PROPERTIES AND MODULATIONS OF BK_Ca CHANNEL,ITS POSTISCHEMIC CHANGES AND MECHAMISMS IN HIPPOCAMPAL CA1 PYRAMIDAL NEURONS OF ADULT RAT

1. Properties Of BKCa channels in hippocampal CAl pyramidal neurons from adult rat BKCa channels are thought to be activated by Ca ions entering the cell during the rising phase of the action potential and membrane depolarization. In hippocampus, BKca channels are largely responsible for action potential repolarization and generation of the fast afierhyperpolarization (fAHP). It is also suggested recently that BKCa channels may play an important role in regulating neuronal excitability at the resting membrane potential. In recent years, a great deal of data concerning BKca channel has been obtained using patch clamp techniques in cultured or young hippocampal neurons. In contrast, most studies concerning roles of BKCa channels in spike repolarization and fAHP have LIsed recordings from slices of adult brain or in vivo recordings. Moreover, a change in duration of action potential was reported previously in hippocampal CAl neurons during development. Little is known about properties of BKCa channels in adult neurons so far. It is therefore desirable that single channel properties of BKca channel should also be examined in adult neurons. The experiments in this study were designed primarily to extend the observations on the properties of individual BKca channel in CAl pyramidal neurons from adult rat hippocampus. BKca channel activity was first observed at [Ca2+]1 = 0.01 Ct M with the membrane potential of +20 mV, and the [Ca2+]+ at which the channel was half activated (P0 = 0.5) was 2 it M. Conductance of single BKCa channel was approx. 245 pS with symmetrical 140 mM KC on both sides of the excised membrane. For a fixed [Ca2+]+ of 2 1-M, P0 increased e-fold for 17 mV positive change in membrane potential. Two exponentials, with time constants of 2.07 ms and 14.36 ms were required to describe the observed open distribution of BKCa channel, suggesting existence of two different open channel states with apparently normal conductance. BKca channel occasionally entered a low-conductance open state with the single channel current amplitude about 45% the amplitude of the normally observed single channel current. The properties of BKCa channel, larger conductance and higher sensitivity to [Ca2+], may partially contribute to shortened duration of action potential in mature hippocampal CAl pyramidal neurons. 2. Modulations of BKca channels in hippocampal CAl pyramidal neurons from adult rat (1) Modulations of BKca channels by phosphorylation BKca channels respond to signals other than intracellular Ca2+ and membrane potential. So far, phosphorylation and dephosphorylation have been shown to influence activity of BKCa channels in other CNS neurons. Hippocampus plays important roles in learning and memory and also is extremely vulnerable to ischemia. However, there is no data concerning modulation of phosphorylation on BKCa channels in hippocampus yet. So, using inside-out configuration of patch clamp, ATP modulation of BKca channels was studied in CAl pyramidal neurons from adult rat hippocampus. Intracellular application of 0.5 mM ATP markedly increased activity of BKCa channels and this action disappeared after pre- and post- treatment with H-89, a specfic inhibitor for protein kinase A. In contrast to H-89, Chele, a specfic inhibitor for protein kinase C, had no apparent effect on ATP-induced augmented BKca channel activity. Channel kinetics analysis showed that the increase in activity of BKca channels induced b...