Distribution and characterisation of Glucagon-like peptide-1 receptor expressing cells in the mouse brain

Abstract

ObjectiveAlthough Glucagon-like peptide 1 is a key regulator of energy metabolism and food intake, the precise location of GLP-1 receptors and the physiological relevance of certain populations is debatable. This study investigated the novel GLP-1R-Cre mouse as a functional tool to address this question.MethodsMice expressing Cre-recombinase under the Glp1r promoter were crossed with either a ROSA26 eYFP or tdRFP reporter strain to identify GLP-1R expressing cells. Patch-clamp recordings were performed on tdRFP-positive neurons in acute coronal brain slices from adult mice and selective targeting of GLP-1R cells in vivo was achieved using viral gene delivery.ResultsLarge numbers of eYFP or tdRFP immunoreactive cells were found in the circumventricular organs, amygdala, hypothalamic nuclei and the ventrolateral medulla. Smaller numbers were observed in the nucleus of the solitary tract and the thalamic paraventricular nucleus. However, tdRFP positive neurons were also found in areas without preproglucagon-neuronal projections like hippocampus and cortex. GLP-1R cells were not immunoreactive for GFAP or parvalbumin although some were catecholaminergic. GLP-1R expression was confirmed in whole-cell recordings from BNST, hippocampus and PVN, where 100 nM GLP-1 elicited a reversible inward current or depolarisation. Additionally, a unilateral stereotaxic injection of a cre-dependent AAV into the PVN demonstrated that tdRFP-positive cells express cre-recombinase facilitating virally-mediated eYFP expression.ConclusionsThis study is a comprehensive description and phenotypic analysis of GLP-1R expression in the mouse CNS. We demonstrate the power of combining the GLP-1R-CRE mouse with a virus to generate a selective molecular handle enabling future in vivo investigation as to their physiological importance.

PPG neurons of the lower brain stem and their role in brain GLP-1 receptor activation

“…1) Circulating GLP-1 might have unrestricted access across the blood-brain barrier (BBB) and act directly on brain GLP-1 receptors (GLP-1R). 2) Circulating GLP-1 might only access areas of the brain that express an incomplete BBB, and, thus, GLP-1R expressed in the area postrema (AP) and the subfornical organ (11,65) might act as relays to generate an electrical signal in the brain linked to peripheral GLP-1. 3) Peripheral GLP-1 might bind to GLP-1R on vagal afferent neurons, for example, at the portal vein, and generate an electrical signal to the brain.…”

“…These catecholaminergic AP cells have been shown to express GLP-1 receptors (11,105), and this finding would suggest that the majority of AP GLP-1 receptors are, indeed, geared toward sampling blood GLP-1 (72), or ventricular GLP-1 (39), rather than receiving synaptic inputs from PPG neurons.…”

“…The distribution of GLP-1 receptors in the brain has been analyzed in detail by in situ hybridization in the rat (65) and using a genetic tag in mouse (11). Recently, two mouse lines have been developed that express Cre-recombinase in GLP-1R-expressing cells, or a FLAG-tagged humanized GLP-1 receptor, respectively (42,79).…”

“…Recently, two mouse lines have been developed that express Cre-recombinase in GLP-1R-expressing cells, or a FLAG-tagged humanized GLP-1 receptor, respectively (42,79). The distribution of PPG projections in mouse (56 -58) shows a high degree of overlap with the expression of GLP-1R in both mouse and rat (11,65). However, notable differences also exist.…”

“…However, notable differences also exist. For example, GLP-1R is highly expressed in the CA3 region of the caudal hippocampus (11,65), a region devoid of PPG innervation and GLP-1 immunoreactivity (41,58). This raises two possible scenarios: 1) GLP-1R expressed in these regions act in a presynaptic manner, i.e., they are located on terminals, which are potentially in distal sites where PPG axons occur; or 2) GLP-1R in these regions receive GLP-1 from nonneuronal sources.…”