Premotor Neuron Divergence Reflects Vocal Evolution.

To identify mechanisms of behavioral evolution, we investigated the hindbrain circuit that generates distinct vocal patterns in two closely related frog species. Male Xenopus laevis and Xenopus petersii produce courtship calls that include a fast trill: trains of ~60 Hz sound pulses. Although fast trill rates are similar, X. laevis fast trills have a longer duration and period than those of X. petersii. To pinpoint the neural basis of these differences, we used whole-cell patch-clamp recordings in a key premotor hindbrain nucleus (the Xenopus parabrachial area, PBX) inex vivo brains that produce fictive vocalizations, vocal nerve activity corresponding to advertisement call patterns.We found two populations of PBX neurons with distinct properties: fast trill neurons (FTNs) and early vocal neurons (EVNs). FTNs, but not EVNs, appear to be intrinsically tuned to produce each species' call patterns because: (1) X. laevis FTNs generate longer and slower depolarizations than X. petersii FTNs during their respective fictive vocalizations, (2) current steps in FTNs induce burst durations that are significantly longer in X. laevis than X. petersii, and (3) synaptically isolated FTNs oscillate in response to NMDA in a species-specific manner: longer and slower in X. laevis than in X. petersii. Therefore, divergence of premotor neuron membrane properties is a strong candidate for generating vocal differences between species. [ABSTRACT FROM AUTHOR]