PAIRED-PULSE MODULATION OF FAST EXCITATORY SYNAPTIC CURRENTS IN MICROCULTURES OF RAT HIPPOCAMPAL-NEURONS

1. Paired-pulse modulation of excitatory non-N-methyl-D-aspartate (non -NMDA) receptor-mediated autaptic currents and conventional monosynapt ic (interneuronal) excitatory postsynaptic currents (EPSCs) was invest igated in microcultures of rat hippocampal neurons, where polysynaptic influences are eliminated. 2. Most autaptic currents and EPSCs exhibi ted paired-pulse depression in response to paired stimuli. Depression was sensitive to the level of transmitter release, which was varied by manipulating extracellular Ca2+ and Mg2+ concentrations. Paired-pulse facilitation emerged in many cells at low levels of transmitter relea se. 3. Paired-pulse depression and facilitation could be differentiall y expressed at two distinct postsynaptic targets of a single presynapt ic cell, and the form of modulation was not dependent upon the transmi tter phenotype of the postsynaptic cell. 4. Paired-pulse depression re covered exponentially with a time constant of similar to 5 s, although in most neurons a much faster component of recovery was detected. Rec overy from paired-pulse facilitation was well described by a single ex ponential of 380 +/- 57 ms. 5. Under conditions of robust paired-pulse depression of evoked responses, spontaneous autaptic and postsynaptic currents (sEPSCs, presumed miniature EPSCs) occurred at an enhanced f requency immediately following evoked responses. The decay of the freq uency increase mirrored the time course of recovery from paired-pulse facilitation of evoked responses examined under conditions of reduced transmitter release. 6. Several lines of evidence suggested a large pr esynaptic component to paired-pulse depression. In eight out of nine c ells no depression in sEPSC amplitudes was detected following conditio ning stimulation. Simultaneously recorded glial glutamate uptake curre nts showed depression similar to neuronal evoked EPSCs. Finally, NMDA receptor mediated EPSC paired-pulse depression at positive potentials was similar to non-NMDA EPSC depression. 7. Neither adenosine nor glut amate feedback onto presynaptic receptors is likely to mediate paired- pulse depression, because neither competitive nor non-competitive inhi bitors of the actions of these agents diminished paired-pulse depressi on.