Brain 5HT Research Articles

Elevation and reduction of plasma tryptophan and their effects on aggression and perceptual sensitivity in normal males

Data from experimental animals suggest that 5-hydroxytryptamine (5HT) may have an inhibitory effect on aggression, while clinical studies have found a correlation between pathological aggression and low brain 5HT. To investigate this relationship further we used amino acid mixtures designed to raise or lower the levels of the 5HT precursor, tryptophan. Normal male subjects were given tryptophan-depleted, balanced, or tryptophan-supplemented ammo acid mixtures and tested for aggression 5 hours later. The balanced amino acid mixture served as a control for the tryptophan depletion and supplementation. Testing for aggression was done using the Buss paradigm in which subjects deliver electric shocks to a (nonexistent) partner in response to stimulus tones. Duration and intensity of shock delivered were the measures of aggression, while responsivity to the stimulus tones was the measure of perceptual sensitivity. Neither tryptophan supplementation nor tryptophan depletion had any effect on these measures of aggression or perceptual sensitivity. We conclude that raising or lowering the synthesis of brain 5HT through alterations in tryptophan availability does not influence aggression in normal males as measured by the Buss task.

Acute effects of ethanol on brain, plasma and adrenal monoamine concentrations in virgin and pregnant rats and their fetuses

The dose-response relationship in brain, plasma, and adrenal monoamine changes after acute oral ethanol administration (1, 2, 4 g/kg body wt) was studied in virgin rats to determine whether the response to the highest dose differed in 21-day pregnant animals, and to assess the potential consequences of ethanol on the neurotransmitter systems of their fetuses. Blood ethanol and acetaldehyde concentrations in blood increased progressively with the ethanol dose in virgin rats, and values in pregnant animals were very similar. Ethanol concentration in fetal blood and amniotic fluid did not differ from that in mother's blood whereas fetal acetaldehyde concentrations were negligible. In a dose-related manner, ethanol decreased brain DA, DOPAC and 5HT concentrations did not affect those of NA and 5HIAA, or adrenal A and NA concentrations, whereas it enhanced plasma NA levels. Basal levels of monoamines and their changes after ethanol intake did not differ in pregnant and virgin rats. Monoamine and metabolite concentrations were much lower in fetal than in maternal brains whereas plasma and adrenal catecholamine concentrations were very similar and maternal ethanol intake did not modify these fetal parameters in the fetus. Results are in agreement with the known similar metabolic response to ethanol in fed pregnant and virgin rats. The lack of fetal monoamine response to maternal ethanol intake may be a consequence of the incapacity of fetal liver to form acetaldehyde and the ability of the placenta to oxidize maternal acetaldehyde which protects the fetus from maternal alcohol intake at late gestation.

The behavioral effects of depletions of brain serotonin induced by 5,7-dihydroxytryptamine vary with time after administration

Ether-anesthetized Sprague-Dawley rats were depleted of brain serotonin (5HT) by intraventricular injections of 50 micrograms 5,7-dihydroxtryptamine (57DHT). Oral pretreatment with 25 mg/kg desmethylimipramine was used to protect brain noradrenergic neurons from 57DHT. Liquid chromatographic assays revealed that this treatment did not significantly alter catecholamine levels but depleted hippocampal 5HT by 92% and striatal 5HT by 45%. Three or eleven days after lesioning, locomotor and exploratory behavior was characterized in separate groups of animals with a behavioral pattern monitor (BPM). On Days 4 and 12, the animals were retested following saline or 1.0 mg/kg amphetamine. Three days after depletion, lesioned rats exhibited a decrease rate of habituation of locomotor activity relative to controls. When challenged with amphetamine (1.0 mg/kg), 5HT-depleted rats exhibited increased corner and decreased center activity, as well as stereotyped patterns of locomotion. Eleven days following lesion, 5HT-depleted rats exhibited habituation rates greater than controls; amphetamine challenge yielded patterns of activity similar to those of control animals. These results show that central serotonergic pathways play an important role in modulating both spontaneous and amphetamine-elicited activity in rats, and that compensatory mechanisms operate over time to alter the behavioral effects of 57DHT-induced depletions of brain 5HT.

Regional turnover of norepinephrine and dopamine in rat brain following acute exposure to air ions.

Exposure to air ions has been reported to influence serotonin (5HT), although critical reviews of these studies and previous measurements in our laboratory of the concentration, release, and utilization of brain 5HT indicate that neither the data nor the interpretations of the data are particularly convincing. Measurements of other possibly relevant neurotransmitter systems--norepinephrine (NE) and dopamine (DA)--were made in brain regions selected because of their importance in the modulation of brain functions relating to motivation, arousal, endocrine function, and motor activity, all responses that have been reported to be influenced by air ion exposure. Results indicate that exposure of male Holtzman rats to high concentrations (5.0 X 10(5)/cm3) of positive or negative air ions or to DC electric fields (3.0 kV/m) for periods up to 66 h failed to affect the concentration of NE or DA significantly in any of the brain regions.

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.

Atlas of serotonergic cell bodies in the cat brainstem: An immunocytochemical analysis

The localization and relative number of serotonergic (5HT) cell bodies in the brainstem of the cat were studied through the use of a specific immunocytochemical technique. A surprisingly large number of 5HT cells were found in regions in addition to the classical raphe nuclei (obscurus, pallidus, magnus, centralis superior, and dorsalis). Foremost among these were: the ventral medulla, just dorsal to the pyramidal tract and inferior olivary complex, and especially the area in and around the lateral reticular nucleus; the dorsal pons, surrounding the central reticular core, and in the central gray area; and a region in the mesencephalon, in and around the interpeduncular nucleus. The advantages and disadvantages of the existing schemas for subdividing and labeling groups of brain 5HT neurons are discussed.

Influence of glucocorticoids on brain serotonin metabolism in rats.

It has been suggested brain serotonin (5HT) plays a role in regulation of the hypothalamic-pituitary-adrenal system, although none of the experimental approaches so far reported has produced unequivocal results, mainly on account of the static tissue 5HT level or the known effects of drugs being employed in interpretation of their data. The present study in rats of the possible feedback influence of glucocorticoids on the brain 5HT was designed to analyze the dynamic turnover of its metabolism. Intracisternal injection of corticosterone (COR) caused no significant changes in 5HT content but increased its turnover dose-dependently in the cortex-cerebellum and the rostral brain stem including the hypothalamus, and lowered the serum COR level at doses less than 5.0 micrograms/kg, reducing its effect with increasing doses. Intracisternal or intraperitoneal administration of miliary doses of dexamethasone (DEX) decreased 5HT turnover in the same brain portions. The effects of glucocorticoids on the brain 5HT metabolism were more potent when injected intracisternally, especially onto the rostral brain stem. Although COR and DEX played controversial roles, the present results suggest that adrenal glucocorticoids may directly act on brain 5HT metabolism and there may be a feed-back relation between adrenal glucocorticoid and brain 5HT.

Benzodiazepine anxiolytic action and affinities for serotonergic and adrenergic receptors

1. Tissue homogenates were prepared from brains of four week old Lewis rats. 2. 5HT receptors were prepared from three brain regions and labelled with 3H-5HT and 3H-spiperone. 3. σ -Receptors were prepared from rat forebrains and labelled with 3H-WB4101 and 3H-clonidine. 4. Incubations were carried out at seven concentrations for each of twelve benzodiazepines. 5. IC 50 values were derived from specific binding versus log (drug concentration) plots. 6. The benzodiazepines appeared to have little affinity for 5HT or NA post-synaptic receptors in rat brain.

Alterations in brain 5HT and tryptamine content during indoleamine-induced myoclonus in guinea pigs

l-5-Hydroxytryptophan (5HTP) (with or without carbidopa pretreatment), l-tryptophan (plus pargyline pretreatment), or tryptamine (plus pargyline pretreatment) all induced dose-dependent myoclonus in guinea pigs. At the time of maximal behavioural response animals were killed for determination of brain indoleamine content. Administration of 5HTP (50–200 mg/kg) to naive guinea pigs, or of 5HTP (20–80 mg/kg) to carbidopa- (25 mg/kg 1 hr previously) pretreated animals, markedly elevated brain 5-hydroxytryptamine (5HT) concentrations but depressed whole brain tryptamine content. l-Tryptophan (50–200 mg/kg) administration to pargyline- (75 mg/kg 30 min previously) pretreated animals also increased cerebral 5HT levels. l-Tryptophan (200 mg/kg plus pargyline), elevated whole brain tryptamine content. Administration of tryptamine (40mg/kg) to pargyline-pretreated guinea pigs caused a small increase in brain 5HT levels, but markedly elevated cerebral tryptamine content. 5HT appears to be the indoleamine mainly responsible for 5HTP-induced myoclonus but tryptamine predominates in tryptamine-induced myoclonus. Both 5HT and tryptamine may contribute to myoclonus induced by l-tryptophan.

Noradrenaline and 5-hydroxytryptamine modulation of brain dopamine function: implications for the treatment of Parkinson's disease.

1 Dopamine deficiency in the brain is the prime biochemical deficit in Parkinson's disease, but loss of noradrenaline and 5HT also may contribute. 2 In rats, 5HT-containing neurones originating from the dorsal and median raphe nuclei innervate forebrain dopamine-containing areas so as to impose an inhibitory regulatory tone on dopamine function. However, this interaction between brain dopamine and 5HT-containing neuronal systems is complex, and the effect produced appears dependent on the relative activity of each system. 3 Anatomical evidence for innervation of dopamine-containing brain regions by noradrenaline fibres in the rat is scanty, but functional studies suggest the existence of inputs which facilitate dopamine function. 4 Drug therapy designed to increase or decrease brain 5HT function has had no consistent effect in Parkinson's disease. 5 Manipulation of brain noradrenergic activity in Parkinson's disease had little effect, although the noradrenaline precursor L-threo-DOPS may reduce freezing attacks. 6 Until more specific drug molecules are available the role of brain noradrenergic and 5HT mechanisms in Parkinson's disease remains uncertain.

Fluoxetine in the treatment of intention myoclonus.

Some types of intention myoclonus respond to serotonin precursor therapy (e.g., L-5-hydroxytryptophan, L-5HTP). Fluoxetine, a specific serotonin (5HT) uptake blocker, was found to have no antimyoclonic effect when administered by itself to four patients with intention myoclonus. However, in two patients with intention myoclonus responsive to L-5HTP and carbidopa, fluoxetine reduced the required dose of L-5HTP to approximately one-third, with greater antimyoclonic activity, decreased side effects, and reduction in platelet 5HT and plasma 5-hydroxyindoleacetic acid and L-5HTP concentrations. These findings further support the hypothesis that some forms of intention myoclonus are caused by a deficiency of brain 5HT, and suggest that the addition of fluoxetine to L-5HTP and carbidopa may improve antimyoclonic therapy.

Corticosteroid response to stress depends upon increased tryptophan availability.

Prior administration of valine to rats has been shown previously to prevent restraint stress-induced increases in brain tryptophan and 5HT turnover. The present study demonstrates that the accompanying attenuation of the corticosteroid response to this stress is substantially reversed by administration of tryptophan with the valine. Tyrosine is not effective in reversing this attenuation, and in fact itself attenuates the corticosteroid response to the stress when given alone. It is concluded that at least part of the corticosteroid response to restraint stress is mediated by an increase in serotonergic activity that is dependent on increased supply of the precursor, tryptophan, and that this can be antagonised by either of two amino acids which compete with tryptophan for access to the brain. Implications for stress-associated human disorders are discussed.

Tryptamine-induced myoclonus in guinea-pigs pretreated with a monoamine oxidase inhibitor indicates pre- and post-synaptic actions of tryptamine upon central indoleamine systems

Tryptamine (1–320 mg/kg) evoked only slight muscle jerking in naive guineα-pigs but, in animals pretreated with pargyline (75 mg/ kg; 1 hr previously), tryptamine induced a dose-dependent (6–160 mg/kg) myoclonus. The myoclonus induced by tryptamine (40 mg/kg) plus pargyline (75 mg/kg) was differentially inhibited by the indoleamine receptor antagonists, methergoline (5 mg/kg) which was more potent than methysergide (10 mg/kg), mianserin (10 mg/kg) which was more potent that cyproheptadine (10 mg/kg) and propranolol (20 mg/kg) which was more potent than cinanserin (10 mg/kg). This rank order of potency differed from that observed for the order of potency of these drugs in inhibiting the myoclonus induced by l-5-hydroxytryptophan (5HTP) plus carbidopa in guinea-pigs (Luscombe, Jenner and Marsden, Neuropharmacology, 1981), perhaps indicating involvement of pharmacologically distinct indoleamine receptors. Manipulation of presynaptic function of 5-hydroxytryptamine (5HT) by tryptophan hydroxylase inhibition with p-chlorophenylalanine to produce depletion of cerebral 5HT, or by an l-tryptophan load to elevate 5HT in brain, suggested that the functional integrity of serotonergic neurones is required for the expression of myoclonus induced by tryptamine plus pargyline. A range of blockers of 5HT re-uptake did not alter the jerking produced by tryptamine (40 mg/kg) in guinea pigs pretreated with pargyline (75 mg/kg; 1hr previously), or the threshold myoclonus induced by a smaller dose of tryptamine (10 mg/kg; plus pargyline 75 mg/kg). It is suggested that myoclonus induced by tryptamine in guinea pigs pretreated with pargyline involves activation of post-synaptic indoleamine receptors by tryptamine by a mechanism which requires intact presynaptic function of 5HT.

Cerebral monoamines and lidocaine toxicity in rats.

The effect of alterations in whole brain monoamine content on the plasma lidocaine concentration resulting in seizures was studied in rats. Reductions in brain monoamine content were produced by treatment with one of the following drugs: reserpine, parachlorophenylalanine (PCPA), or alpha methyl paratyrosine (AMPT). Reserpine depleted norepinephrine (NE), and dopamine (DA) by 75 per cent and serotonin (5HT) by 54 per cent; PCPA reduced brain 5HT 56 per cent without changing NE and DA; AMPT reduced brain NE and DA by 54 and 60 per cent, respectively, without altering 5HT content. Treatment with 5-hydroxytryptophan, a serotonin precursor combined with the peripheral decarboxylase inhibitor RO4-4602 increased brain 5HT content by 400 per cent without changes in DA and NE. Whole brain NE concentrations were assayed fluorometrically, DA brain concentrations were assayed by HPLC, and lidocaine concentrations in plasma were determined by gas chromatography. Plasma lidocaine concentrations at the onset of convulsions were found to be elevated significantly only by drugs causing serotonin depletion; increasing to 128 per cent of control with reserpine treatment and 139 per cent of control with PCPA treatment. Depletion of NE and DA had no effect on the lidocaine seizure threshold. Increases in brain 5HT caused a small but not statistically significant decrease to 94 per cent of control in the mean plasma lidocaine concentration at seizure onset.

Relationships among Maternal Diet, Serotonin Metabolism at Weaning, and Protein Selection of Progeny

Brain serotonin (5HT) metabolism in rats at weaning was examined in relation to their maternal dietary protein (PC) and tryptophan (TRP) concentration and to their protein feeding behavior. Pup brain TRP, 5HT, and 5-hydroxyindoleaceticacid (5HIAA) concentration was unaffected by maternal diet PC (10, 20, or 40%) but was increased by TRP additions (0.66 or 1.86%) to 40% casein diets. Turnover of brain 5HT, measured at weaning after probenecid injection, was inversely related to maternal diet PC (r = .54, P < 0.05) but was not affected by TRP supplementation. Pups allowed to select from 10 and 60% casein diets selected during the first 3 days post-weaning, dietary PC which reflected the PC (r = 0.65, P < 0.01), but not TRP content of their maternal diet, and also reflected brain 5HT turnover (r = -.50, P < 0.05), but not concentration, of their littermates. Thus, the relationship of maternal dietary PC to protein selection by the offspring is associated with maternal diet-induced changes in their brain 5HT turnover, at least as measured after probenecid administration.serotonin feeding behavior maternal diet protein selection tryptophan weanling rats protein

Triphasic changes in plasma ACTH concentration and brain serotonin synthesis rate following adrenalectomy in rats.

Following bilateral adrenalectomy in adult male rats, there occurs a pattern of triphasic change in basal plasma concentration of radioimmunoassayable ACTH. Plasma ACTH is markedly elevated at 2 h, has returned down almost to normal at 20 h and is again markedly elevated 96 h after adrenalectomy. We have examined serotonin (5HT) synthesis rats in several brain regions, anterior hypothalamus, posterior hypothalamus, and brain stem, at these times after adrenalectomy using the accumulation of 5-hydroxytryptophan (5HTP) after inhibition of aromatic L-amino acid decarboxylase with m-hydroxybenzylhydrazine. In both anterior hypothalamus and brain stem, decrease 5HT synthesis rates were observed at 2 and 96 h after adrenalectomy, but at 20 h 5HT synthesis rates were normal. This pattern was not observed in the posterior hypothalamus. Thus, we demonstrated inverse correlations between 5HT synthesis rates in anterior hypothalamus and brain stem, but not posterior hypothalamus, and basal plasma ACTH concentration throughout the period of triphasic change following adrenalectomy in adult male rats. Both the adrenalectomy-induced increases in plasma ACTH concentration and the adrenalectomy-induced decreases in brain 5HT synthesis rates were inhibited by treatment with dexamethasone, suggesting that the changes resulted from glucocorticoid withdrawal. The data are consistent with a role of brain 5HT neurons with cell bodies in brain stem and nerve endings in anterior hypothalamus in the regulation of the triphasic changes in plasma ACTH concentration following adrenalectomy in rats.

Effects of maternal ethanol ingestion on cerebral neurotransmitters and cyclic-AMP in the rat offspring

1. 1. To study the effects of maternal alcohol ingestion on brain parameters in offspring, rats were given ethanol for drinking (25% w/v) from the time of mating until sacrifice. Controls drank tap water. 2. 2. Alcohol ingestion reduced daily food and liquid consumption but total caloric intake was only slightly diminished. 3. 3. Maternal body weight increased and offspring body weight, size and brain weight were reduced in the animals receiving alcohol. 4. 4. Brain concentrations of tryptophan, tyrosine and GABA were augmented in ethanol treated mothers at 1 day post-partum. 5. 5. Comparison of brain parameters in offspring of alcoholic mothers with those of controls showed that tryptophan and 5HT concentrations were augmented in 4 day old neonates, NA was increased in 21 day fetuses and 1 day old neonates, and adenylate cyclase activity was also greater in the brains of 21 day fetuses and the cerebellums of 4 day old neonates. 6. 6. Neither phosphodiesterase nor cyclic-AMP concentrations differed in offspring of alcoholic and control mothers. 7. 7. Data showed alterations in brain NA and 5HT systems in the offspring of alcoholic mothers.

Protein and energy consumption, plasma amino acid ratios, and brain neurotransmitter concentrations

The hypothesis that long-term protein and energy intakes are controlled, respectively, by brain concentrations of serotonin (5HT) and the catecholamines, dopamine (DA) and norepinephrine (NE) has been proposed by Anderson and associates [2,5]. This control is assumed to be mediated through the influence of diet composition on the availability of the amino acid precursors of the neurotransmitters to the brain. The hypothesis was tested in experiments in which plasma amino acid concentrations and brain neurotransmitters were measured in young rats allowed to choose between two isocaloric diets differing in protein content (15% vs 55% casein or 15% vs 55% wheat gluten) and containing additional amounts of certain amino acids. No consistent correlation was observed between protein consumption and either the plasma ratio of tryptophan to neutral amino acid (Trp/NAA) concentrations or brain 5HT concentration. Similarly, no correlation was observed between energy intake and either the plasma ratios of Tyr/Phe and Tyr/NAA concentrations or the brain concentrations of NE and DA. The most apparent association observed was a strong correlation between total protein consumption and the sum of plasma branched-chain amino acid concentrations. Under the conditions of this experiment, feeding behavior seemed to be associated positively with the needs of the animal for nutrients and negatively with ingestion of an excess of amino acids generally rather than with plasma amino acid concentration ratios and brain neurotransmitter levels.

Brain serotonin turnover correlates inversely with plasma adrenocorticotropin during the triphasic response to adrenalectomy in rats.

After bilateral adrenalectomy in adult male rats a triphasic change occurs in the plasma concentration of radioimmunoassayable ACTH. Plasma ACTH is markedly elevated at 2 h, returns almost to normal at 20 h, and is again markedly elevated 92 h after adrenalectomy. We have examined brain serotonin (5HT) turnover during this period using two nonsteady state methods: accumulation of 5HT after monoamine oxidase inhibition with pargyline and decline of 5-hydroxyindoleacetic acid (5HIAA) after pargyline. Both endpoints demonstrated decreases in hypothalamic and brain stem 5HT turnover 2 and 92 h after adrenalectomy, but normal 5HT turnover 20 h after adrenalectomy. Thus, we demonstrated inverse relationships between 5HT turnover in both the hypothalamus and brain stem and the plasma ACTH concentration throughout the period of triphasic change after adrenalectomy. The adrenalectomy-induced increases in plasma ACTH and decreases in brain 5HT turnover both 2 and 92 h after adrenalectomy are inhibited by treatment with small doses of corticosterone. The data strongly suggest that the activity of some brain 5HT neurons changes after adrenalectomy in a triphasic pattern and, further, that these changes are related in part to glucocorticoid withdrawal. 5HT receptor antagonists blunted corticosterone inhibition of the adrenalectomy-induced decreases in brain 5HT turnover, providing further evidence for an interaction between glucocorticoids and brain 5HT neurons. The data are consistent with a role of brain 5HT neurons in the adrenalectomy-induced triphasic changes in ACTH secretion.

Influence of plasma tryptophan on brain 5HT synthesis and serotonergic activity.

Studies are described on the effect of plasma tryptophan changes on brain 5HT synthesis in man and rat. Results show that human brain 5HT synthesis is influenced by the supply of tryptophan to the brain. This is indicated by: (a) significant correlations between plasma free tryptophan and CSF 5HIAA concentrations; (b) raised cortical 5HT concentrations after infusing tryptophan. In rat experiments, determinations of brain tryptophan uptake from a bolus of plasma injected into the carotid artery showed: (a) increased uptake when bolus free tryptophan was raised and total tryptophan kept constant; (b) unchanged uptake when bolus free tryptophan was kept constant and total tryptophan decreased. Brain tryptophan uptake from a buffer bolus was decreased by large neutral amino acids. Plasma total tryptophan could be rapidly decreased and free tryptophan increased by briefly disturbing food deprived rats. When free tryptophan concentration rose markedly there was an associated increase of brain tryptophan and 5HT turnover. Studies of shock provoked analgesia in rats and cortical evoked potentials in man both suggest that physiological variations of serotonergic activity are sufficient to influence these measures. This raises the possibility that moderate changes of tryptophan supply to the brain could, in some circumstances, alter serotonergic activity.