Platelet serotonin levels and gonadal hormones in rats

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.

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.

Opposite modulatory effects of ovarian hormones on rat brain dopamine and serotonin transporters

The present study was designed to investigate the modulatory effect of gonadal steroids on brain dopamine (DA) and serotonin (5-HT) presynaptic transporters in female and male rats. Female and male rats were castrated and treated with either vehicle or gonadal hormones. The pharmacodynamic characteristics of the DA and 5-HT transporters were analyzed by [3H]BTCP and [3H]imipramine binding respectively. Ovariectomy (OVX) resulted in an upregulation of the striatal DA transporter and this alteration was prevented by estradiol (E2) or E2 + progesterone (P) treatment but not by P alone. In contrast to the DA transporter, the hypothalamic 5-HT transporter was down-regulated by OVX in female rats and this decrease was reversed by the administration of E2, P or their combination. The striatal DA transporter and the hypothalamic 5-HT transporter in male rat were not affected by orchidectomy or by administration of testicular hormone. Our findings indicate that ovarian, but not testicular, steroid hormones may play an important role in the regulation of brain DA and 5-HT transporters. It appears that ovarian hormones modulate rat brain 5-HT and DA transporters in opposite directions. These interactions between ovarian steroids and presynaptic transporters may be relevant to DA- and 5-HT-related neuropsychiatric disorders.

Platelet serotonin level in rats is under genetic control

It has recently been shown that platelet serotonin (5-hydroxytryptamine, 5HT) levels (PSL) in Wistar rats represent an individually stable biological parameter (neither subject to periodic oscillations nor markedly influenced by sex and age) that shows a unimodal frequency distribution within the population (range: 1.2-2.2 micrograms 5HT/mg platelet protein). To investigate the genetic background of PSL, selective breeding for the extreme values of this trait was performed. In the fourth generation, two discrete sublines of animals (statistically different from the unselected population) with constitutionally high or low PSL could be discerned: one with congenitally low PSL (1.1-1.6 micrograms 5HT/mg platelet protein; approximately normal distribution) and the other with congenitally high PSL (1.6-2.9 micrograms 5HT/mg platelet protein; irregular distribution). No difference in the pattern of inheritance between sexes could be discerned. The results demonstrated a marked heritable component underlying the expression of individual values of PSL in rats, suggesting that this parameter is a trait characteristic.