Differential regulation of glial reactions in the central facial tract and the facial nucleus after facial neurorrhaphy

Abstract

We previously reported that perineuronal astrocytic and microglial reactions are drastically upregulated in the facial nucleus after facial axotomy at the brain stem surface or the stylomastoid foramen. Furthermore, periaxonal astrocytic and microglial reactions develop retrogradely in the central facial tract which contains proximal facial axons in the brain stem. Because reconnection of interrupted peripheral nerve by microsurgical suture is a common clinical practice, the aim of this study was to investigate the spatiotemporal patterns of glial reactions in the central facial tract and the facial nucleus after facial neurorrhaphy. Here, we show immunofluorescent and immunohistochemical evidence that facial neurorrhaphy at the stylomastoid foramen largely prevented axotomy-induced astrocytic and microglial activation in the central facial tract. In contrast, glial reactions in the facial nucleus were still highly elevated after facial neurorrhaphy. Microglial and astrocytic processes were observed to ensheath the facial motoneurons in the facial nucleus. Nevertheless, the transformation of ramified to amoeboid shape of microglia, occurring at 10 weeks after facial axotomy, was not seen after neurorrhaphy. We further examined the effect of N-nitro-l-arginine methyl ester (L-NAME), an inhibitor of nitric oxide synthase (NOS), on glial reactions after neurorrhaphy. Western blot analyses demonstrate that inhibition of nitric oxide (NO) production significantly reduced microglial but not astrocytic reaction in the facial nucleus after neurorrhaphy. Taken together, these results indicate that in contrast to the intense glial reactions in both the central facial tract and the facial nucleus after facial axotomy, glial reactions are differentially regulated in these two compartments after facial neurorrhaphy. NO is involved in the activation of microglia in the facial nucleus after facial neurorrhaphy.