VARIATION IN GABA MINI AMPLITUDE IS THE CONSEQUENCE OF VARIATION IN TRANSMITTER CONCENTRATION

Miniature postsynaptic currents (minis) in cultured retinal amacrine c ells, as in other central neurons, show large variations in amplitude. To understand the origin of this variability, we have exploited a nov el form of synapse in which pre- and postsynaptic receptors sample the same quantum of transmitter. At these synapses, mini amplitudes measu red simultaneously in the 2 cells show a strong correlation, accountin g for, on average, more than half of the variance in amplitude. Two pi eces of evidence support the conclusion that variations in the amount of transmitter in different quanta underlie this correlation. First, d iazepam, which enhances GABA binding, increases mini amplitude, implyi ng therefore that transmitter concentration is not saturating. Second, we show that amplitude distributions from all cells, even those with a small number of release sites, have the same shape, implying that mo st or all variance is intrinsic to each release site.