Alterations in intracellular calcium chelation reproduce developmental differences in repetitive firing and afterhyperpolarizations in rat neocortical neurons

Many 1-week-old rat sensorimotor cortical neurons exhibit extreme spike-frequency adaptation (neurons only fire for the first 100–250 ms of a 1 s current injection) accompanied by a large, prolonged afterhyperpolarization (AHP). Relatively greater expression of a Ca-dependent K + current appears to underlie the extreme adaptation observed in immature cells. In the present study, we examined whether altering intracellular Ca 2+ buffering by introducing Ca 2+ chelators via the recording electrode could reproduce the age-related differences in firing and AHPs. We studied firing behavior and AHPs in 1-week-old and adult neocortical neurons with sharp microelectrodes, under three recording conditions: no chelator, 2 mM BAPTA, or 100–200 mM BAPTA. Our principal findings in regard to firing behavior and AHPs were that (1) adult-low BAPTA neurons mimicked 1 week-control cells, (2) 1 week-high BAPTA neurons were similar to adult-control cells, (3) a greater percentage of 1 week-low BAPTA neurons showed complete adaptation, and (4) adult neurons impaled with high BAPTA electrodes fired in a burst-spiking mode. These data suggest that Ca 2+ regulation is qualitatively different in immature and adult neurons.