High neonatal leptin exposure enhances brain GR expression and feedback efficacy on the adrenocortical axis of developing rats.

Leptin modifies the activity of the hypothalamic-pituitary-adrenal axis in adult rodents and inhibits the production of glucocorticoids from human and rat adrenals in vitro. During development, high levels of circulating leptin and low levels of corticosterone secretion are observed together with adrenal hyporesponsiveness to stress. As chronic neonatal leptin administration reduced stress-induced corticotropin-releasing factor mRNA expression and ACTH secretion in pups, we determined whether elevated leptin levels enhanced the feedback effect of glucocorticoids on the hypothalamic-pituitary-adrenal axis. In naive pups we found a highly significant inverse relationship between plasma levels of leptin and corticosterone (P < 0.01) during postnatal d 6-20. We tested the ability of dexamethasone (1 or 10 microg/kg BW, ip, -3 h before stress) to suppress ether-induced ACTH secretion in 10-d-old pups that were treated during the neonatal period (d 2-9) with either vehicle or leptin (1 or 3 mg/kg BW, ip, daily). The expressions of brain GR and MR in vehicle- or leptin-treated neonates were determined by in situ hybridization and Western blotting. Chronic leptin treatment enhanced the ability of dexamethasone to suppress ACTH secretion after stress, and the low dose of dexamethasone was discriminant. Leptin treatment increased GR mRNA levels in the hypothalamic paraventricular nucleus (P < 0.05) and in the dentate gyrus of the hippocampus in a dose-dependent fashion. Hippocampal GR protein concentrations were increased by leptin treatment (P < 0.05). Expression of MR mRNA was not modified. Thus, the ability of leptin to enhance glucocorticoid feedback in pups is mediated in part by changes in brain GR. The high circulating leptin concentrations found in developing pups might be critical to regulate glucocorticoid production, GR levels, and stress responses. As leptin levels in pups vary with maternal diet, leptin might represent an important mediator of the maternal environment on the infant.

Novel CD8 molecule on macrophages and mast cells: expression, function and signaling.

BACKGROUND: We previously identified, using flow cytometry and in situ RT-PCR, a novel CD8 molecule on rat alveolar macrophages (AM) and mast cells (MC). RT-PCR also demonstrated that mouse AM express CD8 mRNA. Functional studies on rat AM determined that ligation of CD8 alpha- and beta-chains induced inducible nitric oxide synthase (iNOS) upregulation, nitric oxide (NO), TNF-alpha and IL-1beta (CD8alpha only) secretion. However, CD8 did not induce AM phagocytosis of IgG-opsonized or unopsonized particles. Rat MC stimulated through CD8 secreted NO and TNF-alpha, but not histamine. Because of its potential role in regulating cell function, we investigated the signaling pathways involved in macrophage CD8 stimulation. METHODS AND RESULTS: Inhibitor of src family kinases (PP1) significantly (p<0.05) inhibited CD8alpha (OX8 antibody)-induced iNOS upregulation, NO, TNF-alpha and IL-1beta production in rat AM. In addition, Ro 31-8220 (a PKC inhibitor) inhibited OX8-induced iNOS upregulation, NO and IL-1beta production, but did not inhibit TNF-alpha production. Using Syk antisense, we further determined that OX8 stimulation of NO is Syk kinase dependent. CONCLUSION: Studies on the signaling mechanisms of CD8 determined that src family kinases, PKC, and Syk kinase are involved in CD8 signaling. Additionally, CD8 may have differential signaling pathways, as an inhibitor to PKC downregulated OX8-induced IL-1beta, but not TNF-alpha release. Our studies demonstrate that AM CD8 is similar to T lymphocyte CD8 in that src kinases are involved in CD8-mediated signaling. However, p56(lck), which is expressed in T lymphocytes, has not been found in macrophages, suggesting that other src family kinases may be involved in AM and MC CD8 signaling.