Individual rat hippocampal neurons, grown in isolation from other neurons on small spots of permissive substrate, were studied in order to characterize the electrical properties of the synapses that such cells formed with themselves (autapses). Excitatory (probably glutamatergic) or inhibitory (probably type A y-aminobutyratergic) autapses were frequently found. Excitatory autaptic currents reversed near the potential expected for monovalent cations were blocked by the glutamatergic antagonist kynurenic acid, and possessed a slow component with the pharmacological profile of N-methyl-D-aspartate-type channels. These currents also exhibited trialto-trial statistical fluctuations in their amplitudes, this being well-described by quantal analysis. Inhibitory autaptic currents reversed at hyperpolarized potentials, as expected for chloride-permeable pores and were blocked by picrotoxin, a type A r-aminobutyric receptor antagonist. It is concluded that autaptic currents in culture are identical to those found at synapses.Although neurons have only infrequently been reported to form synapses on their own processes (1-6), such "autaptic" connections-to use the term coined by van der Loos and Glaser (3)-may not be uncommon. For example, van der Loos and Glaser (3) reported that, in Golgi preparations, 6 of 12 well-impregnated occipital cortical pyramidal cells exhibited autapses, and Karabelas and Purpura (6) found that 2 of 14 horseradish peroxidase-filled neurons in substantia nigra formed synapses on themselves, one with at least 20 boutons. Furthermore, these estimates of autapse frequency could be underestimates because part of the dendritic trees of the neurons examined was outside the plane of the section.The possible functional significance of such autaptic loops remains obscure because the operation of autapses has received even less study than their existence. van der Loos and Glaser (3) speculated that autapses might serve a selfinhibitory function. Physiological evidence for the existence of autapses has also been reported in cultured chicken spinal ganglion cells (2) and in sympathetic ganglion cells cocultured with myocytes (4). However, it is not known whether glutamatergic and y-aminobutyratergic (GABAergic) central neurons can form functional autapses or how the operation of functional autapses might compare with that of synapses on the same cell type.In the course of experiments designed for other purposes, we have had the opportunity to study autaptic circuits formed by rat hippocampal neurons maintained in culture. We find that when a neuron's axon is constrained to grow within the region of the cell's own dendritic tree, abundant autaptic connections typically form. The operation of these autapses, both excitatory and inhibitory, seems indistinguishable from that of synapses in the same culture system. We note that such autaptic circuits offer an unusually homogeneous population of synapses for physiological investigations. MATERIALS AND METHODSCell Culture. The method of Furshpan and coll...

Experiments analysing the statistical properties of synaptic transmission, before and after the induction of long-term potentiation (LTP), suggest that expression of LTP largely arises in a presynaptic mechanism--an increased probability of transmitter release.

Spontaneous synaptic events can be difficult to detect when their amplitudes are close to the background noise level. Here we report a sensitive new technique for automatic detection of small asynchronous events. A waveform with the time course of a typical synaptic event (a template) is slid along the current or voltage trace and optimally scaled to fit the data at each position. A detection criterion is calculated based on the optimum scaling factor and the quality of the fit. An event is detected when this criterion crosses a threshold level. The algorithm automatically compensates for changes in recording noise. The sensitivity and selectivity of the method were tested using real and simulated data, and the influence of the template parameter settings was investigated. Its performance was comparable to that obtained by visual event detection, and it was more sensitive than previously described threshold detection techniques. Under typical recording conditions, all fast synaptic events with amplitudes of at least three times the noise standard deviation (3 sigma) could be detected, as could 75% of events with amplitudes of 2 sigma. The scaled template technique is implemented within a commercial data analysis application and can be applied to many standard electrophysiological data file formats.