Anin vitrostudy of retinotectal transmission in the chick: Role of glutamate and GABA in evoked field potentials

Abstract

We have developed two brain slice preparations for studying tectofugal visual pathways in the chick: conventional, 400-μm 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, APS, 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.

Chattering and Differential Signal Processing in Identified Motion-Sensitive Neurons of Parallel Visual Pathways in the Chick Tectum

“…5A, inset) for all cells tested (n ϭ 7). This CNQX sensitivity is consistent with previous studies (Dye and Karten, 1996) indicating that glutamate is the transmitter at the synapses from retinal afferents onto the bottlebrush endings. The CNQX sensitivity confirms that the response is caused by synaptic transmission and not by direct electric stimulation of dendritic endings.…”

“…In birds and mammals, retinal input to the visual midbrain is primarily mediated by glutamate that acts on ionotropic and metabotropic receptors, and blocking of these interrupts retinotectal transmission (Canzek et al, 1981;Binns and Salt, 1994;Dye and Karten, 1996;Cirone and Salt, 2000). Because in our experiments the glutamate receptor blocker CNQX completely abolished cellular responses of SGC neurons, we conclude that synaptic transmission at the bottlebrush endings of SGC-I neurons is mediated by glutamate receptors.…”

“…Tectal slices were prepared as described previously (Dye and Karten, 1996). Animals were anesthetized with a mixture of ketamine and xylazine (40 and 12 mg / kg, respectively, i.m.)…”

“…Previous studies have investigated cellular responses of avian tectal neurons to electrical stimulation in vivo (Hardy et al, 1984(Hardy et al, , 1985Leresche et al, 1986) and in vitro (Dye and Karten, 1996). However, these studies have stimulated the optic nerve or the optic tract, activating a large number of afferent fibers and probably a network of tectal cells.…”

Columnar projections from the cholinergic nucleus isthmi to the optic tectum in chicks (Gallus gallus): A possible substrate for synchronizing tectal channels

“…Considering the dense termination and extensive laminar distribution of the Ipc and SLu efferents in the TeO, as well as the glutamatergic nature of the retinal inputs (Wang et al, 1978;Dye and Karten, 1996), Ipc and SLu may provide a substantial contribution to the tectal cholinergic system. In addition, Ipc and SLu may also contribute to the tectal cholinesterase (AChE).…”