Kainic acid has been described as a highly specific neurotoxin that when injected locally into the nervous system destroys neuronal perikarya but spares axons of passage and terminals in the vicinity of the injection site. The effects of injection of this agent into the rat dorsal lateral geniculate nucleus on geniculocortical and corticofugal pathways have been examined. Neuronal perikarya were absent from injected geniculates, and products of neuronal degeneration were observed in the external stratum of the optic radiation and in layer IV of striate cortex. Furthermore, no visually driven units could be found in physiological recordings from ipsilateral visual cortex, and no orthograde axonal transport of radioactivity to cortex was detectable by autoradiography. These observations are consistent with the complete destruction of the geniculocortical pathway. On the other hand orthograde axonal transport appeared normal in corticogeniculate and corticotectal neurons on the injected site. In addition to silver grains appearing over the geniculate and superior colliculus label was observed over corticofugal axons in the internal stratum of the optic radiation. It is noteworthy that axons of the corticofugal pathways are clearly segregated from geniculocortical axons over a part of their course through white matter. Corticogeniculate neurons on the injected site were able to transport D-aspartate but not nuclear yellow retrogradely to cell bodies in layer VI. These results are consistent with the notion that cortical neurons terminating in and passing through the kainate lesion site survive but it suggests, however, that corticogeniculate neurons may be functionally altered by these lesions.
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