Antinociceptive properties of a new tetrapeptide, Asn-Ala-Gly-Ala, in mice.

The effect of intracerebroventricularly (i.c.v.) administered Asn-Ala-Gly-Ala (NAGA), a partial sequence of beta-lipotropin, was investigated using the tail-pressure, hot-plate and phenylbenzoquinone (PBQ)-induced writhing tests in mice. I.c.v. administration of NAGA produced a dose-dependent inhibition of responses as measured by the three different assays. The ED50 of NAGA on the tail-pressure test did not differ from that obtained on the hot-plate test. NAGA showed a prominent reduction in activity on the PBQ writhing as compared with the hot-plate and tail-pressure tests. A low dose of naloxone (0.1-1.0 mg/kg, s.c.) resulted in a dose-dependent antagonism of the effect of NAGA in all assays. These data suggest that the antinociceptive effect induced by NAGA may involve the endogenous opioid system in mice.