Analgesic effects of enkephalin analogs in rats

The analgesic effects of intracerebroventricular injections of Met-enkephalin and five of its analogs in a dose of 10 micrograms each were quantified with a hot plate test in rats. Two analogs showed analgesic effect. [D-Ala2, Met5]-enkephalin had a weak and short-lasting analgesic effect. [D-Ala2, Met5]-enkephalinamide (DALA) had a striking and long-lasting analgesic effect. However, sulfation of tyrosine residue totally abolished the analgesic action of DALA. The analgesic effect of DALA was not affected by preinjection of its sulfated analog.