Responses of locus coeruleus neurons to caloric stimulation in rats

Firing properties and classification of MVN neurons in rats

In order to know the effects of caloric stimulation on neuronal firing in medial vestibular nuclei (MVN) by middle ear irrigation, the middle ear was irrigated with ice (4 degrees C), hot (44 degrees C), and warm (37 degrees C) water, and the firing rate of MVN neuron was extracellularly recorded. The results showed that the firing rate of MVN neuron was changed by caloric stimulation, and the majority of MVN neurons showed excitation by irrigation with hot water and inhibition by ice water (type A). The neuronal firing was recovered immediately after the cessation of the stimulation. I It was concluded that the neuronal firing rate in MVN was changed by caloric stimulation in middle ear cavity. The response was different in various neurons.

Noradrenergic pathways involved in the development of vertigo and dizziness--a review

In this study, vestibular caloric stimulation (CS) inhibited noradrenergic (NA) neurons of the locus coeruleus (LC) in rats. The vestibular input can be modified by the ventrolateral medulla (VLM), which then inhibits the LC neuronal activity via GABAA receptors. Clinically, CS induces vertigo in humans. Thus, LC-NA inhibition may be involved in the development of vertigo. Moreover, it is speculated that Sopite syndrome, one of the major symptom complexes of motion sickness, is also evoked by LC-NA inhibition. The central LC-NA neuronal system may participate in vertigo and motion sickness independent of the histaminergic neuronal system. In contrast, the cholinergic neuronal system may mediate LC-NA inhibition during the vestibulo-atonomic reflex. The LC-NA system projects to most higher centers and affects sensory information processing. Therefore, it is suggested that the suppression of sensory information processing induced by LC-NA inhibition causes drowsiness, one of the major symptoms of vertigo and motion sickness. It is also speculated that LC-NA inhibition participates in the development of sensory mismatch during vertigo and motion sickness.

Vestibular modulation of plasma vasopressin levels in rats

Recent studies, which have shown an increase of plasma vasopressin (VP) in experimental motion sickness and the efficacy of VP antagonists for motion sickness, suggest an important role of VP in the development of vestibulo-autonomic responses. We have recently found evidence of the co-existence of vasopressinergic neurons with the stress-sensitive chemokinergic neuronal system in the hypothalamo-pituitary pathway in rats, which uses cytokine-induced neutrophil chemoattractant (CINC) as an effector molecule. In this study, to elucidate possible roles of VP and CINC in the vestibulo-autonomic responses, we simultaneously measured plasma VP and CINC concentrations after electrical or caloric vestibular stimulation in urethane-anesthetized rats. Electrical vestibular stimulation with more than 200 microA increased the plasma levels of VP in a current intensity-dependent manner, and stimulation with 500 microA increased the plasma VP levels to 350% of the normal control group, which received no stimulation. Caloric vestibular stimulation with cold water increased the plasma VP levels to 262% of the control group, which received caloric stimulation with water at 37 degrees C, and stimulation with warm water tended to increase the plasma VP levels. Plasma CINC levels were neither affected by electrical nor caloric vestibular stimulation. These findings indicate that vestibular stimulation increased plasma levels of VP but not CINC, and this vestibular-induced activation of VP neurons may be involved in a mechanism of vestibulo-autonomic responses.

Neurons in rostral ventrolateral medulla mediate vestibular inhibition of locus coeruleus in rats

The effects of caloric vestibular stimulation on the central noradrenergic neurons system were examined in the rat. In urethane-anesthetized rats, caloric stimulation inhibited the spontaneous activity of noradrenergic locus coeruleus neurons and increased systemic blood pressure. Electrical and chemical lesions in the ventrolateral medulla attenuated both the locus coeruleus inhibition and the blood pressure increase in response to caloric stimulation. Neither the neuronal inhibition nor the pressor effect was attenuated by any deafferentation of the forebrain or baroreceptors, or lesioning of the nucleus tractus solitarius. These findings indicate that the caloric stimulation-induced locus coeruleus inhibition is mediated by neurons in the ventrolateral medulla, and that these neurons also mediate the vestibulo-pressor responses. The locus coeruleus inhibition via the ventrolateral medulla is, however, considered to be independent of ventrolateral medulla-mediated systemic pressor effect. Collectively these findings suggest that the ventrolateral medulla is the major origin of inhibitory vestibular input to the noradrenergic neurons of the locus coeruleus, and that the ventrolateral medulla plays an important role in the vestibulo-autonomic response.

GABAergic inhibitory response of locus coeruleus neurons to caloric vestibular stimulation in rats

We examined the effects of caloric vestibular stimulation on the neuronal activity of the locus coeruleus (LC) in urethane-anesthetized rats. The middle ear cavity was irrigated with hot (44 degrees C) or cold (30 degrees C) water through a polyethylene tube. Most neurons (hot water: 76%, 55/72; cold water: 90%, 19/21) exhibited suppression of neuronal discharge in response to caloric stimulation. The suppression of LC neuronal discharge following caloric stimulation occurred with a long latency (approximately 80 s), and lasted a long period of time (approximately 3 min). Neither caloric stimulation of the auricle, nor irrigation of the middle ear with water at 37 degrees C, nor caloric stimulation of the middle ear after labyrinthectomy inhibited LC neuronal discharge. The caloric stimulation-induced LC neuronal inhibition was significantly attenuated by the intravenous injection of picrotoxin and by the iontophoretic application of bicuculline methiodide. These findings indicate that the predominant effect of caloric vestibular stimulation on LC neuronal discharge is inhibitory, and that the caloric stimulation-induced LC neuronal inhibition is mediated by GABAA receptors located on the membrane of LC neurons. It is suggested that the suppressed activity of noradrenergic LC neurons is involved in the vestibulo-autonomic reflex.