Effect of age and neuroleptics on tyrosine hydroxylase-IR in sector CA2 of schizophrenic brain

Functional disconnection between subsystems of the default mode network in schizophrenia

BACKGROUND

A dysfunctional default mode network (DMN) has been reported in patients with schizophrenia. However, the stability of the deficits has not been determined across different stages of the disorder.

METHODS

We examined the functional connectivity of the DMN subsystems of 125 patients with first-episode schizophrenia (FES) or recurrent schizophrenia (RES), compared to that of 82 healthy controls. We tested the robustness of the findings in an independent cohort of 158 patients and 39 healthy controls. We performed resting-state functional connectivity analysis, and examined the strength of the connections within and between the three subsystems of the DMN (core, dorsal medial prefrontal cortex [dMPFC], and medial temporal lobe [MTL]). We also analyzed the connectivity correlations to symptoms and illness duration.

RESULTS

We found reduced connectivity strength between the core and MTL subsystems in schizophrenia patients compared to controls, with no differences between the FES and RES patient groups; these findings were validated in the second sample. Schizophrenia patients also showed a significant reduction in connectivity within the MTL and between the dMPFC-MTL subsystems, similarly between FES and RES groups. The connectivity strength within the core subsystem was negatively correlated with clinical symptoms in schizophrenia. There was no significant correlation between the DMN subsystem connectivity and illness duration.

CONCLUSIONS

DMN subsystem connectivity deficits are present in schizophrenia, and the homochronicity of their appearance indicates the trait-like nature of these alterations. The DMN deficit may be useful for early diagnosis, and MTL dysfunction may be a crucial mechanism underlying schizophrenia.

Amygdalocortical circuitry in schizophrenia: from circuits to molecules

Schizophrenia is a disorder in which disturbances in the integration of emotion with cognition plays a central role and probably involves several different regions, including the dorsolateral prefrontal cortex, the rostral anterior cingulate cortex, the hippocampal formation, and basolateral amygdala (BLA). Recent brain imaging studies have reported changes in volume, whereas postmortem studies point to dysfunction of the GABA and glutamate systems in these regions. Microarray-based profiles indicate that complex changes in the expression of genes associated with synaptic transmission and ion channels are involved in GABA cell dysfunction in schizophrenics. Molecular abnormalities vary considerably on the basis of sector and layer, suggesting that the unique connectivity of intrinsic and extrinsic afferents may critical in regulating the activity of genes in specific subpopulations of GABA cells. Projections of the BLA may be of particular importance to the induction of abnormal circuitry in schizophrenia, as their ingrowth during late adolescence and early adulthood may help to 'trigger' the onset of illness in susceptible individuals. A preponderance of cellular and molecular abnormalities has been found in the stratum oriens (SO) of sectors CA3/2 in which BLA afferents provide a robust innervation. These observations have lead to the development of a rodent model for the study of abnormal circuitry in this disorder. For example, single-cell recordings in hippocampal slices exposed to increased activation from the BLA have shown decreases in GABA currents in pyramidal neurons in SO of CA3/2, but not CA1, and support the validity of this model. Overall, the postmortem studies of neural circuitry abnormalities in schizophrenia are beginning to implicate specific cellular, molecular, and electrophysiological mechanism in specific subtypes of cortical neurons defined by their afferent and efferent connectivity within key corticolimbic regions.

Dopamine D2 receptor levels in striatum, thalamus, substantia nigra, limbic regions, and cortex in schizophrenic subjects

BACKGROUND

Studies in schizophrenic patients have reported dopaminergic abnormalities in striatum, substantia nigra, thalamus, anterior cingulate, hippocampus, and cortex that have been related to positive symptoms and cognitive impairments.

METHODS

[(18)F]fallypride positron emission tomography studies were performed in off-medication or never-medicated schizophrenic subjects (n = 11, 6 men, 5 women; mean age of 30.5 +/- 8.0 [SD] years; 4 drug-naive) and age-matched healthy subjects (n = 11, 5 men, 6 women, mean age of 31.6 +/- 9.2 [SD]) to examine dopamine D(2) receptor (DA D(2)r) levels in the caudate, putamen, ventral striatum, medial thalamus, posterior thalamus, substantia nigra, amygdala, temporal cortex, anterior cingulate, and hippocampus.

RESULTS

In schizophrenic subjects, increased DA D(2)r levels were seen in the substantia nigra bilaterally; decreased levels were seen in the left medial thalamus. Correlations of symptoms with ROI data demonstrated a significant correlation of disorganized thinking/nonparanoid delusions with the right temporal cortex ROI (r = .94, p = .0001), which remained significant after correction for multiple comparisons (p < .03). Correlations of symptoms with parametric images of DA D(2)r levels revealed no significant clusters of correlations with negative symptoms but significant clusters of positive correlations of total positive symptoms, delusions and bizarre behavior with the lateral and anterior temporal cortex, and hallucinations with the left ventral striatum.

CONCLUSIONS

The results of this study demonstrate abnormal DA D(2)r-mediated neurotransmission in the substantia nigra consistent with nigral dysfunction in schizophrenia and suggest that both temporal cortical and ventral striatal DA D(2)r mediate positive symptoms.

Synaptic plasticity (and the lack thereof) in hippocampal CA2 neurons

The hippocampus is critical for some forms of memory and spatial navigation, but previous research has mostly neglected the CA2, a unique region situated between CA3 and CA1. Here, we show that CA2 pyramidal neurons have distinctive physiological characteristics that include an unprecedented synaptic stability. Although basal synaptic currents in CA1 and CA2 are quite similar, synaptic plasticity including long-term potentiation and long-term depression is absent or less likely to be induced with conventional methods of stimulation in CA2. We also find that CA2 neurons have larger leak currents and more negative resting membrane potentials than CA1 neurons, and consequently, more current is required for action potential generation in CA2 neurons. These data suggest that the molecular "conspiracy against plasticity" in CA2 makes it functionally distinct from the other hippocampal CA regions. This work provides critical insight into hippocampal function and may lead to an understanding of the resistance of CA2 to damage from disease, trauma, and hypoxia.

Differences in the cellular distribution of D1 receptor mRNA in the hippocampus of bipolars and schizophrenics

Several lines of evidence have pointed to a role of the dopamine system in the pathophysiology of schizophrenia. A recent postmortem study demonstrated a selective decrease of tyrosine hydroxylase fibers on pyramidal neurons in sector CA2 in the hippocampus of schizophrenics. Although both brain imaging and postmortem studies have examined the distribution of the D1 receptor in the prefrontal and cingulate cortex, no study to date has examined its expression of mRNA using a high-resolution autoradiographic approach. In order to further assess whether the regulation of the dopamine D1 receptor is altered in hippocampal neurons in this disorder, we used in situ hybridization (ISH) to measure the expression of messenger RNA for this receptor in the dentate gyrus and sectors CA1-4. Both the number of cells expressing D1 mRNA and the amount of expression per cell were measured in 15 schizophrenic, 15 bipolar disorder, and 15 normal control subjects. The results show a significant (21%) and selective decrease in D1 mRNA expression in sector CA3 of schizophrenic subjects. First-degree relatives of schizophrenics did not show any differences in either the expression of D1 mRNA per cell or the number of cells expressing this mRNA when compared to a separate group of normal controls matched for age and PMI. Subjects with bipolar disorder also showed a significant (25%) and selective increase of D1 mRNA expression in sector CA2. Other hippocampal sectors did not show significant changes. These findings in schizophrenics and bipolars were also associated with inverse changes in the overall number of neurons expressing D1 mRNA in sectors CA3 and CA2, respectively. This study shows diagnosis-specific changes in D1 mRNA expression in the hippocampus of schizophrenic versus bipolar subjects and suggests that this neuromodulatory system may show distinct changes in the pathophysiology of the two disorders.

Molecular abnormalities of the hippocampus in severe psychiatric illness: postmortem findings from the Stanley Neuropathology Consortium

Between 1997 and 2002, 48 data sets from the hippocampus were produced on samples from the Stanley Neuropathology Consortium. From these data sets, 224 total measures were available from the various subdivisions of the hippocampus. An integrative analysis of these measures was performed using a multivariate, nonparametric analysis of variance (ANOVA). ANOVA with correction for multiple comparisons indicated that parvalbumin-containing cells in CA2 were reduced in schizophrenia and bipolar disorder. In addition, reelin protein in the molecular layer of the dentate gyrus was decreased in schizophrenia, bipolar disorder, and depression at the trend level of statistical significance (P=0.065). These results strongly suggest a dysfunction of inhibitory GABA-ergic interneurons in severe mental illness. Without correction for multiple comparisons, 31 measures were abnormal in at least one disease, whereas 11 measures would be expected to appear abnormal by chance. Abnormal molecules included measures of synaptic density or neuronal plasticity (reelin, SNAP-25, BDNF, Complexin I and II), as well as parvalbumin, tyrosine receptor kinase A, glucocorticoid receptors, glutamate NR1 receptor subunits, serotonin 5HT2(A) and 5HT1(B) receptors, and dopamine D(5) receptors.

Hippocampal neurons in schizophrenia

The hippocampus is crucial for normal brain function, especially for the encoding and retrieval of multimodal sensory information. Neuropsychiatric disorders such as temporal lobe epilepsy, amnesia, and the dementias are associated with structural and functional abnormalities of specific hippocampal neurons. More recently we have also found evidence for a role of the hippocampus in the pathophysiology of schizophrenia. The most consistent finding is a subtle, yet significant volume difference in schizophrenia. Here we review the cellular and molecular basis of smaller hippocampal volume in schizophrenia. In contrast to neurodegenerative disorders, total hippocampal cell number is not markedly decreased in schizophrenia. However, the intriguing finding of a selective loss of hippocampal interneurons deserves further study. Two neurotransmitter receptors, the GABAA and AMPA/kainate glutamate receptors, appear to be abnormal, whereas changes of the NMDA glutamate receptor are less robust. The expression of several genes, including those related to the GABAergic system, neurodevelopment, and synaptic function, is decreased in schizophrenia. Taken together, recent studies of hippocampal cell number, protein expression, and gene regulation point towards an abnormality of hippocampal architecture in schizophrenia.

Striatal responses to partial dopaminergic lesion: evidence for compensatory sprouting

Dopaminergic lesions result in the acute loss of striatal dopamine content, the loss of tyrosine hydroxylase-immunoreactive fibers, upregulation of preproenkephalin mRNA expression, and compensatory changes in the synthesis and metabolism of dopamine. Despite the severe loss of fine tyrosine hydroxylase-immunoreactive fibers, larger fibers persist. We found that some tyrosine hydroxylase fiber types increase their branching and become thicker after partial lesion. To determine whether the remaining tyrosine hydroxylase fibers were degenerative or part of a compensatory response, we morphologically characterized striatal tyrosine hydroxylase fibers and compared them to silver-stained degenerative structures. Branched and large tyrosine hydroxylase fiber types were nondegenerative. Furthermore, normal preproenkephalin mRNA expression was maintained despite severe overall loss of tyrosine hydroxylase fibers in striatal regions with abundant branching, whereas preproenkephalin mRNA expression increased in severely depleted regions that lacked branched fibers, indicating that branching or sprouting was involved in the compensation for dopamine depletion and the maintenance of normal preproenkephalin expression. In support of compensatory sprouting by tyrosine hydroxylase fibers, mRNA for growth associated protein-43 was upregulated in dopaminergic midbrain cells. We conclude that an important compensatory response to partial dopaminergic depletion is the formation of new branches or sprouting.