|Afferent projections of the torus semicircularis were traced in the blind cave form of Astyanax fasciatus using HRP tract-tracing techniques, and the results were compared to those found in previous studies of closely related sighted teleosts.
Following unilateral injections of tracer to the nucleus centralis of the torus semicircularis, retrogradely labeled cells were found in two octaval nuclei of the medulla. The descending octavolateralis nucleus was labeled bilaterally and the anterior octaval nucleus was labeled primarily on the ipsilateral side. Both the descending and anterior octavolateralis nuclei are known to receive auditory input from CN VIII in sighted fish. In addition, labeled commissural fibers could be traced across the midline to labeled cells in the nucleus centralis of the contralateral torus semicircularis.
Large Purkinje-like neurons of the contralateral medial octavolateralis nucleus of the medulla were labeled with HRP injections to the nucleus lateralis. The medial octavolateralis nucleus receives input from the anterior and posterior lateral line nerves known to carry mechanosensory information to the CNS. In addition, labeled cells in the contralateral descendens nucleus of V in the caudal medulla and in the ipsilateral ventromedial nucleus of the thalamus were found. Both of these nuclei process somatosensory information. Labeled cells were also located in the anterior tuberal nucleus of the hypothalamus and minor nuclei of the mesencephalon.
These results suggest that the neural input to the torus semicircularis in cave A. fasciatus differ little, if any, from those previously found in sighted teleosts studied. Further, these findings also suggest that the torus semicircularis functions as a multimodal processing center with auditory input from the descending and anterior octavolateralis nuclei of the medulla, mechanosensory information relayed by the medial octavolateralis nucleus, and somatosensory input from the descendens nucleus of V and the ventromedial nucleus of the thalamus.