Nachweis der Expression metabotroper GABAB-Rezeptoren in GABAergen Interneuronen des somatosensorischen Kortex mittels Immunhistochemie

Abstract

In the cerebral cortex, different groups of neurons can be distinguished based on their mode of action. On the one hand, there are the excitatory neurons, which make up the majority (70-80 %) of neurons. The remaining 20-30 % are the inhibitory neurons, which have a great diversity. Thus, GABAergic interneurons can be divided into three subpopulations based on molecular markers: Parvalbumin (PV), Somatostatin (SST) and 5HT3a receptor-expressing neurons. Within the 5HT3a receptor group, a further distinction can be made between vasoactive intestinal polypeptide (VIP)-expressing and non-VIP-expressing cells. In inhibitory neurotransmission, metabotropic GABAb receptors are involved in addition to ionotropic GABAa receptors. GABAb receptors not only have a direct influence on a single target cell, but can also influence the neuronal network through their downstream cellular cascades. The question raised to what extent GABAergic interneurons are colocalised with the GABAb receptor and what effects this has on excitation and inhibition within this network. In the present study, the focus was on demonstrating the expression of metabotropic GABAB receptors in the PV-, the VIP- and the SST-positive GABAergic interneurons. Experiments were performed on frontal brain slices in the somatosensory cortex (barrel cortex) using immunohistochemistry for the GABAB receptor. Transgenic mouse lines (PVcre/tdTomato, VIPcre/tdTomato and SSTcre/tdTomato) in which cells are labelled with the fluorophore tdTomato were used to visualise subpopulations of interneurons. For further characterisation of SST-expressing neurons, the GIN mouse was used, in which the specific group of SST-positive Martinotti cells is labelled with GFP. After immunohistochemical staining, sections were imaged with a confocal laser scanning microscope and checked for colocalisation of the native fluorophore with the GABAB receptor by using a specific software. For the SST-positive interneurons, almost complete co-expression (96.8%) with the GABAB receptor was detected. For the GFP-positive cells of the GIN mouse, there was a 100% overlap of the two signals. This suggests that probably the entire population of SST-expressing interneurons, and in any case the GFP-positive cells of the GIN mouse, which represent the Martinotti cells, express the metabotropic GABAB receptor in all layers of the barrel cortex. In the VIP-positive interneurons, 95.0% express the GABAB receptor. For the largest population of GABAergic interneurons, the PV-expressing cells, the majority (88.4%) were found to be colocalised with the GABAB receptor in all layers. The exceptions are layers II/III and IV, where colocalisation with the GABAB receptor is significantly lower (76.4% and 74.2%, respectively). Thus, the PV-expressing interneurons seem to play a special role within the GABAergic interneurons. The frequency of PV-positive basket cells, particularly in layer IV, suggests that a subpopulation of PV-expressing interneurons exists that lack metabotropic GABAB receptors. The extent to which these possible subpopulations of basket cells are integrated into different neuronal networks must be clarified by future studies. In summary, the majority of SST-, VIP- and PV-expressing interneurons have the metabotropic GABAB receptor. So far, there has been little evidence on their cortical distribution of GABAB-expressing neurons, especially for interneurons. To understand the importance of GABAB receptors for cortical processing, further studies are needed, especially regarding the subcellular localisation of the GABAB receptor.