Bioactive Peptidic Analogues and Cyclostereoisomers of the Minimal Antinociceptive Histogranin Fragment-(7-10)

摘要

Novel analogues of the minimal antinociceptive histogranin (HN) fragment Gly~7-Gln-Gly-Arg~(10) in which amino acids in positions 8, 9, and 10 were replaced by lipophilic amino acids and corresponding D-amino acid residues in combination with N- to C-terminal cyclization, were synthesized and tested in various animal models of pain. All synthetic compounds were potent and efficacious analgesics in the mouse writhing test. Cyclic [-Gly-Ala-Tyr-D-Arg-] (9) and cyclic [-Gly p-Cl-Phe-Tyr-D-Arg-] (10) were the most potent analgesics, being 17 and 135 times as potent as HN, respectively (AD_(50) of 1.37 and 0.17 nmol/mouse icv, as compared with 23 nmol/mouse for HN). The times of action of compounds 9 and 10 were also much improved with half-maximal effects still being observed 60 min and >90 min after their administration, respectively, as compared with 8.1 min for the parent peptide HN-(7-10) and 22.1 min for HN. At analgesic doses, compounds 9 and 10 were devoid of motor effect as assessed by the mouse rotarod assay. As already observed with HN, compounds 9 (10 nmol/rat; i.t.) and 10 (0.5 nmol/rat; i.t.) were effective in blocking persistent inflammatory pain in the formalin test and hyperalgesia induced by intraplantar administration of complete Freund adjuvant. In addition, the analgesic effects evoked by compounds 9 (10 nmol/mouse; icv) and 10 (1μmol/kg; i.v.) in the mouse writhing test and compound 9 (10 nmol/mouse; icv) in the mouse tail flick assay were similarly antagonized by the dopamine D_2 receptor antagonist raclopride (1 nmol/mouse; icv) but not the opiate antagonist naloxone (1 nmol/mouse; icv). Finally, the various cyclic compounds competed with the binding of [~3H]raclopride in rat brain membrane preparations. Their ability to compete with the binding of the D_2 ligand correlated well with their potency in alleviating pain in the mouse writhing test (r = 0.95). These results indicate that the analgesic activity of the minimal active core in HN can be improved by changes that favor its interaction with the dopamine D_2 receptor.