PERTUSSIS TOXIN-INSENSITIVE SIGNALING OF THE ORL1 RECEPTOR - COUPLINGTO G(Z) AND G(16) PROTEINS

Nociceptin/OFQ is the endogenous ligand for the G protein-coupled opio id receptor-like (ORL1) receptor. To elucidate the cellular functions of the ORL1 receptor, we examined its ability to interact with G(z) an d G(16), two pertussis toxin (PTX)-insensitive G proteins that are kno wn molecular partners for the opioid receptors. In HEK 293 cells trans iently expressing the ORL1 and dopamine D-1 receptors, nociceptin/OFQ dose-dependently inhibited dopamine-stimulated cyclic AMP (cAMP) accum ulation in a PTX-sensitive manner. However, PTX failed to block the no ciceptin/OFQ-induced inhibition of dopamine-stimulated cAMP accumulati on in HEK 293 cells co-expressing the alpha-subunit of G(z). This resu lt indicates functional interaction between the ORL1 receptor and G(z) . A similar result was obtained with retinoic acid-differentiated SH-S Y5Y cells, which endogenously express both the ORL1 receptor and G(z). When the ORL1 receptor was transiently co-expressed in COS-7 cells wi th the alpha-subunit of G(16), nociceptin/OFQ dose-dependently stimula ted the formation of inositol phosphates, Nociceptin-induced stimulati on of phospholipase C was absolutely dependent on the co-expression of alpha(16) and exhibited the appropriate ligand selectivity. In terms of its ability to interact with PTX-insensitive G proteins, the ORL1 r eceptor behaves very much like the opioid receptors.