Transynaptic transport of procion yellow in the central nervous system

Intracellular recordings were performed in slices of the lateral geniculate body, the visual cortex, and the hippocampus of guinea pigs. After recording, Procion Yellow was iontophoretically injected. During the injection period the neurones were activated antidromically and/or transynaptically. Up to 6 h after dye injection slices were fixed in a buffered formaldehyde/sucrose solution and later histologically processed. In the lateral geniculate body, after orthodromic stimulation across the optic tract, optic tract fibers were stained secondarily. Orthodromic activation across intrageniculate connections led to secondarily stained geniculate neurones. In the visual cortex orthodromic activation of the injected neurones across white matter led to secondarily stained fibres travelling in the direction to the white matter or to secondarily stained neurones in the vicinity of the injected neurone. In the hippocampus anti- and orthodromic activation across the alveus or the mossy fibre pathway led to a secondary staining of neighbouring pyramidal neurones and to a secondary staining of fibres which where afferent to the injected neurone. All slices where Procion Yellow was detected in the extracellular space were rejected. These findings led to the conclusion that Procion Yellow must have been transported across chemical synapses from the post- to the presynaptic side. The transport occurred only if the concerned synapses were activated.