Obesity is associated with lipid accumulation in non-adipose tissues, which can result in “lipotoxic” outcomes such as impaired insulin action and apoptosis. We have recently shown that “lipotoxicity” extends to the central nervous system (CNS), such that obesity induced by high-fat feeding results in lipid deposition in the hypothalamus of mice1. Lipid deposition in the brain is postulated to affect energy homeostasis and peripheral insulin action. Recent studies have challenged the dogma that the CNS does not oxidise fatty acids2. Interestingly, diets that contain medium-chain fatty acids (MCFA) enhance fatty acid (FA) oxidation, increase energy expenditure and improve insulin action when compared with diets containing long-chain fatty acids (LCFA)3. Hence, the aim of this research was to compare the metabolic fates of MCFA and LCFA in the hypothalamus.
FA metabolism was assessed using radiometric methods in hypothalamic-derived immortalised and primary murine neurons, hypothalamic sections ex vivo and in mice in vivo. Neurons are capable of transporting FA across the plasma membrane, oxidizing FA and storing FA as triglycerides. The oxidation to storage ratio was 1:5 for LCFA and 4:1 for MCFA. Whole hypothalamus isolated from lean mice exhibit a preference for LCFA oxidation rather than storage (10:1 ratio). FA administered directly into the cerebrospinal fluid via the lateral ventricle were both oxidised and stored as glycerides (1:4 and 25:1 oxidation:storage ratio for LCFA and MCFA, respectively). When LCFA were administered via the carotid artery or via oral gavage, CNS FA uptake and storage were low. Conversely, MCFA were readily able to cross from the gut to the CNS via the circulation and were oxidised and stored at a 20:1 ratio. Thus, the parenchyma of the hypothalamus (including neurons) is capable of FA transport, oxidation and storage, and MCFA may have a greater influence on CNS FA metabolism than LCFA.