Neuronal survival and glial reactions after recurrent laryngeal nerve resection in the rat

Objectives/Hypothesis: Complete axonal injury to the recurrent laryngeal nerve (RLN) leads to permanent loss of coordinated function of the intrinsic muscles of the larynx. The aim of the present study was to investigate retrograde reactions, neuronal survival, and glial reactions in the nucleus ambiguus after a distal resection of the RLN to evaluate the potential need for neuroprotective substances. Study Design and Methods: A segment of the left RLN was resected in 31 adult rats. Before sacrifice of the animals at 2 to 28 days postlesion, the motor neurons in the nucleus ambiguus were retrogradely traced by the use of Fluorogold. Brainstems were isolated and processed for neuron quantification and immunohistochemical analysis. Neuron counts were performed in the nucleus ambiguus on serial sections. Glial reactions were investigated in the nucleus ambiguus using immunohistochemistry. Results: No decrease in the number of motor neurons in the nucleus ambiguus could be demonstrated up to 1 month postlesion. Astroglia and microglia showed increased immunoreactivity at 7 to 14 days postinjury, followed by a slight decline in glial reaction. Microglia revealed no signs of transformation into macrophages during the study period, further indicating the absence of neuronal loss. Conclusions: Neuronal death does not occur within 1 month postlesion as a result of resection of the RLN in the adult rat, and neuroprotective substances should therefore be of minor value after RLN injury. Glial reactions appear in a similar fashion as after other peripheral nerve lesions not causing neuronal loss.