MAO inhibitory side effects of neuroleptics and platelet serotonin content in schizophrenic patients

Monoamine Oxidase B (MAO-B): A Target for Rational Drug Development in Schizophrenia Using PET Imaging as an Example

Monoamine oxidase B (MAO-B) is an important high-density enzyme involved in the generation of oxidative stress and central in the catabolism of dopamine, particularly in brain subcortical regions with putative implications in the pathophysiology of schizophrenia. In this chapter, we review postmortem studies, preclinical models, and peripheral and genetic studies implicating MAO-B in psychosis. A literature search in PubMed was conducted and 64 studies were found to be eligible for systematic review. We found that MAO-B could be identified as a potential target in schizophrenia. Evidence comes mostly from studies of peripheral markers, showing reduced platelet MAO-B activity in schizophrenia, together with preclinical results from MAO-B knock-out mice resulting in a hyperdopaminergic state and behavioral disinhibition. However, whether brain MAO-B is altered in vivo in patients with schizophrenia remains unknown. We therefore review methodological studies involving MAO-B positron emission tomography (PET) radioligands used to quantify MAO-B in vivo in the human brain. Given the limitations of currently available treatments for schizophrenia, elucidating whether MAO-B could be used as a target for risk stratification or clinical staging in schizophrenia could allow for a rational search for newer antipsychotics and the development of new treatments.

Platelet Serotonin Concentration Is Associated with Illness Duration in Schizophrenia and Chronological Age in Depression

OBJECTIVE

Impaired serotonergic neurotransmission has been implicated in the pathogenesis of depression and schizophrenia. Blood platelets have been used for years as a peripheral model of neuronal serotonin dynamics. The objective was to investigate platelet count and serotonin concentration in patients with depression and schizophrenia, in an attempt to ascertain their clinical usefulness.

METHODS

953 participants were included in the study, 329 patients with depression, 339 patients with schizophrenia and 285 healthy controls. ELISA was used to assess platelet serotonin concentrations.

RESULTS

There were no statistically significant differences among groups regarding age, total platelet count and serotonin concentration. Linear regression analyses revealed inverse correlations between platelet serotonin concentration and age of patients with depression and healthy individuals, as well as between platelet serotonin concentration and illness duration in patients with schizophrenia. In other words, longer illness duration in patients with schizophrenia, and higher age in patients with depression and healthy individuals was associated with lower platelet serotonin concentrations.

CONCLUSION

Platelet count and serotonin concentration did not prove to be of diagnostic value in differentiating patients and healthy individuals. However, illness duration in patients with schizophrenia may be associated with reduced concentrations of platelet serotonin.

Influence of clozapine on platelet serotonin, monoamine oxidase and plasma serotonin levels

The purpose of this study was to investigate the influence of clozapine on plasma serotonin, platelet serotonin and monoamine oxidase (MAO) levels in schizophrenic patients and to compare their results with those of unmedicated healthy controls. Groups of 20 outpatients with schizophrenia and 20 healthy controls matched for age, sex and smoking status were recruited for the study. Psychopathology, neurocognitive functioning, plasma serotonin, platelet serotonin and MAO levels were assessed after 1-week drug free interval, and 8 weeks after initiation of clozapine treatment in an open design. The mean clozapine dose at week 8 was 382.5+/-96.4 (range: 250-600) mg/day. In the patient group, at baseline, plasma serotonin and platelet MAO levels were significantly lower, and platelet serotonin levels were significantly higher than in controls. After 8 weeks of clozapine treatment, plasma serotonin and platelet MAO levels increased significantly, while a significant decrease in platelet serotonin levels was detected compared with baseline values. Baseline platelet MAO levels explained 22% of the variance in Clinical Global Impression - Improvement (CGI-I) and improvement in attention, while baseline platelet serotonin predicted 23% of the variance in the improvement in positive symptoms during clozapine treatment. Our data indicate that clozapine may be reversing or compensating for a pre-existing alteration in serotonergic neurotransmission in schizophrenic patients. The prediction of response to clozapine through peripheral biochemical markers may have important clinical implications if repeated in larger samples.

Serotonergic effects of clozapine and its metabolites in hippocampal HT22 cells

In the hippocampal neuronal in vitro system of HT22 cells, we studied the effects of clozapine (Cloz) and its metabolites clozapine-N-oxide (Cloz-N-oxide) and norclozapine (Norcloz) on 5-HT transporter affinity (K(M)) and uptake (V(max)), MAO-B affinity (K(M)) and maximal velocity (V(max)), as well as on 5-HT(2) receptor affinity and density. Clinically relevant concentrations of Cloz (200 and 400 ng/ml) and its metabolites (100 and 200 ng/ml) were used for the examination of the effects after short-term (4 h) and long-term (24 h) incubation. Statistical evaluation revealed that a significantly lowered 5-HT transporter affinity (higher K(M)) was related to higher concentrations of Cloz and its metabolites. A significantly higher 5-HT transporter uptake was dependent on both high concentrations of drugs and an increased time of incubation. No significant influence of the investigated independent variables on MAO-B affinity could be demonstrated, whereas a significant drug-related increase of MAO-B velocity was detectable. Additionally, low and high concentrations of Cloz and its metabolites induced a higher 5-HT(2) receptor affinity (lower K(D)). No significant influences of the investigated independent variables on 5-HT(2) receptor density were detectable. The results of the present study show that Cloz and its metabolites induce significant alterations in serotoninergic parameters of hippocampal HT22 cells, validating the system of hippocampal HT22 cells for further examinations of the mechanisms of action of atypical neuroleptics.

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.