Voltage Dependence of Conductance Changes Evoked by Glycine Release in the
Zebrafish Brain.
Legendre P., H. Korn.
Laboratoire de Neurobiologie Cellulaire, Departement des Biotechnologies,
Institut Pasteur, 75015 Paris, France.
APStracts 2:0016N, 1995.
SUMMARY AND CONCLUSIONS
1. The kinetics and mechanisms underlying the voltage dependence of inhibitory
postsynaptic currents (IPSCs) recorded in the Mauthner cell (M cell) were
investigated in the isolated medulla of 52-h-old Zebrafish larvae, with the
use of whole cell and outside-out patch-clamp recordings. 2. Spontaneous
miniature IPSCs (mIPSCs) were recorded in the presence of 10 6 M tetrodotoxin
(TTX), 10 mM MgCl 2, and 0.1 mM [CaCl 2 ] o . Depolarizing the cell from 50
to +50 mV did not evoke any significant change in the distribution of mIPSC
amplitudes, whereas synaptic currents were prolonged at positive voltages. The
average decay time constant was increased twofold at +50 mV. 3. The voltage
dependence of the kinetics of glycine-activated channels was first
investigated during whole cell recording experiments. Currents evoked by
voltage steps in the presence of glycine (50 [mu]M) were compared with those
obtained without glycine. The increase in chloride conductance (g Cl ) evoked
by glycine was time and voltage dependent. Inactivation and reactivation of
the chloride current were observed during voltage pulses from 0 to 50 mV and
from 50 to 0 mV, respectively, and they occurred with similar time constants
(2 3 s). During glycine application, voltage-ramp analysis revealed a shift in
the reversal potential ( E Cl ) occurring at all [Cl ] i tested. 4. The
basis of the voltage sensitivity of glycine-evoked g Cl was first analyzed by
measuring the relative changes in the open probability ( P o ) of glycine-
activated channels with voltage. This study focused on patches displaying a
single conductance level of 40 pS. P o increased steadily from 60 to 0 mV and
then remained stable for higher holding potential values. As for g Cl,
measured during whole cell recordings, changes in P o with voltage were not
instantaneous. 5. The relative contribution of opening frequency and mean open
time and/or burst duration to the voltage dependence of NP o were analyzed by
comparing closed time and burst duration histograms at 50 and +50 mV. Three
mean burst durations (0.6, 3, and 30 ms; V h = 50 mV) characterized glycine
channel activity in the M cell. Short gaps (0.6 ms) within a burst and a short
burst component (0.9 ms) were voltage independent, whereas medium ( b2 ) and
long bursts ( b3 ) were lengthened when the patch was depolarized to +50 mV.
At positive voltages, burst frequency was also increased, whereas the relative
proportion of b1, b2, and b3 remained unchanged. 6. Slow and fast
changes in channel kinetics with membrane potential suggest a complex channel
behavior. A model is proposed in which an inactivated state is linked to the
liganded closed state with voltage-dependent inactivation and closing rate
constants. 7. Our results suggest that a change in the mean open time of the
glycine-gated channel essentially accounts for voltage-dependent properties of
the inhibitory synaptic currents. Voltage-dependent desensitization will only
occur during long-lasting epileptic activity of the inhibitory presynaptic
cells.
Received 9 September 1994; accepted in final form 25 January 1995.
APS Manuscript Number J565-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 3 April 1995.