Pharmacologic evidence for NMDA, APB and kainate/quisqualate retinotectal transmission in the isolated whole tectum of goldfish

Abstract

The optic tectum of goldfish with intact optic and toral marginal fiber tracts was isolated in a perfusion chamber where the effectiveness of antagonists was tested on synaptic field potential responses to stimulation of each afferent system. There were 3 main conclusions about excitatory synapses. First, monosynaptic activation of retinotectal synapses was not detectably antagonized by d-tubocurarine, implying there is no nicotinic cholinergic component to optic transmission nor strong cholinergic gating of optic terminals. Second, a significant component of retinotectical transmission was shown to be mediated by kainate and quisqualate receptors since 6,7-dinitroquinoxaline-2,3-dione and kynurenate strongly suppressed the optic field potential. In addition, activation of these synapses involves two previously undescribed N-methyl- d-aspartate (NMDA) and APB receptor subtypes since optic field potentials were partially suppressed by 2-amino-5-phosphonovalerate (APV), 2-amino-4-phosphonobutyrate (APB) and MK-801. This is the first evidence that APB receptors exist in the visual system central to the retina. Together, these results are consistent with the possibility that retinal ganglion cells use multiple glutamate receptor subtypes. Third, the optic tectum contains a population of intrinsic glutaminergic synapses activated by a non-optic input, the marginal fibers, which can be suppressed by both APV and kynurenate. The existence of tectal NMDA receptors whicha are not at primary optic synapses implies that APV used to interfere with rearrangement of optic fibers during development may act not only at afferent synapses but also at a more central component of the circuit.