GH-Releasing Hormone Neurons Regulate the Hypothalamic-Pituitary-Somatotropic Axis via Short-Loop Negative Feedback

Area of Science:

• Neuroendocrinology
• Molecular Endocrinology
• Physiology

Background:

• Growth hormone (GH) is regulated by GH-releasing hormone (GHRH) neurons.
• The role of GH short-loop negative feedback on GHRH neurons is not fully understood.
• GH receptor (GHR) and insulin-like growth factor-1 receptor (IGF1R) signaling pathways are crucial in endocrine regulation.

Purpose of the Study:

• To investigate the role of GHR in GHRH neurons in regulating GH secretion and body growth.
• To determine the relative importance of GHR and IGF1R signaling in GHRH neurons for feedback control.

Main Methods:

• Generation of GHRH-specific GHR knockout (GHRHΔGHR) mice.
• Analysis of GH secretion, body composition, and growth parameters.
• Electrophysiological recordings and generation of double knockout mice (GHRHΔGHR/IGF1R and GHRHΔIGF1R).

Main Results:

• GHR ablation in GHRH neurons increased body weight, lean mass, and naso-anal length, associated with higher GH secretion.
• IGF1R signaling in GHRH neurons played a more dominant role than GHR signaling in regulating body growth and IGF-1 levels.
• GH and IGF-1 did not acutely alter GHRH neuron activity, suggesting indirect feedback mechanisms.

Conclusions:

• GH exerts negative feedback on the hypothalamic-pituitary-somatotropic axis via GHRH neurons.
• IGF1R signaling is the predominant pathway sensed by GHRH neurons for regulating GH secretion.
• This study clarifies the complex interplay of GH, GHRH, GHR, and IGF1R in growth regulation.