Differential time-course of slow afterhyperpolarizations and associated Ca 2+ transients in rat CA1 pyramidal neurons: further dissociation by Ca 2+ buffer

Abstract

Hippocampal neurons exhibit a slow afterhyperpolarization following membrane depolarization; this is thought to reflect an underlying Ca 2+-dependent K + current. This current is potentiated by intermediate concentrations (0.1–1.0 mM) of exogenous Ca 2+ buffer [Schwindt P. C. et al. (1992) Neuroscience 47, 571–578; Tymianski M. et al. (1993) Neuron 11, 221–235]. The relationship between the slow afterhyperpolarization and associated Ca 2+ transients was investigated in the presence and absence of added exogenous Ca 2+ buffer. Slow afterhyperpolarizations and underlying K + currents were measured using whole-cell patch-clamp recordings from hippocampal CA1 neurons in acute rat brain slices. Inclusion of fluorescent Ca 2+ indicators in the patch pipette solution allowed simultaneous measurement of the evoked subcellular Ca 2+ transients using a confocal microscope. The peak Ca 2+ signal exhibited an incremental increase with each action potential. This increase eventually reached a plateau with increasing numbers of action potentials, suggesting dye saturation with peak Ca 2+ concentrations. As the K D for Ca 2+ of the indicator dyes used was between 200 and 300 nM, it is predicted that saturation will occur when the peak Ca 2+ signal exceeds 1 mM. This occurred with fewer action potentials in dendritic vs somatic compartments. Neither compartment exhibited averaged Ca 2+ transients matching the slow afterhyperpolarization time-course, dendritic Ca 2+ transients being most divergent. Intracellular accumulation of exogenous Ca 2+ buffer, either by inclusion in the patch pipette or by incubation of the brain slice with its membrane-permeable form, caused a prolongation of the slow afterhyperpolarization but not of the somatic Ca 2+ transient. The initial rate of decline of the dendritic Ca 2+ transient was diminished, but remained faster than that of the slow afterhyperpolarization. We conclude that neither dendritic nor somatic Ca 2+ signals match the slow afterhyperpolarization time-course, with this dissociation being further magnified by addition of exogenous Ca 2+ buffer. The implications of this result are discussed.