Lipid signaling and fat storage in the dark-eyed junco

Seasonal hyperphagia and fattening promote survivorship in migratory and wintering birds, but reduced adiposity may be more advantageous during the breeding season. Factors such as photoperiod, temperature, and food predictability are known environmental determinants of fat storage, but the underlying neuroendocrine mechanisms are less clear. Endocannabinoids and other lipid signaling molecules regulate multiple aspects of energy balance including appetite and lipid metabolism. However, these functions have been established primarily in mammals; thus the role of lipid signals in avian fat storage remains largely undefined. Here we examined relationships between endocannabinoid signaling and individual variation in fat storage in captive white-winged juncos (Junco hyemalis aikeni) following a transition to long-day photoperiods. We report that levels of the endocannabinoid 2-arachidonoylglycerol (2-AG), but not anandamide (AEA), in furcular and abdominal fat depots correlate negatively with fat mass. Hindbrain mRNA expression of CB₁ endocannabinoid receptors also correlates negatively with levels of fat, demonstrating that fatter animals experience less central and peripheral endocannabinoid signaling when in breeding condition. Concentrations of the anorexigenic lipid, oleoylethanolamide (OEA), also inversely relate to adiposity. These findings demonstrate unique and significant relationships between adiposity and lipid signaling molecules in the brain and periphery, thereby suggesting a potential role for lipid signals in mediating adaptive levels of fat storage.