Action of the noradrenergic system on adult-born cells is required for olfactory learning in mice

Abstract

We have previously shown that an experience-driven improvement in olfactory discrimination (perceptual learning) requires the addition of newborn neurons in the olfactory bulb (OB). Despite this advance, the mechanisms which govern the selective survival of newborn OB neurons following learning remain largely unknown. We propose that activity of the noradrenergic system is a critical mediator providing a top-down signal to control the selective survival of newly born cells and support perceptual learning. In adult mice, we used pharmacological means to manipulate the noradrenergic system and neurogenesis and to assess their individual and additive effects on behavioral performance on a perceptual learning task. We then looked at the effects of these manipulations on regional survival of adult-born cells in the OB. Finally, using confocal imaging and electrophysiology, we investigated potential mechanisms by which noradrenaline could directly influence the survival of adult-born cells. Consistent with our hypotheses, direct manipulation of noradrenergic transmission significantly effect on adult-born cell survival and perceptual learning. Specifically, learning required both the presence of adult-born cell and noradrenaline. Finally, we provide a mechanistic link between these effects by showing that adult-born neurons receive noradrenergic projections and are responsive to noradrenaline. Based upon these data we argue that noradrenergic transmission is a key mechanism selecting adult-born neurons during learning and demonstrate that top-down neuromodulation acts on adult-born neuron survival to modulate learning performance.

Figure 1.

Perceptual learning is under control of the noradrenergic system. A, Time course of the experiment. Spontaneous discrimination between +/−limonene (+/−lim), pentanol/butanol (pent/but), and decanal/dodecanone (dec/dodec) was tested before and after a period of olfactory enrichment and/or drug treatment. Experimental groups were enriched by introducing +/−lim into the home cage for 1 h periods over 10 d and treated during the same period of time with dexefaroxan, labetalol, or saline. B, C, Behavioral discrimination was tested before (B) and after (C) the treatment period. The two odorants of each pair are cross-habituated before the enrichment period and thus not discriminated (B), as well as in the control non-enriched group (Ci). After enrichment with +/−lim, two of the three odor pairs are discriminated (+/−lim and pent/but) (Cii). Treatment with labetalol with (Ciii) or without (Civ) enrichment prevents the improvement of discrimination. In contrast, after treatment with dexefaroxan without any enrichment, all three odor pairs are discriminated (Cv). *p < 0.05 in response magnitude between trials 4 (Hab4) and 5 (Otest). Di, Quantification of DBH-positive neurons in the locus ceruleus. Odor enrichment increased the density of DBH-positive cells compared with non-enriched animals. *p < 0.05. Dii, DBH labeling in the locus ceruleus. The data are expressed as mean values ± SEM.

Figure 2.

Newborn cell survival is modulated during perceptual learning and regulated by noradrenergic system. A, Experimental paradigm. BrdU was administered 8 d before the enrichment period and mice were killed 25 d after administration of BrdU. Animals were exposed to +limonene or decanal 1 h before they were killed. Bi, Odor enrichment and noradrenergic modulation affect neurogenesis. BrdU-positive cell density in the granule cell layer of the OB is increased in the saline-enriched and dexefaroxan-treated animals compared with the control non-enriched group. In both groups treated with labetalol with or without enrichment, the density of BrdU-positive cells is similar to that of the control group. Bii, Representative image of BrdU labeling. Scale bar, 15 μm. Ci, Quantification of BrdU/NeuN double-labeling in the granule cell layer showed no effect of the enrichment or drug treatment on the neuronal fate of adult-born cells. Cii, Pseudo-confocal imaging of BrdU/NeuN. Scale bar, 4 μm. D, Mapping of BrdU-positive cell density in the granule cell layer of the OB. Average maps of BrdU-positive cell density showed similarities between saline-enriched and dexefaroxan-treated animals compared with the other groups and correspond to animals whose discrimination abilities were improved. E, Overlap maps were constructed by delimitation of regions with a high density of BrdU-positive cells, and these high-density areas were compared between maps and a percentage of overlap was calculated. F, Value of all bins of the enriched and dexefaroxan-treated maps was extracted and grouped in two categories: [mean; mean + 2 SD] representing intervals of mean values and [mean + 2SD; + ∞] representing intervals of more extreme values. The dexefaroxan-treated animals map presents more bins ranged in the interval of [mean; mean + 2SD] and less bins in the interval of [mean + 2SD; + ∞] compared with the enriched-animals map, indicating that adult-born cells are more widely distributed after noradrenergic treatment than after odor enrichment (t test for comparison of proportions; *p < 0.05; **p < 0.005). The data are expressed as mean values ± SEM. G, Gi, Olfactory deprivation decreased the density of Zif268-positive cells in the granule cell layer of the OB compared with the non-deprived side (**p < 0.005). Gii, Representative image of Zif268 labeling. Scale bar, 15 μm. H, Enriched and dexefaroxan-treated animals displayed more granule cells expressing Zif268 in response to +limonene than did non-enriched animals or both groups of labetalol-treated animals (black bars). Only dexefaroxan-treated animals displayed an increase of Zif268-positive cells in response to decanal compared with the other groups (gray bars). Ii, In response to +limonene stimulation, enriched and dexefaroxan-treated animals displayed more adult-born neurons expressing Zif268 than did non-enriched animals or both groups of labetalol-treated animals. After decanal stimulation, only dexefaroxan-treated animals displayed more BrdU-positive cells expressing Zif268 compared with the other groups. Iii, Confocal imaging of BrdU/Zif268. Scale bar, 4 μm. *p < 0.05; **p < 0.005; ***p < 0.001. The data are expressed as mean values ± SEM. salNE, Saline-non-enriched animals; salE, saline-enriched animals; dex, dexefaroxan-non-enriched animals; labNE, labetalol-non-enriched animals; labE, labetalol-enriched animals.

Figure 3.

Olfactory perceptual learning requires the action of the noradrenergic system on adult-born neurons. A, Experimental design. Saline or AraC was locally infused 3 d before the administration of BrdU and lasting for 21 d. Animals of both groups were treated with dexefaroxan once a day during 10 d. B, Mice that received AraC had a significant reduction of BrdU-positive cell density in the granule cell layer of the OB compared with the saline-infused animals. C, NeuN-positive cell density was similar between saline- and AraC-infused animals. Di, In the saline-infused animals, treatment with dexefaroxan improved discrimination for the three odor pairs tested. Dii, The strong reduction of bulbar neurogenesis in the AraC-infused group blocked the dexefaroxan treatment-induced improvement of discrimination. E, Both groups of saline (Ei)- and AraC (Eii)-infused animals discriminate well a dissimilar pair of odorants (+limonene vs decanal and isoamyl acetate vs octanal). F, Zif268-positive cell density is superior in the saline-infused dexefaroxan-treated animals compared with the AraC infused. *p < 0.05; **p < 0.005, ***p < 0.001. The data are expressed as mean values ± SEM.

Figure 4.

Adult-born neurons respond to noradrenaline. A, Representative of NET labeling in the bulbar granule cell layer of animals injected with GFP lentivirus. We found that NET-positive fibers project onto newborn neurons. B, Depolarization induced by current injection produced only few action potential in adult-born neuron. C, Time-plot of noradrenaline effect on the membrane resistance of a newborn neuron. Inset, Membrane hyperpolarization produced by injected current (−0.005 nA) during control period (black trace) and in the presence of noradrenaline, 10 μm (gray trace). D, Modifications induced by noradrenaline on the membrane resistance of eight different adult-born neurons. **p < 0.005.