Peripheral mechanisms of directional hearing in fish
(supported by NIH First Award R29DC03257)

       Sound localization is a very important function for the auditory systems of all vertebrates.  It is known that terrestrial vertebrates localize a sound source by comparing information encoded by two ears.  However, binaural cues such as binaural time and intensity differences are unlikely available to fishes because of higher speed of sound traveling in water and relatively small distance between the two ears.  Auditory receptor cells, hair cells, in the fish ear are oriented in various directions in three dimensional space (Lu and Popper 1998).  It was hypothesized that fish encode acoustic directional information about a sound source using arrays of these spatially oriented hair cells.  To test this hypothesis, we record directional responses from saccular ganglion neurons which innervate sensory hair cells in the ear.  After the response directionality of each ganglion neuron is characterized, a neuronal tracer, Neurobiotin, is intracellularly injected into the ganglion neuron.  Dendritic terminals of the injected neurons are visualized using a fluorescent dye conjugated antibody to the neuronal tracer.  Hair cells in the same sensory epithelia are also labeled using a different fluorescent probe.  Using these double labeling techniques and confocal microscopy, we are able to precisely determine the hair cells that the ganglion neurons innervate and their morphological polarizations.  In general, we have found that response directionality of saccular ganglion neurons is closely correlated with orientation of hair cells they innervate (Lu et al. 1998, Lu and Popper 2001).

Neural mechanisms of sound localization by fish
(supported by NIH Research Grant R01DC03257)

       The primary goal of this work is to determine how the primitive auditory system of fish functions in sound localization.  Previous studies from my lab and others have demonstrated that fish determine the axis at which a sound wave is propagating by using arrays of spatially oriented sensory hair cells in the ear.  However, it is not known how the brain processes the peripherally coded directional information in order to extract the specific direction of the sound.  This study will elucidate mechanisms underlying central auditory processing of directional information.  It has been proposed that the fish ear is stimulated by a sound wave through two distinct pathways: direct particle motion input and indirect pressure input via the swim bladder.  It was hypothesized that fish (with a swim bladder) first determine the axis on which the sound is propagating and then determine its direction by comparing the timings of inputs carrying pressure and particle motion information within the brain.  We will test these two steps of this hypothesis on two functionally distinguishable teleost fishes, the sleeper goby and goldfish.

Florida red tides and hearing
(supported by a Pilot Project Award from the NIH/NIEHS Marine and Freshwater Biomedical Science Center, RSMAS, UM)

       Florida red tides are blooms of the dinoflagellate, Karenia brevis.  This marine alga releases many types of natural neurotoxins, brevetoxins, that cause massive kills of marine animals, including endangered species, and threaten human health.  This study is to reveal whether or not a neurotoxin associated with Florida red tides, brevetoxin-3, affects hearing sensitivity of a teleost fish.  Specific questions are listed as follows:
1) Does brevetoxin-3 cause any hearing loss?
2) If so, what are dose and time effects on hearing loss?
3) What are the mechanisms underlying brevetoxin-induced hearing loss?
Evoked neural responses are collected using auditory brainstem recording techniques, and ear structure/ultrastructure is examined using light/electron microscopy.  To my knowledge, this proposal is the first study to investigate potential effects of red tide neurotoxins on the auditory system.  Results from this study will enhance our awareness of possible neurotoxic effects on sensory systems in vertebrates, including humans.

See new releases:

Cyber Diver News Network: Red Tides Affect Marine Life Sensory Systems

University of Miami News: Red Tides Can Cause Hearing Loss in Fish

Roles of fish otolithic organs in hearing

       The fish ear consists of three otolithic organs and three semicircular canals with associated ampullae.  All three otolithic organs (the saccule, lagena, and utricle) have been considered hearing as well as vestibular organs.  Fish auditory afferents are directionally selective, and their response directionality derives from orientation of hair cells innervated by them.  However, it is not known to what extent each otolithic organ contributes to overall hearing sensitivity in any fish species.  This work is to reveal roles of individual otolithic organs of the sleeper goby in hearing sensitivity by comparing auditory thresholds between normal and experimental fish with unilateral or bilateral otolith removal.   Auditory brain stem responses (ABR) are recorded from the sleeper goby.