Expression of GLP-1 receptors in insulin-containing interneurons of rat cerebral cortex
Glucagon-like peptide 1 (GLP-1) receptors are expressed by pancreatic beta cells and GLP-1 receptor signalling promotes insulin secretion. GLP-1 receptor agonists have neural effects and are therapeutically promising for mild cognitive impairment and Alzheimer’s disease. Our previous results showed that insulin is released by neurogliaform neurons in the cerebral cortex, but the expression of GLP-1 receptors on insulin-producing neocortical neurons has not been tested. In this study, we aimed to determine whether GLP-1 receptors are present in insulin-containing neurons.
We harvested the cytoplasm of electrophysiologically and anatomically identified neurogliaform interneurons during patch-clamp recordings performed in slices of rat neocortex. Using single-cell digital PCR, we determined copy numbers of Glp1r mRNA and other key genes in neurogliaform cells harvested in conditions corresponding to hypoglycaemia (0.5 mmol/l glucose) and hyperglycaemia (10 mmol/l glucose). In addition, we performed whole-cell patch-clamp recordings on neurogliaform cells to test the effects of GLP-1 receptor agonists for functional validation of single-cell digital PCR results.
Single-cell digital PCR revealed GLP-1 receptor expression in neurogliaform cells and showed that copy numbers of mRNA of the Glp1r gene in hyperglycaemia exceeded those in hypoglycaemia by 9.6 times (p < 0.008). Moreover, single-cell digital PCR confirmed co-expression of Glp1r and Ins2 mRNA in neurogliaform cells. Functional expression of GLP-1 receptors was confirmed with whole-cell patch-clamp electrophysiology, showing a reversible effect of GLP-1 on neurogliaform cells. This effect was prevented by pre-treatment with the GLP-1 receptor-specific antagonist exendin-3(9-39) and was absent in hypoglycaemia. In addition, single-cell digital PCR of neurogliaform cells revealed that the expression of transcription factors (Pdx1, Isl1, Mafb) are important in beta cell development.
Our results provide evidence for the functional expression of GLP-1 receptors in neurons known to release insulin in the cerebral cortex. Hyperglycaemia increases the expression of GLP-1 receptors in neurogliaform cells, suggesting that endogenous incretins and therapeutic GLP-1 receptor agonists might have effects on these neurons, similar to those in pancreatic beta cells.
KeywordsAnimal Basic science Gastro-entero pancreatic factors Hormone receptors Other techniques Rat
Gamma-aminobutyric acid type A
Gamma-aminobutyric acid type B
Glucagon-like peptide 1
Insulin gene enhancer binding protein, islet factor 1
The authors thank É. Tóth, M. V. Várady and N. Tóth for technical assistance (MTA-SZTE Research Group for Cortical Microcircuits, University of Szeged, Szeged, Hungary). Some of these data were presented as an abstract at the ADA 77th Scientific Sessions in 2017.
EAC and GT formulated the key hypothesis, designed the experiments, contributed to the data analysis, interpretation and production of figures, and wrote the paper with final approval of the version to be published. ÁKK, NF, EAC, ÁZ, IL, AP and LGP performed single-cell digital PCR and molecular data interpretation, drafted the article and gave final approval of the version to be published. SF, BK, SL and GM performed electrophysiology, harvested cytoplasms, drafted the text, performed analysis and produced figures, drafted the manuscript and gave final approval of the version to be published. GT is responsible for the integrity of the work as a whole.
This work was supported by the European Research Council INTERIMPACT project (GT), the Hungarian Academy of Sciences (GT), the National Research, Development and Innovation Office of Hungary (GINOP-2.3.2-15-2016-00018, VKSZ-14-1-2015-0155), the Ministry of Human Capacities, Hungary (grant 20391-3/2018/FEKUSTRAT) and by the National Brain Research Program, Hungary (GT).
Duality of interest
The authors declare that there is no duality of interest associated with this manuscript.
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