Brain Research, 85 (1975) 307-312 © Elsevier Scientific PublishingCompany,Amsterdam- Printed in The Netherlands 307 THE ORTHOGRADE TRANSPORT OF HORSERADISH PEROXIDASE IN THE VISUAL SYSTEM JACQUES REPERANT Laboratoire de Neuromorphologie (U.106 I.N.S.E.R.M.), H6pital de Port-Royal, 75014 Paris, Laboratoire de Psychophysiologie sensorielle, 4 place Jussieu, Paris 75005, and Laboratoire d'Anatomie Compar~e, 55 rue de Buffon, Paris 75005 (France) Though the phenomenon of retrograde intraaxonal transport of horseradish peroxidase (HRP) is by now well establishedl,a,4,6-11,1a, 14 the question of its orthograde axoplasmic active migration remains a subject of controversy1,4,11-13. In the rat visual system two recent studies demonstrated the extent to which opinion is divided on this subject. On the one hand, for Hansson 4, HRP may be taken up by the ganglion nerve cells and transported upwards to their synaptic endings. On the other hand, Bunt et al. 1 did not observe such phenomenon. In order to review this problem, HRP (Sigma type VI, 3-5 #1 of 20 ~ solution in saline) was injected into the vitreous humor of the eye in a series of young anesthetized albino rats 08-25 days postnatal). Controls were injected with the same volume of saline. From 24 to 72 h after injection the animals were perfused through the heart with 300 ml of 1 ~o paraformaldehyde and l ~ glutaraldehyde in 0.12 M phosphate buffer (pH 7.2). The brains were removed, washed in buffer-sucrose solution at 4 °C, cut in serial sections (40 #m) on a freezing microtome and incubated for 15 rain in a medium con- taining H202 and 3,3'-diaminobenzidine tetrahydrochlorideL For ultrastructural examination in some experiments the optic nerves and the brain slices containing the tractus opticus or primary optic centers were immersed overnight in the same buffer solution without sucrose and cut with a Vibratome in sections 30--80/~m thick. After peroxidase revelation proper in incubation medium~, these sections were post- fixed in 2 ~ osmium tetroxide solution, dehydrated in ethanol and then embedded in Epon-Araldite mixture. Sections 0.2-0.1/~m were mounted on grids and observed without metal impregnation. Ultrathin sections were also prepared and stained with uranyl acetate and lead citrate. These sections were examined with a Siemens Elmiskop IA. In addition the rat visual projections were studied with Fink-Heimer and radio- autographic methods. Primary optic fibers were also impregnated with the Golgi method. For comparison similar experiments were performed in pigeons. Besides the intraocular injection of HRP, the enzyme was also injected stereotaxically into the optic tract. This material was observed only at the light microscopical level. In only 60~ of the HRP experimental rats a peroxidase reaction (Fig. 1) can be observed 24 h after HRP intraocular injection, in the optic nerve, optic tracts and