VARIANCE ANALYSIS OF GAMMA-AMINOBUTYRIC-ACID (GABA)-ERGIC INHIBITORY POSTSYNAPTIC CURRENTS FROM MELANOTROPES OF XENOPUS-LAEVIS

We have studied the variance in the decay of large spontaneous gamma-aminobutyric acid (GABA)-ergic inhibitory postsynaptic currents (IPSCs) in melanotropes of Xenopus laevis to obtain information about the number of GABA(A) receptor channels that bind GABA during the IPSCs. The average decay of the IPSCs is well described by the sum of two exponential functions. This suggests that a three-state Markov model is sufficient to describe the decay phase, with one of the three states being an absorbing state, entered when GABA dissociates from the GABA(A) receptor. We have compared the variance in the decay of large spontaneous IPSCs with the variance calculated for two different three-state models: a model with one open state, one dosed state, and one absorbing state (I), and a model with two open states and one absorbing state (II). The data were better described by the more efficient model II. This suggests that the efficacy of GABA at synaptic GABA(A) receptor channels is high and that only a small number of channels are involved in generating the GABA-ergic IPSCs.