Platelet monoamine oxidase activity in schizophrenia: relationship to family history of the illness and neuroleptic treatment

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.

Inhibition of monoamine oxidases by haloperidol and its metabolites: pharmacological implications for the chemotherapy of schizophrenia

The effect of haloperidol and its metabolites on human platelet monoamine oxidase B (MAO-B) and human placenta monoamine oxidase A (MAO-A) in vitro has been investigated. We found that 4-(4-chlorophenyl)-1-[4-(4-fluorophenyl)-4-oxobutyl]-pyridinium (HP+), 4-(4-chlorophenyl)-1-[4-(4-fluorophenyl)-4-oxobutyl]-1,2,3,6- tetrahydropyridine (HTP) and 4-chlorophenyl-1,2,3,6-tetrahydropyridine (CPTP) are potent inhibitors of MAO. HP+ appeared to be a reversible, uncompetitive and selective MAO-B inhibitor with a Ki of 0.83 microM. HTP was found to be an irreversible, uncompetitive and selective MAO-B inhibitor (Ki of 1.84 microM). CPTP inhibits both MAO-A and MAO-B. Some other haloperidol metabolites, i.e. 4-(4-chlorophenyl)-4-hydroxypyridine (CPHP), 4-(4-chlorophenyl)-1-[4-(4-fluorophenyl)-4-oxobutyl]-1,2,3,6- tetrahydropyridine N-oxide (HTPNO) and reduced haloperidol (RHAL), do not inhibit MAO to any appreciable degree at concentrations up to 100 microM. The results suggest that haloperidol metabolites may contribute to the reduction of platelet MAO-B activity in schizophrenic patients undergoing neuroleptic chemotherapy. An examination of the literature reveals that schizophrenic patients with low platelet MAO activity exhibit a strong association with the use of haloperidol. Other possible pharmacological implications of the inhibition of MAO activity are discussed.

Pharmacological and clinical implications of MAO-B inhibitors

Although the involvement of monoamine oxidase B (MAO-B) in physiological function is not yet well understood, its inhibitors have been shown to be quite useful in the treatment of various neuropsychiatric disorders. Platelet MAO-B activity has been found to be reduced in several psychiatric disorders, related to substance abuse and associated with different personalities. 1-Deprenyl (selegiline), an archetypical MAO-B inhibitor, alone does not seem to exert an antidepressive effect, however, it may become useful when administered in combination with amine neurotransmitter precursors. MAO-B inhibitors are useful adjunct drugs to 1-DOPA in the symptomatic treatment of Parkinson's disease. Interestingly, 1-deprenyl alone can slow down the progress of otherwise disabled syndromes of Parkinson's disease. It has been proposed that 1-deprenyl may play a role in neuroprotection and neurorescue. MAO-B inhibitors can selectively and dramatically increase the level of beta-phenylethylamine, which has been shown to potentiate dopamine and noradrenaline function in the central nervous system. Several new types of highly selective, reversible and irreversible MAO-B inhibitors have recently been developed. The mechanism(s) of neuroprotective and rescue actions of 1-deprenyl and other MAO-B inhibitors will help to shed some light on our understanding of the neurodegenerative process.

Platelet monoamine oxidase in schizophrenia: a meta-analysis

We did a meta-analysis on all publications (English and other languages) concerned with platelet monoamine oxidase (MAO) in schizophrenia. Essentially, when patients were medicated with a neuroleptic, most studies found that schizophrenics had lower platelet MAO levels than controls. Administration of neuroleptic lowers MAO levels. MAO levels in drug-free schizophrenics were similar to controls. Only a minority of studies found drug-free schizophrenics had decreased platelet MAO levels.

Long-latency auditory event-related potentials in schizophrenia and in bipolar and unipolar affective disorder

Long-latency auditory event-related potentials were examined in 96 subjects with schizophrenia, 99 with bipolar affective disorder and 48 with major depressive (unipolar) disorder, and compared with 32 in-patient and 213 normal controls. The latency of the P3 component was significantly greater in the schizophrenic and bipolar subjects compared to other groups. The difference was stable with respect to clinical state at the time of testing and was not due to age differences or the effect of psychotropic medications. The results support the clinical distinction between bipolar and unipolar affective disorders, but also show that P3 change is not specific to schizophrenia and found in bipolar but not unipolar affective disorder.

Platelet serotonin in subtypes of schizophrenia and unipolar depression

In subtypes of schizophrenia and unipolar depression, both increased and decreased levels of platelet serotonin were found. Hyperserotonemia was usually observed in patients with psychotic features (i.e., in paranoid schizophrenia and psychotic depression). Hyposerotonemia, although less common than hyperserotonemia, was present in nonparanoid schizophrenia and nonpsychotic depression (i.e., in patients without psychotic symptoms). A sex difference in platelet monoamine oxidase activity was observed among healthy subjects, but not among schizophrenic patients. The activity of platelet monoamine oxidase in paranoid and nonparanoid schizophrenic patients did not differ from that in healthy subjects. The findings in this study suggest that biological differences between subtypes of unipolar depression or schizophrenia might depend upon the presence of psychotic symptoms.

Platelet glutathione peroxidase and monoamine oxidase activity in schizophrenics with CT scan abnormalities: relation to psychosocial variables

We have previously reported that the activity in platelets of the important antioxidant enzyme glutathione peroxidase (GPx) is inversely correlated with computed tomographic (CT) measures of brain atrophy in a population of patients with chronic schizophrenia, suggesting that low GPx may be a vulnerability factor in those schizophrenic patients with structural brain abnormalities. The significance of this finding has now been explored in a larger clinical population by examining the relation of GPx and CT parameters to psychosocial variables and to the activity of platelet monoamine oxidase (MAO), which has also been reported to be altered in certain schizophrenic populations. In the present study, low platelet GPx and high brain atrophy were found to be associated with DSM-III diagnoses of nonparanoid schizophrenia, a high degree of chronicity, and a predominance of negative symptoms. Contrary to some literature reports, atrophy also correlated with age and length of illness among the schizophrenic patients, although the contribution of these factors was less than that of low GPx, which was itself not age dependent. The ventricle-brain ratio (VBR) and atrophy were highly correlated in a control group of affective disorder patients, but not in the schizophrenic group, where large VBRs were found predominantly in the DSM-III undifferentiated subgroup. The low-GPx/high-atrophy schizophrenic patients had normal platelet MAO levels, and MAO was significantly lower only in the paranoid subgroup, consistent with reported observations. There was no evidence for a neuroleptic-induced effect on either enzyme.

Platelet monoamine oxidase activity and schizophrenia--a myth that refuses to die?

Platelet monoamine oxidase (MAO) activity was determined using kynuramine as a substrate in a group of schizophrenic patients (n = 107), a group of healthy individuals (n = 100), and a group of psychiatric patients who were neither schizophrenics nor alcoholics (n = 110). No significant difference emerged between the schizophrenics and the other two groups, while a significant reduction in platelet MAO activity in a group of alcoholics (n = 60) was confirmed. Breaking down the schizophrenic group according to course of illness, phenomenology (paranoid-hallucinatory or not) and drug use did not lead to a significant deviation in platelet MAO activity in any of these subgroups. It can also be demonstrated from the literature that the results reached by most research teams question the usefulness of platelet MAO activity as a genetic marker for psychiatric illness.

Unravelling the relationship between genetic and environmental risk factors in psychiatric disorders

Murray et al (1985) have proposed a method for using biological markers and information about family history to reduce the heterogeneity in a disease such as schizophrenia. They propose that families which are heavily loaded with illness are most likely to be segregating for a major locus and therefore should be used for studies of genetic marker or other biological traits that are thought to be related to a genetic etiology. They propose that patients without a family history (sporadics) of an illness should be investigated for hypothesised environmental components since they are the cases where environmental factors are most likely to play a large role. They give an example from their own data on Ventricular Brain Ratios (VBR) in a sample of schizophrenic twins (Reveley et al, 1984) where schizophrenic twins without any family history (FH-) of a major psychosis had significantly larger ventricles than did schizophrenic twins with a family history (FH+). They conclude that while there is a genetic determinant to ventricular size within the normal range, large ventricles reflect an environmental aetiology that is more important in sporadic cases than in those with a familial pattern to the illness. Subsequently, Reveley & Chitkara (1985) found that singleton schizophrenic patients who were FH-had a significantly larger VBR than did controls while FH+ patients had a mean midway between FH-patients and controls. While this is an attractive hypothesis for the structural brain changes seen in some schizophrenic patients, results of other studies are not consistent with these findings (Nasrallah et al, 1983; Schulsinger et al, 1984; Owens et al, 1985; DeLisi et al, 1986). For example, in our own data (DeLisi et al, 1986), ‘familial’ schizophrenics had larger ventricles than did controls and risk factors thought to be environmental (head injuries and birth complications) were found to be present in that sample.

Clinical, biological, and neuropsychological features associated with lateral ventricular enlargement in DSM-III schizophrenic disorder

Clinical, historical, neuropsychological, and biological correlates of lateral ventricular enlargement on computed tomography (CT scan) were explored in a sample of DSM-III schizophrenics. Patients with enlarged ventricles, as compared with those whose ventricles were normal, presented a longer duration of illness and mean duration of hospitalization, and higher scores on the subscales alogia, affective flattening, and attentional impairment of the Scale for the Assessment of Negative Symptoms (SANS), on the scales self-care, participation in household activities, work performance, and behavior in crises and emergencies of the Disability Assessment Schedule, on the scales rhythm, writing, reading, arithmetic, and left hemisphere of the Luria-Nebraska Neuropsychological Battery, and on the subtests digit span, digit symbol and block design of the Wechsler Adult Intelligence Scale. Furthermore, on the computerized electroencephalogram, beta relative activity was significantly higher in patients with normal ventricles on the right frontal, left frontal, and right central leads. On stepwise discriminant function analysis, the patient groups with enlarged and normal ventricles could be separated statistically, and duration of illness and summary score on the SANS were found to be the best discriminators.

Factors associated with decreased platelet MAO activity in chronic schizophrenics

Platelet MAO activity was determined in a sample of chronic schizophrenics, including drug-free and neuroleptic-treated patients, and in a normal control group. Patients with MAO values below and above the median were compared with respect to several clinical, historical, neuroradiological and neuropsychological variables. The enzyme activity was significantly decreased in the whole patient group and in the subgroup of neuroleptic-treated patients, but not in the subgroup of drug-free patients. The only significant difference between low MAO and high MAO patients concerned drug status (higher percentage of patients on neuroleptics in the former subgroup). On stepwise discriminant function analysis, drug status (on neuroleptics vs. off neuroleptics) correctly classified 63.4% of patients.

Relationship between CSF noradrenaline levels, C-EEG indicators of activation and psychosis ratings in drug-free schizophrenic patients

A significant increase of cerebrospinal fluid (CSF) noradrenaline (NA) levels, probably reflecting a rise of central noradrenergic activity, has been observed in a sample of acute schizophrenic patients as compared with a population of subjects without personal or family history of major psychoses. CSF NA levels have been found to be significantly correlated with computerized EEG (C-EEG) indicators of arousal (negative correlation with alpha relative activity and positive correlation with alpha barycentric frequency and beta relative activity in frontal and central leads). No significant relationship has emerged between CSF NA concentration and psychosis ratings on CPRS as well as platelet MAO activity. These findings seems to confirm the link between central noradrenergic hyperactivity and the condition of enhanced arousal of the schizophrenic patient, although the role of this condition in the pathophysiology of schizophrenia (primary phenomenon or non-specific consequence of the stress related to the illness?) remains to be elucidated.