A potentially novel nicotinic receptor inAplysianeuroendocrine cells

Abstract

White SH, Carter CJ, Magoski NS. A potentially novel nicotinic receptor in Aplysia neuroendocrine cells. J Neurophysiol 112: 446-462, 2014. First published April 16, 2014 doi:10.1152/jn.00796.2013.-Nicotinic receptors form a diverse group of ligand-gated ionotropic receptors with roles in both synaptic transmission and the control of excitability. In the bag cell neurons of Aplysia, acetylcholine activates an ionotropic receptor, which passes inward current to produce a long-lasting afterdischarge and hormone release, leading to reproduction. While testing the agonist profile of the cholinergic response, we observed a second current that appeared to be gated only by nicotine and not acetylcholine. The peak nicotine-evoked current was markedly smaller in magnitude than the acetylcholine-induced current, cooperative (Hill value of 2.7), had an EC 50 near 500 M, readily recovered from desensitization, showed Ca 2ϩ permeability, and was blocked by mecamylamine, dihydro-␤-erythroidine, or strychnine, but not by ␣-conotoxin ImI, methyllycaconitine, or hexamethonium. Aplysia transcriptome analysis followed by PCR yielded 20 full-length potential nicotinic receptor subunits. Sixteen of these were predicted to be cation selective, and real-time PCR suggested that 15 of the 16 subunits were expressed to varying degrees in the bag cell neurons. The acetylcholine-induced current, but not the nicotine current, was reduced by double-strand RNA treatment targeted to both subunits ApAChR-C and -E. Conversely, the nicotine-evoked current, but not the acetylcholine current, was lessened by targeting both subunits ApAChR-H and -P. To the best of our knowledge, this is the first report suggesting that a nicotinic receptor is not gated by acetylcholine. Separate receptors may serve as a means to differentially trigger plasticity or safeguard propagation by assuring that only acetylcholine, the endogenous agonist, initiates large enough responses to trigger reproduction.

Diacylglycerol‐mediated regulation of Aplysia bag cell neuron excitability requires protein kinase C

“…We previously found a 50–60% desensitization (White & Magoski, ; White et al . ), yet in the present study, the second acetylcholine current was almost 80% of the first current, suggesting OAG may potentiate the acetylcholine receptor. The presence of other membrane lipids, such as phosphatidic acid or cholesterol, can influence nicotinic receptors (Sturgeon & Baenziger, ), stabilizing a resting state over the desensitized conformation (daCosta et al .…”

“…Naive cultured bag cell neurons exhibit a large inward current in response to acetylcholine by the opening of an ionotropic cholinergic receptor (White & Magoski, ; White et al . ); moreover, we find that post‐ I OAG neurons respond similarly upon acetylcholine delivery. The current, however, was significantly stronger than that reported by White & Magoski () (−40 vs .…”

“…For a small number of specific experiments, acetylcholine was pressure ejected from an unpolished patch pipette (1–2 μm bore) for 2 s at 75–150 kPa using a PMI‐100 pressure microinjector (Dagan, Minneapolis, MN, USA), as done previously by our lab (White & Magoski, ; White et al . ) and others (Fisher et al . ).…”

Hydrogen Peroxide Gates a Voltage-Dependent Cation Current inAplysiaNeuroendocrine Cells

“…We previously established that ACh gates, with essentially no delay, an ionotropic receptor in bag cell neurons (White and Magoski, 2012). This ligand-gated channel opens as soon as ACh contacts the cell, and is sensitive to classic nicotinic-type agonists and antagonists (White and Magoski, 2012;White et al, 2014). Thus, the latency from when the ACh-containing reservoir was opened to when the ACh-induced current was first observed represented the lag time of our perfusion apparatus.…”

Mechanisms of nicotine-induced cutaneous vasodilation and sweating in young adults: roles for KCa, KATP, and KVchannels, nitric oxide, and prostanoids

“…Alternatively, Kv channels may be involved in augmenting acetylcholine release from cholinergic nerves associated with sudomotor axon reflex (pre-muscarinic receptor level). Specifically, depolarization associated with the activation of nicotinic receptors (White et al 2014) propagates to cholinergic nerve terminals, which in turn induces the activation of K V channels and thus hyperpolarization associated with K + efflux. This hyperpolarization may increase Ca 2+ D r a f t…”