Presynaptic Serotonergic Dysfunction in Patients with Alzheimer''s Disease

Indices of presynaptic serotonergic nerve endings were assayed in neocortical biopsy samples from patients with histologically verified Alzheimer's disease. The concentrations of 5-hydroxytryptamine (serotonin) and 5-hydroxyindoleacetic acid, serotonin uptake, and K+-stimulated release of endogenous serotonin were all found to be reduced below control values. Changes occurred in samples from both the frontal and temporal lobes, but they were most severe (at least a 55% reduction) in the temporal lobe. This is indicative of substantial serotonergic denervation. Values for serotonergic markers in Alzheimer's disease samples did not show correlations with rating of the severity of dementia, indices of cholinergic innervation, or senile plaque and cortical pyramidal neurone loss. However, neurofibrillary tangle count and an index of glucose oxidation (both probably reflecting pyramidal cells) correlated with the concentration of 5-hydroxyindoleacetic acid.     

Evidence for a serotonin transporter deficit in experimental acute liver failure

It has been suggested that alterations of serotonin transport may be implicated in the pathogenesis of the neuropsychiatric symptoms encountered in acute liver failure. In order to address this issue, microdialysate concentrations of serotonin, its precursor L-tryptophan and metabolite 5-hydroxyindoleacetic acid (5-HIAA) as well as brain regional distribution of serotonin transporter ([3H]-citalopram) sites were measured in rats with acute liver failure resulting from hepatic devascularization. A significant loss of [3H]-citalopram sites was observed in dorsal Raphe nucleus, in frontal and frontoparietal cortices as well as in substantia nigra of rats with severe encephalopathy resulting from acute liver failure. In frontal cortex, this loss of transporter binding sites was accompanied by significant increases of L-tryptophan, serotonin and 5-HIAA concentrations in extracellular fluid. Pharmacological manipulation of the brain serotonin system could afford a novel therapeutic approach to the prevention of the neuropsychiatric symptoms characteristic of acute liver failure in humans.     

Increased uptake sites for serotonin and dopamine with decreased S2 serotonin receptors in microencephalic rat brain

Methylazoxymethanol (MAM)-induced cerebral hypoplasia resulted in a significant increase in densities of both serotonin uptake sites in frontal cortex and dopamine uptake sites in striatum, suggesting serotonergic and dopaminergic axon terminals were compressed in the smaller brain volumes. The density of S2 serotonin receptors in MAM-lesioned frontal cortex was decreased probably due to down-regulation, while densities of D1 and D2 dopamine receptors in striatum were identical between MAM-lesioned rats and control rats.     

Binding of the Novel Serotonin Agonist 8-Hydroxy-2-(Di-n-Propylamino) Tetralin in Normal and Alzheimer Brain

Binding of [3H]8-hydroxy-2-(di-n-propylamino) tetralin, a putative ligand for the 5-hydroxytryptamine (5-HT, serotonin) 1A recognition site, was measured in neocortex from postmortem human brain. The substance was found to bind to a saturable site with a KD value and pharmacological profile similar to that of rat. Binding to membranes from normal human temporal cortex was found to significantly correlate (inversely) with age. A significant reduction in binding, reflecting decreased density of recognition sites, was observed in the frontal cortex of patients with Alzheimer's disease (48% loss). This region in the dement brains showed unaltered presynaptic 5-HT function (5-HT and 5-hydroxyindoleacetic acid content) whereas 5-HT concentration was reduced in the temporal cortex.     

[3H]Ketanserin (Serotonin Type 2) Binding Increases in Rat Cortex Following Basal Forebrain Lesions with Ibotenic Acid

The response of the serotonergic system following injury to the basal forebrain cholinergic system was investigated in rats. The density of 5-hydroxytryptamine (serotonin) type 2 (S2) receptor sites in the frontal cortex and hippocampus was determined 1 week and 4 months after production of lesions by injections of ibotenic acid into the medial septum and nucleus basalis magnocellularis. One week later, the number of S2 receptor sites in the frontal neocortex, as defined by [3H]ketanserin binding, was unchanged. Four months later, the number of [3H]ketanserin binding sites (and Bmax) was increased and high-affinity [3H]serotonin uptake was decreased in the frontal neocortex, but not in the hippocampus, relative to unlesioned controls. Choline acetyltransferase (acetyl-CoA:choline O-acetyltransferase; EC 2.3.1.6) activity was decreased significantly in the frontal neocortex and hippocampus 1 week and 4 months after surgery. The change in frontal neocortical S2 receptor site density was inversely related to the level of choline acetyltransferase activity, was specific for cholinergic denervation associated with the cortex but not the hippocampus, and may represent a localized denervation supersensitivity due to degeneration of median raphe cortical afferents.     

Role of the serotonin uptake carrier in the neurochemical response to methamphetamine: Effects of citalopram and chlorimipramine

The role of the serotonin uptake carrier in the methamphetamine-induced depression of serotonin synthesis was examined. In vivo, coadministration of citalopram or chlorimipramine with methamphetamine blocked the irreversible depression of tryptophan hydroxylase activity observed in the neostriatum and cerebral cortex after repeated administration of high doses of methamphetamine. The methamphetamine-induced reduction of neostriatal serotonin and 5-hydroxyindoleacetic acid was also attenuated by the two uptake inhibitors. In contrast, neither drug antagonized the depression of neostriatal tyrosine hydroxylase activity observed after methamphetamine administration. Citalopram also blocked the reversible inhibition of tryptophan hydroxylase activity observed after the acute administration of methamphetamine. In vitro, citalopram significantly inhibited methamphetamine-induced [³H] serotonin release from neostriatal slices. The results demonstrate that inhibitors of the serotonin uptake carrier can antagonize both the in vivo and in vitro effects of methamphetamine on serotonergic neurons. Furthermore, the methamphetamine-induced depression of serotonin synthesis is dependent upon a functional serotonin uptake system.     

Effects of short- and long-term neuroleptic treatment on brain serotonin synthesis and turnover: focus on the serotonin hypothesis of schizophrenia

Short-term (90 min) administration of haloperidol (2 mg/kg), or chlorpromazine (10 mg/kg) increased the activity of tryptophan hydroxylase as well as the levels of 5-hydroxytryptamine (serotonin) and 5-hydroxyindoleacetic acid in mid-brain of rats. The chronic neuroleptic treatment (21 days) produced more pronounced changes in all parameters related to serotonin synthesis and turnover. The activity of tryptophan hydroxylase in mid-brain was further augmented; the levels of 5-hydroxytryptamine and 5-hydroxyindole-acetic acid were significantly elevated not only in mid-brain, but also in several other discrete regions examined. These data suggest that neuroleptics enhance the synthesis and utilization of brain serotonin. The role of brain serotonergic neurons in the pathophysiology of schizophrenia is further considered.     

Effects of sex factors and hemispheric localization on the involvement of serotonin from the frontal cortex, striatum, and nucleus accumbens into the processing of novel and repeatedly presented information in rats

The effects of novel or relevant (a single exposure to experimental chamber) and irrelevant (20 exposures to experimental chamber) stimuli on the levels of serotonin (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the frontal cortex, striatum, and nucleus accumbens in the left and right hemispheres were studied in male and female rats. It was found that 5-HT and 5-HIAA contents in the frontal cortex changed in response to neither relevant nor irrelevant stimuli. However, there were hemispheric difference in 5-HT and 5-HIAA in the frontal cortex of intact animals. The level of 5-HT in males and the level of 5-HIAA in females were higher in the left frontal cortex. In females, the level of 5-HIAA in the left striatum decreased in response to the novel stimulus. Sex differences in: a) 5-HT metabolism (increase in the level of 5-HIAA in males and increase in 5-HT in females) and b) lateralization (the striatal 5-HT metabolism in males changed bilaterally and only in the left hemisphere in females) were observed in reactions to irrelevant stimuli. Both in male and female rats, serotonin content in the nucleus accumbens changed only in response to the irrelevant stimuli. The 5-HT level increased in the left and right hemispheres independently of sex, but hemispheric difference was revealed only in females, in which the serotonin level was higher in the left nucleus accumbens. It is concluded that serotonergic neurotransmitter mechanisms are involved in hemispheric and sex differences in selective attention.     

Effects of an acute dose of ethanol on dopaminergic and serotonergic systems from rat cerebral cortex and striatum

Intraperitoneal injection 10 min before sacrifice of 1.5 g ethanol/kg weight produced an increase in rat striatal levels of homovanillic acid (HVA) (p < 0.05) but did not affect the striatal concentrations of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA). A similar ethanol treatment led to decreases in 5-HT (p < 0.05) and 5-HIAA (p < 0.05) from cerebral cortex (prefrontal and anterior cingulate areas). The results point to several ethanol-linked alterations in central serotonergic and dopaminergic systems.     

In vivo binding of 125I-LSD to serotonin 5-HT2 receptors in mouse brain

The binding of 125I-LSD (2-[125I]-lysergic acid diethylamide) was studied in various mouse brain regions following intravenous injection of the radioligand. The high specific activity of 125I-LSD enabled the injection of low mass doses (14 ng/kg), which are well below the threshold for induction of any known physiological effect of the probe. The highest levels of 125I-LSD binding were found in the frontal cortex, olfactory tubercles, extra-frontal cortex and striatum while the lowest level was found in the cerebellum. Binding was saturable in the frontal cortex but increased linearly in the cerebellum with increasing doses of 125I-LSD. Serotonergic compounds potently inhibited 125I-LSD binding in cortical regions, olfactory tubercles, and hypothalamus but had no effect in the cerebellum. Dopaminergic compounds caused partial inhibition of binding in the striatum while adrenergic compounds were inactive. From these studies we conclude that 125I-LSD labels serotonin 5-HT2 receptor sites in cortical regions with no indication that other receptor sites are labeled. In the olfactory tubercles and hypothalamus, 125I-LSD labeling occurs predominantly or entirely at serotonin 5-HT2 sites. In the striatum, 125I-LSD labels approximately equal proportions of serotonergic and dopaminergic sites. This data indicates that 125I-LSD labels serotonin receptors in vivo and suggests that appropriate derivatives of 2I-LSD may prove useful for tomographic imaging of serotonin 5-HT2 receptors in the mammalian cortex.     

Dietary tryptophan does not alter the function of brain serotonin neurons

The hypothesis that alterations in dietary tryptophan modify the functional activity of brain serotonin-containing neurons was tested by recording the electrophysiological activity of single serotonergic cells in awake, behaving cats after meal ingestion of diets containing varying proportions of tryptophan and the neutral amino acids that compete with tryptophan for uptake into the brain. The data revealed that while the various diets produced significant changes in brain serotonin and its major metabolite, 5-hydroxyindoleacetic acid, there was no change in the activity of serotonin-containing dorsal raphe cells following meal ingestion. Furthermore, a pulse injection of tritiated labeled tryptophan following the various diets produced no significant change in the release of tritiated serotonin into the lateral ventricles, while tritiated 5-hydroxyindoleacetic acid was significantly increased. These data suggest that dietary tryptophan does not alter the functional activity of central serotonergic neurons, in contrast with current popular beliefs that such dietary manipulations alter brain function.     

Elevated Density of [3H]Imipramine Binding in Aged Human Brain

Aging was associated with an increase in the density of specific binding sites for [3H]imipramine in postmortem specimens of human hypothalamus, frontal cortex, and parietal cortex. In general, [3H]imipramine binding was not affected by factors considered difficult to control in postmortem studies, i.e., time from death to autopsy and cause of death. The in vitro regulation of [3H]imipramine binding by sodium was impaired with age in hypothalamic homogenates. In vitro regulation of [3H]imipramine binding by chloride was intact. Determination of the concentrations of 5-hydroxytryptamine (serotonin) and 5-hydroxyindoleacetic acid in hypothalamus and frontal cortex indicated no apparent age-related changes in indole metabolism. The age-related increase in brain [3H]imipramine binding and impairment in the in vitro regulation of binding by ions are similar to changes observed previously in aged mouse brain. The increase in brain antidepressant binding sites is discussed in relationship to other indices of brain serotonergic function in aging and to the relationship of [3H]imipramine binding and depression.     

Changes in brain serotonin metabolism and [3H]-serotonin receptor binding during recall of conditioned passive avoidance in rats

The content of serotonin and its metabolite 5-hydroxyindoleacetic acid, monoamine oxidase activity, and [3H]-serotonin radioligand receptor binding were examined in the prefrontal cortex, striatum, amygdala, hippocampus and periaqueductal gray matter at different time after one-trial passive avoidance training of rats. Changes in the serotonergic activity were observed only in rats, which showed retrieval of conditioned passive avoidance response. No serotonergic changes were found immediately and one day after training. Also, there were no changes in trained rats without retrieval of conditioned passive avoidance response or rats with experimental amnesia. The pattern of the involvement of brain structures in the retrieval process was also revealed. [3H]-serotonin binding was decreased in the amygdala, periaqueductal gray matter and striatum, whereas it did not change in the prefrontal cortex and hippocampus. At the same time, the serotonin content in these structures did not differ from that of intact rats. Deamination of serotonin by monoamine oxidase and active transport of 5-hydroxyindoleacetic acid from nerve terminals were increased in the amygdala and periaqueductal gray matter, whereas in the striatum serotonin catabolism was decreased. The obtained differences in serotonin catabo- lism suggest that the decrease in receptor binding of serotonin in these brain structures is provided by different synaptic processes: presynaptic changes in the striatum and postsynaptic receptor changes in the amygdala and periaqueductal gray matter. It is concluded that the decrease in the serotonergic activity in the amygdala and periaqueductal gray matter represents one of the mechanisms activating the emotiogenic system mediating the memory trace retrieval in inhibitory avoidance learning.     

Olanzapine-associated neuroleptic malignant syndrome: Is there an overlap with the serotonin syndrome?

BACKGROUND: The neuroleptic malignant syndrome is a rare but serious condition mainly associated with antipsychotic medication. There are controversies as to whether "classical" forms of neuroleptic malignant syndrome can occur in patients given atypical antipsychotics. The serotonin syndrome is caused by drug-induced excess of intrasynaptic 5-hydroxytryptamine. The possible relationship between neuroleptic malignant syndrome and serotonin syndrome is at present in the focus of scientific interest. METHODS: This retrospective phenomenological study aims to examine the seventeen reported olanzapine - induced neuroleptic malignant syndrome cases under the light of possible overlap between neuroleptic malignant syndrome and serotonin syndrome clinical features. RESULTS: The serotonin syndrome clinical features most often reported in cases initially diagnosed as neuroleptic malignant syndrome are: fever (82%), mental status changes (82%) and diaphoresis (47%). Three out of the ten classical serotonin syndrome clinical features were concurrently observed in eleven (65%) patients and four clinical features were observed in seven (41%) patients. CONCLUSION: The results of this study show that the clinical symptoms of olanzapine-induced neuroleptic malignant syndrome and serotonin syndrome are overlapping suggesting similarities in underlying pathophysiological mechanisms.     

Postmortem and Regional Changes of Serotonin, 5-Hydroxyindoleacetic Acid, and Tryptophan in Brain

Using a specific and sensitive high pressure liquid chromatographic technique for the measurement of serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), and tryptophan (TRP), we found that there were no changes in 5-HT or 5-HIAA in the rat cortex when left in situ for 6 h at room temperature or 24 h at 4 degrees C. Only a minimal 14% increase in 5-HT was observed after 24 h at 4 degrees C in the striatum of the same animals. Concentrations of TRP, however, were increased significantly in both brain regions by these postmortem delay procedures. A second study revealed that there were significant regional 5-HT and 5-HIAA concentration differences within the cerebral cortex. The frontal cortex was shown to have the highest concentrations of 5-HT and 5-HIAA. Further, within the frontal cortex, 5-HIAA levels varied, showing apparent progressive rostral to caudal increases. 5-HT concentrations, however, remained constant within the frontal cortex. These results are discussed in reference to the conflicting reports of the previous human suicide and postmortem studies.     

Reduction of Serotonergic Function in Rat Brain by Tryptophan Depletion: Effects in Control and Fluvoxamine-Treated Rats

Abstract: The administration of tryptophan (Trp)-free amino acid mixtures to depressed patients responding to serotonin [5-hydroxytryptamine (5-HT)] uptake inhibitors (SSRIs) worsens their clinical state. This procedure reduces Trp availability to brain and thus impairs 5-HT synthesis. We have examined the influence of Trp depletion on extracellular 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) concentrations in the rat brain using in vivo microdialysis. The treatment with the SSRI fluvoxamine significantly increased 5-HT content in dialysates from frontal cortex, as compared with control rats (10.2 ± 2.7 vs. 3.1 ± 0.4 fmol per fraction), whereas 5-HIAA was unaffected. Food deprivation for 20 h reduced dialysate 5-HT content to almost control values in fluvoxamine-treated rats (10.2 ± 2.7 vs. 4.3 ± 0.6 fmol per fraction) but did not alter dialysate 5-HIAA content (7.8 ± 0.4 vs. 7.2 ± 0.5 pmol per fraction). The administration of Trp-free amino acid mixtures to fluvoxamine-treated rats significantly attenuated the release of 5-HT in frontal cortex (～50%) and, to a lesser extent, in the midbrain raphe nuclei. This effect was more marked in rats not deprived from food before the experiments (67% reduction of dialysate 5-HT content in frontal cortex) and was absent in control rats (treated with saline). In contrast, dialysate 5-HIAA was markedly affected by Trp depletion in all groups, including controls (65–75% reductions). These data show that the administration of an amino acid mixture with the same composition and dose (in milligrams per kilogram of body weight) as those inducing a severe mood impairment in depressed patients reduces 5-HT and 5-HIAA concentrations in brain dialysates. The reduction of 5-HT release, however, occurs only in animals previously treated with the antidepressant fluvoxamine for 2 weeks, which would be consistent with a marked reduction of 5-HT-mediated transmission in treated depressed patients but not in healthy controls.     

Dopaminergic and Serotonergic Neurotransmission Systems Are Differentially Involved in Auditory Cortex Learning: A Long-Term Microdialysis Study of Metabolites

Abstract: Auditory cortex has been shown to be a site of widespread neuronal learning processes even in the context of simple auditory conditioning behavior. In view of their presumed role in determining behavioral and motivational relevance of incoming information we investigated whether the dopaminergic and serotonergic systems are involved in auditory cortex learning. Using a chronic brain microdialysis technique over 4 days, samples from auditory cortex were obtained before, during, and after daily footshock avoidance training simultaneously from trained gerbils and passive control animals or pseudotrained animals. Because of detection limits of dopamine and serotonin in auditory cortex, the response profiles of extracellular homovanillic acid as the metabolite of the dopaminergic system and of 5-hydroxyindoleacetic acid as the metabolite of the serotonergic system were determined from consecutive dialysis samples each day. The response of the dopaminergic system appeared to reflect the initial formation of the behaviorally relevant association exclusively during the first training day, whereas the serotonergic response appeared to correlate with the stress level of animals.     

Serotonin Metabolism and Release in Frontal Cortex of Rats on a Vitamin E-Deficient Diet

Abstract: Rats were fed a control or vitamin E (all- rac -α-tocopheryl acetate)-deficient diet for 3 or 12 weeks. Serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), tryptophan, and α-tocopherol concentrations were determined in the frontal cortex using HPLC. α-Tocopherol concentrations fell significantly to 27% of control values at 12 weeks. Tissue 5-HT, 5-HIAA, and tryptophan concentrations were not significantly altered by the vitamin E-deficient diet at either time point. In vivo microdialysis revealed normal basal and K⁺-stimulated concentrations of 5-HT and 5-HIAA, but extracellular concentrations of tryptophan were significantly decreased after 3 weeks on the vitamin E-deficient diet, which resulted in an increase in the tissue/extracellular ratio and suggested a change in compartmentation. However, after 12 weeks on the deficient diet these values had returned to normal. Results in general indicate that a prolonged and substantial depletion of brain vitamin E can occur without major disturbance of serotonergic function.     

Attenuation and recovery of evoked overflow of striatal serotonin in rats treated with neurotoxic doses of methamphetamine

Repeated administration of methamphetamine to animals can lead to long-lasting decreases in striatal monoamine content. In the present study, the effects of neurotoxic doses of methamphetamine on basal and evoked overflow of striatal serotonin and of its primary metabolite 5-hydroxyindoleacetic acid were examined in awake rats using in vivo microdialysis. Male Fischer-344 rats were administered methamphetamine (5 mg/kg, s.c.) or saline four times in 1 day at 2-h intervals. Microdialysis studies were carried out 1 week, 1 month, and 6 months later. At 1 week posttreatment there were significant decreases in potassium- and amphetamine-evoked overflow of serotonin in the striatum of the methamphetamine-treated animals. Basal extracellular levels of 5-hydroxyindoleacetic acid but not of serotonin were also decreased. Evoked overflow of serotonin recovered by 1 month, and extracellular levels of 5-hydroxyindoleacetic acid had recovered by 6 months. Tissue levels of serotonin and 5-hydroxyindoleacetic acid were decreased at 1 week posttreatment but back to control levels by 1 month after treatment. These results indicate that presynaptic serotonergic functioning is attenuated in the striatum of rats treated 1 week earlier with neurotoxic doses of methamphetamine. However, in the model used, the changes are transient, and recovery can occur within 1-6 months posttreatment.     

Platelet serotonin transporter in schizophrenic patients with and without neuroleptic treatment

Among various hypotheses put forth to account for the etiology of schizophrenia, the abnormal function of serotonergic system has recently gained marked interest. Our previous study showed that drug-free schizophrenic patients had a significant increase in maximum numbers (B(max)) of platelet 5-HT(2A) receptors that declined to normal level after treatment with different neuroleptic drugs. To elucidate the role of the serotonin system in schizophrenia, the serotonin transporters on human platelets were examined in this study. Platelet serotonin transporters obtained from normal control subjects and schizophrenic patients were identified by using [(3)H]imipramine as the radioligand and fluoxetine to define the non-specific binding. The data showed that the mean B(max) of serotonin transporter sites for schizophrenic patients without neuroleptic therapy was significantly higher than in normal controls. The B(max) values for schizophrenic patients on phenothiazine, butyrophenone, thioxanthene and serotonin-dopamine antagonist (SDA) therapies were significantly lower than the B(max) values obtained from schizophrenic patients without neuroleptic therapy, and were comparable to those found in normal control subjects. The dissociation equilibrium constant (K(d)) values in all subject groups remained unchanged. The effect of various medication periods on platelet serotonin transporters was also studied. We found that, B(max) values of 1-4 weeks, 1-4 months, 4-12 months and >1 year of neuroleptic therapies were significantly decreased when compared with the unmedicated group. Significant reduction of brief psychiatric rating scale (BPRS) occurred in all types of neuroleptics and every period of drug treatments compared with the unmedicated group. The present results indicate that alteration of platelet serotonin transporters is associated with schizophrenia. Treatment with various types of neuroleptics suppresses the hypersensitivity of platelet serotonin transporters. The mechanisms of how neuroleptics achieve their therapeutic effects, whether they act via or modulate serotonin system in certain brain area, still need to be further evaluated.