Pretecto-tectal influences

(1)From the dorsal surface of the toad (Bufo b. spinosus, B. marinus) optic tectum (OT), field potentials (FP) were recorded at 9 reference sites in response to electrical stimulation of the optic nerve (ON). The FP showed 4 main components, besides an initial deflection attributed to axonal potentials: two negative waves N1, N2 (attributed to postsynaptic excitatory processes) and two positive waves P2, P3 (attributed to postsynaptic inhibitory processes). The responses across the reference sites were rather similar in different individuals. (2) Electrical stimulation of an area in the ipsilateral pretectal lateral posterodorsal and posterior (Lpd/P) thalamic region evoked tectal FPs showing mainly a negative and a positive wave. Regarding wave amplitudes, the FPs displayed disproportionalities across the reference sites. (3) Electrical stimulation of the contralateral Lpd/P evoked mainly a positive wave in the tectal FP whose disproportionality corresponded roughly to the one obtained to ipsilateral Lpd/P stimulation. (4) The inital negative wave of the tectal FP in response to ON stimulation was nearly abolished, if Lpd/P stimulation preceded ON stimulation at a delay of 17–25 ms. (5) Since FPs showed adaptation to repetitive stimulation, various experiments were carried out to distinguish adaptation phenomena from effects of neuronal interactions between Lpd/P and OT. (6) The results provide evidence that ON- and Lpd/P-mediated inputs interact in superficial tectal layers, whereby pretectotectal input suppresses retinotectal excitatory information transfer. Input of Lpd/P to the contralateral superficial OT suggests postsynaptic inhibition. This study provides no information about pretectal inputs to deeper tectal layers, which anatomically are known to exist.