Melanocyte-inhibiting factor (also known as Pro-Leu-Gly-NH2, Melanostatin, MSH release–inhibiting hormone or MIF-1) is an endogenous peptide fragment derived from cleavage of the hormone oxytocin, but having generally different actions in the body.

[1][2] MIF-1 produces multiple effects, both blocking the effects of opioid receptor activation,[3][4][5][6][7][8] while at the same time acting as a positive allosteric modulator of the D2 and D4 dopamine receptor subtypes,[9][10][11][12][13][14][15][16][17] as well as inhibiting release of other neuropeptides such as alpha-MSH,[18][19][20] and potentiating melatonin activity.

[21] This complex mix of actions produces a profile of antidepressant,[22][23][24] nootropic,[25][26][27][28] and anti-Parkinsonian effects when MIF-1 is administered,[29][30][31] and it has been investigated for various medical uses.

MIF-1 is unusually resistant to metabolism in the bloodstream,[32] and crosses the blood–brain barrier easily,[33][34] though it is poorly active orally and is usually injected.

Several other closely related peptides with important actions in the body include Tyr-MIF-1 and endomorphin-1 and -2.