SPATIAL LOCALIZATION OF CALCIUM CHANNELS IN GIANT FIBER LOBE NEURONS OF THE SQUID (LOLIGO-OPALESCENS)

Whole-cell voltage clamp was used to investigate the properties and sp atial distribution of fast-deactivating (FD) Ca channels in squid gian t fiber lobe (GFL) neurons. Squid FD Ca channels are reversibly blocke d by the spider toxin omega Agatoxin IVA with an IC50 of 240-420 nM wi th no effect on the kinetics of Ca channel gating. Channels with very similar properties are expressed in both somatic and axonal domains of cultured GFL neurons, but FD Ca channel conductance density is higher in axonal bulbs than in cell bodies at all times in culture. Channels presumably synthesized during culture are preferentially expressed in the growing bulbs, but bulbar Ca conductance density remains constant while Na conductance density increases, suggesting that processes det ermining the densities of Ca and Na channels in this extrasomatic doma in are largely independent. These observations suggest that growing ax onal bulbs in cultured GFL neurons are not composed entirely of ''axon al'' membranes because FD Ca channels are absent from the giant axon i n situ but, rather, suggest a potential role for FD Ca channels in med iating neurotransmitter release at the motor terminals of the giant ax on.