Alcohol-induced euphoria: exclusion of serotonin

Christopher J. Morgan and Abdulla A.-B. Badawy*

Cardiff and Vale NHS Trust, Biomedical Research Laboratory, Whitchurch Hospital, Cardiff CF14 7XB, UK

Received 7 April 2000; in revised form 2 July 2000; accepted 22 August 2000


    ABSTRACT
 TOP
 FOOTNOTES
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS AND DISCUSSION
 ACKNOWLEDGEMENTS
 REFERENCES
 
— During the first 30 min after acute ethanol consumption by three fasting normal male volunteers, no increase in circulating tryptophan availability to the brain occurred. On the contrary, a small decrease was observed, which became stronger subsequently. We conclude from this preliminary study that brain serotonin levels are not increased after alcohol intake by normal subjects and that, consequently, this indolylamine is unlikely to mediate the euphoric effects of alcohol.


    INTRODUCTION
 TOP
 FOOTNOTES
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS AND DISCUSSION
 ACKNOWLEDGEMENTS
 REFERENCES
 
Euphoria, the feeling of well-being, has been reported during the early (10–15 min) phase of alcohol consumption, i.e. at the time of rising blood-ethanol concentration (Lukas et al., 1986aGo), and can be correlated with transient changes in the brain electroencephalographic activity (Lukas et al., 1986bGo). The neurochemical basis of euphoria in general and that induced by alcohol in particular remains unclear. At least four neuronal mechanisms have so far been implicated, namely those involving the dopaminergic, {gamma}-aminobutyric (GABA)-ergic, opioidergic, and serotonergic systems. For example, it has been suggested that ethanol may reinforce its own intake by activating GABA receptors, thus causing relaxation, and induce euphoria by releasing dopamine from mesolimbic structures and/or endogenous opioids through activation of the right prefrontal cortex (Littelton and Little, 1994; Tiihonen et al., 1994Go; DiChiara et al., 1996Go). It has also been suggested (Dakis and Gold, 1985Go, 1990Go) that central stimulants induce euphoria and reinforcement by activating dopamine circuits in mesolimbic neurones. The euphorigenic and other subjective effects reinforcing drug intake have led to development of abuse-liability tests for a variety of addictive and other drugs, including serotonin agonists and antagonists (Jasinski, 1991Go). One such agonist, m-chlorophenylpiperazine, has been shown to cause an increase in self-rating of euphoria by normal volunteers (Mueller et al., 1985Go), although such an effect could be demonstrated in another study (Schwartz et al., 1997Go) only in depressed subjects and not in normal volunteers.

One approach with which to demonstrate the possible involvement of serotonin in the euphoric effects of alcohol is if consumption of the latter can be shown to increase brain serotonin concentration. We have examined this possibility in the present work by studying the availability to the brain of the serotonin precursor tryptophan (Trp) during the first 30 min after acute ethanol consumption by normal male volunteers. Previously, we showed (Badawy et al., 1995Go) that acute ethanol consumption by human volunteers actually decreases circulating Trp concentration and availability to the brain, thus decreasing, rather than increasing, brain serotonin synthesis, over a 3-h period. The first time interval of observation was 30 min after ethanol intake, at which the above decreases were already evident. In the present work, it was hypothesized that only an earlier increase in Trp availability to the brain could implicate serotonin in the euphoric effects of alcohol. A summary of this work has appeared in abstract form (Morgan and Badawy, 1999Go).


    SUBJECTS AND METHODS
 TOP
 FOOTNOTES
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS AND DISCUSSION
 ACKNOWLEDGEMENTS
 REFERENCES
 
Subjects
Healthy male volunteers, recruited from hospital staff and science and medical students, were all moderate social drinkers with no family history of alcoholism. Upon screening, all subjects had normal liver function and haematological profiles, were free from any organic or psychiatric disease, and not under any medication. The three volunteers studied in the present work were part of a larger group of subjects who took part in our previous detailed study of the effects of alcohol consumption on Trp metabolism and disposition (Badawy et al., 1995Go), for which ethical approval by the local Ethics Committee and informed consent by the subjects themselves were both obtained. The subjects' age range was 22–35 years. All subjects fasted overnight (12 h) and did not consume any alcoholic beverages for the 24 h preceding the experiments. Other experimental details have been described previously (Badawy et al., 1995Go).

Ethanol administration and blood sampling procedures
Ethanol (99.7% pure, Hayman Ltd, Witham, Essex, UK) was mixed with fresh orange juice and administered orally in a dose of 0.8 g/kg body wt, in the form of a 25% (v/v) solution (total vol.: 4 ml/kg body wt) spaced over 20 min. Subjects arrived at the Academic Unit of this hospital at 09:00 and had a 30-min bed rest, after which a venous blood sample (10 ml) was withdrawn. Immediately thereafter, the subjects started consuming the ethanol solution over the 20-min period. Further blood samples (10 ml each) were then withdrawn at 10, 20, and 30 min after the end of the 20-min drinking session. Subjects remained supine throughout the experiment. Serum was isolated immediately after venesection and frozen at –40°C, along with an ultrafiltrate prepared at the same time from a 1-ml portion as described below. Both the ultrafiltrates and their original sera were analysed for the laboratory parameters described below on the following day.

Laboratory and statistical procedures
Free (ultrafiltrable) and total (free + albumin-bound) serum Trp concentrations were determined fluorimetrically as described by Badawy and Evans (1976). Ultrafiltration was performed using the Amicon Micropartition MPS-1 assembly on fresh serum, to avoid the effect of freezing on Trp binding (Morgan and Badawy, 1994Go). The following additional parameters of importance to Trp disposition were also measured, albumin (Doumas and Biggs, 1972Go), the physiological binder of Trp; non-esterified fatty acids (NEFA) (Mikac-Devic et al., 1973Go), the physiological displacers of albumin-bound Trp; and glucose (Slein, 1963Go), which can cause an insulin-mediated modulation of Trp entry into the brain. Results are expressed as means ± SD and were analysed statistically by one-way analysis of variance (ANOVA) for repeated measures followed by Tukey's multiple comparison test, using the STAT-100 programme (Biosoft 1995/96, 37 Cambridge Place, Cambridge CB2 1NS, UK).


    RESULTS AND DISCUSSION
 TOP
 FOOTNOTES
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS AND DISCUSSION
 ACKNOWLEDGEMENTS
 REFERENCES
 
Previous studies
The dose of ethanol used in the present work (0.8 g/kg body wt) has previously been shown by us (Badawy et al., 1987Go) to produce a maximum blood-ethanol concentration of 78 ± 7 mg/dl at 1 h, with a value at 30 min of 59 ± 4 mg/dl (means ± SEM for 10 subjects). The present experiments were thus conducted early during the ascending phase of the blood-ethanol concentration curve, i.e. at the time euphoria is experienced (Lukas et al., 1986aGo).

In our previous investigation of Trp metabolism and disposition after acute ethanol administration to fasting male volunteers, we found (Badawy et al., 1995Go) that ethanol (0.8 g/kg body wt) decreased free and total serum Trp concentrations over a 3 h period. The maximum decreases occurred at 1.5–2 h after ethanol intake and were 28 and 24% for free and total [Trp] respectively. At 30 min, the first time interval to be examined, the decreases were already evident (10 and 12% respectively).

Brain Trp concentration is the most important single determinant of cerebral serotonin synthesis, because the rate-limiting enzyme of this synthesis, Trp hydroxylase, is unsaturated with its Trp substrate under physiological conditions (Fernstrom and Wurtman, 1971Go; Carlsson and Lindqvist, 1978Go; Curzon, 1979Go). It follows therefore that peripheral factors influencing Trp availability to the brain must play important roles in cerebral serotonin synthesis. These factors include activity of liver Trp pyrrolase at the primary level (Badawy, 1977Go), and, at the secondary, but more immediate, level, Trp binding to albumin (Curzon, 1979Go) and competition with Trp by five other circulating amino acids (Val, Leu, Ile, Phe, and Tyr) collectively known as the competing amino acids (CAA) for entry into the brain (Fernstrom and Wurtman, 1971Go). In human studies, the most accurate predictor of changes in brain Trp, and hence 5-HT, is therefore the ratio of serum [Trp]/[CAA]. In our earlier study (Badawy et al., 1995Go), we found that ethanol decreased both the free and total Trp ratios significantly as early as 30 min and up to the 2 h time point. Because Trp binding was not altered by ethanol and due to other considerations, we then concluded that the decreases in free and total serum [Trp] are likely to be caused by activation of liver Trp pyrrolase by acute ethanol intake.

Present study
From these earlier studies, it appears therefore that acute ethanol consumption by fasting males causes decreases in circulating Trp concentrations and availability to the brain, which are almost certain to lead to inhibition of cerebral serotonin synthesis. Thus, at no time during the 3-h observation period did we observe an increase in the serum concentrations of the serotonin precursor Trp. It could, however, be argued that an increase in circulating [Trp] could have occurred earlier than 30 min after ethanol consumption, i.e. at a time euphoria is experienced (Lukas et al., 1986aGo). For this reason, the experiments whose results are shown in Table 1Go were performed. Free serum and total serum Trp concentrations showed a gradual decrease during the first 30 min after consumption of a 0.8 g/kg body wt dose of ethanol by fasting male volunteers. We showed previously (Badawy et al., 1995Go) that the decrease at 30 min was significant, presumably because of the larger number (10) of subjects tested. The percentage free serum Trp (an expression of Trp binding to albumin) was also not significantly altered by ethanol, nor was the concentration of the Trp binder albumin (Table 1Go). The concentration of the physiological displacers of albumin-bound Trp, namely NEFA, was, however, significantly decreased by ethanol (Student's t-test) (Table 1Go), as noted previously by us (Badawy et al., 1987Go) and others (Jones et al., 1965Go; Hannak et al., 1985Go). However, this decrease does not seem to have influenced Trp binding. The increase in serum glucose concentration by ethanol was not significant. A one-way ANOVA with repeated measures (Tukey's multiple comparison test) revealed no significant group (time) differences for the percentage free serum Trp or serum albumin or glucose concentrations. There were also no significant group differences in free serum and total serum Trp and serum NEFA concentrations at 10 min, 20 min, or 30 min after ethanol intake. The only significant differences in these three latter parameters were observed between the zero time group and those at 20 min and 30 min after ethanol consumption.


View this table:
[in this window]
[in a new window]
 
Table 1. Effects of acute ethanol consumption on concentrations of serum tryptophan, albumin, non-esterified fatty acids, and glucose in fasting male volunteers
 
In conclusion, although only three subjects were investigated in the present experiments, their response to acute ethanol consumption was both adequate and clear-cut in demonstrating a decrease in serum tryptophan concentrations consistent with our previous findings with larger numbers of observation, with no evidence of an increase in circulating Trp availability to the brain. Although the results with this small number of subjects do not support the idea that serotonin mediates the euphoric effects of alcohol, they must be considered as preliminary and therefore still requiring confirmation and replication in a larger number of subjects, in whom euphoria needs to be assessed simultaneously with the biochemical measurements. A number of previous studies have examined the effects of administration of the serotonin precursor Trp on self-rated euphoria and mood. In one such study (Greenwood et al., 1975Go), Trp did not induce euphoria, whereas in the others, it either enhanced (Smith and Prockop, 1962Go; Charney et al., 1982Go), or exerted no effect (Lieberman et al., 1982–83Go; Cowen et al., 1985Go) on mood. In relation to alcohol consumption, it is difficult to consider whether euphoria is a mood state, an expression of drug reward, or both. The present results, however, suggest that, even if serotonin has a recognized role in mood regulation, it is unlikely to be involved in the euphoric effects of alcohol. Other neuronal mechanisms are therefore a more likely explanation.


    ACKNOWLEDGEMENTS
 TOP
 FOOTNOTES
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS AND DISCUSSION
 ACKNOWLEDGEMENTS
 REFERENCES
 
We thank the volunteers for their participation and the nursing staff of the Academic Unit for provision of facilities and assistance during the study.


    FOOTNOTES
 TOP
 FOOTNOTES
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS AND DISCUSSION
 ACKNOWLEDGEMENTS
 REFERENCES
 
* Author to whom correspondence should be addressed. Back


    REFERENCES
 TOP
 FOOTNOTES
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS AND DISCUSSION
 ACKNOWLEDGEMENTS
 REFERENCES
 
Badawy, A. A.-B. (1977) Minireview: the functions and regulation of tryptophan pyrrolase. Life Sciences 21, 755–767.[ISI][Medline]

Badawy, A. A.-B. and Evans, M. (1976) Animal liver tryptophan pyrrolases — absence of detectable apoenzyme and of hormonal induction mechanism from species sensitive to tryptophan toxicity. Biochemical Journal 158, 79–88.[ISI][Medline]

Badawy, A. A.-B., Morgan, C. J., Thomas, D. R. and Lovett, J. W. T. (1987) The acute effects of ethanol on the serum concentration of tryptophan and other constituents in fasting normal male volunteers. Annals of Clinical Biochemistry 24 (Suppl. 1), 63–65.

Badawy, A. A.-B., Morgan, C. J., Lovett, J. W. T., Bradley, D. M. and Thomas, R. (1995) Decrease in circulating tryptophan availability to the brain after acute ethanol consumption by normal volunteers: implications for alcohol-induced aggressive behaviour and depression. Pharmacopsychiatry 28 (Suppl.), 93–97.

Carlsson, A. and Lindqvist, M. (1978) Dependence of 5-HT and catecholamine synthesis on concentrations of precursor amino acids in rat brain. Naunyn-Schmiedeberg's Archives of Pharmacology 303, 157–164.[ISI][Medline]

Charney, D. S., Heninger, G. R., Reinhard, J. F., Jr, Sternberg, D. E. and Hafstead, K. M. (1982) The effect of intravenous l-tryptophan on prolactin and growth hormone and mood in healthy subjects. Psychopharmacology 77, 217–222.[ISI][Medline]

Cowen, P. J., Gadhvi, H., Gosden, B. and Kolakowska, T. (1985) Response of prolactin and growth hormone to l-tryptophan infusion: effects in normal subjects and schizophrenic patients receiving neuroleptics. Psychopharmacology 86, 164–169.[ISI][Medline]

Curzon, G. (1979) Relationship between plasma, CSF and brain tryptophan. Journal of Neural Transmission 15 (Suppl.), 81–92.

Dakis, C. A. and Gold, M. S. (1985) Pharmacological approaches to cocaine addiction. Journal of Substance Abuse Treatment 2, 139–145.[Medline]

Dakis, C. A. and Gold, M. S. (1990) Addictiveness of central stimulants. Advances in Alcohol and Substance Abuse 9, 9–26.

DiChiara, G., Acquas, E. and Tanda, G. (1996) Ethanol is a neurochemical surrogate of conventional reinforcers. Alcohol 13, 13–17.[ISI][Medline]

Doumas, B. T. and Biggs, H. T. (1972) Determination of serum albumin. Standard Methods in Clinical Chemistry 7, 175–188.

Fernstrom, J. D. and Wurtman, R. J. (1971) Brain serotonin content: physiological dependence on plasma tryptophan levels. Science 173, 149–152.[ISI][Medline]

Greenwood, M. H., Lader, M. H., Kantameneni, B. D. and Curzon, G. (1975) The acute effects of oral (–)-tryptophan in human subjects. British Journal of Clinical Pharmacology 2, 165–172.[ISI][Medline]

Hannak, D., Bartlet, U. and Kattermann, R. (1985) Acetate formation after short-term ethanol administration in man. Biological Chemistry Hoppe-Seyler 36, 749–753.

Jasinski, D. R. (1991) History of abuse liability testing in humans. British Journal of Addiction 86, 1559–1562.[ISI][Medline]

Jones, D. P., Perman, E. S. and Lieber, C. S. (1965) Free fatty acid turnover and triglyceride metabolism after ethanol ingestion in man. Journal of Laboratory and Clinical Medicine 66, 804–813.[ISI][Medline]

Lieberman, H. R., Corkin, S., Spring, B. J., Growdon, J. H. and Wurtman, R. J. (1982–83) Mood, performance, and pain sensitivity: changes induced by food constituents. Journal of Psychiatric Research 17, 135–145.

Littleton, J. and Little, H. (1994) Current concepts of ethanol dependence. Addiction 89, 1397–1412.[ISI][Medline]

Lukas, S. E., Mendelson, J. H. and Benedikt, R. A. (1986a) Instrumental analysis of ethanol-induced intoxication in human males. Psychopharmacology 89, 8–13.[ISI][Medline]

Lukas, S. E., Mendelson, J. H., Benedikt, R. A. and Jones, B. (1986b) EEG alpha activity increases during transient episodes of ethanol-induced euphoria. Pharmacology, Biochemistry and Behavior 25, 889–895.[ISI][Medline]

Mikac-Devic, D., Stankovic, H. and Boskovic, K. (1973) A method for determination of free fatty acids in serum. Clinica Chimica Acta 45, 55–59.[ISI][Medline]

Morgan, C. J. and Badawy, A. A.-B. (1994) Effects of storage on binding and stability of tryptophan in human serum. Annals of Clinical Biochemistry 31, 190–192.[ISI][Medline]

Morgan, C. J. and Badawy, A. A.-B. (1999) Alcohol-induced euphoria: exclusion of serotonin. Alcohol and Alcoholism 34, 474.

Mueller, E. A., Murphy, D. L. and Sunderland, T. (1985) Neuroendocrine effects of m-chlorophenylpiperazine, a serotonin agonist, in humans. Journal of Clinical Endocrinology and Metabolism 61, 1179–1184.[Abstract]

Schwartz, P. J., Murphy, D. L., Wehr, T. A., Garcia-Borreguero, D., Oren, D. A., Moul, D. E., Ozaki, N., Snelbaker, A. J. and Rosenthal, N. E. (1997) Effects of meta-chlorophenylpiperazine infusions in patients with seasonal affective disorder and healthy control subjects. Archives of General Psychiatry 54, 375–385.[Abstract]

Slein, M. W. (1963) d-Glucose determination with hexokinase and glucose-6-phosphate dehydrogenase. In: Methods of Enzymatic Analysis, Bergmeyer, H.-U. ed., pp. 117–123. Academic Press, New York.

Smith, B. and Prockop, D. J. (1962) Central-nervous system effects of ingestion of l-tryptophan by normal subjects. New England Journal of Medicine 267, 1338–1341.[ISI]

Tiihonen, J., Kuikka, J., Hakola, P., Paanila, J., Airaksinen, J., Eronen, M. and Hallikainen, T. (1994) Acute ethanol-induced changes in cerebral blood flow. American Journal of Psychiatry 151, 1505–1508.[Abstract]





This Article
Abstract
FREE Full Text (PDF)
Alert me when this article is cited
Alert me if a correction is posted
Services
Email this article to a friend
Similar articles in this journal
Similar articles in ISI Web of Science
Similar articles in PubMed
Alert me to new issues of the journal
Add to My Personal Archive
Download to citation manager
Search for citing articles in:
ISI Web of Science (3)
Request Permissions
Google Scholar
Articles by Morgan, C. J.
Articles by Badawy, A. A.-B.
PubMed
PubMed Citation
Articles by Morgan, C. J.
Articles by Badawy, A. A.-B.