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 neural pathways. To test this hypothesis, in the thymi of adult rats orchidectomized (ORX) at age of 1 (ORX-1), 7 (ORX-7) and 30 days (ORX-30): (i) noradrenaline (NA), dopamine (DA) and serotonin (5-HT) contents and acetylcholinesterase (AChE) activity were measured and (ii) the distribution of monoamine- and AChE-containing nerves and cells was examined by a sucrose phosphate glyoxylic acid (SPG) method and enzyme histochemistry, respectively. In all groups of ORX rats, the thymus weight was significantly increased over that 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 catecholamines and 5-HT; consequently the concentration of each of them remained unaltered. In these animals, the density of both SPG-stained thymus nerve fibers and cells also remained unchanged. In the ORX-7 rats, the rise in the thymus weight was followed by a proportional increase in the content of all monoamines, except for NA which was reduced. Therefore, in these rats neither the thymus concentrations of DA nor that of 5-HT differed from controls, while the concentration of NA was significantly decreased. The reduction in both NA content and concentration reflected a diminished density of SPG-positive nerve profiles. In the ORX-30 rats, the increase in thymus weight was neither paralleled by a proportional increase in the DA content nor 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 significantly reduced. In parallel with these changes, a decreased density of thymic SPG-positive nerve fibers and cells was found. In all ORX rats, the pattern of intrathymic distribution of SPG-positive fibers and cells remained unchanged. Orchidectomy affected neither the activity of AChE (expressed per gram of tissue) nor the density of AChE-positive nerves and cells in the thymus. As the changes in the density of adrenergic nerve fibers in the thymus from ORX rats were not followed by similar alterations in the density of AChE-containing nerve fibers, it does not seem likely that NA and AChE are colocalized in the thymus nerve fibers. The results also suggest that there is a critical period during ontogenesis when changes within the HPG axis evoked by orchidectomy can affect the sympathetic nerve input to the rat thymus and therefore, most likely, development and function of the organ.