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

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 acidic Ca2+ 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, NAADP-AM 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.