The in vitro release of immunoreactive dynorphin and alpha-neoendorphin from the perfused rat duodenum

The release of immunoreactive (ir) dynorphin (DYN) and alpha-neoendorphin (ir-ANEO) from the isolated perfused rat duodenum was demonstrated using specific radioimmunoassays (RIAs). Depolarization of the tissue by increasing the potassium (K+) concentration up to 108 mM enhanced the release of ir-DYN and ir-ANEO in Ca2+-dependent manner. Administration of the serotonin-releasing agent fenfluramine (10(-6) M) and the serotonin receptor agonist m-chlorophenylpiperazine (m-CPP, 10(-6) M) stimulated the release of ir-DYN and ir-ANEO from the duodenum. A subsequent study revealed that serotonin (5-HT, 10(-6)-10(-4) M) induced a dose-dependent increase in the release of ir-DYN and ir-ANEO from the duodenum. The effect of 5-HT on the release of ir-DYN and ir-ANEO from the duodenum was antagonized by 5-HT antagonist cyproheptadine (10(-6) M). The presence of dynorphin and the related peptides in the gastrointestinal tract (GIT) and their release from the duodenum in vitro indicate that these peptides may act as transmitters involved in some GIT functions. Furthermore, our results suggest that at least part of 5-HT effects on the GIT may be mediated by the release of dynorphin and the related peptides.     

Stimulation of beta-endorphin and beta-lipotropin release from the anterior but not the neurointermediate pituitary lobe in the rat after acute administration of serotonin-acting drugs

The respective contribution of the anterior (AP) and the neuro-intermediate (NIL) lobes of the pituitary gland to changes occuring in plasma β-endorphin (β-EP) and β-lipotropin (β-LPH) titers has been evaluated in the rat after administration of serotonin (5-HT)-acting drugs. β-EP-like immunoreactivity (β-EP-LI) was concurrently evaluated in the mediobasal hypothalamus (MBH). The administration of 50 mg/kg DL 5-hydorxytryptophan (5-HTP) or 12.5 mg/kg fluvoxamine, a 5-HT reuptake blocker, decreased markedly β-EP-LI in the AP and induced a striking rise in plasma β-EP and β-LPH concentrations. Combined administration of fluvoxamine and 5-HTP failed to potentiate the effect of individual treatments. Similarly, administration of 5.0 and 10 mg/kg quipazine, a 5-HT receptor agonist, evoked a marked decrease in β-EP-LI in the AP and a concomitant rise in β-EP and β-LPH concentrations in the plasma, while administration of 1.0 and 5 mg/kg of chlorophenylpiperazine, a weak 5-HT stimulant drug, did not alter the above indices. None of these treatments altered significantly β-EP-LI in the NIL and only the higher dose of quipazine increased it in the MBH. We conclude that brain serotonin neurons exert a stimulatory influence on β-EP and β-LPH release from the AP but, likely, not from the NIL and that hypothalamic endorphins are not implicated in the secretory events occuring at AP level after acute activation of 5-HT neurotransmission.     

Serotonergic Regulation of Acetylcholine Release in Rat Frontal Cortex

Abstract: The extent to which serotonin regulates the activity of cortically projecting cholinergic neurons was studied using in vivo microdialysis to monitor interstitial concentrations of acetylcholine in the frontal cortex of freely moving rats. Systemic administration of the serotonin release-inducing agent fenfluramine (3 or 10 mg/kg, i.p.) increased acetylcholine release by 110–130%. The fenfluramine-induced increase in acetylcholine release was significantly attenuated by pretreatment with the selective serotonin uptake inhibitor fluoxetine (10 mg/kg, i.p.). Pretreatment with the selective dopamine D₁ receptor antagonist SCH-23390 (0.3 mg/kg, s.c.) failed to prevent the fenfluramine-induced increase in acetylcholine release. In contrast, the serotonin 5-HT_2A receptor antagonist ketanserin (5 mg/kg, i.p.) blocked fenfluramine-induced increases in acetylcholine release. In contrast to previous studies that have concluded that serotonin has inhibitory actions on cortical acetylcholine release, the present results indicate that fenfluramine increases cortical acetylcholine release in vivo by its ability to enhance serotonin transmission and that serotonin produces these effects at least in part via actions at serotonin 5-HT_2A receptors.     

Effect of intravenous 5-hydroxytryptophan on hypothalamic concentration of norepinephrine, dopamine, serotonin and hydroxyindole acetic acid in the fetal lamb

To investigate the neurochemical mechanism of the response of growth hormone to 5-hydroxytryptophan (5-HTP), we administered 5-HTP (20 mg/kg) to 10 ovine fetuses (110 or 130 days old; term gestation 147 days). Ninety minutes after 5-HTP administration, and following increases in plasma growth hormone concentrations, the fetus was delivered by hysterotomy. After local anesthesia of the fetus and sacrifice by cervical spinal cord transection the hypothalamus rapidly dissected, and stored at -80 degrees C for later analysis of norepinephrine, dopamine, serotonin and hydroxyindole acetic acid. Compared to the administration of saline, 5-HTP caused a significant increase in the hypothalamic content of serotonin, and norepinephrine, at both gestational ages. 5-hydroxyindole acetic acid increased significantly only in the older fetuses. These results indicate that serotonin may not be the only neurotransmitter active in the growth hormone response to 5-HTP.     

Evidence that serotonin neurons stimulate secretion of prolaction releasing factor.

The purpose of the present study was to determine if serotonin was stimulatory to prolactin release by inhibition of the dopaminergic system or by stimulating release of a prolactin releasing factor (PRF). We measured the amount of prolactin secreted after administration of 30 mg/kg of 5-hydroxytryptophan (5-HTP) to male rats pretreated with fluoxetine (10 mg/kg) and compared it with the amount of prolactin released in male rats treated with αmethyl-p-tyrosine methyl ester (αMT) or various dopamine receptor blocking agents. In every experiment the serotonergic stimulus provided by 5-HTP in fluoxetine-pretreated rats released considerably more prolactin than did treatment with αMT or dopaminergic blockers. We conclude that serotonin releases prolactin not by inhibiting dopaminergic neurons but rather by stimulating the release of PRF.     

5-Hydroxytryptamine affects rat migrating myoelectric complexes through different receptor subtypes: evidence from 5-hydroxytryptophan administration

The effect of the serotonin precursor 5-hydroxytryptophan (5-HTP) on jejunal migrating myoelectric complexes (MMCs) was investigated in conscious rats. Subcutaneous administration of low doses of 5-HTP (1-2 mg/kg) shortened the period between migrating complexes, whereas high doses of the compound (4-8 mg/kg) disrupted the MMC pattern. The serotonin (5-HT2) antagonist methysergide (8 mg/kg s.c.) did not alter basal MMC, neither did it prevent the effect of a low dose of 5-HTP; conversely, it antagonized the disruption due to the high dose. The 5-HT3 antagonist ICS 205-930 (30 micrograms/kg s.c.) decreased MMC frequency; administration of 2 mg/kg 5-HTP following ICS 205-930 brought the frequency of myoelectric complexes back to basal values. Both effects of 5-HTP were prevented by the decarboxylase inhibitor benserazide (85 mg/kg i.p.), which per se caused a transient inhibition of spiking activity. The results suggest that rat MMCs can be influenced in a composite fashion by progressively increasing concentrations of 5-HT, which in turn activate different receptor subtypes. A peripheral neuronal receptor, probably belonging to the 5-HT3 subclass, mediates the increase in MMC frequency observed after low doses of 5-HTP; higher levels of serotonin activate 5-HT2 receptors, causing disruption of cycling activity. Additionally, 5-HT3 receptors, but not 5-HT2, appear to be relevant for the regulation of the MMC pattern by the endogenous amine.     

Enhancement in extracellular serotonin levels by 5-hydroxytryptophan loading after administration of WAY 100635 and fluoxetine

It has been demonstrated that synthesis of serotonin (5-HT) is dependent on the availability of precursor, as well as the activity of 5-HT neurons. In the present series of experiments, we examined the effects of precursor (5-HTP) loading on extracellular hypothalamic 5-HT after administration of fluoxetine alone or in combination with WAY 100635, a selective 5-HT1A antagonist. In the first experiment, fluoxetine alone (10 mg/kg i.p.) caused 5-HT levels to significantly increase to 150% of basal levels. Subsequent administration of 5-HTP at 10, 20, and 40 mg/kg i.p. caused 5-HT levels to further increase to a maximum value of 254%, 405%, and 618%, respectively. In the second experiment, either vehicle or WAY 100635 (1 mg/kg/hour s.c.) was infused, then fluoxetine (10 mg/kg i.p.) and 5-HTP (10 mg/kg i.p.) were administered. By itself, WAY 100635 led to a slight but significant increase in hypothalamic 5-HT levels one hour after the start of administration (130% of basal levels). In the WAY 100635-treated group, fluoxetine caused an increase to 240% of basal levels after one hour, which rose to 290% of basal levels after two hours. Subsequent administration of 5-HTP further increased 5-HT levels to 580% of basal levels after one hour. In the vehicle-treated group, fluoxetine caused an increase of 160% of basal levels which was stable over two hours, and subsequent administration of 5-HTP led to a slight increase in 5-HT levels of 220% after one hour. These results suggest that combining blockade of 5-HT1A autoreceptors with 5-HT uptake inhibition results in a synergistic increase in synthesis and release of 5-HT when precursor is administered.     

Naloxone stimulation of luteinizing hormone release in prepubertal female rats; role of serotonergic system

The purpose of this study was to determine whether naloxone stimulated LH release via a serotonergic mechanism. Injection of naloxone hydrochloride (2 mg/kg B.W.) into 25-day old female prepubertal rats resulted in a significant elevation in serum LH 30 min later. Injection of this dose of naloxone together with morphine sulfate (2 or 5 mg/kg B.E.) resulted in inhibition of naloxone-induced LH release. When rats were first injected with 5-hydroxytryptophan (5-HTP) to increase hypothalamic serotonin content, naloxone failed to increase serum LH levels. On the other hand, when parachlorophenylalanine (PCPA) was given first to reduce hypothalamic serotonin content, naloxone-induced LH release was potentiated. Morphine failed to inhibit the naloxone-induced rise in serum LH when PCPA was first administered. Neither 5-HTP nor PCPA, when injected alone, altered serum LH values. These results suggest that naloxone promotes LH release by reducing hypothalamic serotonergic activity, and morphine inhibits LH release by increasing hypothalamic serotonergic activity. This does not exclude possible involvement of other neurotransmitters.     

Down-Regulation of [3H]5-Hydroxytryptamine Binding Sites in Chick Embryo Brain by Monoamine Oxidase Inhibitors or Fenfluramine and Potentiation by d,l-5-Hydroxytryptophan

Three monoamine oxidase (MAO) inhibitors--pargyline, clorgyline and deprenyl--as well as the serotonin (5-HT, 5-hydroxytryptamine) releasing agent fenfluramine were administered to developing chick embryos and the effects on [3H]5-HT binding parameters and endogenous 5-HT levels were assessed. Multiple, but not acute, pretreatments with any of the three MAO inhibitors significantly increased 5-HT levels (p less than 0.01) and decreased receptor number (Bmax) to a maximum of 20% (p less than 0.01) without affecting the affinity (KD). When d,l-5-hydroxytryptophan (d,l-5-HTP) was similarly administered there were large increases in 5-HT levels (p less than 0.01), but no significant effects on either Bmax or KD. However, if d,l-5-HTP was co-administered with any of the MAO inhibitors there was a significant (p less than 0.01) enhancement of the MAO inhibitor-induced down-regulation to a maximum of about 40%. Multiple pretreatments with fenfluramine resulted in dose-related decreases in 5-HT levels (p less than 0.01) and Bmax (p less than 0.01) without affecting KD. The largest decrease in [3H]5-HT binding sites inducible by fenfluramine treatment alone was also about 40%. When given in combination with d,l-5-HTP, there was a potentiation of the down-regulation capabilities of fenfluramine at several different dosage levels; however, maximal down-regulation was also limited to 40%. Evidence was presented suggesting that these effects were not due to endogenous 5-HT or drugs remaining in the tissue preparation. The overall evidence implies that merely increasing endogenous 5-HT levels, as by precursor administration, does not necessarily induce down-regulation unless the 5-HT is also made available as functional 5-HT.     

Effect of administration of 5-hydroxytryptophan and an inhibitor ofl-aromatic amino acid decarboxylase on glucose metabolism in rat brain

The effect of 5-hydroxytryptophan (5-HTP)—the precursor of serotonin (5-hydroxytryptamine, 5-HT)—and of an inhibitor,N-(dl-seryl)-N-(2,3,4-trihydroxybenzyl)hydrazine (Ro4-4602), ofl-aromatic amino acid decarboxylase on the metabolism of glucose to amino acids in brain tissue was investigated. Labeled glucose (20 Ci, 0.24 mg in 0.2 ml 0.9% saline) was injected intravenously into fed rats pretreated with Ro4-4602 (50 mg/kg intraperitoneally) either alone or in combination with 5-HTP (30 mg/kg intravenously) or with the appropriate vehicle. After the injection of Ro4-4602 plus 5-HTP, the concentrations of 5-HT and 5-HTP in brain were increased, but the increase of 5-HTP that Ro4-4602 slightly inhibits the reaction of decarboxylation in the brain, although at the dose used the drug is usually considered to act only peripherally. After administration of Ro4-4602 alone or combined with 5-HTP, the concentration of glucose in plasma was not significantly increased. However, the concentration of glucose in brain was markedly increased with such treatments. The administration of Ro4-4602 alone or combined with 5-HTP reduced the flux of¹⁴C from labeled glucose to amino acids in brain. The concentrations of amino acids in brain were little changed by these treatments.     

Inhibitory role of serotonin in manifestations of predatory aggression in the mink and silver fox

The predatory aggression of minks and silver-black foxes were estimated by their attacks on the rats placed in their cage. Intraperitoneal injection of 5-hydroxytryptophan (serotonin precursor) in a dose of 100 mg/kg to foxes and 50 mg/kg to minks, caused a significant blocking of predatory aggression. Estimation of serotonin level in the brain following administration of corresponding doses of 5-HTP inhibiting the predatory aggression, revealed a considerable increase of serotonin content. It may be assumed that serotonin inhibitory mechanisms of predatory aggression are homologous in different species of animals.     

Influence of Repeated Levodopa Administration on Rabbit Striatal Serotonin Metabolism,and Comparison Between Striatal and CSF Alterations

Parkinson's disease (PD) is characterized by decreased striatal dopamine, but serotonin (5-HT) is also reduced. Because 5-HT decreases following a single levodopa injection, levodopa has been suggested to contribute to PD's serotonergic deficits. However, in a recent study, rat striatal serotonin levels were reported to increase following 15-day levodopa administration. To address this issue, we administered levodopa (50 mg/kg) to rabbits for 5 days, then measured serotonin, its precursors tryptophan and 5-hydroxytryptophan (5-HTP), and its major metabolite 5-hydroxyindole-acetic acid (5-HIAA) in striatum and CSF. Striatal serotonin and tryptophan were unchanged, while 5-HTP and 5-HIAA increased 4- and 7-fold, respectively. CSF 5-HTP and 5-HIAA were also significantly increased. In levodopa-treated animals, 5-HTP concentrations were moderately correlated (r = 0.679) between striatum and CSF, while weak correlations were present between striatal and CSF concentrations of both serotonin and 5-HIAA. These results suggest that repeated levodopa treatment increases striatal serotonin turnover without changing serotonin content. However, levodopa-induced alterations in striatal serotonin metabolism may not be accurately reflected by measurement of serotonin and 5-HIAA in CSF.     

Inhibitton of prolactin release by stimulation of presynaptic serotonin autoreceptors

The effects of 5-methoxy-N, N-dimethyltryptamine (5-MeODMT), a serotonin agonist with a preferential action on presynaptic autoreceptors, on prolactin release in male rats was determined. Basal serum prolactin levels were not altered after administration of 1.0, 2.0, 5.0, 10.0 or 20.0 mg/kg of 5-MeODMT.Pretreatment with 5-MeODMT reduced prolactin release by agents that depend on serotonergic neurotransmission for part of their prolactin release stimulation. Prolactin release in response to L-5-hydroxytryptophan (5-HTP) or morphine was significantly reduced by pretreatment of the rats with 5-MeODMT.The results of this experiment indicate that 5-MeODMT act as a presynaptic serotonin autoreceptor stimulant and not as a postsynaptic serotonin agonist on the neuronal systems that control prolactin release.     

Mechanism of prolactin release by 5-hydroxytryptophan

Male Wistar rats were intraperitoneally administered 300 mg/kg b.w. of α-methyl-p-tyrosine methyl ester(α-MT). These α-MT pretreated rats were anesthetized with urethane and then 5% glucose or dopamine (1 μg/kg b.w./min) was infused for 45 min. At 1 min before or 15 min after dopamine infusion, 10 or 50 mg/kg of 5-hydroxytryptophan (5-HTP) was injected intraperitoneally, and blood samples were taken from the jugular vein for prolactin determination. In rats treated with α-MT, the administration of 5-HTP increases the serum prolactin level in a dose-related manner. Dopamine infusion caused a marked decrease in serum prolactin level. The concomitant administration of dopamine and 5-HTP prevented the dopamine-induced decrease of serum prolactin in α-MT treated rats. These results indicate that the serotonergic stimulus enhanced prolactin release, not by inhibiting the dopaminergic activity, but by stimulating a prolactin-releasing factor or by activating other neurotransmitter systems.     

Behavioral depression in pigeons following L-tryptophan administration

Since elevations of serotonin (5-HT) content in brain have been related to the behavioral depression which follows administration of 5-hydroxytryptophan (5-HTP) to pigeons emitting a food-reinforced learned response, injections of L-tryptophan (100, 200, and 300 mg/kg I.M.), which is partially metabolized to 5-HT, were given to pigeons working on the same behavioral schedule. Qualitatively similar, but shorter, periods of disrupted behavior followed. As is also the case with 5-HTP, pretreatment with 50 mg/kg iproniazid, a monoamine oxidase inhibitor, increases the duration of behavioral depression following L-tryptophan. Pretreatment with 10 mg/kg Lilly 110140, a new highly selective inhibitor for uptake of 5-HT into synaptosomes, also enhanced L-tryptophan induced depression. Initial neurochemical studies indicate that the elevated levels of 5-HT in the telencephalon after an injection of L-tryptophan follow the time course of the depressed behavior. These data support the suggestion that the release of 5-HT plays a role in certain types of behavioral depression.     

Serotoninergic control of growth hormone (GH) secretion in dogs as measured by dose responsiveness

L-5-hydroxytryptophan (5-HTP) - induced GH release has been demonstrated in animals and man, but serotoninergic control remains controversial. We developed a sensitive radioimmuno-assay for measurement of canine GH and investigated the 5-HTP dose-response relationship in unanesthetized mongrel bitches. Intravenous administration of 1, 5, 10, and 25 mg/kg doses of 5-HTP caused GH release but an increased latency in peak GH response was seen with the larger doses. Doses of 10 and 25 mg/kg induced stressful behavior and markedly increased plasma corticoid levels while lower doses were without effect. From these findings in the dog we conclude that the GH response to low dose 5-HTP is not due to stress and that alternate mechanisms are needed to explain the variable serotoninergic control of GH release demonstrated with low and high dose 5-HTP stimulation.     

Analgesic properties of a systemically-administered synthetic dipeptide of 5-hydroxytryptophan

Synthetic peptides of 5-hydroxytryptophan (5-HTP), including N-acetyl-5-HTP-5-HTP amide (5-HTP-ACETYL-DP), specifically inhibit the binding of serotonin to serotonin binding protein. 5-HTP-ACETYL-DP also produces a long-lasting, opiate-sensitive analgesia following central, but not systemic administration. The present study evaluated an apolar derivative of 5-HTP dipeptide, N-hexanoyl-5-HTP-5-HTP amide (5-HTP-HEX-DP), for its analgesic properties in rats following systemic administration. 5-HTP-HEX-DP (5–50 mg/kg) significantly increased jump thresholds in a dose-dependent manner with peak analgesia occurring at 2.5 hr after injection, and lasting up to 5 hr. In the tail-flick assay, 5-HTP-HEX-DP (20 mg/kg) produced a significant antinociceptive effect at 1 hr post-injection using both high and low intensity levels of radiant heat. While 5-HTP-HEX-DP and morphine each elicited analgesia following acute administration, chronic (14 days) incremental dosing with 5-HTP-HEX-DP or morphine resulted in persistent analgesia in 5-HTP-HEX-DP-treated animals, and a loss of analgesia in morphine-treated rats. Thus, significant tolerance to morphine, but not 5-HTP-HEX-DP analgesia developed using this protocol. Hence, 5-HTP-HEX-DP is a systemically-active analgesic which fails to develop tolerance when administered daily over 14 days.     

Specificity of serotoninergic involvement in the decrease of food intake induced by quipazine in the rat.

The effect of quipazine on brain monoamines and the significance of this interaction in its anorectic activity was studied in rats. At doses ranging from 2.5 to 10 mg/kg quipazine markedly reduced brain 5-hydroxyindolacetic acid concentrations without significant effects on steady-state levels of serotonin, noradrenaline and dopamine. Striatal levels of homovanillic acid were significantly reduced by 10 mg/kg of quipazine but not modified by a dose of 5 mg/kg. Quipazine counteracted the decrease of brain serotonin induced by fenfluramine but did not significantly modify the effect of 6-hydroxydopamine on brain nonadrenaline and dopamine. The decrease of food intake induced by 5 mg/kg of quipazine was completely prevented by pretreatment with methergoline but was not affected by an intraventricular injection of 6-hydroxydopamine or pretreatment with penfluridol, propranolol or phentolamine. The results indicate that at doses between 2.5 and 5 mg/kg quipazine specifically acts on brain serotonin and this interaction may be important for its anorectic activity.     

Behavioral correlates of serotonin depletion.

Depletion of telencephalic serotonin (5-HT) content by medical forebrain bundle lesions, which interrupt the ascending serotonergic pathways or by DL-p-chlorophenylalanine produces an increased sensitivity to pain as measured by the flinch-jump, stabilimetric, or hot-plate methods. Examination of the effects of a number of other lesions and drugs indicated that dopamine, norepinephrine and acetylcholine are not involved in pain sensitivity. Dosages of 75 mg/kg DL-5-hydroxytryptophan(5-HTP), 37.5 mg/kg L-5-HTP or 50 mg/kg Ro 4-4602 (NI-(DL-seryl)-N2-(2,3,4-trihydroxybenzyl)hydrazine) plus 37.5 mg/kg L-5-HTP administered to medical forebrain bundle lesioned rats returned both the telencephalic content of 5-HT and the pain threshold to normal values. Injection of 37.5 mg/kg of D-5-HTP or an equimolar dose of L-dopa had no effect on pain threshold. Normal animals display increased sensitivity to pain and decreased 5-HT contents in frontal pole, hippocampus, and amygdala during dark as compared to light hours. All three of these telencephalic areas are innervated by the ascending serotonergic pathways, and cells in these areas show inhibition of firing following the iontophoretic application of 5-HT. Taken together these data suggest that the serotonergic system normally acts to inhibit the effects of painful stimuli. A review of a variety of behavioral effects of 5-HT depletion including an enhanced response to lysergic acid diethylamide and amphetamine suggests that the ascending serotonergic system may have a general role in the inhibition of arousal, rather than a specific role with respect to various categories of behavior.     

Effects of thyrotropin releasing hormone and its analogue, DN 1417, on biopterin concentration and tryptophan hydroxylation in rat raphe slices

The effects of subchronic administration of thyrotropin releasing hormone (TRH) and its analogue, gamma-butyrolactone-gamma-carbonyl-L-histidyl-L-prolinamide citrate (DN 1417), on serotonin biosynthesis in situ were investigated in tissue slices of the midbrain raphe of rats. TRH or DN 1417 (10 mg/kg per day intraperitoneally) were administered to male Wistar rats for ten days. At twenty four hr after the last injection, tissue slices of the midbrain raphe were prepared and the rate of serotonin biosynthesis was estimated by measuring formation of 5-hydroxytryptophan (5-HTP) from tryptophan during inhibition of aromatic L-amino acid decarboxylase using high-performance liquid chromatography with fluorescence detection. Total biopterin content was determined by a specific radioimmunoassay. 5-HTP formation was decreased 22% and 29%, and total biopterin content 69% and 72%, in TRH- and DN 1417-treated rats, respectively. However, tryptophan concentration in raphe slices did not change. In contrast, the Vmax of tryptophan hydroxylase in the homogenate of the raphe nucleus in the presence of a saturating concentration of (6R)-L-erythro-tetrahydrobiopterin, the naturally occurring pterin cofactor, was significantly increased after repeated administration of TRH or DN 1417. These results indicate that reduction of in situ serotonin biosynthesis in tissue slices from the rats treated with TRH or DN 1417 subchronically contray to the increase in in vitro tryptophan hydroxylase may result from the decrease of the biopterin cofactor, and that changes in concentrations of the biopterin cofactor may play a regulatory role in serotonin biosynthesis in vivo under certain conditions.