Neuronal peptide (enkephalin) receptors in the ear artery of the rabbit

Methionine-enkephalin methylester (MEM) and leucine-enkephalin (LE) inhabit the vasoconstrictor responses of the rabbit ear artery to nerve stimulation by acting on a specific neuronal peptide (enkephalin)-receptor insensitive to opiate agonists. The tetrapeptide: H-Tyr-Gly-Phe-Leu-OCH3 is ineffective. This is the first instance of enkephalins acting in an organ devoid of receptors. In a new test for the analysis of opiate receptors, MEM (ID50=6.9 X 10(-9) M) was a potent inhibitor of transmission. The presence was shown of opiate receptors in the brain which were insensitive to high i.v. or intraventricular doses of enkephalins. It is concluded that enkephalins are not natural ligands to the opiate receptors, but that some of the receptors confuse these structures because of similar characteristics which determine the binding of both opiates and peptides.