Sabina Berretta

GABAergic interneurons: implications for understanding schizophrenia and bipolar disorder.

A core component to corticolimbic circuitry is the GABAergic interneuron. Neuroanatomic studies conducted over the past century have demonstrated several subtypes of interneuron defined by characteristic morphological appearances in Golgi-stained preparations. More recently, both cytochemical and electrophysiological techniques have defined various subtypes of GABA neuron according to synaptic connections, electrophysiological properties and neuropeptide content. These cells provide both inhibitory and disinhibitory modulation of cortical and hippocampal circuits and contribute to the generation of oscillatory rhythms, discriminative information processing and gating of sensory information within the corticolimbic system. All of these functions are abnormal in schizophrenia. Recent postmortem studies have provided consistent evidence that a defect of GABAergic neurotransmission probably plays a role in both schizophrenia and bipolar disorder. Many now believe that such a disturbance may be related to a perturbation of early development, one that may result in a disturbance of cell migration and the formation of normal lamination. The ingrowth of extrinsic afferents, such as the mesocortical dopamine projections, may “trigger” the appearance of a defective GABA system, particularly under stressful conditions when the modulation of the dopamine system is likely to be altered. Based on the regional and subregional distribution of changes in GABA cells in schizophrenia and bipolar disorder, it has been postulated that the basolateral nucleus of the amygdala may contribute to these abnormalities through an increased flow of excitatory activity. By using “partial” modeling, changes in the GABA system remarkably similar to those seen in schizophrenia and bipolar disorder have been induced in rat hippocampus. In the years to come, continued investigations of the GABA system in rodent, primate and human brain and the characterization of changes in specific phenotypic subclasses of interneurons in schizophrenia and bipolar disorder will undoubtedly provide important new insights into how the integration of this transmitter system may be altered in neuropsychiatric disease.

Infralimbic cortex activation increases c-fos expression in intercalated neurons of the amygdala

Recently, it was reported that stimulation of the infralimbic cortex produces a feedforward inhibition of central amygdala neurons. The interest of this observation comes from the fact that the central nucleus is the main output station of the amygdala for conditioned fear responses and evidence that the infralimbic cortex plays a critical role in the extinction of conditioned fear. However, the identity of the neurons mediating this infralimbic-evoked inhibition of the central nucleus remains unknown. Likely candidates are intercalated amygdala neurons. Indeed, these cells receive glutamatergic afferents from the infralimbic cortex, use GABA as a transmitter, and project to the central amygdala. Thus, the present study was undertaken to test whether, in adult rats, the infralimbic cortex can affect the activity of intercalated neurons. To this end, disinhibition of the infralimbic cortex was induced by local infusion of the non-competitive GABA-A receptor antagonist picrotoxin. Subsequently, neuronal activation was determined bilaterally within the amygdala using induction of the immediate early gene Fos. Infralimbic disinhibition produced a significant increase in the number of Fos-immunoreactive intercalated cells bilaterally whereas no change was detected in the central nucleus. In the basolateral amygdaloid complex, increases in the number of Fos-immunoreactive cells only reached significance in the contralateral lateral nucleus. These results suggest that glutamatergic inputs from the infralimbic cortex directly activate intercalated neurons. Thus, our findings raise the possibility that the infralimbic cortex inhibits conditioned fear via the excitation of intercalated cells and the consequent inhibition of central amygdala neurons.

Hippocampal interneurons are abnormal in schizophrenia

OBJECTIVE The cellular substrate of hippocampal dysfunction in schizophrenia remains unknown. We tested the hypothesis that hippocampal interneurons are abnormal in schizophrenia, but that the total number of hippocampal neurons in the pyramidal cell layer is normal. METHODS We collected whole hippocampal specimens of 13 subjects with schizophrenia and 20 matched healthy control subjects to study the number of all neurons, the somal volume of neurons, the number of somatostatin- and parvalbumin-positive interneurons and the messenger RNA levels of somatostatin, parvalbumin and glutamic acid decarboxylase 67. RESULTS The total number of hippocampal neurons in the pyramidal cell layer was normal in schizophrenia, but the number of somatostatin- and parvalbumin-positive interneurons, and the level of somatostatin, parvalbumin and glutamic acid decarboxylase mRNA expression were reduced. CONCLUSIONS The study provides strong evidence for a specific defect of hippocampal interneurons in schizophrenia and has implications for emerging models of hippocampal dysfunction in schizophrenia.

Extracellular Matrix-Glial Abnormalities in the Amygdala and Entorhinal Cortex of Subjects Diagnosed With Schizophrenia

CONTEXT Chondroitin sulfate proteoglycans (CSPGs), a main component of the brain extracellular matrix, regulate developmental and adult neural functions that are highly relevant to the pathogenesis of schizophrenia. Such functions, together with marked expression of CSPGs in astrocytes within the normal human amygdala and evidence of a disruption of astrocytic functions in this disease, point to involvement of CSPG-glial interactions in schizophrenia. HYPOTHESIS Chondroitin sulfate proteoglycan-related abnormalities involve glial cells and extracellular matrix pericellular aggregates (perineuronal nets) in the amygdala and entorhinal cortex of subjects with schizophrenia. DESIGN Postmortem case-control study. SETTING The Translational Neuroscience Laboratory at McLean Hospital, Harvard Medical School. Specimens were obtained from the Harvard Brain Tissue Resource Center at McLean Hospital. PARTICIPANTS Two separate cohorts of healthy control (n = 15; n = 10) and schizophrenic (n = 11; n = 10) subjects and a cohort of subjects with bipolar disorder (n = 11). INTERVENTIONS Quantitative, immunocytological, and histological postmortem investigations. MAIN OUTCOME MEASURES Numerical densities of CSPG-positive glial cells and perineuronal nets, glial fibrillary acidic protein-positive astrocytes, and total numbers of parvalbumin-positive neurons in the deep amygdala nuclei and entorhinal cortex. RESULTS In schizophrenia, massive increases in CSPG-positive glial cells were detected in the deep amygdala nuclei (419%-1162%) and entorhinal cortex (layer II; 480%-1560%). Perineuronal nets were reduced in the lateral nucleus of the amygdala and lateral entorhinal cortex (layer II). Numerical densities of glial fibrillary acidic protein-positive glial cells and total numbers of parvalbumin-positive neurons were unaltered. Changes in CSPG-positive elements were negligible in subjects with bipolar disorder. CONCLUSIONS Marked changes in functionally relevant molecules in schizophrenia point to a pivotal role for extracellular matrix-glial interactions in the pathogenesis of this disease. Disruption of these interactions, unsuspected thus far, may represent a unifying factor contributing to disturbances of neuronal migration, synaptic connectivity, and GABAergic, glutamatergic, and dopaminergic neurotransmission in schizophrenia. The lack of CSPG abnormalities in bipolar disorder points to a distinctive aspect of the pathophysiology of schizophrenia in key medial temporal lobe regions.

Amygdalo‐Entorhinal Inputs to the Hippocampal Formation in Relation to Schizophrenia

Abstract: This chapter reviews recent postmortem studies of schizophrenic brain and discusses the potential role of the amygdala in the induction of hippocampal abnormalities in this disorder. Based on available evidence, sectors CA4, CA3, and CA2, but not CA1, show preferential changes in schizophrenic subjects, although the most pronounced changes have been found in CA3 and CA2. It seems likely that the amygdala would contribute in some way to the induction of abnormalities along the trisynaptic pathway via its direct input to sectors CA3 and CA2, as well as an indirect one that involves the entorhinal cortex and its perforant path projection to the area dentata. The postmortem findings reported to date have been integrated into a working model in which decreases of inhibitory GABAergic modulation are invoked to explain the observation from a recent PET scan study (Heckers et al., 1999) that baseline metabolic activity in the hippocampus of schizophrenics is increased. In addition, however, the apparent inability of schizophrenics to increase metabolic activity in the hippocampus when challenged with a memory retrieval task may reflect a disturbance of disinhibitory modulation postulated herein to occur in sector CA3, a key relay point along the trisynaptic pathway. Overall, it seems plausible that an increase of excitatory activity entering the hippocampus from the basolateral complex via both direct and indirect pathways may make a significant contribution to the pathophysiology of schizophrenia.

Extracellular matrix abnormalities in schizophrenia.

Emerging evidence points to the involvement of the brain extracellular matrix (ECM) in the pathophysiology of schizophrenia (SZ). Abnormalities affecting several ECM components, including Reelin and chondroitin sulfate proteoglycans (CSPGs), have been described in subjects with this disease. Solid evidence supports the involvement of Reelin, an ECM glycoprotein involved in corticogenesis, synaptic functions and glutamate NMDA receptor regulation, expressed prevalently in distinct populations of GABAergic neurons, which secrete it into the ECM. Marked changes of Reelin expression in SZ have typically been reported in association with GABA-related abnormalities in subjects with SZ and bipolar disorder. Recent findings from our group point to substantial abnormalities affecting CSPGs, a main ECM component, in the amygdala and entorhinal cortex of subjects with schizophrenia, but not bipolar disorder. Striking increases of glial cells expressing CSPGs were accompanied by reductions of perineuronal nets, CSPG- and Reelin-enriched ECM aggregates enveloping distinct neuronal populations. CSPGs developmental and adult functions, including neuronal migration, axon guidance, synaptic and neurotransmission regulation are highly relevant to the pathophysiology of SZ. Together with reports of anomalies affecting several other ECM components, these findings point to the ECM as a key component of the pathology of SZ. We propose that ECM abnormalities may contribute to several aspects of the pathophysiology of this disease, including disrupted connectivity and neuronal migration, synaptic anomalies and altered GABAergic, glutamatergic and dopaminergic neurotransmission.

Neuron numbers and volume of the amygdala in subjects diagnosed with bipolar disorder or schizophrenia.

BACKGROUND Growing evidence supports a pivotal role for the amygdala in the pathogenesis of bipolar disorder (BD) and schizophrenia (SZ). However, the occurrence of morphologic changes in the amygdala is currently controversial. METHODS Total number and numeric density of neurons, neuronal somata size, and volume of the lateral (LN), basal (BN), accessory basal (ABN), and cortical (CO) nuclei of the amygdala were measured in 12 normal control, 10 BD, and 16 SZ subjects. RESULTS In BD subjects, reductions of total numbers (41.1%; p = .01) and numeric densities of neurons (14.5%, p = .01), as well as volume (29.0%; p = .01), were detected in LN. Density of neurons was also decreased in ABN of the same subjects (20.8%; p = .0005). These changes were not related to antipsychotics or lithium salt exposure. In SZ subjects, a decrease of total numbers of neurons was detected in LN (23.6%; p = .04). This effect was no longer significant once exposure to antipsychotics was taken into account. CONCLUSIONS These findings offer structural evidence for an involvement of the amygdala in BD. Consequent loss of amygdalar function may account for abnormalities in emotion processing typical of BD subjects. In contrast, changes in SZ were limited and may have been induced by pharmacologic treatment.

Losing the sugar coating: Potential impact of perineuronal net abnormalities on interneurons in schizophrenia

Perineuronal nets (PNNs) were shown to be markedly altered in subjects with schizophrenia. In particular, decreases of PNNs have been detected in the amygdala, entorhinal cortex and prefrontal cortex. The formation of these specialized extracellular matrix (ECM) aggregates during postnatal development, their functions, and association with distinct populations of GABAergic interneurons, bear great relevance to the pathophysiology of schizophrenia. PNNs gradually mature in an experience-dependent manner during late stages of postnatal development, overlapping with the prodromal period/age of onset of schizophrenia. Throughout adulthood, PNNs regulate neuronal properties, including synaptic remodeling, cell membrane compartmentalization and subsequent regulation of glutamate receptors and calcium channels, and susceptibility to oxidative stress. With the present paper, we discuss evidence for PNN abnormalities in schizophrenia, the potential functional impact of such abnormalities on inhibitory circuits and, in turn, cognitive and emotion processing. We integrate these considerations with results from recent genetic studies showing genetic susceptibility for schizophrenia associated with genes encoding for PNN components, matrix-regulating molecules and immune system factors. Notably, the composition of PNNs is regulated dynamically in response to factors such as fear, reward, stress, and immune response. This regulation occurs through families of matrix metalloproteinases that cleave ECM components, altering their functions and affecting plasticity. Several metalloproteinases have been proposed as vulnerability factors for schizophrenia. We speculate that the physiological process of PNN remodeling may be disrupted in schizophrenia as a result of interactions between matrix remodeling processes and immune system dysregulation. In turn, these mechanisms may contribute to the dysfunction of GABAergic neurons.

Amygdalar activation alters the hippocampal GABA system: "partial" modelling for postmortem changes in schizophrenia.

Abnormalities in amygdala and hippocampus have been shown to coexist in schizophrenia (SZ). In the hippocampus, compelling evidence suggests that a disruption of GABA neurotransmission is present mainly in sectors CA4, CA3, and CA2. The amygdala sends important inputs to the hippocampus and is also believed to have a defective GABA system in schizophrenia. To explore the possibility that changes in the hippocampal GABAergic system could be related to an increased inflow of activity originating in the amygdala, a “partial” animal model has been developed. In awake, freely moving, rats a GABAA receptor antagonist was infused locally into the basolateral nuclear complex of the amygdala (BLn). Within 2 hours, a decreased density of both the 65‐ and 67‐kDa isoforms of glutamate decarboxylase (GAD65 and GAD67) ‐immunoreactive (IR) terminals was detected on neuron somata in sectors CA3 and CA2, but not in CA1, CA4, or dentate gyrus. An increase of GAD67‐IR somata was also found in the dentate gyrus and CA4. In anterograde tracer studies, amygdalo‐hippocampal projection fibers were exclusively found in CA3 and CA2, but not CA1. Taken together, these results indicate that activation of amygdalo‐hippocampal afferents is associated with the induction of significant changes in the GABA system of the hippocampus, with a subregional distribution that is remarkably similar to that found in SZ. Under pathologic conditions, an excessive discharge of excitatory activity emanating from the amygdala could be capable of altering inhibitory modulation along the trisynaptic pathway. This mechanism may potentially contribute to disturbances of GABAergic function in the major psychoses. Such “partial” rodent modelling provides an important strategy for deciphering the effect of altered cortico‐limbic circuits in SZ. J. Comp. Neurol. 431:129–138, 2001.

Hippocampal Interneurons in Bipolar Disorder

CONTEXT Postmortem studies have reported decreased density and decreased gene expression of hippocampal interneurons in bipolar disorder, but neuroimaging studies of hippocampal volume and function have been inconclusive. OBJECTIVE To assess hippocampal volume, neuron number, and interneurons in the same specimens of subjects with bipolar disorder and healthy control subjects. DESIGN Whole human hippocampi of 14 subjects with bipolar disorder and 18 healthy control subjects were cut at 2.5-mm intervals and sections from each tissue block were either Nissl-stained or stained with antibodies against somatostatin or parvalbumin. Messenger RNA was extracted from fixed tissue and real-time quantitative polymerase chain reaction was performed. SETTING Basic research laboratories at Vanderbilt University and McLean Hospital. SAMPLES Brain specimens from the Harvard Brain Tissue Resource Center at McLean Hospital. MAIN OUTCOME MEASURES Volume of pyramidal and nonpyramidal cell layers, overall neuron number and size, number of somatostatin- and parvalbumin-positive interneurons, and messenger RNA levels of somatostatin, parvalbumin, and glutamic acid decarboxylase 1. RESULTS The 2 groups did not differ in the total number of hippocampal neurons, but the bipolar disorder group showed reduced volume of the nonpyramidal cell layers, reduced somal volume in cornu ammonis sector 2/3, reduced number of somatostatin- and parvalbumin-positive neurons, and reduced messenger RNA levels for somatostatin, parvalbumin, and glutamic acid decarboxylase 1. CONCLUSION Our results indicate a specific alteration of hippocampal interneurons in bipolar disorder, likely resulting in hippocampal dysfunction.