Thymus and Neuroendocrine System

The close relationship between the thymus and the more classic endocrine organs is further illustrated by the results of studies designed to elucidate the effects of endocrine ablative surgery on thymus gland function. Prepubertal orchidectomy resulted in an increase in thymus weight and accelerated rejection of skin grafts (Castro, 1974). The effects are reversible by testosterone administration. Adrenalectomy also delayed the involution of the thymus (Dougherty, 1952 Ishidate and Metcalf, 1963). 

Castration or ovariectomy (when associated with adrenalectomy) induced a decrease in serum FTS-like bioactivity (Bach, 1983) and genetically dwarf (Snell-Bagg) mice exhibited a premature decline in serum bioactivity (Pelletier et al., 1976). High doses of corticosteroids induced a decrease in serum FTS-like bioactivity and serum Toj levels also exhibited an inverse relationship with serum corticosterone levels as well as a circadian rhythm (McGillis et al., 1983). Thus, current evidence indicates that the endocrine functions of the thymus gland are closely integrated to the other well-defined endocrine organs. 

Relationship between the Thymus Gland AND THE Central Nervous System 

A direct link between the immune (thymus) and endocrine system via the brain has been proposed (Rebar, 1983 Hall and Goldstein, 1983). Such a link is supported by the observations that (1) efferent pathways between the central nervous system (CNS) and the immune system exist, (2) hypophysec-tomy alters immune responsiveness (i.e., prolongs skin graft and lowers antibody production), (3) thymic hormones such as thymosin aj and thymosin p4 are found in discrete areas of the brain, and (4) administration of thymic hormones alters endocrine hormone levels (Comsa et al., 1975 Rebar et al., 1980, 1981a,b Hall and Goldstein, 1983 Palaszynski et al., 1983 Healy et al., 1983). 

Of significant interest are the most recent studies in rats that evaluate the thymus-reproductive and thymus-stress axes. Both in vitro administration of thymosin P4 into the medial basal hypothalamus and pituitary in chambers and in vivo administration into the cerebrovascular system have induced elevations of LH in media and serum, respectively (Rebar et al., 1981b Hall et al., 1983). Localization studies have demonstrated that the rat olfactory bulb has the highest concentration of thymosin 4, although it is also present in several other distinct sites (Hannappel et al., 1982). On the other hand, Toj did not influence levels of LH. Nevertheless, Ta is also present in the brain (Hall et al., 1982 Palaszynski et al., 1983), with its highest concentrations in the subcortical nuclei involved with both the autonomic and neuroendocrine system. These sites include both the pituitary gland and the hypothalamus. When injected intracranially, thymosin otj stimulated a rise in serum corticosterone in mice (Hall et al., 1982). The increase was rapid ( 3 hours) and did not occur in experiments where the peptide was incubated with cultured adrenal fasciculata cells (Vahouny et al., 1983). 

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