ASA 127th Meeting M.I.T. 1994 June 6-10

2aPP10. Developmental changes in auditory nuclei of the torus semicircularis in the American bullfrog (Rana catesbeiana).

S. Boatright-Horowitz

A. M. Simmons

Depts. of Neurosci. and Psychol., Brown Univ., Providence, RI 02912

Anuran amphibians undergo significant neural remodeling of their auditory system as they progress from aquatic larvae to semi-terrestrial adults. It has previously been shown that post-metamorphic auditory maturation in bullfrogs shows great similarity to the typical vertebrate postnatal developmental pattern of decreasing maximally sensitive frequency and sharpening of tuning until achieving the u-shaped audiogram of the adult. These changes in neural response properties are accompanied by morphological changes in auditory nuclei in the midbrain. Histological examination of brain sections reveal physical changes in the auditory midbrain of frogs across postmetamorphic development. The midbrain increased in size significantly from a 1-day post-metamorphic froglet (3.1 cm SVL) to an adult (16.1 cm SVL), with most of the increase due to lateral hypermorphosis in the region containing the torus semicircularis, a homologue of the mammalian inferior colliculus. The expansion of the torus is not due to postmetamorphic cellular division, as the cell body density decreases across development. Tracing studies also reveal changes in connectivity patterns across development. These data suggest that there may be discrete developmental stages in the maturation of the bullfrog's central auditory system. [Work supported by NIH and NSF.]