Chronic ACTH treatment: influence on 5-HT2 receptors and behavioral supersensitivity induced by 5,7-dihydroxytryptamine lesions

The capacity of the serotonin (5-HT) precursor 5-HIP to induce the ACTH-responsive myoclonic-convulsive disorder infantile spasms in patients with Down's syndrome has been cited as evidence for altered serotonergic neurotransmission in infantile spasms. Since there is no animal model of infantile spasms, the suitability of behavioral supersensitivity (myoclonus) evoked by 5-HTP in rats with 5,7-dihydroxytryptamine (DHT) lesions as a model was tested by determining the effect of chronic treatment with ACTH (40 IU/kg) on 5-HTP-evoked myoclonus. In rats treated with DHT as adults, ACTH administration did not alter the "serotonergic behaviors," such as myoclonus, induced by 30 mg/kg 5-hydroxytryptophan (5-HTP), but induced a small significant increase in Bmax of neocortical 5-HT2 sites of the DHT group, with no change in rats without lesions. In rats treated with DHT as neonates, there was also no significant difference in behaviors evoked by several doses of 5-HTP. These data suggest that ACTH minimally modifies the effects on 5-HT receptors of DHT lesions, but the intracisternal DHT model is not a suitable model for infantile spasms because chronic ACTH was not antimyoclonic.     

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.     

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.     

Role of serotonin,gamma-aminobutyric acid,and glutamate in the behavioral thermoregulation of female tilapia during the prespawning phase

The role of hypothalamic neurotransmitter systems in behavioral thermoregulation was investigated in the prespawning female tilapia, Oreochromis mossambicus. Intrahypothalamic microinjection with serotonin (5-HT, 3 microliters of 1.0 x 10(-6) M) resulted in a significant increase in the selected temperature. This effect was mimicked by the agonist of 5-HT1A, 1B, and 2C receptors, N-3-trifluoromethylphenyl piperazine. Intrahypothalamic microinjection of tilapia with gamma-aminobutyric acid (GABA) resulted in a biphasic effect of the temperature selection, whereas microinjection with muscimol, an agonist of GABAA receptor, had no effect on temperature selection. Both agonist and antagonist of glutamate (Glu), N-methyl-D-aspartate (NMDA), and MK-801 (1.0 x 10(-6) M), a noncompetitive blocker of NMDA receptor, significantly decreased the preferred temperature. These results indicate that the hypothalamic 5-HT, GABA, and Glu systems play a role in the temperature selection of prespawning female tilapia.     

Differential regulation of the brain and gut immunoreactive dynorphin by the serotonin system

Administration of drugs such as fenfluramine, 20-40 mg/kg, and m-chlorophenylpiperazine (m-CPP), 2.5-5 mg/kg, which release serotonin or activate postsynaptic serotonin receptors, respectively, induced a dramatic decrease in the duodenal content of immunoreactive dynorphin (ir-DYN). The effect was antagonized by cyproheptadine, 1 mg/kg. Similarly, acute administration of the specific serotonin reuptake blockers fluvoxamine, 15 mg/kg, or femoxetine, 10 mg/kg, and 5-hydroxytryptophan (5-HTP), 40-160 mg/kg, evoked a marked decrease in the duodenal content of ir-DYN. A combined administration of fluvoxamine or femoxetine and 5-HTP failed to potentiate the effect of individual treatment. Only a higher dose of fenfluramine, 40 mg/kg, increased the ir-DYN content in the hypothalamus. These results suggest that the brain and gut ir-DYN is independently regulated by the serotonin system and that a serotonin mechanism might stimulate release of the gut dynorphin content.     

Specificity of serotoninergic inhibition in Limulus lateral eye

The receptor specificity for synaptically mediated lateral inhibition in Limulus lateral eye retina was studied by structure-activity correlations of the action of the putative indoleaminergic neurotransmitter, serotonin (5-HT), and its isomers and structural analogs, tryptamine (TRYP), 6-hydroxytryptamine (6HT), 5,6-dihydroxytryptamine (5,6-DHT), 5-hydroxydimethyltryptamine (5-HDMT), and 5-hydroxytryptophan (5-HTP). The 5-HT blockers, lysergic acid diethylamide (LSD), bromo-LSD (BOL), and cinanserin, were also tested. The inhibitory action of the indoleaminergic agonists is highly structure-specific. An hydroxyl group in the 5 position of the indole nucleus, sterically unencumbered by hydroxyls in neighboing positions, is essential. In order of decreasing potency, 5-HT, 5-HDMT, and 5-HTP are active agonists; TRYP, 6-HT, and 5,6-DHT are inactive. Configuration and mobility of the side chains of the active agonists also affect the interaction, and these side-chain characteristics correlate with agonist potency. The receptors for inhibitory action and for transmembranal transport in reuptake are different. Both active agonists and inactive analogs appear to be taken up (Adolph and Ehinger, 1975. Cell Tissue Res. 163:1-14). LSD and BOL have bimodal actions: direct inhibition and agonist blockade. These actions may be mediated via low-specificity presynaptic uptake receptor sites rather than highly specific, postsynaptic, agonist receptor sites.     

Serotonin involvement in a motor disorder of Scottish terrier dogs

The effect of altering the concentration of 5-HT or the catecholamines upon an inherited neurological condition of Scottish terrier dogs which is characterized by episodes of muscular hypertonicity was assessed in a blind study. Alpha-methyl-paratyrosine and imipramine did not modify the condition. Amphetamine sulfate induced episodes; however, the episode was generally of shorter duration than the behavioral effect. The severity of the clinical rating was markedly increased by p-CPA. This increased severity was reduced by 5-HTP administration. The peripheral serotonin antagonist xylamidine tosylate did not alter the severity of the disease. Nialamide and 5-HTP had a significant beneficial effect. The increase in severity of the disease which follows a decrease in 5-HT coupled with a decrease in severity with an increase in 5-HT suggest certain serotoninergic neurons are involved in modulation of skeletal muscle tone.     

Evidence for GABA control of serotonin metabolism in the rat suprachiasmatic area

Changes in endogenous serotonin (5-HT) metabolism after in vivo stimulation of GABAergic transmission were investigated in the rat suprachiasmatic area (SCA). Activation of GABA transmission was performed by systemic administration of either amino-oxyacetic acid: AOAA, a GABA-transaminase inhibitor or RS baclofen, a GABA B agonist. After drugs administration, the amounts of endogenous 5-HT and 5-HIAA were measured. The release and synthesis of 5-HT were investigated in vitro, using a static incubation of tissue fragments. AOAA or RS baclofen induced an increase in endogenous 5-HT content but did not affect 5-hydroxyindole-acetic acid (5-HIAA). Both drugs induced an increase in the release and synthesis of 5-HT. Detailed study of the effects of AOAA over time on 5-HT metabolism showed that the increase in 5-HT release preceded the increase in amine synthesis. These results suggest that the in vivo stimulation of GABA transmission induces an increase in metabolic activity of the 5-HT neuronal system in the SCA. This effect may likely be mediated via activation of GABA B receptors.     

Serotoninergic stimulation of aldosterone secretion in vivo: role of the hypothalamo-pituitary adrenal axis.

The control of aldosterone secretion in vivo by serotonin was studied in conscious rats. Serial blood samples were taken from indwelling arterial cannulae before and after i.p. administration of 1 ml (4 g/l) 5-hydroxytryptophan (5-HTP), the precursor of serotonin (5-HT), or saline, and analysed for 5-HTP, serotonin, 5-hydroxyindoleacetic acid, plasma renin activity (PRA), corticosterone, aldosterone, sodium and potassium concentration. The relative contribution of the hypothalamo-pituitary adrenal axis was investigated in animals pretreated with the synthetic glucocorticoid dexamethasone. 5-HTP caused a significant increase in all parameters within 45 min except for plasma sodium and potassium. Saline administration showed no significant effect. Dexamethasone pretreatment significantly impaired the corticosterone and aldosterone response to 5-HTP, although the aldosterone response was merely attenuated. No other parameter was affected by dexamethasone pretreatment. The results show that administration of 5-HTP, which increases serum serotonin levels, stimulates PRA, corticosterone and aldosterone secretion. Dexamethasone pretreatment inhibits the aldosterone response, though not completely, suggesting that the stimulatory action of 5-HTP involves the release of ACTH, which stimulates corticosterone and aldosterone secretion by the adrenal cortex. The failure of dexamethasone to block the aldosterone response completely, suggests the involvement of other mechanisms such as the renin-angiotensin system or a direct action of serotonin on the adrenal zona glomerulosa.     

Serotonergic disturbances in autistic disorder: L-5-hydroxytryptophan administration to autistic youngsters increases the blood concentrations of serotonin in patients but not in controls

Some studies have suggested that disorders in the peripheral and central metabolism of serotonin (5-HT) may play a role in the pathophysiology of autistic disorder. This study examines the whole blood concentrations of 5-HT and 5-hydroxy-indoleacetic acid (5-HIAA) in baseline conditions and during a challenge with L-5-OH-tryptophane (5-HTP; 4 mg/kg in non enteric-coated tablets), the precursor of 5-HT, in a study group of 18 male, post-pubertal, Caucasian autistic patients (age 13-19 y.; I.Q.>55) and 20 matched healthy volunteers. In baseline conditions, no significant differences in 5-HT or 5-HIAA levels could be found between autistic youngsters and normal controls. 5-HTP administration significantly increased the levels of 5-HT in autistic youngsters but not in normal controls. Following 5-HTP challenge the 5-HT levels were significantly higher in autistic patients than in healthy volunteers. After challenge with 5-HTP, no significant differences were found in the concentrations of 5-HIAA or the test substance between autistic youngsters and normal controls. Differences in the peripheral metabolism of 5-HT which may not be observed in baseline conditions but which became clear after loading with 5-HTP, suggest that an increased synthesis of 5-HT from its precursor 5-HTP might be a one factor responsible for differences in the serotonergic system between autistic post-pubertal youngsters and normal controls.     

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.     

Behavioral and biochemical actions of p-chlorophenylethylamine (p-CPEA) in mice.

p-chlorophenylethylamine (p-CPEA), a metabolite of p-chlorophenylalanine (p-CPA) induces the “serotonin syndrome” which consists of lateral head weaving, Straub tail, hindlimb abduction, tremor, hyperactivity, reciprocal fore-paw treading, salivation and piloerection. These p-CPEA-induced behavioral signs were partially prevented by pretreatment with serotonin (5-HT) uptake blockers (fluoxetine, chlorimipramine, Org 6582) and 5-HT receptor blockers (methiothepin, methysergide, cinnanserin) but not by two depletors of brain 5-HT (p-CPA, reserpine). p-CPEA (50 mg/kg) produces an initial decrease in 5-HT associated with a concurrent increase in 5-hydroxyindoleacetic acid with a maximum change at 30 minutes after injection; these early biochemical changes are prevented by pretreatment with fluoxetine (10 mg/kg). p-CPEA also competes with (³H)-5-HT for 5-HT receptors. The reported paradoxical effects of p-CPA on several behavioral paradigms could be due to its decarboxylation to p-CPEA which may both stimulate 5-HT receptors and enhance 5-HT release.     

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.     

The occurrence, synthesis and metabolism of 5-hydroxytryptamine and 5-hydroxytryptophan in the cestode Hymenolepis diminuta: a high performance liquid chromatographic study

The biosynthesis and metabolism of 5-hydroxytryptamine (serotonin; 5-HT) in the cestode Hymenolepis diminuta was investigated by High Performance Liquid Chromatography (HPLC). Incubation of intact H. diminuta in [3H]tryptophan resulted in substantial radioactivity recovered in 5-HT, 5-hydroxytryptophan (5-HTP), and 5-hydroxyindoleacetic acid (5-HIAA). Furthermore, the tissue levels of 5-HT and 5-HTP, as determined by HPLC with electrochemical detection, were significantly depressed when the animals were deprived of tryptophan. On the other hand, the tissue levels of 5-HTP were significantly increased following incubation with the 5-HTP decarboxylase inhibitor m-hydroxybenzylhydrazine. The synthesis and metabolism of 5-HT are discussed in the light of 5-HT as a physiological transmitter in H. diminuta.     

Hyperleptinemia elicited by the 5-HT precursor, 5-hydroxytryptophan in mice: involvement of insulin

Mechanisms for hyperleptinemia elicited by a serotonin (5-hydroxytryptamine, 5-HT) precursor, 5-hydroxytryptophan (5-HTP), were investigated. 5-HTP elicited apparent increases in serum leptin levels of mice. Administration of 5-HTP did not alter expression of leptin mRNA in white adipose tissues. Furthermore, neither 5-HTP nor 5-HT increased leptin secretion from isolated fat pads of mice. Since insulin is known to enhance leptin release, involvement of insulin in 5-HTP-induced hyperleptinemia was examined. 5-HTP significantly elevated serum insulin levels. In mice treated with streptozotocin, which depletes insulin, 5-HTP did not increase serum leptin levels. These results suggest that hyperinsulinemia participates the elevation of serum leptin levels elicited by 5-HTP.     

Effect of orally administered l-tryptophan on serotonin,melatonin, and the innate immune response in the rat

To assess the effects of external administration of L-tryptophan on the synthesis of serotonin and melatonin as well as on the immune function of Wistar rats, 300 mg of the amino acid were administered through an oral cannula either during daylight (08:00) or at night (20:00) for 5 days. Brain, plasma, and peritoneal macrophage samples were collected 4 h after the administration. The accumulation of 5-hydroxytryptophan (5-HTP) after decarboxylase inhibition was used to measure the rate of tryptophan hydroxylation in vivo. Circulating melatonin levels were determined by radioimmunoassay, and the phagocytic activity of macrophages was measured by counting, under oil-immersion phase-contrast microscopy, the number of particles ingested. The results showed a diurnal increase (p < 0.05) in the brain 5-HTP, serotonin (5-hydroxytryptamine, 5-HT), and 5-hydroxyindolacetic acid (5-HIAA) of the animals which had received tryptophan at 08:00 and were killed 4 h later. In the animals which received tryptophan during the dark period, the 5-HT declined but the 5-HT/5-HIAA ratio remained unchanged. There was also a significant increase (p < 0.05) in nocturnal circulating melatonin levels and in the innate immune response of the peritoneal macrophages in the animals which had received tryptophan at 20:00. The results indicated that the synthesis of serotonin and melatonin, as well as the innate immune response, can be modulated by oral ingestion of tryptophan.     

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.     

Mapping convulsants' binding to the GABA-A receptor chloride ionophore: a proposed model for channel binding sites

Gamma-aminobutyric acid (GABA) type A receptors play a key role in brain inhibitory neurotransmission, and are ligand-activated chloride channels blocked by numerous convulsant ligands. Here we summarize data on binding of picrotoxin, tetrazoles, beta-lactams, bicyclophosphates, butyrolactones and neurotoxic pesticides to GABA-A ionophore, and discuss functional and structural overlapping of their binding sites. The paper reviews data on convulsants' binding sensitivity to different point mutations in ionophore-lining second trans-membrane domains of GABA-A subunits, and maps possible location of convulsants' sites within the chloride ionophore. We also discuss data on inhibition of glycine, glutamate, serotonin (5-HT3) and N-acetylcholine receptors by GABA-A channel blockers, and examine the applicability of this model to other homologous ionotropic receptors. Positioning various convulsant-binding sites within ionophore of GABA-A receptors, this model enables a better understanding of complex architectonics of ionotropic receptors, and may be used for developing new channel-modulating drugs.     

Ovine Pineal Indoles: Effects of l-Tryptophan or l-5-Hydroxytryptophan Administration

L-5-Hydroxytryptophan (L-5-HTP) (20 or 200 mg/kg i.p.) but not L-tryptophan (500 mg/kg i.p.) loading substantially increases serum melatonin in sheep. In the present study we examined the effects of these compounds on pineal serotonin and six serotonin metabolites. L-Tryptophan failed to increase 5-hydroxytryptamine (5-HT; serotonin) or any of its metabolites despite a five-fold increase in pineal tryptophan. In contrast, L-5-HTP loading produced a marked increase in pineal 5-HT and its metabolites, including N-acetylserotonin (NAS) and melatonin, indicating that an increased synthesis of melatonin is responsible for the increased serum melatonin concentration after loading with this precursor. No change in pineal indoleamine N-acetyltransferase (NAT) activity was seen. These results are consistent with the suggestion that, during daytime in the sheep, 5-HT availability may limit the production of melatonin.     

Pharmacological effects of GABA on serotonin metabolism in the rat brain

The pharmacological effects of GABA-related drugs were studied on the serotonin (5-HT) and 5-hydroxyindole-acetic acid (5-HIAA) contents of various regions of the rat brain. These effects were examined in the nuclei raphe dorsalis, magnus and centralis and in structures receiving a dense serotonin innervation such as the habenula complex and subcommissural organ. The GABA agonist, muscimol, increased the 5-HT contents and reduced 5-HIAA levels in structures containing serotoninergic terminals suggesting an inhibitory effect of GABA on the firing of serotoninergic neurons with concomitant reduction of 5-HT utilisation. In contrast, the GABA antagonist, bicuculline, probably stimulated 5-HT turnover since its intraperitoneally administration produced significant increase of 5-HT and/or 5-HIAA levels in the same brain regions. These data are in agreement with a transsynaptic inhibitory control of GABA on serotoninergic neurons. Drugs which inhibit the GABA catabolism such as amino-oxyacetic acid or gamma-vinyl-GABA and which should elevate GABA levels in the synaptic gap were capable of increasing or decreasing the 5-HT and the 5-HIAA levels depending on the experimental conditions. These results suggest that several processes are probably involved in the control of serotoninergic neurons by GABA in the rat brain. Among them, an intracellular effect of GABA on 5-HT metabolism might well occur in cells containing both GABA and 5-HT.