the dopaminergic system either under baseline conditions or after ethanol challenges. However, increased opioidergic tone in the ventral striatum and a deficiency in endocannabinoid signaling in the prefrontal cortex of AA rats may comprise mechanisms leading to increased ethanol consumption. Because complex behaviors, such as ethanol drinking, are not likely to be controlled by single factors, system-oriented molecular-profiling strategies have been used recently. Microarray based expression analysis of AA and ANA brains and novel data-mining strategies provide a system biological view that allows us to formulate a hypothesis on the mechanism underlying selection for ethanol preference. Two main factors appear active in the selection: a recruitment of signal transduction networks, including mitogen-activated protein kinases and calcium pathways and involving transcription factors such as Creb, Myc and Max, to mediate ethanol reinforcement and plasticity. The second factor acts on the mitochondrion and most likely provides metabolic flexibility for alternative substrate utilization in the presence of low amounts of ethanol.

Phenotypic characterization of genetically selected Sardinian alcohol-preferring (sP) and -non-preferring (sNP) rats

Sardinian alcohol-preferring (sP) and -non-preferring (sNP) rats are one of the pairs of rat lines selectively bred for high and low alcohol preference and consumption, respectively, under the homecage, continuous two-bottle choice regimen. sP rats meet most of the fundamental criteria for an animal model of alcoholism, in that they voluntarily consume sufficient amounts of alcohol to achieve significant blood alcohol levels and produce psychopharmacological effects, including anxiolysis and motor stimulation. sP rats are also willing to 'work' (such as lever-pressing) for alcohol. Chronic alcohol drinking in sP rats results in the development of tolerance to a given effect of alcohol (specifically, motor incoordination) and relapse-like drinking (the alcohol deprivation effect). Conversely, sNP rats avoid alcohol virtually completely; their avoidance for alcohol being resistant even to an environmental manipulation such as long-term exposure to alcohol plus sucrose. sP and sNP rats have been characterized for different phenotypes, possibly associated to their different alcohol preference and consumption. In comparison with sNP rats, alcohol-naive sP rats displayed (1) more anxiety-related behaviors; (2) higher initial sensitivity to the locomotor stimulating and sedative/hypnotic effects of alcohol; and (3) lower sensitivity to the aversive effects of alcohol. The present paper reviews the data collected to date on alcohol drinking behavior and other alcohol-related behaviors in sP and sNP rats. The behavioral profile of sP rats is also compared with that of other lines of selectively bred alcohol-preferring rats and the heterogeneity resulting from this comparison is discussed in terms of different animal models for the different forms of alcoholism.

Chronic ethanol intake alters circadian period-responses to brief light pulses in rats

Although chronic alcohol intake is associated with widespread disruptions of sleep-wake cycles and other daily biological rhythms in both human alcoholics and experimental animals, the extent to which the chronobiological effects of alcohol are mediated by effects on the underlying circadian pacemaker remains unknown. Nevertheless, recent studies indicate that both adult and perinatal ethanol treatments may alter the free-running period and photic responsiveness of the circadian pacemaker. The present experiment was designed to further characterize the effects of chronic ethanol intake on the response of the rat circadian pacemaker to brief light pulses. Ethanol-treated and control animals were exposed to 15-min light pulses during either early or late subjective night on the first day of constant darkness following entrainment to a 12:12 light-dark cycle. Relative to pulses delivered during early subjective night and to "no-pulse" conditions, light pulses delivered during late subjective night resulted in period-shortening after-effects under constant darkness, but only in control animals, not in ethanol-treated animals. These results indicate that chronic ethanol intake reduces the responsiveness of the circadian pacemaker to acute photic stimulation, and suggest that the chronobiological disruptions seen in human alcoholics are due in part to alterations in circadian pacemaker function.

Temporal pattern in consumption of the first drink of the day in alcohol-dependent persons

Loss of control over drinking and the craving for alcohol are cardinal signs of alcohol dependence. Our clinical practice indicates that these cravings do not occur randomly during the day, but at the same times each day for the same patient. To validate this hypothesis that alcohol-dependent patients have a circadian rhythmic craving in their desire for their first drink of the day, we asked 217 persons diagnosed as alcohol-dependent according to DSM-IV criteria to complete a questionnaire that surveyed whether this craving occurred at a fixed time each day. Of the respondents, 82% reported it did; 87% of them could state the time of day they consumed their first daily drink; and 80% reported that the time of their first drink of the day did not vary much from one day to the next. The most frequent time of consuming the first drink of alcohol was between 09:00 and 11:00 h, and it was independent of the subjects' sleep-wake routine (the delay between the hours of wake-up and the time of the first urge for alcohol was 3:45 +/- 3:30 h) and lunch or dinner time. This rhythmicity seems to be a pertinent criterion for alcohol dependence syndrome.

Effects of lifelong ethanol consumption on drinking behavior and motor impairment of alcohol-preferring AA and alcohol-avoiding ANA rats

The effects of drinking ethanol throughout a lifetime on voluntary drinking behavior and ethanol-induced motor impairment were studied in alcohol-preferring AA (Alko, Alcohol) and alcohol-avoiding ANA (Alko, Non-Alcohol) rats of both sexes. At the age 3 months, the rats were tested for individual voluntary ethanol (10% vol./vol.) intake and ethanol-induced motor impairment (2 g/kg, i.p.). The rats were housed in group cages, half of them having 12% (vol./vol.) ethanol as the only source of fluid and the other half having free access to water. Food was always available for all animals. At the age of 23 months, their individual voluntary ethanol intake and ethanol-induced motor impairment were tested again. During forced drinking, the females of both strains consumed more ethanol than did the males. The ethanol consumption of the AA and ANA females and the ANA males increased significantly (P < .001) with age, but a slight decrease was seen in the ethanol consumption of the AA males. Time x strain interaction showed a significant (P < .05) difference in the ethanol consumption of male rats, with the AA males having a slight decrease in ethanol consumption with age, whereas the ANA males increased their ethanol consumption. After 19 months of forced ethanol exposure, AA males significantly decreased their individual voluntary ethanol consumption, and individual voluntary ethanol consumption by ethanol-exposed AA males was more pronounced (P < .001) than that of the AA rats that had free access to water (P < .05). For the female AA rats, those having free access to water significantly decreased their voluntary ethanol consumption (P < .05), but those having ethanol only did not. No significant changes in voluntary ethanol consumption with age or with different exposures were seen in the ANA rats. Body weights were higher in the groups having access to water than in the ethanol-only groups, but the differences were not significant within the AA and ANA strains. The ANA rats were significantly heavier in all groups. These results indicate that the voluntarily nondrinking ANA rats can drink almost as much ethanol as the voluntarily drinking AA rats when they are forced to drink ethanol and that lifelong forced ethanol drinking does not change their inherent drinking habits. When sensitivity to ethanol was measured with the tilting-plane test, the old AA female rats were more sensitive to ethanol than were the young ones. The young ANA females were more sensitive than the AA females when tested at 4 months. In males, aging did not produce any differences in ethanol sensitivity.

Brain ethanol levels after voluntary ethanol drinking in AA and Wistar rats

Brain ethanol was monitored in the nucleus accumbens with one minute microdialysis and headspace gas chromatography in male Wistar and alcohol preferring AA (Alko Alcohol) rats after voluntary limited access consumption without food restriction. The rats drank 0.93 +/- 0.14 (Wistar) and 0.73 +/- 0.07 g/kg (AA), with a resulting mean maximal brain ethanol level of 15.9 mM and 14.1 mM, respectively. Maximum brain ethanol levels for individual AA rats were in the range 9.4-33.6 mM, median 15.5 mM and for the individual Wistar rats in the range 2.5-35.2 mM, median 17.8 mM. There was a significant but not perfect correlation between the amount ethanol drunk and the resulting ethanol level in the nucleus accumbens, probably because of the rats not being food deprived before the experiment. The results show that rats drink pharmacologically meaningful doses in a voluntary limited access situation and that blood samples can give us a hint about the level attained in the brain, but to know the early brain concentration after drinking, microdialysis is an excellent tool.

Effects of continuous versus intermittent ethanol exposure on rat sympathetic neurons

BACKGROUND

Binge ethanol exposure is known to induce degeneration of central nervous system (CNS) neurons. Sympathetic hyperactivity has been related to ethanol withdrawal symptoms, but the effects of repeated withdrawals on peripheral sympathetic neurons have not been studied previously.

METHODS

The effects of continuous versus intermittent ethanol consumption on sympathetic neurons of the superior cervical ganglion (SCG) were studied in male Wistar rats. Two-month-old rats were divided into three groups: one group with ethanol (10% v/v) as the drinking fluid throughout the 51/2-month experiment (continuous, n = 9), one group drinking ethanol on 4 days/week and water on 3 days/week (intermittent, n = 9), and a control group (n = 9) with water as the only available fluid. All groups had food ad libitum. SCG volume, neuron density, and total number of neurons were measured by using unbiased morphometric methods.

RESULTS

As the mean daily ethanol consumption did not differ between the two ethanol-exposed groups (continuous 5.7 g/kg/day versus intermittent 5.8 g/kg/day), the total dose of ethanol consumed was 42% smaller in the intermittent group. The total number of SCG neurons decreased by 28%, and neuron density by 23%, in the intermittent group compared with the control group, whereas no significant neuron loss was observed in the continuous group. The volume of the SCG was similar in all study groups. The results suggest that repeated ethanol withdrawals, rather than ethanol exposure per se, are deleterious to sympathetic neurons.

CONCLUSIONS

Ethanol-induced degeneration of neurons is not only related to the amount of ethanol consumed, but also to the patterns of drinking.

Lifelong ethanol consumption and loss of locus coeruleus neurons in AA and ANA rats

The effects of lifelong ethanol exposure and aging on the morphology of the locus coeruleus (LC) were studied in the AA (Alko, Alcohol) and ANA (Alko, Nonalcohol) rats of both sexes. The ethanol-consuming (EtOH) rats were given 12% (v/v) ethanol as the only drinking fluid from 4 to 22 months of age, whereas the young (3-month-old) and aged (24-month-old) controls had only water available. The total LC neuron numbers were obtained by using the unbiased disector method. In the AA line, as we have previously reported. the EtOH female and male rats displayed a 26-30% loss of LC neurons compared with the controls. In the ANA line, the EtOH females had 30% fewer LC neurons than the controls (EtOH 1579 +/- 377 vs. controls 2264 +/- 269, ANOVA p < 0.01), whereas the EtOH males showed no neuron loss compared to the controls (EtOH 1848 +/- 525 vs. controls 2216 +/- 152, ANOVA NS). However, taking into account (sex by line ANCOVA) the markedly higher ethanol intake of the female rats in both lines, no gender or line differences in the ethanol-induced LC degeneration were detected. Neither was there any difference in LC neuron numbers between the young and old control rats of either line of rats. In conclusion, chronic alcohol consumption, not aging per se, damages the LC neurons in experimental animals.

Alcohol as a food: a commentary on Richter

The present study examined the maintenance of voluntary alcohol intake in male Long-Evans rats. A microstructural analysis of consummatory behaviors (food, alcohol, water) was carried out using a computerized drinkometer system. In this sample of animals, there was no association (r = 0.07) between total food intake and total alcohol intake. There was no compensation for the extra calories ingested in the form of alcohol via a reduction in total food intake, or a reduction in food bout sizes associated with pre- or postprandial alcohol consumption. Further microstructural analyses determined that there were no significant difference between water and alcohol in terms of their distribution in relation to food (non-, pre-, or postprandial bouts). Of the total of 586 bouts of fluid intake analyzed, 45.6% were consumed postprandially, with a similar number (43.2%) consumed nonprandially. A comparison of the size of food bouts associated with different fluid bout types (pre- or postprandial) indicated that food bouts were the same size regardless of whether they were accompanied by water or alcohol. A final analysis determined that 55% of the total daily alcohol intake was consumed postprandially, and that the sizes of non-, pre-, or postprandial fluid bouts were significantly different for water vs. alcohol. Post hoc pairwise comparisons found that alcohol postprandial bouts were significantly larger than all types of water bouts. Alcohol and water bouts ranged in size from < 0.5 ml to > 5.5 ml. There was a significant difference in the distribution of bout sizes with more alcohol bouts at the high end of the distribution. Only 24% of the water bouts were > 2.5 ml compared to 48.4% of the alcohol bouts. The results of this study demonstrate that rats organize their consummatory behavior in many discrete, short bouts. There were considerable individual differences in alcohol preference, alcohol-bout frequency, duration, and size, as well as the prandial distribution of bouts. All of these variables together produce the "pattern" of alcohol intake in individual animals, and is likely to influence the level of intoxication achieved. Although rats do not dissociate their alcohol intake from normal feeding patterns, alcohol bouts occurring postprandially are significantly larger than other bouts of fluid consumption, suggesting that animals perceive the pharmacological effects of and are affected by the alcohol they consume. In animals with a preference for alcohol solutions, it is unlikely that alcohol is consumed as a food.

Chronic alcohol feeding in liquid diet or in drinking water has similar effects on electron microscopic appearance of the hepatic sinusoid in the rat

Electron microscopic appearance of the liver sinusoid was examined in rats fed alcohol chronically in a complete liquid diet or in sucrose-containing drinking water. The animals were kept on liquid diet (+/- alcohol) for 14 weeks or on sucrose-containing drinking water (+/- alcohol) for 12.5 weeks and sacrificed thereafter. To rule out possible artifact induced by fixation procedure, livers were fixed by immersion (no perfusion), immersion preceded by perfusion, and by perfusion with glutaraldehyde and examined with both scanning and transmission electron microscopy. Regardless of the mode of its administration, and of the fixation procedure used, alcohol induced similar changes in liver sinusoid ultrastructure. Such changes included disruption of the sieve-plate pattern of the sinusoidal endothelial cell fenestrations with the appearance of large gaps and resulting in a meshwork lining, wherein large areas of the sinusoid communicated freely with the underlying hepatocytes. Transmission electron microscopy complemented these findings. The results reported in this study demonstrate that alcohol-induced structural changes of the liver sinusoid in the rat are similar whether alcohol is fed via a liquid diet or in drinking water. Therefore, alcohol administration in drinking water may provide a simple, inexpensive, and convenient method of inducing structural changes in the rat liver sinusoid.

Ethanol-induced vacuolation in rat peripheral nervous system

The effect of chronic (3-4 weeks), heavy ethanol exposure on neuronal vacuolation in rat peripheral nervous system was studied in male Wistar rats. The rats were force-fed with 25% ethanol 3 times a day, which resulted in blood ethanol levels of 53.1 +/- 18.8 mmol/l, i.e., marked intoxication. In the superior cervical ganglia (SCG), the ethanol exposure increased the proportion of vacuolated neurons c. 13-fold (0.2 +/- 0.0% in the control ganglia, 2.7 +/- 0.6% in the EtOH-ganglia, P < 0.001). A considerable population of vacuolated neurons (VN) was seen in the sensory inferior vagal (nodose) ganglia, and occasional neurons with large cytoplasmic vacuoles in the sensory dorsal root ganglia (DRG) of the EtOH-rats. In the hypogastric ganglia, where VN are regularly found in the adult rat, ethanol exposure did not affect the amount or the appearance of the vacuolated neurons. The number of VN in the SCG decreased significantly between 2 days and 1 week after cessation of the exposure, but did not return to control level by 1 month after ethanol withdrawal. In electron microscopy, most of the vacuolated SCG neurons showed normal ultrastructure, apart from the large cytoplasmic vacuoles. Some vacuolated neurons, however, showed neuropathologic changes, e.g., dilated endoplasmic reticulum, mitochondrial alterations and increased numbers of myelin figures. These degenerative changes were more frequent in the vacuolated DRG neurons than in the sympathetic ones. The occurrence of VN in rat peripheral ganglia may represent a reaction to increased stimulation during prolonged ethanol exposure and, especially, during repeated phases of ethanol withdrawal.

The limited access paradigm: description of one method

Restricting access to alcohol to a short period daily causes rats, in effect, to drink on command. They usually begin drinking alcohol immediately when it is first made available each day and consume a rather constant amount during each access period. The procedure thus has a variety of useful applications. The specific method reported here in detail provides continual access to food and water, but access to unflavored 10% ethanol solution only 1 h/d, all in the home cage, and produces mean alcohol intakes from 0.5-1.0 g/kg in the access hour.

Lifelong ethanol consumption enhances the age-related changes in rat sympathetic neurons

The effects of aging and chronic ethanol administration on the histochemical and morphometric features of rat superior cervical ganglion were studied in a rat strain selected for voluntary alcohol consumption. Ethanol was administered to the experimental group ad libitum (10% v/v in drinking water) from 3 months to 28 months of age, the average ethanol intake being 6.4-5.4 g/kg per day. The sympathetic neurons of the ethanol consuming rats showed several signs of enhanced degeneration, e.g. decreased neuronal packing density, increased amount of age-pigment and decreased intensity of catecholamine histofluorescence and tyrosine hydroxylase immunoreactivity. The results may indicate a selective vulnerability of peripheral sympathetic neurons rather than a universal accelerated aging due to chronic ethanol exposure.

Interaction of aging and lifelong ethanol ingestion on ethanol-related behaviors and longevity

The interactions of aging and long-term voluntary ethanol consumption were studied in the alcohol-preferring AA (Alko Alcohol) rats. The mean daily ethanol intake was 6.45 +/- 0.31 g/kg/day (mean +/- SE) at the beginning of the exposure at 3 months of age. The control animals were given only food and water ad libitum. There was no difference in survival or weight gain between the control and ethanol groups. When tested for voluntary ethanol intake at the age of 24 months, the rats in the ethanol group consumed significantly more ethanol than the controls. The two groups did not differ in ethanol-induced motor impairment, sleep-time, or hypothermia, nor in the rate of ethanol elimination. The 24-month-old animals, however, showed higher sensitivity to ethanol than the 3-4-month-old rats in the sleep-time test. It is concluded that the feeding regimen used in this study did not produce any detectable interactions between ethanol and the aging processes in the AA rats.

Hypothalamic monoamines and food intake in alcohol-preferring AA and alcohol-avoiding ANA rats

The concentrations and synthesis of monoamines in various hypothalamic nuclei and the influence of monoaminergic drugs on food intake were studied in two rat lines produced by selective outbreeding for voluntary high and low alcohol drinking. The hypothalamic nuclei of the alcohol-preferring AA rats contained slightly more serotonin than those of the alcohol-avoiding ANA rats, but the accumulation of 5-hydroxytryptophan after inhibition of aromatic amino acid decarboxylase was the same in both lines. There was no significant difference in the basal concentrations of catecholamines between the lines, but the accumulation of L-DOPA was significantly greater in the ANA than the AA rats, suggesting differences in catecholamine turnover. This difference was significant in the paraventricular nucleus, which is involved in the regulation of food intake. Clonidine (an alpha 2-agonist) and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, a 5-HT1A agonist) induced hyperphagia and 1-(3-trifluoromethylphenyl)piperazine (TFMPP, a 5-HT1B agonist) induced hypophagia dose-dependently in both rat lines. Clonidine tended to be more potent in the ANA than the AA rats. Food intake following a 20-h fast was significantly lower in the ANA than AA rats. These results suggest that the alcohol-avoiding ANA and alcohol-preferring AA rats have different hypothalamic monoamine mechanisms controlling food intake, which could also partially account for their differential alcohol acceptance.

Differential reinforcement and diurnal rhythms of lever pressing for ethanol in AA and Wistar rats

High-drinking AA (Alko, Alcohol) and moderate-drinking Wistar rats, after ethanol drinking experience in their home cages, were housed continually in operant chambers with free access to water and food. Ethanol and water could be obtained by lever pressing on a concurrent FR1:FR1 schedule. The AA rats readily learned the operant response for oral ethanol, responded significantly more for ethanol than water, and increased ethanol responding when the fixed-ratio schedule for it was increased from FR1 to FR2 and FR4. This indicates that ethanol was serving as a reinforcer for the AAs. In contrast, the Wistars showed little evidence for ethanol reinforcement. Both AAs and Wistars had a three-peak pattern of ethanol responding during the dark phase, but peaks for the Wistars preceded those for the AAs by 1 or 2 hr. The patterns were similar when on an FR4 schedule, which greatly reduced the amount of alcohol, suggesting that they are not controlled by blood alcohol levels. The difference between the AA and Wistar patterns may, however, be related to the differential ethanol reinforcement.

Social, circadian, nutritional, and subjective correlates of the spontaneous pattern of moderate alcohol intake of normal humans

The relationship of moderate alcohol intake to the subjective states of hunger, thirst, depression, and anxiety, to social facilitation, circadian rhythms, and the ingestion of other nutrients by humans spontaneously behaving in their natural environment was investigated. Ninety-six adults were paid to maintain 7-day diaries of everything they ingested, when and where they ingested it, the number of other people present, and their subjective states at the beginning and end of the meal. The data from the 64 subjects who reported alcohol intake were analyzed individually with univariate and multivariate regression techniques. Subjective states were not found to be associated with subsequent alcohol ingestion, but alcohol was found to be associated with a reduction in subsequent thirst and anxiety. The amount of alcohol ingested was found to be positively related to the amount of nonalcohol calories ingested, particularly carbohydrates, the hour of the day, and the number of other people present. These results suggest that moderate alcohol intake by normal humans in their natural environment is affected by a variety of influences, but is primarily related to the time of day and socio-cultural factors.

The AA and ANA rat lines, selected for differences in voluntary alcohol consumption

The offspring of rats that voluntarily select larger quantities of alcohol are heavier consumers of alcohol than the offspring of rats that tend to avoid it. Such selective breeding, repeated over many generations, was used to develop the AA (Alko, Alcohol) line of rats which prefer 10% alcohol to water, and the ANA (Alko, Non-Alcohol) line of rats which choose water to the virtual exclusion of alcohol. In addition to demonstrating the likely role of genetic factors in alcohol consumption, these lines have been used to find behavioral, metabolic, and neurochemical correlates of differential alcohol intake. Some of the line differences that have been found involve the reinforcing effects of ethanol, the changes in consumption produced by alcohol deprivation and nutritional factors, the behavioral and adrenal monoamine reactions to mild stress, the development of tolerance, the accumulation of acetaldehyde during ethanol metabolism, and the brain levels of serotonin. It is hoped that these studies will lead to a better understanding of the genetically-determined mechanisms that influence the selection of alcohol.

Demonstration of lever pressing for oral ethanol by rats with no prior training or ethanol experience

Male rats of the alcohol-preferring AA line were placed in an operant conditioning chamber with one lever delivering 10% alcohol solution and a second giving water. Free food and water were also continually available in the chamber so the animals should not have been motivated to obtain alcohol for reasons of hunger or thirst. The rats had never had alcohol previously. No shaping was used. The rats simply lived for the next 2 weeks in the operant chamber. All of them eventually learned to work for alcohol. Ethanol responding was significantly higher than pressing for water throughout the second week: on the last day, all rats pressed more than 300 times for alcohol and less than 40 times for water, took in a mean of 5.3 +/- 0.2 g/kg of ethanol, and obtained 72% of their total fluid as earned ethanol solution despite the presence of free water. Their acquisition was, however, much slower than that observed in male AA rats that had previously had prolonged access to drinking alcohol in their home cages. Living continually in the operant chamber is thought probably to have been an important factor in enabling the naïve rats to learn to work for alcohol.

Hepatic carbohydrate metabolism in rats bred for alcohol preference

The influence of ethanol on the carbohydrate metabolism was studied in two strains of rats: the AA strain with an inherited preference for alcohol and the ANA strain with an aversion to alcohol. In both strains, a single intraperitoneal dose of ethanol (1.5 g/kg body wt.) slightly increased the blood glucose concentration. In AA rats alcohol increased the rate of gluconeogenesis from alanine and had no effect on the liver glycogen stores, whereas in ANA rats the rate of gluconeogenesis remained unchanged and the glycogen stores decreased. It thus appears that the two rat strains maintain their blood glucose concentration by different mechanisms; the ANA rats utilise both glycogenolysis and gluconeogenesis but the AA rats only gluconeogenesis.