Autapses and Networks of Hippocampal Neurons Exhibit Distinct Synaptic Transmission Phenotypes in the Absence of Synaptotagmin I

Synaptotagmin-I (syt-I) is required for rapid neurotransmitter release in mouse hippocampal neurons. However, contradictory results have been reported regarding evoked and spontaneous secretion from syt-I knock-out (KO) neurons. Here, we compared synaptic transmission in two different hippocampal neuron preparations: autaptic cultures in which a single isolated cell innervates itself, and dissociated mass cultures in which individual cells are innervated by neighboring cells. In autaptic cultures, the total extent of evoked release, size of readily releasable pool of synaptic vesicles, and release probability were unchanged in syt-I KO neurons. In contrast, in cultures containing multiple interconnected neurons, total evoked release, the number of docked vesicles, and release probability, were significantly reduced in syt-I KO neurons. Using a micronetwork system in which we varied the number of cells on an island, we found that the frequency of spontaneous synaptic vesicle fusion events (minis) was unchanged in syt-I KO neurons when two or fewer cells were present on an island. However, in micronetworks composed of three or more neurons, mini frequency was increased threefold to fivefold in syt-I KO neurons compared with wild type. Moreover, interneuronal synapses exhibited higher rates of spontaneous release than autaptic synapses. This higher rate was attributable to an increase in release probability because excitatory hippocampal neurons in micronetworks formed a set number of synapses per cell regardless of the number of connected neurons. Thus, aspects of synaptic transmission differ between autaptic and dissociated cultures, and the synaptic transmission phenotype, resulting from loss of syt-I, is dictated by the connectivity of neurons.