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.