Abstract
Key points The gut hormone called glucagon‐like peptide 1 (GLP‐1) is a strong moderator of energy homeostasis and communication between the peripheral organs and the brain.GLP‐1 signalling occurs in the brain; using a newly developed genetic reporter line of mice, we have discovered GLP‐synthesizing cells in the olfactory bulb.GLP‐1 increases the firing frequency of neurons (mitral cells) that encode olfactory information by decreasing activity of voltage‐dependent K channels (Kv1.3).Modifying GLP‐1 levels, either therapeutically or following the ingestion of food, could alter the excitability of neurons in the olfactory bulb in a nutrition or energy state‐dependent manner to influence olfactory detection or metabolic sensing.The results of the present study uncover a new function for an olfactory bulb neuron (deep short axon cells, Cajal cells) that could be capable of modifying mitral cell activity through the release of GLP‐1. This might be of relevance for the action of GLP‐1 mimetics now widely used in the treatment of diabetes. The olfactory system is intricately linked with the endocrine system where it may serve as a detector of the internal metabolic state or energy homeostasis in addition to its classical function as a sensor of external olfactory information. The recent development of transgenic mGLU‐yellow fluorescent protein mice that express a genetic reporter under the control of the preproglucagon reporter suggested the presence of the gut hormone, glucagon‐like peptide (GLP‐1), in deep short axon cells (Cajal cells) of the olfactory bulb and its neuromodulatory effect on mitral cell (MC) first‐order neurons. A MC target for the peptide was determined using GLP‐1 receptor binding assays, immunocytochemistry for the receptor and injection of fluorescence‐labelled GLP‐1 analogue exendin‐4. Using patch clamp recording of olfactory bulb slices in the whole‐cell configuration, we report that GLP‐1 and its stable analogue exendin‐4 increase the action potential firing frequency of MCs by decreasing the interburst interval rather than modifying the action potential shape, train length or interspike interval. GLP‐1 decreases Kv1.3 channel contribution to outward currents in voltage clamp recordings as determined by pharmacological blockade of Kv1.3 or utilizing mice with Kv1.3 gene‐targeted deletion as a negative control. Because fluctuations in GLP‐1 concentrations monitored by the olfactory bulb can modify the firing frequency of MCs, olfactory coding could change depending upon nutritional or physiological state. As a regulator of neuronal activity, GLP‐1 or its analogue may comprise a new metabolic factor with a potential therapeutic target in the olfactory bulb (i.e. via intranasal delivery) for controlling an imbalance in energy homeostasis.
Highlights
Energy homeostasis is dependent upon neuronal and hormonal communication between the peripheral organs and the brain
yellow fluorescent protein (YFP) fluorescence revealed a large population of PPG-neurons located in the granule cell layer (GCL) of the olfactory bulb (OB) (Fig. 1A)
The somata were positioned in the upper portion of the GCL with axonal arbors reaching into the mitral cell layer (MCL) and external plexiform layer (EPL)
Summary
Energy homeostasis is dependent upon neuronal and hormonal communication between the peripheral organs and the brain. In the central nervous system, cell bodies of GLP-1 producing neurons have been identified in the caudal nucleus of the solitary tract and intermediate reticular nucleus (Jin et al 1988; Larsen et al 1997; Merchenthaler et al 1999; Hisadome et al 2010; Trapp & Richards, 2013). These neurons send axons to several areas in the brain, including the paraventricular nucleus, the dorsomedial hypothalamus and the nucleus accumbens (Larsen et al 1997; Tauchi et al 2008; Llewellyn-Smith et al 2011). Direct central infusion of GLP-1 has been shown to inhibit food and water intake and to reduce blood glucose (Tang-Christensen et al 1996; Kinzig et al 2002; Dossat et al 2011)
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