Abstract
A quantitative histochemical method was developed and standardized and then used to characterize the heterogeneity of cytochrome oxidase activity among primary afferent neuronal cell bodies in dorsal root ganglia of rat. In addition, the relationship between cytochrome oxidase and carbonic anhydrase activities in these neurons was determined. In tests of the procedure, the density of cytochrome oxidase reaction product evaluated repeatedly in individual neurons within sections of ganglia was found to increase linearly over incubation periods of up to 6 h. The heterogeneity in cytochrome oxidase activity in ganglia was not simply a reflection of the heterogeneity in ganglion cell sizes. On the whole, each class of ganglion cell exhibited the full range of staining densities encountered but intense staining was observed in many more large type A cells than small type B cells. The latter, together with their termination fields within the substantia gelatinosa of the spinal cord, were lightly stained. A significant positive correlation was found between neuronal size and staining density (r = 0.43). However, the large scatter in the plot of these two variables suggests that the expression of cytochrome oxidase in sensory neurons is governed to a considerable extent by properties of these neurons that are unrelated to their size. Analysis of cytochrome oxidase and carbonic anhydrase activities in the same ganglion cells revealed that all neurons with dense staining for the oxidase were anhydrase positive. Conversely, however, some intensely anhydrase-positive cells exhibited only light staining for cytochrome oxidase. The heterogeneity of cytochrome oxidase activity among neurons in dorsal root ganglia may be related to the steady state electrophysiological activity of distinct populations of sensory neurons which in turn may be related to the specific sensory modalities these populations transmit. The observation that some neurons with the greatest abundance of carbonic anhydrase do not contain high or even moderate levels of cytochrome oxidase suggests some degree of dissociation between the functional requirement for carbonic anhydrase in sensory neurons and the rate of energy expenditure in these cells.
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