Platelet serotonin level in rats is under genetic control

Constitutive Serotonin Tone as a Modulator of Brown Adipose Tissue Thermogenesis: A Rat Study

Brown adipose tissue (BAT), an important regulator of thermogenic and metabolic processes, is considered a promising target to combat metabolic disorders. The neurotransmitter and hormone serotonin (5HT) is a major modulator of energy homeostasis, with its central and peripheral pools acting in opposing ways. To better understand how individual variations in 5HT homeostasis influence the thermogenic functionality of BAT, we used a rat model consisting of two sublines with constitutively increased (high-5HT) or decreased (low-5HT) whole-body 5HT tone, developed by selective breeding for platelet 5HT parameters. We have shown that animals with constitutively low 5HT activity maintained at a standard housing temperature (22 °C) have greater interscapular BAT (iBAT) mass and higher iBAT metabolic activity (as evidenced by measurements of iBAT temperature and glucose uptake), accompanied by increased iBAT mRNA expression of key thermogenic genes, compared to animals with high 5HT tone. In response to further thermogenic challenges-intermittent cold exposure or treatment with a β₃-adrenergic agonist-5HT sublines show several functional and molecular differences linking constitutively low endogenous 5HT tone to higher BAT activity/capacity. Overall, the results support a role of 5-HT in the control of BAT thermogenesis They also suggest that individuals with lower 5HT activity may be more sensitive to β3-adrenergic drugs.

Constitutively Elevated Blood Serotonin Is Associated with Bone Loss and Type 2 Diabetes in Rats

Reduced peripheral serotonin (5HT) in mice lacking tryptophan hydroxylase (TPH1), the rate limiting enzyme for 5HT synthesis, was reported to be anabolic to the skeleton. However, in other studies TPH1 deletion either had no bone effect or an age dependent inhibition of osteoclastic bone resorption. The role of 5HT in bone therefore remains poorly understood. To address this issue, we used selective breeding to create rat sublines with constitutively high (high-5HT) and low (low-5HT) platelet 5HT level (PSL) and platelet 5HT uptake (PSU). High-5HT rats had decreased bone volume due to increased bone turnover characterized by increased bone formation and mineral apposition rate, increased osteoclast number and serum C-telopeptide level. Daily oral administration of the TPH1 inhibitor (LX1032) for 6 weeks reduced PSL and increased the trabecular bone volume and trabecular number of the spine and femur in high-5HT rats. High-5HT animals also developed a type 2 diabetes (T2D) phenotype with increased: plasma insulin, glucose, hemoglobin A1c, body weight, visceral fat, β-cell pancreatic islets size, serum cholesterol, and decreased muscle strength. Serum calcium accretion mediated by parathyroid hormone slightly increased, whereas treatment with 1,25(OH)2D3 decreased PSL. Insulin reduction was paralleled by a drop in PSL in high-5HT rats. In vitro, insulin and 5HT synergistically up-regulated osteoblast differentiation isolated from high-5HT rats, whereas TPH1 inhibition decreased the number of bone marrow-derived osteoclasts. These results suggest that constitutively elevated PSL is associated with bone loss and T2D via a homeostatic interplay between the peripheral 5HT, bone and insulin.

Tail biting in pigs: blood serotonin and fearfulness as pieces of the puzzle?

Tail biting in pigs is a widespread problem in intensive pig farming. The tendency to develop this damaging behaviour has been suggested to relate to serotonergic functioning and personality characteristics of pigs. We investigated whether tail biting in pigs can be associated with blood serotonin and with their behavioural and physiological responses to novelty. Pigs (n = 480) were born in conventional farrowing pens and after weaning at four weeks of age they were either housed barren (B) or in straw-enriched (E) pens. Individual pigs were exposed to a back test and novel environment test before weaning, and after weaning to a novel object (i.e. bucket) test in an unfamiliar arena. A Principal Component Analysis on behaviours during the tests and salivary cortisol (novel object test only) revealed five factors for both housing systems, labeled ‘Early life exploration’, ‘Near bucket’, ‘Cortisol’, ‘Vocalizations & standing alert’, and ‘Back test activity’. Blood samples were taken at 8, 9 and 22 weeks of age to determine blood platelet serotonin. In different phases of life, pigs were classified as tail biter/non-tail biter based on tail biting behaviour, and as victim/non-victim based on tail wounds. A combination of both classifications resulted in four pig types: biters, victims, biter/victims, and neutrals. Generally, only in phases of life during which pigs were classified as tail biters, they seemed to have lower blood platelet serotonin storage and higher blood platelet uptake velocities. Victims also seemed to have lower blood serotonin storage. Additionally, in B housing, tail biters seemed to consistently have lower scores of the factor ‘Near bucket’, possibly indicating a higher fearfulness in tail biters. Further research is needed to elucidate the nature of the relationship between peripheral 5-HT, fearfulness and tail biting, and to develop successful strategies and interventions to prevent and reduce tail biting.

Serotonergic modulation of pain and analgesic responses: a study in rats with constitutionally altered serotonin transporters

BACKGROUND

A role of the serotonin (5HT) transporter, a key regulator of serotonergic transmission, in the physiology, pharmacology and genetics of pain responses has been proposed recently. The present study aimed to explore the impact of constitutive differences in the activity of the serotonin transporter, and 5HT homeostasis in general, on the modulation on pain sensitivity and analgesic responses to drugs that utilize 5HT mechanisms.

METHODS

A novel genetic animal model, Wistar-Zagreb 5HT rats, obtained by selective breeding of animals for extreme activity of the platelet serotonin transporter was used. As a consequence of breeding, two sublines of this model, termed high-5HT and low-5HT, differ in both central and peripheral serotonin homeostasis. Thermal pain sensitivity of 5HT sublines was assessed at baseline and following administration of analgesic drugs, as determined by paw withdrawal latency to radiant heat stimulation.

RESULTS

Animals from 5HT sublines show differences in both basal pain sensitivity and analgesic responses. Rats with the low-5HT phenotype displayed decreased baseline paw withdrawal latencies (hyperalgesia) in comparison to their high-5HT counterpart (25%; p < 0.001). They also showed better analgesic response to acute and prolonged treatment with tramadol (p = 0.027) and clomipramine (p = 0.019), respectively, whereas administration of fluvoxamine did not produce an analgesic effect in either 5HT subline.

CONCLUSIONS

These findings support the idea that functionality of the serotonin transporter is one of the physiological/genetic determinants of individual differences in pain responses and modulation. They also validate Wistar-Zagreb 5HT rats, with constitutionally up-regulated/down-regulated serotonin transporter, as a potential new genetic model for studying serotonergic modulation of pain responses.

Platelet serotonin transporter function predicts default-mode network activity

Background

The serotonin transporter (5-HTT) is abundantly expressed in humans by the serotonin transporter gene SLC6A4 and removes serotonin (5-HT) from extracellular space. A blood-brain relationship between platelet and synaptosomal 5-HT reuptake has been suggested, but it is unknown today, if platelet 5-HT uptake can predict neural activation of human brain networks that are known to be under serotonergic influence.

Methods

A functional magnetic resonance study was performed in 48 healthy subjects and maximal 5-HT uptake velocity (V_max) was assessed in blood platelets. We used a mixed-effects multilevel analysis technique (MEMA) to test for linear relationships between whole-brain, blood-oxygen-level dependent (BOLD) activity and platelet V_max.

Results

The present study demonstrates that increases in platelet V_max significantly predict default-mode network (DMN) suppression in healthy subjects independent of genetic variation within SLC6A4. Furthermore, functional connectivity analyses indicate that platelet V_max is related to global DMN activation and not intrinsic DMN connectivity.

Conclusion

This study provides evidence that platelet V_max predicts global DMN activation changes in healthy subjects. Given previous reports on platelet-synaptosomal V_max coupling, results further suggest an important role of neuronal 5-HT reuptake in DMN regulation.

Relations between peripheral and brain serotonin measures and behavioural responses in a novelty test in pigs

Pigs differ in their behavioural responses towards environmental challenges. Individual variation in maladaptive responses such as tail biting, may partly originate from underlying biological characteristics related to (emotional) reactivity to challenges and serotonergic system functioning. Assessing relations between behavioural responses and brain and blood serotonin parameters may help in understanding susceptibility to the development of maladaptive responses. The objective of the current study was, therefore, to assess the relationship between the pigs' serotonergic parameters measured in both blood and brain, and the behaviour of pigs during a novelty test. Pigs (n=31) were subjected to a novelty test at 11weeks of age, consisting of 5-min novel environment exposure after which a novel object (a bucket) was introduced for 5min. Whole blood serotonin, platelet serotonin level, and platelet serotonin uptake were determined at 13weeks of age. Levels of serotonin, its metabolite and serotonin turnover were determined at 19weeks of age in the frontal cortex, hypothalamus and hippocampus. The behaviour of the pigs was different during exposure to a novel object compared to the novel environment only, with more fear-related behaviours exhibited during novel object exposure. Platelet serotonin level and brain serotonergic parameters in the hippocampus were interrelated. Notably, the time spent exploring the test arena was significantly correlated with both platelet serotonin level and right hippocampal serotonin activity (turnover and concentration). In conclusion, the existence of an underlying biological trait - possibly fearfulness - may be involved in the pig's behavioural responses toward environmental challenges, and this is also reflected in serotonergic parameters.

5HT-1A receptors and anxiety-like behaviours: studies in rats with constitutionally upregulated/downregulated serotonin transporter

Altered activity of brain serotonergic (5HT) system has been implicated in a wide range of behaviours and behavioural disorders, including anxiety. Functioning of 5HT-1A receptor has been suggested as a modulator of emotional balance in both, normal and pathological forms of anxiety. Here, we studied serotonergic modulation of anxiety-like behaviour using a genetic rat model with constitutional differences in 5HT homeostasis, named Wistar-Zagreb 5HT (WZ-5HT) rats. The model, consisting of high-5HT and low-5HT sublines, was developed by selective breeding of animals for extreme activities of peripheral (platelet) 5HT transporter, but selection process had affected also central 5HT homeostasis, as evidenced from neurochemical and behavioural studies. Anxiety-like behaviour in WZ-5HT rats was evaluated by two commonly used paradigms: open field and elevated-plus maze. The involvement of 5HT-1A receptors in behavioural response was assessed by measuring mRNA expression in cell bodies (raphe nuclei) and projection regions (frontal cortex, hippocampus) by use of RT-PCR and in situ hybridization, and by measuring functionality of cortical 5HT-1A receptors by use of [(3)H]8-OH-DPAT radioligand binding. Animals from the high-5HT subline exhibit increased anxiety-like behaviour and decreased exploratory activity when exposed to novel environment. No measurable differences in constitutional (baseline) functionality or expression of 5HT-1A receptors between sublines were found. The results support contribution of increased serotonergic functioning to the anxiety-like behaviour. They also validate the high-5HT subline of WZ-5HT rats as a potential model to study mechanisms of anxiety, especially of its nonpathological form, while the low-5HT subline may be useful to model sensation seeking phenotype.

Effects of genetic group selection against mortality on behavior and peripheral serotonin in domestic laying hens with trimmed and intact beaks

Severe feather pecking is a maladaptive behavior in laying hens that may result in cannibalism and ultimately death of the victims. Selection methods in which the genetic effect of an animal on the survival of its group members is taken into account, i.e. 'group selection', have been shown to be very effective in reducing mortality due to feather pecking and cannibalism in laying hens. It has been suggested that fearfulness and serotonergic functioning are involved in the causation and development of feather pecking. We investigated effects of selection based on survivability in non-beak trimmed groups on fear-related behavior and peripheral 5-HT concentration and uptake in hens with trimmed or intact beaks, in a 2 x 2 factorial arrangement. Hens from the second generation of the low mortality line showed less fear-related behavior than control line hens. In addition, they displayed higher whole blood 5-HT concentrations and a lower platelet 5-HT uptake, indicating differences in functional activity of the 5-HT system. Beak trimming resulted in lowered levels of fear, and in a reduction of feather damage. Hens with trimmed and intact beaks did, however, not differ in peripheral 5-HT. The results imply that fearfulness and 5-HT activity are related to feather pecking without distinguishing between cause and effect. However, given that selection altered fear-related behaviors and peripheral 5-HT concentration and uptake, whereas beak trimming affected fearfulness and feather damage, but not 5-HT parameters, we suggest that peripheral 5-HT activity might reflect the predisposition to develop severe feather pecking under adverse conditions in laying hens.

Central aminopeptidase and serotonin system activities: possible relationship

The co-localization of serotonin (5-hydroxytryptamine, 5HT) and neuroactive peptides in the same neuron points to the importance of interactions between serotonergic and peptidergic systems in maintaining body homeostasis. In this work, we used an original genetic rat model to search for possible interrelations between 5HT system functioning and the activities of aminopeptidases, i.e. enzymes which are the key regulators of (neuro)peptides level/function. The activities of three cytosolic exopeptidases: alanyl aminopeptidase (alanyl-AP), arginyl aminopeptidase (arginyl-AP) and dipeptidyl peptidase III (DPP III) were measured in brains and peripheral tissues of the sublines of rats with constitutionally upregulated/downregulated 5HT transporter activity. These rat sublines, termed as high-5HT and low-5HT subline, have been obtained previously by selective breeding for the extreme values of platelet 5HT level and velocity of 5HT uptake. Besides in the periphery they show marked alterations also in brain 5HT function, indicating the differences in central 5HT transmission/homeostasis. In this study, we have found that animals from the high-5HT subline have significantly lower activity of brain alanyl-AP (p<0.05) and arginyl-AP (p<0.01) as compared to control animals. No other differences were noticed regardless of the 5HT subline, investigated organ or analyzed aminopeptidase. Results suggest that the constitutional upregulation of serotonergic activity may be related to a lowered brain cytosolic aminopeptidase activity which may have an influence on the cleavage of their physiological substrates.

Serotonin transporter kinetics in rats selected for extreme values of platelet serotonin level

By selective breeding of Wistar rats for the extreme values of platelet serotonin (5HT) level (PSL), we have developed earlier two sublines of animals differing markedly in this parameter. Further studies, performed on the protein and mRNA levels, revealed platelet serotonin transporter (5HTt) as parameter underlying mentioned differences in PSL between sublines. In this work, we have performed full-kinetic analysis of platelet serotonin uptake (PSU) in animals from the genetically selected sublines. The results demonstrated marked differences in maximal velocity (V(max)) of the 5HT transporter, as contrasted to the lack of any difference in the Michaelis constant (K(m)). High correlation between PSL and V(max) of PSU was demonstrated, revealing that the number of membrane 5HT transporter sites is under genetic control and responsible for marked differences in PSL between high- and low-5HT sublines. These results enabled further selective breeding of animals for the extremes of V(max) of platelet 5HT transporter, and so the development of more specific model "Wistar-Zagreb 5HT rats".

Expression of brain and platelet serotonin transporters in sublines of rats with constitutionally altered serotonin homeostasis

By selective breeding, two sublines of rats, termed Wistar-Zagreb 5HT rats, with constitutionally high or low values of platelet serotonin (5HT) level and activity of platelet serotonin transporter (5HTt) have been developed. Previous studies demonstrated significant differences between the sublines in the expression of platelet 5HTt at the level of both mRNA and protein. Neurochemical and behavioural studies demonstrated differences in functional activity of brain 5HTt indicating that, similarly to platelets, differences in mRNA level might be expected in brains of selected animals. In this work, semi-quantitative RT-PCR method for measuring the 5HTt expression in rat tissues was described and then used to quantify the 5HTt mRNA in brains and platelets of animals from high-5HT and low-5HT sublines. Three different housekeeping genes: GAPDH, beta-actin and cyclophylin B, were used as internal standards to normalise 5HTt signals. Significant differences in platelet 5HTt mRNA between the sublines were confirmed, as contrasted to only a tendency toward higher 5HTt mRNA levels in midbrain of animals from the high-5HT subline. Results indicate differences in transcriptional regulation of central and peripheral 5HT transporters, suggesting that homeostatic control in the brain counteract more efficiently the selection pressure than in the periphery.

Effects of group selection for productivity and longevity on blood concentrations of serotonin, catecholamines, and corticosterone of laying hens

Selection of a line of White Leghorn chickens for high group productivity and longevity resulted in reducing cannibalism and flightiness in multiple-hen cages. Improvements in survival might have been due to changes of physiological homeostasis. The objective of the present study was to test the hypothesis that genetic selection for high (HGPS) and low (LGPS) group productivity and survivability also altered regulation of neuroendocrine homeostasis. Hens were randomly assigned to individual cages at 17 wk of age. At 21 wk of age, blood concentrations of dopamine, epinephrine, norepinephrine, and serotonin were measured using HPLC assay. Blood concentrations of corticosterone were measured using radioimmunoassay. The LGPS hens had greater blood concentrations of dopamine and epinephrine than the HGPS hens (P < 0.01). The blood concentration of norepinephrine was not significantly different between the lines, but the ratio of epinephrine to norepinephrine was greater in the LGPS hens (P < 0.01). The blood concentrations of serotonin were also higher in the LGPS hens compared to those in the HGPS hens (P < 0.01). Although the HGPS hens tended to have a higher level of blood corticosterone, the difference was not significant (1.87 +/- 0.19 vs. 1.49 +/- 0.21 ng/mL; P = 0.08). The results suggest that selection for group productivity and survivability alters the chickens' neuroendocrine homeostasis, and these changes may correlate with its line-unique coping ability to domestic environments and survivability.

Physiological characteristics of platelet/circulatory serotonin: study on a large human population

The aim of this work was the study of platelet/circulatory serotonin (5-hydroxytryptamine, 5-HT), specifically alternative ways of its measurement and main physiological characteristics. The study was performed on a large human population (N=500) of blood donors of both sexes over the course of a longer time period (17 months). Owing to the heterogeneity in measurement of circulatory serotonin encountered in the literature, three ways of expression were comparatively studied: per unit number of platelets, per unit mass of platelet protein and per unit volume of whole blood. Results demonstrated unimodal distribution of individual frequencies of platelet/circulatory serotonin in the human population with the mean values of 579+/-169 ng 5-HT/10(9) platelets; 332+/-89.9 ng 5-HT/mg protein and 130+/-42.3 ng 5-HT/ml blood (mean+/-S.D.). A progressive decrease of serotonin level with age (18-65 years) was demonstrated, reaching statistical significance between the extreme age groups. No significant differences in the serotonin level between the sexes were observed. No seasonal oscillations in platelet/circulatory serotonin were found. Platelet serotonin demonstrated intra-individual stability over time. Finally, regarding the methodology of measurement, our results demonstrated a good correlation among the above-mentioned ways of expression of platelet/circulatory serotonin. This indicates the possibility of intercomparison of the literature reports expressing this physiological parameter either as 5-HT concentration in platelets or as 5-HT level in the circulation.

Pretreatment platelet 5-HT concentration predicts the short-term response to paroxetine in major depression. Grupo de Trastornos Afectivos

BACKGROUND

A previous retrospective study revealed that a high pretreatment platelet serotonin (5-HT) concentration was associated with a low response to serotonergic antidepressants in drug-free major depressives. We have examined such a relationship in depressive patients treated with paroxetine.

METHODS

Seventy-four drug-free major depressives (DSM-IV) were admitted to the study. Clinical ratings were performed and blood was drawn prior to the initiation of treatment and after 4 weeks of paroxetine (20 mg/day). The concentrations of 5-HT, 5-hydroxyindoleacetic acid, and tryptophan were determined in plasma and blood.

RESULTS

Paroxetine treatment reduced platelet 5-HT to 17% of baseline after 4 weeks of treatment. Responder patients had a pretreatment platelet 5-HT concentration 22% lower than nonresponders (p < .035). Admission HAMD scores, plasma paroxetine concentration, or platelet 5-HT concentration at endpoint did not differ between responders and nonresponders. Yet, the response rate was 11% in patients with high pretreatment platelet 5-HT (> 900 ng/10(9) platelets) and 50% in those below that value (p < .004).

CONCLUSIONS

These findings support that depressed patients with a high pretreatment platelet 5-HT concentration have a poor therapeutic outcome after treatment with a standard paroxetine dose. These differences may be related to the existence of molecular differences in the 5-HT transporter.

Serotonin transporter on rat platelets: levels of mRNA underlie inherited differences in uptake kinetics

By the breeding selection for the extreme values of platelet serotonin transporter activity, two sublines of Wistar-derived rats, with constitutionally high or low platelet serotonin uptake (PSU), were previously developed. In order to study the genetic background of these inherited differences, comparative Northern blot analysis of the platelet serotonin transporter messenger RNA levels of the animals from the two sublines was performed. If the values of animals from the high-PSU subline are taken as 100%, animals from the low-PSU subline demonstrated lower values of both platelet serotonin uptake and transporter mRNA levels (amounting to 62 and 76% respectively). Correlation between platelet serotonin uptake and the respective levels of messenger RNA for the serotonin transporter (r = 0.829, P < 0.01, N = 8) points to the same direction, indicating that the process of breeding selection for the extreme values of transporter kinetics has influenced transcription mechanisms of the serotonin transporter gene.

Physiological characteristics of serotonin transporters on rat platelets

Physiological characteristics of serotonin (5-hydroxytryptamine, 5HT) transport through the platelet membrane was investigated in Wistar rats with our recently developed method permitting repetitive measurements of transporter kinetics in individual animals. Full kinetic analysis in the population of 91 animals revealed Michaelis constant (K(m)) of 0.158 +/- 0.025 microM and maximal velocity (Vmax) of 5HT uptake of 225 +/- 32 pmol per 10(8) platelets min-1 (mean +/- S.D.). Both kinetic parameters demonstrated normal distribution curves, which for Vmax were slightly skewed toward higher than average values. No gender effect was shown in frequency distributions, mean values and variability of kinetic parameters. A significant intraindividual correlation between kinetic parameters was found suggesting compensation at the level of the plasma membrane. Kinetic parameters were not influenced by age (until the middle age) or annual cycle (under laboratory conditions) and were shown to be fairly stable in time, supporting the view that platelet 5HT transport kinetics could be a useful biological trait marker.

Relationship between blood serotonergic variables, melancholic traits, and response to antidepressant treatments

The relationship between peripheral serotonergic variables, melancholic traits, and clinical improvement after antidepressant treatment was examined in 83 drug-free major depressive patients. Plasma serotonin (5-HT) concentrations was lower in untreated melancholic patients (1.00 +/- 0.11 vs. 1.84 +/- 0.28 ng/mL, p < 0.008; N = 40 and 43, respectively). A tendency was observed for plasma 5-hydroxyindoleacetic acid (p < 0.06), whereas platelet 5-HT and plasma tryptophan did not differ between groups. After blood sampling and clinical ratings, treatment began with fixed doses of 5-HT uptake inhibitors (clomipramine or fluvoxamine), monoamine oxidase inhibitors, or tianeptine, a 5-HT uptake enhancer. There was no significant difference in response rates between patients with and without melancholic traits. The relationship between the clinical response at 6 weeks (>50% reduction of baseline Hamilton score) and the pretreatment values of biochemical variables was examined. Responders had a lower pretreatment platelet 5-HT (530 +/- 36 vs. 664 +/- 50 ng/10(9) platelets, p < 0.03; N = 44 and 39, respectively). Patients with a platelet 5-HT concentration above 800 ng/10(9) platelets had a lower response rate than those below this value (p < 0.003). This difference was maximal in the subgroup of patients treated with 5-HT uptake inhibitors (N = 49). In this subgroup, the response rates of patients with 5-HT concentrations below and above the cutoff point were, respectively, 70% and 17% (p < 0.001). A pretreatment platelet 5-HT value above 800 ng/10(9) platelets had a predictive value for a negative response of 92%. These results suggest the presence of biochemical differences in the peripheral serotonergic system between melancholic and nonmelancholic patients. The inverse relationship between the pretreatment platelet 5-HT content and clinical response may be useful in the investigation of the relationship between the 5-HT system and antidepressant response.

Basal and stimulated extracellular serotonin concentration in the brain of rats with altered serotonin uptake

We examined the relationship between the density of serotonergic (5-hydroxytryptamine [5-HT]) uptake sites and extracellular 5-HT concentration in the rat brain using microdialysis with two different models, lesions with 5,7-dihydroxytryptamine (50 microg in the dorsal raphe nucleus (DRN) 15 days before) and sublines of rats genetically selected displaying extreme values of platelet 5-HT uptake. Compared to controls, lesioned rats had a reduced cortical concentration of 5-hydroxyindoles (45%), unchanged basal extracellular 5-HT in the DRN and ventral hippocampus (VHPC), and reduced basal 5-hydroxyindoleacetic acid (5-HIAA) concentrations (46%, DRN; 22%, VHPC). Yet the perfusion of 100 mmol/L KCl or 1 micromol/L citalopram elevated dialysate 5-HT significantly more in the DRN and VHPC of controls. In genetically selected rats, platelet 5-HT content and uptake were highly correlated (r2 = 0.9145). Baseline dialysate 5-HT (VHPC) was not different between high and low 5-HT rats and from normal Wistar rats. However, KCl or citalopram perfusion increased dialysate 5-HT significantly more in high 5-HT than in low 5-HT rats, and the former displayed a greater in vivo tissue 5-HT recovery. Significant but small differences in the same direction were noted in [3H]citalopram binding in several brain areas, as measured autoradiographically. Thus, basal extracellular 5-HT (but not 5-HIAA) concentrations are largely independent on the density of serotonergic innervation and associated changes in uptake sites. However, marked differences emerge during axonal depolarization or reuptake blockade. The significance of these findings for the treatment of mood disorders in patients with neurological disorders is discussed.

Identification of serotonin transporter mRNA in rat platelets

Total RNA isolated from rat platelets by guanidinium-acid-phenol extraction, and mRNA for the serotonin (5-hydroxytryptamine) transporter (5HTt) was identified. From a typical starting sample of 20 mL of rat blood (approximately 9 x 10(9) platelets), 14 to 17 micrograms of total platelet RNA was obtained. Northern blot analysis, using 32P-labeled 5HTt cDNA as a probe, identified approximately 3.3 kb long 5HTt mRNA. After rehybridization with cyclophilin cDNA, approximately 1 kb long mRNA for cyclophilin, which could serve as a reference for 5HTt mRNA quantification, was also identified. Densitometric analysis demonstrated clearly measurable signals for both mRNAs. The possibility of quantification of rat platelet 5HTt mRNA should enable parallel studies on 5HTt gene expression in platelets and brain of the same animal, and the evaluation of the platelet model at the molecular genetic level.

Effects of the antidepressant drug tianeptine on plasma and platelet serotonin of depressive patients and healthy controls

We have examined the effects of a single 12.5 mg dose and of 12 weeks treatment up to 37.5 mg daily with tianeptine, a new antidepressant drug that potentiates in vivo the uptake of serotonin (5-HT). On day 0, tianeptine reduced plasma 5-HT concentration. This acute effect occurred also on subsequent examination days. However, long-term treatment tended (P < 0.06) to increase basal plasma 5-HT concentrations, in covariation with decreases of MADRS (Montgomery-Asberg Depression Rating Scale) and HARS (Hamilton Anxiety Rating Scale). Platelet 5-HT increased only in elderly patients, probably due to the higher plasma concentration of the drug in this group than in younger patients. These results show that the acute effects of therapeutic doses of tianeptine are consistent with an enhancement of the 5-HT uptake. However, long-term treatment does not result in a decreased plasma 5-HT, as might be expected from the acute effects of the drug.

Human brain cortex and platelet serotonin2 receptor binding properties and their regulation by endogenous serotonin

In patients with affective disorder and suicidality up-regulation of the serotonin2 receptor has been observed in brain and on platelets. Although the pharmacological profiles of the receptor in brain synaptosomes and platelet membranes are similar, it is a matter of debate whether the platelet serotonin2 receptor reflects the characteristics of the receptor in the brain and whether serotonin regulates the receptor activity. To answer these questions we measured in healthy human subjects the platelet serotonin2 receptor activity and blood serotonin concentrations. In an attempt to find whether the serotonin2 receptor activity in brain cortex synaptosomes and on platelets is similarly expressed we investigated the receptor's binding characteristics in neurosurgical patients. The results suggest that in men and women increased platelet serotonin concentrations correlate with a decrease in platelet membrane serotonin2 receptor affinity. The affinities of the brain cortex synaptosomal and platelet membrane serotonin2 receptor correlate intra-individually. These data suggest that the platelet serotonin2 receptor affinity appears to be regulated at the cellular level by blood serotonin and that the binding characteristics of the serotonin2 receptor in brain cortex synaptosomes corresponds to that on platelets. The latter finding supports the hypothesis of the platelet as a model for neuronal function.

Alteration of NK- and ADCC-activities in rats genetically selected for low or high platelet serotonin level

By selective breeding we have recently obtained two discrete sublines of rats that differ in serotonin content in their platelets. As both serotonin and platelets may influence, or even take part, in immune reactions, we tested in this work the natural cytotoxicity in rats with constitutionally different platelet serotonin levels (PSL). Rats with low platelet serotonin level (mean +/- SD, 1.26 +/- 0.14 micrograms 5HT/mg protein; 81% vs. controls) had significantly higher (P less than 0.001) natural killer (NK) activity (mean +/- SD, 9.1 +/- 3.9%) than control rats with average PSL (1.57 +/- 0.18 micrograms 5HT/mg protein). On the contrary, rats with constitutionally high PSL (2.42 +/- 0.21 micrograms 5HT/mg protein, 154% vs. controls) had somewhat lower (P less than 0.02) NK activity (4.1 +/- 1.7%) than control animals (5.7 +/- 1.9%). Antibody-dependent cellular cytotoxicity (ADCC) against nucleated targets of the RCH line, detecting lymphoid effectors, as well as ADCC against chicken red blood cells (CRBC), detecting predominantly non-lymphoid effectors, were also significantly higher (P less than 0.001) in rats with low PSL (19.6 +/- 6.8% vs. 6.6 +/- 3.1% in controls for lymphoid effectors, and 71.8 +/- 6.1% vs. 48.7 +/- 8.8% in control rats for non-lymphoid effectors). However, no significant alteration of either ADCC was determined in rats with high PSL. The results suggest in vivo regulation of natural cytotoxicity by serotonin.