Proton- and ammonium-sensing by histaminergic neurons controlling wakefulness

Yevgenij Yanovsky,Anna Kernder,Jeffrey M Zigman,Olga A Sergeeva,Sherri Osborne-Lawrence,Helmut L Haas,Alisa Bein,Ichiro Sakata

doi:10.3389/fnsys.2012.00023

Abstract

The histaminergic neurons in the tuberomamillary nucleus (TMN) of the posterior hypothalamus are involved in the control of arousal. These neurons are sensitive to hypercapnia as has been shown in experiments examining c-Fos expression, a marker for increased neuronal activity. We investigated the mechanisms through which TMN neurons respond to changes in extracellular levels of acid/CO2. Recordings in rat brain slices revealed that acidification within the physiological range (pH from 7.4 to 7.0), as well as ammonium chloride (5 mM), excite histaminergic neurons. This excitation is significantly reduced by antagonists of type I metabotropic glutamate receptors and abolished by benzamil, an antagonist of acid-sensing ion channels (ASICs) and Na+/Ca2+ exchanger, or by ouabain which blocks Na+/K+ ATPase. We detected variable combinations of 4 known types of ASICs in single TMN neurons, and observed activation of ASICs in single dissociated TMN neurons only at pH lower than 7.0. Thus, glutamate, which is known to be released by glial cells and orexinergic neurons, amplifies the acid/CO2-induced activation of TMN neurons. This amplification demands the coordinated function of metabotropic glutamate receptors, Na+/Ca2+ exchanger and Na+/K+ ATPase. We also developed a novel HDC-Cre transgenic reporter mouse line in which histaminergic TMN neurons can be visualized. In contrast to the rat, the mouse histaminergic neurons lacked the pH 7.0-induced excitation and displayed only a minimal response to the mGluR I agonist DHPG (0.5 μM). On the other hand, ammonium-induced excitation was similar in mouse and rat. These results are relevant for the understanding of the neuronal mechanisms controlling acid/CO2-induced arousal in hepatic encephalopathy and obstructive sleep apnoea. Moreover, the new HDC-Cre mouse model will be a useful tool for studying the physiological and pathophysiological roles of the histaminergic system.

Highlights

The mammalian brain depends on a constant supply of glucose and oxygen
ACSF ACIDIFICATION EXCITES RAT E2 tuberomamillary nucleus (TMN) NEURONS As it was shown previously that only the ventrolateral TMN of the rat shows increased c-Fos expression in response to systemic hypercapnia (Johnson et al, 2005), we performed patch-clamp recordings from ventrolateral TMN, which corresponds to the E2 group (Figure 1A)
We found that activation of P2Y1 receptors in hypothalamic slices containing TMN causes release of glutamate which amplifies the excitatory response through type I metabotropic glutamate receptors

Summary

Introduction

The mammalian brain depends on a constant supply of glucose and oxygen. Lack of either one of them rapidly leads to failure of neuronal function. Extracellular levels of CO2 change in response to alterations of blood oxygen levels and are fundamental physicochemical signals controlling breathing and wakefulness. The mechanisms connecting changes in extracellular CO2 to arousal are unclear. Arousal-inducing systems are modulated, either directly or indirectly, in response to hypercapnia or acidification. Serotonin neurons from the dorsal raphe respond to CO2 and genetic deletion of serotonin neurons abolishes the ventilatory response to hypercapnia (Corcoran et al, 2009; Buchanan and Richerson, 2010). Previous studies have shown that the orexin (hypocretin) neurons in the perifornical area of the posterior hypothalamus are highly sensitive to minor changes in the extracellular pH (Williams et al, 2007), displaying a 100% increase in firing rate in response to a 0.1 unit acidic shift. In wild type but not in orexin-deficient mice augmented ventilation is observed during the 2 h following a brief hypercapnia episode (Terada et al, 2008), implicating the orexin system in the ventilatory response

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