Postnatal Development of Cochlear Function in the Mustached Bat, Pteronotus parnellii

1 Zoological Institute, University of Frankfurt, 60323 Frankfurt am Main, Germany 2 Institute of Biochemistry and Biology, University of Potsdam, 14471 Potsdam, Germany 3 Department of Animal Physiology, University of Habana, Vedado, La Habana 10400, Cuba 4 School of Biology, University of Sussex, Falmer, Brighton BN19QG, United Kingdom Submitted 3 February 2003; accepted in final form 16 June 2003 Postnatal development of the mustached bat's cochlea was studied by measuring cochlear microphonic and compound action potentials. In adults, a cochlear resonance is involved in enhanced tuning to the second harmonic constant frequency component (CF2) of their echolocation calls at 61 kHz This resonance is present immediately after birth in bats that do not yet echolocate. Its frequency is lower (46 kHz) and the corresponding threshold minimum of cochlear microphonic potentials is broader than in adults. Long-lasting ringing of the cochlear microphonic potential after tone stimulus offset that characterizes the adult auditory response close to CF2 is absent in newborns. In the course of the first 5 postnatal weeks, there is a concomitant upward shift of CF2 and the frequency of cochlear threshold minima. Up to the end of the third postnatal week, sensitivity of auditory threshold minima and the Q value of the cochlear resonance increase at a fast rate. Between 2 and 4 wk of age, two cochlear microphonic threshold minima are found consistently in the CF2 range that differ in their level-dependent dynamic growth behavior and are 1.5–5.7 kHz apart from each other. In older animals, there is a single minimum that approaches adult tuning in its sharpness. The data provide evidence to show that during maturation of the cochlea, the frequency and the sensitivity of the threshold minimum associated with CF2 increases and that these increases are associated with the fusion of two resonances that are partly dissociated in developing animals. Address for reprint requests and other correspondence: M. Kössl, Zoologisches Institut, Siesmayerstr. 70, 60323, Frankfurt/M., Germany (E-mail: koessl{at}zoology.uni-frankfurt.de ).