Structure and function of aberrant perforant path synapses in the hippocampus of neonatally X-irradiated rats.

We have examined the functional and morphological characteristics of synapses made by perforant path fibres projecting from the entorhinal cortex to the medial zone of the dentate area in adult rats which had been X-irradiated at birth, a procedure which prevents the proliferation of granule cells in that zone. We provide ultrastructural evidence that perforant path fibres in this region make synaptic contact with dendritic spines, and demonstrate that these synapses generate functional responses when the perforant path is stimulated. The evidence suggests that aberrant functional connections are made with the elongated basal dendrites of pyramidal cells.