A sodium background conductance controls the spiking pattern of mouse adrenal chromaffin cells in situ.

KEY POINTS: Mouse chromaffin cells in acute adrenal slices exhibit two distinct spiking patterns, a repetitive mode and a bursting mode. A sodium background conductance operates at rest as demonstrated by the membrane hyperpolarization evoked by a low Na+ -containing extracellular saline. This sodium background current is insensitive to TTX, is not blocked by Cs+ ions and displays a linear I-V relationship at potentials close to chromaffin cell resting potential. Its properties are reminiscent of those of the sodium leak channel NALCN. In the adrenal gland, Nalcn mRNA is selectively expressed in chromaffin cells. The study fosters our understanding of how the spiking pattern of chromaffin cells is regulated and adds a sodium background conductance to the list of players involved in the stimulus-secretion coupling of the adrenomedullary tissue. ABSTRACT: Chromaffin cells (CCs) are the master neuroendocrine units for the secretory function of the adrenal medulla and a finely-tuned regulation of their electrical activity is required for appropriate catecholamine secretion in response to the organismal demand. Here, we aim at deciphering how the spiking pattern of mouse CCs is regulated by the ion conductances operating near the resting membrane potential (RMP). At RMP, mouse CCs display a composite firing pattern, alternating between active periods composed of action potentials spiking with a regular or a bursting mode, and silent periods. RMP is sensitive to changes in extracellular sodium concentration, and a low Na+ -containing saline hyperpolarizes the membrane, regardless of the discharge pattern. This RMP drive reflects the contribution of a depolarizing conductance, which is (i) not blocked by tetrodotoxin or caesium, (ii) displays a linear I-V relationship between -110 and -40 mV, and (iii) is carried by cations with a conductance sequence gNa  > gK  > gCs . These biophysical attributes, together with the expression of the sodium-leak channel Nalcn transcript in CCs, state credible the contribution of NALCN. This inaugural report opens new research routes in the field of CC stimulus-secretion coupling, and extends the inventory of tissues in which NALCN is expressed to neuroendocrine glands.

Roles of Na+, Ca2+, and K+ channels in the generation of repetitive firing and rhythmic bursting in adrenal chromaffin cells.

Adrenal chromaffin cells (CCs) are the main source of circulating catecholamines (CAs) that regulate the body response to stress. Release of CAs is controlled neurogenically by the activity of preganglionic sympathetic neurons through trains of action potentials (APs). APs in CCs are generated by robust depolarization following the activation of nicotinic and muscarinic receptors that are highly expressed in CCs. Bovine, rat, mouse, and human CCs also express a composite array of Na+, K+, and Ca2+ channels that regulate the resting potential, shape the APs, and set the frequency of AP trains. AP trains of increasing frequency induce enhanced release of CAs. If the primary role of CCs is simply to relay preganglionic nerve commands to CA secretion, why should they express such a diverse set of ion channels? An answer to this comes from recent observations that, like in neurons, CCs undergo complex firing patterns of APs suggesting the existence of an intrinsic CC excitability (non-neurogenically controlled). Recent work has shown that CCs undergo occasional or persistent burst firing elicited by altered physiological conditions or deletion of pore-regulating auxiliary subunits. In this review, we aim to give a rationale to the role of the many ion channel types regulating CC excitability. We will first describe their functional properties and then analyze how they contribute to pacemaking, AP shape, and burst waveforms. We will also furnish clear indications on missing ion conductances that may be involved in pacemaking and highlight the contribution of the crucial channels involved in burst firing.