PRESYNAPTIC LOCALIZATION OF KV1.4-CONTAINING A-TYPE POTASSIUM CHANNELS NEAR EXCITATORY SYNAPSES IN THE HIPPOCAMPUS

Mammalian Shaker voltage-gated potassium channels that contain the Kv1 .4 subunit exhibit rapid activation and prominent inactivation process es, which enable these channels to integrate brief (approximiately mil liseconds) depolarizations over time intervals of up to tens of second s. In the hippocampus, Kv1.4 immunoreactivity is detected at greatest density in two regions: (1) the middle molecular layer (MML), where pe rforant path axons synapse with dentate granule cells, and (2) the str atum lucidum (SL) of CA3, where the messy fibers travel in tight fasci culi and form en passante synapses onto CA3 pyramidal cells. We have s tudied the localization of Kv1.4 within these regions in detail. First , we compared the distribution of Kv1.4 and synaptophysin (a synaptic Vesicle protein primarily localized near termini) under confocal immun ofluorescence microscopy. In the MML, Kv1.4 and synaptophysin immunofl uorescence appeared to overlap. In the SL, however, Kv1.4 and synaptop hysin staining was detected in nonoverlapping, irregular patches (simi lar to 5-10 mu m in diameter). Ultrastructural studies of these two re gions revealed that Kv1.4 immunoreactivity was absent from the surface membranes of cell bodies and dendrites and occurred prominently on ax ons, including axonal ''necks'' near termini. Small excitatory synapti c boutons also were labeled in the MML; by contrast, the messy fiber s ynaptic expansions in the SL were not stained. These localizations may enable Kv1.4-containing channels to regulate the process of neurotran smitter release at these excitatory synapses.