Brain-wide connectivity of molecularly defined GABAergic neurons in mouse barrel cortex visualized with optimized rabies virus tracing
von Georg Hafner
Datum der mündl. Prüfung:2019-05-23
Betreuer:Prof. Dr. Jochen F. Staiger
Gutachter:Prof. Dr. Silvio O. Rizzoli
Gutachter:Camin Phd Dean
EnglischCortical GABAergic neurons are indispensable in controlling the activity of cortical networks. Parvalbu- min (PV), somatostatin (SST) and vasoactive intestinal polypeptide (VIP) expressing neurons are three main classes of GABAergic cells. They differ in morphology, physiology and output connectivity. We wanted to investigate the largely unknown input-connectivity of these neurons on a brain-wide scale using rabies virus tracing. We focused on the PV and VIP neurons in the mouse barrel cortex. First, we used intersectional rabies virus tracing, to specifically target GABAergic PV cells and exclude a small fraction of excitatory PV cells from our starter cell population. We combined the Vgat- Cre/PV-Flp line with Cre- and Flp-dependent helper viruses. After thoroughly evaluating the specificity of these novel viral constructs, we mapped the local and long-range inputs to PV neurons. Local inputs were mainly from layer (L) IV and excitatory. A small number of inputs originated from LI inhibitory neu- rons, which we found to connect to LII/III PV neurons. Long-range inputs originated mainly from other sensory cortices and the thalamus. Surprisingly, in visual cortex most retrogradely labeled neurons were located in LIV, which sent direct connection to PV cells in all layers of barrel cortex as demonstrat- ed with anterograde tracing experiments. Second, we assessed the long-range input connectivity of VIP neurons in wildtype and reeler mutant mice, in which layers are not formed during development. VIP neurons received input from the same areas in both genotypes. The major input sources were other sensory cortices, motor cortex, posterior parietal association area and the thalamus. VIP neurons in reeler mice received a much low- er number of ipsilateral cortical inputs and a much higher number of contralateral cortical inputs. We hypothesize that the disorganized arrangement of neurons in reeler compromises the establishment of cell-type specific ipsilateral long-range projections and necessitates a compensation by an excess of contralateral inputs. Both studies provide valuable insights in the brain-wide circuits in which GABAergic neurons are embedded and introduce new and very specific rabies virus tracing tools.
Keywords: barrel cortex; rabies; inhibitory neurons; reeler; intersectional