Strain-specific steroidal control of pituitary function

Abstract

We have previously shown that 7B2 null mice on the 129/SvEvTac (129) genetic background die at 5 weeks of age with hypercorticosteronemia due to a Cushing's-like disease unless they are rescued by adrenalectomy; however, 7B2 nulls on the C57BL/6NTac (B6) background remain healthy, with normal steroid levels. Since background exerts such a profound influence on the phenotype of this mutation, we have evaluated whether these two different mouse strains respond differently to high circulating steroids by chronically treating wild-type 129 and B6 mice with the synthetic steroid dexamethasone (Dex). Dex treatment decreased the dopamine content of the neurointermediate lobes (NIL) of 129 mice, leading to NIL enlargement and increased total D(2)R mRNA in the 129, but not the B6, NIL. Despite the decrease in this inhibitory transmitter, Dex-treated 129 mice exhibited reduced circulating alpha-melanocyte-stimulating hormone (alpha-MSH) along with reduced POMC-derived peptides compared with controls, possibly due to reduced POMC content in the NIL. In contrast, Dex-treated B6 mice showed lowered cellular ACTH, unchanged alpha-MSH and beta-endorphin, and increased circulating alpha-MSH, most likely due to increased cleavage of NIL ACTH by increased PC2. Dex-treated 129 mice exhibited hyperinsulinemia and lowered blood glucose, whereas Dex-treated B6 mice showed slightly increased glucose levels despite their considerably increased insulin levels. Taken together, our results suggest that the endocrinological response of 129 mice to chronic Dex treatment is very different from that of B6 mice. These strain-dependent differences in steroid sensitivity must be taken into account when comparing different lines of transgenic or knockout mice.

Figure 1

Dex-treated 129 and B6 mice show growth retardation (A) and no significant difference in water volumes drunk (B) (P<0·6584; n=5). The 129 and B6 mice were given Dex in the drinking water for a week before killing. Ctr, control.

Figure 2

PC2 protein levels are increased when compared with their controls in Dex-treated B6 NIL, but not in Dex-treated 129 NIL (A). The NILs of both Dex-treated 129 and B6 mice contain reduced POMC levels (B). NIL lysates were analyzed by SDS-PAGE followed by western blotting using PC2 antiserum.

Figure 3

NIL dopamine levels are decreased in Dex-treated 129 mice. (A) The area volume (*significant differences between 129 and B6 NILs, P<0·0001; ^†significant differences between Ctr and Dex-treated 129 NILs, P<0·0298). (B) NIL D₂R densities (grains per area) of Dex-treated 129 animals were decreased over their WT controls (^‡P<0·0001; n=10); however, total D₂R mRNA levels in Dex-treated 129 NIL were slightly increased. (C) D₂R density (*significant differences between Ctr and Dex-treated 129 NILs, P=0·0556). (D) In situ hybridization of D₂R mRNA.

Figure 4

Dex-treated 129 melanotrophs show a prominent endoplasmic reticulum and Golgi complex (upper panel), while Dex-treated B6 melanotrophs do not differ from untreated controls at the ultrastructural level (lower panel). These data confirm the downregulation of dopaminergic control known to be associated with secretory membrane proliferation in 129 mice, but not in B6 mice. Inset, Image enlarged and enhanced by Corel Photo-Paint software.

Figure 5

Dex-treated 129 animals exhibit lowered blood sugar levels and are hyperinsulinemic and hyperglucagonemic compared with their WT controls (Ctr). (A) Glucose levels (*significant differences between Ctr and Dex-treated 129 mice, P<0·0001). (B) Insulin levels (^†significant differences between Ctr and Dex-treated 129 mice, P<0·0001; ^‡significant differences between Ctr and Dex-treated B6 mice, P< 0·0001). (C) Glucagon levels (^§significant differences between Ctr and Dex-treated 129 mice, P=0·0071; ^||significant differences between Ctr and Dex-treated B6 mice, P<0·0001; n≥10).