Current source density analysis of contra- and ipsilateral isthmotectal connections of the frog

Abstract

The nucleus isthmi (NI) of the frog receives input from the ipsilateral optic tectum and projects back to both optic tecta. After ablation of NI, frogs display no visually elicited prey-catching or threat avoidance behavior. Neural mechanisms that underlie the loss of such important behavior have not been solved. Electrophysiological examination of the contralateral isthmotectal projection has proved that it contributes to binocular vision. On the other hand, there are very few physiological investigations of the ipsilateral isthmotectal projection. In this study, current source density (CSD) analysis was applied to contra- and ipsilateral isthmotectal projections. The contralateral projection produced monosynaptic sinks in superficial layers and in layer 8. The results confirmed former findings obtained by single unit recordings. The ipsilateral projection elicited a prominent monosynaptic sink in layer 8. Recipient neurons were located in layers 6-7. These results, combined with those from the former intracellular study, led to the following neuronal circuit. Afferents from the ipsilateral NI inhibit non-efferent pear shaped neurons in the superficial layers, and strongly excite large ganglionic neurons projecting to the descending motor regions. Thus feedback to the output neurons strengthens the visually elicited responses.

Electrophysiological properties of isthmic neurons in frogs revealed by in vitro and in vivo studies

“…In addition, the NI has been shown to possess a population of GABA-containing neurons that project to the OT (Li and Fite 2001). This may account for the dual modulation of tectal cells observed when the NI is electrically stimulated (Hoshina et al 2006; Xiao and Wang 1999). The homogeneity that we observed in both intracellular and extracellular recordings suggests two possible interpretations.…”