Further investigations on the antinociceptive activity of tuftsin analogs.

C-Terminal dipeptide fragment of tuftsin, Pro-Arg, substituted by D-amino acids, and tuftsin analogs with n-hexyl- and n-heptylamine coupled to their C-termini were synthesized by a classical method in solution and their antinociceptive activity was measured by tail flick immersion test (0.4 microM/icv). D-Pro-D-Arg and D-Pro-L-Arg showed an analgesic activity, with the duration of 60 and 40 min, respectively. The strong behavioral effects observed after injection of D-Pro-D-Arg were decreased by naloxone. L-Pro-D-Arg and Thr-Lys-Pro-Arg-HxA display no antinociceptive effect; the tetrapeptide amide showed some toxicity effects. Thr-Lys-Pro-Arg-HpA was very toxic and caused death of all experimental animals. This effect was not influenced by previous injection of naloxone.