PHORBOL ESTER MODULATION OF BOTH DELTA-MUTANT AND SUBUNIT-OMITTED NICOTINIC RECEPTORS EXPRESSED IN XENOPUS OOCYTES

The action of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate ( TPA), the potent stimulator of protein kinase C (PKC), on acetylcholin e-activated currents (I-ACh) Was investigated in voltage clamped Xenop us laevis oocytes injected with RNAs encoding murine embryonic nicotin ic acetylcholine receptor (AChR) subunits. Comparable potentiation and acceleration of decay of I-ACh were observed within minutes of phorbo l ester application in oocytes injected with various RNA subunit combi nations: (i) alpha beta gamma delta; (ii) alpha beta gamma; (iii) alph a beta delta; and (iv) alpha beta gamma delta(AAA) a mutant of the del ta subunit with serine residues 360-361-362 mutated to alanine. Our fi ndings indicate that the effects on I-ACh induced by PKC stimulation a re independent of both gamma and delta subunits and, accordingly, of t he presence of PKC phosphorylation sites on delta subunit. It is here suggested a novel PKC-dependent modulatory mechanism of cholinergic re ceptor which does not involve direct phosphorylation of the AChR and r equires phosphorylation of intermediate regulatory protein(s).