Brief report

Measurement of ³H-serotonin uptake in blood platelets in major depressive episodes

The objective of this study was to determine whether the presence of gastrointestinal symptoms, as defined by item 12 of Hamilton–Rating-Scale for Depression, is related to kinetic characteristics of platelet-5HT uptake in patients with major depression.

The clinical picture of depression in patients with severe form of appetite loss with difficulties of eating (item 12 = 2) and weight loss was characterized by the combination of depressed mood with somatic symptoms of anxiety, sleep disturbances, decreased activity and the presence of nausea. The high frequency of relatively low Vmax and Km of 5HT uptake in this group (n = 12), all in the lower range of controls, resulted in significantly lower mean values compared with patients without gastrointestinal symptoms (n = 16; item 12 = 0) or 57 healthy subjects (Vmax = 1.36 ± 0.27 vs. 2.14 ± 0.85 vs. 2.05 ± 0.74 nMol 5HT/10⁹plat.x min; Km = 382 ± 68 vs. 467 ± 94 vs. 492 ± 123 nM respectively).

Although our finding needs confirmation, it seems that in the research for serotonergic mechanisms in major depression, it makes sense to look at depressed patients with or without somatic symptoms separately. Based on findings in 5HT transporter knock-out mice (J. Neurosci. 15 (2001) 6348), we assume that the low apparent Vmax of platelet-5HT uptake reflects the low expression of 5HT transporter not only in platelets, but also in the gut mucosa and enteric serotonergic neurons, which probably increases the risk of typical gastrointestinal symptoms such as appetite loss and nausea occurring in some depressed patients.

The dependence of platelet-5HT content on apparent kinetic parameters of 5HT uptake was analyzed in 56 healthy subjects and 47 depressed patients, who had not been taking psychotropic medication for several months. There were no significant relationships between apparent V_max or K_m and platelet-5HT content in both groups. However, the ratio of V_max to K_m, as a measure of apparent 5HT uptake efficiency, significantly correlated with the platelet-5HT concentration in healthy subjects (r=0.627 p<0.001). Female controls showed a higher correlation coefficient (r=0.723) than male controls (r=0.457). A marked deviation from the linear relationship between 5HT content and the ratio V_max/K_m was observed in female depressed patients (r=0.250 n.s.). In male depressed patients the correlation coefficient (r=0.485 p<0.05) was similar to male healthy subjects, but the regression equations differed significantly in slope and intercept. Dividing controls and patients in subgroups with low, median and high net uptake rates, it was found that the frequencies of these uptake rate classes were 24.6%, 33.3%, 42.1% in controls and 38.3%, 44.7%, 17.0% in patients respectively. Patients and controls with low net uptake rate showed very similar uptake kinetics and uptake efficiencies, but the lack of a significant correlation between 5HT content and the ratio V_max/K_m differentiated patients from controls. The status of the serotonergic system, expressed as relationship between 5HT content and uptake efficiency, was very similar in patients and controls in the range of medium net uptake rate. A trend toward higher values of uptake efficiency was apparent in patients with high net uptake rate but the platelet-5HT content was similar to corresponding controls. Mean scores on the HAMD scale (total score and psychic anxiety item) were significantly higher in the low net uptake rate group of patients than in those with a high net uptake rate.

The uptake of [¹⁴C]5-HT, [³H]paroxetine and [³H]LSD binding was determined in platelets from 30 untreated patients with major depression and compared with corresponding variables from 30 healthy age-, sex- and season-matched control subjects. The maximum velocity (V_max) for the 5-HT uptake was significantly decreased in patients (P=0.014) compared to control subjects. Depressed women had significantly lower V_max than female control subjects. In men, V_max did not differ between patients and control subjects. V_max was significantly lower in male inpatients compared with male outpatients (P=0.05). The density (B_max) of 5-HT uptake sites was found to be significantly increased in patients (P<0.05) compared to control subjects and male patients had significantly higher B_max than male control subjects, but there was no difference between female control subjects and female patients. No significant difference was found in B_max of 5-HT₂-receptors between patients and control subjects. A positive correlation was found between B_max of 5-HT₂-uptake sites and the degree of anxiety and between B_max of 5-HT₂ receptors and MADRS scores. B_max of 5-HT₂-receptors was positively correlated with the degree of suicidality. The results in the present study indicate that there may be a gender difference in serotonergic dysfunction in depression.

Genes that regulate the serotonin (5-HT) system including 5-HT receptors may be involved in mood disorders. We studied 5-HT_2A receptor exons and the adjacent intron regions in 102 patients with mood disorders (71 depressive disorders and 31 bipolar disorders). In 34 mood disorder cases, the gene encoding the 5-HT_1A receptor had been sequenced, but no disease-specific polymorphism was found. The substitution of C for T at position 102 in exon 1, which had been reported by Warren et al., was confirmed. The corresponding amino acid, serine, did not change. The allele frequency of T at position 102 was significantly higher in patients with depressive disorders than in those with bipolar disorders and healthy control subjects. Furthermore, the mean age of onset in the patients heterozyous for the T and C alleles was lower than that in those homozygous for the C allele. No other polymorphism in the gene was found.

This study examined, in the largest sample of major depressives reported so far, platelet serotonergic parameters (5-HT uptake, [³H]paroxetine binding and 5-HT_2A receptors measured by [³H]LSD binding) in 60 antidepressant-free depressed patients and 40 age- and gender-matched control subjects before treatment, and in 45 major depression patients during treatment with antidepressants. We found that, at baseline, the density (B_max) of 5-HT_2A receptors was significantly higher (by 39%) in depressed patients than in controls. Suicidal patients had significantly higher B_max values than controls or non-suicidal patients. The rate of serotonin uptake (V_max), but not the uptake at a single concentration, was significantly higher in depressed patients, particularly in females. There was no significant difference between the K_d or B_maxof [³H]paroxetine binding in control and depressed subjects. Treatment with antidepressant drugs of different pharmacological profile had no significant effect on the density of 5-HT_2A receptors, nor did the receptor number predict the response to treatment. The affinity of serotonin uptake site for 5-HT and [³H]paroxetine significantly decreased during treatment with antidepressants, particularly SSRIs. Suppression of 5-HT uptake correlated with decreases in Hamilton depression (HAMD) scores. Our data suggest that the increased density of platelet 5-HT_2A receptors may be associated with untreated major depression in antidepressantfree depressed patients, in particular those with suicidal thoughts. The persistence after antidepressant treatment and clinical improvement would suggest that up-regulation of 5-HT_2A receptors is a trait rather than state phenomenon. Correlation of 5-HT uptake suppression with decreases in HAMD scores suggests that serotonin uptake inhibition is a relevant factor in antidepressant drug effect and clinical improvement.

The lack of correlation between 5-HT uptake by platelets and the ability of platelet membrane to bind antidepressant drugs, particularly imipramine, has been reported. However, more recently it has been suggested that ³H-paroxetine could be a better drug with which to study the platelet 5-HT uptake mechanism in disease states. We have therefore compared the ability of platelet-rich plasma (PRP) from normal individuals to take up 5-HT with the ability of platelet membranes to bind paroxetine.

A significant correlation was apparent between the V_max of ¹⁴C-5-HT uptake by PRP and the B_max of ³H-paroxetine binding to platelet membrane from 30 individuals (r = 0.6468, p = 0.0001). Furthermore, this correlation was highly significant in the 20 female (r = 0.7768, p = 0.00006) but not in the 10 male volunteers. There was also a significant association between V_max and B_max and the month of blood sampling but this did not totally account for the correlation between V_max and B_max.

The simultaneous measurement of 5-HT uptake by PRP and paroxetine binding to platelet membranes from both depressed patients and matched controls should be carried out to confirm and extend these findings.