Ultrastructural characterization of identified cholinergic neurons transplanted to the hippocampal formation of the rat

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Abstract
The ultrastructural features of cholinergic neurons transplanted to the rat hippocampal formation were studied by using a monoclonal antibody to choline acetyltransferase (ChAT). Septal cell suspensions were prepared from E‐18 rat embryos and injected into the hippocampus of host rats that had been previously subjected to a bilateral transection of the fimbria‐fornix. Rats with fimbria‐fornix lesions alone and unoperated rats served as controls and were examined to characterize the native hippocampal cholinergic system. Both unoperated controls and rats with fimbria‐fornix lesions showed a sparse population of intrinsic ChAT‐immunoreactive neurons that were most numerous in the subgranular zone, the hilus fascia dentata, and near the hippocampal fissure. ChAT‐positive terminals from controls formed synapses on dendritic structures that were primarily symmetrical. ChAT‐positive dendrites in controls received synaptic input from nonimmunoreactive axon terminals. In rats with septal transplants, ChAT‐immunoreactive transplant neurons were found that were either bipolar or multipolar. Axons of transplanted neurons were unmyelinated and arose either from the cell body or a primary dendritic process where they gave off numerous collaterals. Terminals from transplant neurons formed synapses with many nonimmunoreactive neurons. In transplant animals, two main targets of ChAT‐immunoreactive terminals were identified: (1) The great majority of synapses were symmetrical junctions with dendritic spines and shafts. (2) A number of terminals were found that appeared to be juxtaposed to nonimmunoreactive axon terminals, possibly forming symmetrical axo‐axonic connections. In contrast, such axo‐axonic contacts were not observed in the controls. It is concluded that transplanted cholinergic neurons may reinnervate the host hippocampus; however, this reinnervation is different from what is seen in the intact hippocampal formation.