Differential sensitivities of the two visual pathways of the chick to labelling by fluorescent retrograde tracers

This study investigates the neurone structure-specific differences of sensi tivities of fluorescent tracers. The tracers were used for retrograde label ling of contralateral projections in the two visual pathways of the chick. Rhodamine B Isothiocyanate (RITC), Fluorogold (FG) and True blue (TB) were injected into either the visual Wulst (thalamofugal pathway) or the nucleus rotundus (Rt; tectofugal pathway) and the retrogradely labelled neurones i n the nucleus geniculatus lateralis pars dorsalis (GLd) or the optic tectum , respectively, were counted. Differential retrograde labelling in the two pathways was observed. In the thalamofugal pathway, both the contralateral and ipsilateral GLd cells were labelled by all three tracers (RITC, FG and TB). However, in the tectofugal pathway, whereas RITC labelled both the ips ilateral and contralateral tectal neurones, FG or TR labelled effectively o nly the ipsilateral tectal neurones. It was clear that FG and TB were taken ; up by the nerve endings and transported part-way along the axon but faile d to be transported to the cell bodies of the contralateral tectal neurones . In addition, red beads and green beads were also injected into Rt and the differential labelling was also observed. Red beads labelled both ipsilate ral and contralateral tectal neurones but green beads labelled only the ips ilateral tectal neurones. Since the contralateral tectal projections consis t of divergent axon collaterals, the present study suggests that various re trograde tracers are not transported in these axon collaterals to label cel l bodies. The contralaterally projecting neurones in the thalamofugal pathw ay are not axon collaterals and they were labelled by all of the tracers us ed. (C) 1999 Elsevier Science B.V. All rights reserved.