Primary dendrites of mitral cells synapse unto neighboring glomeruli independent of their odorant receptor identity

Circuit formation and sensory perception in the mouse olfactory system

In the mouse olfactory system, odor information is converted to a topographic map of activated glomeruli in the olfactory bulb (OB). Although the arrangement of glomeruli is genetically determined, the glomerular structure is plastic and can be modified by environmental stimuli. If the pups are exposed to a particular odorant, responding glomeruli become larger recruiting the dendrites of connecting projection neurons and interneurons. This imprinting not only increases the sensitivity to the exposed odor, but also imposes the positive quality on imprinted memory. External odor information represented as an odor map in the OB is transmitted to the olfactory cortex (OC) and amygdala for decision making to elicit emotional and behavioral outputs using two distinct neural pathways, innate and learned. Innate olfactory circuits start to work right after birth, whereas learned circuits become functional later on. In this paper, the recent progress will be summarized in the study of olfactory circuit formation and odor perception in mice. We will also propose new hypotheses on the timing and gating of olfactory circuit activity in relation to the respiration cycle.

Activity-dependent local protection and lateral inhibition control synaptic competition in developing mitral cells in mice

In developing brains, activity-dependent remodeling facilitates the formation of precise neuronal connectivity. Synaptic competition is known to facilitate synapse elimination; however, it has remained unknown how different synapses compete with one another within a post-synaptic cell. Here, we investigate how a mitral cell in the mouse olfactory bulb prunes all but one primary dendrite during the developmental remodeling process. We find that spontaneous activity generated within the olfactory bulb is essential. We show that strong glutamatergic inputs to one dendrite trigger branch-specific changes in RhoA activity to facilitate the pruning of the remaining dendrites: NMDAR-dependent local signals suppress RhoA to protect it from pruning; however, the subsequent neuronal depolarization induces neuron-wide activation of RhoA to prune non-protected dendrites. NMDAR-RhoA signals are also essential for the synaptic competition in the mouse barrel cortex. Our results demonstrate a general principle whereby activity-dependent lateral inhibition across synapses establishes a discrete receptive field of a neuron.

Genome-Wide Identification of the Odorant Receptor Gene Family and Revealing Key Genes Involved in Sexual Communication in Anoplophora glabripennis

Insects use a powerful and complex olfactory recognition system to sense odor molecules in the external environment to guide behavior. A large family of odorant receptors (ORs) mediates the detection of pheromone compounds. Anoplophora glabripennis is a destructive pest that harms broad-leaved tree species. Although olfactory sensation is an important factor affecting the information exchange of A. glabripennis, little is known about the key ORs involved. Here, we identified ninety-eight AglaORs in the Agla2.0 genome and found that the AglaOR gene family had expanded with structural and functional diversity. RT-qPCR was used to analyze the expression of AglaORs in sex tissues and in adults at different developmental stages. Twenty-three AglaORs with antennal-biased expression were identified. Among these, eleven were male-biased and two were female-biased and were more significantly expressed in the sexual maturation stage than in the post-mating stage, suggesting that these genes play a role in sexual communication. Relatively, two female-biased AglaORs were overexpressed in females seeking spawning grounds after mating, indicating that these genes might be involved in the recognition of host plant volatiles that may regulate the selection of spawning grounds. Our study provides a theoretical basis for further studies into the molecular mechanism of A. glabripennis olfaction.

Neural Circuitry for Stress Information of Environmental and Internal Odor Worlds

In mammals, odor information detected in the olfactory epithelium is converted to a topographic map of activated glomeruli in the olfactory bulb. Odor signals are then conveyed by projection neurons to the olfactory cortex for decision making. Odor information is processed by two distinct pathways, one is innate and the other is learned, which are separately activated during exhalation and inhalation, respectively. There are two types of odor signals, exteroceptive and interoceptive, which are also processed in different phases of respiration. Exteroceptive sensory information whether attractive/pleasant or aversive/stressful, is evaluated by the valence regions in the amygdala. Stress is an alert signal telling the body to take an action so that the normal condition can be recovered. When the odor quality is negative, the brain sets up a behavioral strategy to avoid the danger or to improve the situation. In this review article, we will describe the recent progress in the study of olfactory perception focusing on stress responses to external and internal odors.

Processing of Odor Information During the Respiratory Cycle in Mice

In the mouse olfactory system, odor signals detected in the olfactory epithelium are converted to a topographic map of activated glomeruli in the olfactory bulb. The map information is then conveyed by projection neurons, mitral cells and tufted cells, to various areas in the olfactory cortex. An odor map is transmitted to the anterior olfactory nucleus by tufted cells for odor identification and recollection of associated memory for learned decisions. For instinct decisions, odor information is directly transmitted to the valence regions in the amygdala by specific subsets of mitral cells. Transmission of orthonasal odor signals through these two distinct pathways, innate and learned, are closely related with exhalation and inhalation, respectively. Furthermore, the retronasal/interoceptive and orthonasal/exteroceptive signals are differentially processed during the respiratory cycle, suggesting that these signals are processed in separate areas of the olfactory bulb and olfactory cortex. In this review article, the recent progress is summarized for our understanding of the olfactory circuitry and processing of odor signals during respiration.

Development of the mammalian main olfactory bulb

The mammalian main olfactory bulb is a crucial processing centre for the sense of smell. The olfactory bulb forms early during development and is functional from birth. However, the olfactory system continues to mature and change throughout life as a target of constitutive adult neurogenesis. Our Review synthesises current knowledge of prenatal, postnatal and adult olfactory bulb development, focusing on the maturation, morphology, functions and interactions of its diverse constituent glutamatergic and GABAergic cell types. We highlight not only the great advances in the understanding of olfactory bulb development made in recent years, but also the gaps in our present knowledge that most urgently require addressing.

Summary: This Review describes the morphological and functional maturation of cells in the mammalian main olfactory bulb, from embryonic development to adult neurogenesis.

Acquisition of innate odor preference depends on spontaneous and experiential activities during critical period

Animals possess an inborn ability to recognize certain odors to avoid predators, seek food, and find mates. Innate odor preference is thought to be genetically hardwired. Here we report that acquisition of innate odor recognition requires spontaneous neural activity and is influenced by sensory experience during early postnatal development. Genetic silencing of mouse olfactory sensory neurons during the critical period has little impact on odor sensitivity, discrimination, and recognition later in life. However, it abolishes innate odor preference and alters the patterns of activation in brain centers. Exposure to innately recognized odors during the critical period abolishes the associated valence in adulthood in an odor-specific manner. The changes are associated with broadened projection of olfactory sensory neurons and expression of axon guidance molecules. Thus, a delicate balance of neural activity is needed during the critical period in establishing innate odor preference and convergent axon input is required to encode innate odor valence.

Olfactory Circuitry and Behavioral Decisions

In mammals, odor information detected by olfactory sensory neurons is converted to a topographic map of activated glomeruli in the olfactory bulb. Mitral cells and tufted cells transmit signals sequentially to the olfactory cortex for behavioral outputs. To elicit innate behavioral responses, odor signals are directly transmitted by distinct subsets of mitral cells from particular functional domains in the olfactory bulb to specific amygdala nuclei. As for the learned decisions, input signals are conveyed by tufted cells as well as by mitral cells to the olfactory cortex. Behavioral scene cells link the odor information to the valence cells in the amygdala to elicit memory-based behavioral responses. Olfactory decision and perception take place in relation to the respiratory cycle. How is the sensory quality imposed on the olfactory inputs for behavioral outputs? How are the two types of odor signals, innate and learned, processed during respiration? Here, we review recent progress on the study of neural circuits involved in decision making in the mouse olfactory system.

Developmental regulation of olfactory circuit formation in mice

In mammals, odorants induce various behavioral responses that are critical to the survival of the individual and species. Binding signals of odorants to odorant receptors (ORs) expressed in the olfactory epithelia are converted to an odor map, a pattern of activated glomeruli, in the olfactory bulb (OB). This topographic map is used to identify odorants for memory‐based learned decisions. In the embryo, a coarse olfactory map is generated in the OB by a combination of dorsal‐ventral and anterior‐posterior targeting of olfactory sensory neurons (OSNs), using specific sets of axon‐guidance molecules. During the process of OSN projection, odor signals are sorted into distinct odor qualities in separate functional domains in the OB. Odor information is then conveyed by the projection neurons, mitral/tufted cells, to various regions in the olfactory cortex, particularly to the amygdala for innate olfactory decisions. Although the basic architecture of hard‐wired circuits is generated by a genetic program, innate olfactory responses are modified by neonatal odor experience in an activity‐dependent manner. Stimulus‐driven OR activity promotes post‐synaptic events and dendrite selection in the responding glomeruli making them larger. As a result, enhanced odor inputs in neonates establish imprinted olfactory memory that induces attractive responses in adults, even when the odor quality is innately aversive. In this paper, I will provide an overview of the recent progress made in the olfactory circuit formation in mice.

Lineage does not regulate the sensory synaptic input of projection neurons in the mouse olfactory bulb

Lineage regulates the synaptic connections between neurons in some regions of the invertebrate nervous system. In mammals, recent experiments suggest that cell lineage determines the connectivity of pyramidal neurons in the neocortex, but the functional relevance of this phenomenon and whether it occurs in other neuronal types remains controversial. We investigated whether lineage plays a role in the connectivity of mitral and tufted cells, the projection neurons in the mouse olfactory bulb. We used transgenic mice to sparsely label neuronal progenitors and observed that clonally related neurons receive synaptic input from olfactory sensory neurons expressing different olfactory receptors. These results indicate that lineage does not determine the connectivity between olfactory sensory neurons and olfactory bulb projection neurons.