Activation and blockade by choline of bovine α7 and α3β4 nicotinic receptors expressed in oocytes

Choline, the precursor and the metabolite of acetylcholine, is reputed as a selective α7 nicotinic receptor agonist. In this study, however, we have seen that choline exerted a dual effect on bovine nicotinic receptors expressed in Xenopus oocytes. On the one hand, choline behaved as a weak full agonist on bovine α7-mediated inward currents, with an EC50 of 0.43 mM. On the other, choline blocked bovine α3β4 currents, with an IC50 of 0.97 mM. The blockade by choline was fast (τon, 0.36 s), fully reversible (τoff, 1.23 s), exhibited voltage-dependence (60% blockade at − 100 mV and 30% blockade at − 40 mV), and was of a non-competitive nature, suggesting an open-channel type of α3β4  receptor blockade. Thus, choline by activating α7 receptors and/or blocking α3β4 receptors might play a physiological role in the control of neurotransmission at cholinergic synapses where α7 and α3β4 receptor are expressed.