Short Cationic ACTH-Related Peptides Can Modulate the Na_V1.8 Channel Functioning, Resulting in an Analgesic Effect

Full-length ACTH molecule and ACTH-related hexapeptide H-PKKRRP-OH are demonstrated by the patch-clamp method to decrease the Na_V1.8 channel activation gating system effective charge in the nociceptive neuron membrane, while ACTH-related tetrapeptide Ac-KKRR-NH₂ has no effect. ACTH(1-24), a fully functional ACTH mimetic, and H-PKKRRP-OH show analgesic effects in the formalin test in vivo. All peptides contain the cationic KKRR motif, but only H-PKKRRP-OH and ACTH(1-24) relieve acute pain, targeting the Na_V1.8 channel as a receptor. This seemingly controversial result is explained by application of conformational analysis and blind docking. Though conformational analysis indicates that both H-PKKRRP-OH and Ac-KKRR-NH₂ contain the cationic functional groups at the earlier suggested characteristic distance of 9-12 Å, Ac-KKRR-NH₂ does not interact with the S4_I voltage sensor of the Na_V1.8 channel activation gating system. The docking demonstrates that an extensive network of ligand-receptor ionic and hydrogen bonds involving D151, E157, R218, and R221 VSD_I residues, essential for the analgesic tripeptide Ac-KKK-NH₂ binding, is formed upon the H-PKKRRP-OH binding. Particularly important are the ionic bonds between the H-PKKRRP-OH C-terminal carboxylate anion and the S4_I R218 and R221 guanidinium groups. The described mechanism of Na_V1.8 channel modulation is fundamentally different from the effect of channel blockers.