Abstract
Symptom onset in amyotrophic lateral sclerosis (ALS) may occur in the muscles of the limbs (spinal onset) or those of the head and neck (bulbar onset). Most preclinical studies have focused on spinal symptoms, despite the prevalence of and increased morbidity and mortality associated with bulbar disease. We measured lick rhythm and tongue force to evaluate bulbar disease in the SOD1-G93A rat model of familial ALS. Body weight and grip strength were measured concomitantly. Testing spanned the early (maturation), middle (pre-symptomatic), and late (symptomatic and end-stage) phases of the disease. We measured a persistent tongue motility deficit that became apparent in the early phase of the disease, providing behavioral evidence of bulbar pathology. At end-stage, however, cytochrome oxidase (CO) activity was normal in the hypoglossal nucleus, and in the tongue, neuromuscular innervation, citrate synthase (CS) protein levels and activity, and uncoupling protein 3 (UCP3) protein levels remained unchanged. Interestingly, significant denervation and atrophy were evident in the end-stage sternomastoid muscle, providing peripheral anatomical evidence of bulbar pathology. Changes in body weight and grip strength occurred in the late phase of the disease. Extensive atrophy and denervation were observed in the end-stage gastrocnemius muscle. In contrast to our findings in the tongue, CS protein levels were decreased in the extensor digitorum longus (EDL) and soleus, although CS activity was maintained or increased. UCP3 protein was decreased also in the EDL. These data provide evidence of differential effects in muscles that were more or less affected by disease.