GLUTAMATE-LIKE IMMUNOREACTIVITY IN THE CAT SUPERIOR COLLICULUS AND VISUAL-CORTEX - FURTHER EVIDENCE THAT GLUTAMATE IS THE NEUROTRANSMITTER OF THE CORTICOCOLLICULAR PATHWAY

Biochemical studies provide evidence that the pathway from visual cort ex to the superior colliculus (SC) utilizes glutamate as a neurotransm itter. In the present study, we have used immunocytochemistry, visual cortex lesions, and retrograde tracing to show directly by anatomical methods that glutamate or a closely related analog is contained in cor ticocollicular neurons and terminals. A monoclonal antibody directed a gainst gamma-L-glutamyl-L-glutamate (gamma glu glu) was used to locali ze glutamate-like immunoreactivity in both the superior colliculus (SC ) and visual cortex (VC). Unilateral lesions of areas 17-18 were made in four cats to determine if gamma glu glu labeling was reduced in SC by this lesion. WGA-HRP was injected into the SC of 10 additional cats in order to determine if corticocollicular neurons were also labeled by the gamma glu glu antibody. A distinctive dense band of gamma glu g lu immunoreactivity was found within the deep superficial gray and upp er optic layers of SC where many corticotectal axons are known to term inate. Both fibers and cells were labeled within the band. Immunoreact ivity was also found in cells and fibers throughout the deep layers of SC. Measures of total immunoreactivity (i.e. optical density) in the dense band were made in sections from the SC both ipsilateral to and c ontralateral to the lesions of areas 17-18. A consistent reduction in optical density was found in both the neuropil and in cells within the dense band of the SC ipsilateral to the lesion. A large percentage of all corticocollicular neurons that were retrogradely labeled by WGA-H RP also contained gamma glu glu. These results provide further evidenc e that the corticocollicular pathway in mammals is glutamatergic. The results also suggest that visual cortex ablation alters synthesis or s torage of glutamate within postsynaptic SC neurons, presumably as a re sult of partial deafferentation.