Pharmacological evidence for two types of postsynaptic glycinergic receptors on the Mauthner cell of 52-h-old zebrafish larvae.

The presence of homooligomeric and heterooligomeric glycine receptors (GlyRs) on the Mauthner (M) cell in the isolated medulla of 52-h-old zebrafish larvae was investigated by analysis of the effects of picrotoxin on glycine-gated channels and on glycinergic miniature inhibitory postsynaptic currents (mIPSCs). Two functionally different GlyRs have been previously described on the M cell. The effects of picrotoxin on these two GlyRs were first analyzed by measuring the relative change in their total open probability (NP(o)) 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 NP(o) of the GlyR with multiple subconductance levels. On this GlyR, picrotoxin decreased in a concentration-dependent manner the occurrence of the 80- to 86-pS substate (median inhibiting concentration = 0.89 microM) and had no apparent effect on the 40- to 46-pS opening probability. Opening frequency and the mean open times of the 80- to 88-pS main conductance state were reduced in the presence of 10 microM picrotoxin, but their relative weight remained unchanged. These effects of picrotoxin were not voltage dependent. Picrotoxin also modified 40- to 46-pS kinetics. At 100 microM, picrotoxin evoked voltage-independent flickering during channel openings. Short and long mean open times were significantly decreased, whereas the relative proportion of long mean open times was increased. The medium closed time was decreased, whereas medium burst duration was increased. The burst frequency remained unchanged. Spontaneous glycinergic mIPSCs were recorded in the presence of 1 microM tetrodotoxin + 25 microM bicuculline (holding potential = -50 mV). Application of 10 microM picrotoxin did not change the frequency of the synaptic activity, whereas it decreased the amplitude of large mIPSCs. No effect was observed on the time to peak (0.8 ms) or the mean decay time constant (tau(d) = 7.7 ms). Increasing picrotoxin concentration to 100 microM resulted in a decrease of mIPSC frequency (35.6%), amplitude (39.8%), and tau(d) (from 7.7 to 5 ms). These results suggest that these two functionally different GlyRs correspond to alpha1 homooligomeric-like and alpha1/beta-heterooligomeric-like GlyRs, 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.