Antinociception Produced by 14,15-Epoxyeicosatrienoic Acid Is Mediated by the Activation of β-Endorphin and Met-Enkephalin in the Rat Ventrolateral Periaqueductal Gray

Abstract

Cytochrome P450 genes catalyze formation of epoxyeicosatrienoic acids (EETs) from arachidonic acid. The effects of 5,6-EET, 8,9-EET, 11,12-EET, and 14,15-EET microinjected into the ventrolateral periaqueductal gray (vlPAG) on the thermally produced tail-flick response were studied in male Sprague-Dawley rats. 14,15-EET microinjected into vlPAG (3-156 pmol) dose-dependently inhibited the tail-flick response (ED50 = 32.5 pmol). In contrast, 5,6-EET, 8,9-EET, and 11,12-EET at a dose of 156 pmol were not active when injected into the vlPAG. 14,15-EET failed to displace the radiobinding of [3H][D-Ala2,NHPe4, Gly-ol5]enkephalin (mu-opioid receptor ligand) or [3H]naltrindole (delta-opioid receptor ligand) in crude membrane fractions of rat brain. Tail-flick inhibition produced by 14,15-EET from vlPAG was blocked by intra-vlPAG pretreatment with antiserum against beta-endorphin or Met-enkephalin or the mu-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) or the delta-opioid receptor antagonist naltrindole but not with dynorphin A[1-17] antiserum or the kappa-opioid receptor antagonist nor-binaltorphimine. In addition, tail-flick inhibition produced by 14,15-EET treatment was blocked by intrathecal pretreatment with Met-enkephalin antiserum, naltrindole, or CTOP but not with beta-endorphin antiserum. It is concluded that 1) 14,15-EET itself does not have any affinity for mu- or delta-opioid receptors and 2) 14,15-EET activates beta-endorphin and Met-enkephalin, which subsequently act on mu- and delta-opioid receptors to produce antinociception.