ALL-OR-NONE EXCITATORY POSTSYNAPTIC POTENTIALS IN THE RAT VISUAL-CORTEX

Intracellular recordings were obtained from supragranular neurons in s lices of the rat visual cortex. In similar to 25% of the cells large ( 0.5-1.6 mV) excitatory postsynaptic potentials (EPSPs) of constant amp litude were observed after minimal, presumably single-fibre stimulatio n. The amplitude variance of these large EPSPs was surprisingly small and within the range of the variance of the noise. These EPSPs could b e reduced in amplitude by paired-pulse and low-frequency stimulation o r by raising extracellular Mg2+ concentration. Reduced EPSPs could eit her continue to behave as all-or-none responses, or they could fluctua te between several amplitude levels. Conversely, responses where the a mplitude fluctuated from trial to trial under control conditions could be converted into large all-or-none responses by paired-pulse facilit ation. This indicates that the large all-or-none EPSPs were composed o f several subunits, probably reflecting the action of several differen t release sites. It is concluded that these release sites are either i ndependent and operate with a probability close to 1 or, if operating with a lower probability, are coordinated by a mechanism which synchro nizes release. Several observations suggest that release probabilities can switch from values close to 1 to 0 with repetitive stimulation or high Mg2+ concentration. Thus, a substantial fraction of single-fibre inputs to supragranular cells possess synapses which operate with hig h synaptic efficiency and extremely low variance under control conditi ons but can undergo drastic changes in efficacy when release probabili ties are interfered with. Such modifications of release probability co uld serve as an effective mechanism to regulate the gain of synaptic t ransmission.