Activity-dependent formation of perforated synapses in cultured hippocampal neurons

The study investigated the formation of perforated synapses in rat hippocam pal cell cultures. Perforated synapses are defined by their discontinuous p ostsynaptic densities (PSDs) and are believed to occur in parallel with cha nges in synaptic activity and possibly also synaptic efficacy. Several in v ivo studies have demonstrated an increase in the frequency of perforated sy napses induced by development and environmental stimulation as well as long -term potentiation (LTP). Also in in vitro brain slices, LTP was associated with an elevated number of perforated spine synapses. Our study demonstrat ed for the first time that the formation of perforated synapses can be indu ced by a short-term increase in spontaneous neural activity in a hippocampa l cell culture model. Stimulation with the GABA(A)-antagonist picrotoxin (P TX) induced a significant increase in the percentage of perforated synapses . This strong increase was blocked when APV was added together with PTX, in dicating that the formation of perforated synapses depended on the activati on of NMDA receptors. We also showed that inhibition of the tissue type pla sminogen activator (tPA-stop/PAI-1) significantly interfered with the activ ity-induced increase in perforated synapses. This implies that the proteoly tic activities of tPA might be involved in steps which are downstream from the NMDA receptor-mediated synaptic plasticity leading to structural change s at synaptic contacts. In contrast, even long-term inhibition of electrica l network activity by tetrodotoxin had no effect on the number of perforate d synapses, but almost completely abolished the formation of spine synapses . These results indicate that a short-term increase in neural activity via NMDA receptors and a proteolytic cascade involving tPA lead to the formatio n of perforated synapses.