Dependence of fasting-induced hypothalamic anti-inflammatory microglia mechanisms on adrenal glucocorticoid secretion

Fasting triggers complex physiological and neuroimmune adaptations, yet its impact on hypothalamic microglia and the underlying regulatory role of glucocorticoids remains incompletely understood. The present study focused on fasting-induced systemic changes and cellular adaptations seen in the hypothalamus where components of metabolic- hormonal- and immune regulations are integrated.

Adult male microglia reporter (CX3CR1^+/Gfp) mice were subjected short term (18 h) overnight fasting. Metabolic changes were followed using indirect calorimetry. Hypothalamic expression of pro-and anti-inflammatory markers, hypothalamic neuropeptides and select genes involved in metabolic regulation was measured by qPCR. Number of microglia and their morphological characteristics was analysed by Sholl analysis. The dependence of these markers on fasting-induced corticosterone was studied in adrenalectomized (ADX) or metyrapone-treated mice. Plasma levels of corticosterone and ketone body, β-hydroxybutyrate was assayed by radioimmunassay and a colorimetric kit respectively.

Overnight fasting resulted in a decrease in energy expenditure and respiratory exchange ratio (RER) indicating conservation of energy and a metabolic shift towards utilization of fatty acids as alternative energy source. Fasting increased hypothalamic expression of orexigenic neuropeptides and mRNA levels of Pdk4, Glut1, and Mct2 genes, in line with metabolic compensation. Upregulation of hypothalamic Crh and increased plasma concentration of corticosterone indicated sustained activation of the HPA axis. Importantly, fasting promoted an anti-inflammatory milieu in the hypothalamus characterized by elevated Il-4, Il-10 and IkBα genes without significant activation of pro-inflammatory cytokines (e.g., Il-1β, Il-6, Tnfα). Morphological analysis revealed region-specific changes in microglia number and branching complexity, particularly in hypothalamic regions directly exposed to blood-borne signals. Functional profiling showed increased microglial expression of IkBα and decreased pIkBα, indicating suppressed NFkB signaling. Adrenalectomy (1 week) and acute pharmacological inhibition of corticosterone synthesis (methyrapone) revealed that fasting-induced anti-inflammatory and metabolic gene expression, as well as microglial plasticity were largely glucocorticoid dependent. Hypothalamic expression of fasting-related neuropeptides (Npy, Agrp) and genes, related to the metabolic shift (Pdk4, Glut-1, Mct2, Angptl4) as well as some immune-related genes (Il10, Iba1) was dependent on presence of the adrenal gland or fasting-induced elevation of corticosterone.

These findings highlight short term fasting as a potent modulator of hypothalamic immune-metabolic crosstalk and reveal critical role of adrenal glucocorticoids in orchestrating microglial responses to energetic challenges. The results have potential implications for therapeutic interventions targeting metabolic and inflammatory disorders.