Proglucagon signalling in the rat Dorsomedial Hypothalamus – Physiology and high-fat diet-mediated alterations

A relatively new pharmacological target in obesity treatment has been the preproglucagon (PPG) signalling, predominantly with glucagon-like peptide (GLP) 1 receptor agonists. As far as the PPG role within the digestive system is well recognised, its actions in the brain remain understudied. Here, we investigated PPG signalling in the Dorsomedial Hypothalamus (DMH), a structure involved in feeding regulation and metabolism, using in situ hybridisation, electrophysiology, and immunohistochemistry. Our experiments were performed on animals fed both control, and high-fat diet (HFD), uncovering HFD-mediated alterations. First, sensitivity to exendin-4 (Exn4, a GLP1R agonist) was shown to increase under HFD, with a higher number of responsive neurons. The amplitude of the response to both Exn4 and oxyntomodulin (Oxm) was also altered, diminishing its relationship with the cells' spontaneous firing rate. Not only neuronal sensitivity, but also GLP1 presence, and therefore possibly release, was influenced by HFD. Immunofluorescent labelling of the GLP1 showed changes in its density depending on the metabolic state (fasted/fed), but this effect was eliminated by HFD feeding. Interestingly, these dietary differences were absent after a period of restricted feeding, allowing for an anticipation of the alternating metabolic states, which suggests possible prevention of such outcome. Sprague-Dawley rats were fed either control (CD) or high-fat diet (HFD) for 2–4 weeks, after which preproglucagon (PPG) signalling was investigated in the Dorsomedial Hypothalamus. In situ hybridisation was used to detect glucagon-like peptide (GLP) 1 and 2 receptor (Glp1r/Glp2r) mRNA, shown to localise within different parts of the structure, with the same level and spatial distribution for both dietary groups. Electrophysiological recordings revealed changes in the sensitivity of the DMH neurons to exendin-4 (Exn4, a GLP1R agonist), with a higher number of responsive cells in the HFD-fed group. Moreover, the amplitude of the excitatory response to both Exn4 and oxyntomodulin (Oxm) correlated positively with the neurons' spontaneous firing rate, but only for the CD. The PPG-derived peptides were also shown to influence the synaptic network in the DMH. GLP2 caused opposite effects depending on the time of day for both diets, whereas daytime response to Exn4 was altered by HFD. Last, immunofluorescent labelling of GLP1 indicated variation in its density depending on the animals' metabolic s