Photolytic manipulation of Ca2+ and the time course of slow, Ca(2+)‐activated K+ current in rat hippocampal neurones.

Abstract

1. Experiments were performed on hippocampal CAI pyramidal cells to investigate the time course of a slow, Ca2+-activated K+ current that follows a burst of action potentials. At a temperature of 27-30°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 recordings were made using electrodes containing the photolabile calcium buffers nitr-5 or DM-nitrophen loaded appropriately with Ca2 . Under these conditions, photolysis of the compound using an ultraviolet flashlamp caused an instantaneous increase in cytoplasmic Ca2+ throughout the cell. The response to flash photolysis was a membrane hyperpolarization with an onset limited by the membrane time constant. Multiple (up to twenty) flash responses could be generated. 3. The postspike slow after-hyperpolarization (AHP) and flash-induced hyperpolarizations showed a common sensitivity to the fl-adrenergic receptor agonist isoprenaline.4. Following a burst of spikes, the current underlying an AHP in progress could be terminated or reduced by photolysis-induced production 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) despite the fact that the manipulation occurred 400-500 ms following the end of Ca2+ entry.5. Partial block of the slow AHP by buffer production 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 experiments which altered the spatial distribution of Ca2+ entry, such as production of calcium spikes or dendritic depolarization by glutamate in the presence of tetrodotoxin.7. The Ca2+-dependent K+ current responsible 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.

Alterations in Ca2+-Buffering in Prion-Null Mice: Association with Reduced Afterhyperpolarizations in CA1 Hippocampal Neurons

“…This method is well established in the investigation of the molecular basis of the slow AHP (Lancaster and Zucker, 1994;Sah and Clements, 1999).…”

Location and Function of the Slow Afterhyperpolarization Channels in the Basolateral Amygdala

“…3). Although we cannot rule out the possibility that calcium entering via voltage-gated calcium channels in the membrane during voltage steps reaches high concentrations in microdomains in the vicinity of the channels, the sI AHP is readily activated in CA1 pyramidal neurons by uncaging calcium with whole-field UV excitation, which releases calcium in both the soma and dendrites (Lancaster and Zucker, 1994;Sah and Clements, 1999). Addition- Figure 6.…”

“…The sI AHP is a calcium-dependent potassium current that can also be evoked by uncaging cytosolic calcium (Lancaster and Zucker, 1994;Sah and Clements, 1999). We next examined the location of the sI AHP by examining currents evoked in response to spatially restricted calcium rises.…”

β-Adrenergic Stimulation Selectively Inhibits Long-Lasting L-Type Calcium Channel Facilitation in Hippocampal Pyramidal Neurons

“…Therefore, it seems likely that if these calcium transients principally arise from calcium entry during the depolarization used to evoke the slow AHP (with only a minor component arising from delayed facilitation), they would not be apparently affected by the receptor activation that suppresses the slow AHP. However, activation of potassium channels by the intracellular photolytic release of Ca from DM-nitrophen is apparently sensitive to ␤-adrenergic receptor stimulation (Lancaster and Zucker, 1994). It is not known whether the potassium channels activated by this method were SK channels.…”

“…It is not known whether the potassium channels activated by this method were SK channels. In addition, because repeated photolysis causes depletion of Ca-loaded DM-nitrophen, and calcium release was not monitored (Lancaster and Zucker, 1994), the lack of a reversible effect of the applied agonist makes it possible that the apparent response could result from this depletion. The results presented here raise the possibility that the time course of the slow AHP is determined by delayed facilitation.…”