Developmental changes in the expression of GABAA receptor alpha 1 and gamma 2 subunits in human temporal lobe, hippocampus and basal ganglia: An implication for consideration on age-related epilepsy
Introduction
The mechanism of age-dependency of epilepsy is poorly understood. Several point mutations in the subunits of GABAA receptor have been recently found to be linked to various age-dependent epilepsy syndromes (Baulac et al., 2001, Wallace et al., 2001, Cossette et al., 2002, Harkin et al., 2002). In this regard, the disruption of GABA-mediated pathways is known to be associated with epilepsy (Olsen et al., 1999).
GABAA receptor is a ligand-gated ion channel and Cl− ion currents generated by GABA, a physiological ligand, are associated with the main inhibitory system in the central nervous system (Barnard et al., 1998). GABAA receptor also has binding sites of benzodiazepines. GABAA receptor is a hetero-pentamer, which consists of various subunits such as α, β, γ, δ, ɛ, θ and π. However, the main subunits in the brain are two of α1, two of β2 and one of γ2 subunit (Sieghart et al., 1999).
Several different mutations in the gene that encodes the GABAA receptor γ2-subunit (GABARG2), have been recently identified in four families with generalized epilepsy syndromes associated with febrile seizures (FS), including generalized epilepsy with febrile seizures plus (GEFS+) and classical childhood absence epilepsy (CAE) (Baulac et al., 2001, Wallace et al., 2001, Harkin et al., 2002, Kananura et al., 2002). Both share a unique seizure phenotype referred to as febrile seizures plus that consists of FS extending beyond 6 years of age and/or subsequent frequent afebrile seizures. Furthermore, a truncation mutation of GABRG2 was reported to be associated with severe myoclonic epilepsy of infancy (SMEI), of which seizure phenotype usually starts in infancy (Singh et al., 2001).
In addition, a mutation of the GABAA receptor α1-subunit (GABARA1) was found in affected individuals with autosomal dominant juvenile myoclonic epilepsy (ADJME) in a French-Canadian family. Functional studies of the mutant receptor expressed in Xenopus laevis oocytes show that the mutation results in decreased GABA-activated currents (Cossette et al., 2002).
Thus, epilepsy syndromes in which mutations of GABAA receptor have so far been identified are all age-dependent and thus development of GABAA receptor may be closely linked to such age-dependency. In fact, it has been shown in the rat brain that the expression patterns of mRNAs of different subunits change with development (Laurie et al., 1992). Nevertheless, development of human GABAA receptor per se has not been studied in detail.
The purpose of this study was to analyze the developmental change in GABARA1 and GABARG2, both of which are associated with age-dependent epilepsy syndrome as mentioned above.
Section snippets
Human tissue specimen
Twenty human brains obtained from fetuses/subjects who died aged 22 gestation weeks (GW) to 75 years were used for immunohistochemical studies. Informed consent for postmortem examination was given in writing by the patients or their family representatives. The causes of death were spontaneous abortion, cardiomyopathy, congenital heart disease, or acute lymphocytic leukemia without CNS involvement in fetuses, infants and children, and squamous cell carcinoma without CNS involvement, or liver
Results
The immunoreactivities for GABARA1 and GABARG2 in the developing human hippocampus, temporal lobe and basal ganglia are described in Table 1, Table 2. In the hippocampus, a few GABARA1 immunoreactive neuronal somas were seen in the dentate gyrus and subiculum from 22 GW. GABARA1 expression gradually increased with age and marked GAGARA1 immunoreactivity was noted in the dentate gyrus from 29 GW to 4 months of age. However, it decreased subsequently during childhood but increased again during
Discussion
In the human brain, we found developmental changes in GABARA1 and GABARG2 immunoreactivities in the temporal lobe, hippocampus and basal ganglia. In hippocampus, both subunits were markedly expressed mainly in CA2-3. The appearance of GABARG2 occurred later than that of GABARA1 in the hippocampus. In the temporal cortex, GABARA1 appeared in the pyramidal cell layer from early fetal period, while GABARG2 was delayed to become moderate from 38 GW. Marked GABARA1 was noted in pyramidal cells of
Acknowledgements
This study was supported in part by grants from the Ministry of Health, Labor and Welfare, and the Ministry of Education, Science and Culture of Japan.
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