UNLABELLED: Previous reports indicate that caloric restriction attenuates anxiety and other behavioral responses to acute stress, and blunts the ability of stress to increase anterior pituitary release of adrenocorticotropic hormone. Since hindbrain glucagon-like peptide-1 (GLP-1) neurons and noradrenergic prolactin-releasing peptide (PrRP) neurons participate in behavioral and endocrine stress responses, and are sensitive to the metabolic state, we examined whether overnight food deprivation blunts stress-induced recruitment of these neurons and their downstream hypothalamic and limbic forebrain targets. A single overnight fast reduced anxiety-like behavior assessed in the elevated-plus maze and acoustic startle test, including marked attenuation of light-enhanced startle. Acute stress [i.e., 30 min restraint (RES) or 5 min elevated platform exposure] robustly activated c-Fos in GLP-1 and PrRP neurons in fed rats, but not in fasted rats. Fasting also significantly blunted the ability of acute stress to activate c-Fos expression within the anterior ventrolateral bed nucleus of the stria terminalis (vlBST). Acute RES stress suppressed dark-onset food intake in rats that were fed ad libitum, whereas central infusion of a GLP-1 receptor antagonist blocked RES-induced hypophagia, and reduced the ability of RES to activate PrRP and anterior vlBST neurons in ad libitum-fed rats. Thus, an overnight fast "silences" GLP-1 and PrRP neurons, and reduces both anxiety-like and hypophagic responses to acute stress. The partial mimicking of these fasting-induced effects in ad libitum-fed rats after GLP-1 receptor antagonism suggests a potential mechanism by which short-term negative energy balance attenuates neuroendocrine and behavioral responses to acute stress.
SIGNIFICANCE STATEMENT: The results from this study reveal a potential central mechanism for the "metabolic tuning" of stress responsiveness. A single overnight fast, which markedly reduces anxiety-like behavior in rats, reduces or blocks the ability of acute stress to activate hindbrain neurons that are immunoreactive for either prolactin-releasing peptide or glucagon-like peptide 1, and attenuates the activation of their stress-sensitive projection targets in the limbic forebrain. In nonfasted rats, central antagonism of glucagon-like peptide 1 receptors partially mimics the effect of an overnight fast by blocking the ability of acute stress to inhibit food intake, and by attenuating stress-induced activation of hindbrain and limbic forebrain neurons. We propose that caloric restriction attenuates behavioral and physiological responses to acute stress by "silencing" central glucagon-like peptide 1 signaling pathways.
