Leu-Enkephalin

Mechanism of Action

• DOR preferential agonist (DOR Ki ~1 nM vs MOR Ki ~10 nM); Gi/Go coupling reduces cAMP, activates Kir channels, inhibits N-type Ca2+ channels
• Released in spinal dorsal horn to inhibit substance P-mediated pain transmission; classic "gate control" effector peptide
• DOR activation in limbic brain (amygdala, nucleus accumbens) contributes to anxiolysis and reward; distinct from mu-opioid euphoria
• Rapidly degraded by neprilysin (NEP/enkephalinase) and aminopeptidase N; physiological half-life seconds; enzymatic inhibitors profoundly extend activity
• Immune modulation: DOR expressed on T cells, NK cells, and macrophages; leu-enkephalin modulates cytokine secretion and natural killer activity

Research Findings

• First endogenous opioid discovery (1975, Hughes/Kosterlitz): identified in pig brain as the "endogenous morphine-like substance" at MOR
• Enkephalin knockout mice show reduced pain thresholds and increased anxiety, confirming endogenous analgesic and anxiolytic roles
• RB-101 (dual enkephalinase inhibitor): raises endogenous leu- and met-enkephalin levels; analgesic and antidepressant in rodents without tolerance
• DOR selective agonists (SNC80, ARM390) developed based on enkephalin pharmacophore show antidepressant effects with lower abuse liability than MOR agonists
• Leu-enkephalin plasma levels reduced in fibromyalgia and major depression; potential biomarker

Research Protocols

• Pain assay: 5-50 nmol intrathecal leu-enkephalin in rodents; tail-flick or hot-plate latency measurement
• DOR binding: Ki determination by 125I-DADLE or [3H]-DPDPE displacement; leu-enkephalin Ki ~1-3 nM
• Enkephalinase inhibitor studies: RB-101 or thiorphan + leu-enkephalin to demonstrate synergistic analgesia
• Immune studies: 1-100 nM leu-enkephalin on NK cell cytotoxicity assays or T cell proliferation

Interactions

• Met-enkephalin: sibling peptide from proenkephalin; same DOR pharmacology; met-enkephalin has slightly higher MOR affinity
• Neprilysin (NEP) and aminopeptidase N (APN): primary degrading enzymes; their inhibitors (thiorphan, bestatin) markedly potentiate leu-enkephalin
• Naloxone/naltrexone: block leu-enkephalin at both DOR and MOR; reverse analgesia and immune effects

Safety Profile

Endogenous opioid; not used clinically due to rapid degradation and poor CNS penetration. DOR-based drugs derived from enkephalin pharmacology are being developed as lower-abuse-potential analgesics and antidepressants. Leu-enkephalin itself non-toxic; its fragments (Tyr-Gly-Gly) biologically inactive.

Legal & Regulatory

Research peptide; not approved as therapeutic