Inhibitory response of locus coeruleus neurons to caloric stimulation in rats

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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ﾟC) or cold (30ﾟC) water through a polyethylene tube. Most neurons exhibited suppression of neuronal discharge in response to caloric stimulation. The suppression of LC neuronal discharge following caloric stimulation occured with a long latency and lasted a long period of time (about 3 min). Neither caloric stimulation of the auricle, nor irrigation of the middle ear with water at 37ﾟC,nor caloric stimulation of the middle ear after labyrinthectomy inhibited LC neuronal discharge. The caloric stimulation-induced LC neuronal inhibition was attenuated by the iontophoretic applocation of bicuculline. These findings indicate that the predominant effect of caloric stimulation on LC neuronal discharge is inhibitory, and that the caloric stimulation-induced LC ncuronal in inhibition is mediated by GABA_A receptors. We then examined the effects of lesions in the rostral vcntrolateral medulla (VML) or in the nucleus prepositus hypoglossi (PrII) on LC neuronal inhibition caused by caloric stimulation. VML lesions attenuated caloric stimulation-induced LC neuronal inhibition, but PrII lesions did not. This finding indicates that vestibular inhibition of LC neurons is mediated by VML.It is suggested that the suppressed activity of noradrenergic LC neurons is involved in the vestibulo-autonomic reflex.