synaptojanin1 Is Required for Temporal Fidelity of Synaptic Transmission in Hair Cells

To faithfully encode mechanosensory information, auditory/vestibular hair cells utilize graded synaptic vesicle (SV) release at specialized ribbon synapses. The molecular basis of SV release and consequent recycling of membrane in hair cells has not been fully explored. Here, we report that comet, a gene identified in an ENU mutagenesis screen for zebrafish larvae with vestibular defects, encodes the lipid phosphatase Synaptojanin 1 (Synj1). Examination of mutant synj1 hair cells revealed basal blebbing near ribbons that was dependent on Cav1.3 calcium channel activity but not mechanotransduction. Synaptojanin has been previously implicated in SV recycling; therefore, we tested synaptic transmission at hair-cell synapses. Recordings of post-synaptic activity in synj1 mutants showed relatively normal spike rates when hair cells were mechanically stimulated for a short period of time at 20 Hz. In contrast, a sharp decline in the rate of firing occurred during prolonged stimulation at 20 Hz or stimulation at a higher frequency of 60 Hz. The decline in spike rate suggested that fewer vesicles were available for release. Consistent with this result, we observed that stimulated mutant hair cells had decreased numbers of tethered and reserve-pool vesicles in comparison to wild-type hair cells. Furthermore, stimulation at 60 Hz impaired phase locking of the postsynaptic activity to the mechanical stimulus. Following prolonged stimulation at 60 Hz, we also found that mutant synj1 hair cells displayed a striking delay in the recovery of spontaneous activity. Collectively, the data suggest that Synj1 is critical for retrieval of membrane in order to maintain the quantity, timing of fusion, and spontaneous release properties of SVs at hair-cell ribbon synapses

Ontogenetic Development of Weberian Ossicles and Hearing Abilities in the African Bullhead Catfish

BACKGROUND: The weberian apparatus of otophysine fishes facilitates sound transmission from the swimbladder to the inner ear to increase hearing sensitivity. It has been of great interest to biologists since the 19(th) century. No studies, however, are available on the development of the weberian ossicles and its effect on the development of hearing in catfishes. METHODOLOGY/PRINCIPAL FINDINGS: We investigated the development of the weberian apparatus and auditory sensitivity in the catfish Lophiobagrus cyclurus. Specimens from 11.3 mm to 85.5 mm in standard length were studied. Morphology was assessed using sectioning, histology, and X-ray computed tomography, along with 3D reconstruction. Hearing thresholds were measured utilizing the auditory evoked potentials recording technique. Weberian ossicles and interossicular ligaments were fully developed in all stages investigated except in the smallest size group. In the smallest catfish, the intercalarium and the interossicular ligaments were still missing and the tripus was not yet fully developed. Smallest juveniles revealed lowest auditory sensitivity and were unable to detect frequencies higher than 2 or 3 kHz; sensitivity increased in larger specimens by up to 40 dB, and frequency detection up to 6 kHz. In the size groups capable of perceiving frequencies up to 6 kHz, larger individuals had better hearing abilities at low frequencies (0.05-2 kHz), whereas smaller individuals showed better hearing at the highest frequencies (4-6 kHz). CONCLUSIONS/SIGNIFICANCE: Our data indicate that the ability of otophysine fish to detect sounds at low levels and high frequencies largely depends on the development of the weberian apparatus. A significant increase in auditory sensitivity was observed as soon as all weberian ossicles and interossicular ligaments are present and the chain for transmitting sounds from the swimbladder to the inner ear is complete. This contrasts with findings in another otophysine, the zebrafish, where no threshold changes have been observed

Acoustic particle motion measurement for bioacousticians: Principles and pitfalls

It is currently thought that all fishes detect acoustic particle motion, and it is therefore critical to measure this vector quantity when studying hearing, acoustic behavior, and noise impacts. Proper measurement of particle motion in any of its forms (acceleration, velocity, or displacement) is subject to a range of errors whose magnitude depends on the sound source(s) of interest and the environments in which they are observed. Particle motion measurement principles and errors have primarily been addressed in the literature for free field plane waves. However, such fields are rarely encountered in bioacoustic studies, and experiments or calculations made based on plane wave assumptions can lead to substantially erroneous measurement results and flawed study conclusions. This paper presents a unified treatment of underwater acoustic particle motion measurement by reviewing acoustic field attributes of commonly encountered source/environment scenarios, quantifying measurement error sources, and concluding with guidelines and recommendations for bioacoustic studies

Acoustic particle motion measurement for bioacousticians: Principles and pitfalls

It is currently thought that all fishes detect acoustic particle motion, and it is therefore critical to measure this vector quantity when studying hearing, acoustic behavior, and noise impacts. Proper measurement of particle motion in any of its forms (acceleration, velocity, or displacement) is subject to a range of errors whose magnitude depends on the sound source(s) of interest and the environments in which they are observed. Particle motion measurement principles and errors have primarily been addressed in the literature for free field plane waves. However, such fields are rarely encountered in bioacoustic studies, and experiments or calculations made based on plane wave assumptions can lead to substantially erroneous measurement results and flawed study conclusions. This paper presents a unified treatment of underwater acoustic particle motion measurement by reviewing acoustic field attributes of commonly encountered source/environment scenarios, quantifying measurement error sources, and concluding with guidelines and recommendations for bioacoustic studies