Role of glucocorticoids in tuning hindbrain stress integration

Abstract

The nucleus of the solitary tract (NTS) is a critical integrative site for coordination of autonomic and endocrine stress responses. Stress-excitatory signals from the NTS are communicated by both catecholaminergic [norepinephrine (NE), epinephrine (E)] and noncatecholaminergic [e.g., glucagon-like peptide-1 (GLP-1)] neurons. Recent studies suggest that outputs of the NE/E and GLP-1 neurons of the NTS are selectively engaged during acute stress. This study was designed to test mechanisms of chronic stress integration in the paraventricular nucleus, focusing on the role of glucocorticoids. Our data indicate that chronic variable stress (CVS) causes downregulation of preproglucagon (GLP-1 precursor) mRNA in the NTS and reduction of GLP-1 innervation to the paraventricular nucleus of the hypothalamus. Glucocorticoids were necessary for preproglucagon (PPG) reduction in CVS animals and were sufficient to lower PPG mRNA in otherwise unstressed animals. The data are consistent with a glucocorticoid-mediated withdrawal of GLP-1 in key stress circuits. In contrast, expression of tyrosine hydroxylase mRNA, the rate-limiting enzyme in catecholamine synthesis, was increased by stress in a glucocorticoid-independent manner. These suggest differential roles of ascending catecholamine and GLP-1 systems in chronic stress, with withdrawal of GLP-1 involved in stress adaptation and enhanced NE/E capacity responsible for facilitation of responses to novel stress experiences.

Figure 1.

CVS exposure affected the both PPG and TH mRNA expression in the NTS. a–d, Representative images for PPG mRNA and TH mRNA expression in the NTS. Left, Control; right, CVS. e, f, Semiquantitative results showed that CVS exposure declined PPG mRNA expression, whereas it enhanced TH mRNA expression in NTS. *p < 0.01 versus control.

Figure 2.

QRT-PCR analysis of the regulation of NTS PPG and TH transcription by CVS exposure. a, b, NTS PPG mRNA expression decreased while PPG hnRNA expression increased by 2 week CVS exposure compared with control. c, d, Prolonged stress exposure elevated TH mRNA and hnRNA expression in NTS compared with the naive control. *p < 0.01 versus control.

Figure 3.

Quantification of the expression of GLP-1 fiber in the mpPVN after 2 weeks of CVS exposure. The field area occupied by GLP-1-immunoreactive positive fibers was determined and expressed as a percentage of total measured area (FldAreaP). Data are shown as percentage of control. a, b, Representative images for low-magnification GLP-1 fiber staining in the PVN. mpPVN, Medial parvocellular PVN; 3v, third ventricle. c, d, Representative images for high magnification of projection images. Left, Control; right, CVS. e, Quantification of the expression of GLP-1 fiber in the mpPVN after CVS exposure. The percentage of area occupied by GLP-1-positive fibers declined after 2 weeks of CVS exposure compared with the control. Scale bars: a, b, 100 μm; c, d, 20 μm. *p < 0.05 versus control.

Figure 4.

Quantification of the densitometrics of GLP-1- and TH-immunoreactive positive neurons in the NTS after 2 weeks of CVS exposure. a–d, f–i, Representative images of GLP-1- and TH-immunoreactive positive staining at the level of bregma (−14.40); low-magnification images are shown at the top, and high-magnification images are shown at the bottom. Left, Control; right, CVS. e, Decrement of the densitometrics of NTS GLP-1-immunoreactive positive neurons in the CVS-treated rats compared with the control group. j, Increment of densitometrics of NTS TH-immunoreactive positive neurons in the CVS-treated rats compared with the control animals. Data are shown as the percentage of control. cc, Central canal. Scale bars: a, b, f, g, 50 μm; c, d, h, i, 20 μm. *p < 0.05 versus control.

Figure 5.

QRT-PCR analysis of the regulation of NTS PPG and TH transcription by CVS exposure in ADX-Cort-replaced rats. a, Diurnal rhythms of plasma corticosterone was determined. ADX-Cort rats showed normal plasma corticosterone circadian as indicated by no difference for nadir and peak of plasma corticosterone between sham-ADX and ADX-Cort rats. b, The basal plasma corticosterone was elevated by CVS exposure only in sham-ADX CVS rats. The prolonged stress challenge did not affect the basal plasma corticosterone in ADX-Cort rats. c, d, CVS exposure enhanced PPG mRNA degradation and PPG hnRNA expression in sham-ADX rats. There was no difference for PPG mRNA and hnRNA expression between unstressed and stressed ADX-Cort rats. e, f, Expression of TH mRNA and hnRNA were elevated by the 2 weeks CVS of exposure in both sham-ADX and ADX-Cort rats. *p < 0.01 versus control (Con).

Figure 6.

QRT-PCR analysis of the regulation of NTS PPG and TH transcription by chronic subcutaneous administration of corticosterone (3.5 mg/kg). a, b, Chronic injection of corticosterone decreased PPG mRNA and hnRNA expression compared with the vehicle treatment. c, d, Chronic corticosterone injection did not affect the NTS TH gene expression. e, The plasma corticosterone level was increased by acute subcutaneous injection of corticosterone at 10 mim, reached the peak at 30 min, and maintained elevated levels at 60 and 120 min compared with the vehicle injection. *p < 0.05 versus control.