High Fat Diet-induced Decreases In Insulin Signaling In Afferent Renal Nerves Are Prevented By Intrathecal Injection Of Metformin In Rats

Metformin, an anti-type 2 diabetic agent that increases insulin sensitivity in target tissues, has been suggested having neuroprotective effects. Our previous data showed that metformin protects against high fat diet (HFD)-induced impairment in renal afferent nerves that express the transient receptor potential vanilloid 1 (TRPV1) channels, and prevents HFD-induced renal functional deterioration and hypertension. However, mechanisms of metformin-mediated neuroprotection are largely unknown. This study tests the hypothesis that HFD impairs insulin signaling in afferent renal nerves, which is prevented by metformin via the pathway of IRS-1-tyr, AMPK, and GLUT-1/3. Metformin (1 ng/kg, daily intrathecal injection via indwelled catheters to segments T8-L3 supplying the kidneys) or vehicle was given to rats fed a HFD or normal fat diet (Con) for 8 weeks. Immunohistochemical staining showed that insulin receptor substrate-1 phosphorylated with serine (p-IRS-1-ser), insulin receptor substrate-1 phosphorylated with tyrosine (p-IRS-1-tyr), phosphorylated AMP-activated protein kinase (p-AMPK), glucose transporter 1 (GLUT-1), and GLUT-3 were expressed in dorsal root ganglia (DRG). Levels of p-IRS-1-ser were increased by HFD but decreased by metformin in Con rats, and HFD-induced increases in p-IRS-1-ser were prevented by metformin. Levels of p-IRS-1-tyr, p-AMPK, and the trafficking of GLUT-1 and GLUT-3 from cytoplasma to cell membrane in DRG were decreased by HFD but increased by metformin in Con rats, and HFD-induced decreases in these parameters were prevented by metformin (p-IRS-1-tyr, Con: 0.23±0.02, Con+Met: 0.34±0.04, HFD: 0.14±0.02, HFD+Met: 0.22±0.03, p<0.05). Afferent renal nerve activity (ARNA) in response to insulin perfusion into the renal pelvis was decreased by HFD but enhanced by metformin in Con rats, and HFD-induced decreases in ARNA was prevented by metformin. Our data showed that HFD decreases insulin signaling in afferent renal nerves, which is prevented by metformin injected intrathecally to segments innervating kidneys. These data indicate that metformin may constitute neuroprotecitve effects via improving insulin signaling in afferent renal nerves via activation of the pathway of IRS-1-tyr, AMPK, and GLUT-1/3.