Histamine excites rat superior vestibular nuclear neurons via postsynaptic H1 and H2 receptors in vitro【推荐论文】 .doc

精品论文Histamine excites rat superior vestibular nuclear neurons via postsynaptic H1 and H2 receptors in vitroZHUANG Qianxing, WU Yonghui, WU Guanyi, ZHU Jingning, WANG Jianjun5(School of Life Sciences, Nanjing University, NanJing 210093)Abstract: The superior vestibular nucleus (SVN), which holds a key position in vestibulo-ocular reflexes and nystagmus, receives direct hypothalamic histaminergic innervations. By using rat brainstem slice preparations and extracellular unitary recordings, we investigated the effect of histamine on SVN neurons and the underlying receptor mechanisms. Bath application of histamine10evoked an excitatory response of the SVN neurons, which was not blocked by the low-Ca2+/high-Mg2+medium, indicating a direct postsynaptic effect of the amine. Selective histamine H1 receptor agonist2-pyridylethylamine (2-PyEA) and H2 receptor agonist dimaprit, rather than VUF8430, a selective H4 receptor agonist, mimicked the excitation of histamine on SVN neurons. In addition, selective H1 receptor antagonist mepyramine and H2 receptor antagonist ranitidine, but not JNJ7777120, a selective15H4 receptor antagonist, partially blocked the excitatory response of SVN neurons to histamine.Moreover, mepyramine together with ranitidine nearly totally blocked the histamine-induced excitation.Immunostainings further showed that histamine H1 and H2 instead of H4 receptors existed in the SVN. These results demonstrate that histamine excites the SVN neurons via post-synaptic histamine H1 and H2 receptors, and suggest that the central histaminergic innervation from the hypothalamus may20actively bias the SVN neuronal activity and subsequently modulate the SVN-mediated vestibularfunctions and gaze control.Keywords: Histamine; Superior vestibular nucleus; Histamine receptors; Vestibular reflexes0Introduction25Among the vestibular disorders, pathologic nystagmus is a symptom which often accompanies vertigo and motion sickness. Interestingly, a recent investigation reported an efficacy of the histaminergic drug, betahistine dihydrochloride, in not only antivertigo and anti-motion sickness but also improving oculomotor activity, including increased gain during pursuit movements and faster and more accurate saccades 1. In fact, anti-histaminergic drugs have been30used for clinical treatment of vestibular related diseases, including vertigo, emesis and motion sickness, for almost a century. The therapeutic mechanisms are involved not only in peripheral vestibular system, such as labyrinth in the inner ear 2-3, but also in central vestibular nuclear complex, including at least the medial (MVN) and lateral (LVN) vestibular nucleus 4-6.Immunohistochemical studies have already revealed a moderately dense direct histaminergic35projection from the tuberomammillary nucleus of the hypothalamus to the vestibular nuclei in many species including rat, cat and pigeons 7-9. Nevertheless, histaminergic fiber distribution in the vestibular nuclei shows spatial variations, with significantly heavier labelling in the superior vestibular nucleus (SVN) and MVN than in the LVN and descending vestibular nucleus 9. On the other hand, molecular, autoradiographic and pharmacological studies have also demonstrated40that the LVN and MVN are both endowed with histamine H1, H2 and/or H3 receptors 10-11.Among the four nuclei in the central vestibular nuclear complex, the SVN, together with the MVN, receives fibers predominantly from the semicircular canals and sends fibers through the mediallongitudinal fasciculus rostrally to oculomotor centers and caudally to the spinal cord 12-13.Foundations: This work was supported by grants 31070959, 31071021, 31171050, J1103512 and NSFC/RGC JointResearch Scheme 30931160433 from the National Natural Science Foundation of China; RFDP grant20100091110016, NCET Program, and Fundamental Research Funds for the Central Universities 1094020806 and1095020821 from the State Educational Ministry of China; grant BK2011014 from the Natural Science Foundation of Jiangsu Province, China. This work was also supported by a grant 9112020802 ofBrief author introduction:ZHUANG Qianxing (1980-), male, PhD, NeuroscienceCorrespondance author: WANG Jianjun, male, PhD, Neuroscience. E-mail: jjwangnju.edu.cn- 14 -Both of these two nuclei actually hold a key position in vestibulo-ocular reflexes and gaze control4512, 14-16 and are closely related to nystagmus 17-18.In recent years, role of histamine/hypothalamic histaminergic innervation in the MVN neuronal activity and the MVN-mediated vestibular compensation has been received increasing attention 4, 19-20, however, the action of histamine on the SVN, another key vestibular nucleus primarily involved in reflexes controlling gaze, still remains enigmatic. Thus, by using50extracellular recordings and immunostainings, this work was designed to investigate the effect of histamine on neurons in the SVN and the underlying receptor mechanism. The results demonstrated that histamine excited the SVN neurons via activation of both postsynaptic H1 and H2 receptors.1Material and methods551.1 Slice preparations and incubationsSagittal brain slices (400 M thickness) containing the SVN were prepared from Sprague-Dawley rats (120-250 g) of either sex. Under sodium pentobarbital (40 mg/kg) anesthesia, rats were decapitated. After carefully removing the skull, the brain extending from obex to the superior colliculi was rapidly removed into ice-cold artificial cerebrospinal fluid (ACSF,60composition in mM: 124 NaCl, 5 KCl, 1.2 NaH2PO4, 1.3 MgSO4, 26 NaHCO3, 2 CaCl2 and 10D-glucose) equilibrated with 95% O2/5% CO2. According to the rat brain atlas of Paxinos and Watson 21, the sagittal slices 22 containing the SVN, cerebellum and part of midbrain (including colliculi) were cut (Fig. 1A) with a vibroslicer (VT 1200S, Leica, Germany) at 4 °C. The slices were subsequently transferred into a recording chamber, which was continuously65perfused with 95% O2/5% CO2 oxygenated ACSF (pH 7.4, 33 ± 0.2 °C, flow rate 1.5-2 ml/min).All slices were incubated for at least 40 min before neuronal electrophysiological recordings. All experiments completely conformed to the U.S. National Institutes of Health Guide for the Care and Use of Laboratory Animals (NIH Publications 80-23, revised 1996). All efforts were made to minimize the number of animals used and their suffering.701.2 Electrophysiological recordings, data acquisition and statistical analysisThe SVN was visually identified with the aid of a stereomicroscope (SD-3045F, Olympus, Japan) and spontaneous unitary activity of the SVN neurons was recorded extracellularly from the slices by using glass microelectrodes filled with 2 M NaCl (resistance 5-10 M). Before bath application of histaminergic compounds at known concentrations, the discharge rate of the75recorded neuron was observed for at least 40 min to assure stability. In some experiments, low-Ca2+/high-Mg2+ medium was used to decrease presynaptic neurotransmitter release. In these cases, the concentration of Ca2+ was lowered to 0.3 mM and Mg2+ was raised to 9.0 mM 23-27. Histamine or histamine receptor agonist was added to the perfusing ACSF to stimulate the recorded SVN neuron for a test period of 1 min. After each stimulation, cells were given at least8020 min for recovery and preventing receptors from desensitization. If the SVN neuron responded to the stimulation, the perfusing medium was switched from normal ACSF to the ACSF containing histamine receptor antagonist(s). After the slice was equilibrated with the ACSF containing the antagonist(s) for at least 15 min, histamine or histamine receptor agonist was re-applied and the effect of antagonist(s) on the response of SVN neuron to histamine or histamine85receptor agonist was observed.The neuronal discharges of single unit were amplified and displayed conventionally, and fed into a window discriminator simultaneously. The standard rectangle pulses (5 V, 1 ms) triggered9095100105110115120125from the spikes were sent through an interface (1404 Plus, CED, UK) to a laboratory computer, which was used to analyze the discharge rate online by the software Spike 2 (CED, UK). Peri-stimulus time histograms (PSTHs, sampling interval = 1 s) and the interspike intervals (ISIs, sampling interval = 1 ms) distributions of neuronal discharges were generated by the computer to assess the effects of histamine and histamine receptor agonists on the SVN neurons. Drug-induced effects on spontaneous unitary activity of SVN neurons were considered to be substance specific provided they were reversible and reproducible. The response magnitude of a neuron to the stimulation of histamine or histamine receptor agonist was calculated as the percentage change in the cells peak discharge rate following stimulation with respect to its basal firing rate. All data were expressed as means ± S.E.M.s. Students t-test was employed for statistical analysis of the data and P-values of < 0.05 were considered to be significant.1.3 ImmunofluorescenceThe experimental procedures for immunostaining followed our previous work 28. Rats (weighing 150-200 g) were deeply anesthetized with sodium pentobarbital and perfused transcardially with 100 ml normal saline, followed by 450-500 ml 4% paraformaldehyde in 0.1 M phosphate buffer. Subsequently, the brain was removed, trimmed, and post-fixed in the same fixative for 12 hours at 4 °C and then cryoprotected with 30% sucrose for 48 hours. Frozen coronal sections (25 m thickness) containing the SVN were obtained by using a freezing microtome (CM 3050S, Leica, Germany) and mounted on gelatin-coated slides. The slices were rinsed in phosphate buffered saline containing 0.1% Triton X-100 (PBST) and then incubated in10% normal bovine serum in PBST for 30 min. Sections were incubated overnight at 4 °C with primary antibodies to histamine H1, H2 or H4 receptor, respectively: a rabbit anti-H1 receptor polyclonal antibody (1:50, Santa Cruz, USA), a rabbit anti-histamine H2 receptor polyclonal antibody (1:50, Santa Cruz, USA), or a rabbit anti-histamine H4 receptor polyclonal antibody (1:50, Santa Cruz, USA). After a complete wash in PBS, these sections were incubated in the Alexa 488-conjugated secondary antibody solutions (goat anti-rabbit 1:2000, Invitrogen, USA) for2 h at room temperature in dark. The slides were washed and mounted in UltraCruz mounting medium (Santa Cruz, USA). All micrographs were taken with an inverted laser scanning confocal microscope (FV1000, Olympus, Japan). Incubations replacing the primary antiserum with control immunoglobulins and/or omitting the primary antiserum were used as negative controls.1.4 Histaminergic reagentsStocks solutions of histaminergic compounds were made in distilled water, and dilutions were freshly prepared in ACSF and equilibrated with 95% O2/5% CO2 before superfusing the slices. The histaminergic compounds used in this experiment included histamine (Sigma, USA), highly selective histamine H1 receptor antagonist mepyramine (Tocris, UK), highly selective histamine H2 receptor antagonist ranitidine (Tocris, UK), highly selective histamine H1 receptor agonist 2-pyridylethylamine (2-PyEA; Tocris, UK), and highly selective histamine H2 receptor agonist dimaprit (Tocris, UK). JNJ7777120, a highly selective histamine H4 receptor antagonist, and VUF8430, a highly selective histamine H4 receptor agonist, were kind gifts from Dr. Rob Leurs (VU University Amsterdam, Amsterdam, The Netherlands). The concentrations ofhistaminergic reagents were chosen according to the previous reports 4, 6, 23-27, 29-30.1302Results1351401451501551601651702.1 Histamine directly excites the SVN neurons in a concentration-dependent mannerOne hundred and twenty SVN neurons with tonic spontaneous firing activity were extracellularly recorded from 55 brainstem slices of rats in the present study. The spontaneous firing rate of the SVN neurons ranged from 5 to 80 spikes/s and the mean firing rate was 25.32 ±0.86 spikes/s, which are similar to the previous study on the SVN neurons in vivo 31 and slightlyhigher than MVN neuronal spontaneous firing rate in vitro 4, 32-33.All of the 120 recorded SVN neurons (120/120, 100%) sampled from various sub-regions of the SVN (Fig. 1A) were excited by the histamine stimulation (1-30 M; Fig. 1B). As illustrated in Fig. 1C (see also Fig. 3), the recorded SVN neuron showed a concentration-dependent excitation to 1, 3, and 10 M histamine with peak firing values of 24.5, 26.8 and 29.8 spikes/s, i.e. 8.4%,18.6% and 31.9% increment in the peak discharge rate compared with its basal firing rate of 22.6spikes/s (P < 0.05 or 0.01), respectively. Furthermore, ISIs revealed that during the period of histamine-induced excitation, the interspike intervals of SVN neurons were significantly shortened (P < 0.05 or 0.01, Fig. 1D). The results indicate that histamine is capable of exciting SVN neurons.To exclude the possibility that the excitatory response of SVN neurons was indirectly induced by the effect of histamine on presynaptic elements, we tested the effects of histamine on the SVN cells when the normal ACSF had been replaced with a low-Ca2+/high-Mg2+ medium (n =5). The results showed that the low-Ca2+/high-Mg2+ medium did not block the histamine-inducedexcitation (P > 0.05; Fig. 1E vs C), although the spontaneous firing rates of some tested neurons were slightly decreased, which might be related to a disturbance of normal Ca2+ concentration in local milieu and/or to actions of Mg2+ on intracellular Ca2+-dependent processes 23, 32, 34-35 This result suggests a direct postsynaptic excitatory effect of histamine on the SVN neurons.2.2 Thehistamine-inducedpostsynapticexcitationonSVNneuronsis co-mediated by histamine H1 and H2 receptorsSince the histamine H1, H2 and H4 receptors are all postsynaptic whereas H3 receptors are presynaptic 36-37, we further used histamine receptor agonists and antagonists to examine which postsynaptic histamine receptor(s) mediated the histamine-induced excitation on SVN neurons. The results showed that both the selective H1 receptor agonist 2-PyEA (3-100