Evidence of the preferential involvement of .mu.-receptors in analgesia using enkephalins highly selective for peripheral .mu. or .delta. receptors

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Abstract
To study the preferential involvement of .mu. or .delta. receptors in the analgesic effects of enkephalins, several peptides which selectively interact with these 2 kinds of receptors in peripheral organs were synthesized. The inhibitory potency on the electrically stimulated mouse vas deferens (.delta. receptors) of the short peptide Tyr-D-Ala-Gly-NH-CH(CH3)CH2CH(CH3)2 is 2100 times lower (IC50 [mean inhibitory concentration] = 1220 nM) than that of the longer and more hydrophilic peptide Tyr-D-Ser-Gly-Phe-Leu-Thr (IC50 = 0.58 nM). The IC50 values of all the synthesized compounds on the guinea pig ileum assay (.mu. receptors) are in the same range (100-360 nM) and their analgesic activities in mice, measured on the hot-plate test after intracerebroventricular injection, are similar. The dissociation between antinociceptive properties in mice and potencies on the mouse vas deferens unambiguously reflects a preferential implication of .mu. receptors in analgesia. The possible involvement of brain .delta. receptors in behavioral effects is discussed.

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