ABSTRACT
Growth hormone (GH) secretion decreases spontaneously during lifespan, and the resulting GH deficiency participates in aging-related morbidity. This deficiency appears to involve a defect in the activity of hypothalamic GH-releasing hormone (GHRH) neurons. Here, we investigated this hypothesis, as well as the underlying mechanisms, in identified GHRH neurons from adult ( approximately 13 weeks old) and aged ( approximately 100 weeks old) transgenic GHRH-green fluorescent protein mice, using morphological, biochemical and electrophysiological methods. Surprisingly, the spontaneous action potential frequency was similar in adult and aged GHRH neurons studied in brain slices. This was explained by a lack of change in the intrinsic excitability, and simultaneous increases in both stimulatory glutamatergic- and inhibitory GABAergic-synaptic currents of aged GHRH neurons. Aging did not decrease GHRH and enhanced green fluorescent protein contents, GHRH neuronal number or GHRH-fibre distribution, but we found a striking enlargement of GHRH-positive axons, suggesting neuropeptide accumulation. Unlike in adults, autophagic vacuoles were evident in aged GHRH-axonal profiles using electron microscopy. Thus, GHRH neurons are involved in aging of the GH axis. Aging had a subtle effect at the nerve terminal level in GHRH neurons, contrasting with the view that neuronal aging is accompanied by more widespread damage.
Subject(s)
Cellular Senescence/physiology , Growth Hormone-Releasing Hormone/metabolism , Neurons/physiology , Presynaptic Terminals/ultrastructure , Action Potentials , Afferent Pathways/physiology , Animals , Excitatory Postsynaptic Potentials , Green Fluorescent Proteins/analysis , Green Fluorescent Proteins/genetics , Growth Hormone/physiology , Growth Hormone-Releasing Hormone/genetics , Male , Mice , Mice, Transgenic , Neurons/metabolism , Patch-Clamp Techniques , Presynaptic Terminals/metabolismABSTRACT
Pituitary growth hormone (GH)-secreting cells regulate growth and metabolism in animals and humans. To secrete highly ordered GH pulses (up to 1,000-fold rise in hormone levels in vivo), the pituitary GH cell population needs to mount coordinated responses to GH secretagogues, yet GH cells display an apparently heterogeneous scattered distribution in 2D histological studies. To address this paradox, we analyzed in 3D both positioning and signaling of GH cells using reconstructive, two-photon excitation microscopy to image the entire pituitary in GH-EGFP transgenic mice. Our results unveiled a homologous continuum of GH cells connected by adherens junctions that wired the whole gland and exhibited the three primary features of biological networks: robustness of architecture across lifespan, modularity correlated with pituitary GH contents and body growth, and connectivity with spatially stereotyped motifs of cell synchronization coordinating cell activity. These findings change our view of GH cells, from a collection of dispersed cells to a geometrically connected homotypic network of cells whose local morphology and connectivity can vary, to alter the timing of cellular responses to promote more coordinated pulsatile secretion. This large-scale 3D view of cell functioning provides a powerful approach to identify and understand other networks of endocrine cells that are thought to be scattered in situ. Many dispersed endocrine systems exhibit pulsatile outputs. We suggest that cell positioning and associated cell-cell connection mechanisms will be critical parameters that determine how well such systems can deliver a coordinated secretory pulse of hormone to their target tissues.
