Histamine excites rat lateral vestibular nuclear neurons through activation of post-synaptic H2 receptors

Histaminergic System and Vestibular Function in Normal and Pathological Conditions

Most neurotransmitter systems are represented in the central and peripheral vestibular system and are thereby involved both in normal vestibular signal processing and the pathophysiology of vestibular disorders. However, there is a special relationship between the vestibular system and the histaminergic system. The purpose of this review is to document how the histaminergic system interferes with normal and pathological vestibular function. In particular, we will discuss neurobiological mechanisms such as neuroinflammation that involve histamine to modulate and allow restoration of balance function in the situation of a vestibular insult. These adaptive mechanisms represent targets of histaminergic pharmacological compounds capable of restoring vestibular function in pathological situations. The clinical use of drugs targeting the histaminergic system in various vestibular disorders is critically discussed.

Effects of Several Therapeutic Agents on Mammalian Vestibular Function: Meclizine, Diazepam, and JNJ7777120

Management of vestibular dysfunction may include treatment with medications that are thought to act to suppress vestibular function and reduce or eliminate abnormal sensitivity to head motions. The extent to which vestibular medications act centrally or peripherally is still debated. In this study, two commonly prescribed medications, meclizine and diazepam, and a candidate for future clinical use, JNJ7777120, were evaluated for their effects on short latency compound action potentials generated by the peripheral vestibular system and corresponding central neural relays (i.e., vestibular sensory-evoked potentials, VsEPs). The effects of the selected drugs developed slowly over the course of two hours in the mouse. Findings indicate that meclizine (600 mg/kg) and diazepam (> 60 mg/kg) can act on peripheral elements of the vestibular maculae whereas diazepam also acts most effectively on central gravity receptor circuits to exert its suppressive effects. The novel pharmacological agent JNJ7777120 (160 mg/kg) acts in the vestibular periphery to enhance macular responses to transient stimuli (VsEPs) while, hypothetically, suppressing macular responses to sustained or slowly changing stimuli.

Corticotropin-releasing factor depolarizes rat lateral vestibular nuclear neurons through activation of CRF receptors 1 and 2

Corticotropin-releasing factor (CRF) is a neuropeptide mainly synthesized in the hypothalamic paraventricular nucleus and has been traditionally implicated in stress and anxiety. Intriguingly, genetic or pharmacological manipulation of CRF receptors affects locomotor activity as well as motor coordination and balance in rodents, suggesting an active involvement of the central CRFergic system in motor control. Yet little is known about the exact role of CRF in central motor structures and the underlying mechanisms. Therefore, in the present study, we focused on the effect of CRF on the lateral vestibular nucleus (LVN) in the brainstem vestibular nuclear complex, an important center directly contributing to adjustment of muscle tone for both postural maintenance and the alternative change from the extensor to the flexor phase during locomotion. The results show that CRF depolarizes and increases the firing rate of neurons in the LVN. Tetrodotoxin does not block the CRF-induced depolarization and inward current on LVN neurons, suggesting a direct postsynaptic action of the neuropeptide. The CRF-induced depolarization on LVN neurons was partly blocked by antalarmin or antisauvagine-30, selective antagonists for CRF receptors 1 (CRFR1) and 2 (CRFR2), respectively. Furthermore, combined application of antalarmin and antisauvagine-30 totally abolished the CRF-induced depolarization. Immunofluorescence results show that CRFR1 and CRFR2 are co-localized in the rat LVN. These results demonstrate that CRF excites the LVN neurons by co-activation of both CRFR1 and CRFR2, suggesting that via the direct modulation on the LVN, the central CRFergic system may actively participate in the central vestibular-mediated postural and motor control.

Histamine Increases Neuronal Excitability and Sensitivity of the Lateral Vestibular Nucleus and Promotes Motor Behaviors via HCN Channel Coupled to H2 Receptor

Histamine and histamine receptors in the central nervous system actively participate in the modulation of motor control. In clinic, histamine-related agents have traditionally been used to treat vestibular disorders. Immunohistochemical studies have revealed a distribution of histaminergic afferents in the brainstem vestibular nuclei, including the lateral vestibular nucleus (LVN), which is critical for adjustment of muscle tone and vestibular reflexes. However, the mechanisms underlying the effect of histamine on LVN neurons and the role of histamine and histaminergic afferents in the LVN in motor control are still largely unknown. Here, we show that histamine, in cellular and molecular levels, elicits the LVN neurons of rats an excitatory response, which is co-mediated by the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels and K⁺ channels linked to H2 receptors. Blockage of HCN channels coupled to H2 receptors decreases LVN neuronal sensitivity and changes their dynamic properties. Furthermore, in behavioral level, microinjection of histamine into bilateral LVNs significantly promotes motor performances of rats on both accelerating rota-rod and balance beam. This promotion is mimicked by selective H2 receptor agonist dimaprit, and blocked by selective H2 receptor antagonist ranitidine. More importantly, blockage of HCN channels to suppress endogenous histaminergic inputs in the LVN considerably attenuates motor balance and coordination, indicating a promotion role of hypothalamo-vestibular histaminergic circuit in motor control. All these results demonstrate that histamine H2 receptors and their coupled HCN channels mediate the histamine-induced increase in excitability and sensitivity of LVN neurons and contribute to the histaminergic improvement of the LVN-related motor behaviors. The findings suggest that histamine and the histaminergic afferents may directly modulate LVN neurons and play a critical role in the central vestibular-mediated motor reflexes and behaviors.

Principles of vestibular pharmacotherapy

Ideally, vestibular pharmacotherapy is intended, through specific and targeted molecular actions, to significantly alleviate vertigo symptoms, to protect or repair the vestibular sensory network under pathologic conditions, and to promote vestibular compensation, with the eventual aim of improving the patient's quality of life. In fact, in order to achieve this aim, considerable progress still needs to be made. The lack of information on the etiology of vestibular disorders and the pharmacologic targets to modulate, as well as the technical challenge of targeting a drug to its effective site are some of the main issues yet to be overcome. In this review, my intention is to provide an account of the therapeutic principles that have shaped current vestibular pharmacotherapy and to further explore crucial questions that must be taken into consideration in order to develop targeted and specific pharmacologic therapies for each type and stage of vestibular disorders.

Na(+) -Ca(2+) Exchanger, Leak K(+) Channel and Hyperpolarization-Activated Cyclic Nucleotide-Gated Channel Comediate the Histamine-Induced Excitation on Rat Inferior Vestibular Nucleus Neurons

AIMS

Antihistaminergic drugs have traditionally been used to treat vestibular disorders in the clinic. As a potential central target for antihistaminergic drugs, the inferior vestibular nucleus (IVN) is the largest subnucleus of the central vestibular nuclear complex and is considered responsible for vestibular-autonomic responses and integration of vestibular, cerebellar, and multisensory signals. However, the role of histamine on the IVN, particularly the underlying mechanisms, is still not clear.

METHODS

Using whole-cell patch-clamp recordings on rat brain slices, histamine-induced effect on IVN neurons and the underlying receptor and ionic mechanisms were investigated.

RESULTS

We found that histamine remarkably depolarized both spontaneous firing neurons and silent neurons in IVN via both histamine H1 and histamine H2 receptors. Furthermore, Na(+) -Ca(2+) exchangers (NCXs) and background leak K(+) channels linked to H1 receptors and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels coupled to H2 receptors comediate the histamine-induced depolarization on IVN neurons.

CONCLUSION

These results demonstrate the multiple ionic mechanisms underlying the excitatory modulation of histamine/central histaminergic system on IVN neurons and the related vestibular reflexes and functions. The findings also suggest potential targets for the treatment of vestibular disorders in the clinic, at the level of ionic channels in central vestibular nuclei.

Orexin excites rat inferior vestibular nuclear neurons via co-activation of OX1 and OX 2 receptors

Orexin deficiency results in cataplexy, a motor deficit characterized by sudden loss of muscle tone, strongly indicating an active role of central orexinergic system in motor control. However, effects of orexin on neurons in central motor structures are still largely unknown. Our previous studies have revealed that orexin excites neurons in the cerebellar nuclei and lateral vestibular nucleus, two important subcortical motor centers for control of muscle tone. Here, we report that both orexin-A and orexin-B depolarizes and increases the firing rate of neurons in the inferior vestibular nucleus (IVN), the largest nucleus in the vestibular nuclear complex and holding an important position in integration of information signals in the control of body posture. TTX does not block orexin-induced excitation on IVN neurons, suggesting a direct postsynaptic action of the neuropeptide. Furthermore, bath application of orexin induces an inward current on IVN neurons in a concentration-dependent manner. SB334867 and TCS-OX2-29, specific OX1 and OX2 receptor antagonists, blocked the excitatory effect of orexin, and [Ala(11), D-Leu(15)]-orexin B, a selective OX2 receptor agonist, mimics the orexin-induced inward current on IVN neurons. qPCR and immunofluorescence results show that both OX1 and OX2 receptor mRNAs and proteins are expressed and localized in the rat IVN. These results demonstrate that orexin excites the IVN neurons by co-activation of both OX1 and OX2 receptors, suggesting that via the direct modulation on the IVN, the central orexinergic system may actively participate in the central vestibular-mediated postural and motor control.

Postsynaptic mechanisms underlying the excitatory action of histamine on medial vestibular nucleus neurons in rats

BACKGROUND AND PURPOSE

Anti-histaminergic drugs have been widely used in the clinical treatment of vestibular disorders and most studies concentrate on their presynaptic actions. The present study investigated the postsynaptic effect of histamine on medial vestibular nucleus (MVN) neurons and the underlying mechanisms.

EXPERIMENTAL APPROACH

Histamine-induced postsynaptic actions on MVN neurons and the corresponding receptor and ionic mechanisms were detected by whole-cell patch-clamp recordings on rat brain slices. The distribution of postsynaptic histamine H₁, H₂ and H₄ receptors was mapped by double and single immunostaining. Furthermore, the expression of mRNAs for H₁, H₂ and H₄ receptors and for subtypes of Na⁺ -Ca²⁺ exchangers (NCXs) and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels was assessed by quantitative real-time RT-PCR.

KEY RESULTS

A marked postsynaptic excitatory effect, co-mediated by histamine H₁ and H₂ receptors, was involved in the histamine-induced depolarization of MVN neurons. Postsynaptic H₁ and H₂ rather than H₄ receptors were co-localized in the same MVN neurons. NCXs contributed to the inward current mediated by H₁ receptors, whereas HCN channels were responsible for excitation induced by activation of H₂ receptors. Moreover, NCX1 and NCX3 rather than NCX2, and HCN1 rather than HCN2-4 mRNAs, were abundantly expressed in MVN.

CONCLUSION AND IMPLICATIONS

NCXs coupled to H₁ receptors and HCN channels linked to H₂ receptors co-mediate the strong postsynaptic excitatory action of histamine on MVN neurons. These results highlight an active role of postsynaptic mechanisms in the modulation by central histaminergic systems of vestibular functions and suggest potential targets for clinical treatment of vestibular disorders.

Histaminergic afferent system in the cerebellum: structure and function

Histaminergic afferent system of the cerebellum, having been considered as an essential component of the direct hypothalamocerebellar circuits, originates from the tuberomammillary nucleus in the hypothalamus. Unlike the mossy fibers and climbing fibers, the histaminergic afferent fibers, a third type of cerebellar afferents, extend fine varicose fibers throughout the cerebellar cortex and nuclei. Histamine receptors, belonging to the family of G protein-coupled receptors, are widely present in the cerebellum. Through these histamine receptors, histamine directly excites Purkinje cells and granule cells in the cerebellar cortex, as well as the cerebellar nuclear neurons. Therefore, the histaminergic afferents parallelly modulate these dominant components in the cerebellar circuitry and consequently influence the final output of the cerebellum. In this way, the histaminergic afferent system actively participates in the cerebellum-mediated motor balance and coordination and nonsomatic functions. Accordingly, histaminergic reagents may become potential drugs for clinical treatment of cerebellar ataxia and other cerebellar disease. On the other hand, considering the hypothalamus is a high regulatory center for autonomic and visceral activities, the hypothalamocerebellar histaminergic fibers/projections, bridging the nonsomatic center to somatic structure, may play a critical role in the somatic-nonsomatic integration.

Plasticity of the histamine H3 receptors after acute vestibular lesion in the adult cat

After unilateral vestibular neurectomy (UVN) many molecular and neurochemical mechanisms underlie the neurophysiological reorganizations occurring in the vestibular nuclei (VN) complex, as well as the behavioral recovery process. As a key regulator, the histaminergic system appears to be a likely candidate because drugs interfering with histamine (HA) neurotransmission facilitate behavioral recovery after vestibular lesion. This study aimed at analyzing the post-lesion changes of the histaminergic system by quantifying binding to histamine H₃ receptors (H₃R; mediating namely histamine autoinhibition) using a histamine H₃ receptor agonist ([³H]N-α-methylhistamine). Experiments were done in brain sections of control cats (N = 6) and cats submitted to UVN and killed 1 (N = 6) or 3 (N = 6) weeks after the lesion. UVN induced a bilateral decrease in binding density of the agonist [³H]N-α-methylhistamine to H₃R in the tuberomammillary nuclei (TMN) at 1 week post-lesion, with a predominant down-regulation in the ipsilateral TMN. The bilateral decrease remained at the 3 weeks survival time and became symmetric. Concerning brainstem structures, binding density in the VN, the prepositus hypoglossi, the subdivisions of the inferior olive decreased unilaterally on the ipsilateral side at 1 week and bilaterally 3 weeks after UVN. Similar changes were observed in the subdivisions of the solitary nucleus only 1 week after the lesion. These findings indicate vestibular lesion induces plasticity of the histamine H₃R, which could contribute to vestibular function recovery.

Hypothalamic histaminergic and orexinergic modulation on cerebellar and vestibular motor control

Somatic-nonsomatic integration is critical for generation and execution of an appropriate and coordinated behavioral response to changes in internal and external environments. However, the underlying neural substrates and mechanisms are still enigmatic. Intriguingly, the central histaminergic and orexinergic systems originating from the hypothalamus, a high autonomic regulatory center, innervate almost the whole brain including various subcortical motor structures, particularly the cerebellum and vestibular nuclei. Here, we suggest that the hypothalamic histaminergic and orexinergic system bridging the nonsomatic center to somatic motor structures may actively modulate the cerebellar and vestibular nuclear neurons and subsequently participate in motor control and somatic-nonsomatic integration.

Histamine excites neurons of the inferior vestibular nucleus in rats by activation of H1 and H2 receptors

By using brain slice preparations and extracellular recordings, the effect of histamine on spontaneous firing activities of neurons in the inferior vestibular nucleus (IVN), a key structure responsible for integration of vestibular, multisensory, and cerebellar inputs, in rats was investigated. Perfusing slices with histamine (1-10μM) elicited an excitatory response on IVN neurons. The responses were not blocked by low Ca(2+)/high Mg(2+) medium, indicating a direct postsynaptic effect of the amine. Furthermore, the histamine-induced excitation was partially blocked by selective histamine H1 receptor antagonist mepyramine (1μM) and H2 receptor antagonist ranitidine (1μM), respectively. Co-application of mepyramine and ranitidine nearly totally antagonized the histamine-induced excitation. Additionally, both selective H1 receptor agonist 2-pyridylethylamine (30-300μM) and H2 receptor agonist dimaprit (10-100μM) effectively mimicked the excitatory action of histamine on IVN neurons. Moreover, selective H4 antagonist JNJ7777120 (10μM) and agonist VUF8430 (30-300μM) had no effect on IVN neurons. These results demonstrate that histamine excites IVN neurons via postsynaptic H1 and H2 rather than H4 receptors, and suggest that the central histaminergic system actively modulate all four major vestibular nuclei including the IVN and may subsequently influence the vestibular nuclei-related reflexes and functions.

A role for orexin in central vestibular motor control

The absence of orexin results in narcolepsy-cataplexy. While the function of the central orexinergic system in sleep regulation has been well studied, the role of orexin in motor control is largely unknown. Here, we show that orexin-A acts via OX(1) and OX(2) receptors to directly depolarize neurons in the rat lateral vestibular nucleus (LVN), a subcortical motor center, and enhance their sensitivity. A dual ionic mechanism involving both Na+-Ca²+ exchangers and inward rectifier K+ channels underlies these effects. Furthermore, orexin-A regulates central vestibular-mediated posture, motor balance and negative geotaxis. Orexin is critical when an animal is facing a major motor challenge as opposed to during rest and general movements. Therefore, orexin participates not only in sleep and emotion (nonsomatic) but also in motor (somatic) regulation, suggesting that the central orexinergic system plays an important role in somatic-nonsomatic integration. These findings may account for why the absence of orexin results in narcolepsy-cataplexy.

Histamine regulates activities of neurons in the ventrolateral preoptic nucleus

The neurons responsible for the onset of sleep are thought to be located in the ventrolateral preoptic nucleus (VLPO), which receives a dense histaminergic innervation from the tuberomammillary nucleus (TMN). Yet, the role of histamine in the VLPO remains unclear. Here we report that microinjection of histamine into the VLPO increases the motor activity of rats. Moreover, a bath application of histamine to acute brain slices inhibits the majority of VLPO neurons, which are also inhibited by noradrenaline. Histamine hyperpolarizes the membrane potential and lowers the firing rate. These effects are associated with an increase in the frequency but not in the amplitude of spontaneous GABA(A) receptor-mediated inhibitory postsynaptic currents, and are blocked by gabazine or tetrodotoxin, indicating an indirect action. Conversely, on the noradrenaline-excited VLPO neurons, histamine depolarizes the membrane potential and increases the firing rate via activation of H(1) and H(2) subtype histamine receptors. Moreover, histamine-induced depolarization persists in the presence of gabazine or tetrodotoxin, indicating a direct action. Based on these findings, we propose that in the VLPO, noradrenaline-inhibited neurons may normally be under the inhibitory control of noradrenaline-excited neurons. By facilitating the inhibitory control of the noradrenaline-excited neurons, histamine may inhibit the noradrenaline-inhibited neurons, resulting in excitation of histamine-releasing neurons in the TMN through disinhibition. This effect of histamine in the VLPO may contribute to the maintenance of wakefulness.

Neuropharmacology of vestibular system disorders

This work reviews the neuropharmacology of the vestibular system, with an emphasis on the mechanism of action of drugs used in the treatment of vestibular disorders. Otolaryngologists are confronted with a rapidly changing field in which advances in the knowledge of ionic channel function and synaptic transmission mechanisms have led to the development of new scientific models for the understanding of vestibular dysfunction and its management. In particular, there have been recent advances in our knowledge of the fundamental mechanisms of vestibular system function and drug mechanisms of action. In this work, drugs acting on vestibular system have been grouped into two main categories according to their primary mechanisms of action: those with effects on neurotransmitters and neuromodulator receptors and those that act on voltage-gated ion channels. Particular attention is given in this review to drugs that may provide additional insight into the pathophysiology of vestibular diseases. A critical review of the pharmacology and highlights of the major advances are discussed in each case.