Short-term lithium administration to healthy volunteers produces long-lasting pronounced changes in platelet serotonin uptake but not imipramine binding

Predicting response to lithium in mood disorders: role of genetic polymorphisms

Lithium is considered to be the first choice mood stabilizer in recurrent mood disorders. Its widespread and large-scale use is the result of its proven efficacy. In spite of this fact, patients have been observed to show a variable response to lithium treatment: in some cases it is completely effective in preventing manic or depressive relapses, while in other cases it appears to show no influence on the disease course. The possible definition of a genetic liability profile for adverse effects and efficacy will be of great help, as lithium therapy needs at least 6 months to be effective in stabilizing mood disorders. During the last few years, a number of groups have reported possible liability genes. Lithium long-term prophylactic efficacy has been associated with serotonin transporter protein, tryptophan hydroxylase and inositol polyphosphate 1-phosphatase variants. A number of other candidate genes and anonymous markers did not yield positive associations. Therefore, even if some positive results have been reported, no unequivocal susceptibility gene for lithium efficacy has been identified. Although the available data may not currently allow a meaningful prediction of lithium response, future research is aimed at the development of individualized treament of mood disorders, including the possibility of 'pharmacological genetic counseling'.

Lithium long-term treatment in mood disorders: clinical and genetic predictors

Lithium is the most widely used long-term treatment for recurrent mood disorders. Despite its proven efficacy, patients show a variable response, ranging from complete efficacy to no influence at all. This paper reviews possible predictors of response focusing on molecular genetic studies. The functional polymorphism in the upstream regulatory region of the serotonin transporter gene (5-HTTLPR) has been associated with lithium long-term efficacy in two independent studies, marginal associations have been reported for tryptophan hydroxylase and inositol polyphosphate 1-phosphatase (INPP1). A number of other candidate genes and anonymous markers did not yield positive associations. Therefore, even though some positive results have been reported, no unequivocal susceptibility gene for lithium efficacy has been identified.

Effects of long-term lithium treatment on monoaminergic functions in major depression

Platelet [14C]serotonin uptake, the density of serotonin transporters and 5HT(2) receptors, and 5HT(2) and alpha(2) receptor function in platelets were investigated in 29 outpatients (15 women and 14 men) diagnosed as having a major affective disorder (21 bipolar and 8 unipolar). The data were compared with data for 26 healthy volunteers matched for age, sex and season. No differences were found in the mean values for the uptake velocity (V(max)) and the affinity (K(m)) of the transport carrier for serotonin between patients and controls. However, female patients had lower V(max) compared to male patients and female control subjects. A positive correlation between plasma lithium and V(max) and a tendency toward a negative correlation between plasma lithium and K(m) was observed. Furthermore, there were no differences in platelet B(max) and K(d) for [3H]paroxetine binding and K(d) for [3H]LSD binding between patients and controls. However, there was an increased number of platelet 5-HT(2) receptors and a difference in serotonin-mediated potentiation of platelet ATP secretion between patients compared to controls, especially in women. The findings in the present study suggest that lithium has a net ameliorating impact on serotonin uptake which may render it resistant to change. They also postulate that the effect of lithium may be attained by a dual influence on postsynaptic serotonergic structures, as it increases both the density and the sensitivity of 5-HT(2) receptors.

Serotonin in mania and in the mechanism of action of mood stabilizers: a review of clinical studies

OBJECTIVES

Serotonin (5-hydroxytryptamine, 5-HT) was implicated in the pathophysiology of manic-depressive illness as early as 1958. Although extensive evidence has accumulated since then to support 5-HT's role in depression, relatively fewer studies examined its role in mania. The purpose of this paper was to review and summarize the current state of knowledge on the role of 5-HT in mania and its treatment.

METHODS

We systemically reviewed clinical studies of 1) 5-HT function in mania and 2) 5-HT in the mechanism of action of mood stabilizers, including lithium and anticonvulsants.

RESULTS

Review showed that cerebrospinal fluid, postmortem, platelet, neuroendocrine challenge, and tryptophan depletion studies provided some evidence to support the hypothesis that a 5-HT deficit is involved in mania and that enhancement of 5-HT neurotransmission exerts a mood-stabilizing effect.

CONCLUSIONS

There is some evidence from clinical studies for the contribution of 5-HT in mania and in the mechanism of action of mood stabilizers. However, it is very likely that other neurotransmitters also play important roles. Future directions for research include 1) in vivo study of 5-HT receptor subtypes using positron emission tomography, 2) investigation of the interaction between 5-HT and other neurotransmitter systems, and 3) determination of the relationships between diagnostic subtypes of mania and 5-HT function and other neurotransmitter systems.

Effect of antidepressant treatment on platelet 5-HT content and relation to therapeutic outcome in unipolar depressive patients

Platelet 5-HT levels and scores on the 17-item Hamilton Rating Scale for Depression (HRS) were studied in patients with unipolar depression before and after antidepressant treatment. Before treatment there were no differences in platelet 5-HT values or in HRS scores between patients who showed a good and a poor therapeutic response. Repeated administration of 5-HT uptake inhibitors (amitriptyline, clovoxamine, fluvoxamine) for 28 days markedly decreased platelet 5-HT levels. Chronic treatment with trazodone or maprotiline (weak inhibitors of platelet 5-HT uptake) produced no changes in platelet 5-HT levels. No significant correlation was observed between platelet 5-HT concentrations and the HRS scores before or during treatment. The findings suggest that the changes in platelet 5-HT levels after antidepressant treatment are mainly due to the effects of antidepressants on the 5-HT uptake system.

Platelet tritiated imipramine binding in patients suffering from mania

Platelet imipramine binding was measured in 16 patients suffering from DSM-IIIR mania and compared with binding values reported in depressed and healthy control subjects recruited in a parallel study (Ellis et al., 1990). Binding levels in the manic group did not differ from control values, but were higher than in the depressed group. Within the manic group, binding did not differ with severity of illness or the presence of depressive symptoms but there was a trend to lower values (comparable to those in the depressed group) with increasing duration of illness. This raises the possibility that changes in imipramine binding in depression and mania may be similar, consistent with the permissive hypothesis of serotonin function.

Platelet imipramine binding in children and adolescents with impulsive behavior

The serotonergic system has been implicated in the regulation of impulsive aggressive behavior either toward oneself or others. Imipramine binding sites were measured in the platelets of 23 impulsive aggressive children. Subjects ratings of total behavior, externalizing behavior, hostility, and aggression, as measured by the Child Behavior Checklist, were inversely correlated with the platelet imipramine binding. These findings are consistent with previous studies that suggest that decreased serotonergic activity is associated with impulsive aggressive behavior.

Lithium and serotonin function: implications for the serotonin hypothesis of depression

Lithium enjoys wide clinical use in the treatment of affective disorders, but the mechanism of its action in these conditions is still controversial. Recent studies have shown that lithium can interact with other antidepressant drugs to enhance their efficacy, perhaps by specific effects on serotonin (5-HT) function. A large body of independent evidence suggests that 5-HT function is abnormal in depression. This review documents preclinical evidence of lithium's effects on 5-HT function at the levels of precursor uptake, synthesis, storage, catabolism, release, receptors, and receptor-effector interactions. The weight of this evidence suggests that lithium's primary actions on 5-HT may be presynaptic, with many secondary postsynaptic effects. Studies in humans, using very different methodological approaches, generally suggest that lithium has a net enhancing effect on 5-HT function. These actions of lithium may serve to correct as-yet unspecified abnormalities of 5-HT function involved in the pathogenesis of depression.

New applications of lithium therapy

Several placebo-controlled double-blind studies have indicated that lithium sometimes augments antidepressants, converting nonresponding patients to responders. Lithium therapy has also benefitted some schizoaffective patients and some alcoholics. Side effects are minimal. Mechanisms involved in lithium's effectiveness have not yet been discovered.

Differences between sodium-dependent and desipramine-defined [3H]imipramine binding in intact human platelets

Measurements of sodium-dependent [3H]imipramine binding to intact human platelets from 20 human volunteers were made and compared to desipramine-defined binding, a method commonly employed in population studies of platelet [3H]imipramine sites. The density (Bmax) of sodium-dependent [3H]imipramine sides in platelets was significantly lower (449 +/- 36 sites/platelet) and the affinity (Kd) significantly higher (1.15 +/- 0.12 nM) than those obtained when excess desipramine was used to define specific binding (Bmax 654 +/- 33 sites/platelet, p less than 0.001; Kd 1.52 +/- 0.11 nM, p less than 0.001). There was no significant correlation between the density (Bmax) of sodium-dependent and desipramine-defined binding in individual subjects, suggesting that a different proportion of sites are labeled under the two assay conditions. No age-dependent variation was found in either Kd or Bmax values of sodium-dependent or desipramine-defined [3H]imipramine binding. The results suggest determination of sodium-dependent [3H]imipramine binding to intact platelets may be a useful measure for the estimation of [3H]imipramine recognition sites relevant to the serotonin uptake in studies of patients with affective disorders.

Increased plasma free serotonin but unchanged platelet serotonin in bipolar patients treated chronically with lithium

The effect of lithium salts administered chronically to bipolar patients on peripheral measures of the serotoninergic system has been studied. Plasma free serotonin (5HT), whole blood 5HT, plasma 5-hydroxyindoleacetic acid (5HIAA) and plasma total tryptophan (TP) have been analyzed in 22 patients treated with lithium carbonate (mean daily dose: 1280 mg, mean serum concentration: 0.73 mmol/l) and compared to 14 healthy controls and 11 patients treated chronically with antipsychotic drugs. Lithium salts induced significant increases in plasma free 5HT (+159% with respect to control values) and in plasma 5HIAA (+39%) without affecting 5HT contained in platelets. Plasma TP was also unchanged by chronic lithium treatment. The ratio between 5HT stored in platelets and 5HT free in plasma, a variable reduced after uptake inhibitors like clomipramine, was decreased in lithium-treated patients (-50%). These results are compatible with an enhanced synthesis of 5HT in the periphery (mainly enterochromaffin cells) as well as with an inhibition of platelet 5HT uptake (or increased 5HT efflux from intracellular stores) induced by lithium. The lack of effect of several antipsychotic drugs upon these variables is consistent with their predominant effect on the dopaminergic system and reinforces the specificity of the effect observed with lithium salts. Taken together, these results support the usefulness of using this "in vivo" 5HT peripheral model for the study of the actions elicited by drugs acting on the presynaptic components of the 5HT system.

Studies on the serotonin transporter in platelets

[3H]-Imipramine and [3H]-paroxetine label with high affinity a recognition site which is associated with the serotonergic transporter in blood platelets. The pharmacological profile of [3H]-imipramine and [3H]-paroxetine binding is highly correlated with the potency of drugs to inhibit the uptake of serotonin. Dissociation kinetic experiments suggest that the substrate recognition site for serotonin may be different from the modulatory site which is labeled with [3H]-imipramine or [3H]-paroxetine. The existence of an endocoid acting on the imipramine receptor to modulate the serotonin transporter has been proposed by several laboratories. In clinical studies most laboratories have reported a decrease in Bmax of [3H]-imipramine binding in platelets from depressed untreated patients when compared with matched healthy volunteers. The Bmax of [3H]-imipramine binding in platelets appears to be a state-dependent biological marker in depression.