Introduction: Botulinum neurotoxin “BoNT” is a potent toxin which causes muscle paralysis. However, it is widely used in management of muscle-related disorders such as masseter hypertrophy. Intra-masseteric BoNT injection may predispose certain adverse effects on the adjacent tissues including parotid gland. The exact mechanisms as well as the reversibility of these effects on parotid glands still have been not fully elucidated. Purpose: To assess, histologically, histochemically and immunohistochemically, the effects of intra-masseteric botulinum neurotoxin injection on the parotid glands of albino rats. Materials & Methods: Fifteen adult male albino rats were divided into 2 groups: control group (5 rats received intra-masseteric injection of sodium chloride), and BoNT group (10 rats received intra-masseteric injection of 2.5 units BoNT). The latter group was divided equally into 2 subgroups according to the time of rats’ scarification (3 days and 14 days after injection). The parotid glands were studied by histomorphometric analysis, Periodic Acid Schiff reaction and immunohistochemical expression of neuron specific enolase marker. Results: BoNT injected rats showed acinar vaculations and decreased size, blood vessels congestion and mild fibrosis in both durations. 14 days subgroup displayed inflammatory cell infiltrate. BoNT group showed less intense reaction to PAS was observed in addition to under-expression of neuron specific enolase which was more severe in the rats sacrificed after 3 days. Conclusion: BoNT has an adverse effect on parenchymal and connective tissue elements of parotid salivary gland which is slightly modified by time factor.
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