Occurrence of adrenergic nerve fibers and of noradrenaline in thymus gland of juvenile and aged rats
Introduction
Since 1966, the presence of monoamines has been described in the rat thymus [1]. In 1984 it was demonstrated that adrenergic nerve fibers (ANF) coming from the superior cervical ganglion project to the rat thymus [2]. Subsequent histochemical studies verified a major sympathetic innervation of the thymus gland in mice and rats [3]. By injecting horseradish peroxidase into the thymus, labelled cells were observed in the sympathetic chain and particularly in the superior cervical ganglion. A spurious labelling of the cervical spinal cord and of the brain stem was obtained by spread of the tracer into surrounding structures.
In the rabbit thymus noradrenergic nerve fibers enter with nerve bundles and plexi around blood vessels, travel into the cortex from subcapsular plexi with the vasculature, and branch into the parenchyma of the thymic cortex. The vasculature and parenchymal regions of both the outer and deeper cortex are innervated by these fibers. Peptidergic immunoreactivity is also found in varicose profiles in the thymus within the cortex [4].
Total innervation of the rat thymus was studied by light and electron microscopy in juvenile and aged rats [5]. In aged rats the nerve fibers are thicker, tortuous, and more branched than in juvenile animals. No nerve fibers were found within the thymic parenchyma.
Thymuses from 344 Fischer rats were examined at 3, 8, 12, 17, 21 and 27 months old with fluorescence histochemistry for catecholamines and with LCEC for quantification of monoamines. Noradrenergic fibers entered the thymus with the vasculature and arborized in the cortex. Innervation increased with age parallel to thymic involution. Noradrenaline, 5-OH-triptamine and MHPG (3-methoxy-4-hydroxy-phenyleneglycol), a major metabolite of noradrenaline, were unchanged [6].
Sympathetic and parasympathetic innervation of the rat thymus has been also described using immunohistochemical, fluorescence histochemical and histochemical methods. Only parasympathetic Ache-positive nerve fibers are described in the medulla [7].
The density of total innervation of the rat thymus increases with age, exceeding the reduction of thymic volume due to involution, and correlates inversely with thymic volume. In fact, the total innervation remains firm and so it appears to be (relatively) increased in comparison with the decrease of the thymic parenchyma. In any case, thymic innervation is confined only to the extraparenchymal component [8].
In the rat thymus, noradrenergic innervation and development of β-adrenergic receptors have been studied. The sympathetic nervous system runs in the subcapsular cortex, at the corticomedullary junction and in the cortex itself, but is extremely scarce in the medulla. Receptors are present in the medulla. Chemical sympathectomy with 6-OHDA or guanethidine induces a decrease of prolipheration of peripheral T-cells and a destruction of noradrenergic nerve fibers [9].
Sympathetic nervous profiles are present in rat thymus from the 18th day of gestation [10]. In the outer cortex of adult rats, nerve fibers were found mainly in apposition to thymocytes, while in the deeper cortex and medulla they were found adjacent to thymic epithelial cells [10]. The histochemical patterns of noradrenaline and dopamine increase with age [11]. Moreover, a link has been demonstrated between the sympathetic nerve fibers and thymocytes in the rat thymus [12].
Finally, alterations have been identified in the sympathetic innervation of the thymus in aged mice. Age is also associated with a reduced immune reactivity. The noradrenergic innervation of the thymus was examined histologically and neurochemically revealing an increase of sympathetic innervation with age and with thymic involution [13].
The occurrence of noradrenergic and adrenergic innervation of the thymus has been investigated and reported in previous studies. In particular thymic innervation has been studied both during late fetal and postnatal development [10] and in juvenile and aged rats [8].
This study was carried out with the aim to confirm whether ANF and noradrenaline are localized in the rat thymus as well as to determine whether changes in adrenergic innervation or in NPY-like immunoreactivity in juvenile and aged rat thymus occur. In two of our previous papers, we pointed out the GABA-transaminase activity [14] and the acetylcholinesterase activity [15] in the thymus of juvenile and aged rats.
All these studies suggest a modulation of the functions of the thymocytes via the activation of their neuroreceptors.
Section snippets
Materials and methods
Male Wistar rats of 3 (young), 12 (adult), 24 (old) months of age and weighing 150–360 g (body weight) were used in our experiments. All the animals were treated in agreement with the Helsinki accord on the use of animals in research approved by all Institutional Review Boards. The procedures performed in this study included: (1) experimental groups of animals; (2) treatment with 6-OHDA; (3) drawing of samples; (4) sections from thymus; (5) staining of thymus; (6) staining of nerve fibers; (7)
Results
The thymuses drawn from juvenile, adult and old rats treated with neurotoxin 6-OHDA show marked modifications of their sympathetic innervation and cannot be used for further analysis. On the contrary the samples of thymus drawn from normal rats (untreated with 6-OHDA) were suitable for use in all our experiments. By examining the data reported in Table 1, we can see that body weight increases with age (150 g in young, 310 g in adult and 360 g in old rats). As most of the respective values
Discussion
The present study was undertaken to investigate whether aging is accompanied by alterations in the thymic autonomic adrenergic innervation. Our results showed that, in comparison to young and adult rats, in aged rats the density of monoaminergic histofluorescent nerve profiles decreased remarkably, while their pattern of intrathymic distribution remained unchanged. The thymic concentration of noradrenaline (NA) significantly increased between the age of 12 and 24 months. Moreover, the density
Acknowledgements
The present study was supported by a grant from University ‘La Sapienza’ of Rome. The authors are greatly indebted to Dr M.A. Aita and Dr M. Kovacs for their useful suggestions and criticisms. The technical assitance of Dario Caporuscio, the excellent secretarial work of Silvana Casamento, the photographic assistance of Giuseppe Leoncini and the kind help of Sharon Hobby in the revision of the English language are also gratefully acknowledged.
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