Although the cat visual system has been the subject of intensive investigation, little attention has been given to the morphological features of
ganglion cell projections to the suprachiasmatic nucleus. The present study has utilized anterograde transport of
horseradish peroxidase and wheat germ agglutinin-conjugated
horseradish peroxidase to label
ganglion cell terminals in the cat suprachiasmatic nucleus. Visualization of the reaction product was facilitated through the use of
gold-substituted
silver intensification.
Ganglion cell terminals were found to be morphologically diverse, making both asymmetric and symmetric contacts with postsynaptic processes. Synaptic vesicles were either scattered or densely packed, sometimes forming paracrystalline arrays. In contrast to other retinorecipient areas in which
ganglion cell terminals have been characterized by the presence of lightly staining mitochondria, many of the
retinal terminals in the suprachiasmatic nucleus were seen to contain darkly stained mitochondria. Postembedding antiglutamate immunocytochemistry was used to evaluate the level of endogenous
glutamate in these
ganglion cell terminals. Although morphologically diverse, all of the
retinal terminals in the suprachiasmatic nucleus were
glutamate positive, consistent with the postulated role of
glutamate as the
neurotransmitter of retinal ganglion cells.