Integration of ascending and descending inputs in the auditory midbrain of anurans

In anurans, the midbrain torus semicircularis is involved in auditory processing and audio-motor integration. In this study, we examined the influence of descending forebrain projections on the auditory response properties and hence the audiomotor transmission of mesencephalic interface neurons. In order to investigate response integration, we performed intracellular recordings from torus neurons in an isolated brain preparation of Discoglossus pictus and Bombina orientalis and stimulated the auditory nerve, striatum, and the dorsal thalamus electrically with single pulses. Stimulation of all three sites could evoke responses in torus neurons that were either excitatory, inhibitory, or a mixture of both, with durations of up to several hundred milliseconds. Further, striatum and thalamus were activated by pulse trains (10-20 Hz, 50 pulses) immediately before stimulating the auditory nerve with single pulses. Thus, responses of torus neurons to "auditory" input were facilitated or suppressed for up to 2 min by striatum stimulation or only suppressed by thalamus stimulation. Intracellular labeling of recorded neurons revealed that response modulation by descending input mostly occurred in laminar nucleus neurons. These results suggest that descending forebrain projections to mesencephalic audiomotor interface neurons may play an important role in modifying acoustically guided behavior in anurans.