Abstract
The traditional function of neurotransmitter-gated ion channels is to induce rapid changes in electrical activity. Channels that are Ca(2+)-permeable, such as N-methyl-D-aspartate receptors at depolarized membrane potentials, can have a broader repertoire of consequences, including changes in synaptic efficacy, developmental plasticity, and excitotoxicity. Neuronal nicotinic receptors for acetylcholine (nAChRs) are usually less Ca(2+)-permeable than N-methyl-D-aspartate receptors but have a significant Ca2+ permeability, which is greater at negative potentials. Here we report that in neuroendocrine cells, activation of nAChRs can trigger exocytosis at hyperpolarized potentials. We used whole-cell patch-clamp recordings to record currents and the capacitance detection technique to monitor exocytosis in isolated bovine chromaffin cells. Stimulation of nAChRs at hyperpolarized potentials (-60 or -90 mV) evokes a large current and a maximal capacitance increase corresponding to the fusion of approximately 200 large dense-core vesicles. The amount of exocytosis is controlled both by the Ca2+ influx through nAChRs and by a contribution from thapsigargin-sensitive Ca2+ sequestering stores. This is a form of neurotransmitter action in which activation of nAChRs triggers secretion through an additional coupling pathway that coexists with classical voltage-dependent Ca2+ entry.