Glycine-Evoked Currents in Acutely Dissociated Neurons of the Guinea Pig
Ventral Cochlear Nucleus.
Harty, T. Patrick and Paul B. Manis.
Department of Otolaryngology Head and Neck Surgery, and The Center for
Hearing Sciences, The Johns Hopkins University School of Medicine, Baltimore,
MD 21205.
APStracts 2:0376N, 1995.
SUMMARY AND CONCLUSIONS
1. Glycine was applied to acutely dissociated neurons of the guinea pig
ventral cochlear nucleus (VCN) using iontophoresis. With approximately equal
chloride concentrations in the extra- and intracellular solutions (i.e., E Cl
= 0 mV), cells held at -60 mV responded with inward currents that were 1-10 nA
in amplitude, had rise times of approximately 50 ms and decayed to half of the
peak amplitude in 50-600 ms. More than 95% of cells with diameters greater
than 12 [mu]m responded to glycine. Response amplitude and area increased with
increasing duration of the iontophoretic pulse. Response amplitude saturated
at a pulse durations of 60-80 ms, whereas response area did not exhibit
saturation for pulse durations of 10-100 ms. 2. The glycine antagonist
strychnine was added to the extracellular solution at concentrations of 0.5-
500 nM to evaluate its effect on glycine-evoked responses. Strychnine produced
a 50% reduction in the response at a concentration of 12 nM and the dose-
response function had a limiting slope (Hill coefficient) of 1.4. 3. Changes
in glycine-evoked currents as a function of cell membrane potential were
examined in the presence of tetrodotoxin (TTX), tetraethylammonium chloride
(TEA) and 4-aminopyridine (4-AP), which block sodium and potassium
conductances activated by depolarization. Both the amplitude and the decay of
glycine-evoked currents displayed a voltage-dependence. Under conditions where
the glycine currents reversed at -35 mV, the amplitude of responses evoked at
membrane potentials of 0 mV were 2.3 times larger than responses evoked at -70
mV. The decay time constant at 0 mV was 1.49 times longer than at -70 ? mV. 4.
Acutely dissociated neurons of the VCN previously have been classified based
on the absence (type I) or presence (type II) of a low threshold outward
current. Type I cells fire repetitively in response to current pulses, whereas
type II cells fire transiently. Glycine-evoked responses were compared in
cells identified electrophysiologically as type I or type II based on
previously established criteria under voltage-clamp. The average amplitude of
responses recorded at a membrane potential of -70 mV was 1.1 nA and 1.3 nA for
type I and type II cells, respectively. The rise time of the glycine current
for the two groups of cells was similar (52 ms for type I and 57 ms for type
II), but the decay of currents to half-maximum amplitude following the offset
of the iontophoretic pulse was longer in type II cells (340 ms) than in type I
cells (173 ms). No differences between the two groups were noted with regard
to the outward rectification of peak currents or the voltage-dependence of
current decay. 5. The reversal potential of glycine-evoked responses was
determined in extracellular solutions with varying chloride concentrations.
The change in the glycine reversal potential (54 mV) for a 10x change in
chloride concentration was similar to the change in the chloride equilibrium
potential (58 mV) over the same range of extracellular chloride
concentrations. A similar result was obtained by maintaining the extracellular
chloride concentration constant and varying the chloride concentration in the
intracellular solution. Glycine-evoked responses were not affected by changes
in the potassium or sodium equilibrium potentials. The glycine receptors are
therefore principally permeable to chloride. 6. In the VCN, glycine-mediated
currents are readily evoked from the majority of larger neurons, indicating an
abundance of glycine receptors on the somata and proximal processes of these
neurons. The properties of glycine receptors in VCN and other areas of the
nervous system are generally similar. The voltage-dependence of glycine-evoked
currents implies that the inhibitory effectiveness of glycine receptors in VCN
increases nonlinearly with depolarization.
Received 16 February 1995; accepted in final form 6 December 1995.
APS Manuscript Number J113-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 23 December 95