Metabolic changes in the nucleus of the optic tract after monocular enucleation as revealed by cytochrome oxidase histochemistry

The histochemistry for the mitochondrial enzyme cytochrome oxidase (CO) was used to evaluate the levels of metabolic activity in neurons of the nucleu s of the optic tract (NOT) and dorsal terminal nucleus (DTN) in the opossum (Didelphis aurita). The observations were performed in four groups: normal juveniles (4 months old), monocularly enucleated juveniles analysed when a dults, normal adults (8 to 18 months old) and monocularly enucleated adults . CO labeled cells were observed to have a similar distribution along the N OT-DTN anteroposterior axis in both juvenile and adult normal animals. Mono cular enucleation performed in adults produced a significant reduction of t he reactive neuropil but not of the number of CO labeled cells in the deaff erented NOT-DTN: the number of labeled neurons per section in the deafferen ted side matched those of the ipsilateral complex. In juveniles, however, t his procedure caused a systematic reduction of the number of CO labeled cel ls in the contralateral NOT-DTN in comparison to the spared complex. The la ck of reduction in the number of neurons found on the deafferented side of the NOT-DTN of monocularly enucleated adult opossums compared with the ipsi lateral side might result from the presence of compensatory inputs to maint ain their metabolic equivalence. However, when the monocular enucleation wa s performed in juvenile opossums, a statistically significant asymmetry of CO neurons in the NOT-DTN was observed. In other words, the compensatory me chanisms proposed for the adults were either absent or insufficient to achi eve symmetry in juveniles, suggesting a more heavily reliance in the retina l input.