Stimulation of mu- and delta-opioid receptors enhances phosphoinositide metabolism in mouse spinal cord : evidence for subtypes of delta-receptors

The accumulation of inositol phosphates (IPs) induced by agonist-activated opioid receptors was analysed in mouse spinal cord slices pre-labelled with myo-[3H]inositol. Agonists showing selectivity to μ-opioid receptors, morphine and [d-Ala2,MePhe4, Gly(ol)5]enkephalin (DAMGO), promoted concentration-dependent increases in the formation of IPs. The activation of δ-opioid receptors by the selective agonists [d-Pen2,5]enkephalin (DPDPE) and [d-Ala2]deltorphin II produced similar increases in phosphoinositide (PI) metabolism. Pre-treatment of the slices with pertussis toxin (PTX) blocked the effect of opioid agonists on IP production. The involvement of Gi/Go-protein (guanine nucleotide-binding protein) classes in this opioid effect is therefore suggested. The activity of the opioid agonists was reduced by the opioid antagonists naltrexone and naloxone. The antagonist at δ1-receptors, 7-benzylidenenaltrexone (BNTX), exhibited greater potency than the antagonists at δ2-receptors, naltriben methanesulphonate (NTB) or naltrindrole 5′-isothiocyanate (NT II), in reducing the activating effect of DPDPE on phosphoinositide metabolism. Conversely, NTB and NT II were more potent antagonists of the activity of [d-Ala2]deltorphin II than BNTX. This work demonstrates the coupling of spinal μ- and δ-opioid receptors to phospholipase C and the generation of IPs. It also provides biochemical evidence for pharmacological subtypes of δ-opioid receptors in the activation of this signalling pathway.