L-Tryptophan
Over the last 30 years, serotonin function has been a topic of intense research for understanding a variety of behavioral and psychiatric disorders, many of which have a core feature of impulsive behavior.
Our continuing research program at the Neurobehavioral Research Laboratory and Clinic investigates the role of serotonin in the expression of impulsive behavior using an experimental technique designed to temporarily increase or decrease brain serotonin synthesis. We examine how reduced serotonin synthesis affects impulsivity in healthy adults, as well as how reduced serotonin affects impulsivity following alcohol consumption in both healthy adults and problem drinkers.
L-Tryptophan Manipulation
The mechanisms that lead to synthesis of brain serotonin are fundamental to our ability to test how changes in brain serotonin affect impulsivity and other behaviors. Serotonin is unique among neurotransmitters in that its synthesis is dependent on a single amino acid, L-tryptophan, which is an essential amino acid available to humans only through our daily dietary intake. The first step in synthesis of serotonin is the transformation of L-tryptophan to 5-hydroxytryptophan by the enzyme tryptophan hydroxylase. This is the rate-limiting step in the synthetic process and important because tryptophan hydroxylase in the human brain is typically only 50% saturated with its tryptophan substrate. This makes tryptophan hydroxylase
(and therefore serotonin synthesis) sensitive to fluctuations in tryptophan
availability,
which is essential to the experimental process for temporarily
manipulating serotonin.
Because of the sensitivity to fluctuations in tryptophan availability, the most commonly used method for producing transient reductions in concentrations of brain serotonin synthesis is acute tryptophan depletion (ATD). This method involves administration of an amino-acid beverage that contains 15 amino acids, but lacks tryptophan. Consumption of the tryptophan depletion beverage initiates two separate processes that reduce the availability of tryptophan for brain serotonin synthesis and availability. First, the large influx of amino acids stimulates protein synthesis in the liver. But, without a proportionate intake of tryptophan, the concentration of endogenous plasma tryptophan is sharply reduced because of incorporation into newly synthesized proteins. In this process, the usual plasma tryptophan concentrations can be reduced by 96%. Second, the large influx of amino acids produces high plasma concentrations of five large neutral amino acids that compete with the (now) greatly reduced concentration of plasma tryptophan for transport into the brain. The small proportion of plasma tryptophan relative to these competing amino acids (i.e., CAAs: isoleucine, leucine, phenylalanine, tyrosine, and valine) minimizes transport of tryptophan across the blood-brain barrier. Together, these two processes greatly reduce tryptophan's availability in the brain for serotonin synthesis. In addition to ATD, serotonin synthesis can be maintained or increased using a tryptophan balanced or loading formulation, either of which can be used as a control condition for depletion. Tryptophan loading is accomplished by adding a disproportionately large amount of tryptophan to the same amino-acid formulation. This large amount of tryptophan relative to the competing LNAAs maximizes tryptophan's competitive advantage and increases the availability of tryptophan for brain serotonin synthesis.
Since the mid 1980s, when conducting studies using ATD, most researchers used a 100g amino-acid formulation to test behavioral changes. However, considerable unpleasant side effects (e.g., drowsiness, nausea, emesis) can result following consumption of this size beverage. These have the potential to confound experimental findings and interpretations through increased participant attrition or physical discomfort of study participants, which can produce less than optimal performance during behavioral testing. To ameliorate the unwanted side effects, smaller-size beverages (most commonly 50g) have been used in a number of recent studies, although the time course and effectiveness of these smaller drinks was never tested. In one of our recent studies, we compared the 100g and 50g sizes across a 7-hour time-course assessment of plasma amino acid changes following both depletion and loading (Dougherty et al., 2008). In this study we compared hourly changes in the ratio of plasma tryptophan to CAA, which is the best indicator of tryptophan transport into the brain and subsequent reduction in serotonin synthesis. We found that changes in the ratio were similar for both the 100g (96% depletion) and 50g (89% depletion) formulations, but side effects and attrition were significantly lower following consumption of the 50g beverage. Determining that the 50g beverage is an effective means for depleting tryptophan and reducing unwanted side effects is a major addition to the literature of tryptophan research. Our work will provide a ready reference for researchers when planning future studies for examining mood and behavioral changes that result from serotonin dysregulation.
Using the 50g formulation, we have also tested the combined effects of tryptophan depletion (i.e., reduced serotonin) and alcohol consumption to examine changes in impulsive behavior (Dougherty et al., 2007). Initial results showed that ATD plus Alcohol significantly increased impulsivity relative to all other experimental conditions (ATD plus Non-Alcohol Placebo, ATL plus Alcohol, and ATL plus Non-Alcohol Placebo). One particularly novel aspect of this study is that all study volunteers were healthy adults without any pre-existing physical or psychiatric disorders. By testing healthy adults, we are more confident that the changes in impulsivity we observe following alcohol consumption are the result of reduced serotonin and not some other factor(s) related to an individual's underlying physical or psychiatric condition. When comparing impulsivity measured before any experimental intervention to impulsivity measured after consumption of each of the beverages we can infer that reduced serotonin function is likely to be an underlying causal factor in these behavioral changes particularly following alcohol consumption. The implication is that some individuals may be more vulnerable to alcohol-induced impulsive behavior that is (at least in part) related to brain serotonin dysregulation.
In one of our current studies we are testing whether three different types of impulsive behavior are differentially affected by lowered brain serotonin and alcohol consumption, and in two new studies just getting underway, we are testing alcohol-induced impulsive behavior of binge and non-binge drinkers. In one study we will determine how reduced serotonin synthesis (via ATD) interacts with alcohol consumed during a simulated binge to produce increased impulsivity. In the other we will determine the relationship of serotonin reductions, impulsivity, and the ability for binge drinkers to reduce their alcohol consumption during a contingency management procedure (i.e., rewards received at the end of each week for successful drinking reduction as measured by a continuous alcohol monitoring device). These and future studies will reveal how serotonin dysregulation impacts alcohol consumption and impulsive behaviors with an ultimate goal of applying this knowledge in community prevention and intervention settings.
Dietary Sources of Tryptophan
This table provides examples of common foods and the amounts of tryptophan and the competing amino acids (CAAs) in those foods. More importantly, the table provides the ratio of tryptophan to CAAs. It is this ratio that provides the best indication of tryptophan availability for transport across the blood-brain barrier for use in serotonin synthesis. The foods listed below are ranked in descending order by the ratio of tryptophan (Trp) to Competing Amino Acids (CAAs), which is a convenient way to compare different foods and the relative amounts of tryptophan available to the brain. Popular lore holds that it is tryptophan that causes the sleepiness we feel after a turkey dinner, but interestingly, turkey ranks quite near the bottom of this list in terms of tryptophan availability. To put these measurements in context, for example, the recommended daily allowance of tryptophan for a 175 lb (79 kg) adult is 278 to 476 mg. Of course this amount takes the needs of the whole body into account, not just the brain.
| Food Item |
Unit Measured |
L-tryptophan (mg) |
Sum of CAAs* (mg) |
Trp/CAA Ratio |
| Whole Milk |
Quart |
732 |
8,989 |
0.081 |
| Dried Prune
|
One Piece
|
2
|
27
|
0.074
|
|
Semisweet Chocolate
|
Ounce
|
18
|
294
|
0.061
|
|
Wheat Bread
|
Slice
|
19
|
317
|
0.060
|
|
Sweet Chocolate
|
Ounce
|
16
|
270
|
0.059
|
|
Oats for Oatmeal
|
Cup
|
147
|
2,617
|
0.056
|
|
White Bread
|
Slice
|
22
|
439
|
0.050
|
|
Chicken, Skinless, Boneless, Dark Meat (raw)
|
Pound
|
256
|
5,492
|
0.047
|
|
Chicken, Skinless, Boneless, Light Meat (raw)
|
Pound
|
238
|
5,122
|
0.046
|
|
Banana
|
One Medium
|
11
|
237
|
0.046
|
|
Canned Tuna
|
Ounce
|
472
|
10,591
|
0.045
|
|
2% Milk
|
Quart
|
551
|
12,516
|
0.044
|
|
Turkey, Skinless, Boneless, Light Meat (raw)
|
Pound
|
410
|
9,525
|
0.043
|
|
Turkey, Skinless, Boneless, Dark Meat (raw)
|
Pound
|
303
|
7,036
|
0.043
|
| Peanuts |
Ounce |
65 |
1,574 |
0.041 |
| Cheddar Cheese |
Ounce |
91 |
2,298 |
0.040 |
| Apple |
One Medium |
2 |
70 |
0.029 |
*The CAAs are isoleucine, leucine, phenylalanine, tyrosine, and valine, the five large neutral amino acids that compete with tryptophan for entry into the brain.
Amino Acids in Tryptophan Depletion and Loading Formulations
Table 2. This table provides the amount of each of the 15 amino acids plus tryptophan for both the 50 g and 100 g L-tryptophan depletion and loading formulations used in experimental research. For perspective, recall (see Table 1) that recommended daily intake for a 175 lb (79 kg) adult is 278 to 476 milligrams. The amounts listed below are measured in grams.
| 50 g |
L-tryptophan Formulation |
100 g |
| 0.00 |
L-tryptophan Depletion |
0.00 |
| 5.15 |
L-tryptophan Loading |
10.30 |
| 50 g |
Competing Amino Acids (CAAs) |
100 g |
| 4.00 |
L-isoleucine |
8.00 |
| 6.75 |
L-leucine |
13.50 |
| 2.85 |
L-phenylalanine |
5.70 |
| 3.45 |
L-tyrosine |
6.90 |
| 4.45 |
L-valine |
8.90 |
| 50 g |
Remaining Amino Acids |
100 g |
| 2.75 |
L-alanine |
5.50 |
| 2.45 |
L-arginine |
4.90 |
| 1.35 |
L-cysteine |
2.70 |
| 1.60 |
Glycine HCl |
3.20 |
| 1.60 |
L-histidine |
3.20 |
| 4.45 |
L-lysine |
8.90 |
| 1.50 |
L-methionine |
3.00 |
| 6.10 |
L-proline |
12.20 |
| 3.45 |
L-serine |
6.90 |
| 3.25 |
L-threonine |
6.50 |
| 50.00 |
Depletion, total grams |
100.00 |
| 55.15 |
Loading, total grams |
110.30 |
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