POSTMETAMORPHIC CHANGES IN AUDITORY-SENSITIVITY OF THE BULLFROG MIDBRAIN

During metamorphosis, the lateral line system of ranid frogs (Rana cat esbeiana) degenerates and an auditory system sensitive to airborne sou nds develops. We examined the onset of function and developmental chan ges in the central auditory system by recording multi-unit activity fr om the principal nucleus of the torus semicircularis (TSp) of bullfrog s at different postmetamorphic stages in response to tympanically-pres ented auditory stimuli. No responses were recorded to stimuli of up to 95 dB SPL from late-metamorphic tadpoles, but auditory responses were recorded within 24 hours of completion of metamorphosis. Audiograms f rom froglets (SVL < 5.5 cm) were relatively flat in shape with high th resholds, and showed a decrease in most sensitive frequency (MSF) from about 2500 Hz to about 1500 Hz throughout the first 7-10 days after c ompletion of metamorphosis. Audiograms from frogs larger than 5.5 cm s howed continuous downward shifts in MSF and thresholds, and increases in sharpness around MSF until reaching adult-like values. Spontaneous activity in the TSp increased throughout postmetamorphic development. The torus increased in volume by approximately 50% throughout developm ent and displayed changes in cell density and nuclear organization. Th ese observations suggest that the onset of sensitivity to tympanically presented airborne sounds is limited by peripheral, rather than centr al, auditory maturation.