A. Sik et al., Distribution of chloride channel-2-immunoreactive neuronal and astrocytic processes in the hippocampus, NEUROSCIENC, 101(1), 2000, pp. 51-65
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.