Odour-driven activity of HDB axonal projections in the OB

Monika Müller1, Inna Schwarz2, Irina Pavlova2, Fabrizio Musacchio1, Manuel Mittag1, Martin Schwarz2, Martin Fuhrmann1

1 Neuroimmunology and Imaging Group, German Center of Neurodegenerative Diseases, Bonn, Germany
2 Functional Neuroconnectomics Group, Department of Epileptology, Medical School, University of Bonn, Bonn, Germany

The horizontal limb of diagonal band of Broca (HDB) is the main source of cholinergic innervations of the olfactory bulb (OB), but also provides gabaergic and glutamatergic input. The OB integrates odour information emerging from the external world with modulatory input from other areas, such as the HDB, and relays it to higher brain regions involved in odour processing. This process will result in various odour driven behaviours, enabling the organism to respond to its environment. Previous studies demonstrated the HDBs involvement in the modulation of neuronal activity within the olfactory bulb affecting odour-driven behaviour.

To address this question, we employed in vivo 2-Photon calcium imaging in C57BL6, ChAT-Cre and GAD2-Cre mice to monitor neuronal activity in awake behaving mice during odour stimulation. Hereby, we identified odour-tuned HDB axons in the Olfactory bulb. GCaMP6 expressing cholinergic and GABAergic projections innervating the glomerular layer in the OB displayed changes in fluorescence upon odour stimulation. Interestingly, we found different classes of odour-tuned axons responding to one, both or none of the presented odours. Moreover, these differently odour-tuned axons were spatially intermingled within the glomerular layer. Additionally, we observed reduced glomerular activity upon silencing the HDB by tetanus toxin injections compared to sham-treated Thy1-GCaMP6f-mice.

Thus we conclude, that odours activate different types of HDB neurons innervating the glomerular layer and modulating the activity of target cells within the olfactory bulb. Our work highlights the odour-driven activation of HDB neurons and the resulting modulatory effect on the OB, improving our understanding of odour processing.