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 |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
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., 1995) 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, 1999
).
![]() |
SUBJECTS AND METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
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at the Academic Unit of this hospital at 09:00 and had a 30-min bed rest, after which a venous blood sample (10 ml) was withdrawn.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 Further blood samples (10 ml each) were then withdrawn at 10, 20, and 30 min after the end of the 20-min drinking session.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, 1994). The following additional parameters of importance to Trp disposition were also measured, albumin (Doumas and Biggs, 1972
), the physiological binder of Trp; non-esterified fatty acids (NEFA) (Mikac-Devic et al., 1973
), the physiological displacers of albumin-bound Trp; and glucose (Slein, 1963
), 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 |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
In our previous investigation of Trp metabolism and disposition after acute ethanol administration to fasting male volunteers, we found (Badawy et al., 1995) 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 1.52 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, 1971; Carlsson and Lindqvist, 1978
; Curzon, 1979
). 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, 1977
), and, at the secondary, but more immediate, level, Trp binding to albumin (Curzon, 1979
) 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, 1971
). 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., 1995
), 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., 1986a). For this reason, the experiments whose results are shown in Table 1
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., 1995
) 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 1
). The concentration of the physiological displacers of albumin-bound Trp, namely NEFA, was, however, significantly decreased by ethanol (Student's t-test) (Table 1
), as noted previously by us (Badawy et al., 1987
) and others (Jones et al., 1965
; Hannak et al., 1985
). 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.
|
![]() |
ACKNOWLEDGEMENTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
![]() |
FOOTNOTES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
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, 7988.[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), 6365.
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.), 9397.
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, 157164.[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, 217222.[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, 164169.[ISI][Medline]
Curzon, G. (1979) Relationship between plasma, CSF and brain tryptophan. Journal of Neural Transmission 15 (Suppl.), 8192.
Dakis, C. A. and Gold, M. S. (1985) Pharmacological approaches to cocaine addiction. Journal of Substance Abuse Treatment 2, 139145.[Medline]
Dakis, C. A. and Gold, M. S. (1990) Addictiveness of central stimulants. Advances in Alcohol and Substance Abuse 9, 926.
DiChiara, G., Acquas, E. and Tanda, G. (1996) Ethanol is a neurochemical surrogate of conventional reinforcers. Alcohol 13, 1317.[ISI][Medline]
Doumas, B. T. and Biggs, H. T. (1972) Determination of serum albumin. Standard Methods in Clinical Chemistry 7, 175188.
Fernstrom, J. D. and Wurtman, R. J. (1971) Brain serotonin content: physiological dependence on plasma tryptophan levels. Science 173, 149152.[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, 165172.[ISI][Medline]
Hannak, D., Bartlet, U. and Kattermann, R. (1985) Acetate formation after short-term ethanol administration in man. Biological Chemistry Hoppe-Seyler 36, 749753.
Jasinski, D. R. (1991) History of abuse liability testing in humans. British Journal of Addiction 86, 15591562.[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, 804813.[ISI][Medline]
Lieberman, H. R., Corkin, S., Spring, B. J., Growdon, J. H. and Wurtman, R. J. (198283) Mood, performance, and pain sensitivity: changes induced by food constituents. Journal of Psychiatric Research 17, 135145.
Littleton, J. and Little, H. (1994) Current concepts of ethanol dependence. Addiction 89, 13971412.[ISI][Medline]
Lukas, S. E., Mendelson, J. H. and Benedikt, R. A. (1986a) Instrumental analysis of ethanol-induced intoxication in human males. Psychopharmacology 89, 813.[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, 889895.[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, 5559.[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, 190192.[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, 11791184.[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, 375385.[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. 117123. 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, 13381341.[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, 15051508.[Abstract]