Direct ipsilateral retinal projections in goldfish ( Carassius auratus)

Abstract

ALAN D. SPRINGER* and GARY E. LANDRETH Neuroscience Laboratory, University of Michigan, Ann Arbor, Mich. 48109 (U.S.A.) (Accepted December 30th, 1976) The goldfish visual system has been extensively used in biochemical, electro- physiological, anatomical and behavioral studies 3,Ta°,n,lB,19, as well as in those con- cerned with neuronal plasticity and regeneration 4,6. Conventional silver-degeneration methodsl°,~8,19 and radioautography 8,~z have been used to map retinofugal projec- tions in goldfish. The optic nerves are described as completely decussating at the chiasm and projecting to the contralateral optic tectum, as well as to the diencephalon and preoptic areal0,X8,19. In addition to the known contralateral projections, we have found direct retinal projections to the ipsilateral diencephalon and preoptic area using [ZH]proline radioautography. Common goldfish (Carassius auratus), weighing 8-11 g, and 6-7 cm in body length, were obtained from Ozark Fisheries, Stoutland, Mo.; the fish were maintain- ed at 20 ± 1 °C. Six fish were injected intraocularly with 25 #Ci of e-[4,5-ZH]proline (46 Ci/mmole; New England Nuclear) in 5/~1 of saline. Fish were sacrificed either 24 h or 35 days after injection. Following fixation, the brains were processed for radioauto- graphy 8,1'~. Brain sections were coated with Kodak NTB2 emulsion, exposed for 34 days, developed and stained with hematoxylin-eosin. Silver grains were heavily concentrated over the contralateral optic tract, optic tectum and retinal targets of the diencephalon and preoptic area. Retinofugal fibers were also found to terminate in several ipsilateral regions by three routes : an uncross- ed pathway via the ipsilateral optic tract, fibers that decussate at the chiasm but sub- sequently recross the midline through the posterior commissure and fibers which re- cross through the minor commissure. A direct projection to the ipsilateral optic tectum was not seen. In all specimens examined, decussation of the optic nerve at the chiasm was in- complete. A small number of fibers are distributed throughout the ipsilateral optic tract near the chiasm. As the tract courses dorsally, the fibers collect and follow its * Present address: Department of Physiology, University of Illinois Medical Center, Chicago, Ill. 60680, U.S.A.