Engs, Ruth C. [Ed.], "Controversies in the Addiction's Field". Ting-Kai Li, M. D. CHAPTER 3: Neurobiological and Genetic Basis for Alcoholism Based Upon Research in Animal Models
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CHAPTER 3 Neurobiological and Genetic Basis for Alcoholism Based Upon Research in Animal Models
Ting-Kai Li, M. D.
Alcoholism is a disorder of alcohol (drug)-seeking behavior. This abnormal "appetite" for alcohol results in loss of control over drinking and tolerance to the pharmacological actions of the drug. Both psychological and physical dependence develop over time. Alcoholism, also known as the alcohol dependence syndrome (Edwards and Gross, 1976), is expressed as intense reinforced-responding for alcohol.
Alcohol-drinking or alcohol self-administration behavior is a quantitative trait. As with most quantitative traits (e.g., height, blood pressure or serum cholesterol), expression is influenced by both genetic and environmental factors. It is now known that there are significant genetic contributions to the normal ranges of variation in human behavior, including various dimensions of personality, cognitive abilities, and intelligence. There is also a genetic component to the variance in alcohol-drinking behavior (Jardine and Martin, 1984). Genetic influence is also seen in a number of behavioral disorders, including alcoholism (Goodwin et al., 1973). Subtypes of familial alcoholism have been identified, in which genetic and environmental factors appear to have different degrees of influence (Cloninger, 1986). Because ethanol is a weak drug which is effective only at concentrations thousands of times higher than that of other drugs of abuse, such as opiates, cocaine or amphetamine, genetic susceptibility becomes an important etiological consideration in ourquest to understand the pathogenesis of alcoholism. The identification of genetic vulnerability in subgroups of alcoholics does not diminish the importance of environmental influences, as is evident from the large number of "sporadic" (nonfamilial) cases in the affected population.
There is at this time still very little understanding about the biological mechanisms that promote abnormal alcohol-seeking behavior; however, studies in genetically-developed animal models are beginning to provide interesting leads that appear to be relevant to the human disorder. This
chapter will review some of the advances in this area. The animal models were developed with the notion that the neuropsychopharmacological actions of ethanol and how different individuals react to them can be important biological determinants. Ethanol's action is biphasic,i.e., it can be reinforcing (rewarding) in the low concentration range, but aversive at high concentrations (Pohorecky, 1977). Perception by the individual of the reinforcing actions of ethanol might be expected to maintain alcohol seeking behavior, where as aversive effects would be expected to extinguish this behavior. Continued exposure can evoke neuroadaptive and metabolic changes of different degree that attenuate the effects of ethanol.
Genetic Variability in Response to Alcohol in Humans
In looking at various biological responses to ethanol that can serve as feedback loops to influence alcohol drinking behavior in humans, it is noteworthy that a number of them, as well as drinking behavior itself, have shown a large degree of between-individual variability which are, in part, genetically determined (McClearn and Erwin, 1982; Li, 1985). These include: alcohol elimination rate, the sensitivity of the brain to the actions of alcohol (as exemplified by patterns of ethanol-induced change in the electroencephalogram), and systemic reactivity to ethanol's metabolism, as evidenced by the alcohol-flush reaction.
The alcohol-flush reaction is aversive and occurs in approximately 50% of Asian populations. It is caused by a genetic variation in the mitochondrial form of aldehyde dehydrogenase (ALDH2) that renders it inactive. The ALDH2 deficiency, acetaldehyde, which is quite toxic, accumulates in the body and provokes the flush reaction (Harada et al., 1981). The ALDH2 deficiency trait is protective against heavy drinking and individuals with this genetic trait represent a very small proportion (less than 10%) of Asian alcoholics as compared with the approximately 50% prevalence rate in the general population (Harada et al., 1983) The mutation on the ALDH2 gene that causes this trait has been identified, and it has been shown that ALDH2-deficiency is inherited as an autosomal dominant trait (Crabb et al., 1989). In other words, the dominant gene allele that codes for the inactive or low activity ALDH2 is protective against heavy drinking and alcoholism, whereas the recessive gene allele that codes for the active ALDH2 enzyme is permissive of heavy drinking. This is the first gene identified to have an important impact on drinking behavior and alcoholism.
It can be expected that other responses to alcohol's actions, such as individual differences in sensitivity to the reinforcing effects of ethanol, in capacity for developing tolerance to the aversive effects of ethanol, and in seventy of withdrawal reactions owing to physical dependence might
also be attributable, in part, to genetic factors. Experiments on the heritability of the high dose aversive effects of alcohol and of the chronic effects of ethanol administration (tolerance and physical dependence) are difficult to justify in humans for ethical reasons. Clearly, such studies and the study of the relation to alcohol-seeking behavior of these responses to ethanol are more appropriately carried out in experimental laboratory animals. For similar reasons, exploration of the neuroanatomical, neurophysiological, and neurochemical substrates of alcohol-seeking behavior can only be pursued with use of laboratory animals that exhibit differences in degree of alcohol preference or alcohol-seeking behavior.
Genetic Variability in Responses to Alcohol in Experimental Animals
In experimental animals, genetic influence has been documented for a variety of responses to ethanol including alcohol preference, alcohol metabolic rate, neuronal sensitivity to the acute effects of ethanol, the capacity to develop tolerance, and severity of withdrawal reactions (Li, 1985; McClearn and Erwin, 1982). A recent review (Crabbe, 1989) summarizes the various inbred mouse strains and selectively bred mouse and rat lines that differ in the above traits. The process of selective breeding systematically mates chosen individuals that exhibit the most extreme levels of a given phenotype (e.g., high or low measures of alcohol drinking preference) through successive generations. Over time, the selected lines would have a high or low frequency of the genes influencing that trait, while the frequencies for genes not affecting that trait should be randomly distributed. Apart from demonstrating experimentally that the phenotype in question is genetically influenced, these pharmacogenetically different animals provide useful tools for investigating mechanisms, since associated traits are likely to share common mechanisms through common gene action.
Genetic Animal Models for the Study of Alcohol-Seeking Behavior and Alcoholism
To elucidate the neurobiological mechanisms that underlie abnormal alcohol-seeking behavior, it is necessary to have experimental animals that would orally self-administer alcoholic solutions, (for the drug properties of ethanol) in amounts comparable to that in the human alcoholic. A major roadblock to studies of this nature in the past has been that most species of laboratory animals do not like to drink unflavored aqueous solutions containing moderate (10%) to high concentrations of ethanol. However, through selective breeding, rat lines that exhibit high and low alcoholdrinking preference have been raised. Two pairs of these, the alcohol preferring P and HAD lines and the
alcohol nonpreferring NP and LAD lines were raised in our laboratory from Wistar and N/Nih foundation stocks, respectively.
Studies in the past 12 years have shown that the P rats satisfy the major criteria foran animal model of alcoholism (Lesterand Freed, 1973; Cicero, 1979). Specifically, the P rats:
- 1. Voluntarily drink ethanol solutions (10-30%, v/v in water in quantities that elevate blood alcohol concentrations (BACs) into pharmacologically meaningful ranges (Li et al., 1979, Lumeng and Li, 1986). Blood ethanol concentrations as high as 200 mg% have been observed.
- 2. Develop, with chronic drinking, metabolic and neuronal tolerance (Lumeng and Li, 1986: Gatto et al., 1987a), and physical dependence as evidenced by withdrawal signs (Waller et al., 1982).
- 3. Work by operant responding (bar-pressing) to obtain ethanol in concentrations as high as 30%, but not because of the caloric value, taste or smell of the ethanol solutions (Penn et al., 1978; Murphy et al., 1989). The NP rats will not bar-press for ethanol in concentrations higher than 5%. Importantly, the P, but not the NP rats would self-administer ethanol even by direct intragastric infusion (Waller et al., 1984). Blood alcohol concentrations as high as 400 mg% were observed.
Associated Traits of Alcohol-Seeking Behavior
Comparison of the P rats with the selectively bred, alcohol-nonpreferring NP line of rats has revealed differences that suggest new avenues for exploration of the neurobiological basis of alcohol-seeking behavior. Compared with the NP rats:
- 1. The P rats are behaviorally stimulated by low doses of ethanol and are less affected by sedative-hypnotic doses of ethanol (Waller et al., 1986). The NP rats are more sensitive to the aversive properties of ethanol than the P rats (Froehlich et al., 1988a).
- 2. The P rats develop tolerance more quickly within a single session of exposure to a sedative-hypnotic dose of ethanol (acute tolerance), and this tolerance persists (as tested by a second dose of ethanol) for a much longer period of time (Waller et al.,1983; Gatto et al., 1987b). With chronic free-choice drinking, P rats develop tolerance to the aversive properties of ethanol. Concomitantly, the free-choice drinking of ethanol increases (unpublished data).
- 3. Ethanol-naive P rats exhibit lower levels of serotonin (5-HT) and dopamine (DA) in certain
brain regions (Murphy et al., 1982), most notably the nucleus accumbens (Murphy et al.. 1987). Higher densities of 5-HT receptors in some of these regions (cerebral cortex and hippocampus) have been found in the P rats as compared with NP rats (Wong et al., 1988). Preliminary immunocytochemical studies suggest that the P rats may have fewer 5-HT fibers in the affected regions as compared with NP rats (unpublished observations). The administration of 5-HT uptake inhibitors attenuates the alcohol-seeking behavior of the P rats (McBride et al., 1988).
The difference between lines in response to ethanol suggests that both the enhanced responsiveness to the low-dose reinforcing effects of ethanol, and the rapid development and persistence of tolerance to the highdose, aversive effects of ethanol are important in promoting high alcoholseeking behavior. Recent comparison studies in HAD and LAD rats have shown that, as with the P rats, selection for ethanol drinking preference (10%,v/v, vs. H20) produced lines (HAD) that exhibit operant responding for ethanol as reward in concentrations as high as 30%. As with NP rats, LAD rats responded very little for ethanol when alcohol concentration exceeded 5% (Levy et al., 1988). Furthermore, as was found in the P and NP rats, HAD animals exhibit longer persistence of tolerance after a single, sedative-hypnotic dose of ethanol than do the LAD animals (Froehlich et al., 1987). Therefore, the salient features of the hypothesis formulated from the studies of the P and NP rats appear generalizable.
More recently, the contents of dopamine (DA), serotonin (5-HT), and their primary acid metabolites were assayed in 10 brain regions of the HAD and LAD lines of rats (Gongwer et al., 1989). Compared with the LAD line, the contents of 5-HT and/or 5-hydroxyindoleacetic acid were lower in several brain regions (cerebral cortex, striatum, nucleus accumbens, septal nuclei, hippocampus and hypothalamus) of the HAD line. The levels of DA, 3,4-dihydroxyphenylacetic acid, and homovanillic acid were also lower in the nucleus accumbens and anterior striatum of the HAD rats. These data agree generally with the neurochemical findings in P and NP rats and implicate the involvement of 5-HT and DA pathways in mediating alcohol-drinking behavior. Other neurotransmitter systems that may have an important role in ethanol preference are gamma-aminobutyric acid (GABA) and the opioid peptides (Froehlich et a1., 1988b). Immunocytochemical and morphometric studies have revealed more GABAergic terminals in the nucleus accumbens of the P and HAD rats than in that of the NP and LAD rats (Hwang et al., 1988). How these neurotransmitter systems interface in producing alcohol-seeking behavior is currently under active investigation. It is our hope that understanding gained through such studies can lead to better treatment modalities for alcoholism.
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