Effects Of Histamine And Orexin On The Superior Vestibular Nucleus And Cerebellar Interpositus Nucleus Neurons And The Underlying Receptor Mechanisms

The central histaminergic and orexinergic systems originate from the hypothalamus. The former originates from tuberomammillary nucleus of hypothalamus, whereas the latter originates from perifornical area and lateral hypothalamic area. Both of them innervate almost the whole brain and their receptors widely present in the central nervous system.The cerebellum and vestibular nuclei are two important subcortical motor structures, which play an important role in muscle tone, balance and motor control. Previous studies have revealed the cerebellum and vestibular nuclei are not only innervated by both histaminergic and orexinergic fibers but also endowed with both histamine and orexin receptors. Therefore, the study is focused on the effects and receptor mechanisms of histamine and orexins on some neurons in two subcortical motor structures by using electrophysiological recordings, immunofluorescence and qPCR. Part1:Histamine postsynaptically excites rat superior vestibular nuclear neurons via H1and H2receptors in vitroWe investigated the effect of histamine on superior vestibular nucleus (SVN) neurons and the underlying receptor mechanisms by using rat brainstem slice preparations and extracellular recordings. Bath application of histamine (1-30μM) evoked 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 H1receptor agonist2-pyridylethylamine (2-PyEA)(3-100μM) and H2receptor agonist dimaprit (3-100μM), rather than VUF8430(3-100μM), a selective H4receptor agonist, mimicked the excitation of histamine on SVN neurons. In addition, selective H1receptor antagonist mepyramine (1μM) and H2receptor antagonist ranitidine (1μM), but not JNJ7777120(10μM), a selective H4receptor antagonist, partially blocked the excitatory response of SVN neurons to histamine. Moreover, mepyramine (1μM) together with ranitidine (1μM) nearly totally blocked the histamine-induced excitation. Immunostainings further showed that histamine H1and H2instead of H4receptors existed in the SVN. These results demonstrate that histamine excites the SVN neurons via post-synaptic histamine H1and H2receptors, and suggest that the central histaminergic innervation from the hypothalamus may actively bias the SVN neuronal activity and subsequently modulate the SVN-mediated vestibular functions and gaze control.Part2:Receptor mechanism underlying the orexin-induced excitation on cerebellar interpositus nucleus neurons in rats at different agesIt is reported that the central orexinergic nervous system changes with development. In order to observe the change of effect of orexin on cereballar nuclei at different developmental stages, the effect and receptor mechanisms of orexin on cerebellar interpositus nucleus (IN) neurons were investigated in rats at three different age stages (7-14days,21-28days, and adult) in the present study. First, qPCR results showed that both orexin1receptors (OX1R) and orexin2receptors (OX2R) were existed in the rat cerebellar IN at all three age stages, and there was no significant difference in two receptors mRNA expression in rat cerebellar nuclei neurons at three different age stages.Next, using extracellular recordings, the response of IN neurons to orexin-A (300nM) stimulation was excitatory at all three age stages. Moreover, an increase of neuronal firing rate elicited by orexin-A was not only partly dose-dependently blocked by selective OX1R antagonist SB334867(1μM,3μM,10μM), but also dose-dependently mimicked by selective OX2R agonist [Ala11, D-Leu15]-orexin-B (300nM,1uM) at all three age stages, implicating that the orexin-A-induced excitation was mediated by both OX1R and OX2R.Furthermore, according to statistics analysis, both the amplitude of reaction elicited by orexin-A or [Ala11, D-Leu15]-orexin-B and the antagonistic effect induced by SB334867on IN neurons did not change at different age stages. All these results demonstrate a consistent excitatory effect of orexin on IN neurons and the underlying receptor mechanisms at different age stages, and suggest that regulation of orexin on motor control via cereballar circuitry may not change with development.