Calcium-induced calcium release in noradrenergic neurons of the locus coeruleus

Abstract

The locus coeruleus (LC) is a nucleus within the brainstem that consists of norepinephrine-releasing neurons. It is involved in broad processes including autonomic regulation, and cognitive and emotional functions such as arousal, attention and anxiety. Understanding the mechanisms that control the excitability of LC neurons is important because they innervate widespread regions of the central nervous system. One of the key regulators is the cytosolic calcium concentration ([Ca2+]c), the increases in which can be amplified by calcium-induced calcium release (CICR) from the intracellular calcium stores. Although the electrical activities of LC neurons are regulated by changes in [Ca2+]c, the extent of CICR involvement in this regulation has remained unclear. Here we show that CICR hyperpolarizes acutely dissociated LC neurons of the rat brain and demonstrate the pathway whereby it does this. When CICR was activated by extracellular application of 10 mM caffeine, LC neurons were hyperpolarized in the current-clamp mode of the patch-clamp recording, and the majority of neurons showed an outward current in the voltage-clamp mode. This outward current was accompanied by an increase in membrane conductance, and its reversal potential was close to the K+ equilibrium potential, indicating that it is mediated by the opening of K+ channels. The outward current was generated in the absence of extracellular calcium and was blocked when the calcium stores were inhibited by applying ryanodine. Pharmacological experiments indicated that the outward current was mediated by Ca2+-activated K+ channels of the non-small conductance type. Finally, the application of caffeine led to an increase in the [Ca2+]c in these neurons, as visualized by fluorescence microscopy. These findings delineate a mechanism whereby CICR suppresses the electrical activity of LC neurons, and indicate that it could play a dynamic role in modulating the LC-mediated noradrenergic tone in the brain.