Relative contributions of excitatory and inhibitory neuronal activity to
alkaline transients evoked by stimulation of Schaffer collaterals in the rat
hippocampal slice.
Taira, T., P. Paalasmaa, J. Voipio and K. Kaila.
Department of Biosciences, Division of Animal Physiology, P.O. Box 17 and
the Institute of Biomedicine, Department of Physiology, P.O. Box 9, FIN-00014
University of Helsinki, Finland, Fax: +358-0-1917301.
APStracts 2:0098N, 1995.
SUMMARY AND CONCLUSIONS
1. The neuronal basis of alkaline shifts in extracellular pH (pH o ) evoked by
stimulation of Schaffer collaterals was studied by means of double-barreled H
+ -selective microelectrodes in the area CA1 of rat hippocampal slices. 2.
Alkaline transients in stratum pyramidale evoked by stimulation at a low
frequency (5-10 Hz) were enhanced by pentobarbital (100 [mu]M). In the absence
of the drug, inhibition of extracellular carbonic anhydrase (CA o ) by
benzolamide or by prontosil-dextran 5000 (PD 5000) resulted in an increase in
the alkaline shifts. In contrast to this, alkaloses evoked by low-frequency
stimulation in the presence of pentobarbital were attenuated by a subsequent
inhibition of CA o . 3. Blockade of _-aminobutyric acid-A (GABA A ) receptors
with picrotoxin (PiTX; 100 [mu]M) resulted in an enhancement of alkaline
transients in s. pyramidale evoked by low-frequency stimulation (10 Hz) but
suppressed alkaline shifts evoked by brief high-frequency (1 s, 100 Hz) trains
of stimuli. 4. Application of trains of stimuli consisting of a constant
number of pulses (50 or 100) revealed a striking dependence of the effect of
benzolamide on stimulation frequency (10-200 Hz) in s. pyramidale: the
enhancement of the alkaloses seen upon inhibition of CA o became progressively
smaller with an increase in frequency, and at 100-200 Hz benzolamide produced
a suppression or a complete block of the alkaline transients. However,
alkaline transients evoked using a constant train duration (5 s) were enhanced
by benzolamide at all stimulation frequencies examined (5-200 Hz).
5. In contrast to the above findings, in stratum radiatum inhibition of CA o
enhanced alkaline transients evoked by a constant number (100) of stimulation
pulses irrespective of stimulation frequency. 6. The present results indicate
that the relative contributions of excitatory and inhibitory neuronal activity
to alkaline transients triggered by the activity of Schaffer collaterals show
a striking dependence on the pattern of stimulation and on regional synaptic
architecture. In s. pyramidale, alkaline shifts evoked at low frequencies (5-
10 Hz) are mainly (but not solely) caused by excitatory transmission, whereas
the GABA A receptor-mediated component assumes a dominant role during brief
high-frequency trains of stimuli. In the presence of pentobarbital, GABA A
receptors make a significant contribution to alkaline transients also at low
frequencies. The finding that the alkaline transients recorded in s. radiatum
are enhanced upon inhibition of CA o even when evoked by brief high-frequency
stimulus trains is in agreement with the preponderance of excitatory synapses
in the dendritic region.
Received 29 September 1994; accepted in final form 23 March 1995.
APS Manuscript Number J611-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 25 April 1995.