TABLE 1.

Changes in endogenous opioid peptide and receptor expression in MS patients and animal models.

Opioid peptide/receptor	Disease	Expression change	References
Met-enkephalin	MS (human)	↓ protein in serum	Ludwig et al., 2017
	EAE (mouse)	↓ protein in serum	Ludwig et al., 2017; Patel et al., 2020
β-endorphin	MS (human)	No change in serum	Ludwig et al., 2017
	MS (human)	↓ protein in peripheral blood mononuclear cells	Gironi et al., 2000
	EAE (mouse)	No change in serum	Ludwig et al., 2017
MOR, KOR, and DOR	TMEV (mouse)	↓ mRNA in spinal cord	Lynch et al., 2008