Objective: Obesity is often accompanied by hyperactivity of the neuroendocrine stress axis and has been linked to an increased risk of psychiatric disorders. Insulin is reciprocally regulated with the stress hormone corticosterone (CORT), raising the possibility that insulin normally provides inhibitory tone to the hypothalamus-adrenal-pituitary (HPA) axis. Here we examined whether disrupting signaling via the insulin receptor (InsR) in hypothalamic subpopulations impacts the neuroendocrine response to acute psychological stress.
Methods: We used Nkx2.1-Cre, Sim1-Cre and Agrp-Cre transgenic driver lines to generate conditional knockouts of InsR signaling throughout the hypothalamus, paraventricular nucleus of the hypothalamus (PVH) and in neurons expressing Agouti-related peptide (AgRP) in the arcuate nucleus of the hypothalamus (ARH), respectively. We used a combination of molecular, behavioral and neuroendocrine criteria to evaluate the consequences on HPA axis responsiveness.
Results: Endpoints related to body weight and glucose homeostasis were not altered in any of the conditional mutant lines. Consistent with observations in the neuronal Insr knockout mice (NIRKO), baseline levels of serum CORT were similar to controls in all three lines. In male mice with broad disruptions of InsR signals in Nkx2.1-expressing regions of the hypothalamus (IR(Nkx2.1) KO), we observed elevated arginine vasopressin (AVP) levels at baseline and heightened neuroendocrine responses to restraint stress. IR(Nkx2.1) KO males also exhibited increased anxiety-like behaviors in open field, marble burying, and stress-induced hyperthermia testing paradigms. HPA axis responsivity was not altered in IR(Sim1) KO males, in which InsR was disrupted in the PVH. In contrast to observations in the IR(Nkx2.1) KO males, disrupting InsR signals in ARH neurons expressing Agrp (IR(Agrp) KO) led to reduced AVP release in the median eminence (ME).
Conclusions: We find that central InsR signals modulate HPA responsivity to restraint stress. InsR signaling in AgRP/NPY neurons appears to promote AVP release, while signaling in other hypothalamic neuron(s) likely acts in an opposing fashion. Alterations in InsR signals in neurons that integrate metabolic and psychiatric information could contribute to the high co-morbidity of obesity and mental disorders.
Keywords: ACTH, adrenocorticotropic hormone; ARH, arcuate nucleus of the hypothalamus; AVP, arginine vasopressin; AgRP; AgRP, agouti-related peptide; CORT, corticosterone; CRH, corticotropin-releasing hormone; FST, forced swim test; Gr, Glucocorticoid receptor; HPA axis; HPA axis, Hypothalamus–Pituitary–Adrenal axis; Hypothalamus; IRAgrp KO, knockout of InsR using Agrp-Cre; IRNkx2.1 KO, knockout of InsR using Nkx2.1-Cre; IRSim1 KO, knockout of InsR using Sim1-Cre; InsR, insulin receptor; Insulin; MB, marble burying test; MBH, mediobasal hypothalamus; ME, median eminence; NPY, neuropeptide Y; NSF, novelty suppressed feeding test; OF, open field test; POMC, pro-opiomelanocortin; SIH, stress-induced hyperthermia test; Stress response.