Modulation of olfactory bulb activity by serotoninergic inputs in odor-associative learning

Olfaction is critical for animal survival, enabling them to discern complex environmental cues such as food, mates, and predators. This sensory modality can trigger innate animal behaviors through detecting distinct odorants and activating hardwired neural circuits. In addition, the olfactory system mediates odor-associative learning, allowing animals to link odors with beneficial or harmful contexts and form long-term memories. Several olfactory cortical regions including piriform cortex, anterior olfactory nucleus, and lateral entorhinal cortex have been implicated in the formation of odor-associative learning and memory.^1-3 Surprisingly, the olfactory bulb (OB)—the first relay station in the olfactory system—also exhibits neuronal plasticity during odor learning, suggesting that the OB is able to encode odor values in addition to basic odor information.^{4, 5} However, it is still not fully understood how the odor values are encoded in the OB neurons and how the neuronal plasticity is shaped during odor learning.

In the current issue of Acta Physiologica, Jing et al. elucidate the serotonergic inputs from the dorsal raphe nucleus (DRN) to the OB as the neural mechanisms underlying plasticity of odor response in the OB during odor-associative learning.⁶ Odors are initially detected by olfactory receptors on olfactory sensory neurons and transmitted to the OB, where the olfactory information is integrated. This information is then relayed by secondary neurons in the OB, mainly mitral and tufted (M/T) cells, to higher brain regions, including the piriform cortex, olfactory tubercle, and anterior olfactory nucleus. Beyond direct projections from olfactory sensory neurons, the OB is modulated by feedback from the olfactory cortex and centrifugal inputs from systems such as serotonergic, cholinergic, and noradrenergic pathways. These higher central inputs are believed to regulate OB responses to odors and play a role in odor-associative learning and memory, though direct evidence has been limited. The authors focused on serotonergic inputs to the OB that are mainly originated from the DRN (Figure 1A). Previous studies have indicated that the DRN modulates OB neural activity and odor response. The DRN neurons activated by optogenetics and electrical stimulation can release serotonin, regulate synaptic activity in the OB, and modulate outputs of M/T cells.^{7, 8} However, there is limited evidence connecting this neural regulation to olfactory perception and discrimination under physiological conditions such as odor-associative learning process.

Using GCaMP to detect DRN neuronal activity, the authors found that the serotoninergic neurons in DRN is specifically activated during odor-associative tasks with a reward (both go/go task and go/no-go tasks) but not during passive odor recognition. However, during early learning stage of the go/no go task, the DRN serotonergic neurons respond to both reward-associated and non-reward-associated odors. As mice learn to discriminate between these odors, DRN serotonergic neuron activation by non-reward odors decreases (Figure 1B). When establishing a new reward contingency by switching the two odors, the DRN serotonergic neurons respond differently, suggesting DRN serotoninergic neurons encode information about the odor's value rather than its identity. By utilizing retrograde tracing and targeted activation of serotonergic neurons projecting from the DRN to the OB, the authors observed increased area preference in mice, further indicating that these neurons encode reward information. These observations provide strong evidences for the involvement of the DRN-OB serotonergic pathway in odor-associative learning and memory.

Next, the authors investigated how the DRN serotonergic neurons regulate odor-induced responses in the OB. Ablation of the DRN serotonergic neurons completely blocks the differential OB responses to reward and non-reward odors after mice acquire odor reward information. The authors further explored the role of the DRN serotonergic neurons in OB neural network activity by analyzing local field potentials (LFPs). In rodents, beta and gamma oscillations are critical frequency bands for odor processing during odor-related tasks. In the go/no-go task, beta and gamma bands in the neural networks show a shaping ability as the animals learn the task. However, following the ablation of DRN serotonergic neurons, these LFP changes are completely disrupted. Through these electrophysiological experiments, the study comprehensively elucidates the physiological mechanisms underlying changes in OB neural activity during odor-associative learning, demonstrating a strong dependence on DRN serotonergic regulation.

Similarly, in fruit flies, dorsal paired medial neurons regulate synaptic plasticity of Kenyon cells in mushroom body through serotonin, contributing to olfactory learning and highlighting the conservation of this pathway across species.⁹ It is noteworthy that neurodevelopmental disorders such as autism spectrum disorder (ASD) are accompanied by decreased serotonin levels.¹⁰ Studies have shown that individuals with ASD misinterpret human-generated odors, exhibiting opposite physical and psychological feedback compared to normal individuals when exposed to fear and calmness odors.¹¹ This evidence also suggests the relevance of serotonin in the establishment of olfactory learning and memory. Therefore, it is also important to explore whether the DRN-OB serotonergic pathway is impaired in psychiatric disorders in the future.

There are still several limitations in this study. It focuses exclusively on reward-associated odors, leaving the role of the DRN-OB serotonin pathway in punishment-associated odor learning unexplored. Additionally, the precise mechanisms of OB M/T cell regulation by DRN serotonergic neurons—whether direct or indirect—remain unclear. In the OB, serotonergic input can also target M/T cells via inhibitory granule cells, and histone serotonylation in astrocytes can regulate gene expression involved in olfactory sensory processing.¹² The potential involvement of feedback from the olfactory cortex and other higher centers, particularly cholinergic and noradrenergic centrifugal inputs to the OB, in odor learning also warrants further investigation.

In summary, the study by Jing et al. reveals the significant physiological role of the DRN-OB serotonin circuit in odor-associative learning and how the serotonergic input influences OB neural activity during odor discrimination. These findings deepen our understanding of the mechanisms by which animals perceive and store the odor information under different behavioral states and may provide insights into olfactory-related disorders. Future studies should investigate the detailed cellular and molecular mechanisms of DRN serotonergic projections as well as cooperation of different neuromodulators in the OB neural network.

Yue Hao: Conceptualization; writing – original draft. Zheng Wang: Writing – review and editing. Qian Li: Conceptualization; writing – original draft; writing – review and editing.

This study is supported by the National Natural Science Foundation of China (32122038 and 32371042), the Basic Research Project from the Science and Technology Commission of Shanghai Municipality (21JC1404500 and 23ZR1480000), Shuguang Program from Shanghai Education Development Foundation and Shanghai Municipal Education Commission (21SG16), the Chinese Academy of Sciences Grant (JCTD-2021-06), and the Postdoctoral Fellowship Program of CPSF (GZC20241066).

The authors declare no conflict of interest.