Pharmacological evidence for two types of postsynaptic glycinergic receptors on the Mauthner-cell of 52 hour-old zebrafish larvae. P. Legendre. Laboratoire de Biologie Cellulaire et Mol‚culaire (INSERM U 261), Departement des Biotechnologies, Institut Pasteur, 25 rue du Dr. Roux, 75015 Paris, France.
APStracts 4:0027N, 1997.
ABSTRACT
The presence of homo-oligomeric and hetero-oligomeric glycine receptors (GlyR) on the Mauthner cell (M-cell) in the isolated medulla of 52-h-old zebrafish larvae was investigated by the analysis of the effects of picrotoxin on glycine-gated channels and on miniature glycinergic inhibitory postsynaptic currents (mIPSCs). Two functionally different GlyR have been previously described on M-cell. The effects of picrotoxin on these two GlyRs was first analysed by measuring the relative change in their total open probability (NPo) with picrotoxin concentration. Picrotoxin had no significant effect on the glycine channel with a single conductance level of 40-46 pS. In contrast, picrotoxin application decreased the NPo of the GlyR with multiple subconductance levels. On this GlyR, picrotoxin decreased in a concentration- dependent manner the occurrence of 80-86 pS substate (IC50 = 0.89 ĉM) and had no apparent effect on the 40-46 pS opening probability. Opening frequency and the mean open times of the 80-88 pS main conductance state were reduced in the presence of 10 ĉM picrotoxin but their relative weight remained unchanged. These effects of picrotoxin were not voltage-dependent. Picrotoxin also modified 40-46 pS kinetics. 100 ĉM Picrotoxin evoked voltage-independent flickering during channel openings. Their short and long mean open times were significantly decreased while the relative proportion of long mean open times was increased. The medium closed time was decreased while medium burst duration was increased. The burst frequency remained unchanged. Spontaneous glycinergic mIPSCs were recorded in the presence of 1 ĉM TTX + 25 ĉM bicuculline (Vh = -50mV). Application of 10 ĉM picrotoxin did not change the frequency of the synaptic activity while it decreased the amplitude of large mIPSCs. No effect was observed on the time-to-peak (0.8 ms) or the mean decay time constants (td = 7.7 ms). Increasing picrotoxin concentration to 100 ĉM resulted in a decrease of mIPSC frequency (35.6 %), amplitude (39.8 %) and td (from 7.7 ms to 5 ms). Our results suggest that these two functionally different GlyRs correspond to à1 homo-oligomeric-like and à1/á hetero- oligomeric-like GlyR and that both are synaptically activated. Variation in the proportions of GlyR subtypes from one synapse to another could partly account for the broad amplitude distribution of mIPSCs recorded from the zebrafish M-cell 

Received 28 October 1996; accepted in final form 30 December 1996.
APS Manuscript Number J857-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 24 January 1997