A Measured Milestone in Schizophrenia Research

THE PAST 2 decades have witnessed a resurgence of interest in identifying the structural underpinnings of schizophrenia. Neuroimaging studies have shown that in schizophrenic subjects the brain as a whole and the frontal cortex in particular are smaller than in normal subjects.^{1 - 3} A wealth of data has established that prefrontal cortical dysfunction—for example, impairments in working memory, abstract thinking, attention, and language coherency—are prominent symptoms of schizophrenia.⁴ Yet just what is missing in the schizophrenic prefrontal cortex to account for these deficits has been hard to pinpoint. There are no obvious signs of pathology, neuronal loss, or gliosis in the prefrontal cortex in schizophrenic patients. Perhaps because of the subtle nature of the structural deficit, progress in identifying an anatomical substrate for schizophrenia has come only with the application of quantitative methods. Recent stereologic analyses of the prefrontal cortex have begun to home in on the deficit, revealing the presence of increased neuronal density in schizophrenic patients.^{5 - 6} The implication of this finding is that the interneuronal space, rather than the neurons themselves, is underrepresented in the prefrontal cortex. Although appearing empty in Nissl preparations, this space is actually chock-full of presynaptic and postsynaptic neuronal processes. Thus, the predominant pathology in schizophrenia is not the classic form involving neuronal cell death, but instead a sublethal reduction in the elements of neuronal connectivity, the reduced neuropil deficit.⁷ Now the challenge has moved to determining what components in this composite neuropil compartment are impoverished.

Glantz and Lewis⁸ have combined a classic anatomical method, Golgi impregnation, with new analytical tools to examine the density of dendritic spines in prefrontal cortical area 46 of schizophrenic patients in comparison with normal controls and with patients suffering from other mental illnesses. Glantz and Lewis have been at the forefront of research applying quantitative techniques to identify the specific morphologic and biochemical basis of schizophrenia. These same authors have recently reported a reduction in synaptophysin immunoreactivity in the schizophrenic prefrontal cortex,⁹ and prior work by the Lewis laboratory has demonstrated diminished staining for dopaminergic and γ-aminobutyric acid–containing (GABAergic) terminal fields in prefrontal areas.^{10 - 11} In this study, they report a 23% reduction in spine density on the basilar dendrites of pyramidal neurons located in deep layer III in schizophrenic subjects in comparison with normal controls. This reduction is specific in terms of disease (ie, layer IIIc pyramidal cells in the prefrontal cortex of patients with other psychiatric disorders did not show a similar decrease) and in terms of cortical location (as spine density was not diminished in layer III pyramidal cells in the visual cortex of schizophrenic patients). There is also modest laminar specificity in the described deficit, as a smaller, nonsignificant reduction of spine density was found in superficial layer III of the schizophrenic prefrontal cortex.

Indeed, the strength of this study derives largely from the well-conceived experimental design, in which the area of interest is limited to 1 cytoarchitectonically defined area (46), 1 sublayer (IIIc), and 1 type of dendritic branch (basilar). Glantz and Lewis capitalize on previous work in the field that has identified area 46 as a prime site of functional impairment in schizophrenic patients¹² and shown that large pyramidal neurons in layer IIIc are atrophic in postmortem schizophrenic brains¹³ to focus on this group of neurons as likely candidates for spine reduction. Moreover, as the pathological blueprint for schizophrenia may involve only certain prefrontal areas or exhibit regionally distinct pathology, as some evidence from the cingulate and temporal cortices suggests,^{14 - 16} limiting the experimental survey to a single cortical area enhanced the prospect of identifying an anatomical correlate of the disease. Within each cortical area, additional specificity is conferred by the precise wiring of afferent terminals into the 6 layers and onto different portions of dendrites, not only apical and basilar branches but also shafts and spines of these dendritic arbors. For example, deep layer III receives projections from the thalamus, other cortical regions, and brainstem monoaminergic nuclei, as well as local excitatory and inhibitory connections. Ultrastructural studies have shown that spines on pyramidal neurons represent the postsynaptic component of a triadic arrangement in which dopaminergic inhibitory afferents and presently unspecified excitatory terminals converge on a single spine, presumably enabling dopamine to modulate its partnered input.^{17 - 18} The reported reduction in layer IIIc spines in the schizophrenic prefrontal cortex therefore suggests that the large pyramidal cells residing in this sublayer receive fewer synapses from modulatory dopaminergic and excitatory afferent sources.

Conversely, a weakness in the study is the inhomogenity of patient populations included in the "schizophrenic" and "other psychiatric" groups. The clinical diagnosis of schizophrenia encompasses a diverse array of symptoms that vary in expression from one patient to the next, leading to subclassification of the disease based on clinical presentation. In some patients, the presence of mood disorders akin to those associated with affective illness are sufficiently prominent to warrant a separate categorization as "schizoaffective disorder." Whether this diagnosis represents a totally distinct mental illness or a transitional form is currently the subject of debate.¹⁹ Certainly, limitations in availability of postmortem brains dictate that some inhomogeneity in the sample of schizophrenic patients must be tolerated. However, inclusion of schizoaffective patients in this group seems an unduly broad definition of the disease. Lumping of multiple diseases in the psychiatrically impaired comparison group is similarly problematic. It is possible that morphometric changes in alcohol-induced psychosis are opposite in direction and magnitude to those in depressed patients and therefore cancel one another out. Restriction of the psychiatric comparison group to one disease, eg, major depression, would have been more informative. Other quantitative data indicate that the cortical pathology in schizophrenic brains is distinct from that of patients with affective disorders.^{20 - 21} Definitive evidence showing that reduced spine density also distinguishes schizophrenia from affective psychosis must await replication with a more homogeneous psychiatric comparison group.

The search for a pathological basis for schizophrenia is a field still in its infancy. Progress at this stage is comparable to that of Alzheimer disease 15 years ago, when cholinergic neurons were first identified as the vulnerable neuronal phenotype. In schizophrenia research, the path has been longer and more tortuous, requiring sophisticated quantitative techniques to tease out the deficit. The good news is that the pathology in the prefrontal cortex of schizophrenic patients seems to be limited to neuronal processes and synaptic connections, stopping short of actual neuronal loss. If so, the possibility, however remote, exists that therapies for remodeling and regrowth of these connections will be developed and ultimately lead to better treatments for the disease. A critical first step in this direction is identification of the exact chemoarchitectonic basis of the disease so that specific neuronal cells and connections may be targeted for intervention. Novel strategies must be developed for isolating components of the neuropil compartment to assess the viability of specific connections in schizophrenia. In addition, appropriate animal models need to be developed to mimic the reduction in cortical circuitry and to test putative therapies for restoring normal connectivity. However, at present, the application of quantitative methods in schizophrenia research continues to be the major factor in gaining new insight into the disease.

This study was supported by grant 44866 from the Center for Neuroscience of Mental Disorders, National Institute of Mental Health, Rockville, Md.