Hypothalamic-pituitary-adrenal axis dysfunction in streptozotocin(STZ)-diabetic rats: effect of lipoic acid and ?-tocopherol treatment

Abstract

Background and aims: Glucocorticoid release is involved in glucose counterregulation, and a diminished or absent adrenocortical function renders non-diabetic as well as insulin-treated diabetic subjects prone to hypoglycemia. We previously demonstrated that STZ-diabetic rats show increased basal, but markedly reduced ACTH-stimulated corticosterone (CS) levels. Present experiments, performed in STZ-diabetic rats receiving lipoic acid (LA) or ?-tocopherol (aT) treatment, were designed 1) to evaluate the effect of these treatments on oxidative stress parameters, on nitric oxide synthase (NOS) activity, and on steroidogenic function of the adrenal cortex of STZtreated rats, and 2) to test the hypothesis that STZ-induced oxidative stress might modify CS release by affecting pituitary ACTH secretion. Materials and methods: Male Wistar rats (220-240g) were randomly assigned to the following groups: CON (control), STZ (2 injections of 40 mg/ kg STZ separated by 48 h), LA (90 mg/kg ip every 48 h), aT (200 mg/kg/d po), STZ-LA or STZ-aT. LA, aT or vehicle were initiated immediately after the confirmation of hyperglycemia. After 4 weeks, all animals were sacrificed. Oxidative stress parameters and NOS activity, as well as the expression levels of CYP11A1, a key steroidogenic enzyme, and of the ACTH receptor MC2R were evaluated by RT-qPCR in the adrenal cortex. Plasma ACTH and serum CS levels (basal and ACTH-stimulated) were measured by RIA. After normality testing a one-way ANOVA was used for statistical evaluation. Results: STZ rats displayed elevated basal CS levels, and a diminished CS response to ACTH injection. Antioxidants decreased CS levels to those observed in controls (CON: 7.3 ± 2.5*, STZ: 71.7 ± 9.2, STZ aT: 5.8 ± 3.4*, STZ-LA: 19.1 ± 5.5* ng/ml; *p<0.001 vs STZ), and restored the defective response to ACTH. Lower basal ACTH levels were detected in STZ animals; neither LA nor aT had an impact on these changes. At the adrenal level, both drugs precluded the increase in TBARS and carbonyl content, and in the expression levels of catalase and heme oxygenase-1 detected in STZ rats. Both antioxidants also prevented the increase in NOS activity detected in diabetic animals (CON: 88.5 ± 0.5*, STZ: 216.7 ± 9.2, STZ-aT: 108.6 ± 7.4* and STZLA: 117.4 ± 16.8* pmol/min/mg protein; *p<0.01 vs STZ), and corrected the down-regulation of MC2R expression observed in the STZ group. CYP11A1 mRNA levels were not affected by any of these treatments. Conclusion: Our results show that, in STZ-diabetic rats, generation of oxidative stress is associated with an increased activity of the adrenocortical NO generating system, a known negative modulator of CS release. Systemic antioxidant treatment not only normalized oxidative stress parameters and NOS activity, but also corrected the observed effects of STZ-induced diabetes on CS levels. Compared to control animals, STZ-diabetic rats showed lower baseline circulating levels of ACTH, probably resulting from exaggerated basal CS output. Antioxidant therapy restored basal and ACTH-induced glucocorticoid release without modifying plasma ACTH levels, further supporting the role of local regulatory signals, such as NO, in the dysregulation of adrenal steroidogenesis observed in this animal model of diabetes. Supported by: PICT 2008 N1034