Supersensitivity to Mu-opioid Receptor-mediated Inhibition of the Adenylyl Cyclase Pathway Involves Pertussis Toxin-resistant Galpha Protein Subunits

Sustained administration of opioids leads to antinociceptive tolerance, while prolonged association of L-type Ca2+ channel blockers (e.g. nimodipine) with opioids results in increased antinociceptive response. Herein, we investigated the changes in mu-opioid receptor signalling underlying this shift from analgesic tolerance to supersensitivity. Thus, the interaction of mu-opioid receptors with G proteins and adenylyl cyclase was examined in lumbar spinal cord segments of rats. In control animals, the mu-opioid selective agonists, sufentanil and DAMGO, stimulated [35S]5'-(gamma-thio)-triphosphate ([35S]GTP gamma S) binding and inhibited forskolin-stimulated adenylyl cyclase activity, through a mechanism involving pertussis toxin (PTX) sensitive G alpha(i/o) subunits. Seven days of chronic sufentanil treatment developed antinociceptive tolerance associated with a reduction in mu-agonist-induced [35S]GTP gamma S binding, mu-agonist-induced adenylyl cyclase inhibition, and co-precipitation of G alpha o, G alpha i2 G alpha z and G alpha q11 subunits with mu-opioid receptors. In contrast, combined nimodipine treatment with sufentanil over the same period increased the sufentanil analgesic response. This antinociceptive supersensitivity was accompanied by a significant increase of mu-agonist-induced inhibition of adenylyl cyclase that was resistant to the antagonism by PTX. In good agreement, co-precipitation of the PTX-resistant, G alpha z and G alpha q/11 subunits with mu-opioid receptors was not lowered. On the other hand, the PTX-sensitive subunits, G alpha i2 and G alpha o, as well as agonist-stimulated [35S]GTP gamma S binding were still reduced. Our results demonstrate that mu-opioid analgesic tolerance follows uncoupling of spinal mu-opioid receptors from their G proteins and linked effector pathways. Conversely, the enhanced analgesic response following combined nimodipine treatment with sufentanil is associated with adenylyl cyclase supersensitivity to the opioid inhibitory effect through a mechanism involving PTX-resistant G protein subunits.