| Research Abstract |
The NOT and AON, which both receive direct retinal inputs, perform a regulatory function on optokinetic responses (OKR). The geniculostriate pathway influences on this regulation, apparently via extrastriate areas involved in motion processing.
Retina and extrastriate lateral suprasylvian visual cortical efferents to NOT and AON have been studied ultrastructually in cats using WGA-HRP as anterograde tracer in combination with GABA immunohistochemistry. Retinal terminals (RTs) are numerous in both NOT and AON. These terminals contain pale mitochondria and round synaptic vesicles. RTs in NOT are often large and make asymmetrical synaptic contact with GABA-negative conventional dendrites and vesicle-containing GABA-positive dendrites which participate in glomeruli, while RTs in AON are small and make asymmetrical contacts mainly with conventional dendrites. No typical retinal glomeruli are found in AON. There appear to be no differences in morphology or synaptic organization of the cortica
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l terminals (CTs) between NOT and AON. CTs always are small, and make asymmetrical contact with GABA-negative conventional dendrites. The present results indicate that RTs and CTs differ morphologically in synaptic structure, reflecting their qualitative difference in processing visual information. Furthermore, RTs are morphologically identical in NOT and AON, reflecting their unique role in visual processing.
Then, we have examined the projections from the retina and visual cortical areas (areas 17, 18a and 18b) to NOT in rats using tracers (wheat germ agglutinin-conjugated horseradish peroxidase, WGA-HRP and cholera toxin B subunit, CTB) in order to clarify how these two different functional inputs to OKR are organized. CTB injection into the vitreous body resulted in anterograde label almost exclusively in the contralateral NOT. Ultrastructually, the size of the retinal axon terminals was small (up to 0.7 μm in diameter), contained round synaptic vesicles and pale mitochondria, and made asymmetrical synaptic contacts with both GABA-positive and GABA-negative NOT neurons. Visual cortical area 17 and the transitional area between area 17 and 18a, or between area 17 and 18b projected their axons to the ipsilateral NOT. Ultrastructually, the size of the cortical axon terminals was small (up to 0.5 μm in diameter), contained round synaptic vesicles, and made asymmetrical synaptic contacts only with GABA-negative NOT neurons. With light and electron microscopical observation, there was a considerable overlap in the cortico-NOT and retino-NOT projection pattern : GABA-negative (presumably NOT projection) neurons simultaneously receive input from both cortical and retinal terminals. From these results, it seems reasonable to postulate that inputs from visual cortical areas in the pigmented rat cooperate with those from the retina in controlling OKR. Less
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