Supraspinal Gβγ-dependent stimulation of PLCβ3 originating from G inhibitory protein-μ opioid receptor-coupling is necessary for morphine induced acute hyperalgesia

Although alterations in mu-opioid receptor (mu OR) signaling mediate excitatory effects of opiates in opioid tolerance, the molecular mechanism for the excitatory effect of acute low dose morphine, as it relates to mu OR coupling, is presently unknown. A pronounced coupling of mu OR to the alpha subunit of G inhibitory protein emerged in periaqueductal gray (PAG) from mice systemically administered with morphine at a dose producing acute thermal hyperalgesia. This coupling was abolished in presence of the selective mu OR antagonist d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH2 administered at the PAG site, showing that the low dose morphine effect is triggered by mu OR activated G inhibitory protein at supraspinal level. When G beta gamma downstream signalling was blocked by intra-PAG co-administration of 2-(3,4,5-trihydroxy-6-oxoxanthen-9-yl)cyclohexane-1-carboxylic acid, a compound that inhibits G beta gamma dimer-dependent signaling, a complete prevention of low dose morphine induced acute thermal hyperalgesia was obtained. Phospholipase C beta 3, an enzyme necessary to morphine hyperalgesia, was revealed to be associated with G beta gamma in PAG. Although opioid administration induces a shift in mu OR-G protein coupling from Gi to Gs after chronic administration, our data support that this condition is not realized in acute treatment providing evidence that a separate molecular mechanism underlies morphine induced acute excitatory effect.