Castration of sexually immature rats affects sympathetic innervation of the adult thymus

It has been hypothesized that maturational processes within the hypothalamo -pituitary-gonadal (HPG) axis and thymus are reciprocally regulated via neu ral pathways. To test this hypothesis, in the thymi of adult rats orchidect omized (ORX) at age of 1 (ORX-1), 7 (ORX-7) and 30 days (ORX-30): (i) norad renaline (NA), dopamine (DA) and serotonin (5-HT) contents and acetylcholin esterase (AChE) activity were measured and (ii) the distribution of monoami ne- and AChE-containing nerves and cells was examined by a sucrose phosphat e glyoxylic acid (SPG) method and enzyme histochemistry, respectively. In a ll groups of ORX rats, the thymus weight was significantly increased over t hat in sham-ORX control rats. In the ORX-1 rats, the increase in the thymus weight was accompanied by a proportional increase in the content of both c atecholamines and 5-HT; consequently the concentration of each of them rema ined unaltered. In these animals, the density of both SPG-stained thymus ne rve fibers and cells also remained unchanged. In the ORX-7 rats, the rise i n the thymus weight was followed by a proportional increase in the content of all monoamines, except for NA which was reduced. Therefore, in these rat s neither the thymus concentrations of DA nor that of 5-HT differed from co ntrols, while the concentration of NA was significantly decreased. The redu ction in both NA content and concentration reflected a diminished density o f SPG-positive nerve profiles. In the ORX-30 rats, the increase in thymus w eight was neither paralleled by a proportional increase in the DA content n or in 5-HT, while the content of NA was decreased. Thus, in their thymi the concentration of both NA and DA, as well as that of 5-HT, were significant ly reduced. In parallel with these changes, a decreased density of thymic S PG-positive nerve fibers and cells was found. In all ORX rats, the pattern of intrathymic distribution of SPG-positive fibers and cells remained uncha nged. Orchidectomy affected neither the activity of AChE (expressed per gra m of tissue) nor the density of AChE-positive nerves and cells in the thymu s. As the changes in the density of adrenergic nerve fibers in the thymus f rom ORX rats were not followed by similar alterations in the density of ACh E-containing nerve fibers, it does not seem likely that NA and AChE are col ocalized in the thymus nerve fibers. The results also suggest that there is a critical period during ontogenesis when changes within the HPG axis evok ed by orchidectomy can affect the sympathetic nerve input to the rat thymus and therefore, most likely, development and function of the organ. Copyrig ht (C) 2000 S. Karger AG, Basel.