Tyr-MIF-1 binding in brain is not altered by ligands selective for the GABAA/benzodiazepine receptor

From MIF-1 to endomorphin: the Tyr-MIF-1 family of peptides

The Tyr-MIF-1 family of small peptides has served a prototypic role in the introduction of several novel concepts into the peptide field of research. MIF-1 (Pro-Leu-Gly-NH(2)) was the first hypothalamic peptide shown to act "up" on the brain, not just "down" on the pituitary. In several situations, including clinical depression, MIF-1 exhibits an inverted U-shaped dose-response relationship in which increasing doses can result in decreasing effects. This tripeptide also can antagonize opiate actions, and the first report of such activity also correctly predicted the discovery of other endogenous antiopiate peptides. The tetrapeptide Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH(2)) not only shows antiopiate activity, but also considerable selectivity for the mu-opiate binding site. Tyr-W-MIF-1 (Tyr-Pro-Trp-Gly-NH(2)) is an even more selective ligand for the mu receptor, leading to the discovery of two more Tyr-Pro tetrapeptides that have the highest specificity and affinity for this site. These are the endomorphins: endomorphin-1 is Tyr-Pro-Trp-Phe-NH(2) and endomorphin-2 is Tyr-Pro-Phe-Phe-NH(2). Tyr-MIF-1 proved, contrary to the then prevailing dogma, that peptides can be saturably transported across the blood-brain barrier by a quantifiable transport system. Unexpectedly, the Tyr-MIF-1 transporter is shared with Met-enkephalin. In the era in which it was doubtful whether a peripheral peptide could exert CNS effects, the Tyr-MIF-1 family of peptides also explicitly showed that they can exert more than one central action that persists longer than their half-lives in blood. These peptides clearly illustrate that the name of a peptide restricts neither its actions nor its conceptual implications.

Hemorphins, cytochrophins, and human beta-casomorphins bind to antiopiate (TYR-MIE-1) as well as opiate binding sites in rat brain

Novel peptides with opiate activity, derived from endogenous sources (human and bovine casomorphins from milk, hemorphins from hemoglobin, and cytochrophins from mitochondrial cytochrome b), were tested for their ability to inhibit binding of the brain peptide Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) to its high affinity sites in rat brain. The order of potency in inhibiting binding of 125I-Tyr-MIF-1 was: hemorphin and bovine casomorphins greater than Tyr-MIF-1 greater than cytochrophins greater than human casomorphins. Naloxone and DAMGO were ineffective at inhibiting Tyr-MIF-1 binding. The results provide evidence that, in addition to their ability to bind to mu opiate receptors, these novel endogenous peptides with opiate activity and a peptide (Tyr-MIF-1) with antiopiate properties also bind to a non-opiate site labeled by Tyr-MIF-1. These sites could be involved in a balance between opiate and antiopiate peptides.

Differential effects of Tyr-MIF-1 and naloxone in two animal models involving benzodiazepine

It has been shown previously that the endogenous brain peptide Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) can act as an antiopiate and can also increase binding and function at the GABAA/benzodiazepine receptor complex. We now describe the effects of this tetrapeptide in two models in which the antiopiate naloxone has been reported to decrease the activity of benzodiazepines. Unlike naloxone, Tyr-MIF-1 and MIF-1 neither prevented chlordiazepoxide-induced locomotor hyperactivity in mice on a tilting floor nor suppressed chlordiazepoxide-induced eating in rats. Thus, in these two systems, Try-MIF-1 did not act as an antiopiate or alter the effects of a benzodiazepine, indicating a selectivity in the actions of Tyr-MIF-1.