CHANGES IN THE PATTERN OF GLUTAMATE-LIKE IMMUNOREACTIVITY IN RAT SUPERIOR COLLICULUS FOLLOWING RETINAL AND VISUAL CORTICAL-LESIONS

We have investigated the pattern of glutamate-like immunoreactivity in the superficial layers of the rat superior colliculus by means of pos tembedding immunocytochemical methods for light and electron microscop y. At the light microscopic level, labelling was faintly to moderately intense in most perikarya of the stratum zonale, stratum griseum supe rficiale and stratum opticum. Furthermore, strong glutamate-immunoreac tive terminal-like elements were accumulated most densely in stratum z onale, stratum griseum superficiale and stratum opticum. At the electr on microscopic level, a postembedding immunogold method revealed that the vast majority of those labelled elements corresponded to retinal a nd visual cortical terminals. These profiles were about twice as heavi ly labelled as their postsynaptic partners. To determine the contribut ion of retinal and cortical afferents to the pattern of glutamate-like immunoreactivity, rats were subjected to right retinal ablation, left cortical ablation or combined right retinal and left cortical ablatio ns. After retinal ablation, strongly labelled perikarya were observed in the retinorecipient layers. Furthermore, a prominent loss of glutam ate-immunoreactive terminal-like elements occurred in stratum zonale a nd stratum griseum superficiale. Ipsilateral superior colliculus to co rtical ablation exhibited subtle changes characterized by a moderate i ncrease in perikaryal immunostaining in stratum zonale, stratum griseu m superficiale and stratum opticum and by an apparent discrete reducti on of labelled dots in stratum griseum superficiale and stratum opticu m. In cases with combined lesions, strongly immunoreactive cell bodies and dendrites were accompanied by a massive disappearance of labelled terminal-like elements in stratum zonale, stratum griseum superficial e and stratum opticum. The effect of retinal and visual cortical ablat ions on the pattern of glutamate-like immunoreactivity suggests that t hese afferents are the major sources for glutamate-immunoreactive term inals in the rat superior colliculus. In addition, these findings prov ide further evidence for glutamate as neurotransmitter in the visual p athways studied.