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 characteristic features, including a unique form of long-term potentiation that does not require activation of the N-methyl-D-aspartate receptor by glutamate, a complex postsynaptic architecture, and sprouting in response 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 we have enriched messy fiber synapses by increasing the ratio of granule-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 connections are glutamatergic, inhibited by dynorphin A, and exhibit N-methly-D-aspartate-independent long-term potentiation. Thus, the simplicity and experimental accessibility of this enriched in vitro messy fiber pathway provides a new perspective for studying nonassociative plasticity in the mammalian central nervous system.