The non-significant changes in the protein levels of MOR in the spinal cord from diabetic mice treated with vehicle are in accordance to the unchanged expression of MOR observed in the majority of investigations performed after 3–4 weeks of STZ-induced diabetes [5, 42] but in contrast to the down-regulated expression of these receptors demonstrated by Shaqura et al., (2013) [43] in the spinal cord from diabetic rats at 12 weeks after STZ injection. This evidence concerns the gene OPRM1 and diabetes mellitus.