Sensory Neuron Sensitization By Pkc-induced Trpv1 Phosphorylation In Type 2 Diabetic Rats

Abstract

Skeletal muscle reflex-induced increases in blood pressure are exaggerated during exercise in type 2 diabetes mellitus (T2DM). We previously demonstrated that skeletal muscle afferent discharge in response to capsaicin, a transient receptor potential cation channel subfamily V member 1 (TRPV1) agonist, is heightened in T2DM likely contributing to the potentiated pressor response. However, the underlying mechanisms remain unclear. Evidence suggests that the high glucose levels in T2DM sensitize sensory neurons through the receptor for advanced glycation end products (RAGE)/protein kinase C (PKC) pathway in dorsal root ganglia (DRG). Moreover, early-stage diabetes associated with TRPV1 overactivity is mediated through PKC. Therefore, it was hypothesized that the augmentation in muscle afferent discharge in T2DM previously reported is due to the phosphorylation of TRPV1 via an overactive RAGE/PKC pathway. PURPOSE: To investigate 1) the impact of T2DM on plasma levels of advanced glycation end products (AGE) and high-mobility group box-protein 1 (HMGB-1), both RAGE ligands, and 2) the impact of T2DM on the RAGE/PKC pathway including the phosphorylation of TRPV1 in DRG subserving skeletal muscle afferents. METHODS: For 14-16 weeks, Sprague-Dawley rats were given either a normal diet (control) or a high fat diet in combination with a low dose (35 mg/kg) of streptozotocin (T2DM). Plasma insulin, HMGB1 and AGE were determined using ELISA. RAGE, phosphorylated PKC and TRPV1 protein levels were quantified in DRG by western blotting. RESULTS: After overnight fasting, T2DM rats exhibited hyperglycemia (95±6 vs. 156±18 mg/dL, P<0.05) and hyperinsulinemia (1.6±0.2 vs. 4.1±0.4 ng/mL, P<0.05). HMGB1, AGE and RAGE did not differ between groups. Phosphorylated PKC (1.0±0.1 vs. 1.2±0.2 arb unit, P=0.08) and TRPV1 (1.0±0.3 vs. 1.3±0.4 arb unit, P=0.08) levels in DRG tended to be greater in T2DM than control. CONCLUSIONS: These findings suggest that phosphorylated TRPV1 may be increased by PKC overactivity in DRG of T2DM but acts independently of RAGE. Importantly, these changes may mediate the sensitization of skeletal muscle afferents facilitating the exaggerated pressor response to exercise characteristic of T2DM. Supported by Lawson & Rogers Lacy Research Fund and SHP Interdisciplinary Research Grant Program.