Download Orexin-A excites rat lateral vestibular nucleus neurons and improves

Orexin-A excites medial vestibular nucleus neurons through
nonselective cation channels and Na+/Ca2+ exchangers
Xiao-Yang ZHANG, Qian-Xing ZHUANG, Jing-Ning ZHU*, Jian-Jun WANG*
Department of Biological Science and Technology and State Key Laboratory of Pharmaceutical
Biotechnology, School of Life Sciences, Mailbox 426, Nanjing University, 22 Hankou Road,
Nanjing 210093, China
* Corresponding authors
E-mail: [email protected]; [email protected]
Orexin is a newly discovered peptide which is synthesized exclusively within the
lateral hypothalamic area and perifornical area. Lack of orexin neurons causes
narcolepsy-cataplexy, which is characterized by excessive daytime sleepiness,
premature transitions to REM sleep, and sudden skeletal muscle weakness without
impairment of consciousness. However, most studies so far on the pathogenesis of
narcolepsy-cataplexy have been limited merely to the role of orexin in sleep-awake
cycle, and the function of orexin on motor control has still been little known. In the
present study, effect of orexin-A on the medial vestibular nucleus (MVN), which
holds a key position in controlling head and eye movements, were investigated.
Immunofluorescence histochemical results showed that both orexin 1 receptors
(OX1Rs) and orexin 2 receptors (OX2Rs) are presented in the MVN in rats. Using
brain slice preparations and whole-cell patch clamp recordings, type-A and type-B
MVN neurons were identified according to the shape of their after hyperpolarization
action potentials. A dose-dependent increase in neuronal firing rates and an inward
current elicited by orexin-A were observed in both type-A and type-B MVN neurons.
The orexin-A-elicited excitation on the type-A and type-B MVN neurons were not
only partly blocked by selective OX1R antagonist SB334867 but also mimicked by
selective OX2R agonist [Ala11, D-Leu15]-orexin B, indicating that the
orexin-A-induced excitation on MVN neurons was mediated by both OX1R and
OX2R. Moreover, tetrodotoxin (TTX) did not block the excitations evoked by
orexin-A, suggesting the actions of the peptide on MVN neurons are postsynaptic.
Coupled with an inward current elicited by orexin-A, a slight decrease of membrane
conductance of the MVN neurons was observed and also was not blocked by TTX,
suggesting activities of ion channels on postsynaptic membrane are increased by
orexin directly. The I-V curves before and after application of orexin-A on MVN
neurons showed that more than one ionic basis was involved in the orexin-A-induced
excitation. Both Ba2+, a K+ conductance blocker, and high K+ had no effect on the
orexin-A-elicited inward currents on both type-A and type-B MVN neurons,
excluding the possibility involvement of a K+ current. After NaCl in the artificial
cerebrospinal fluid was replaced with Tris-Cl, the orexin-A-elicited inward current
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was totally blocked, suggesting a dependence on external Na+. Therefore, the effect of
blocking the Na+/Ca2+exchanger (NCX) was further tested. KB-R7943, a selective
blocker of NCX, partly blocked the orexin-A-elicited inward current on MVN
neurons. Furthermore, the reversal potential of the orexin-A-induced current in the
condition of blocking the NCX was around −40 mV, which is exactly the reversal
potential of nonselective cation channels (NSCC). And the orexin-A-evoked current
was found to be voltage independence, which is also the characteristic of NSCC.
These results demonstrate that orexin-A excites both type-A and type-B MVN
neurons via activation of OX1R and OX2R coupled with NCX and NSCC. Thus, it is
suggested that the central orexinergic system arising from the hypothalamus may
actively participate in the control of head and eye movement through its direct
modulation on the activities of MVN neurons. These findings may also propose a new
potential mechanism for why the absence of orexin results in narcolepsy-cataplexy.
(Supported by NSFC grants 30670671, 30700201, 31070959, and 31071021,
NSFC/RGC grant 30931160433, and RFDP grants 20070284057 and
20100091110016)
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