Effects of neonatal capsaicin administration on the numbers and volumes of neurons in left and right T10 dorsal root ganglia in the rat

The long-term effects of neonatal capsaicin were studied in left and right dorsal root ganglia (T10) from control and capsaicin-treated groups of Wistar rats. At 12 hours post partum, 5 females per group were injected subcutaneously with capsaicin or vehicle solution and killed at 6 months of age. Tissues were perfusion-fixed, embedded in resin and serially sectioned. A Nissl stain was used to distinguish between A and B neurons and systematic random sampling schemes were employed to obtain stereological estimates of numbers of neurons and mean volumes of their perikarya. Numbers were calculated from ganglion volumes (estimated via the Cavalieri principle) and neuron packing densities (estimated using physical disectors). Mean perikaryal volumes were calculated from packing densities and volume densities (estimated by point counting). Data were analysed to isolate main and interaction effects of neuron subtype, laterality and treatment. There was no evidence of lateral asymmetry or interaction effects. Control ganglia contained 3320 (coefficient of variation, CV, 8%) neurons. Most (73%) were B cells with a mean volume of 13,100 mu m(3) (CV 17%) of which the nucleus accounted for 1,800 mu m(3) (CV 18%). About 22% were A cells with a mean volume of 79,800 mu m(3) (CV 24%) and a nucleus of 6,100 mu m(3) (CV 26%). After capsaicin, over half the original population of cells was destroyed and B cell loss was significantly greater than that of A cells (about 80% of all cells lost were B cells). The mean size of A cells was greater after capsaicin due to selective loss of smaller cells and a greater volume of cytoplasm. B cell perikaryal volume was not affected but nuclear volume declined. The findings show that capsaicin destruction of peripheral sensory neurons is bilaterally symmetrical. In general, smaller neurons are selectively destroyed but this operates differently in A and B cells. It is size-dependent in A cells but size-independent (possibly random) in B cells.