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 microM. 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.
Jahromi
B S
BS
Department of Surgery (Neurosurgery), University of Toronto, Ontario, Canada.
Zhang
L
L
Carlen
P L
PL
Pennefather
P
P
D000200:Action Potentials / Q000187:drug effects / Q000502:physiology*; D000818:Animals; D001921:Brain / Q000502:physiology*; D002021:Buffers; D002118:Calcium / Q000378:metabolism*; D002614:Chelating Agents / Q000494:pharmacology; D004396:Coloring Agents; D003712:Dendrites / Q000187:drug effects / Q000502:physiology; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D018408:Patch-Clamp Techniques; D017966:Pyramidal Cells / Q000502:physiology*; D051381:Rats; D017208:Rats, Wistar; D013997:Time Factors
D002021:Buffers; D002614:Chelating Agents; D004396:Coloring Agents; D004533:Egtazic Acid; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; D002118:Calcium