Abstract
Glucagon-like peptide-1 (GLP-1) regulates reproduction centrally, although, the neuroanatomical basis of the process is unknown. Therefore, the putative networking of the central GLP-1 and gonadotropin-releasing hormone (GnRH) systems was addressed in male mice using whole mount immunocytochemistry and optogenetics. Enhanced antibody penetration and optical clearing procedures applied to 500–1000 µm thick basal forebrain slices allowed the simultaneous visualization of the two distinct systems in the basal forebrain. Beaded GLP-1-IR axons innervated about a quarter of GnRH neurons (23.2 ± 1.4%) forming either single or multiple contacts. GnRH dendrites received a more intense GLP-1 innervation (64.6 ± 0.03%) than perikarya (35.4 ± 0.03%). The physiological significance of the innervation was examined by optogenetic activation of channelrhodopsin-2 (ChR2)-expressing axons of preproglucagon (GCG) neurons upon the firing of GnRH neurons by patch clamp electrophysiology in acute brain slices of triple transgenic mice (Gcg-cre/ChR2/GFP-GnRH). High-frequency laser beam stimulation (20 Hz, 10 ms pulse width, 3 mW laser power) of ChR2-expressing GCG axons in the mPOA increased the firing rate of GnRH neurons (by 75 ± 17.3%, p = 0.0007). Application of the GLP-1 receptor antagonist, Exendin-3-(9-39) (1 μM), prior to the photo-stimulation, abolished the facilitatory effect. In contrast, low-frequency trains of laser pulses (0.2 Hz, 60 pulses) had no effect on the spontaneous postsynaptic currents of GnRH neurons. The findings indicate a direct wiring of GLP-1 neurons with GnRH cells which route is excitatory for the GnRH system. The pathway may relay metabolic signals to GnRH neurons and synchronize metabolism with reproduction.
Highlights
Glucagon-like peptide-1 (GLP-1) (Kreymann et al 1987), a cleavage product of the preproglucagon peptide encoded by the Gcg gene, is a potent hormone produced by intestinal L-cells (Baggio and Drucker 2007; Mojsov et al 1990) and neurons residing in the nucleus of the solitary tract (NTS) and the reticular nucleus of the medulla oblongata (Larsen et al 1997; Merchenthaler et al 1999; Vrang and Larsen 2010)
We have recently addressed the effects of GLP-1 receptor activation on the electrophysiological properties of gonadotropin-releasing hormone (GnRH) neurons in acute brain slice preparations (Farkas et al 2016)
(1) GLP-1-axons arising from the NTS innervate a subpopulation of hypophysiotropic GnRH neurons (23%) in the OVLT-medial preoptic area (mPOA) area; (2) Mapping the surface of GnRH neurons for juxtaposed GLP-1 axons in orthogonal view revealed that dendrites of GnRH neurons receive almost twice as many GLP-1-IR boutons than cell bodies; (3) Optogenetic stimulation of axons projecting from GCGChR2-expressing neurons of the NTS to the OVLT-mPOA area, the main residence of the hypophysiotropic GnRH neurons, significantly increases the firing rate of GnRH cells and (4) blocking the GLP-1 receptor (GLP-1R) by Exendin 3 prior to the optogenetic activation prevents the increment in firing of GnRH cells, indicating that GLP-1 released from the photo-stimulated axons is responsible for the facilitatory action
Summary
Glucagon-like peptide-1 (GLP-1) (Kreymann et al 1987), a cleavage product of the preproglucagon peptide encoded by the Gcg gene, is a potent hormone produced by intestinal L-cells (Baggio and Drucker 2007; Mojsov et al 1990) and neurons residing in the nucleus of the solitary tract (NTS) and the reticular nucleus of the medulla oblongata (Larsen et al 1997; Merchenthaler et al 1999; Vrang and Larsen 2010). The chemogenetic activation of preproglucagon (GCG) synthesizing neurons of the NTS has been shown to reduce food intake and body weight via their communication with downstream homeostatic regulatory centers (Gaykema et al 2017). We have recently addressed the effects of GLP-1 receptor activation on the electrophysiological properties of GnRH neurons in acute brain slice preparations (Farkas et al 2016). These studies elucidated that the potent GLP-1 receptor agonist, Exendin 4, elevated the firing rate of GnRH neurons and increased the frequency of miniature GABAergic postsynaptic currents (mPSCs) in GnRH cells. The fine structural details, the intensity and the physiological significance of networking have not been elucidated yet
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
