Retrograde transneuronal transport of the fluorescent dye rhodamine beta-isothiocyanate from the primary and centrifugal visual systems in the pigeon.

The transneuronal labeling properties of the fluorescent dye Rhodamine beta-isothiocyanate (RITC) were investigated in the pigeon following the intraocular injection of the tracer either alone or in combination with kainic acid (RITC/KA). The RITC was transported bidirectionally from the eye producing both orthograde axonal and terminal labeling of the primary visual system (PVS) and retrograde labeling of the centrifugal visual system (CVS) comprising the n. isthmo-opticus (NIO) and associated ectopic neurons (EN) as well as the tractus isthmo-opticus (TIO). In addition, RITC-positive cell bodies were identified in layers 9/10 of the optic tectum, n. isthmi parvocellularis (Ipc) and the hyperstriatum accessorium (HA) subdivision of the telencephalic visual Wulst. In the RITC/KA experiments, the orthograde transport of the dye within the PVS was selectively suppressed and this coincided with the absence of somatic labeling in n. Ipc and HA. The results are explained in terms of transneuronal transport of RITC, involving the terminal uptake and subsequent retrograde axonal transport of the tracer within second-order neurons in n. Ipc and HA which project to the PVS and in tectal layers 9/10 projecting to the CVS. Moreover, the transneuronal transport of RITC, as demonstrated using the present experimental conditions, appears to be very specific and only labeled distant cell populations known to project to either the PVS or CVS.