5-Methionine-enkephalin amide

5-Methionine-enkephalin amide ([Met5]-Enkephalin, amide) is an agonist for δ opioid receptors as well as putative ζ (zeta) opioid receptors. 5-Methionine-enkephalin amide circulates in several forms, potentially derived from proteins other than proenkephalin A, and plasma levels of both free native and peptidase-derivable 5-Methionine-enkephalin amide are physiologically modulated.

Method: Distal colonic longitudinal muscle strips (with pelvic nerves) isolated from cats were treated with 5-Methionine-enkephalin amide (1×10⁻¹⁰–3×10⁻⁸ M). Contractions were evoked by electrical stimulation of the pelvic nerves (2 Hz, 120 mA, 0.5 ms, 10 s, 1 min intervals), and the percentage of inhibition was recorded.
Result: 5-Methionine-enkephalin amide concentration-dependently inhibited pelvic nerve‑evoked longitudinal muscle contractions, with an IC₅₀ of 2.2×10⁻⁹ M (95% CI: 9.4×10⁻¹⁰–5.8×10⁻⁹ M). The effect was rapid in onset and completely reversible upon washing[1].
Method: Primary mixed glial cell cultures from the brains of 1‑day‑old neonatal ICR mice were treated continuously with 5-Methionine-enkephalin amide (10⁻¹⁰, 10⁻⁸, 10⁻⁶ M). Total cell numbers were counted using a hemocytometer on days 3, 6, and 8 of culture.
Result: 5-Methionine-enkephalin amide dose‑dependently reduced the total number of mixed glial cells. On day 6, cell counts in the 10⁻⁶ M and 10⁻⁸ M groups decreased to 45% and 44% of the control, respectively, while the 10⁻¹⁰ M group decreased to 65% of the control[2].