INS and diabetes mellitus: Most DE studies used STZ‐induced diabetic rats, and the neuron loss might not completely reflect the effects of diabetes, since the direct neurotoxicity of STZ was confirmed by intracerebral‐ventricular injection.24, 25 In addition, treatment with STZ mainly damages pancreas and impedes the synthesis and secretion of insulin, which is distinguished from KK‐Ay mice with a high level of insulin resistance.