Differential uptake, metabolism and behavioral effects of the D and L isomers of 5-hydroxytryptophan

Serotonin a la carte: supplementation with the serotonin precursor 5-hydroxytryptophan

This paper reviews the preclinical and clinical evidence regarding the use of the dietary supplement 5-hydroxytryptophan (5-HTP) for the treatment of depression. In the absence of supplementation with exogenous 5-HTP, the amount of endogenous 5-HTP available for serotonin synthesis depends on the availability of tryptophan and on the activity of various enzymes, especially tryptophan hydroxylase, indoleamine 2,3-dioxygenase, and tryptophan 2,3-dioxygenase (TDO). Factors affecting each of these are reviewed. The amount of 5-HTP reaching the central nervous system (CNS) is affected by the extent to which 5-HTP is converted to serotonin in the periphery. This conversion is controlled by the enzyme amino acid decarboxylase, which, in the periphery, can be blocked by peripheral decarboxylase inhibitors (PDIs) such as carbidopa. Preclinical and clinical evidence for the efficacy of 5-HTP for depression is reviewed, with emphasis on double-blind, placebo-controlled (DB-PC) trials. Safety issues with 5-HTP are also reviewed, with emphasis on eosinophilia myalgia syndrome (EMS) and serotonin syndrome.

5-Hydroxytryptophan plus SSRIs for interferon-induced depression: Synergistic mechanisms for normalizing synaptic serotonin

Interferon-alpha (IFN) is widely used in the treatment of certain cancers and viral infections, including hepatitis C (HCV). Unfortunately, depression is a common side effect of IFN therapy, affecting approximately a third of HCV patients receiving IFN therapy. Studies have shown that selective serotonin reuptake inhibitors (SSRIs) can effectively treat IFN-induced depression in only 63-75% of cases. For the remaining percentage, depression often necessitates dose reduction of or discontinuation from IFN therapy. Emerging evidence indicates that IFN may cause depression by affecting brain serotonin. IFN has been shown to increase serotonin reuptake and to decrease serotonin synthesis. We hypothesize that SSRIs are not fully effective because they affect only serotonin reuptake, not serotonin synthesis, and that effective treatment must address both uptake and synthesis. 5-Hydroxytryptophan (5-HTP) effectively increases central nervous system synthesis of serotonin. It is the immediate precursor of serotonin and is widely available as a dietary supplement, which is well absorbed after an oral dose. Several double-blind studies have shown 5-HTP to be effective in the treatment of nondrug-induced depression. We hypothesize that patients who become depressed on IFN will respond to the synergistic combination of SSRIs plus 5-HTP.

Differential effect of 5-hydroxytryptophan on approach and avoidance behavior in rats

The current hypersensitive postsynaptic serotonin receptor theory of depression developed and expanded by Aprison and Hingtgen was based on an animal model of behavior in which food-reinforced approach behavior was suppressed following 5-hydroxytryptophan (5-HTP) administration. In this paper, data are presented to show that when the same animal is taught to emit, alternatingly, approach and avoidance behavior, and the serotonin precursor, 5-HTP, is administered, only the approach behavior is affected. Adult, male Wistar rats were trained on Sidman avoidance (RS20:SS10) and food-reinforced approach (VI 1) schedules. During the first part of this study, rats received separately 50-min sessions for approach and avoidance responding. For the second part, both schedules were given in the same experimental chamber. In the third part, 10-min alternating approach and avoidance components were combined in the same 50-min sessions. Significant behavioral suppression of approach responding was observed following administration of L-5-HTP (50 mg/kg IP), as well as after D,L-5-HTP (25 and 50 mg/kg IP) in a dose-dependent relationship, whereas no significant effect was seen for Sidman avoidance responding during this type of session. These results support the role of serotonin in food-reinforced approach behavior and suggest that suppression of Sidman avoidance behavior may be mediated by other neurotransmitter systems.

Action of chronically administered antidepressants on the serotonergic postsynapse in a model of depression

A theory of excessive transmission of serotonin (5-HT) in depression has been previously proposed. The purpose of the present study was to test this theory further by using the model of depression in rats induced by L-5-hydroxytryptophan (5-HTP), the precursor of 5-HT. The drug effects on 5-HTP (25 mg/kg) induced behavioral depression were tested by chronic administration using methysergide which is a postsynaptic blocker of 5-HT, or by comparable clinical doses of antidepressant drugs. Methysergide (2 mg/kg) blocked 5-HTP induced depression on days 8 and 22 after initiation of medication by 70% and 83%, respectively. Among antidepressants, mianserin (2 mg/kg) was the first to produce an effect, displaying a 38% effect as early as 1 day after the start of medication and having blocking effects of 52% and 72% on days 8 and 22. Desipramine (5 mg/kg), doxepine (5 mg/kg), imipramine (5 mg/kg) and trazodone (10 mg/kg) showed no significant effect on days 1 and 8, and on day 22, 64, 36, 33 and 32% blocking, respectively. Amitriptyline had an initial effect of 41% at a dose of 10 mg/kg. Clomipramine (5 mg/kg), zimelidine (6 mg/kg) and chlorpromazine (2.5 mg/kg), which is a neuroleptic, showed no effect. Considering these results in light of recent data reported on the 5-HT synapse, it was suggested that 5-HTP induced depression may be induced by excessive transmission of 5-HT and that some antidepressant drugs may produce their effect by blocking this postsynaptic transmission. Based on these results, the mechanisms of human depression were discussed.

Manipulations of 5-HT activity and memory in the rat

The activity of 5-HT was manipulated by means of the peripheral injection of 5-HT reuptake inhibitors, fenfluramine and fluoxetine. These drug treatments, at doses higher than 1 mg/kg, produced retrograde amnesia in a one-trial appetitive learning task in rats. A non-specific inhibitor of 5-HT reuptake, imipramine, did not produce this amnesic effect, nor did the combination of fenfluramine with the MAOI tranylcypromine, although it produced, as expected, the "serotonergic syndrome." Results for the metabolic precursor of 5-HT, 5-HTP, also administered peripherally, were inconsistent, with amnesic effects seen at 5 and 20 mg/kg but none at 10 mg/kg.

Conditioned taste aversions induced by 1-5-hydroxytryptophan are mediated by the area postrema

Previous studies have shown that injections of 5-HTP can induce conditioned taste aversions when paired with a novel taste. Adult male albino rats received either lesions of the area postrema or were subjected to a sham lesion procedure. All rats were adjusted to a 23 1/2 hr/day water deprivation schedule and on the conditioning day were given a 0.15% saccharin solution for 1/2 hr. After drinking the saccharin fluid 9 area postrema lesioned and 10 sham lesioned rats were injected i.p. with 25 mg/kg 1-5-hydroxytryptophan. Similarly 10 area postrema lesioned and 6 sham lesioned rats were injected with the vehicle solution. A two-bottle choice test between the saccharin solution and water was given to all animals on the third and fourth days after the conditioning day. The sham lesioned rats injected with the 1-5-hydroxytryptophan exhibited a strong aversion to the saccharin taste whereas the vehicle injected sham lesioned rats showed an equal preference for the two fluids. The difference in group mean saccharin preference ratio was significant (p less than .01). Both area postrema lesioned groups exhibited saccharin preference ratios that were comparable to and not significantly different from the sham lesioned animals injected with the vehicle solution. These results show that an intact area postrema is necessary for induction of conditioned taste aversions with 1-5-hydroxytryptophan.

Evidence that taste aversion learning induced by l-5-hydroxytryptophan is mediated peripherally

Rats learned to avoid a saccharin solution if their initial consumption of it was followed by intraperitoneal (IP) administration of 25 mg/kg l-5-hydroxytryptophan (l-5-HTP); this taste aversion learning did not occur in rats pretreated with 50 mg/kg (IP) of the aromatic l-amino acid decarboxylase inhibitor RO 4-4602 (benserazide). RO 4-4602 antagonized the l-5-HTP-induced elevation of 5-hydroxytryptamine (5-HT) in the mesentery but significantly increased the l-5-HTP-induced elevation of 5-HT in the brain. These results indicate that l-5-HTP-induced taste aversion is correlated with peripheral, but not central, elevation of 5-HT.

Postsynaptic serotonergic blockade following chronic antidepressive treatment with trazodone in an animal model of depression

Acute pretreatment with clinically equivalent doses of antidepressive drugs has been observed to block D,L-5-hydroxytryptophan (5-HTP) induced behavioral depression in rats working on a food-reinforced operant schedule. Data from studies designed to distinguish presynaptic from postsynaptic events, indicated that the antidepressants were acting in part as blockers of postsynaptic serotonergic receptors. Using the same 5-HTP model of depression, we studied both the chronic and acute effects of a recently introduced antidepressant, triazolopyridine compound. Rats working for milk reinforcement and exhibiting behavioral depression following administration of 50 mg/kg 5-HTP were pretreated (one hr before 5-HTP) with 1, 2, or 4 mg/kg trazodone with resulting blockade of 5-HTP induced depression of 35, 62 and 70% respectively. Chronic administration of trazodone (2 mg/kg trazodone/day) also resulted in a significant blockade of the 5-HTP effect (75%). Neither 2 mg/kg or 4 mg/kg trazodone was found to potentiate the shorter period of depression following 25 mg/kg 5-HTP. Chronic treatment with the antidepressant drugs, amitriptyline or mianserin also blocked 5-HTP depression. Thus, as in our earlier studies, these data suggest an important postsynaptic mechanism associated with chronic administration of trazodone, amitriptyline and mianserin which could be implicated in the therapeutic effectiveness of these drugs. The potency of trazodone in relation to other antidepressant drugs in our behavioral model of depression paralleled their potency in displacing radioligand binding to 5-HT receptors, and gives additional support for the new hypersensitive postsynaptic serotonin receptor theory of depression.

Pre- and postsynaptic serotonergic manipulations in an animal model of depression

Rats working on a food-reinforced operant schedule and exhibiting behavioral depression following administration of D,L-5-hydroxytryptophan (5-HTP) were pretreated with one of three drugs: methysergide, fluoxetine, or amitriptyline. The former two drugs were used to establish a basis for distinguishing between pre- and postsynaptic events. We found that methysergide, a known postsynaptic blocker of serotonin, almost completely abolished the depressive effect of 5-HTP, whereas fluoxetine, a known specific uptake blocker of serotonin, potentiated the depressive effect of the 5-hydroxytryptamine (5-HT) precursor. Amitriptyline, one of the commonly prescribed antidepressive drugs, reduced the behavioral depression following 5-HTP by approximately 50%. These data indicate that amitriptyline can act as an antagonist of 5-HT at the postsynaptic receptor. The results of this study, as well as those recently reported from CNS membrane binding studies, suggest that the therapeutic effects of some antidepressive drugs may be explained by their postsynaptic rather than presynaptic properties at central serotonergic receptors. Thus, these studies support the hypothesis that some types of human depression may be primarily due to an excess of free 5-HT acting at postsynaptic receptors.

Role of peripheral mechanisms in the behavioral effects of 5-hydroxytryptophan

The effects of 5-hydroxytryptophan (5-HTP) were studied in rats trained to press a lever under a fixed-ratio (Fr-32) schedule of water presentation. d-, l-and d,l-5-HTP all decreased responding in a dose-related manner. The levo isomer (12.5-25 mg/kg) was twice as potent as the racemic form (25-50 mg/kg) in this respect. Moderate doses of d-5-HTP (less than 100 mg/kg) did not affect responding, whereas 200 mg/kg produced almost complete suppression. The response decrement produced by 25 mg/kg l-5-HTP was completely antagonized by pretreatment with either 50 mg/kg or 400 mg/kg of the decarboxylase inhibitor, benserazide (Ro4-4602). The specific peripheral decarboxylase inhibitor, carbidopa (MK-486) (50 mg/kg) and the peripheral serotonergic antagonist, xylamidine tosylate (1 mg/kg) also antagonized the effects of 25 mg/kg l-5-HTP. These results suggest that at least some of the behavioral effects of 5-HTP are due to increases in levels or turnover of 5-HTP in peripheral serotonergic neuronal systems.