Induction of type 2 iodothyronine deiodinase in the mediobasal hypothalamus by bacterial lipopolysaccharide: role of corticosterone

Abstract

To determine whether endotoxin-induced activation of type 2 iodothyronine deiodinase (D2) in the mediobasal hypothalamus is dependent on circulating levels of corticosterone, the effect of bacterial lipopolysaccharide (LPS) on D2 gene expression was studied in adrenalectomized, corticosterone-clamped adult, male, Sprague Dawley rats. In sham-adrenalectomized animals, LPS (250 microg/100 g body weight) increased circulating levels of corticosterone and IL-6, as well as tanycyte D2 mRNA in the mediobasal hypothalamus. Adrenalectomized, corticosterone-clamped animals showed no significant rise in corticosterone after LPS, compared with saline-treated controls but increased IL-6 levels and tanycyte D2 mRNA similar to LPS-treated sham controls. To further clarify the potential role of corticosterone in the regulation of D2 gene expression by LPS, animals were administered high doses of corticosterone to attain levels similar to that observed in the LPS-treated group. No significant increase in D2 mRNA was observed in the mediobasal hypothalamus with the exception of a small subpopulation of cells in the lateral walls of the third ventricle. These data indicate that the LPS-induced increase in D2 mRNA in the mediobasal hypothalamus is largely independent of circulating corticosterone and indicate that mechanisms other than adrenal activation are involved in the regulation of most tanycyte D2-expressing cells by endotoxin.

Figure 1

Dark-field photomicrographs showing D2 mRNA in the mediobasal hypothalamus (MBH) after a single dose of LPS (250 μg/100 g BW). A, D2 mRNA (arrows) is present in tanycytes lining the floor and infralateral walls of the third ventricle and increases 3 (B), 6 (C), and 9 h (D) after LPS administration, with the maximal response reached between 6 and 9 h. E, Twelve hours after LPS, the D2 response is beginning to decline. F, Computerized image analysis of D2 mRNA in the MBH after saline and LPS administration. Data represent mean ± sem of three animals per group. **, P < 0.01; ***, P < 0.001. III, Third ventricle; ME, median eminence. Scale bar, 200 μm.

Figure 2

Effect of corticosterone replacement on IL-6 levels in LPS-treated animals. A, IL-6 levels rise approximately 7-fold after administration of LPS in both sham and adrenalectomized animals treated with a 30% corticosterone pellet. SH, Sham; CHOL, cholesterol; SAL, saline; ADX, adrenalectomized; CORT, corticosterone. B, In contrast, significant diminution of the response is observed in adrenalectomized animals replaced with a 45% corticosterone pellet. Data represent the mean ± sem of four to eight animals (sham groups) or five to 10 animals (adrenalectomized groups). **, P < 0.01; ***, P < 0.001.

Figure 3

Effect of LPS on circulating corticosterone levels in sham-adrenalectomized controls and adrenalectomized animals treated with a 30% corticosterone pellet. Data represent mean ± sem of animals described in Fig 2. ***, P < 0.001. CHOL, Cholesterol; SAL, saline; ADX, adrenalectomized; CORT, corticosterone.

Figure 4

Dark-field photomicrographs showing D2 mRNA in the mediobasal hypothalamus (MBH) of sham-adrenalectomized controls implanted sc with a cholesterol pellet and treated with either saline (A) or LPS (B), compared with adrenalectomized rats replaced with a 30% corticosterone pellet and similarly treated with saline (C) or LPS (D). D2 mRNA is significantly increased after LPS in the walls of the third ventricle (III) and median eminence (ME) in both sham and adrenalectomized animals. E, Computerized image analysis of D2 mRNA in the MBH after saline and LPS administration. SH, Sham; CHOL, cholesterol; SAL, saline; ADX, adrenalectomized; CORT, corticosterone. Data represent mean ± sem of animals described in Fig 2. **, P < 0.01. Scale bar, 200 μm.

Figure 5

Circulating corticosterone levels after ip administration of LPS or sc administration of high-dose corticosterone (Cort). Data represent the mean ± sem of four to eight animals (saline control and LPS treated) or six animals [vehicle (50% ethanol in saline) and high dose corticosterone]. **, P < 0.01.

Figure 6

Dark-field photomicrographs of D2 mRNA expression in the mediobasal hypothalamus (MBH) 9 h after animals received either ip injections of saline (A) and 250 μg/100 g BW LPS (B) or sc injections of vehicle (50% ethanol in saline) (C) and high-dose corticosterone (D). Note that D2 mRNA expression is significantly increased only in the MBH of the LPS-treated animals, whereas high-dose corticosterone has no significant effect. E, Computerized image analysis of D2 mRNA in the MBH of saline, LPS, vehicle, and high-dose corticosterone (Cort)-treated animals. Data represent mean ± sem of six to eight animals in each group. ***, P < 0.001. III, Third ventricle. Scale bar, 120 μm.

Figure 7

A, D2 mRNA expression in the lateral walls of the third ventricle. Groups are as shown in Fig. 6. Cort, Corticosterone. B, In situ hybridization autoradiogram of D2 mRNA showing the region (bracketed) analyzed in A. Data represent mean ± sem. *, P < 0.05; ***, P < 0.001.