Effect of dietary tryptophan on plasma and brain tryptophan, brain serotonin, and brain 5-hydroxyindoleacetic acid in rainbow trout

In order to determine the effect of dietary tryptophan level on plasma and brain tryptophan, brain serotonin, and brain 5-hydroxyindoleacetic acid levels, juvenile rainbow trout (Salmo gairdneri) were raised for 16 weeks on semipurified diets containing 0.06%, 0.16%, 0.21%, 0.26%, 0.39%, or 0.59% tryptophan. After 14 weeks, feed intake was depressed in fish fed the diets containing 0.06% or 0.16% tryptophan. No further differences in feed intake were noted between the remaining treatments. In addition, body weight was lower in fish fed diets containing 0.06%, 0.16%, or 0.21% tryptophan compared with fish fed higher levels. After 16 weeks of feeding the test diets, plasma tryptophan levels were found to be directly related to dietary tryptophan levels. Similarly, increased dietary levels of tryptophan resulted in increased brain levels of tryptophan, serotonin, and 5-hydroxyindoleacetic acid. These results demonstrate that in rainbow trout, as in mammals, altered dietary levels of tryptophan result in alterations in plasma and brain tryptophan, brain serotonin, and brain 5-hydroxyindoleacetic acid.     

Effect of intraventricular injection of 5,7-dihydroxytryptamine on regional tryptophan hydroxylase of rat brain

5,6-DIHYDROXYTRYPTAMINE has been shown to cause selective degeneration of serotonergic neurons in the central nervous sytem (BAUMGARTEN, LACHENMAYER and SCHLOSSBERGER, 1972b). This degeneration is accompanied by depletion of serotonin (BAUMGARTEN et al., 1971; 1972a) and loss of tryptophan hydroxylase activity (VICTOR, BAUMGARTEN and LOVENBERG, 1973) in certain regions of the brain. In the current experiments, the effect of 5,7-dihydroxytryptamine (another dihydroxylated tryptamine derivative) on tryptophan hydroxylase activity has been examined. Since tryptophan hydroxylase is the rate-limiting enzyme in serotonin biosynthesis and has a similar distribution to that of serotonin in the brain, it is used as a biochemical marker of serotonergic neurons, Recent experiments also indicate that 5,7-dihydroxytryptamine causes morphological damage to serotonergic neurons of the central nervous system (BAUMGARTEN and LACHENMAYER, 1972).     

Gamma-hydroxybutyrate increases tryptophan availability and potentiates serotonin turnover in rat brain

Gamma-hydroxybutyrate (GHB) is both a therapeutic agent and a recreative drug. It has sedative, anxiolytic and euphoric effects. These effects are believed to be due to GHB-induced potentiation of cerebral GABAergic and dopaminergic activities, but the serotonergic system might also be involved. In this study, we examine the effects of pharmacological doses of GHB on the serotonergic activity in rat brain. Administration of 4.0 mmol/kg i.p. GHB to rats induces an accumulation of tryptophan and 5-HIAA (5-hydroxyindole acetic acid) in the frontal cortex, striatum and hippocampus without causing significant change in the tissue serotonin content. In the extracellular space, GHB induced a slight decrease in serotonin release. The tryptophan and 5-HIAA accumulation induced by GHB is mimicked by the GHB receptor agonist para-chlorophenyl-transhydroxycrotonate (NCS-356) and blocked by NCS-382 (6,7,8,9-tetrahydro-5-[H]-benzocycloheptene-5-ol-4-ylidene acetic acid) a selective GHB receptor antagonist. GHB induces the accumulation of either a derivative of or [3H]-tryptophan itself in the extracellular space, possibly by increasing tryptophan transport across the blood-brain barrier. The blood content of certain neutral amino-acids, including tryptophan, is also increased by peripheral GHB administration. Some of the effect of GHB could be reproduced by baclofen and reduced by the GABAB antagonist CGP 35348. Taken together, these results indicate that the GHB-induced stimulation of tissue serotonin turnover may be due to an increase in tryptophan transport to the brain and in its uptake by serotonergic cells. As the serotonergic system may be involved in the regulation of sleep, mood and anxiety, the stimulation of this system by high doses of GHB may be involved in certain neuropharmacological events induced by GHB administration.     

Immunohistochemical localization of tryptophan hydroxylase and serotonin transporter in the carotid body of the rat

It has been proposed that serotonin (5-HT) facilitates the chemosensory activity of the carotid body (CB). In the present study, we investigated mRNA expression and immunohistochemical localization of the 5-HT synthetic enzyme isoforms, tryptophan hydroxylase 1 (TPH1) and TPH2, and the 5-HT plasma membrane transport protein, 5-HT transporter (SERT), in the CB of the rat. RT-PCR analysis detected the expression of mRNA for TPH1 and SERT in extracts of the CB. Using immunohistochemistry, 5-HT immunoreactivity was observed in a few glomus cells. TPH1 and SERT immunoreactivities were observed in almost all glomus cells. SERT immunoreactivity was seen on nerve fibers with TPH1 immunoreactivity. SERT immunoreactivity was also observed in varicose nerve fibers immunoreactive for dopamine beta-hydroxylase, but not in nerve fibers immunoreactive for vesicular acetylcholine transporters or nerve terminals immunoreactive for P2X₃ purinoreceptors. These results suggest that 5-HT is synthesized and released from glomus cells and sympathetic nerve fibers in the CB of the rat, and that the chemosensory activity of the CB is regulated by 5-HT from glomus cells and sympathetic nerve fibers.     

Calcium activation of brain tryptophan hydroxylase.

Using both radioisotopic and fluorometric techniques to measure the activity of midbrain soluble enzyme, we have demonstrated that calcium activates tryptophan hydroxylase. The observed activation apparently results from an increased affinity of the enzyme for both its substrate, tryptophan, and the cofactor 2-amino-4-hydroxy-6-methyl-5,6,7,8-tetrahydropteridine (6-MPH₄). The calcium activation of tryptophan hydroxylase appears to be specific for both enzyme and effector: other brain neurotransmitter biosynthetic enzymes, such as aromatic amino acid decarboxylase(s) and tyrosine hydroxylase, are not affected by calcium (at concentrations ranging from 0.01 mM to 2.0 mM); other divalent cations, such as Ba⁺⁺, Mg⁺⁺, and Mn⁺⁺, have no activating effect on tryptophan hydroxylase. This work suggests that increases in brain serotonin biosynthesis induced by neural activation may be due to influx of Ca⁺⁺ associated with membrane depolarization and resulting activation of nerve ending tryptophan hydroxylase.     

Effect of restraint stress on rat brain serotonin

Restraint-induced stress in rats was found to enhance steady state concentrations of whole brain and hypothalamic serotonin, at 1,2 and 4 h after immobilization. The increase was maximal at 1 h and tended to decline thereafter. The rate of accumulation of rat brain serotonin, in pargyline pretreated animals, was significantly enhanced after restraint stress. Bilateral adrenalectomy and metyrapone, an endogenous corticoid synthesis inhibitor, failed to affect restraint stress (1h)-induced increase in rat brain serotonin levels. Thus restraint stress-induced autoanalgesia and potentiation of the pharmacological actions of several centrally acting drugs, in rats, are serotonin-mediated responses. The results also indicate that restraint stress-induced effects on rat brain serotonin are not dependent on endogenous corticoid activity.     

Effects of clofibrate on plasma tryptophan, growth hormone, and prolactin and on brain tryptophan and serotonin in prepubertal rats

The effects of clofibrate administration (200 mg/kg, po) on somatic growth, plasma levels of lipids, tryptophan, growth hormone (GH), and prolactin (PRL), as well as on brain concentrations of tryptophan and 5-hydroxytryptamine (5-HT) were studied in prepubertal male rats. The drug did not significantly alter ponderal growth, but an appreciable reduction of tail length was observed in rats treated for 30 days. Triglyceride concentrations in plasma showed a 43% diminution after 30 days of treatment, whereas free fatty acid (FFA) levels were not modified. Clofibrate administration for 7, 15, or 30 days caused a fall in total tryptophan and a significant increase of the free fraction in plasma with no change in brain tryptophan levels. Brain 5-HT was generally unaffected but a marked elevation of this parameter was noted in rats treated for 15 days. Plasma GH and PRL concentrations remained unaltered. It may be concluded from these findings that the slight reduction of somatic growth, the diminution of triglycerides, and the increase of free tryptophan in plasma, induced by chronic clofibrate treatment, are not associated with variations in brain tryptophan and 5-HT levels or with modifications of plasma GH and PRL titers.     

Caffeine injection raises brain tryptophan level,but does not stimulate the rate of serotonin synthesis in rat brain

Acute caffeine injection (100 mg/kg) elevates brain levels of tryptophan (TRP), serotonin (5HT), and 5-hydroxyindoleacetic acid (5HIAA). Experiments were performed to determine if the increases in 5HT and 5HIAA result from a stimulation of the rate of 5HT synthesis. Both the rate of 5-hydroxytryptophan (5HTP) accumulation following NSD-1015 injection, and the rate of ³H-5-hydroxyindole synthesis from ³H-tryptophan were measured $\text{in vivo}$ following caffeine administration and found to be normal. Tryptophan hydroxylase activity, as measured $\text{in vitro}$ in brain homogenates, was also unaffected by caffeine. The results suggest that the elevations in brain 5HT and 5HIAA levels produced by caffeine do $\text{not}$ reflect enhanced 5HT synthesis, despite significant elevations in brain TRP level. Some other mechanism(s) must therefore be responsible for these elevations in brain 5-hydroxyindole levels.     

Effect of food restriction on serotonin metabolism in rat brain

The brain concentration of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) increased in rats maintained on restricted volume of low-protein or normal-protein diet, whereas these two agents decreased in rats fed low-protein diet ad libitum. In these two food-restricted groups brain 5-HT and 5-HIAA concentrations were not correlated with brain tryptophan hydroxylase activity, but the concentrations correlated closely with cerebral tryptophan concentrations. The cerebral tryptophan concentration in the two food-restricted groups was not consistent with the total or free tryptophan concentration in plasma. In these restricted rats cerebral tryptophan concentration was elevated, and, unlike the plasma tryptophan, it showed no diurnal variation. These results suggested that tryptophan uptake into the brain from plasma was enhanced by limiting food volume intake. Tryptophan uptake was increased by glucagon injection without changing the plasma tryptophan level, but injection of hydrocortisone or insulin had little or no effect on tryptophan concentration in either the plasma or brain.d-Glucose injection elevated plasma tryptophan concentration but decreased brain tryptophan concentration.     

Depletion of tryptophan hydroxylase by 5,6-dihydroxytryptamine in rat brain. Time course and regional differences

—Wistar rats were injected intraventricularly with 75 μg 5,6-dihydroxytryptamine. Tryptophan hydroxylase was assayed in seven regions of the brain as well as spinal cord at intervals following injection. Spinal cord was depleted to 1 per cent of control by 12 days. Tectum was depleted to 32 per cent of control by 9 days. The time course of depletion in most regions was biphasic, with an early reduction of activity 1 h after injection with continued reduction of activity 1-2 days following injection, and a recovery of activity at 4-6 days. The activity again drops at 9-12 days, and this reduction of activity persists to varying degrees to 60 days, with slight recovery at this time in certain regions.     

Effect of hypoxia on tyrosine and tryptophan hydroxylation in unanaesthetized rat brain

T he T yrosine and tryptophan hydroxylase enzymes have been proposed as rate-limiting steps in the biosynthesis of catecholamines and 5-hydroxytryptamine (5-HT), respectively; thus under normal physiological conditions the rate of amine synthesis appears to be controlled by the activity of these hydroxylase enzymes (see U denfriend , 1966; L ovenberg , J equier and S joerdsma , 1968). Subtle changes in neuronal activity may result not in changes in the levels of the amine neurotransmitters, but rather in alteration in their production and metabolism without measurable change in their levels. Previous studies of the effect of hypoxia on monoamines have dealt with amine levels, but there have been no studies of the effect of lowered oxygen on the synthesis of these substances.