Central urocortin 3 and type 2 corticotropin-releasing factor receptor in the regulation of energy homeostasis: critical involvement of the ventromedial hypothalamus

Abstract

The vital role of the corticotropin-releasing factor (CRF) peptide family in the brain in coordinating response to stress has been extensively documented. The effects of CRF are mediated by two G-protein-coupled receptors, type 1 and type 2 CRF receptors (CRF(1) and CRF(2)). While the functional role of CRF(1) in hormonal and behavioral adaptation to stress is well-known, the physiological significance of CRF(2) remains to be fully appreciated. Accumulating evidence has indicated that CRF(2) and its selective ligands including urocortin 3 (Ucn 3) are important molecular mediators in regulating energy balance. Ucn 3 is the latest addition of the CRF family of peptides and is highly selective for CRF(2). Recent studies have shown that central Ucn 3 is important in a number of homeostatic functions including suppression of feeding, regulation of blood glucose levels, and thermoregulation, thus reinforcing the functional role of central CRF(2) in metabolic regulation. The brain loci that mediate the central effects of Ucn 3 remain to be fully determined. Anatomical and functional evidence has suggested that the ventromedial hypothalamus (VMH), where CRF(2) is prominently expressed, appears to be instrumental in mediating the effects of Ucn 3 on energy balance, permitting Ucn 3-mediated modulation of feeding and glycemic control. Thus, the Ucn 3-VMH CRF(2) system is an important neural pathway in the regulation of energy homeostasis and potentially plays a critical role in energy adaptation in response to metabolic perturbations and stress to maintain energy balance.

Keywords: CRF; Ucn 3; VMH; energy balance; feeding; glucose.

FIGURE 1

Representative darkfield photomicrographs showing Ucn 3 mRNA expression (white clusters) in a wildtype mouse (A), an ob/ob obese mouse (B), and an ob/ob mouse treated with leptin (0.1 mg/kg/day for 2 days) (C). (D) Summary of Ucn 3 mRNA levels in the MeA of WT and Ob/ob obese mice treated with vehicle or leptin. The Ucn 3 mRNA levels in an additional group of Ob/ob mice that was paired-fed were also determined. Different letters represent statistical significance: p < 0.05. MeApd, medial nucleus of amygdala, posterodorsal part; opt, optic tract. Scale bar = 20 μm.

FIGURE 2

(A) Darkfield photomicrograph showing CRFR2 mRNA signal revealed by in situ hybridization in the basal hypothalamic area of a transgenic mouse expressing Cre recombinase (Cre) and enhanced yellow fluorescent protein (EYFP) in SF1-positive cells. Note that CRFR2 mRNA hybridization signal (white clusters) was abundant in the dorsomedial part of the VMH (VMHdm). (B) Bright field photomicrograph of the same area in (A) showing SF1-positive cells, revealed by immunostaining with anti-green fluorescent protein antibody (darkbrown precipitates) in the VMH. (C) High magnification of boxed area in (A) showing colocalization of CRFR2 (black dot clusters) and SF1 (brown precipitates) in the VMH. ARH, arcuate nucleus of hypothalamus; V3, third ventricle; VMHc, central part of the VMH; VMHdm, dorsomedial part of the VMH; VMHvl, ventrolateral part of the VMH. Scale bar = 50 μm (B), 20 μm (C).

FIGURE 3

(A) Darkfield photomicrograph showing the injection site (green circle) of an adenoviral vector encoding Cre-regulated expression of farnesylated green fluorescent protein (GFP_f) delivered into the VMH of a CRFR2-Cre mouse. The expression of GFP_f is normally silenced due to a Cre-regulated transcription block sequence inserted upstream of GFP_f reporter cassette. In the presence of Cre, the transcription block sequence is removed to allow GFP_f to be expressed in Cre-positive cells. (B) Darkfield photomicrograph depicting the distribution of VMH CRFR2-positive fibers (golden staining) within the rostral ventrolateral medulla (RVLM). ARH, arcuate nucleus of hypothalamus; MARN, magnocellular reticular nucleus; ME, median eminence; Py, pyramidal tract; Tu, tuberal nucleus; V3, third ventricle; VII, facial nucleus; VMHdn, dorsomedial part of the ventromedial nucleus of hypothalamus. Scale bar = 50 μm.