Neurotransmitter systems in schizophrenia

Amygdala volume in schizophrenia: post-mortem study and review of magnetic resonance imaging findings

BACKGROUND

Claims that schizophrenia is a disease of the limbic system have been strengthened by meta-analyses of magnetic resonance imaging (MRI) studies finding reduced hippocampus and amygdala volumes. Some post-mortem studies do not find these abnormalities.

AIMS

To assess the volume of the amygdala in a series of brains post-mortem.

METHOD

Amygdala volume was estimated using point-counting in both hemispheres of the brains of 10 male and 8 female patients with schizophrenia, and a comparison group of 9 males and 9 females.

RESULTS

No significant reduction of amygdala volume was found.

CONCLUSIONS

Significant volume reduction of the amygdala is not a consistent feature of schizophrenia; findings from early MRI studies using coarse delineation methods may introduce bias to subsequent meta-analyses.

Relations of clinical features, subgroups and medication to serum monoamines in schizophrenia

BACKGROUND

Plasma and serum indices of monoaminergic activity reflect partly the illness of schizophrenia (e.g. HVA/deficit syndrome) and sometimes the symptoms (e.g. HVA/anhedonia). But, such studies have rarely taken both metabolites and parent amines or inter-amine activity ratios into account. We hypothesized that comparing the major symptom dimensions to measures of transmitter activity (with and without control for antipsychotic drug treatment) would show differential patterns of activity useful for the design of pharmacological treatments.

METHODS

Dopamine (DA), noradrenaline (NA), serotonin (5-HT), their three major metabolites and prolactin were measured in the serum of 108 patients with schizophrenia and 63 matched controls: DA D2-receptor blocking-activity was estimated from a regression of butyrophenone displacement in striatum in vitro on to PET reports of drug-binding in vivo. Symptoms were factored into four dimensions (disorganized/thought disorder, nonparanoid/negative, ideas-of-reference and paranoid/positive symptoms).

RESULTS

(1). Patients' DA activity did not differ from controls: but their 5-HT and NA turnovers increased/decreased, respectively, and the DA/5HT-metabolite ratio was lower. Increased DA-D2-receptor occupancy was predicted by decreased DA-metabolism and its ratio to 5-HT-metabolism. (2). Patients had higher levels of NA, DA-metabolites and DA-/5-HT-metabolite ratios on atypical vs typical drugs. (3). Increased D2-occupancy was associated with lower DA metabolism in paranoid patients but was unrelated to relative increases of DA/5-HT- and NA-metabolism in nonparanoid patients. (4). Low DA-/5-HT-metabolite ratios, high prolactin and low DA-metabolism characterized thought-disordered patients. (5). High DA-/5-HT-metabolite ratios paralleled many ideas-of-reference. The metabolites were sensitive, respectively, to control for D2-occupancy and prolactin.

CONCLUSIONS

The role of DA in paranoid, and 5-HT in thought-disordered and ideas-of-reference dimensions point both to the mechanisms underlying the features typical of these subgroups and the type of medication appropriate.

Schizophrenia and viral infection during neurodevelopment: a focus on mechanisms

The task of defining schizophrenia pathogenesis has fascinated and frustrated researchers for nearly a century. In recent years, unprecedented advances from diverse fields of study have given credence to both viral and developmental theories. This review considers possible mechanisms by which viral and developmental processes may interact to engender schizophrenia. Many of the current controversies in schizophrenia pathogenesis are reviewed in light of the viral hypothesis, including: epidemiological findings and the role of a genetic diathesis, phenotype heterogeneity, abnormalities in excitatory and inhibitory neurotransmitter systems, anomalous cerebral latereralization, and static vs progressive disease. The importance of animal models in elucidating the impact of viral infections on developing neurons is illustrated by recent studies in which neonatal rats are infected with lymphocytic choriomeningitis virus in order to examine alterations in hippocampal circuitry. Finally, consideration is given to a new hypothesis that some cases of schizophrenia could be instigated by a viral infection that disrupts developing inhibitory circuits, consequently unleashing glutamatergic neurotransmission leading to selective excitotoxicity, and a degenerative disease course.

GABAergic neuronal subtypes in the human frontal cortex--development and deficits in schizophrenia

Recent studies have provided evidence for a deficit of GABA-containing interneurons in the frontal cortex in schizophrenia. That this deficit might be brought about during early foetal or neonatal life is a hypothesis consistent with the substantial indications for a neurodevelopmental aetiology of the disease. GABAergic neurons can be defined by the presence of one of three types of calcium binding proteins, which are thought to have neuroprotective properties. We have undertaken an investigation into the postnatal ontology of these neuronal subtypes and find that calretinin expression is relatively constant and present from before birth, calbindin expression is also present early but redistributes in the cortex over the first months of life, while parvalbumin-immunoreactivity is not observed until between 3 and 6 months of age. Investigation of frontal cortical tissue taken post mortem from a series of schizophrenic patients and matched control subjects revealed that parvalbumin-, but not calretinin-immunoreactive cells are significantly diminished in schizophrenia. These observations support the hypothesis that GABAergic deficits in schizophrenia may stem from toxic events occurring during cortical development which selectively target immature neurons before protection by parvalbumin is conferred.

Plasma homovanillic acid in untreated schizophrenia--relationship with symptomatology and sex

Plasma homovanillic acid (pHVA) concentrations are considered to reflect, in part, central dopamine metabolism and thus may be of value in assessing the role of dopamine neurotransmission in schizophrenia. Furthermore, some recent studies have suggested a relationship of pHVA with symptomatology. We have undertaken a study of pHVA in a large cohort of unmedicated DSM-IV schizophrenic patients in order to assess the relationship of pHVA to various clinical parameters. pHVA in 58 drug-free patients (10.11+/-0.52 ng/ml) was significantly elevated in comparison with 62 matched control subjects (8.77+/-0.39 ng/ml). pHVA was found to be higher in patients with a more negative syndrome. No significant correlation of pHVA with overall SAPS or SANS scores was apparent in the patients although, within the SANS subscales, a significant relationship to anhedonia-asociality was apparent. Interestingly, the male drug-free patients showed a correlation of pHVA with negative symptoms defined by SANS and several SANS subscales, while females showed no significant relationship with any SANS subscales. The results may suggest that an increased dopaminergic turnover is apparent in (male) schizophrenic patients with predominantly negative symptoms, providing some support for reports that this change in neuronal activity may be related to the neuropathological abnormalities seen in the disease, which may themselves differ between males and females. Such neuronal deficits of developmental or degenerative origin may thus result in an elevation/disinhibition of central dopamine metabolism in schizophrenia.

Postmortem studies in schizophrenia

For over a century, postmortem studies have played a central part in the search for the structural and biochemical pathology of schizophrenia. However, for most of this time, little progress has been made. Recently, the situation has begun to change, helped by the emergence of more powerful methodologies and research designs, and by the availability of brain imaging to provide complementary information. As a result, it can now be clearly concluded that there are structural cerebral abnormalities in schizophrenia that are intrinsic to the disorder. The neuropathological process is not primarily degenerative, but involves a change in the normal cytoarchitecture of the brain, probably originating in development. Neurochemically, there is postmortem evidence for alterations in several transmitter systems including dopamine, glutamate, serotonin, and γ-aminobutyric acid (GABA). The cardinal findings are reviewed here, together with a consideration of the conceptual and methodological issues that face postmortem studies of schizophrenia.

The neuropathology of schizophrenia. A critical review of the data and their interpretation

Despite a hundred years' research, the neuropathology of schizophrenia remains obscure. However, neither can the null hypothesis be sustained--that it is a 'functional' psychosis, a disorder with no structural basis. A number of abnormalities have been identified and confirmed by meta-analysis, including ventricular enlargement and decreased cerebral (cortical and hippocampal) volume. These are characteristic of schizophrenia as a whole, rather than being restricted to a subtype, and are present in first-episode, unmedicated patients. There is considerable evidence for preferential involvement of the temporal lobe and moderate evidence for an alteration in normal cerebral asymmetries. There are several candidates for the histological and molecular correlates of the macroscopic features. The probable proximal explanation for decreased cortical volume is reduced neuropil and neuronal size, rather than a loss of neurons. These morphometric changes are in turn suggestive of alterations in synaptic, dendritic and axonal organization, a view supported by immunocytochemical and ultrastructural findings. Pathology in subcortical structures is not well established, apart from dorsal thalamic nuclei, which are smaller and contain fewer neurons. Other cytoarchitectural features of schizophrenia which are often discussed, notably entorhinal cortex heterotopias and hippocampal neuronal disarray, remain to be confirmed. The phenotype of the affected neuronal and synaptic populations is uncertain. A case can be made for impairment of hippocampal and corticocortical excitatory pathways, but in general the relationship between neurochemical findings (which centre upon dopamine, 5-hydroxytryptamine, glutamate and GABA systems) and the neuropathology of schizophrenia is unclear. Gliosis is not an intrinsic feature; its absence supports, but does not prove, the prevailing hypothesis that schizophrenia is a disorder of prenatal neurodevelopment. The cognitive impairment which frequently accompanies schizophrenia is not due to Alzheimer's disease or any other recognized neurodegenerative disorder. Its basis is unknown. Functional imaging data indicate that the pathophysiology of schizophrenia reflects aberrant activity in, and integration of, the components of distributed circuits involving the prefrontal cortex, hippocampus and certain subcortical structures. It is hypothesized that the neuropathological features represent the anatomical substrate of these functional abnormalities in neural connectivity. Investigation of this proposal is a goal of current neuropathological studies, which must also seek (i) to establish which of the recent histological findings are robust and cardinal, and (ii) to define the relationship of the pathological phenotype with the clinical syndrome, its neurochemistry and its pathogenesis.

Receptor mechanisms of antipsychotic drug atypicality

Recent advances in antipsychotic treatment of schizophrenia have offered several new compounds which avoid many of the limitations of the classical antipsychotics. These so-called ‘atypical’ antipsychotics have fewer extrapyramidal side effects, greater efficacy against negative symptoms and greater efficacy in otherwise treatment-resistant patients. However, the mechanism of action of these atypical antipsychotics is still unclear. The several receptors currently implicated in the pharmacological profile of these atypical antipsychotics include subtypes of those for dopamine, serotonin, noradrenaline, and acetylcholine among others. The current hypotheses for possible mechanisms of action of atypical antipsychotics are discussed along with the experimental correlates of antipsychotic efficacy in animal models.

What is an atypical antipsychotic?

This is an exciting time for research into the drug treatment of schizophrenia. Modern techniques in imaging and molecular biology have contributed to our understanding of the receptor mechanisms of antipsychotic drug action. Several new antipsychotics are, or will shortly be, available and each of these new drugs promise improvements over classical antipsychotics. This review will discuss the concept of atypicality in antipsychotic drugs, and will describe some of the models for identification of improved antipsychotic action. The various receptor mechanisms proposed to underlie atypical antipsychotic action, several of which have featured in recent debates in this journal, will be considered in the light of the pharmacology of some of the newly-emerging antipsychotic drugs.