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Parmly Hearing Institute, Loyola University Chicago, 6525 N. Sheridan Rd., Chicago, Illinois 60626
We have been investigating aspects of auditory processing and directional hearing in the toadfish Opsanus tau. We have shown that the saccule is an auditory endorgan that encodes both frequency and direction of a sound source (1). This information is sent via the VIIIth nerve to nuclei in the medulla, in particular, the descending octaval nucleus (1). Our previous work on cells in the descending octaval nucleus in Opsanus tau has revealed that most are highly directional (1) and that these directional auditory cells project to the midbrain. The torus semicircularis (TS) is a sensory processing site in the midbrain of fishes and amphibians. Nucleus centralis in the TS receives input from auditory areas in the medulla, and nucleus ventrolateralis receives input from lateral line areas in the medulla (2). Here we report some preliminary results from extracellular recordings of auditory cells in the TS.
Our protocol is described in detail elsewhere (1). In brief, the toadfish is anesthetized and immobilized (pancuronium bromide injection and lidocaine applied topically), and the dorsal surface of the midbrain is exposed. Following surgery, the fish is placed in a cylindrical dish filled with fresh seawater and is secured with a head holder. The water surface in the dish lies just below the surgical opening in the skull. The dish is part of a three-dimensional shaker table that provides sinusoidal motion of the animal with the surrounding water along linear pathways to simulate the particle motion component of underwater sound at appropriate frequencies (50–300 Hz) and levels, in the horizontal and mid-sagittal planes at specified angles (0°, 30°, 60°, 90°, 120°, 150° in each plane). In addition, we tested for external mechanoreceptive sensitivity (tentatively identified as lateral line) by producing hydrodynamic disturbances using puffs of air at the water surface along the length of the fish in the absence of an auditory stimulus. Units were classified as responding to hydrodynamic stimuli if the evoked spike rate was two standard deviations or more above the mean background rate.
For extracellular recording we used pulled glass electrodes with tip sizes of 3–5 µm and resistances of 3–10 M
. Our recording sites in the TS were confirmed in two ways. First, we used neurobiotin-filled electrodes (4% in 3 M NaCl) to mark the location of the first auditory cell analyzed. Second, the location of the electrode at all recording sites was plotted using the scale on a three-dimensional micromanipulator (accuracy to 10 µm). The neurobiotin was visualized using standard ABC immunohistochemistry (Vector Labs) in 50-µm floating sections, which were then placed on slides, dehydrated, and coverslipped.
We have recorded from 71 units in the TS. Of the cells that responded to the auditory stimuli, we have found that 33% have auditory sensitivity only and 67% respond to both auditory and hydrodynamic stimulation. Units unresponsive to auditory stimuli but responsive to hydrodynamic stimuli were observed frequently, but were not analyzed further. Figure 1 illustrates the responses of two TS units to varying levels of whole-body vibration in three orthogonal directions and to the hydrodynamic assay for putative lateral line sensitivity. Some units demonstrate a relatively large response to hydrodynamic stimulation compared with auditory (e.g., P1 in Fig. 1), while others respond little if at all to the hydrodynamic stimulus (e.g., B5 in Fig. 1). Both units of Figure 1 are highly sensitive and directional with respect to the vibrational axes producing the greatest responses: P1 responds best in the front-back axis with a displacement threshold of 3 dB re: 1 nm; and B5 is most responsive to vertical motion with a displacement threshold of -10 dB re: 1 nm.
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Supported by an R01 grant from NIH, NIDCD to R.R.F. and from an NIH, NIDCD Program Project Grant to the Parmly Hearing Institute.
Literature Cited
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