Enhancement of whole-cell synaptic currents by low osmolarity and by low
[NaCl] in rat hippocampal slices.
Rong Huang, Daniel F. Bossut and George G. Somjen.
Departments of Cell Biology and Neurobiology, Duke University Medical
Center, Durham, NC 27710.
APStracts 4:0046N, 1997.
ABSTRACT
We recorded whole-cell currents of patch-clamped neurons in stratum pyramidale
of CA1 region of rat hippocampal tissue slices. Synaptic currents were evoked
by orthodromic stimulation while holding potential of the neuron was varied
from hyperpolarized to depolarized levels. Extracellular osmolarity (o) was
lowered by superfusion with artificial cerebrospinal fluid (ACSF) in which
NaCl concentration was reduced. The effect of low extracellular NaCl was
tested in additional trials in which NaCl was substituted by isosmolar
fructose. Both, lowering of ão and isosmotic lowering of [NaCl]o caused
reversible increase of excitatory postsynaptic currents (EPSCs). The effect of
lowering ão was concentration-dependent, and it was significantly stronger
than the effect of equivalent isosmotic lowering of [NaCl]o. Inhibitory
postsynaptic currents (IPSCs) also increased in many but not in all cases.
Lowering of ão caused a prolongation of the time constant of relaxation of the
capacitive charging current induced by small hyperpolarizing voltage steps. A
virtual input capacitance, calculated by dividing this time constant by the
input resistance, increased during hypotonic exposure. Isosmotic lowering of
[NaCl]o had no effect on time constant or input capacitance. Depolarizing
voltage commands evoked spike-like inward currents presumably representing
Na+-dependent action potentials generated outside the voltage-clamped region
of the cell. These current spikes became smaller in low ão and in low [NaCl]o.
Broader, voltage-dependent, presumably Ca2+-mediated inward currents became
more prominent during hypotonic exposure. We conclude that lowering of [NaCl]o
causes enhancement of excitatory synaptic transmission. Transmission may be
facilitated by the uptake of Ca2+ into presynaptic terminals as well as into
postsynaptic target neurons, induced by the low [NaCl]o. Lowering of o
enhances synaptic transmission more than does a corresponding isosmotic
lowering of [NaCl]. The excess increase recorded from the cell soma in low o
may in part be due to changing electrotonic length caused by the swelling of
dendrites.
Received 21 August 1996; accepted in final form 6 January 1997.
APS Manuscript Number J675-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 5 February 1997