Brain Re.search. 435 ( 198713~7-392 387 Elsevier BRE 22618 Intracellular injection of a Ca 2+ chelator inhibits spike repolarization in hippocampal neurons Johan F. Storm Department of Neurobiology and Behavior and Howard Hughes Medical Institute, State University of New York at Stony Brook. Stony Brook, NY 11794 (U.S.A.) (Received 15 May 1987) (Accepted 25 August 1987) K,3' words: Action potential repolarization: Afterhyperpolarization:Ca-,~ctivatcdK-current: Hippocampalpyramidal cell: Cak-iumche!ater: !.2-Bi:;,'c;-a~i;~ophci~t,xyiethane-N.N.N'.N'-tetraacetic acid (BAPTA): EGTA The Ca-dependenceof spike repolarization and afterhyperpolarizations (AHPs) in CA t pyramidal cells, was tested with intracellu- lar electrodes containing the Ca buffers EGTA or 1.2-bis(o-aminophenoxytethanc-N.N.N'.N'-tetraacetic acid (BAPTA). EGTA blocked only the slow AHP: but the fast-actingCa chelator BAPTA also inhibited spike repolarization and the fast AHP. This sup- ports the hypothesisthat a fast Ca-activated K-current contributes to spike repolarization. The mechanisms underlying repolarization of the action potential are potentially important for a num- ber of neuronal functions, e.g. control of repetitive fir- ing, transmitter release 2°, and even certain forms of synaptic plasticity and learning7. Yet little is known about repolarization mechanisms in the vertebrate brain. In squid axons, the voltage-dependent delayed rectifier current (IK) is the main repo!arizing current; but data from autonomic I and hippocampal n''-'-?'5 neurons suggest that a fast calcium-activated potassi- um c u r r e t ~ t 1"1°'13"25 is important in vertebrate somata. Thus, in hippocampal CAt cells, suppression of in- ward Ca currents or blocking of the fast Ca-activated K-current inhibit repolarization and broaden the spikell.22 -25. The repolarizing K-current is presumed to be acti- vated by Ca ions which enter the cell during the ac- tion potential. However, previous experiments have not tested whether an increase in ~..~*~ acellular free Ca during the spike is essential for activation of tLe repolarizing current. The present study provides di- rect evidence for this mechanism by showing that in- jection of a fast-acting Ca-chelating agent inhibits spike repolarization in rat hippocampal pyramidal cells. Some of this material was presented at the So- ciety for Neuroscience meeting in Washington in 1986. lntracellular recordings were obtained from CA~ pyramidal cells (n = 21) in hippocampal slices from rats, using 3 M KCl-filled microelectrodes (tip resis- tance 20-80 Mr)). Various concentrations (50-200 raM) of EGTA (Sigma) or 1.2-bis(o-aminophen- oxy)ethane-N.N.N'.N'-tetraacetic acid (BAPTA) 19 (Gallard-Schlezinger) were added to the electrode- filling KC! solution (pH = 7.2-7.4). The recording chamber was perfused with saline, containing (in raM): NaC! 124, NaHCO3 22.5, KCI 1.8, KH2PO4 1.2, MgSO4 1.0, CaCI2 2.0, MgCI2 1.0, glucose 10, and was bubbled with 95% 0,,/5% CO,. The temper- ature was kept constant (+ 0.5 °C) at 30-34 °C. in some experiments, 5/~M bicucutline was added to the saline in order to suppress spontaneous inhibitory synaptic potentials. Single action potentials were elicited by brief (2-4 ms) depolarizing current pulses Correspondence: J.F. Storm. Present address: Institute of Neurophysiology, Karl Johans gt. 47, Oslo 1. Norway. 0006-8993/87/$0,3.50 © 1987ElsevierSciencePublishers B.V. (BiomedicalDivision)