Abstract
GABAergic neurons in basal forebrain (BF) nuclei project densely to all layers of the mouse main olfactory bulb (OB), the first relay in odor information processing. However, BF projection neurons are diverse and the contribution of each subtype to odor information processing is not known. In the present study, we used retrograde and anterograde tracing methods together with whole-brain light-sheet analyses, patch-clamp recordings coupled with optogenetic and chemogenetic approaches during spontaneous odor discrimination, and go/no-go odor discrimination/learning tests to characterize the synaptic targets in the OB of BF calretinin-expressing (CR+) GABAergic cells and to reveal their functional implications. We used mice of either sex to show that OB-projecting CR+ neurons innervate the bulbar granule cell layer but not the glomerular layer. Optogenetic stimulation of CR+ axonal projections in OB slices elicited monosynaptic GABAergic currents in granule cells (GCs). Retrograde rabies virus-based transsynaptic tracing experiments confirmed these synaptic connections and further suggested that CR+ neurons provide the principal, if not the unique, BF input onto GCs. Chemogenetic inhibition of CR+ neurons in the BF of male mice did not affect odor discrimination in habituation/dishabituation tasks but led to impairment in odor learning during go/no-go odor-associative tasks. Our results revealed a subtype-specific projection pattern in the OB of a select population of BF neurons and suggested that distinct BF GABAergic projections have distinct effects on odor information processing and learning.
Significance statement The basal forebrain projects densely into the olfactory bulb and plays an important role in odor processing. The basal forebrain contains neurochemically distinct cellular populations but the contribution of each subtype to odor information processing is not known. We identified predominant, if not unique, synaptic connections between neurons in the basal forebrain that are characterized by the expression of calretinin (CR) and granule cells in the olfactory bulb. The detailed morpho-functional characterization of these connections based on anterograde, retrograde, and rabies-based transsynaptic labeling, patch-clamp recordings, optogenetics, and behavioral analyses indicated that these connections are GABAergic in nature and suggested that CR-expressing neurons in the basal forebrain are involved in odor learning.
Footnotes
This work was supported by a Canadian Institute of Health Research (CIHR) grant to A.S. and by a Centre National de la Recherche Scientifique (CNRS) grant to D.D.S-J,. We are grateful to Dr. Karl-Klaus Conzelmann and Alexandru Adrian Hennrich (Max Von Pettenkofer Institute Virology and Gene Center, Medical Faculty, Ludwig-Maximilians-University Munich, Germany) for the generous gift of the (EnvA)SAD-ΔG-mCherry virus and Dr. Bennedikt Bennigner (University Medical Center of the Johannes Gutenberg University, Mainz, Germany) for the retro-TVA-G plasmid construction. We thank the Cell Biology and Image Acquisition (CBIA) Core Facility (RRID:SCR_021845) at the University of Ottawa for the use of the various imaging systems and the Viral Vector for Gene Transfer core facility of the Structure Fédérative de Recherche Necker for the retroviral vector production. We also thank Aurelia Ces and Pierre Hener (ComptOpt platform and morpho-functional analysis platform, respectively, Institut des Neurosciences Cellulaires et Intégratives, Strasbourg) for their technical support.
The authors declare no competing interests.