Differential time-course of slow afterhyperpolarizations and associated Ca²⁺ transients in rat CA1 pyramidal neurons: Further dissociation by Ca²⁺ buffer

Hippocampal neurons exhibit a slow afterhyperpolarization following membrane depolarization; this is thought to reflect an underlying Ca²⁺- dependent K⁺ current. This current is potentiated by intermediate concentrations (0.1-1.0 mM) of exogenous Ca²⁺ buffer [Schwindt P. C. et al. (1992) Neuroscience 47, 571-578; Zhang L. et al. (1995) J. Neurophysiol. 74, 2225-2241]. The relationship between the slow afterhyperpolarization and associated Ca²⁺ transients was investigated in the presence and absence of added exogenous Ca²⁺ 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 Ca²⁺ indicators in the patch pipette solution allowed simultaneous measurement of the evoked subcellular Ca²⁺ transients using a confocal microscope. The peak Ca²⁺ 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 Ca²⁺ concentrations. As the K(D) for Ca²⁺ of the indicator dyes used was between 200 and 300 nM, it is predicted that saturation will occur when the peak Ca²⁺ signal exceeds 1 μM. This occurred with fewer action potentials in dendritic vs somatic compartments. Neither compartment exhibited averaged Ca²⁺ transients matching the slow afterhyperpolarization time-course, dendritic Ca²⁺ transients being most divergent. Intracellular accumulation of exogenous Ca²⁺ 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 Ca²⁺ transient. The initial rate of decline of the dendritic Ca²⁺ transient was diminished, but remained faster than that of the slow afterhyperpolarization. We conclude that neither dendritic nor somatic Ca²⁺ signals match the slow afterhyperpolarization time-course, with this dissociation being further magnified by addition of exogenous Ca²⁺ buffer. The implications of this result are discussed.