Journal
JOURNAL OF NEUROSCIENCE
Volume 28, Issue 25, Pages 6360-6371Publisher
SOC NEUROSCIENCE
DOI: 10.1523/JNEUROSCI.0793-08.2008
Keywords
calcium; GABAergic modulation; olfactory bulb; optical imaging; presynaptic regulation; sensory neurons
Categories
Funding
- NIDCD NIH HHS [R01 DC006441-05, DC6441, R01 DC006441] Funding Source: Medline
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The first reorganization of odor representations in the nervous system occurs at the synapse between olfactory receptor neurons and second-order neurons in olfactory bulb glomeruli. Signal transmission at this synapse is modulated presynaptically by several mechanisms, a major one being mediated by GABA(B) receptors, which suppress presynaptic calcium influx and subsequent transmitter release from the receptor neuron terminal. Here, we imaged stimulus-evoked calcium influx into the receptor neuron terminal in anesthetized mice and used odorant and electrical stimulation combined with in vivo pharmacology to characterize the functional determinants of GABAB-mediated presynaptic inhibition and to test hypotheses on the role of this inhibition in olfactory processing. As expected from previous studies, blocking presynaptic GABAB receptors in vivo increased odorant-evoked presynaptic calcium signals, confirming that GABAB-mediated inhibition modulates the strength of receptor inputs. Surprisingly, we found that the strength of this inhibition was affected little by the nature of the input, being independent of the spatial distribution of activated glomeruli, independent of the sniff frequency used to sample the odorant, and similar for weak and strong odorant-evoked inputs. Instead, we found that tonic inhibition was a major determinant of receptor input strength; this tonic inhibition in turn was dependent on glutamatergic transmission from second-order neurons in the glomerular layer. Thus, rather than adaptively shaping odor representations in an activity-dependent manner, a primary role of presynaptic inhibition in vivo may be to modulate the magnitude of sensory input to the brain as a function of behavioral state.
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