GRM5 and type 2 diabetes mellitus: Enhancing the expression of Ng, mGluR5, NT-3, and nNOS in the hippocampal CA1 region of type 2 diabetic rats; reducing aldose reductase activity (AR), sorbitol (SNS) concentration, superoxide dismutase (SOD) activity, and serum malondialdehyde (MDA) levels; protecting against nerve damage; improving sciatic nerve conduction velocity