Twenty-one hormones fail to inhibit the brain to blood transport system for Tyr-MIF-1 and the enkephalins in mice

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Abstract
Tyr-MIF-1 (Tyr-Pro-Leu-Gly-amide) and methionine enkephalin are transported intact across the blood-brain barrier by a saturable, stereospecific system. This system has been found to be modulated by a few non-peptide substances and by certain conditions such as ageing and some stresses. We investigated the possibility that hormones structurally unrelated to Tyr-MIF-1 and the enkephalins might also be capable of modulating this transport. Twenty-one hormones were tested including steroids, proteins, glycoproteins, peptides, and thyroid hormones, in doses ranging from 0.01 pmol to 1 nmol/mouse by injecting each hormone directly into the lateral ventricle simultaneously with [125I]Tyr-MIF-1. No clear effect on transport could be established for any of the substances at the doses tested. None of these substances seemed able to act as competitive inhibitors, to share their respective transport systems with Tyr-MIF-1, or to modulate immediately the saturable transport system.