A mechanism underlying the sexually dimorphic ACTH response to lipopolysaccharide in rats: sex steroid modulation of cytokine binding sites in the hypothalamus

Abstract

It is well established that the hypothalamic-pituitary-adrenal responses to immune stressors are sexually dimorphic in rodents (females > males), but the underlying mechanism is still unclear. To investigate the mechanism, in this study we examined whether the sex steroid environment affects the following variables in male and female rats: (1) plasma levels of ACTH, interleukin (IL)-1beta, IL-6 and tumour necrosis factor-alpha (TNF-alpha) after systemic lipopolysaccharide (LPS) administration; (2) static concentrations of corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) in the mediobasal hypothalamus (MBH) and those of ACTH in the anterior pituitary (AP); and (3) the binding characteristics of IL-1beta, IL-6 and TNF-alpha in the MBH and AP. LPS-induced ACTH release was significantly higher in female than in male rats, and this sexual difference was abolished by performing gonadectomy in both sexes. Administration of physiological doses of testosterone and oestradiol to gonadectomized males and females, respectively, restored the altered ACTH responses to normal. Changes in the sex steroid milieu did not affect plasma cytokine responses to LPS, tissue contents of CRH, AVP and ACTH, or the IL-6 binding characteristics in the MBH and AP. However, the number of IL-1beta and TNF-alpha binding sites, but not their binding affinities, in the MBH showed significant changes according to altered sex hormone milieu, in the same direction as the LPS-induced ACTH response. These results suggest that the hypothalamic sensitivity to peripheral IL-1beta and TNF-alpha may be an important mechanism underlying the sexually dimorphic ACTH response to LPS in rats.

Figure 1. Effects of ODX and T replacement on plasma ACTH and cytokine responses to an i.v. bolus administration of LPS (100 µg (kg body wt)⁻¹) in male rats

○, sham ODX; Δ, ODX; □, ODX + T. For abbreviations, see Table 1. *Statistically significant vs. the other two groups.

Figure 3. Effects of gonadectomy and sex steroid replacement on the integrated ACTH secretion (area under the curve, AUC) induced by LPS (100 µg (kg body wt)⁻¹, i.v. bolus) in male and female rats

For abbreviations, see Table 1. *Statistically significant vs. the other male groups. †Statistically significant vs. the OVX, OVX + P and the three male groups. ‡Statistically significant vs. the sham ODX and ODX + T groups.

Figure 2. Effects of OVX and E₂ and/or P replacement on plasma ACTH and cytokine responses to an i.v. bolus administration of LPS (100 µg (kg body wt)⁻¹) in female rats

○, sham OVX; Δ, OVX; •, OVX + E₂; ▴, OVX + P; ▪, OVX + E₂ + P. For abbreviations, see Table 1. *Statistically significant vs. the sham OVX, OVX + E₂ and OVX + E₂ + P groups.

Figure 4. Specific binding of ¹²⁵I-IL-1β to membrane preparations of the MBH and AP from sham ODX rats

A, saturation curve for specific ¹²⁵I-IL-1β binding as a function of ligand concentration. B, Scatchard plot of the binding data. In this figure and in Figs 5 and 6, each value is the mean of triplicate determinations from a typical experiment that was replicated six times with similar results. The same binding studies were also performed for the remaining seven experimental groups. K_d, equilibrium dissociation constant; B_max, number of binding sites. For abbreviations, see Tables 1 and 2.

Figure 5. Specific binding of ¹²⁵I-IL-6 to membrane preparations of the MBH and AP from sham ODX rats

A, saturation curve for specific ¹²⁵I-IL-6 binding as a function of ligand concentration. B, Scatchard plot of the binding data. For abbreviations, see Tables 1 and 2 and Fig. 4.

Figure 6. Specific binding of ¹²⁵I-TNF-α to membrane preparations of the MBH and AP from sham ODX rats

A, saturation curve for specific ¹²⁵I-TNF-α binding as a function of ligand concentration. B, Scatchard plot of the binding data. For abbreviations, see Tables 1 and 2 and Fig. 4.