Avaliação neuroimunoendócrina de ratos hipotireoidianos submetidos ao estresse de imobilização e a endotoxemia: participação da L-arginina e da sintase do óxido nítrico.

by Rodriguez, Tania Tavares

The identification of nitric oxide (NO) within the hypothalamus and pituitary gland has suggested its role as a modulator of the activity on hypothalamic-pituitaryaxis. Hypothalamic nitric oxide synthase (NOS) is known to be regulated by thyroid hormones. This work was undertaken to investigate the effects of previous injectionof Nv-nitro-L-arginine methyl ester (L-NAME), a NOS inhibitor, and L-Arginine (LArg), the substrate for NO synthesis, on prolactin (PRL) and corticosterone (CORT)secretion, c-Fos hypothalamic expression and on blood leukocyte subsets redistribuition (lymphocytes, neutrophils, Total T, T-helper and cytotoxic cells)through the immobilization (IMO) stress-induced and lipopolysaccharide (LPS)- induced inflammatory responses in thyroidectomized rats. Male Wistar rats (220- 280g) thyroidectomied (TX) or sham operated (N), (n = 10/15 per group), were intraperitoneally (i.p.) injected with L-NAME (10 mg/Kg) or L-Arg (200 mg/Kg) or the same volume of vehicle (saline solution), 30 min before stress-induction or LPS injection i.p., 250 mg/100 g of body weight. Blood samples for hormone assays were collected before (baseline time -30min) and after treatment with L-NAME, L-Arg orsaline (SL) (0 min) and after 5, 15, 30, 60, 120 and 240 of endotoxemia. Blood samples for leukocyte analysis were collected before and after treatment with drugs (0 min) and 240 min after stress or endotoxemia. Brains were removed for c-Fos immunohistochemistry before and after 30 min of L-NAME, L-Arg or SL injection plus 120 min of endotoxemia or IMO stress. Hormones were measured by radioimmuneassays and analysis of leukocyte subtypes were obtained by two-color flow citometry. Our results show that the thyroid hormones are necessary for a proper response of PRL and CORT releasing during IMO and endotoxemia. In N rats, NO20 increases PRL release in response to endotoxemia whereas, in hypothyroid rats, NO appears to have an opposite effect. Although NO is not a limiting factor in stressinduced PRL secretion in N and TX rats, L-Arg overload modulates PRL secretion by increasing it temporarily in both groups. NO stimulates CORT secretion during endotoxemia and IMO stress, but high concentrations of NO reduce CORT release directly by adrenal gland. NOS activity is down regulated in hypothyroid rats, sohypothyroidism modulates negatively the hypothalamus-pituitary-adrenal axis. Hypothyroidism reduces inflammatory response induced by LPS and NO acts as an antiadhesive molecule. NO modulates leukocyte trafficking which results in a redistribuition of leukocytes between the blood and other immune compartments. The modulation of immune cell distribuition by acute stress and endotoxemia may be anadaptative response designed to enhance immune vigilance and increase the capacity of the immune system to respond to challenge. Thus, the thyroidectomized animals are immune deficient because of its significant blockade in PRL and CORT release during stress and endotoxemia and the pattern of redistribuition of leukocytes between different immune compartments.Endotoxemia and stress increase Fos-like immunoreactive positive neurons in hypothalamic nuclei, PVN and SON, of normal rats. Hypothyroidism inhibits partiallyFos-like expression in the PVN of endotoxemic rats, and in the SON during IMO and endotoxemia, except in the group of endotoxemic TX rats treated with L-Arg.