Signal transduction of opiate receptors in spinal cord cells

The spinal cord plays an important role in opiate-induced analgesia and in the development of opiate tolerance. We have shown that the majority of the opiate receptors in the spinal cord (> 70% are of the K type. We have also demonstrated that cocultures consisting of neurons of spinal cord and dorsal root ganglia express primarily the K type of opiate receptors. Using these cocultures, we found that activation of k-opiate receptors leads to inhibition of the basal-and forskolin-stimulated adenylate cyclase, as well as to the inhibition of the activity of voltage-dependent Ca²⁺ channels. In addition, activation of the k-opiate receptor led to stimulation of the phosphatidylinositol turnover in these cells. All these opiate-induced activities could be blocked by opiate antagonists, as well as by pretreatment of the cells with pertussis toxin, demonstrating the involvement of authentic opiate receptors and pertussis toxin-sensitive GTP-binding protein(s) in the transduction pathways. We found that prolonged exposure of the cells to k-opiate agonists led to a marked reduction in the ability of a second application of opiates or other agonists (e.g., muscarinic, αadrenergic agonists) to inhibit adenylate cyclase and Ca²⁺ channel activities (i.e., heterologous desensitization). Moreover, the capacity of opiate agonists to stimulate phosphatidylinositol turnover, was also markedly reduced. We have also shown that chronic opiate treatment greatly attenuated the pertussis toxin catalyzed ADP-ribosylation of the α subunits of the GTP-binding proteins G_i and G_o. In addition, the immunoreactive levels of the α subunits of G_i and G_o were markedly reduced. These results led us to propose that prolonged opiate exposure leads to alteration in the GTP-binding protein transduction system, which may in turn account for the desensitization observed.