Dual actions of isthmic input to tectal neurons in a reptile, Gekko gekko

We analyzed postsynaptic potentials and dye-labeled morphology of rectal ne urons responding to electrical stimulation of the optic nerve and of the nu cleus isthmi in a reptile. Gekko gekko, in order to compare with previously reported interactions between the optic tectum and the nucleus isthmi in a mphibians and birds. The results indicate that isthmic stimulation exerts i nhibitory and excitatory actions on tectal cells, similar to dual isthmotec tal actions in amphibians. It appears that dual actions of the isthmotectal pathway in amphibians and reptiles are shared by two subdivisions of the n ucleus isthmi in birds. The morphology of tectal cells responding to isthmi c stimulation is generally similar to that of tectoisthmic projecting neuro ns, but they differ particularly in that some tectoisthmic cells bear numer ous varicosities whereas cells receiving isthmic afferents do not. Thus, it is likely that at least some tectoisthmic cells may not be in the populati on of tectal cells that can be affected by isthmic stimulation. Forty-four percent of injections resulted in dye-coupled labeling, suggesting extensiv e electrical connections between rectal cells in reptiles.