Acoustic modulation of immediate early gene expression in the auditory midbrain of female túngara frogs.

To better understand the molecular consequences of auditory processing in frogs, we investigated the acoustic modulation of two immediate early genes (IEGs), egr-1 and fos, in the auditory midbrain of female túngara frogs. Since túngara frog egr-1 had already been identified, we first isolated a túngara-specific fos clone using degenerate PCR followed by Rapid Amplification of cDNA Ends. In order to examine the temporal kinetics of acoustically modulated IEG mRNA expression, we first acoustically isolated females collected from a mating chorus and analyzed the decline in IEG expression in the torus semicircularis (homolog of the inferior colliculus). We found that IEG mRNA levels declined rapidly and reached baseline within 2 h. Next, we presented females with a 30-min recording of a mating chorus and analyzed IEG expression following different survival times. We found that IEG expression increased within 15-30 min of sound presentation but, compared to other vertebrates, in the túngara frog it took longer to reach the highest and lowest mRNA levels in response to sound and isolation, respectively. We also found that acoustic stimulation of egr-1 and fos differed in the three subdivisions of the torus semicircularis, suggesting that, as in birds, the two genes could provide largely different information when used in IEG mapping studies. While our results confirm the generality of sensory-induced IEG expression in vertebrates, whether the longer time course of IEG expression that we observed represents a species difference in the mechanisms of IEG transcription awaits further study.