Tetrapeptide Endomorphin Analogs Require Both Full Length and Truncated Splice Variants of the Mu Opioid Receptor Gene Oprm1 for Analgesia

Abstract

<p class="first" id="P1">The mu opioid receptor gene undergoes extensive alternative splicing. Mu opioids can be divided into three classes based on the role of different groups of splice variants. Morphine and methadone require only full length 7 transmembrane (7TM) variants for analgesia, whereas IBNtxA (3’-iodobenzyol-6β-naltrexamide) needs only truncated 6TM variants. A set of endomorphin analogs fall into a third group that requires both 6TM and 7TM splice variants. Unlike morphine, endomorphin 1 and 2, DAPP (Dmt,D-Ala-Phe-Phe-NH ₂) and IDAPP (3’-iodo-Dmt-D-Ala-Phe-Phe-NH ₂) analgesia was lost in an exon 11 knockout mouse lacking 6TM variants. Restoring 6TM variant expression in a knockout mouse lacking both 6TM and 7TM variants failed to rescue DAPP or IDAPP analgesia. However, re-establishing 6TM expression in an exon 11 knockout mouse that still expressed 7TM variants did rescue the response, consistent with the need for both 6TM and 7TM variants. In receptor binding assays, ¹²⁵I-IDAPP labeled more sites (B _max) than ³H-DAMGO in wildtype mice. In exon 11 knockout mice ¹²⁵I-IDAPP binding was lowered to levels similar to ³H-DAMGO, which remained relatively unchanged compared to wildtype mice. ¹²⁵I-IDAPP binding was totally lost in an exon 1/exon 11 knockout model lacking all <i>Oprm1</i> variant expression, confirming the drug was not cross labeling non-mu opioid receptors. These findings suggested that ¹²⁵I-IDAPP labeled two populations of mu binding sites in wildtype mice, one corresponding to 7TM variants and the second dependent upon 6TM variants. Together, these data indicate that endomorphin analogs represent a unique, genetically defined and distinct class of mu opioid analgesic. </p>