DEVELOPMENT OF MOSSY FIBER SYNAPSES IN HIPPOCAMPAL SLICE CULTURE

The messy fiber synaptogenesis has been studied in hippocampal slice c ultures. In vivo messy fiber terminals contact the thorny excrescences of CA3 pyramidal neurons over a restricted portion, i.e. the proximal part of the apical dendrite. In organotypic cultures messy fibers exp and their terminal field and invade the infrapyramidal area of the CA3 region and the supragranular layer of the dentate gyrus. Newly formed messy fiber synapses in CA3 region were examined, through electron mi croscopy, in cultures taken at various time intervals. The main events of the formation of newly formed messy fiber synapses can be summariz ed as follows. During the first week following explantation messy fibe r axons contact the dendritic shaft of the pyramidal dendrite and esta blish both symmetrical and asymmetrical contacts. Subsequent modificat ions occur in the postsynaptic portion facing the messy fiber bouton: (i) a massive accumulation of polyribosomes and coated vesicles in the subsynaptic cytoplasm; (ii) undulations of the plasma membrane; (iii) disappearance of neurotubules at postsynaptic sites and appearance of a fine network of filamentous material. Later on in culture, complex giant spines invaginate within the synaptic bouton. In conclusion this study shows that CA3 pyramidal neurons following deafferentation reta in the capacity to form thorny excrescences, when contacted by messy f ibers. Moreover these results suggest a crucial role for messy fibers to induce the formation of thorny excrescences in an heterotopic local ization, i.e. over the basilar dendrites of CA3 pyramidal neurons.