Abstract
The anatomical organization of receptor neuron input into the olfactory bulb (OB) allows odor information to be transformed into an odorant-specific spatial map of mitral/tufted (M/T) cell glomerular activity at the upper level of the OB. In other sensory systems, neuronal representations of stimuli can be reorganized or enhanced following learning. While the mammalian OB has been shown to undergo experience-dependent plasticity at the glomerular level, it is still unclear if similar representational change occurs within (M/T) cell glomerular odor representations following learning. To address this, odorant-evoked glomerular activity patterns were imaged in mice expressing a GFP-based calcium indicator (GCaMP2) in OB (M/T) cells. Glomerular odor responses were imaged before and after olfactory associative conditioning to aversive foot shock. Following conditioning, we found no overall reorganization of the glomerular representation. Training, however, did significantly alter the amplitudes of individual glomeruli within the representation in mice in which the odor was presented together with foot shock. Further, the specific pairing of foot shock with odor presentations lead to increased responses primarily in initially weakly activated glomeruli. Overall, these results suggest that associative conditioning can enhance the initial representation of odors within the OB by enhancing responses to the learned odor in some glomeruli.
Highlights
All olfactory receptor neurons (ORNs) expressing the same receptor protein project their axons to specific glomeruli in the olfactory bulb (OB) where they form excitatory synapses onto a circuit of diverse cell types including mitral/tufted (M/T) cells and several types of juxtaglomerular interneurons (Buck and Axel, 1991; Ressler et al, 1994; Kiyokage et al, 2010)
Using in vivo imaging, this study investigated the impact of olfactory aversive learning on OB M/T cell glomerular odor responses in adult mice
The results demonstrate that olfactory learning significantly alters OB M/T cell glomerular odor representations to the trained odor by altering the response amplitudes of glomeruli within the representation
Summary
All olfactory receptor neurons (ORNs) expressing the same receptor protein project their axons to specific glomeruli in the olfactory bulb (OB) where they form excitatory synapses onto a circuit of diverse cell types including mitral/tufted (M/T) cells and several types of juxtaglomerular interneurons (Buck and Axel, 1991; Ressler et al, 1994; Kiyokage et al, 2010). Associative conditioning in rat pups leads to increased 2-DG glomerular uptake (Coopersmith et al, 1986; Sullivan and Leon, 1986; Woo et al, 1987; Woo and Leon, 1991) and increased c-fos expression in juxtaglomerular neurons and deeper layer granule cells in regions responding to the trained odorant (Johnson et al, 1995; Funk and Amir, 2000; Salcedo et al, 2005) Together these experiments point toward learning leading to long-term changes in glomerular layer interneuronal responses to learned odorants
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