Development of bone-conduction mobile phones: assessment of hearing mechanisms by measuring psychological characteristics and acoustical properties in the outer ear canal.

We have been developing novel mobile phones using bone conduction, with flat-panel loudspeakers that convey speech sound by vibrating the pinna. In bone conduction via the pinna, i.e., pinna conduction, it is thought that speech sounds are conveyed via both air- and bone-conduction pathways. To obtain useful information for further development of bone-conduction mobile phones, peripheral mechanisms of the pinna conduction need to be clarified. In this study, hearing thresholds, sound field in the outer ear canals, and vibrations of the inner wall of the outer ear canals were measured while normal-hearing participants used pinna-conduction mobile phones. Thresholds decreased linearly as contact pressure increased below 1 kHz, but contact pressure did not affect thresholds above 2 kHz. Additionally, sound fields in the ipsilateral ear canal showed similar results. These results indicate that there is a considerable degree of bone-conduction components from the pinna to the inner ear, which only allow sounds below 1 kHz through. Because similar characteristics were observed in the threshold and the sound field in the outer ear canal, we suggest that osseotympanic emission, sound emission into the ear canal from the inner wall, and air conduction via external auditory foramen are the dominant components of pinna conduction. However, in the vibration measurement, differences between the ipsi- and contra-lateral responses were smaller than the sound field measurement. The smaller inter-lateral differences of the vibration in the outer ear canal suggest the existence of a significant amount of bone-conduction components that directly reach the middle or inner ear. Although the amount of such bone-conduction components does not seem sufficient for pinna.

Nicotinic receptor agonist-induced salivation and its cellular mechanism in parotid acini of rats.

Cigarette smoking and nicotine enhance parotid saliva secretion, however, the underlying mechanism is unclear. To address the mechanism of nicotine-induced salivation and to explore the possibility that nicotinic receptor agonists act as sialogogues, we investigated the effects of nicotinic receptor agonists on salivary secretion in vivo and on intracellular Ca²+ concentration in digested parotid acini in vitro in rats. In urethane-anesthetized rats, intravenous administration of nicotinic receptor agonists, nicotine and cytisine, at 3 µmol/kg increased whole saliva output accompanied by a pressor response with nicotine, but not with cytisine. Using Ca²+-imaging system on digested parotid acini in which autonomic nerve terminals were kept intact, nicotine and cytisine dose-dependently increased intracellular Ca²+ concentration at µM level. This was not observed in single acinar cells containing no nerve terminal. The nicotine-induced Ca²+ response was largely blocked by a muscarinic receptor antagonist and partly blocked by an adrenergic receptor antagonist. Furthermore, the same nicotine-induced Ca²+ response was blocked by mecamylamine, a relatively selective nicotinic antagonist for α3ß4 subtype receptor, but not by other selective antagonists, dihydro-ß-erythroidine for α4-containing receptor and methyllycaconitine for α7 nicotinic receptors. These results suggest that nicotinic agonists-induced salivation is due to a release of acetylcholine and noradrenaline from autonomic nerve terminals through activation of α3ß4 nicotinic receptor subtype. In addition, considering the blood pressure response and development of addiction with nicotine, cytisine may be a better therapeutic candidate to serve as a sialogogue for xerostomia patients.

Differences in the Ca2+ response resulting from neurotransmitter stimulations of rat parotid acini and ducts.

There are few data available regarding the differences in intracellular Ca2+ responses of parotid acinar and ductal cells. This study investigated the Ca2+ mobilization that was induced by the chemical stimulation of acinar and ductal cells from rat parotid glands. In fura-2 loaded parotid cells, carbachol increased the intracellular Ca2+ concentration ([Ca2+](i)) to a greater extent in the acinar cells than in the ductal cells, but noradrenaline increased the [Ca2+](i) in the ductal cells more than in the acinar cells. Although there was no difference in the alpha1-adrenergic receptor agonist phenylephrine-induced Ca2+ mobilization between acini and ducts, the beta-adrenergic receptor agonist isoproterenol increased the [Ca2+](i) in only the ductal cells. Additionally, the effects of non-adrenergic, non-cholinergic neurotransmitters were investigated. Substance P and ATP increased the [Ca2+](i) in parotid acini and/or ducts. A substance P-induced Ca2+ response was observed in only acini, while the ATP-induced Ca2+ response was significantly higher in ducts than in acini. These results suggest that parotid acini have a greater sensitivity to cholinergic and substance P stimulation and a lesser sensitivity to beta-adrenergic and ATP stimulation than the ductal cells. In light of these results, substance P and isoproterenol will be useful for identifying parotid acini and ducts, respectively.

Effects of pilocarpine and cevimeline on Ca2+ mobilization in rat parotid acini and ducts.

Previous reports suggested that there is no significant difference in the binding affinity of pilocarpine and cevimeline on muscarinic receptors (1). However, in vivo studies from our laboratory suggested that pilocarpine-induced salivation from the parotid gland is greater than that induced by cevimeline in rats. Therefore, in the present study, sialogogue-induced intracellular Ca(2+) mobilization was investigated in isolated parotid gland cells in vitro. Pilocarpine and cevimeline increased the intracellular Ca(2+) concentration of parotid gland acinar and duct cells over 1 microM in a dose-dependent manner. Pilocarpine-induced responses were higher than cevimeline-induced responses. Ca(2+) responses to both agents were completely blocked by 1 microM 4-DAMP, an M3 muscarinic receptor antagonist. In the absence of extracellular Ca(2+), both sialogogues induced transient Ca(2+) increase in parotid gland acinar cells. These results suggest that the sialogogues stimulate some common routes via the Ca(2+) signaling in parotid gland acinar cells. We also report a significant difference of Ca(2+) responses in concentration between pilocarpine and cevimeline in parotid gland acinar cells. The different Ca(2+) responses between the sialogogues would explain the different saliva volumes from the parotid gland between them that we have observed in previous in vivo studies.

Ca2+ mobilization by nicotine through synaptic activation in rat parotid acini.

Nicotine has been reported to increase the intracellular Ca(2+) concentration ([Ca(2+)](i)) in sublingual acini due to neurotransmitter release from nerve terminals associated with the cell preparation (1). However, it is unclear whether or not the same reaction exists in parotid cells. Therefore, we investigated effect of nicotine on Ca(2+) mobilization in digested parotid acini from rats. After removing the parotid gland from Wistar rats, the tissues were minced and digested with collagenase. Then, the intracellular Ca(2+) indicator fura-2 was added to the preparation, and the change in [Ca(2+)](i) was monitored using fluorescent microscope. In many but not all parotid acini, K(+) stimulation induced transient increases in [Ca(2+)](i). The K(+)-induced Ca(2+) response in parotid acini was completely blocked by Cd(2+)-containing solution. These results suggest that the parotid cell preparation has nerve terminals. In all high-K(+)-sensitive parotid acini, over 3 microM of nicotine increased [Ca(2+)](i), and the response was blocked by a Cd(2+)-containing solution and nicotinic receptor antagonists. All high-K(+)-insensitive acinar cells were resistant to the effect of nicotine on Ca(2+) mobilization. These results suggest that nicotine induces increases in [Ca(2+)](i) in parotid acini due to neurotransmitter release from associated nerve terminals.