Quantal analysis of inhibitory synaptic transmission in the dentate gyrus of rat hippocampal slices: a patch‐clamp study.

Abstract

SUMMARY1. Synaptically connected neurones were identified in the granule cell layer of slices of 17-to 21-day-old rat hippocampus. Whole-cell current recording using the patch-clamp technique revealed synaptic currents ranging from less than 10 to 200 pA in symmetrical Cl-conditions, at a holding potential of -50 mV. These currents were blocked by 2 ftM-bicuculline, indicating that they result from the activation of postsynaptic y-aminobutyric acid receptor (GABAA-receptor) channels.2. Addition of tetrodotoxin (TTX, 1 1uM) resulted in the loss of most currents of more than 40 pA in amplitude. Currents which disappeared after TTX treatment were assumed to be the result of spontaneous presynaptic action potentials. The currents seen in the absence of TTX are referred to as spontaneously occurring inhibitory postsynaptic currents (IPSCs); those remaining in the presence of TTX were defined as miniature IPSCs. 3. Similar currents were observed when recording in the whole-cell configuration while extracellular stimulation was applied to a nearby neurone. These currents were also completely blocked by 2 /tM-bicuculline and by 0-5 JLm-TTX They were thus defined as stimulus-evoked IPSCs.4. The half rise time of both miniature and stimulus-evoked IPSCs was fast (<1Ims). The time course of decay of both miniature IPSCs and stimulus-evoked IPSCs could be well fitted with the sum of two exponentials. At a membrane potential of -50 mV, the mean decay time constants of the two components were 2-0+0-38 and 54-4+ 18 ms (mean+ S.D.) for miniature IPSCs (six cells) and 2-2+ 1-3 and 66 + 20 ms (three cells) for stimulus-evoked IPSCs.5. Stimulus-evoked IPSCs varied in amplitude from less than ten to hundreds of picoamperes. In eight of eleven cells histograms of IPSC amplitudes showed several clear peaks which, when fitted with the sum of Gaussian curves, were found to be equidistant. This is consistent with the view that stimulus-evoked IPSC amplitudes vary in a quantal fashion. The quantal size varied between 7 and 20 pA, at a membrane potential of -50 mV. 7. In one cell where the recording was stable for more than an hour, changing the membrane voltage from -50 to -120 mV increased the quantal size by a factor of 241, close to that expected if the current-voltage relation of IPSC peak amplitudes were linear.8. The peak of miniature IPSC amplitude histograms measured in the presence of 1 /sM-TTX was comparable with the quantal size of stimulus-evoked IPSCs.However, in all cells, a tail of larger amplitude miniature IPSCs was observed. The amplitudes in the tail in six of twelve cases were quantally distributed. 9. Single-channel currents activated by GABA, applied locally to outside-out patches isolated from the soma membrane of granule cells, indicated that GABAAreceptor channels had two conductance levels of 14 and 23 pS. Thus for IPSCs in hippocampal granule cells, one quantal current represents the simultaneous opening of less than thirty GABAA-receptor channels.10. The small number of postsynaptic channels mediating a quanta...

Nonsaturation of AMPA and NMDA receptors at hippocampal synapses

“…5). Finally, variations in the site of exocytosis of synaptic vesicles relative to the location of postsynaptic glutamate receptors could cause variable amounts of glutamate to be sensed by postsynaptic receptors (32,33).…”

Muscarinic receptor type 1 (M1) stimulation, probably through KCNQ/Kv7 channel closure, increases spontaneous GABA release at the dendrodendritic synapse in the mouse accessory olfactory bulb

“…We applied a method used by Edwards et al (1990; Asertine's analysis) to diagnose the occurrence of peaks in mIPSC amplitude histograms. The autocorrelation function (ACF) of the amplitude histograms was calculated using the expression: ACF(j) = {$i H(i)H(i+j)}/($i H(i)H(i)), where H(i) is the height of the i th bin which is centred at the amplitude value given by i " bin width.…”

Properties of GABAAReceptors Underlying Inhibitory Synaptic Currents in Neocortical Pyramidal Neurons

“…In "dinapses" of amacrine cells it has been suggested that GABA A receptors are not saturated, based on the observation that transmitter concentration is the major determinant of mIPSC amplitude (Frerking et al, 1995). On the other hand, the low quantal variance of evoked IPSCs (Edwards et al, 1990), nonstationary fluctuation analysis, and pharmacological approaches support the hypothesis of saturation in hippocampal granule cells (Otis and Mody, 1992;De Koninck and Mody, 1994). Although differences in the anatomy of the synapses, the amount of GABA released in the cleft, and the kinetic properties of the GABA A receptors may generate a variety of scenarios (Frerking and Wilson, 1996), it is clear that more experiments and measurement of cleft GABA concentration during synaptic transmission are needed to resolve this issue.…”