Analgesic effects of D-amino acids in four inbred strains of mice.

1. Prominent strain differences of mice were found in analgesic effects of D-amino acids. 2. In C57BL/6CrSlc and C3H/HeSlc mice, pain threshold, which was determined by using a hot-plate method, increased to 140-175% of the control after the systemic treatment of all three D-amino acids employed, such as D-phenylalanine, -leucine and -methionine, whereas in DBA/2CrSlc or BALB/cCrSlc mice, out of three only one D-amino acid, D-phenylalanine or -leucine, produced significant increase of pain threshold. 3. This lack of ability to perceive analgesic effects of specific amino acids observed in the latter two strains suggests that there probably exist different analgesia-inducing mechanisms for each of three D-amino acids in mice and the latter two strains lack two of them.