The effects of bilateral intranigral microinjection of selective opioid agonists on behavioral responses to noxious thermal stimuli

Document Type

Article

Publication Date

8-23-1991

Abstract

This study examined the effects of bilateral intranigral microinjection of selective opioid agonists on the tail-flick and hot-plate antinociception tests. The principal findings are: (1) the μ-selective agonist d-Ala2, N-Me-Phe4, Gly5-ol-enkephalin (DAGO) had antinociceptive effects on both tests which were reversible by β-funaltrexamine (β-FNA; a μ-selective antagonist) and naloxone (a non-selective opioid antagonist); (2) the antinociceptive potency of DAGO injected into the nigra is comparable to its potency in the periaqueductal gray; (3) intranigral d-Pen2, d-Pen5-enkephalin (a δ-selective agonist), U-50,488H and dynorphin A-(1-13) (κ-selective agonists) had no antinociceptive effects; (4) antinociceptive effects were produced by the mixed δ/μ agonists d-Thr2-leucine enkephalin-Thr (DTLET) and d-Ser2-leucine enkephalin-Thr (DSLET); (5) the effect of DTLET on the hot-plate but not the tail-flick test was reversed by Cys2,Tyr3,Orn5,Pen7-amide (CTOP; a μ-selective antagonist), β-FNA, and naloxone, but not by the δ-selective antagonist naltrindole. Based on the potent antinociceptive effects of DAGO, the complete lack of such effects by the highly selective δ abd κ agonists, and the antagonism of DTLET by CTOP and β-FNA, it is concluded that the antinociceptive effects of intranigral opioid agonists are mediated by μ receptors. © 1991.

Publication Source (Journal or Book title)

Brain Research

First Page

136

Last Page

145

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