An in vitro study of retinotectal transmission in the chick: role of glutamate and GABA in evoked field potentials.

We have developed two brain slice preparations for studying tectofugal visual pathways in the chick: conventional, 400-microns slices ("thin slices"), and "thick slices" which encompass the rostral pole of the optic tectum and the contralateral optic nerve. Stimulation was delivered with a bipolar electrode positioned in stratum opticum in thin slices and in the contralateral optic nerve in thick slices. While the latter preparation provided a means of exclusively and unambiguously activating retinal afferents, several lines of evidence also indicated that the evoked field potentials in thin slices were chiefly consequent to retinal afferent excitation: (1) the similarity of evoked field potentials in thin slices to those in thick slice preparations; (2) their precise localization in retinorecipient layers as shown by prelabeling from retina with FITC-coupled cholera toxin; (3) transmission delays appropriate for retinal afferents as established with the thick slice preparation; (4) patterns of labeled afferents resulting from applications of Dil crystals to slices fixed after recording; and (5) the similarity in transmitter pharmacology between thin and thick slice preparations. Pharmacological manipulations carried out with bath-applied antagonists indicated that glutamate is the principal retinotectal transmitter. The broadly active glutamate receptor blocker, kynurenic acid, reversibly eliminated the postsynaptic component of the field potential as confirmed with 0 Ca2+ saline. A complete block was also effected by the non-NMDA antagonists CNQX and DNQX. The specific NMDA antagonist, AP5, caused a smaller and variable reduction in response amplitude. The GABA antagonist, bicuculline, caused a prolongation of the monosynaptic field epsp in retinorecipient layers and an enhancement of the long-latency, negative wave in cellular layers below, supporting a late, excitation-limiting role for this inhibitory transmitter.