Immunohistochemical study of GABAA receptor α1 subunit in the hippocampal formation of aged brains with Alzheimer-related neuropathologic changes

doi:10.1016/S0006-8993(98)00437-5

Brain Research

Volume 799, Issue 1, 13 July 1998, Pages 148-155

https://doi.org/10.1016/S0006-8993(98)00437-5 Get rights and content

Abstract

Immunocytochemical techniques were employed to examine the distribution of the γ-aminobutyric acid (GABA)_A receptor α1 subunit within the hippocampus of 19 elderly subjects with Alzheimer-related neuropathologic changes. In mild cases (i.e., Braak stages I and II), the most intense neuropil immunolabeling was observed in the molecular layer of the dentate gyrus, the stratum pyramidale of the CA1 subregion and subiculum, while the weakest labeling was observed in the CA3 subfield. In CA4 region, the proximal dendrites and cell bodies of mossy cells were intensely α1 positive. Throughout the hippocampus, we observed a number of α1 labeled interneurons. These cells consisted of both large and small multipolar cells as well as small bipolar neurons. In moderate cases (i.e., Braak stages III and IV), the pattern and intensity of α1 immunolabeling appeared indistinguishable from mild cases. In severe cases (i.e., Braak stages V and VI), we observed a marked decrease in neuropil immunolabeling within the CA2, CA1 subregions and prosubiculum, while the labeling of the molecular layer of the dentate gyrus, subiculum proper and presubiculum was indistinguishable from mild and moderate cases. These data together with our previous immunocytochemical study in which we demonstrated a marked preservation of the GABA_A receptor subunit β2/3 suggest that responses of selected GABA_A receptor subunits to AD pathology are variable with the α1 subunit displaying a high degree of vulnerability.

Introduction

While recent research suggests that some of the pathological changes in Alzheimer's disease (AD) may be due to glutamate-mediated excitotoxicity [17], it is important to bear in mind that much of the adverse effects of glutamate receptor stimulation may be mitigated via the action of the inhibitory neurotransmitter, γ-aminobutyric acid (GABA). It is our hypothesis that neuronal vulnerability in AD is defined in part by a delicate balance of excitatory (i.e., glutamate) and inhibitory (i.e., GABA) input as mediated through selected glutamatergic and GABAergic receptor subtypes. To investigate this hypothesis we examined the anatomical distribution of the α-amino-3-hydroxy-5-methyl-4-isoaxolepropionate (AMPA)-sensitive receptor subunits GluR1 and GluR2/3 within the hippocampal formation of subjects with AD 1, 12. These latter studies demonstrated a marked preservation, if not increase, in AMPA receptor immunolabeling within the AD hippocampus, particularly within termination zones of glutamatergic pathways (e.g., molecular layer of dentate gyrus, stratum radiatum of CA3). In addition, as an initial investigation into the GABA system in AD, we examined the distribution and density of the GABA_A receptor subunits β2/3 within the AD hippocampus. These studies, like our investigations of AMPA receptor subunits, demonstrated a relative preservation of β2/3 immunoreactivity throughout the hippocampus even in cases with frank neuropathology [15]. Although these latter studies suggest that some subunits of the GABA_A receptor are well preserved in the AD hippocampus, it is important to consider that within this structure GABA_A receptor subunits display a differential distribution with respect to specific subregions 7, 11, 16, 22, 23and thus may be uniquely affected in AD. Supporting this concept is the knowledge that within the human hippocampus there exists a hierarchy of AD-mediated neuronal vulnerability such that CA1 neurons are more vulnerable than CA2→CA3→DG. Although the mechanism underlying the selected vulnerability of hippocampal neurons is unknown we predict that `vulnerable' vs. `resistant' neurons are unique with respect to their GABA and glutamate receptor phenotypes. To examine this hypothesis, in this study immunocytochemical techniques were employed to examine the distribution of the GABA_A receptor α1 subunit within the hippocampus of 19 elderly subjects with Alzheimer-related neuropathologic changes. This study examined the same cases as used previously for studies of β2/3 receptor subunit, thus allowing us to compare directly the results of the two studies. To our knowledge this is the first description of the α1 subunit in the hippocampus of patients with AD.

Section snippets

Subjects

Brain tissue was obtained post mortem from 19 elderly subjects (81.6±7.0 years): six non-demented subjects (82.5±6.9 years) and 13 clinically diagnosed as AD (81.2±7.3 years) (Table 1). The postmortem interval and brain weight of the elderly cases were 3.5±2.3 h and 1132±151 g, respectively. For non-demented subjects, clinical evaluations were largely based on retrospective analysis of medical records and interviews with their physicians or family members. AD subjects were all participants in a

Results

In the mild cases (i.e., Braak stages I and II), the most intense α1 immunolabeling was observed in the molecular layer of the dentate gyrus, CA1 field and subicular complex (i.e., prosubiculum, subiculum proper, presubiculum) (Fig. 1C). In the molecular layer of the dentate gyrus, despite its overall dark appearance, we were able to distinguish between the inner and outer sublayers with the inner being more immunoreactive (Fig. 2A). In addition, in this region a number of darkly-labeled

Discussion

In general, our findings are in agreement with previous studies investigating the distribution of α1 subunits in the dentate gyrus and Ammon's horn of the normal human hippocampal formation 11, 23. In brief, we observed subregional variations in the intensity of α1 immunolabeling. The most intense immunolabeling was observed in the molecular layer of the dentate gyrus and the strata pyramidale and moleculare within CA1 region and subiculum. Notably, this pattern of neuropil labeling was

Acknowledgements

This work was supported by NIH grant AG 08206. Tissue was provided in part by the University of Pittsburgh Alzheimer Disease Research Center funded by the National Institute on Aging AG-05133.

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¹: Current address: Institute of Clinical Medicine, Department of Psychiatry, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba-shi, Ibaraki 305, Japan.

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Short communicationImmunohistochemical study of GABAA receptor α1 subunit in the hippocampal formation of aged brains with Alzheimer-related neuropathologic changes

Abstract

Introduction

Section snippets

Subjects

Results

Discussion

Acknowledgements

Neurosci. Lett.

J. Neurol. Sci.

Neuroscience

Exp. Neurol.

Brain Res.

AMPA-selective glutamate receptor subtype immunoreactivity in the hippocampal dentate gyrus of patients with Alzheimer disease. Evidence for hippocampal plasticity

Mol. Chem. Neuropathol.

Neuropathological staging of Alzheimer-related changes

Acta Neuropathol.

A sensitive and highly specific assay for PHF proteins

Soc. Neurosci. Abstr.

Subpopulations of GABA neurons in the dentate gyrus express high levels of the α1 subunit of the GABAA receptor

Hippocampus

Subunit selectivity and epitope characterization of mAbs directed against the GABAA/benzodiazepine receptor

J. Cell Biol.

Short communication
Immunohistochemical study of GABA_A receptor α1 subunit in the hippocampal formation of aged brains with Alzheimer-related neuropathologic changes

Subpopulations of GABA neurons in the dentate gyrus express high levels of the α1 subunit of the GABA_A receptor

Subunit selectivity and epitope characterization of mAbs directed against the GABA_A/benzodiazepine receptor