DIRECT COUPLING OF OPIOID RECEPTORS TO BOTH STIMULATORY AND INHIBITORY GUANINE NUCLEOTIDE-BINDING PROTEINS IN F-11 NEUROBLASTOMA SENSORY NEURON HYBRID-CELLS

Evidence is presented for linkage of opioid receptors directly to the stimulatory G protein (guanine nucleotide-binding protein), G(s), in addition to the generally accepted linkage to the inhibitory and ''other'' G proteins, G(i) and G(o), in F-11 (neuroblastoma-dorsal root ganglion neuron) hybrid cells. Treatment of intact F-11 cells with cholera toxin decreased specific binding of the opioid agonist [D-Ala2, D-Leu5]enkephalin to F-11 cell membranes by 35%, with the remaining binding retaining high affinity for agonist. Under these conditions cholera toxin influenced the a subunit of G(s) (G(s)alpha) but had no effect on the alpha subunit of G(i/o) (G(i/o)alpha), based on ADP-ribosylation studies. Pertussis toxin treatment decreased high-affinity opioid agonist binding by about 50%; remaining binding was also of high affinity, even though pertussis toxin had inactivated G(i/o)alpha selectively and essentially completely. Simultaneous treatment with both toxins had an additive effect, reducing specific binding by about 80%. While opioid agonists inhibited forskolin-stimulated adenylate cyclase activity of F-11 cells as expected, opioids also stimulated basal adenylate cyclase activity, indicative of interaction with G(s) as well as G(i). Cholera toxin treatment attenuated opioid-stimulation of basal adenylate cyclase, whereas pertussis toxin treatment enhanced stimulation. In contrast, inhibition by opioid of forskolin-stimulated activity was attenuated by pertussis toxin but not by cholera toxin. It is concluded that a subset of opioid receptors may be linked directly to G(s) and thereby mediate stimulation of adenylate cyclase. This G(s)-adenylate cyclase interaction is postulated to be responsible for the novel excitatory electrophysiologic responses to opioids found in our previous studies of sensory neurons and F-11 cells.