Analgesia produced by centrally administered DAGO, DPDPE and U50488H in the formalin test

Abstract

Three opioid agonists [(D-Ala 2,N-MePhe 4,Glu-ol 5]enkephalin (DAGO), [D-Pen 2,D-Pen 5]enekphalin (DPDPE) and U50488H) were tested independently for their ability to produce analgesia in the formalin test. These agonists were chosed based upon their ability to act selectively at μ, δ and κ opioid receptor types respectively. Rats received one intracerebroventricular (i.c.v.) injection of an agonist 20 min after subcutaneous injection of 15% formalin into a rear paw. Formalin injection produces continuous pain that results in two stereotypeic behaviors, paw licking and paw lifting. Ten minutes after i.c.v. injection rats were observed for an 8 min period and scored for formalin-induced behavior. All agonists produced analgesia as indicated by a dose-dependent attenuation of formalin-induced behavior. At the doses tested, the rank order of analgesic effecact was DAGO > DPDPE > U50488H. We suggest that centrally located μ, δ and κ opioid receptors can each modulate the perception of this clinically relecant form of continuous pain. Additionaklly, the highers dose of DPDPE tested significantly increased rearing whereas DAGO and U50488H failed to affect rearing.