Tamotsu Urushibata

Retinal ganglion cells related to optokinetic nystagmus in the rat.

The nucleus of the optic tract (NOT) in the pretectum is the visuo-motor relay between the retina and preoculomotor structure in the pathway conveying signals responsible for optokinetic nystagmus (OKN) in both afoveate and foveate animals. Unilateral lesions of the NOT abolish OKN toward the side ipsilateral to the lesion. However, what parts of the retina and what kinds of retinal ganglion cells project their fibers into the NOT are still unknown. To examine this, horseradish peroxidase conjugated with wheatgerm agglutinin was injected into the NOT of the rat. Labeled retinal ganglion cells were diffusely documented in the contralateral side, whereas those situated only in the lower temporal crescent, were found in the ipsilateral side. Ganglion cells dominated in the range of small cells, i.e., W cells. Therefore, ganglion cells responsible for OKN are believed to be related to W cells. However, medium-sized and large cells were definitely labeled in the rat, suggesting possible participation of these cells in OKN.

Immunohistochemical Study and an Electron Microscopic Observation of the Nucleus of the Optic Tract in the Rat

In the present study, gamma aminobutyric acid (GABA)-positive cells and terminals were detected in the nucleus of the optic tract (NOT) in the rat. However, GABA-positive cells in the NOT receiving projections directly from retinal ganglion cells could not be identified. To determine the role of GABA in the projection from the retinal ganglion cells to the NOT, we used electron microscopy to observe this nucleus of a rat in which one eyeball had been removed. In the nerve synapse binding region of the NOT on the side opposite to the enucleated eyeball, the symmetrical and asymmetrical types were recognized; the ratio between the two was the same as before the eyeball was enucleated. From these results, we conclude that the inhibitory axon terminals in the NOT do not have GABA activity. The GABA-positive cells seem to act indirectly as intrinsic interneurons, when processing or transmitting the information from the retina to the inferior olivary nucleus.