The detection of osmotic stimuli is essential for all organisms, yet few osmoreceptive proteins are known, none of them in vertebrates. By employing a candidate-gene approach based on genes encoding members of the TRP superfamily of ion channels, we cloned cDNAs encoding the vanilloid receptor-related osmotically activated channel (VR-OAC) from the rat, mouse, human, and chicken. This novel cation-selective channel is gated by exposure to hypotonicity within the physiological range. In the central nervous system, the channel is expressed in neurons of the circumventricular organs, neurosensory cells responsive to systemic osmotic pressure. The channel also occurs in other neurosensory cells, including inner-ear hair cells, sensory neurons, and Merkel cells.

Our hearing organ, the cochlea, evidently poises itself at a Hopf bifurcation to maximize tuning and amplification. We show that in this condition several effects are expected to be generic: compression of the dynamic range, infinitely sharp tuning at zero input, and generation of combination tones. These effects are "essentially" nonlinear in that they become more marked the smaller the forcing: there is no audible sound soft enough not to evoke them. All the well-documented nonlinear aspects of hearing therefore appear to be consequences of the same underlying mechanism. PACS numbers: 87.19.Dd, 43.66. + y, 87.17.Nn The classic Helmholtz theory [1] posits that our hearing organ, the cochlea, is arranged like a harp or the back plane of a piano, with a number of highly tuned elements arrayed along a frequency scale, performing Fourier analysis of the incoming sound. Although the notion that the inner ear works like a musical instrument offers a beautiful esthetic symmetry, it has serious flaws. In the 1940s, Gold [2] pointed out that the cochlea's narrow passageways are filled with fluid, which dampens any hope of simple mechanical tuning. He argued that the ear cannot operate as a passive sensor, but that additional energy must be put into the system. As in the operation of a regenerative receiver [3], active amplification of the signal can compensate for damping in order to provide highly tuned responses. von Békésy's classic measurements in the cochlea [4] demonstrated the mapping of sound frequencies to positions along the cochlea. He observed the tuning to be quite shallow and found cochlear responses to behave linearly over the range of physiologically relevant sound intensities. Gold's notions were largely set aside in favor of the hypothesis of coarse mechanical tuning followed by a "second filter," whose nature was surmised to be electrical.von Békésy conducted his measurements on cadavers, whose dead cochleas lacked power sources or amplifiers that might have provided positive feedback. Only fairly recently, laser-interferometric velocimetry performed on live and reasonably intact cochleas has led to a very different picture [5,6]. There is, in fact, sharp mechanical tuning, but it is essentially nonlinear: there is no audible sound soft enough that the cochlear response is linear. Although the response far from the resonance's center is linear, at the resonance's peak the response rises sublinearly, compressing almost 80 dB into about 20 dB (Fig. 1). The width of the resonance increases with increasing amplitude, being least for sounds near the threshold of hearing. Observation of the response's essential nonlinearity at the level of cochlear mechanics contradicts von Békésy's finding. Furthermore, this nonlinearity does not originate in the rigidity of membranes or in fluid-mechanical effects. Because it reversibly disappears if the cochlea's ionic gradient is temporarily disturbed, the nonlinearity depends on a biological power supply [7].Gold conjectured that a regenerative mechanism for hearing...

Amplification of auditory stimuli by hair cells augments the sensitivity of the vertebrate inner ear. Cell-body contractions of outer hair cells are thought to mediate amplification in the mammalian cochlea. In vertebrates that lack these cells, and perhaps in mammals as well, active movements of hair bundles may underlie amplification. We have evaluated a mathematical model in which amplification stems from the activity of mechanoelectricaltransduction channels. The intracellular binding of Ca 2؉ to channels is posited to promote their closure, which increases the tension in gating springs and exerts a negative force on the hair bundle. By enhancing bundle motion, this force partially compensates for viscous damping by cochlear f luids. Linear stability analysis of a six-state kinetic model reveals Hopf bifurcations for parameter values in the physiological range. These bifurcations signal conditions under which the system's behavior changes from a damped oscillatory response to spontaneous limit-cycle oscillation. By varying the number of stereocilia in a bundle and the rate constant for Ca 2؉ binding, we calculate bifurcation frequencies spanning the observed range of auditory sensitivity for a representative receptor organ, the chicken's cochlea. Simulations using prebifurcation parameter values demonstrate frequency-selective amplification with a striking compressive nonlinearity. Because transduction channels occur universally in hair cells, this active-channel model describes a mechanism of auditory amplification potentially applicable across species and haircell types.

As in other excitable cells, the ion channels of sensory receptors produce electrical signals that constitute the cellular response to stimulation. In photoreceptors, olfactory neurons, and some gustatory receptors, these channels essentially report the results of antecedent events in a cascade of chemical reactions. The mechanoelectrical transduction channels of hair cells, by contrast, are coupled directly to the stimulus. As a consequence, the mechanical properties of these channels shape our hearing process from the outset of transduction. Channel gating introduces nonlinearities prominent enough to be measured and even heard. Channels provide a feedback signal that controls the transducer's adaptation to large stimuli. Finally, transduction channels participate in an amplificatory process that sensitizes and sharpens hearing.

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Ca2+ signaling serves distinct purposes in different parts of a hair cell. The Ca 2+ concentration in stereocilia regulates adaptation and, through rapid transduction-channel reclosure, underlies ampli®ca-tion of mechanical signals. In presynaptic active zones, Ca 2+ mediates the exocytotic release of afferent neurotransmitter. At efferent synapses, Ca 2+ activates the K + channels that dominate the inhibitory postsynaptic potential. A copious supply of diffusible protein buffer isolates the three signals by restricting the spread of free Ca 2+ and limiting the duration of its action. Using cDNA subtraction and a gene expression assay based on in situ hybridization, we detected abundant expression of mRNAs encoding the Ca 2+ buffer parvalbumin 3 in bullfrog saccular and chicken cochlear hair cells. We cloned cDNAs encoding this protein from the corresponding inner-ear libraries and raised antisera against recombinant bullfrog parvalbumin 3. Immunohistochemical labeling indicated that parvalbumin 3 is a prominent Ca 2+ -binding protein in the compact, cylindrical hair cells of the bullfrog's sacculus, and occurs as well in the narrow, peanut-shaped hair cells of that organ. Using quantitative Western blot analysis, we ascertained that the concentration of parvalbumin 3 in saccular hair cells is approximately 3 mM. Parvalbumin 3 is therefore a signi®cant mobile Ca 2+ buffer, and perhaps the dominant buffer, in many types of hair cell. Moreover, parvalbumin 3 provides an early marker for developing hair cells in the frog, chicken, and zebra®sh.

NO 3 • can compete with omnipotent • OH/SO 4 •– in decomposing aqueous pollutants because of its lengthy lifespan and significant tolerance to background scavengers present in H 2 O matrices, albeit with moderate oxidizing power. The generation of NO 3 • , however, is of grand demand due to the need of NO 2 • /O 3 , radioactive element, or NaNO 3 /HNO 3 in the presence of highly energized electron/light. This study has pioneered a singular pathway used to radicalize surface NO 3 – functionalities anchored on polymorphic α-/γ-MnO 2 surfaces (α-/γ-MnO 2 -N), in which Lewis acidic Mn 2+/3+ and NO 3 – served to form • OH via H 2 O 2 dissection and NO 3 • via radical transfer from • OH to NO 3 – ( • OH → NO 3 • ), respectively. The elementary steps proposed for the • OH → NO 3 • route could be energetically favorable and marginal except for two stages such as endothermic • OH desorption and exothermic • OH-mediated NO 3 – radicalization, as verified by EPR spectroscopy experiments and DFT calculations. The Lewis acidic strength of the Mn 2+/3+ species innate to α-MnO 2 -N was the smallest among those inherent to α-/β-/γ-MnO 2 and α-/γ-MnO 2 -N. Hence, α-MnO 2 -N prompted the rate-determining stage of the • OH → NO 3 • route ( • OH desorption) in the most efficient manner, as also evidenced by the analysis on the energy barrier required to proceed with the • OH → NO 3 • route. Meanwhile, XANES and in situ DRIFT spectroscopy experiments corroborated that α-MnO 2 -N provided a larger concentration of surface NO 3 – species with bi -dentate binding arrays than γ-MnO 2 -N. Hence, α-MnO 2 -N could outperform γ-MnO 2 -N in improving the collision frequency between • OH and NO 3 – species and in facilitating the exothermic transition of NO 3 – functiona...