IONIC MECHANISMS OF SPONTANEOUS GABAERGIC EVENTS IN RAT HIPPOCAMPAL SLICES
EXPOSED TO 4-AMINOPYRIDINE.
Karri Lamsa and Kai Kaila.
Department of Biosciences, Division of Animal Physiology, University of
Helsinki, P.O. Box 17, FIN-00014 Helsinki, Finland.
APStracts 4:138N, 1997.
ABSTRACT
Ion-selective (H+ and K+) microelectrode techniques as well as conventional
extra- and intracellular recordings were used to study the ionic mechanisms of
propagating spontaneous GABAergic events (SGEs) in rat hippocampal slices
exposed to 4-aminopyridine (4-AP, 50 - 100 æM). All experiments were made in
the presence of antagonists of ionotropic glutamate receptors (10 æM NBQX and
40 æM AP5). The SGEs were composed of a negative-going change in field
potential with a temporally coincident increase (0.7 ñ 0.3 mM) in
extracellular K+ ([K+]o) and an alkaline transient (0.01 - 0.08 units) in
extracellular pH (pHo) in stratum radiatum (s. radiatum) of the area CA1.
Simultaneous intracellular recordings showed a triphasic hyperpolarization-
depolarization-late hyperpolarization response in pyramidal cells. Application
of pentobarbital (PB, 100 æM) decreased the interval between SGEs from a mean
value of about 35 s to 20 s and shortened the period of refractoriness of
stimulus-evoked propagating events. This was accompanied by an increase in the
amplitude of the field potential response, of the [K+]o and the pHo shifts,
and of the depolarizing phase of the pyramidal-cell response. The SGEs were
completely blocked by the GABAA receptor antagonist, picrotoxin (100 æM PiTX).
The amplitudes of the negative-going field potential and of the depolarizing
phase of the pyramidal-cell response as well as the ionic shifts associated
with SGEs were strongly suppressed in the nominal absence of CO2/HCO3-. There
was a five-fold increase in the inter-event interval, and propagating SGEs
could not be evoked by stimuli given at intervals shorter than about 2 - 3
minutes. Exposure to inhibitors of carbonic anhydrase, benzolamide (BA, 10 æM)
or ethoxyzolamide (EZA, 50 æM) fully blocked the alkaline pHo transients, and
turned them into acid shifts. The poorly membrane-permeant BA had no
discernible effect on the other components of the SGEs, but application of EZA
had effects reminiscent to those of CO2/HCO3- -free medium. Addition of the
GABAA receptor-permeant weak-acid anion, formate (20 mM) re-established the
SGEs that were first suppressed by exposure to the CO2/HCO3- -free medium. No
SGEs were seen in the presence #JO72-7
of a similar concentration of the GABAA receptor-impermeant anion, propionate.
Unlike the alkaline transients associated with HCO3- -driven SGEs, those
supported by formate were not blocked by BA. The present data suggest that an
inward current carried by bicarbonate is necessary for the generation of SGEs,
and that the GABAA receptor-mediated excitatory coupling among GABAergic
interneurons is essentially dependent on the availability of intracellular
bicarbonate.
Received 24 January 1997; accepted in final form 3 July 1997.
APS Manuscript Number J72-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 27 August 1997