Metabolic activity in rat tectal grafts is influenced by host sensory innervation

It has been shown previously that fetal tectal tissue grafted to the midbrain of newborn host rats grows, differentiates, and receives input from the host brain. In the present study, 4 neuroanatomical techniques have been combined to examine how metabolic activity in tectal transplants is influenced by an identified host sensory pathway. Tectal tissue from E15 pigmented rat embryos was transplanted to the midbrain region of anesthetized newborn rats of the same strain. Six to 22 weeks later, the functional relationship between tectal transplants and the visual system of the host animal was examined by mapping metabolic activity in the grafts and relating this activity to the presence or absence of host retinal innervation. Metabolic activity in tectal grafts was assessed using the radioactive 2-deoxy-glucose (2-DG) method and cytochrome oxidase (CO) histochemistry. Graft regions receiving input from host retinal axons were demonstrated by anterograde labeling after bilateral intraocular injections of HRP or WGA-HRP; all areas in grafts that were homologous to the superficial layers of normal superior colliculus (SC) were identified using AChE histochemistry. The levels of metabolic activity demonstrated with 2-DG and CO varied between animals and within individual grafts. Grafts that did not connect with the host showed only low metabolic activity. In grafts that received host input, localized areas of high metabolic activity were seen with both 2-DG and CO. Highest levels of activity were consistently found in area containing both intense AChE activity and a high density of host retinal innervation.