NAADP-mediated channel 'chatter' in neurons of the rat medulla oblongata

Abstract

NAADP (nicotinic acid-adenine dinucleotide phosphate) is a potent Ca2+-mobilizing messenger that stimulates Ca2+ release in a variety of cells. NAADP-sensitive Ca2+ channels are thought to reside on acidicCa2+ stores and to be functionally coupled to IP3 (inositol 1,4,5-trisphosphate) and/or ryanodine receptors located on the endoplasmic reticulum. Whether NAADP-sensitive Ca2+ channels 'chatter' to other channels, however, is not clear. In the present study, we have used a cell-permeant NAADP analogue to probe NAADP-mediated responses in rat medulla oblongata neurons. NAADP-AM (NAADP-acetoxymethyl ester) evoked global cytosolic Ca2+ signals in isolated neurons that were reduced in amplitude by removal of external Ca2+, abolished by disruption of acidic compartments and substantially inhibited by blockade of ryanodine receptors. In rat medullary slices,NAADPAM depolarized neurons from the nucleus ambiguus in the presence of intracellular EGTA, but not of the faster Ca2+ chelator BAPTA [1,2-bis-(o-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid]. Depolarization was also dependent upon extracellular Ca2+, acidic stores and ryanodine receptors. In voltage-clamp mode, NAADP-AM induced an inward current with a reversal potential of approx. 0 mV. The results of the present study reveal the presence of acidic NAADP-sensitive Ca2+ stores in medulla neurons, the mobilization of which results not only in global Ca2+ signals but also in local signals that activate non-selective cation channels on the cell surface resulting in depolarization. Thus NAADP is capable of co-ordinating channels both within the cell interior and at the cell membrane representing a novel mechanism for excitation of central neurons. © The Authors Journal compilation.