Evidence for a neurotransmitter role of aspartate and/or glutamate in the projection from the torus longitudinalis to the optic tectum of the goldfish.

Different experimental approaches have been used to demonstrate that aspartate and/or glutamate is a transmitter(s) in the projection from the torus longitudinalis to the marginal layer of the optic tectum in the goldfish. Slices of the optic tectum incubated in vitro in the presence of D-[3H]aspartate and processed for light microscopic autoradiography, demonstrated a preferential accumulation of the labeled compound in the marginal layer. Under the same experimental conditions several neurons in the central part of the torus longitudinalis selectively accumulated D-[3H]aspartate. Synaptosome-enriched preparations from the optic tectum showed high-affinity uptake for D-[3H]aspartate and the rate of the uptake was significantly decreased after disconnection from the ipsilateral torus longitudinalis. The same subcellular preparations showed Ca2+-dependent release of previously accumulated D-[3H]aspartate under high potassium stimulation. This release was significantly reduced in preparations from optic tecta 5 days after cutting their connection with the ipsilateral torus longitudinalis. Finally, D-[3H]aspartate injected in the optic tectum retrogradely labeled the fiber systems connecting the marginal layer with the ipsilateral torus longitudinalis as well as neuronal cell bodies in the torus longitudinalis itself. From autoradiographic experiments it was, in addition, noticed that several tectal neurons selectively accumulated D-[3H]aspartate in the cell bodies as well as in main dendritic trunks. This observation suggests tht aspartate and/or glutamate may be a transmitter(s) in some intrinsic circuits and extrinsic projections of the optic tectum.