Facial nerve injury-associated hippocampal microglial activation

Introduction: Facial nerve injury induces changes in hippocampal long-term synaptic plasticity and affects both object recognition memory and spatial memory consolidation (i.e., hippocampus-dependent tasks). Although facial nerve injury-associated microglial activation has been described regarding the primary motor cortex, it has not been ascertained whether something similar occurs in the hippocampus. Peripheral nerve injuryassociated microglial changes in hippocampal tissue could explain neuronal changes in the contralateral hippocampus. Objective: To characterize the effect of unilateral facial nerve injury on microglial proliferation and activation in the contralateral hippocampus. Materials and methods. Immunohistochemical experiments detected microglial cells in the hippocampal tissue of rats that had undergone facial nerve injury. The animals were sacrificed at specific times after injury to evaluate hippocampal microglial cell proliferation (cell density) and activation (cell area); sham-operated animals were compared to lesioned animals sacrificed 1, 3, 7, 21, or 35 days after injury. Results: Facial nerve-injured rats’ hippocampal microglial cells proliferated and adopted an activated phenotype 3- to 21-days post-lesion. Such modifications were transient since the microglial cells returned to their resting state five weeks after injury, despite the injury’s irreversible nature. Conclusions: Facial nerve injury causes the transient proliferation and activation of microglial cells in the hippocampus. This finding might partly explain the morphological and electrophysiological changes described for CA1 pyramidal neurons and the impairment of spatial memory consolidation which has previously been observed in facial nerve-injured rats.