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OPEN
received: 15 February 2016 accepted: 20 April 2016 Published: 11 May 2016
Increased olfactory bulb acetylcholine bi-directionally modulates glomerular odor sensitivity Mounir Bendahmane, M. Cameron Ogg, Matthew Ennis & Max L. Fletcher The glomerular layer of the olfactory bulb (OB) receives heavy cholinergic input from the horizontal limb of the diagonal band of Broca (HDB) and expresses both muscarinic and nicotinic acetylcholine (ACh) receptors. However, the effects of ACh on OB glomerular odor responses remain unknown. Using calcium imaging in transgenic mice expressing the calcium indicator GCaMP2 in the mitral/tufted cells, we investigated the effect of ACh on the glomerular responses to increasing odor concentrations. Using HDB electrical stimulation and in vivo pharmacology, we find that increased OB ACh leads to dynamic, activity-dependent bi-directional modulation of glomerular odor response due to the combinatorial effects of both muscarinic and nicotinic activation. Using pharmacological manipulation to reveal the individual receptor type contributions, we find that m2 muscarinic receptor activation increases glomerular sensitivity to weak odor input whereas nicotinic receptor activation decreases sensitivity to strong input. Overall, we found that ACh in the OB increases glomerular sensitivity to odors and decreases activation thresholds. This effect, along with the decreased responses to strong odor input, reduces the response intensity range of individual glomeruli to increasing concentration making them more similar across the entire concentration range. As a result, odor representations are more similar as concentration increases. Odors are detected by olfactory sensory neurons (OSNs) in the nasal cavity that express a single receptor type. OSNs project their axons into specific glomeruli in the olfactory bulb (OB) where they form excitatory synapses onto a complex circuit of interneurons and mitral/tufted (M/T) cells. This convergence forms the basis of the glomerular odor map whereby odor information is represented by distinct spatio-temporal patterns of M/T cell apical dendrite glomerular activity. Cholinergic innervation of the OB arises from the horizontal limb of the diagonal band of Broca (HDB)1. These fibers terminate densely in the glomerular layer and moderately in the sub-glomerular layers. This projection pattern is paralleled by expression of muscarinic and nicotinic ACh receptor (AChR) subtypes2–8. ACh release by the basal forebrain cholinergic system has been demonstrated to be involved in arousal, attention, and learning. During active, awake states, cholinergic neurons display increased activity9,10 and are active during odor investigation and learning11. Similarly, cortical ACh release is increased by novel sensory stimuli12,13 and by arousing or aversive events14,15. ACh release is hypothesized to have several effects including cue detection, enhancing sensory coding of salient stimuli, and facilitating memory encoding16,17. Previous studies have demonstrated that ACh release and activation of AChRs facilitate olfactory learning, memory, odor discrimination, and generalization18–24. However, the mechanisms by which ACh release facilitates these behaviors are not understood, especially in terms of OB odor processing. Previous in vitro electrophysiology studies have shown that ACh or cholinergic agonists can exert excitatory or inhibitory effects that depend on cell (M/T vs. inhibitory interneurons) and AChR subtype3,6,7,22,25,26. How these varying cellular effects impact odor responses has been less well studied. More recent in vivo studies using optogenetic approaches have demonstrated that activation of HDB ACh neurons or ACh fibers in the OB can lead to both increases and decreases in M/T cell odor responses26,27.
Department of Anatomy and Neurobiology, University of Tennessee Health Science Center, Memphis, TN 38163, USA. Correspondence and requests for materials should be addressed to M.B. (email: [email protected]) Scientific Reports | 6:25808 | DOI: 10.1038/srep25808
1
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Figure 1. HDBS affects glomerular odor responses. (A) Glomerular responses recorded from the dorsal surface of the OB in a GCaMP2 expressing mouse. Odor responses to 1% methyl valerate are increased when the odorant is paired with HDBS while HDBS alone with no odorant does not induce any response. (B) Example traces after photo-bleaching correction of the glomerulus indicated by the arrow in A, in the control, HDBS + odor, and HDBS alone conditions. The oscillations reflect the respiration cycle; black bar at bottom indicates time of HDBS. (C) Plot of the HDBS response modulation as function of the control responses (1236 pre-post HDBS paired responses). Odor responses are both above and below the dashed unity gain line, showing both increased and decreased odor response magnitude by HDBS. Most decreases are observed when the control response magnitudes are relatively high (above 5% Δf/f) and observed more frequently as the control responses increase. (D) Bar graphs showing the mean absolute odorant response magnitude change (91.3 ± 7.96% increase) induced by HDBS compared to the pre-HDBS (Control); responses return to control levels two minutes following HDBS (Post). *p
OPEN
received: 15 February 2016 accepted: 20 April 2016 Published: 11 May 2016
Increased olfactory bulb acetylcholine bi-directionally modulates glomerular odor sensitivity Mounir Bendahmane, M. Cameron Ogg, Matthew Ennis & Max L. Fletcher The glomerular layer of the olfactory bulb (OB) receives heavy cholinergic input from the horizontal limb of the diagonal band of Broca (HDB) and expresses both muscarinic and nicotinic acetylcholine (ACh) receptors. However, the effects of ACh on OB glomerular odor responses remain unknown. Using calcium imaging in transgenic mice expressing the calcium indicator GCaMP2 in the mitral/tufted cells, we investigated the effect of ACh on the glomerular responses to increasing odor concentrations. Using HDB electrical stimulation and in vivo pharmacology, we find that increased OB ACh leads to dynamic, activity-dependent bi-directional modulation of glomerular odor response due to the combinatorial effects of both muscarinic and nicotinic activation. Using pharmacological manipulation to reveal the individual receptor type contributions, we find that m2 muscarinic receptor activation increases glomerular sensitivity to weak odor input whereas nicotinic receptor activation decreases sensitivity to strong input. Overall, we found that ACh in the OB increases glomerular sensitivity to odors and decreases activation thresholds. This effect, along with the decreased responses to strong odor input, reduces the response intensity range of individual glomeruli to increasing concentration making them more similar across the entire concentration range. As a result, odor representations are more similar as concentration increases. Odors are detected by olfactory sensory neurons (OSNs) in the nasal cavity that express a single receptor type. OSNs project their axons into specific glomeruli in the olfactory bulb (OB) where they form excitatory synapses onto a complex circuit of interneurons and mitral/tufted (M/T) cells. This convergence forms the basis of the glomerular odor map whereby odor information is represented by distinct spatio-temporal patterns of M/T cell apical dendrite glomerular activity. Cholinergic innervation of the OB arises from the horizontal limb of the diagonal band of Broca (HDB)1. These fibers terminate densely in the glomerular layer and moderately in the sub-glomerular layers. This projection pattern is paralleled by expression of muscarinic and nicotinic ACh receptor (AChR) subtypes2–8. ACh release by the basal forebrain cholinergic system has been demonstrated to be involved in arousal, attention, and learning. During active, awake states, cholinergic neurons display increased activity9,10 and are active during odor investigation and learning11. Similarly, cortical ACh release is increased by novel sensory stimuli12,13 and by arousing or aversive events14,15. ACh release is hypothesized to have several effects including cue detection, enhancing sensory coding of salient stimuli, and facilitating memory encoding16,17. Previous studies have demonstrated that ACh release and activation of AChRs facilitate olfactory learning, memory, odor discrimination, and generalization18–24. However, the mechanisms by which ACh release facilitates these behaviors are not understood, especially in terms of OB odor processing. Previous in vitro electrophysiology studies have shown that ACh or cholinergic agonists can exert excitatory or inhibitory effects that depend on cell (M/T vs. inhibitory interneurons) and AChR subtype3,6,7,22,25,26. How these varying cellular effects impact odor responses has been less well studied. More recent in vivo studies using optogenetic approaches have demonstrated that activation of HDB ACh neurons or ACh fibers in the OB can lead to both increases and decreases in M/T cell odor responses26,27.
Department of Anatomy and Neurobiology, University of Tennessee Health Science Center, Memphis, TN 38163, USA. Correspondence and requests for materials should be addressed to M.B. (email: [email protected]) Scientific Reports | 6:25808 | DOI: 10.1038/srep25808
1
www.nature.com/scientificreports/
Figure 1. HDBS affects glomerular odor responses. (A) Glomerular responses recorded from the dorsal surface of the OB in a GCaMP2 expressing mouse. Odor responses to 1% methyl valerate are increased when the odorant is paired with HDBS while HDBS alone with no odorant does not induce any response. (B) Example traces after photo-bleaching correction of the glomerulus indicated by the arrow in A, in the control, HDBS + odor, and HDBS alone conditions. The oscillations reflect the respiration cycle; black bar at bottom indicates time of HDBS. (C) Plot of the HDBS response modulation as function of the control responses (1236 pre-post HDBS paired responses). Odor responses are both above and below the dashed unity gain line, showing both increased and decreased odor response magnitude by HDBS. Most decreases are observed when the control response magnitudes are relatively high (above 5% Δf/f) and observed more frequently as the control responses increase. (D) Bar graphs showing the mean absolute odorant response magnitude change (91.3 ± 7.96% increase) induced by HDBS compared to the pre-HDBS (Control); responses return to control levels two minutes following HDBS (Post). *p
