Abstract
Granule cells are the dominant cell type of the olfactory bulb inhibiting mitral and tufted cells via dendrodendritic synapses; yet the factors regulating the strength of their inhibitory output, and, therefore, their impact on odour discrimination, remain unknown. Here we show that GABAAR β3-subunits are distributed in a somatodendritic pattern, mostly sparing the large granule cell spines also known as gemmules. Granule cell-selective deletion of β3-subunits nearly abolishes spontaneous and muscimol-induced currents mediated by GABAA receptors in granule cells, yet recurrent inhibition of mitral cells is strongly enhanced. Mice with disinhibited granule cells require less time to discriminate both dissimilar as well as highly similar odourants, while discrimination learning remains unaffected. Hence, granule cells are controlled by an inhibitory drive that in turn tunes mitral cell inhibition. As a consequence, the olfactory bulb inhibitory network adjusts the speed of early sensory processing.
Highlights
Granule cells are the dominant cell type of the olfactory bulb inhibiting mitral and tufted cells via dendrodendritic synapses; yet the factors regulating the strength of their inhibitory output, and, their impact on odour discrimination, remain unknown
The two apical regions shown in panels d and e reveal dense b3-immunoreactive clusters; yet, none of these are positioned within the apical Granule cells (GCs) dendrite
Identical results were obtained in a total of five cells taken from three mice analysed with this approach
Summary
Granule cells are the dominant cell type of the olfactory bulb inhibiting mitral and tufted cells via dendrodendritic synapses; yet the factors regulating the strength of their inhibitory output, and, their impact on odour discrimination, remain unknown. Granule cells (GCs) are key players for the early processing of odour information in the olfactory bulb (OB) These interneurons are exclusively GABAergic[1] and account for the second layer of inhibition within the OB, dominating the OB circuitry[2,3,4,5]. GCs are involved in synchronization and establishment of slow temporal patterns of mitral cells (MCs)[6,7,8], contrast enhancement for spatial representations of odours[8,9,10] and sharpening of activity onset in the OB network[11,12] These are features of recurrent and lateral inhibition in the OB, mediated by the reciprocal synapse established between the GCs and MCs13–15. We use the go/no-go operant-conditioning paradigm to show that specific deletion of the b3-subunit in the GCL accelerates odour discrimination times (ODTs) for monomolecular odours and highly similar mixtures, but leaves discrimination learning unaffected
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