RECONSTITUTION OF THE HIPPOCAMPAL MOSSY FIBER AND ASSOCIATIONAL-COMMISSURAL PATHWAYS IN A NOVEL DISSOCIATED CELL-CULTURE SYSTEM

Synapses of the hippocampal messy fiber pathway exhibit several charac teristic features, including a unique form of long-term potentiation t hat does not require activation of the N-methyl-D-aspartate receptor b y glutamate, a complex postsynaptic architecture, and sprouting in res ponse to seizures. However, these connections have proven difficult to study in hippocampal slices because of their relative paucity (<0.4%) compared to commissural collateral synapses. To overcome this problem , we have developed a novel dissociated cell culture system in which w e have enriched messy fiber synapses by increasing the ratio of granul e-to-pyramidal cells. As in vivo, messy fiber connections are composed of large dynorphin A-positive varicosities contacting complex spines (but without a restricted localization). The elementary synaptic conne ctions are glutamatergic, inhibited by dynorphin A, and exhibit N-meth ly-D-aspartate-independent long-term potentiation. Thus, the simplicit y and experimental accessibility of this enriched in vitro messy fiber pathway provides a new perspective for studying nonassociative plasti city in the mammalian central nervous system.