THE CLONED KAPPA-OPIOID RECEPTOR COUPLES TO AN N-TYPE CALCIUM CURRENTIN UNDIFFERENTIATED PC-12 CELLS

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, opioid receptor. To further analyse its functional propert ies, we have stably expressed the kappa opioid receptor in undifferent iated PC-12 cells, a pheochromocytoma clonal cell line, which do not e ndogenously express this receptor. We have previously shown that kappa opioid agonists selectively bind to these PC-12 membranes with high a ffinity.(17) Here we show that kappa selective agonists are able to in hibit accumulation of cyclic adenosine monophosphate in a stereoselect ive manner. Further, the kappa agonist U-50,488 is able to inhibit an N-type calcium current in a pertussis toxin sensitive manner; this inh ibition is blocked by the kappa-selective antagonist norbinaltorphimin e. Inhibition of the calcium current via the kappa receptor is stereos elective as the agonist levorphanol is able to mediate inhibition wher eas in the same cells dextrorphan is ineffective. This is the first de monstration that the cloned kappa opioid receptor functionally couples to a calcium current, as has been reported for kappa receptors expres sed endogenously in the nervous system. Kappa opioid receptors are tho ught to be important in pain pathways, learning and memory deficits, a nd seizure activity. A major physiological action of the dynorphins, t he endogenous ligands of the kappa receptor, is thought to be inhibiti on of neurotransmitter release at presynaptic terminals. N-type calciu m channels may be important in neurotransmitter release. For example i t has been demonstrated at the messy fiber synapse in the hippocampus that inhibition of N-type calcium currents blocks excitatory postsynap tic potentials measured in the pyrimidal cells.(2) We show here that t he cloned kappa receptor acts to inhibit N-type calcium channels in cl onal PC-12 cells, which may be an important mechanism by which the dyn orphins regulate synaptic events.