Test for decreased serotonin/tryptophan metabolite ratios in abstinent alcoholics

Tryptophan metabolism in alcoholism

Acute and chronic alcohol (ethanol) intake and subsequent withdrawal exert major effects on tryptophan (Trp) metabolism and disposition in human subjects and experimental animals. In rats, activity of the rate-limiting enzyme of Trp degradation, liver Trp pyrrolase (TP), is enhanced by acute, but inhibited after chronic, ethanol administration, then enhanced during withdrawal. These changes lead to alterations in brain serotonin synthesis and turnover mediated by corresponding changes in circulating Trp availability to the brain. A low brain-serotonin concentration characterizes the alcohol-preferring C57BL/6J mouse strain and many alcohol-preferring rat lines. In this mouse strain, liver TP enhancement causes the serotonin decrease. In man, acute ethanol intake inhibits brain serotonin synthesis by activating liver TP. This may explain alcohol-induced depression, aggression and loss of control in susceptible individuals. Chronic alcohol intake in dependent subjects may be associated with liver TP inhibition and a consequent enhancement of brain serotonin synthesis, whereas subsequent withdrawal may induce the opposite effects. The excitotoxic Trp metabolite quinolinate may play a role in the behavioural disturbances of the alcohol-withdrawal syndrome. Some abstinent alcoholics may have a central serotonin deficiency, which they correct by liver TP inhibition through drinking. Further studies of the Trp and serotonin metabolic status in long-term abstinence in general and in relation to personality characteristics, alcoholism typology and genetic factors in particular may yield important information which should facilitate the development of more effective screening, and preventative and therapeutic strategies in this area of mental health.

Effect of DOV 102,677 on the volitional consumption of ethanol by Myers' high ethanol-preferring rat

BACKGROUND

Inhibitors of monoamine neurotransmitter transporters are well established as antidepressants. However, the evidence that single (serotonin) or dual (serotonin-norepinephrine) neurotransmitter uptake inhibitors can treat ethanol abuse, either as a comorbidity with depression or as a separate entity, is inconsistent. Drugs that have, in addition, the ability to inhibit dopamine uptake may have an advantage in the treatment of alcohol abuse. Therefore, the inhibitor of norepinephrine, serotonin and dopamine uptake, DOV 102,677, was tested for its effects on the volitional consumption of ethanol by an ethanol-preferring rat strain.

METHODS

Myers' high ethanol-preferring rats were screened by a 10-day, 3 to 30% step-up test and then given free access to the preferred concentration of ethanol in a 3-bottle choice task. Consumption of ethanol (g/kg), water, food, and body weight were measured daily during a 3-day predrug treatment period, a 3-day treatment period, and a 3-day posttreatment period. Additional Sprague-Dawley rats were observed for 24 hours for the behavioral effects of 2.0 mg/kg s.c. reserpine after a 30-minute pretreatment with different doses of DOV 102,677.

RESULTS

The triple monoamine uptake inhibitor DOV 102,677 dose-dependently decreased the volitional consumption of ethanol by as much as 71.2% (20 mg/kg i.p., b.i.d.) over 3 days of administration. This effect carried over into the posttreatment period. Similarly, the proportion of ethanol to total fluids consumed declined by 66.2% (20 mg/kg s.c., b.i.d.), while food consumption and body weight were unaltered. In contrast, amperozide (2 mg/kg i.p., b.i.d.) suppressed the amount of ethanol consumed by 56%, while naltrexone (5 mg/kg i.p., b.i.d.) was without effect. DOV 102,677 (40 mg/kg s.c.) inhibited reserpine-induced akinesia and ptosis, but not hypothermia in Sprague-Dawley rats, consistent with its transient inhibition of serotonin transport, and more long-lived inhibition of norepinephrine and dopamine uptake.

CONCLUSIONS

DOV 102,677 significantly decreased the volitional consumption of ethanol with minimal alterations in the intake of food or on body weight in an ethanol-preferring rat strain, suggesting that triple reuptake inhibitors may find utility in treating alcohol abuse.

Compulsive drug-seeking behavior and relapse. Neuroadaptation, stress, and conditioning factors

The development of addiction and vulnerability to relapse following withdrawal is proposed to be the result of neuroadaptive processes within the central nervous system that oppose the acute reinforcing actions of drugs of abuse. These changes lead to impairment in the mechanisms that mediate positive reinforcement and the emergence of affective changes such as anxiety, dysphoria, and depression during withdrawal. Considerable evidence exists implicating perturbations in DA and 5-HT transmission in the nucleus accumbens--neurochemical systems that are activated by cocaine and ethanol self-administration and deficient during withdrawal--as potential substrates for these affective changes. In addition, growing evidence suggests that enhanced CRF release in the central nucleus of the amygdala represents a mechanism underlying the anxiogenic and stress-like consequences of withdrawal that are common to all drugs of abuse. A growing body of evidence also implicates dysregulation of the non-neuroendocrine CRF stress system within the central nucleus of the amygdala as a common factor in the anxiogenic and aversive consequences of withdrawal from drugs of abuse. Moreover, a possible link may exist between long-lasting abnormalities in CRF function in the CeA and vulnerability to relapse during protracted abstinence. Another presumably critical element contributing to the chronic relapsing nature of drug addiction is the learned responses to drug-related stimuli. The long-lasting efficacy of drug- and alcohol-associated contextual stimuli in eliciting drug-seeking behavior in animal models of relapse resembles the endurance of conditioned cue reactivity and cue-induced cocaine craving in humans and confirms a significant role of learning factors in the long-lasting addictive potential of cocaine. With cocaine, D1-dependent neural mechanisms within the medial prefrontal cortex and basolateral amygdala may be important substrates for the motivating effects of drug-related environmental stimuli. With ethanol, available data suggest a role for opioid receptors in the mediation of conditioned drug-seeking behavior. Finally, conditioning factors (i.e., exposure to drug-associated stimuli) and stress can interact to augment vulnerability to relapse. This finding emphasizes that it will be important to consider the simultaneous effects of multiple environmental triggers for relapse in the development of treatment and medication strategies.

Tryptophan metabolism in alcoholism

Studies of tryptophan (Trp) metabolism in relation to the serotonin status in alcoholism are of 2 types: (1) those related to the pharmacological effects of ethanol; (2) those concerning the serotonin status in the absence of alcohol intake. In experimental animals, acute and chronic ethanol administration and subsequent withdrawal exert a variety of effects on brain serotonin synthesis and turnover mediated by corresponding changes in Trp availability to the brain secondarily mainly to modulation of liver Trp pyrrolase (TP) activity. Alcohol-preferring mice and rats exhibit a central serotonin deficiency caused by, or in some cases associated with, a higher TP activity. Liver TP also appears to be a target of ethanol in man and evidence has recently emerged that alcoholics with positive family history are serotonin-deficient because of a lower availability of circulating Trp to the brain. Acutely, ethanol depletes brain serotonin in normal subjects, which may explain alcohol-induced aggression in susceptible individuals and also the incidence of depression in alcoholism. Trp availability to the brain is increased before the appearance of the alcohol-withdrawal syndrome in man, raising the possibility that the associated behavioural disturbances may involve the excitotoxic Trp metabolite quinolinate. Further studies of the Trp and serotonin status in relation to these important clinical features of alcohol dependence and alcoholism may therefore yield fruitful results.

Comparison of the alcohol-preferring P rat to the Wistar rat in behavioral tests of impulsivity and anxiety

To determine whether the selectively bred alcohol preferring P rat displays impulsive and anxiety-related behaviors, as have been noted in Type 2 human alcoholics, P rats were compared with outbred Wistar rats, the strain from which P rats were derived, on a series of behaviors reflecting impulsivity and anxiety. The two groups were also compared on their volitional consumption of ethanol. When compared with the Wistar rats, the P rats preferred a higher concentration of ethanol and imbibed a much greater amount of ethanol when they were offered their preferred concentration. However, the behavioral tasks produced inconsistent results. The P rats completed 100 bar presses for food in less time when tested on a constant reinforcement schedule, which suggests that they are hyperactive compared to Wistar rats. However, the P rats also emitted a higher percentage of reinforced responses on differential reinforcement of low rate responding (DRL)-10s and gnawed less from a cork stopper, which suggests that they are less impulsive and possibly neophobic. The two groups did not differ on emergence into or activity in an open field, their activity in or open-arm duration in the elevated plus maze, or performance on DRL-5s and DRL-15s. Collectively, the behavioral data suggest that P rat does not serve as a model for the anxiety and impulsiveness associated with the Type 2 alcoholic individual.

Tryptophan metabolism and disposition in relation to alcohol and alcoholism

Tryptophan (Trp) metabolism and disposition in relation to alcohol and alcoholism are briefly reviewed. The changes observed could generally be classified into those: (1) exerted by acute or chronic alcohol administration and/or subsequent withdrawal; (2) already present in the absence of alcohol consumption, such as in naive alcohol-preferring animals or in abstinent alcoholics. In normal rats, acute ethanol administration activates liver Trp pyrrolase and exerts a biphasic effect on brain 5-HT (5-hydroxytryptamine or serotonin) synthesis, whereas chronic ethanol administration and subsequent withdrawal exert opposite effects on 5-HT synthesis mediated by corresponding changes in liver Trp pyrrolase activity. A cerebral 5-HT deficiency has been demonstrated in the alcohol-preferring C57BL mouse strain and in a number of alcohol-preferring rat lines, the mechanism of which is understood only in two models; the C57B1 mouse strain has a higher liver Trp pyrrolase activity and the P rat line from Indiana has a lower density of serotonergic fibres in cerebral cortex. In man, acute ethanol intake lowers circulating [Trp] and its availability to the brain, almost certainly by activating liver Trp pyrrolase. Some evidence exists for possible inhibition of pyrrolase activity in non-abstinent chronic alcoholics. Evidence in recently abstinent alcoholics suggests that Trp availability to the brain may be impaired and that this may be particularly so in patients with positive family history. Exploration of this latter possibility may be important in understanding the biological basis of predisposition to alcoholism.

Volitional consumption of ethanol by fawn-hooded rats: effects of alternative solutions and drug treatments

Behavioral and neurochemical measures of brain 5-hydroxytryptamine (5-HT) function in the Fawn-Hooded rat are abnormal relative to outbred strains of rats. Fawn-Hooded rats freely drink large amounts of 10% ethanol in the presence of water and have been proposed to be an animal model for studies related to alcoholism. In this study, Fawn-Hooded rats were given solutions of ethanol increasing in concentration from 3% to 30% (w/v in tap water) over 10 days with tap water in a second drinking tube and a third tube left empty. The solutions of ethanol that produced maximal drinking with a preference (ml ethanol/ml total fluid) near 50% ranged from 5% to 13%, which became the fixed individual concentrations for each rat. After a 5-day baseline period the rats were offered a solution in the third drinking tube of either 0.5% aspartame or chocolate Ultra SlimFast (diluted with water 2:1). The chocolate drink, but not aspartame, significantly reduced the consumption of alcohol by 73%. For the drug experiments, the rats were given successive 4-day periods of: baseline drinking; drug or saline injections b.i.d.; and a posttreatment period. Neither ipsapirone, a 5-HT1a partial agonist, nor naltrexone injected inhibited the intakes of ethanol solutions. Treatment with 2.5 mg/kg of amperozide, a 5-HT2 antagonist, decreased the consumption of ethanol by 38%, but also caused a decrease in consumption of food. These results show a pattern of drinking of increasing concentrations of ethanol different than other strains of rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonin and alcohol intake, abuse, and dependence: clinical evidence

A large body of literature has emerged concerning the role of the neurotransmitter serotonin (5-hydroxytryptamine, or 5-HT) in the regulation of alcohol intake and the development of alcoholism. Despite the wealth of information, the functional significance of this neurotransmitter remains to be fully elucidated. This paper, part one of a two-part review, summarizes the available clinical research along two lines: the effects of alcohol on serotonergic functioning and the effects of pharmacological manipulation of serotonergic functioning on alcohol intake in normal (nonalcohol dependent) and alcohol-dependent individuals. It is concluded that considerable evidence exists to support the notion that some alcoholic individuals may have lowered central serotonin neurotransmission.

The urinary excretion of tryptophan and tryptophan metabolites in the chronic ethanol-fed rat

An investigation was made into the hypothesis that chronic ethanol ingestion disturbs the metabolism of tryptophan which is reflected by alterations in the urinary excretion of the metabolites 5-hydroxyindoleacetic acid (5-HIAA), anthranilic acid (AA) and indoleacetic acid (IAA). In particular, we investigated whether experimental chronic alcoholism is associated with a decrease in the tryptophan metabolite ratios as suggested in the literature. Male Wistar rats were chronically fed a nutritionally-complete liquid diet in which ethanol comprised 35% of total calories: controls were pair-fed identical amounts of the same diet in which ethanol was replaced by isocaloric glucose. At 6 weeks, 24 h urine samples were collected for the analysis of tryptophan, 5-HIAA, AA and IAA by HPLC. During ethanol-feeding there were reductions in the daily urinary excretion (i.e. mumol/24 h) of tryptophan (-57%, P = 0.026) and concomitant increases in 5-HIAA excretion (62%, P = 0.057). Expression of data in terms of lean tissue mass (i.e. urinary creatinine) revealed identical conclusions. An analysis was performed on the molar ratios of these urinary analytes. The tryptophan: total metabolite ratio was significantly decreased (by -53%), but the AA: total metabolite ratio was not significantly altered (P = 0.102). The ratios 5-HIAA/AA and 5-HIAA/IAA were slightly increased, but they did not attain statistical significance (P > 0.351). It was concluded that chronic ethanol feeding is associated with significant changes in the urinary excretion of tryptophan and its related metabolites.(ABSTRACT TRUNCATED AT 250 WORDS)

Time course of ethanol-induced changes in serotonin metabolism

The effect of acute ethanol consumption on serotonin metabolism was examined in healthy volunteers in the fasted and fed state by determination of plasma and urinary levels of the serotonin metabolites 5-hydroxyindole-3-acetic acid (5-HIAA) and 5-hydroxytryptophol (5-HTOL). The plasma and urinary levels of 5-HIAA were reduced by about 40% and 25%, while the 5-HTOL levels were increased on an average 7-fold and 50-fold, respectively, after oral intake of ethanol (0.8 g/kg) over 30 min in a fasted state. The maximal effect on both 5-HIAA and 5-HTOL levels was found 4-6 h after starting drinking. Urinary 5-HTOL and the 5-HTOL/5-HIAA ratio did not return to baseline until 19 h after the start of the administration (i.e., about 10 h after ethanol reached zero level). The mean 24-h excretion of 5-HTOL was increased 15-fold by the ethanol intake, while the 5-HIAA excretion was not significantly different. A clear dose dependent effect was observed in one individual who also ingested a lower amount of ethanol (0.5 g/kg). When ethanol (0.8 g/kg) was ingested over 3 h together with food, the urinary level of 5-HTOL and the 5-HTOL/5-HIAA ratio did not return to baseline until after 20-22 h. In other subjects who had unlimited access to ethanol and ingested between 1.3-2.3 g/kg together with food, the time to reach baseline 5-HTOL/5-HIAA ratio in urine ranged from 20 h to over 26 h.

Neuroendocrine responses to serotonergic agents in alcoholics

Previous studies have suggested a possible deficit in serotonergic function in alcoholism. In order to further assess the serotonergic system in alcoholism, the plasma cortisol and prolactin (PRL) responses following 6-chloro-2-[1-piperazinyl]pyrazine (MK-212), a direct-acting serotonin2 (5-HT2)/5-HT1c receptor agonist, L-5-hydroxytryptophan (L-5-HTP), a precursor of 5-HT, and placebo were compared in male alcoholics and normal controls. The increase in plasma cortisol following L-5-HTP was significantly lower in the alcoholic subjects compared with the normal controls. The plasma PRL, but not the plasma cortisol response, following MK-212 was also significantly lower in the alcoholics. L-5-HTP had no significant effect on plasma PRL levels in either group. The basal plasma cortisol and PRL concentrations of the alcoholics and normal controls were not significantly different. These data are consistent with previous reports of a serotonergic abnormality in alcoholism.

Liver tryptophan pyrrolase. A major determinant of the lower brain 5-hydroxytryptamine concentration in alcohol-preferring C57BL mice

The lower brain 5-hydroxytryptamine concentration in alcohol-preferring C57BL, compared with -non-preferring CBA, mice is caused by a decrease in circulating tryptophan availability to the brain secondarily to a higher liver tryptophan pyrrolase activity associated with a higher circulating corticosterone concentration. Activity or expression of liver tryptophan pyrrolase and/or their induction by glucocorticoids may be important biological determinants of predisposition to alcohol consumption.