Glucose-6-phosphate dehydrogenase activity in the olfactory system of the young rat: an enzyme histochemical study using computerized image analysis.

An understanding of olfactory system glucose metabolism is necessary for the interpretation of radiolabeled 2-deoxyglucose studies of odor processing since the relationship between glucose uptake and neural activity is based on assumptions regarding cellular glucose utilization. As part of an ongoing study examining divergent pathways of glucose metabolism in the olfactory system, the relative activity of glucose-6-phosphate dehydrogenase, the rate-limiting enzyme of the hexose monophosphate shunt, was examined among cells of the rat olfactory bulb and anterior olfactory nucleus, by using enzyme histochemistry on fresh frozen tissue. Optical density measurement of formazan reaction product in stained tissue were quantified by computerized image analysis. To aid in the identification of histochemically stained neurons, alternate sections were Nissl-stained. The highest olfactory bulb dehydrogenase levels were found in the olfactory nerve and glomerular layers. Individual mitral and tufted cells also showed high dehydrogenase activity. In most stained neurons, formazan reaction product filled the cytoplasm and sometimes extended into the proximal part of dendrites and axons. The external plexiform and granule cell layers had low enzyme activity. High activity also was seen in pyramidal cells of pars dorsalis and pars lateralis of the anterior olfactory nucleus, one of the first, and most rostral of the olfactory bulb projection sites. High glucose-6-phosphate dehydrogenase activity in the olfactory system indicates that a significant amount of glucose can be channeled through the hexose monophosphate shunt in these neurons, with a concomitant production of NADPH. This may reflect high activity of cellular detoxification enzymes that rely on NADPH for reducing power. Such detoxification processes may be engaged in response to the potential entry and transsynaptic movement of airborne chemicals into the brain via the olfactory system.