Bioactive peptidic analogues and cyclostereoisomers of the minimal antinociceptive histogranin fragment-(7-10).

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 (1 micromol/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; i.c.v). Finally, the various cyclic compounds competed with the binding of [(3)H]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.