Cytochrome oxidase and NADPH-diaphorase on the afferent relay branch of the optokinetic reflex in the opossum

In the present study, histochemical techniques combined with more conventional anatomical methods were used to refine the identification of the nucleus of the optic tract and the nuclei of the accessory optic system in the opossum. The distribution of the enzyme cytochrome oxidase (CO) was examined in the cells and the neuropil of the opossum's mesodiencephalic region. Strong CO labeling was present in the nucleus of the optic tract (NOT)‐dorsal terminal nucleus (DTN). Alternate sections, taken from animals that had received bilateral injections of horseradish peroxidase centered in the region of the inferior olive, were subjected to assays for CO and horseradish peroxidase. The region occupied by CO‐labeled cells in the NOT‐DTN superimposed with the one defined by retrogradely labeled cells. Cell counts along the NOT‐DTN anteroposterior axis revealed that although the olivary and CO‐positive cells were confined within similar boundaries, the latter are up to twofold more numerous than the former. As revealed by cytochrome oxidase histochemistry, the outlines of the NOT‐DTN, the other pretectal nuclei and the nuclei belonging to the accessory optic system coincided with those revealed by the histochemistry for nicotinamide dinucleotide phosphate diaphorase (NADPH‐d). After an intraocular injection of cholera toxin beta subunit and alternate sections processing for NADPH‐d and CO, the distribution of labeled retinal terminal fields in the mesodiencephalic region was shown to be coincident with regions of high levels of histochemical labeling. These results are discussed in the light of previous anatomofunctional assessments of the pretectum and accessory optic system. J. Comp. Neurol. 398:206–224, 1998. © 1998 Wiley‐Liss, Inc.