LARGE-AMPLITUDE VARIABILITY OF GABAERGIC IPSCS IN MELANOTROPES FROM XENOPUS-LAEVIS - EVIDENCE THAT QUANTAL SIZE DIFFERS BETWEEN SYNAPSES

1. We made in situ whole-cell recordings from melanotropes in the intermediate lobe of the pituitary gland of Xenopua laevis. Melanotropes received spontaneous synaptic inputs that had a fast rise time and a much slower decay. These inputs were GABAergic inhibitory postsynaptic currents(IPSCs): they followed the reversal potential for chloride ions and they were blocked by the gamma-aminobutyric acid-A (GABA,) receptor antagonist bicuculline. 2. Because of the very low baseline noise it was possible to see discrete levels in the tails of IPSCs that corresponded to the opening of one or more synaptic GABA(A) receptor channels. ''All-points'' histograms of the IPSCs showed that the chord conductance of the channels in the tails of the IPSCs was 21.6 +/- 0.6 pS (mean +/- SE, n = 6). 3. The amplitudes of the spontaneous IPSCs were very variable, ranging from 3 to 390 pA at a holding potential of -80 mV. The average of the median amplitudes was -67.5 +/- 5.9 pA (n = 28). The amplitude distributions of the IPSCs were well described by the sum of two lognormal distributions with large SDs. The average of the means of the first lognormal distribution was 27.8 +/- 5.3 pA(n = 10); the average of the SDs was 24.7 +/- 8.1 pA. For the second lognormal distribution these values were 87.0 +/- 13.4 and 33.7 +/- 7.4 pA. An average of 41.8 +/- 6.9% ofthe IPSCs originated from the first lognormal distribution. 4. The large variability in the amplitudes of spontaneous IPSCs was not the result of presynaptic action potentials because it was not reduced by tetrodotoxin (TTX), Ca2+-free extracellular solution, or the combined application of TTX and Mn2+ 5. The time course of the IPSCs from the first and the second lognormal distributions were very similar: averages of the median 20- to 80% rise times were 585 +/- 64 and 488 +/- 28 mu s, respectively (n = X), whereas the decays were well described by the sum of two exponential functions, with fast time constants of 8.9 +/- 1.1 (n = 7) and 9.3 +/- 3.3 ms and slow time constants of 29.5 +/- 3.3 and 3 1.7 +/- 2.6 ms, respectively 6. The decay of the IPSCs was voltage dependent; it was similar to 3 times slower at a holding potential of +40 mV than at -80 mV (n = 5). Desensitization of the GABA(A) receptor does not seem to play a role in the decay of the IPSCs because in outside-out patches this desensitization was much slower (1.3 +/- 0.3 s, n = 6) than the decay of the IPSCs and not dependent on voltage. 7. Melanotropes from animals that had been adapted to a black background for greater than or equal to 3 wk had larger whole-cell capacitances (8.6 +/- 1.1 pF, n = 36) than cells from white-adapted animals (4.7 +/- 0.4 pF, n = 23), which indicates a more than twofold increase in cell volume. During most experiments the frequency of the spontaneous IPSCs remained constant. Average intervals were 9.8 +/- 2.9 s (n = 17) in cells from black-adapted animals and 2.8 +/- 0.9 s(n 11) in cells from white-adapted animals; however, this difference was not significant. No clear differences were observed between the amplitudes and time courses of the IPSCs in white-adapted versus black-adapted animals. 8. Stimulus-evoked IPSCs were blocked by TTX (1 mu M) and by bicuculIine (20 mu M), a GABA, receptor antagonist, which means that they were evoked by action potentials in the afferent GABAergic fibers. The stimulus-evoked IPSCs had sharp thresholds, their average amplitudes did not increase with higher stimulation intensities, and their delays were well described by a single Gaussian distribution, suggesting that a single fiber was stimulated. 9. The probability of release was dependent on the extracellular Ca2+ concen