Reorganization of input synapses of parvalbumin-containing neurons in the rat fascia dentata following entorhinal lesion.

Removal of ipsilateral entorhinal afferents to the fascia dentata results in a retraction of postsynaptic parvalbumin (PARV)-containing dendrites of GABAergic neurons from the outer molecular layer. This study analyzes the reorganization of input synapses of these identified neurons following deafferentation. The density of synaptic input (total length of synaptic membrane specializations) of PARV-immunostained dendrites increased by 34% in the outer molecular layer of the fascia dentata 8 days, and by 21% 55 days following lesion when compared with unoperated controls. Eight days postlesion this increase was mainly due to an enlargement of synaptic membrane specializations of single terminals whereas 55 days after the lesion there was an increase in the number of synapses on the identified dendrites. Our results suggest expansion of terminals of remaining afferent systems (i.e. commissural fibers) in the early postlesional period and reactive synaptogenesis (i.e. de novo formation of synaptic contacts) on PARV-positive dendrites after long survival time. This increased innervation may be of functional importance as it might compensate for the reduction of the receptive field of the PARV-positive, supposedly inhibitory neurons in the postlesional dentate gyrus.