Rapid nonisotopic assay of tryptophan-5-hydroxylase activity in tissues

On the mechanism of the acute decrease of rat brain tryptophan hydroxylase activity by 4-chloroamphetamine

The decrease of rat brain soluble tryptophan hydroxylase activity 4 h after the injection of 20 mg/kg i.p. of 4-chloroamphetamine was antagonized by 20 mg/kg i.p. of zimelidine, a selective inhibitor of membrane serotonin (5-HT) uptake, by 2.5 mg/kg i.p. of chlorpromazine and by 5 mg/kg i.p. of methergoline, a proposed antagonist of central 5-HT receptors, when these compounds were injected 15 min prior to 4-chloroamphetamine. Zimelidine partially reversed the fall in tryptophan hydroxylase when injected 4 h after 4-chloroamphetamine and the enzyme activity was recorded after further 4 h. Reserpine, 5 mg/kg i.p., 18 h before 4-chloroamphetamine prevented the decrease of the enzyme activity but had no effect when injected 1 h beforehand. It is suggested that the decrease of the tryptophan hydroxylase activity is secondary to the release of 5-HT and mediated by 5-HT receptor stimulation.

Altered serotonin synthesis, turnover and dynamic regulation in multiple brain regions of mice lacking the serotonin transporter

To evaluate the consequences of inactivation of the serotonin transporter (SERT) gene on 5-HT homeostasis and function, 5-HT synthesis and turnover rates were measured using the decarboxylase inhibition method in multiple brain regions (frontal cortex, striatum, brainstem, hippocampus and hypothalamus) from mice with a genetic disruption of SERT. 5-HT synthesis rates were increased 30-60% in the different brain regions of SERT -/- mice compared to littermate +/+ control mice despite 55-70% reductions in tissue 5-HT concentrations. Brain regions that possessed a greater capacity to increase synthesis and turnover (frontal cortex, striatum) demonstrated lesser reductions in tissue 5-HT. Female SERT -/- mice had greater increases (79%) in brain 5-HT synthesis than male -/- mice did (25%), a finding associated with higher brain tryptophan concentrations in females. Despite increased 5-HT synthesis, there was no change in either TPH2 or TPH1 mRNA levels or in maximal in vitro TPH activity in the brainstem of SERT -/- mice. Catecholamine homeostasis as reflected in brain tissue concentrations and in synthesis and turnover of dopamine and norepinephrine was unchanged in SERT -/- mice. Taken together, the results demonstrate a markedly altered homeostatic situation in SERT -/- mice that lack 5-HT reuptake, resulting in markedly depleted tissue stores that are inadequately compensated for by increased 5-HT synthesis, with brain region and gender specificity observed.

Early nutritional changes modify the kinetics and phosphorylation capacity of tryptophan-5-hydroxylase

Gestational malnutrition induces an acceleration of the serotonin biosynthetic pathway in the developing brain with an increase in brain L-tryptophan (L-Trp), tryptophan-5-hydroxylase (TrpOH) activity and serotonin content. In the present work we report results on the possible mechanism of TrpOH activation. Kinetic experiments were done with different L-Trp concentrations in the rat brain at different ages. Also various phosphorylating conditions of the enzyme were tested in order to compare its activation in developmentally malnourished and normal brains. The results showed lower Km values and no changes in the Vmax in the malnourished as compared to controls. Interestingly, in the malnourished group, TrpOH showed an increased activity under the phosphorylating conditions employed. We propose that in the activation of brain TrpOH by developmental malnutrition, not only is an elevation of L-Trp involved, but also a change in the enzyme itself reflected in a higher affinity for L-Trp and in a greater response to phosphorylation. This allows us to propose the possibility that early chronic malnutrition induces structural changes in the enzymatic molecule.

Radioenzymatic assay for tryptophan hydroxylase: [3H]H2O release assessed by charcoal adsorption

The rate-limiting reaction in the biosynthesis of the neurotransmitter, serotonin, is catalyzed by the enzyme, tryptophan hydroxylase. Studies on the characteristics of this enzyme have been hampered by its relative instability and paucity in the brain. We have modified a charcoal adsorption radioenzymatic assay used for the measurement of tyrosine hydroxylase to assess rat brain tryptophan hydroxylase activity. This protocol is based on the principle that aromatic amino acid hydroxylases are mixed-function oxygenases and will utilize O2 and reduced pterin to convert tritiated amino acid substrate to product and tritiated H2O. All product and unreacted substrate are adsorbed by acidified charcoal. The [3H]H2O is analyzed by liquid scintillation spectrometry and is indicative (stoichiometrically) of the amount of product formed and, thus, the activity of the enzyme. This assay has a high signal-to-noise ratio and is sensitive enough to determine enzymatic activity in homogenates of individual raphe nuclei. In addition, its simplicity in design allows for the simultaneous testing of large numbers of samples. The enzyme activity and kinetic determinations derived from this protocol agree with those of other investigators using more lengthy, involved procedures.

Brain Na+/K(+)-ATPase regulation by serotonin and norepinephrine in normal and kindled rats

In this work we confirmed the activation of rat brain Na+/K(+)-ATPase by norepinephrine (NE) and observed a variable response of the enzyme according to the brain region considered. In isolated neuronal or glial fractions from normal cerebral cortices, we studied the response of the enzyme to increasing concentrations of serotonin (5-HT) (10(-9)-10(-3) M). A dose-dependent response over basal values was present in glial fractions, beginning at 10(-6) M. No such response was obtained in the neuronal fractions. In amygdaloid kindled brains, the pattern of activation by NE was different than in controls: less pronounced (cortex, brainstem, and diencephalon), inhibition-activation (cerebellum), or no change (striatum). The activation of Na+/K(+)-ATPase by 5-HT observed in the control glial fraction was not present in the kindled glial fraction. In conclusion, 5-HT seems to activate Na+/K(+)-ATPase preferentially in glial cells, and the kindling process markedly modifies this regulation. The normal response to NE in brain homogenates is less altered by kindling than is the response to 5-HT in the same regions.

Regulatory role of a neurotransmitter (5-HT) on glial Na+/K(+)-ATPase in the rat brain

In the present work we studied the effect of serotonin (5-HT) on the kinetics of Na+/K(+)-ATPase in subcellular preparations of the cerebral cortex from male Wistar rats using various concentrations of ATP and K+ with and without added 5-HT. Also we studied the effect of 5-HT on the enzyme in glial or neuronal preparations. The results indicated that there was a significant increase (P < 0.05) of the Vmax in the presence of 5-HT in the whole tissue preparation (homogenate) but not in the subcellular fractions, suggesting that the interaction could be preferentially with the glial pump. Further results supported that this was the case since activation by 5-HT was mainly in the glial preparations. Kinetic data and the binding of [3H]ouabain supported that the enzyme is activated by 5-HT through the exposure of more enzymatic active sites.

Tryptophan hydroxylase synthesis is induced by 3',5'-cyclic adenosine monophosphate during circadian rhythm in the rat pineal gland

Tryptophan hydroxylase in the rat pineal gland undergoes diurnal rhythmic activity. Rat pineal glands exhibit increased tryptophan hydroxylase activity when incubated with a cyclic AMP analogue in vitro. Cyclic AMP-dependent protein kinase phosphorylates tryptophan hydroxylase, purified from rat brain, without any modification of its enzyme activity under our experimental conditions. Actinomycin D or cycloheximide decreases the stimulating effect of the cyclic AMP analogue on pineal tryptophan hydroxylase activity. Incubation of pineal glands in the presence of [35S]methionine showed a cyclic AMP-induced increase in tryptophan hydroxylase synthesis. These results explain the circadian rhythm of tryptophan hydroxylase activity in the rat pineal gland and suggest that the regulation of tryptophan hydroxylase expression by cyclic AMP occurs probably either at the translational level or via transient expression of a transcriptional regulatory element.

Long-term pinealectomy alters hypothalamic serotonin metabolism in the rat

In the present study, the effects of long-term pinealectomy on tryptophan, 5-hydroxytryptamine (5-HT or serotonin), 5-hydroxy-3-indoleacetic acid (5-HIAA), and tryptophan hydroxylase and monoamine oxidase activities were studied in preoptic area-anterior hypothalamus (POA-AH) and in the medial and posterior hypothalamus of the rat. After pinealectomy, 5-HT levels decreased significantly in medial hypothalamus but increased in the POA-AH. The levels of 5-HIAA decreased significantly in the POA-AH and medial hypothalamus. Tryptophan levels remained unchanged while tryptophan hydroxylase activity diminished significantly in POA-AH and medial hypothalamus. Monoamine oxidase activity remained unchanged in the hypothalamic regions. These results suggest that pinealectomy induces differential inhibitory actions on the serotoninergic terminal regions, mainly in anterior and medial hypothalamic areas.

Differential effects of pinealectomy on amygdala and hippocampus serotonin metabolism

The purpose of the present study was to determine the effects of long-term pinealectomy on serotonin metabolism in the amygdala and the hippocampus of male rats. Pinealectomy did not significantly alter either tryptophan or serotonin concentrations in the amygdala or the hippocampus. However, statistically significant decreases in 5-hydroxyindole-3-acetic acid levels and tryptophan hydroxylase activity were found in the amygdala. Monoamine oxidase activity was unchanged in both regions. These results support the involvement of the amygdaloid serotoninergic system in mediating the functions of the pineal gland.

Formal demonstration of the phosphorylation of rat brain tryptophan hydroxylase by Ca2+/calmodulin-dependent protein kinase

Tryptophan hydroxylase is activated in a crude extract by addition of ATP and Mg2+. This activation is reversible and requires in addition both Ca2+ and calmodulin. Thus, phosphorylation by an endogenous calmodulin-dependent protein kinase has long been suspected. Now that we have prepared a specific polyclonal antibody to rat brain tryptophan hydroxylase, we have been able to prove that this hypothesis is correct. After incubation of purified tryptophan hydroxylase with Ca2+/calmodulin-dependent protein kinase together with [gamma-32P]ATP, Mg2+, Ca2+, and calmodulin, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and blotting of the enzymes onto nitrocellulose sheets, we could label the band of tryptophan hydroxylase by the antiserum and the peroxidase technique and show by autoradiography that 32P was incorporated into this band. By measuring the radioactivity, we calculated that about 1 mol of phosphate was incorporated per 8 mol of subunits of the enzyme (2 mol of native enzyme). Because the concentration of ATP which we employed (50 microM) gives about half-maximal activation in crude extract compared to saturating ATP conditions (about 1 mM), this result indicates that the incorporation of at least 1 mol of phosphate/mol of tetramer of native tryptophan hydroxylase is required for maximal activation.

Newborn humans and rats malnourished in utero: free plasma L-tryptophan, neutral amino acids and brain serotonin synthesis

In the present study we report results concerning 5-hydroxytryptamine (5-HT) metabolism in two groups of small for date (SFD) human babies (gestational age 36 and 3 weeks), who suffered intrauterine nutritional restriction. A complementary study in the brain of rat fetuses with two types of intrauterine deprivation, in which brain L-tryptophan (L-Trp), tryptophan-5-hydroxylase (T5-H) activity and 5-HT content were determined on days 17, 19 and 21 of gestation. The same parameters studied prenatally were followed in both species during the immediate postnatal period. In the SFD babies the results were: (a) the free fraction of plasma L-Trp was significantly elevated; (b) plasma neutral amino acids were not substantially modified; (c) the bound fraction of L-Trp and plasma proteins were significantly low, as compared to controls. In the fetal brain of intrauterine malnourished rats, L-Trp, activity of T5-H and 5-HT content, were significantly elevated, since day 17, as related to normal littermates. These alterations in 5-HT metabolism persisted during the early postnatal period in both species. Elevation of the free fraction of plasma L-Trp in early malnourished SFD human babies suggest an increased transport of this amino acid to the brain with a possible enhancement of serotonin synthesis, during a critical period of brain differentiation.

Altered tyrosine and tryptophan metabolism during hypothermic hibernation in the 13-lined ground squirrel (Spermophilus tridecemlineatus)

(1) Tyrosine and tryptophan metabolism in brain and peripheral tissues were studied in hypothermic hibernating and normothermic nonhibernating 13-lined ground squirrels (Spermophilus tridecemlineatus). (2) In the hypothermic hibernating state, there were significant elevations of brain stem tyrosine, norepinephrine, and dopamine levels; forebrain norepinephrine and dopamine levels; and cerebellum norepinephrine and tyrosine levels. (3) On the other hand, plasma norepinephrine levels were significantly decreased in hypothermic hibernating squirrels while plasma tyrosine levels were increased. Kidney norepinephrine levels were significantly increased in hypothermic hibernating squirrels, while kidney tyrosine levels were decreased. Total plasma tryptophan and free plasma tryptophan were significantly reduced in hypothermic hibernating squirrels. Hepatic tyrosine aminotransferase Km and Vmax were decreased in hypothermic hibernating squirrels, while tryptophan 2,3-dioxygenase activity was not altered. Plasma and liver albumin were increased in hypothermic hibernating squirrels, while plasma and liver total protein were not altered. (4) These results demonstrate that significant changes in tyrosine and tryptophan metabolism occur in both central and peripheral tissues with concomitant alterations in metabolites during hypothermic hibernation in 13-lined ground squirrels.

Cerebral serotonin regulation by phenylalanine analogues and during hyperphenylalaninemia

Severe hyperphenylalaninemia induced in infant rats by 3 days of treatment with p-chlorophenylalanine (p-cl phe) plus phenylalanine (phe) did not lower the tryptophan concentration of the brain, and the cerebral serotonin (5-HT) deficiency was attributable entirely to the known suppression to tryptophan hydroxylase (TPH) by p-cl phe. The decrease in 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) was thus no more pronounced than in rats which, treated with p-cl phe alone, were devoid of hyperphenylalaninemia. Suppression of TPH was found to also underlie the decrease in cerebral 5-HT caused by treatment with alpha-methylphenylalanine (alpha-mephe) alone: a 22% loss of midbrain TPH activity was detectable 24 hr after an injection only, reverted toward the normal during the next 2 days, and was clearly unrelated to the weak competitive inhibition of the enzyme by alpha-mephe in vitro. However, alpha-mephe (unlike p-cl phe), when administered together with phe, did not suppress TPH, nor did it counterbalance the reduction of cerebral tryptophan uptake by excess phe. Thus the 5-HT diminution in the rat model of phenylketonuria produced by treatment with alpha-mephe plus phe was attributable to hyperphenylalaninemia and the inhibition of tryptophan transport to the brain. Injection of tryptophan was found to restore the cerebral 5-HT level in the face of persistently severe hyperphenylalaninemia.

Increased tryptophan hydroxylase activity in serotonergic nerve terminals spared by 5,7-dihydroxytryptamine

Adult rats received intraventricular injections of 5,7-dihydroxytryptamine (5,7-DHT) to destroy serotonin (5-HT)-containing nerve terminals throughout the brain. When the animals were killed 3 or 21 days later, we observed a marked decrease in 5-HT content in septum and hippocampus and a parallel decline in in vitro high affinity 5-HT uptake. 5-Hydroxyindoleacetic acid (5-HIAA) concentrations also were reduced but by a much smaller extent, resulting in significant increases in the ratio of 5-HIAA to 5-HT. These changes were accompanied by similar increases in the ratio of tryptophan hydroxylase (TPH) activity to 5-HT content. The relative increases in TPH activity resulted from two temporally distinct processes, the first of which appeared to be an activation that could be mimicked in vitro by Ca2+-dependent phosphorylation. We conclude that, after partial damage to 5-HT neurons, there is a compensatory increase in the synthesis and release of 5-HT from those terminals that remain.

Streptozotocin-induced diabetes reduces brain serotonin synthesis in rats

The rate of brain 5-hydroxytryptamine (serotonin) synthesis and turnover in streptozotocin-diabetic rats was assessed using three separate methods: the rate of 5-hydroxytryptophan accumulation following decarboxylase inhibition with Ro 4-4602; the decline in 5-hydroxyindoleacetic acid levels following monoamine oxidase inhibition with pargyline; and the rate of 5-hydroxyindoleacetic acid accumulation following blockade of acid transport with probenecid. Each of the three methods revealed that 5-hydroxytryptamine synthesis and turnover is decreased by 44-71% in diabetic rats with plasma glucose levels of between 500 and 600 mg%. In addition, the levels of free and bound plasma tryptophan were measured and the levels of the free amino acid were found to be the same in control and diabetic rats. Since diabetic rats exhibit a 40% decrease in brain tryptophan, the free tryptophan level in plasma does not predict brain tryptophan levels in diabetic rats. These data are discussed within the context of psychiatric disturbances experienced by diabetic patients.

Tryptophan 5-hydroxylase. Rapid purification from whole rat brain and production of a specific antiserum

Tryptophan 5-hydroxylase (EC 1.14.16.4; L-tryptophan tetrahydropteridine: oxygen oxidoreductase) was purified to electrophoretic homogeneity from whole brain supernatant using the following steps: pteridine-argarose affinity chromatography, hydrophobic and finally hydroxyapatite chromatography. Exogenous catalase was necessary throughout most of the purification procedure in order to protect the enzyme against inactivation. The iron chelator desferrioxamine at a concentration of 10 microM or higher brought about an irreversible loss of enzyme activity of a partially purified preparation containing an excess of catalase, whereas this same chelator at a lower concentration afforded considerable protection of the enzyme's activity during the final purification stage despite the quasi-total absence of catalase and the presence of an excess of ferrous iron. Antiserum raised in the rabbit to purified tryptophan 5-hydroxylase appears to be monospecific for the enzyme after immunoadsorption of anti-catalase antibodies which were present due to the trace of catalase which remained in the final enzyme preparation.

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 in vivo following caffeine administration and found to be normal. Tryptophan hydroxylase activity, as measured 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 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.

Activities of tryptophan-metabolizing enzymes in liver and brain of rats treated with aflatoxins

Male Sprague-Dawley rats were treated ip twice daily for 10 days with mixed aflatoxins (10 micrograms/kg body weight) and then the activities of liver tryptophan 2,3-dioxygenase and brainstem tryptophan 5-hydroxylase were determined in vitro. Total tryptophan 2,3-dioxygenase activity was reduced by aflatoxin treatment while holoenzyme activity was not. Induction of tryptophan 2,3-dioxygenase activity by L-tryptophan was not altered by the treatment though induction by hydrocortisone was blocked. It is suggested that aflatoxin may alter the tryptophan 2,3-dioxygenase-haem bond in vivo rather than affect enzyme or cofactor synthesis. Kinetics studies performed in vitro on brain tryptophan 5-hydroxylase showed that aflatoxin treatment in vivo increased the Km of the hydroxylase when L-tryptophan and synthetic 6,7-dimethyl-5,6,7,8-tetrahydrobiopterin were used as the substrate and cofactor, respectively. However, aflatoxin treatment did not alter the Vmax of tryptophan-5-hydroxylase.

Inhibition of tryptophan hydroxylase: neurochemical action of catecholamide seizure-inducing agent

H13/04, an audiogenic seizure-inducing catecholamide, has previously been demonstrated to decrease the accumulation of 5-hydroxytryptophan (5-HTP), while increasing the accumulation of dihydroxyphenylalanine (DOPA) after aromatic acid decarboxylase inhibition in vivo. The present study examined the effect of H13/04 on intracellular storage, release, and metabolism of serotonin (5-HT) and noradrenaline (NA) in vitro in order to differentiate between the primary effects of the drug and possible secondary effects due to neurotransmitter interaction. H13/04 had not effect on NA synthesis by brain minces from C57BL/6 mice, but did have a marked effect on [P3H]5HT synthesis from [3H]tryptophan in mouse brain minces. H13/04 was subsequently shown to competitively inhibit tryptophan hydroxylase. The data presented in this study indicate that the primary action of H13/04 on biogenic amines is to decrease the synthesis rate of 5-HT by competitive inhibition of tryptophan hydroxylase. The lack of any effect on NA in vitro is consistent with the hypothesis that the primary biochemical action of the drug is on the 5-HT system and that the action on NA in vivo is an indirect effect possibly secondary to the inhibition of 5-HT synthesis.

Mechanism of irreversible inactivation of phenylalanine-4- and tryptophan-5-hydroxylases by [4-36Cl, 2-14C]p-chlorophenylalanine: a revision

Intraperitoneal injection of [4-36Cl, 2-14C]p-chlorophenylalanine (pCPA) (300 mg/kg) in rats revealed absence of chlorine in pure hepatic phenylalanine hydroxyase, while the carbon label appeared a 1--4 moles/mole of [14C]tyrosine in the inactivated phenylalanine and cerebral tryptophan-5-hydroxylase. Crystalline muscle aldolase and tyrosine hydroxylase also revealed the presence of [2-14C]tyrosine from [2-14C]pCPA without inactivating these enzymes. Injection of L-[(U)-14C] tyrosine led to its incorporation into the above enzymes, but to a different degree without altering the enzyme activity. Repeated injections of p-chlorophenylacetic acid had no effect on phenylalanine or tryptophan-hydroxylase, Administration of pCPA did not change the levels of cerebral biopterins. Reexamination of the effect of cycloheximide on reversing enzymic inactivation by pCPA failed to confirm our earlier observation.

Alterations of tryptophan metabolism in a rat strain (Osborne-Mendel) predisposed to obesity

1. Osborne-Mendel (O-M) rats displayed differences in brain and systemic tryptophan metabolism. O-M rats had decreased brainstem tryptophan-5-hydroxylase activity and decreased serotonin (5-HT) levels as compared to Sprague-Dawley rats. However, brain tryptophan levels were actually increased in O-M rats. Norepinephrine, dopamine and 5-hydroxyindole-3-acetic acid levels were not different between strains. 2. Pineal serotonin levels were increased in O-M rats. 3. Liver tryptophan 2,3-dioxygenase activity was increased in O-M rats while tyrosine aminotransferase activity was not different between strains. 4. Total blood cholesterol was decreased in O-M rats while triglycerides, free fatty acids and albumin was not different between strains. Total serum tryptophan was not different between strains while O-M rats had an increased level of free (unbound) tryptophan.

Circadian variations in the activity of tyrosine hydroxylase, tyrosine aminotransferase, and tryptophan hydroxylase: relationship to catecholamine metabolism

Circadian variations in the activity of tyrosine hydroxylase, tyrosine aminotransferase, and tryptophan hydroxylase were observed in the rat brain stem. Tyrosine hydroxylase exhibited a bimodal pattern with peaks occurring during both the light and dark phases of the circadian cycle. Tyrosine aminotransferase had one daily peak of activity occurring late in the light phase, whereas tryptophan hydroxylase activity was maximal late in the dark phase. Circadian fluctuations in tyrosine hydroxylase activity did not correlate well with circadian variations in the turnover rates of norepinephrine or dopamine nor with levels of these catecholamines. This supports the idea that although tyrosine hydroxylase is the rate-limiting enzyme in the synthesis of catecholamines, other factors must also be involved in the in vivo regulation of this process. Administration of alpha-methyl-p-tyrosine (AMT) methyl ester HCl (100 mg/kg) had no effect on the activity of tryptophan hydroxylase, but effectively eliminated the peak of tyrosine hydroxylase activity that occurred during the light phase. AMT also lowered levels of tyrosine aminotransferase, but only at times near the daily light to dark transition. These chronotypic effects of AMT emphasize the importance of "time of day" as a factor that must be taken into account in evaluating the biochemical as well as the pharmacological and toxicological effects of drugs.

Effect of undernutrition on tryptophan and tyrosine hydroxylases in the developing rat brain

Rats born to well-fed mothers (20% protein diet ad libitum), protein-restricted mothers (7.5% protein diet ad libitum) or pair-fed with protein-restricted mothers were killed on days 0, 7, 14, 21, 28 and 35 and activities of the two enzymes of neurotransmitter synthesis, tryptophan-5-hydroxylase (EC 1.14.16.4) and tyrosine hydroxylase (EC 1.14.16.2) were assayed. Enzyme activities in normal animals were low at birth and progressively increased to reach adult levels by day 15. Protein-restricted and pair-fed animals also showed a similar pattern. However, significantly higher activities were observed from day 15 onwards in both experimental groups.

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.

GABA-accumulating neurons in the nucleus raphe dorsalis and periaqueductal gray in the rat: a biochemical and radioautographic study

The possibility of a GABAergic innervation of the nucleus raphe dorsalis (NRD) has been investigated by using the following approaches: (i) the identification of the principal neuronal groups afferent to the NDR by using horseradish peroxidase retrograde transport, (ii) the determination of glutamate decarboxylase activity (GAD) in the NRD after lesioning these groups or their putative pathways, and (iii) the radioautographic identification of terminals axons and nerve cells accumulating intraventricularly injected [3H]GABA. The hypothesis of a local GABAergic network is supported by the failure to obtain important changes in GAD after lesions of NRD afferents and the presence in this nucleus of terminals, fibers and nerve cell bodies accumulating [3H]GABA. It appears that these GABA-accumulating neurons could represent a portion of aperiventricular GABAergic system in the periaqueductal gray and the pontine ventricular gray.

Biopterin. V. De novo synthesis of dihydrobiopterin: evidence for its quinonoid structure and lack of dependence of its reduction to tetrahydrobiopterin on dihydrofolate reductase

Samples of quinonoid‐l‐erythrodihydrobiopterin (q‐BH2) and quinonoid‐6‐methyl‐dihydro‐pterin (q‐6‐MPH2) were prepared by oxidation of l‐erythro‐5,6,7,8‐tetrahydrobiopterin (BH4) and 5,6,7,8‐tetrahydro‐6‐methylpterin (6‐MPH4) and separated from D‐erythro‐7,8‐dihydrobiopterin (7,8‐BH2) and 6‐methyl‐7,8‐dihydropterin (7,8‐6‐MPH2) as well as from the tetrahydropterins on phosphocellulose column by high‐pressure liquid chromatography. The quinonoid dihydropterins were identified and quantitated by scan of their ultraviolet absorption and fluorescence emission spectra through their rearrangement to their 7,8‐tautomer and also by gas chromatography of their rapidly synthesized trimethylsilyl derivative. Identification was also achieved by the enzymatic reduction of [3H]q‐BH2to [3H]BH4 by dihydrofolate reductase (DHFR). Direct proof for the enzymatic synthesis of the q‐BH2 from GTP or from 2‐amino‐6‐(5′‐triphosphoribosyl)‐amino‐5‐ or ‐6‐formamido‐6‐hydroxypyrimi‐dine (FPyd‐P3) was obtained by isolation of the compound which was identical in all respects to the q‐BH2 obtained by chemical synthesis from BH4. The reduction of enzymatically synthesized q‐BH2 by dihydropteridine reductase (DHPR) to BH4 was not inhibited by methotrexate (MTX). When the enzymatically synthesized q‐BH2 was converted to 7,8‐BH2, it was reduced only by DHFR. This reduction, however, was inhibited by MTX. On the biosynthetic pathway from GTP to dihydrobiopterin, the enzyme responsible for the appearance of the quinonoid structure is the d‐erythro‐dihydroneopterin triphosphate synthetase, the product of which (quinonoid d‐erythro‐dihydroneopterin triphosphate) is converted to quinonoid dihydrobiopterin by l‐erythro‐dihydrobiopterin synthetase. Experiments in vivo established that DHFR does not participate in the reduction of dihydrobiopterin to tetra‐hydrobiopterin when the former is synthesized from GTP de novo. MTX at 5 × 10−6M exerted no inhibition on the reduction of the biosynthetic dihydrobiopterin to tetrahydrobiopterin in vivo, yet completely inhibited the reduction of intraventricularly injected tritiated dihydrofolate ([3H]FH2) to tritiated tetrahydrofolate ([3H]FH4).

Characteristics of the activation by dithiothreitol and Fe(2+) of tryptophan hydroxylase from the rat brain

The preincubation of tryptophan hydroxylase extracted from various areas of the central nervous system of the rat with 30 mM dithiothreitol and 50 muM ferrous ammonium sulfate under nitrogen atmosphere resulted in a persistent increase of its activity. Studies on the enzyme characteristics indicated that this activation was associated with a doubling in its Vmax and a shift (from 7.6 to 7.2) of the optimal pH for its activity. In contrast, the molecular weight and the apparent affinities of tryptophan hydroxylase for its pterin cofactor and for tryptophan were not significantly altered by the preincubation with dithiothreitol and ferrous ammonium sulfate. Since this treatment did not prevent the stimulatory effects of various compounds (phosphatidylserine, ATP and MG(2+), Ca(2+)) on tryptophan hydroxylase activity, this might be a good procedure to activate this enzyme with only minor changes in its regulatory properties.

Structure-activity relationships among the halogenated amphetamines

p-Chloroamphetamine and various analogs influence brain serotonin neurons through multiple actions. Comparison of these compounds has permitted the distinction between short-term and long-term depletion of serotonin and among inhibition of tryptophan hydroxylation, release of serotonin, inhibition of serotonin reuptake, and inhibition of monoamine oxidase as mechanisms involved in the actions of these agents on serotonin neurons.