The distribution of cytochrome oxidase (CO) in the lateral geniculate nucleus (LGN) of normal adult macaque monkeys was analyzed in quantitative light- and electron-microscope (EM) studies. Both reactive and nonreactive neurons were found throughout laminae 1–6. Darkly reactive neurons were more numerous in laminae 1, 2, and 6. Within each lamina, there was a positive correlation between cell size and level of CO activity in neurons. The cell density was greater in laminae 1, 4, and 6, which received input from the contralateral eye, than in corresponding laminae representing the ipsilateral eye (2, 3, and 5). The cell density in each of the magnocellular laminae was less than that in each of the parvicellular laminae.

Three types of neurons could be distinguished within magnocellular laminae at the EM level. Types I and II were interpreted to be relay neurons, while the small size of type III qualified them to be interneurons. Two populations were found within parvicellular laminae, corresponding to relay neurons and interneurons, respectively. EM quantitative analyses revealed that the darkly reactive neurons contained a high proportion of moderate to darkly reactive mitochondria, which comprised 44.2%, 42.5%, and 39.2% of the mitochondrial population within neurons of laminae 1, 2, and 6 respectively. In contrast, a relatively low proportion of reactive mitochondria were found in neurons within laminae 3 (22.8%), 4 (26.4%), and 5 (32.4%). Moreover, magnocellular laminae contained a higher density of synapses on cell bodies than parvicellular laminae. All laminae, except layer 5, contained predominantly symmetric synapses on cell bodies. However, the proportion of symmetrical synapses in the magnocellular laminae (65.6%) was greater than that in the parvicellular laminae (59%).

Our quantitative light and EM results indicate that the level of metabolic activity in neurons of the monkey LGN is higher in magnocellular, ON-center, and contralateral than in parvicellular, OFF-center, and ipsilateral visual pathways, respectively. The metabolic activity of neurons is likely to reflect the chronic level of spontaneous and synaptically evoked discharges of these cells.

The B fragment of cholera toxin (CT-B) provides a highly sensitive anterograde tracer for labeling retinofugal axons, revealing dense projections to known central retinorecipient nuclei, and sparse but distinct inputs to regions that have not been traditionally recognized as targets of direct retinal projections. In hamsters, we can identify CT-B labeled retinal axons in more than 25 cell groups in the mesencephalon, diencephalon, and basal telencephalon. CT-B labeling additionally delineates the complete arbor morphology, especially in regions that receive a sparse input, offering hitherto unknown views of retinal axon ramifications. We present here the terminal morphology of retinal axons in the lateral geniculate body and superior colliculus, verifying earlier studies, and also document novel findings on the configuration of retinal axon endings in the ventral nucleus of the lateral geniculate body, intergeniculate leaflet, suprachiasmatic nucleus, and in the nuclei of the accessory optic tract. Additionally, the trajectory and terminal morphology of retinal afferents to the hypothalamus, preoptic area, and basal telencephalon are detailed. The results are discussed in the context of possible functional roles for some of these projections.