The cloned kappa opioid receptor couples to an N-type calcium current in undifferentiated PC-12 cells

Abstract

We have recently reported the cloning of a mouse kappa opioid receptor cDNA. 25 Following transfection of the kappa receptor cDNA into COS-1 cells, a receptor is expressed with the pharmacological specificity of a kappa 1 opioid receptor. To further analyse its functional properties, we have stably expressed the kappa opioid receptor in undifferentiated PC-12 cells, a pheochromocytoma clonal cell line, which do not endogenously express this receptor. We have previously shown that kappa opioid agonists selectively bind to these PC-12 membranes with high affinity. 17 Here we show that kappa selective agonists are able to inhibit accumulation of cyclic adenosine monophosphate in a stereoselective manner. Further, the kappa agonist U-50,488 is able to inhibit an N-type calcium current in a pertussis toxin sensitive manner; this inhibition is blocked by the kappa-selective antagonist norbinaltorphimine. Inhibition of the calcium current via the kappa receptor is stereoselective as the agonist levorphanol is able to mediate inhibition whereas in the same cells dextrorphan is ineffective. This is the first demonstration that the cloned kappa opioid receptor functionally couples to a calcium current, as has been reported for kappa receptors expressed endogenously in the nervous system. Kappa opioid receptors are thought to be important in pain pathways, learning and memory deficits, and seizure activity. A major physiological action of the dynorphins, the endogenous ligands of the kappa receptor, is thought to be inhibition of neurotransmitter release at presynaptic terminals. N-type calcium channels may be important in neurotransmitter release. For example it has been demonstrated at the mossy fiber synapse in the hippocampus that inhibition of N-type calcium currents blocks excitatory postsynaptic potentials measured in the pyrimidal cells. 2 We show here that the cloned kappa receptor acts to inhibit N-type calcium channels in clonal PC-12 cells, which may be an important mechanism by which the dynorphins regulate synaptic events.