SYNAPTIC PLASTICITY IN DISSOCIATED HIPPOCAMPAL CULTURES - PRESYNAPTICAND POSTSYNAPTIC CONTRIBUTIONS

The distinction between pre- or postsynaptic expression of synaptic pl asticity is difficult to make, unless the postsynaptic receptors can b e investigated in isolation. We have studied single synaptic contacts in dissociated cultures of rat hippocampus. The reaction of postsynapt ic receptor assemblies to the induction of synaptic plasticity was mea sured and compared with changes in the rate of spontaneous miniature e xcitatory postsynaptic currents (mEPSCs), which can reflect changes in the transmitter release mechanism. The response of a receptor assembl y to locally applied exogenous glutamate was measured before and after synchronized application of glutamate and a train of postsynaptic dep olarizations ('pairing'). Pairing induced a variety of changes: (i) th e majority of the receptor assemblies showed no change in their respon se to glutamate before and after pairing; (ii) the postsynaptic curren t due to exogenous glutamate showed a rapid increase in five out of 26 cases. This was not due to changes in the single channel conductance; (iii) the rate of mEPSCs increased, if it had previously been below 2 5 Hz; (iv) the rate of mEPSCs decreased, if it had previously been abo ve 25 Hz. Effects 2 and 3 were blocked by antagonists of NMDA receptor s. These findings provide direct evidence for an increase of the numbe r of glutamate receptors at a subset of the investigated postsynaptic sites during synaptic potentiation.