Functional and morphological evidence of age‐related denervation in rat laryngeal muscles

Fast and constant laryngeal activity depends on reliable communication between motor neurons and laryngeal muscles at the neuromuscular junction (NMJ). It is unclear how aging alters NMJ function in the intrinsic laryngeal muscles. This study tested the hypothesis that two laryngeal muscles (thyroarytenoid, TA, and posterior cricoarytenoid, PCA) become functionally denervated with age. The sensitivity of TA and PCA from Fischer 344‐Brown Norway rats (6 and 30 mo of age) to the neuromuscular blocker tubocurarine (TC, 0 – 20 μM) was examined in vitro. The ratio of force response to nerve stimulation (NS, pulse duration 0.2 ms) to force response to direct muscle stimulation (MS, pulse duration, 1.0 ms) was used as a TC sensitivity index. NMJ density was examined with fluorescence microscopy in 10 μm‐thick laryngeal cryosections labeled with FITC‐conjugated α‐bungarotoxin, an NMJ marker. TA and PCA were significantly weaker at 30‐ compared to 6‐mo (~20% decrease in peak force with MS). Without TC, TA and PCA peak forces in response to NS and MS were not different within each age group. TC (≥0.5 μM) steadily decreased NS/MS in TA and PCA muscles from 6‐mo old rats; full NS block was achieved at 5.0 μM. Muscles from 30‐mo old rats were more sensitive to TC: NS/MS started to decrease at 0.1 μM TC and full NS block occurred at 1.0 μM. Laryngeal cryosections at the motor innervation point demonstrated decreased NMJ abundance in both TA and PCA from 30‐mo old rats. The results are consistent with the hypothesis that aging rat PCA and TA muscles become functionally denervated. It is still unclear whether the degree of functional denervation corresponds to the loss of NMJ density in these muscles. Supported by NIDCD