PHOTOLYTIC MANIPULATION OF CA2-COURSE OF SLOW, CA2+-ACTIVATED K+ CURRENT IN RAT HIPPOCAMPAL-NEURONS( AND THE TIME)

1. Experiments were performed on hippocampal CAI pyramidal cells to in vestigate the time course of a slow, Ca2+-activated K+ current that fo llows a burst of action potentials. At a temperature of 27-30-degrees- C, this current rises to a peak 200-400 ms following the cessation of Ca2+ entry before decaying to baseline in 4-8 s. 2. Intracellular reco rdings were made using electrodes containing the photolabile calcium b uffers nitr-5 or DM-nitrophen loaded appropriately with Ca2+. Under th ese conditions, photolysis of the compound using an ultraviolet flashl amp caused an instantaneous increase in cytoplasmic Ca2+ throughout th e cell. The response to flash photolysis was a membrane hyperpolarizat ion with an onset limited by the membrane time constant. Multiple (up to twenty) flash responses could be generated. 3. The postspike slow a fter-hyperpolarization (AHP) and flash-induced hyperpolarizations show ed a common sensitivity to the beta-adrenergic receptor agonist isopre naline. 4. Following a burst of spikes, the current underlying an AHP in progress could be terminated or reduced by photolysis-induced produ ction of calcium buffer from diazo-4 within the cell. This action was rapid (within the settling time of the flash artifact, i.e. < 10 ms) d espite the fact that the manipulation occurred 400-500 ms following th e end of Ca2+ entry. 5. Partial block of the slow AHP by buffer produc tion was accompanied by an increase in the time to peak of the event. 6. The time to peak of the slow AHP could also be manipulated by exper iments which altered the spatial distribution of Ca2+ entry, such as p roduction of calcium spikes or dendritic depolarization by glutamate i n the presence of tetrodotoxin. 7. The Ca2+-dependent K+ current respo nsible for the slow AHP responds immediately to increases or decreases in cytoplasmic Ca2+. It seems likely, therefore, that the slow AHP is controlled solely by changes in free Ca2+ and that the time course is governed by the redistribution of cytoplasmic Ca2+ following activity -induced entry through voltage- or receptor-operated channels.