Prominently decreased hippocampal neurogenesis in a spontaneous model of type 1 diabetes, the nonobese diabetic mouse.

In human diabetes, degenerative and functional disorders of the central nervous system, including depression, are common findings. Defective dentate gyrus (DG) neurogenesis is associated with affective-related disorders and depression. We previously demonstrated reduced DG neurogenesis in a pharmacological type 1 diabetes model, the streptozotocin (STZ)-treated mouse. Here, we explored DG neurogenesis in a spontaneous T1D model, the nonobese diabetic (NOD) mouse, at prediabetic and diabetic stages. Cell proliferation was assessed in the DG of 5, 8 and 12-week-old control C57BL/6 and BALB/c strains and NOD mice, killed 2 h after bromodeoxyuridine (BrdU) administration. Survival of the newly generated cells was studied in 15-week-old animals that were killed 21 days after BrdU injection. The number of proliferative BrdU-positive cells in the DG was, regardless of age, constantly and significantly lower in NOD than in control strains, showing the presence of hippocampal alterations far before clinical diabetes onset in NOD mice. Diabetes also strongly decreased cell survival in NOD DG. However, cell phenotype proportion, as assessed by co-localization with neuronal or glial markers and confocal microscopy, was not modified. Hippocampal neurogenesis is strongly diminished in the spontaneous NOD model, like in the STZ model. Notably, NOD hippocampal DG cell proliferation defect takes place during the prediabetic stage. Whether this early alteration might result, in this autoimmune strain, from hypothalamo-pituitary adrenal axis alterations and/or ongoing brain inflammatory process sharing many characteristics of aging is discussed and deserves further investigation.

Gene therapy for long-term restoration of circulating thymulin in thymectomized mice and rats.

Thymulin is a thymic peptide possessing hypophysiotropic activity and antiinflammatory effects in the brain. We constructed a synthetic DNA sequence encoding met-FTS, a biologically active analog of thymulin, and subsequently cloned it into different expression vectors. A sequence optimized for expression of met-FTS in rodents, 5'-ATGCAGGCCAAGTCGCAGGGGGGGTCGAACTAGTAG-3', was cloned in the mammalian expression vectors pCDNA3.1(+) and phMGFP (which expresses the Monster Green Fluorescent Protein), thus obtaining pcDNA3.1-metFTS and p-metFTS-hMGFP, which express met-FTS and the fluorescent fusion protein metFTS-hMGFP, respectively. The synthetic sequence was also used to construct the adenoviral vector RAd-metFTS, which expresses met-FTS. Transfection of HEK293 and BHK cells with pcDNA3.1-metFTS (experimental groups) or pcDNA3.1 (control), led to high levels of thymulin bioactivity (>600 versus <0.1 pg/ml in experimental and control supernatants, respectively). Transfection of HEK293 and BHK cells with pmetFTS-hMGFP revealed a cytoplasmic and nuclear distribution of the fluorescent fusion protein. A single intramuscular (i.m.) injection (10(7) plaque forming units (PFU)/mouse or 10(8) PFU/rat) of RAd-metFTS in thymectomized animals (nondetectable serum thymulin) restored serum thymulin levels for at least 110 and 130 days post-injection in mice and rats, respectively. We conclude that RAd-metFTS constitutes a suitable biotechnological tool for the implementation of thymulin gene therapy in animal models of chronic brain inflammation.

Altered functional responses with preserved morphology of gonadotrophic cells in congenitally athymic mice.

Neonatal thymectomy or congenital absence of the thymus induces severe reproductive deficiencies in female mice, which are associated with reduced levels of circulating and pituitary gonadotropins. In contrast, the reproductive function is well preserved in nude males. It was therefore of interest to assess gonadotrophic cell morphology and function in congenitally athymic male mice. Circulating gonadotropins were measured under basal and stressful conditions, taking as a reference their haired counterparts. Adult normal (+/+), heterozygous nude (nu/+), and homozygous (nu/nu) CD-1 mice were subjected to 1-h immobilization stress. Serum levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) were assessed by RIA at 0, 30, and 60 min poststress. Athymic animals showed significantly lower basal levels of serum LH and FSH than their heterozygous littermates. Immunohistochemical assessment of LH and FSH cell populations revealed a normal morphology and cell number in the athymic animals compared to their normal littermates. Immobilization stress induced a significant reduction in gonadotrophin levels, particularly LH, in normal mice but had only a weak effect in athymic animals. It is concluded that congenital athymia in the adult male mouse is associated with decreased basal levels of serum LH and FSH, in the presence of a normal gonadotroph number and morphology. The anomalous responses of athymic mice to stress do not appear to be due to primary hypopituitarism but, rather, to an altered modulation of pituitary hormone secretion. .

Neuroendocrine control of thymus physiology.

The thymus gland is a central lymphoid organ in which bone marrow-derived T cell precursors undergo differentiation, eventually leading to migration of positively selected thymocytes to the peripheral lymphoid organs. This differentiation occurs along with cell migration in the context of the thymic microenvironment, formed of epithelial cells, macrophages, dendritic cells, fibroblasts, and extracellular matrix components. Various interactions occurring between microenvironmental cells and differentiating thymocytes are under neuroendocrine control. In this review, we summarize data showing that thymus physiology is pleiotropically influenced by hormones and neuropeptides. These molecules modulate the expression of major histocompatibility complex gene products by microenvironmental cells and the extracellular matrix-mediated interactions, leading to enhanced thymocyte adhesion to thymic epithelial cells. Cytokine production and thymic endocrine function (herein exemplified by thymulin production) are also hormonally controlled, and, interestingly in this latter case, a bidirectional circuitry seems to exist since thymic-derived peptides also modulate hormonal production. In addition to their role in thymic cell proliferation and apoptosis, hormones and neuropeptides also modulate intrathymic T cell differentiation, influencing the generation of the T cell repertoire. Finally, neuroendocrine control of the thymus appears extremely complex, with possible influence of biological circuitry involving the intrathymic production of a variety of hormones and neuropeptides and the expression of their respective receptors by thymic cells.

Studies on the gonadotropin-releasing activity of thymulin: changes with age.

We assessed the ability of thymulin, a zinc-dependent nonapeptide produced by the thymic epithelial cells, to influence the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from dispersed anterior pituitary (AP) cells from young, adult, and senescent female rats. Perifusion of young and senescent AP cells with thymulin doses of 10(-6) to 10(-5) M gave a significant stimulatory response for LH but not FSH. Gonadotropin release was always lower in the senescent cells. AP cells from both age groups incubated with 10(-8) to 10(-3) M thymulin showed a time- and dose-dependent response for both gonadotropins, with a maximal stimulation at 10(-7) M. Preincubation of thymulin with an antithymulin serum completely quenched the secretagogue activity of the hormone. Coincubation of thymulin with the secretagogue gonadotropin-releasing hormone (GnRH) revealed a synergistic effect on LH release and an additive effect on the release of FSH. The calcium chelator EGTA blocked the gonadotropin-releasing activity of thymulin in AP cells. The cAMP enhancers, caffeine, NaF, and forskolin significantly increased the thymulin-stimulated release of gonadotropins. The inositol phosphate enhancer LiCl potentiated the action of thymulin on gonadotropins. It is concluded that the gonadotropin-releasing activity documented here for thymulin is an age- and receptor-dependent effect mediated in part by calcium, cAMP, and inositol phosphates.

Is there a role for growth hormone upon intrathymic T-cell migration?

Intrathymic T-cell differentiation is essentially driven by the thymic microenvironment, a tridimensional network formed by thymic epithelial cells and to a lesser extent, dendritic cells, macrophages, fibroblasts, and extracellular matrix components. Thymocyte migration throughout the thymus is partially dependent on extracellular-matrix (ECM)-mediated interactions. Herein we investigated the putative role of growth hormone (GH) upon events related to intrathymic T-cell migration. We demonstrated that GH upregulates the expression of ECM ligands and receptors in distinct preparations of cultured thymic epithelial cells TECs). We also showed that adhesion of thymocytes to thymic epithelial cells was significantly increased by GH treatment, an effect that could be consistently abrogated when TECs were treated to antifibronectin, anti-VLA5, antilaminin, or anti-VLA6 antibodies before addition of thymocytes to the cultures. We also studied thymic nurse cells (TNCs), lymphoepithelial complexes that can be isolated ex vivo from the thymus. In this system, we had previously demonstrated that ECM ligands and receptors control both inward and outward thymocyte traffic. We then showed that GH enhances thymocyte release from TNCs, as well as the reconstitution of these lymphoepithelial complexes. Lastly, we evaluated the in vivo influence of GH on thymocyte exit. This was done by means of intrathymic injection of GH plus fluorescein isothiocyanate (FITC), and further analysis of recent thymic emigrants (FITC+ cells) in peripheral lymphoid organs, as defined by CD4/CD8-based cytofluorometric phenotyping. The proportions of FITC+ T cells appeared augmented in lymph nodes in GH-treated mice, as compared to controls. Taken together, these data indicate that GH stimulates intrathymic T-cell traffic, an effect that is at least partially mediated by extracellular matrix-mediated interactions.

Growth hormone-releasing activity of thymulin on pituitary somatotropes is age dependent.

Thymulin is a Zn-bound nonapeptide produced by the thymic epithelial cells (TEC) whose secretion is modulated by growth hormone (GH), among others. We assessed the ability of thymulin to influence the release of GH from dispersed anterior pituitary (AP) cells from young, middle-aged and senescent Sprague-Dawley female rats. Perifused and incubated AP cells were used in different sets of experiments and GH release was measured by RIA. Perifusion of young and senescent AP cells with thymulin doses, ranging from 10(-8) to 10(-5) M, gave a logarithmic dose-response pattern of GH. Supernatants from TEC lines also showed GH secretagogue activity. The GH release was always lower in the senescent cells. AP cells incubated with 10(-8)-10(-3) M thymulin showed a time- and dose-dependent response, the latter being bell-shaped with a maximum at 10(-7) M thymulin. Preincubation of thymulin with an antithymulin serum completely quenched the secretagogue activity of the hormone. Coincubation of thymulin with GHRH revealed a semiadditive release of GH in young and middle-aged AP cells and an additive effect in senescent cells. In middle-aged AP cells, the synthetic GH secretagogue GHRP-6 showed a synergistic effect with thymulin on GH release. The calcium chelator EGTA, but not the calcium ionophore A23187, blocked the GH-releasing activity of thymulin in AP cells. The cAMP enhancers, caffeine, NaF and forskolin significantly increased the thymulin-stimulated release of GH while trifluoperazine, a protein kinase C inhibitor, had no effect. The inositol phosphate enhancer LiCl potentiated the action of thymulin on GH release. The data suggest that the GH-releasing activity of thymulin is receptor-mediated and involves calcium, cAMP and inositol phosphates. In addition, senescence appears to impair somatotrope responsiveness to thymulin.

Immunoneuroendocrine connectivity: the paradigm of the thymus-hypothalamus/pituitary axis.

It is now largely established that the immune and neuroendocrine systems cross-talk by using similar ligands and receptors. In this context, the thymus-hypothalamus/pituitary axis can be regarded as a paradigm of connectivity in both normal and pathological conditions. For example, cytokines and thymic hormones modulate hypothalamic-pituitary functions: (a) interleukin (IL)-1 seems to upregulate the production of corticotropin-releasing factor and by adrenocorticotropin by hypothalamic neurons and pituitary cells, respectively; (b) thymulin enhances LH secretion. Conversely, a great deal of data strongly indicate that the hypothalamic-pituitary axis plays a role in the control of thymus physiology. Growth hormone (GH) for example, enhances thymulin secretion by thymic epithelial cells (TEC), both in vivo and in vitro, also increasing extracellular matrix-mediated TEC/thymocyte interactions. Additionally, gap junction-mediated cell coupling among TEC is upregulated by ACTH. In a second vein, it was shown that GH injections in aging mice increased total thymocyte numbers and the percentage of CD3-bearing cells, as well concanavalin-A mitogenic response and IL-6 production. In addition to mutual effects, thymus-pituitary similarities for cytokine and hormone production have been demonstrated. Cytokines such as IL-1, IL-2, IL-6, interferon-gamma, transforming growth factor-beta and others can be produced by hypothalamic and/or pituitary cells. Conversely, hormones including GH, PRL, LH, oxytocin, vasopressin and somatostatin can be produced intrathymically. Moreover, receptors for various cytokines and hormones are expressed in both the thymus and the hypothalamus/pituitary axis. Lastly, it is noteworthy that a thymus-pituitary connectivity can also be seen under pathological situations. In this regard, an altered HPA axis has been reported in AIDS, human falciparum malaria and murine rabies, that also show a severe thymic atrophy.

Neuroendocrine control of the thymus.

Thymocytes undergo a complex process of differentiation, largely dependent on interactions with the thymic microenvironment, a tridimensional cellular network formed by epithelial cells, macrophages, dendritic cells, and fibroblasts. One key cellular interaction involves the TCR-CD3 complex expressed by thymocytes with MHC-peptide complexes present on microenvironmental cells. Additionally, thymic epithelial cells (TEC) interact with thymocytes via soluble polypeptides such as thymic hormones and interleukins, as well as through extracellular matrix (ECM) ligands and receptors. Such types of heterotypic interactions are under neuroendocrine control. For example, thymic endocrine function, represented by thymulin production, is up-regulated, both in vivo and in vitro, by thyroid and pituitary hormones, including prolactin and growth hormone. We also showed that these peptides enhance the expression of ECM ligands and receptors, as well as the degree of TEC-thymocyte adhesion. In addition, we studied the thymic nurse cell complex, used herein as an in vitro model for ECM-mediated intrathymic T-cell migration. We observed that T-cell migration is also hormonally regulated as ascertained by the thymocyte entrance into and exit from these lymphoepithelial complexes. Taken together these data clearly illustrate the concept that neuroendocrine circuits exert a pleiotropic control on thymus physiology. Lastly, the intrathymic production of classic hormones such as prolactin and growth hormone suggests that, in addition to endocrine circuits, paracrine and autocrine interactions mediated by these peptides and their respective receptors may exist in the thymus, thus influencing both lymphoid and microenvironmental compartments of the organ.

Involvement of laminin and its receptor in abrogation of heart graft rejection by autoreactive T cells from Trypanosoma cruzi-infected mice.

Extracellular matrix ligands and receptors have been identified as determining in vivo lymphocyte positioning and activation, including effector functions in alloreactive responses. Herein we evaluated the involvement of laminin and its receptor, the very late antigen 6 (VLA-6) integrin, in CD4+ T cell-dependent autoreactivity, using a transplantation model for the autoimmune myocarditis occurring in mice chronically infected with Trypanosoma cruzi. Previous work showed that syngeneic mouse hearts grafted in the ears of chronic chagasic recipients were rejected through a CD4+ T cell-dependent mechanism. Rejection also occurred when cells from chagasic animals were transferred adjacent to hearts transplanted into naive recipients. Here, we observed the formation of a thick laminin network during rejection, with donor-derived CD4+ T cells concentrated in the laminin-rich areas. Most importantly, anti-laminin as well as anti-laminin receptor Ab inhibited the rejection of syngeneic hearts by T cells from chagasic animals. Our results suggest that interaction of the VLA-6 molecule with laminin is involved in triggering the antimyocardial autoreactive process by driving the influx of CD4+ T cells to the heart. They also support the concept that an Ag-specific T cell response, even an autoreactive one, can be modulated by in vivo interactions involving extracellular matrix ligands and receptors. In this regard, our study represents, to our knowledge, the first in vivo evidence for laminin-mediated T cell echotaxis, with simultaneous experimental demonstration of ligand and receptor involvement. Lastly, our findings indicate that treatment with anti-VLA-6 Abs can be effective in suppressing autoimmune disease activity.

Pituitary hormones modulate cell-cell interactions between thymocytes and thymic epithelial cells.

The thymic microenvironment plays a key role in the intrathymic T-cell differentiation. It is composed of a tridimensional network of epithelial cells whose physiology is controlled by extrinsic circuits such as neuroendocrine axes. Herein we show that the expression of extracellular matrix ligands and receptor by cultured thymic epithelial cells is upregulated by prolactin (PRL) and growth hormone (GH), the latter apparently occurring via insulin-like growth factor I (IGF-I). Thymocyte release from the lymphoepithelial complexes, thymic nurse cells, as well as the reconstitution of these complexes are enhanced by PRL, GH or IGF-I. Treatment of a mouse thymic epithelial cell line with these hormones induced an increase in thymocyte adhesion, an effect significantly prevented in the presence of antibodies to fibronectin, laminin or respective receptors VLA-5 and VLA-6. Our data suggest that the in vitro changes in thymocyte/thymic epithelial cell interactions induced by pituitary hormones are partially mediated by the enhancement of extracellular matrix ligands and receptors.

Quantitative immunohistochemical changes in the endocrine pancreas of nonobese diabetic (NOD) mice.

The nonobese diabetic (NOD) mouse is an animal model that shares a number of clinical, genetic, and immunologic characteristics with human insulin-dependent diabetes mellitus. Since little is known about the morphometric cell profiles in the endocrine pancreas of these NOD animals, it was of interest to assess their changes in morphometry within the pancreatic islet cell types during two stages of this syndrome. Prediabetic (6-week-old) and diabetic (16-week-old) NOD female mice, as well as normal C57BL/6 female mice (15 weeks old), were used. Light microscopic immunocytochemical and morphometric methods were employed to study the endocrine cell populations. The immunoperoxidase technique for the identification of insulin, glucagon, somatostatin, and pancreatic polypeptide, as well as the point-counting method, was used on serial sections of pancreas tissue. Compared to those of normal and prediabetic mice, pancreata from diabetic animals showed a decrease in both the number of islets and the volume density of the endocrine component. Analysis of islet tissue revealed a significant diminution of B-cell volume density, as well as an increased A-, D-, and PP-cell volume density. A parallel variation in the number of B and non-B cells was also found. In addition, when the total pancreatic tissue surface was taken as reference, the fractional area occupied by all the different types of islet cells was seen to be diminished in a variable fashion. We conclude that the diabetic syndrome of NOD mice not only severely affects the B-cell mass, but also causes marked changes in the non-B endocrine-cell populations.

Control of the thymic microenvironment by growth hormone/insulin-like growth factor-I-mediated circuits.

The thymus gland is a central lymphoid organ in which bone marrow-derived T cell precursors undergo maturation, eventually leading to the migration of positively selected thymocytes to the T-dependent areas of peripheral lymphoid organs. This process occurs under the influence of the thymic microenvironment, by means of secretory polypeptides and cell-cell contacts. The thymic microenvironment is a tridimensional cellular network composed of epithelial cells (its major component), macrophages, dendritic cells, fibroblasts and extracellular matrix elements. The epithelial reticulum is a heterogeneous tissue, in which a particular lymphoepithelial structure has been isolated in vitro: the thymic nurse cell complex, which possibly creates particular microenvironmental conditions for thymocyte differentiation. Additionally, thymic nurse cells are useful tools to study mechanisms involved in intrathymic T cell migration, including neuroendocrine influences. Previous data showed that thymic hormonal function can be modulated by hormones and neuropeptides, including growth hormone. Interestingly, GH acts pleiotropically on the thymic epithelium increasing cell growth and expression of extracellular matrix ligands and receptors, the latter resulting in an enhancement of thymocyte adhesion to the epithelial cells and thymocyte release from thymic nurse cells. The role of GH on thymus development is further stressed by the findings obtained with GH-deficient dwarf mice. Besides the precocious decline in serum thymulin found in these animals, a progressive thymic hypoplasia occurs, with decreased numbers of CD4+CD8+thymocytes, both defects being largely restored by long-term GH treatment. The effects of GH in the thymus are apparently mediated by IGF-1. Enhancement of thymulin secretion induced by GH, as well as the stimulation of thymocyte adhesion to thymic epithelial cells can be prevented in vitro by treatment with antibodies for IGF-I or IGF-I receptor. Moreover, in both systems IGF-I alone can yield similar effects. Also, the enhanced concanavalin-A mitogenic response and IL-6 production by thymocytes observed in GH-treated mice can be detected in animals treated with IGF-I. Lastly, mouse substrains selected for high or low IGF-I circulating levels exhibited differential thymus developmental patterns correlating with IGF-I levels. A further conceptual aspect concerning the GH-IGF-I-mediated control of thymus physiology is the recent demonstration of an intrathymic production of these molecules, leading to the hypothesis that, in addition to the classical endocrine pathway, GH-IGF-I-mediated paracrine and autocrine pathways may also be implicated in the control of thymus physiology. In any case, such control is exerted pleiotropically, with modulation in the expression of several genes in different cell types of the organ. In this respect, it is exciting to imagine a role of GH-IGF-I loops in shaping the intrathymically generated T cell repertoire.

Reduced ability of hypothalamic and pituitary extracts from old mice to stimulate thymulin secretion in vitro.

There is substantial evidence that growth hormone (GH) is particularly important in the control of the age-related decline of thymus function. It was therefore of interest: (a) to assess the overall capacity of tissue extracts from mediobasal hypothalamus (MBH), anterior pituitary (AP) and testis, obtained from young (3 months, Yc), middle-aged (13 months, MAc) and old (18 months, Oc) intact C57BL/6 mice to stimulate in vitro the release of thymulin, a Zn-bound immunoregulatory thymic peptide, from pure cultures of mouse thymic epithelial cells (TEC); (b) to perform the same evaluation utilizing MBH, AP and testicular extracts from mice of the same age-range but treated for 45 days with a sc dose of ovine GH (2 micrograms/g body wt) known to stimulate thymulin secretion in vivo. Pituitary hormones were measured by heterologous rat RIAs, whereas thymulin release was estimated by a rosette assay. Untreated animals showed a significant age-dependent increase in the AP content of follicle stimulating hormone but not in other AP hormones. In both control and treated animals, pituitary GH content decreased significantly with age. MBH extracts from C57BL/6 males evidenced thymulin-releasing activity on mouse TEC lines. This activity was maximal in the MBH from young animals and declined with the age of the MBH donors. The thymulin-releasing activity of MBHs from GH-treated mice was higher than that of the control animals and showed a less pronounced decline with age. AP extracts from the same animals showed a higher thymulin-releasing activity than did MBH preparations. This activity showed a progressive age-associated reduction in the APs from untreated mice, whereas in the GH-treated group, an age-related decline was only seen in the old donors. Control testicular extracts had little effect on thymulin release whereas GH treatment induced a definite thymulin-release inhibiting activity in the testicular homogenates of our animals which increased progressively with the age of the testis donors. We conclude that the MBH, AP and testis of the young mouse contain factors able to affect directly the endocrine activity of the thymic epithelium. The amount of these substances declines with age and seems to be modulated by GH.

Altered thyrotropic and somatotropic responses to environmental challenges in congenitally athymic mice.

Neonatal thymectomy or congenital absence of the thymus induces morphologic alterations in pituitary somatotrophs as well as in thyroid epithelium. It was therefore of interest to assess somatotropic and thyrotropic cell morphology and the corresponding serum hormone levels in athymic nude mice under basal and stressful conditions, taking as a reference their haired counterparts. Normal (+/+), heterozygous nude (nul+) and homozygous (nu/nu) CD-1 mice were subjected to either 1-h immobilization stress or 2-h cold stress. Serum levels of growth hormone (GH), thyrotropin (TSH), thyroxine (T4), and triiodothyronine (T3) were assessed by RIA at 0, 30, and 60 min poststress. Athymic animals showed lower basal levels of serum TSH, GH, and T3, but not T4, than their heterozygous littermates. Immunohistochemical assessment of somatotropic and thyrotropic cell populations revealed a normal morphology in the athymic animals. Immobilization stress induced a marked reduction in GH and TSH levels in normal mice but had only a weak effect in athymic animals. Two hours of cold exposure caused a comparable increase in serum TSH in normal and athymic animals, whereas the serum T4 and T3 response to cold was greater in the athymic nudes. Cold exposure drastically reduced serum GH levels in normal animals but had only a weak effect in the athymic mice. We conclude that congenital athymia in the mouse is associated with decreased basal levels of serum TSH and GH in the presence of a normal somatotroph and thyrotroph morphology. The anomalous responses of athymic mice to stress do not appear to be due to primary hypopituitarism but rather, to an altered modulation of pituitary hormone secretion.

Functional gap junctions in thymic epithelial cells are formed by connexin 43.

A multiparametric study was carried out to investigate the presence and possible role of communicating junctions in the thymus, particularly in the thymic epithelium, the major component of the thymic microenvironment. The presence of direct cell-cell communication mediated by gap junctions was demonstrated in human and murine thymic epithelial cells (TEC) by means of in situ and in vitro immunohistochemical labeling as well as in vitro fluorochrome injection and double whole-cell patch clamp experiments. Moreover, both immuno- and Northern blot studies revealed that the gap junction protein connexin 43 and its mRNA were present in TEC. Importantly, we showed that thymic endocrine activity, as ascertained by thymulin production, could be specifically down-modulated in vitro by a gap junction inhibitor, octanol. We also investigated the existence of gap junctions between TEC and thymocytes. In thymic nurse cells we were able to detect cell-cell communication, although only a minor percentage of epithelial/thymocyte pairs were coupled in a given moment. In contrast, intercellular communication was not detected between cultured phagocytic cells of the thymic reticulum and the respective rosetting thymocytes. We suggest that gap junctions formed by connexin 43 may represent a novel (and rather cell type-specific) pathway for intrathymic cellular communication, including TEC/TEC as well as possible TEC/thymocyte interactions.

Effects of growth hormone and thyroxine on thymulin secretion in aging rats.

It is well-established that the activity of the endocrine thymus is under neuroendocrine control. In particular, growth hormone (GH) and thyroxine (T4) have been shown to be capable of reconstituting thymus function in hormone-deficient animals. It was therefore of interest to assess the effect of combined administration of ovine GH (0.1 mg/100 g BW/day) and T4 (10 micrograms/100 g BW/day) on serum thymulin levels in young (5 months), old (21 months) and senescent (29-30 months) male Sprague-Dawley rats. Age-matched controls received 0.1 mg bovine serum albumin/100 g BW daily during the same period (14 days). Prolactin (Prl), GH, T4 and triiodothyronine (T3) were measured in serum by radioimmunoassay, whereas serum thymulin was determined by rosette bioassay. As expected, GH and T4 were lower in the old and senescent controls whereas serum Prl displayed a slight age-related increase. No age changes were detected in serum T3. Hormone-treated animals showed supraphysiologic levels of both T4 and T3, but serum levels were comparable among the three treated age groups for each thyroid hormone. Endogenous GH levels were moderately elevated in the treated rats. In the control rats serum thymulin showed a marked reduction from 5 to 21 months of age but no further reduction was observed between 21 and 29-30 months. Hormone treatment induced a mean relative increase (% increase relative to age-matched controls) in serum thymulin of 44, 38 and 48% in young, old and senescent rats, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)