Possible role of Ca ions in the vestibular system

Dysgenesis of melanocytes and cochlear dysfunction in mutant microphthalmia (mi) mice

In order to evaluate the cytological homology of intermediate cells and melanocytes, and to investigate the function of melanocytes in the inner ear, hearing acuity and cochlear pathology were studied in three strains of mice, namely, wild type mice (+/+), albino mice without melanin (c2J/c2J), and microphthalmia mice with no melanocytes (mibw/mibw). Our histochemical data indicated that intermediate cells showed cytological characteristics almost identical to those of melanocytes and that disorders of melanin and/or melanocytes were reflected in the stria vascularis of each mouse. While c2J/c2J presented the same normal hearing acuity and normal structure of the stria vascularis as +/+, the hearing acuity of mibw/mibw mutants was severely impaired. Their stria vascularis was abnormally thin, lacking intermediate cells. According to these results, lack of melanin has little influence on hearing acuity; however, the absence of intermediate cells or melanocytes causes severe hearing loss, presumably due to a strial dysfunction.

Hearing fluctuation during migraine attacks. Activity of propranolol-HCl in rats

Transitory hearing impairment has been well documented in humans during basilar migraine attacks. Ischemia induced by activation of central nervous system adrenoreceptors by cAMP is one of the mechanisms that has been implicated, as well as calcium metabolism alterations. Propranolol-HCl has proven to be an effective treatment. Massive local adrenaline release is regarded as the primum movens mechanism triggering the liberation of other beta-receptor activators resulting in sterile edema and exudate. This exudate can be visualized (via ophthalmoscope) and is viewed as pathognomonic of a migraine attack. We investigated both the action of adrenaline and Propranolol-HCl on brainstem auditory potentials in 3 groups of young rats. The first group was perfused intra-arterially with adrenaline. The second group received the same treatment plus a prophylactic daily dose of Propranolol-HCl. The third group served as the control and received Propranolol-HCl, but was perfused with a Ringer solution. Alterations in blood flow and hearing sensitivity (BERA) following perfusion occurred in the first group only. Propranolol seems to exhibit a protective action during experimental attempts to induce migraine-like attacks in rats.

On the pathophysiology of tinnitus; a review and a peripheral model

In this paper I investigate the consequences of the assumption that tinnitus is the result of correlated neural activity in auditory nerve fibers under 'no sound' conditions. Two possible pathological conditions capable of causing this correlation are ephaptic excitation of one nerve fiber by neighboring nerve fibers and synchronization of the various synapses in individual hair cells. The first condition is likely to be found in cases suffering from acoustic neuroma where the myelin sheath of the auditory neurons is damaged. The second condition is attributed to a spontaneous excess influx of K+ or Ca2(+)-ions into the hair cell resulting in transient hair cell depolarizations causing synchronous transmitter release at all hair cell synapses. This condition is postulated in noise trauma and ototoxic drug damage of the inner hair cell membrane. The model produces the excess of short interspike intervals found in auditory nerve fiber recordings in animal models of tinnitus as well as the theoretically required correlation in the activity of neighboring neurons.

Phantom auditory perception (tinnitus): mechanisms of generation and perception

Phantom auditory perception--tinnitus--is a symptom of many pathologies. Although there are a number of theories postulating certain mechanisms of its generation, none have been proven yet. This paper analyses the phenomenon of tinnitus from the point of view of general neurophysiology. Existing theories and their extrapolation are presented, together with some new potential mechanisms of tinnitus generation, encompassing the involvement of calcium and calcium channels in cochlear function, with implications for malfunction and aging of the auditory and vestibular systems. It is hypothesized that most tinnitus results from the perception of abnormal activity, defined as activity which cannot be induced by any combination of external sounds. Moreover, it is hypothesized that signal recognition and classification circuits, working on holographic or neuronal network-like representation, are involved in the perception of tinnitus and are subject to plastic modification. Furthermore, it is proposed that all levels of the nervous system, to varying degrees, are involved in tinnitus manifestation. These concepts are used to unravel the inexplicable, unique features of tinnitus and its masking. Some clinical implications of these theories are suggested.

Calcium antagonists and the vestibular system: a critical review of flunarizine as an antivertigo drug

Flunarizine, a diphenylalkylamine, is one of the most popular antivertiginous drugs used nowadays in France. However, until now, there are very few preliminary data about the physiological or pathophysiological functions of calcium in the vestibular system. Moreover, experimental and clinical arguments are still insufficient to clearly demonstrate that 1) flunarizine is an effective antivertiginous drug and 2) this putative antivertiginous property is really due to the anticalcic action of the drug and not to a more classical antagonistic effect on H1 receptors. Much more work is needed before accepting the indication of any anticalcic drug as an effective antivertigo treatment.