Distribution of chloride channel-2-immunoreactive neuronal and astrocytic processes in the hippocampus

The chloride homeostasis of neurons and non-neuronal cells is maintained in part by a voltage-sensitive inwardly rectifying chloride conductance throu gh the chloride channel-2. This channel is activated by hyperpolarization a nd extracellular hypotonicity. In the present study, hippocampal sections w ere immunostained for chloride channel-2, and somata and dendrites of both pyramidal and non-pyramidal cells were found to be immunoreactive. In addit ion, glial processes in the vicinity of small blood vessels were also immun ostained, whereas the neuropil of strata pyramidale and lacunosum-molecular e contained chloride channel-2-positive punctate structures. Electron micro scopy and double immunostaining using antibodies against chloride channel-2 , and glial fibrillary acidic protein confirmed that the dense network of c hloride channel-2-positive processes corresponds to the end feet of astrocy tes. The distribution of chloride channel-2-immunoreactive astrocytes was i nhomogeneous throughout the hippocampus: strata oriens, pyramidale and lacu nosum-moleculare of CA1-CA3 and the outer molecular layer of the dentate gy rus contained the majority of immunoreactive end feet, whereas the other la yers showed sparse labeling. Subcellular studies demonstrated that, in addi tion to astrocytes, chloride channel-2 was localized in the membrane of den drites, dendritic spines, cell bodies and axon initial segments of neurons, frequently close to, or within active zones of, symmetrical synapses. Thus, chloride channel-2 appears to be involved in transmembrane chloride m ovements associated with GABAergic synaptic transmission. The specific lami nar distribution of chloride channel-2-positive astroglial processes coinci ding with that of GABAegic axon terminals suggests that the network of astr ocytes may be able to siphon and deliver Cl- ions to layers with intense GA BAergic transmission. thereby increasing the efficacy of GABAA receptor-med iated inhibition. (C) 2000 IBRO. Published by Elsevier Science Ltd. All rig hts reserved.