Postnatal Development of Centrifugal Inputs to the Olfactory Bulb

Olfactory bulb activity shapes the development of entorhinal-hippocampal coupling and associated cognitive abilities

The interplay between olfaction and higher cognitive processing has been documented in the adult brain; however, its development is poorly understood. In mice, shortly after birth, endogenous and stimulus-evoked activity in the olfactory bulb (OB) boosts the oscillatory entrainment of downstream lateral entorhinal cortex (LEC) and hippocampus (HP). However, it is unclear whether early OB activity has a long-lasting impact on entorhinal-hippocampal function and cognitive processing. Here, we chemogenetically silenced the synaptic outputs of mitral/tufted cells, the main projection neurons in the OB, during postnatal days 8-10. The transient manipulation leads to a long-lasting reduction of oscillatory coupling and weaker responsiveness to stimuli within developing entorhinal-hippocampal circuits accompanied by dendritic sparsification of LEC pyramidal neurons. Moreover, the transient silencing reduces the performance in behavioral tests involving entorhinal-hippocampal circuits later in life. Thus, neonatal OB activity is critical for the functional LEC-HP development and maturation of cognitive abilities.

Olfactory-driven beta band entrainment of limbic circuitry during neonatal development

Cognitive processing relies on the functional refinement of the limbic circuitry during the first two weeks of life. During this developmental period, when the auditory, somatosensory and visual systems are still largely immature, the sense of olfaction acts as 'door to the world', providing an important source of environmental inputs. However, it is unknown whether early olfactory processing shapes the activity in the limbic circuitry during neonatal development. Here, we address this question by combining simultaneous in vivo recordings from the olfactory bulb (OB), lateral entorhinal cortex (LEC), hippocampus (HP) and prefrontal cortex (PFC) with olfactory stimulation as well as opto- and chemogenetic manipulations of mitral/tufted cells in the OB of non-anaesthetized neonatal mice of both sexes. We show that the neonatal OB synchronizes the limbic circuity in the beta frequency range. Moreover, it drives neuronal and network activity in LEC, as well as subsequently, HP and PFC via long-range projections from mitral cells to HP-projecting LEC neurons. Thus, OB activity shapes the communication within limbic circuits during neonatal development. KEY POINTS: During early postnatal development, oscillatory activity in the olfactory bulb synchronizes the limbic circuit. Olfactory stimulation boosts firing and beta synchronization along the olfactory bulb-lateral entorhinal cortex-hippocampal-prefrontal pathway. Mitral cells drive neuronal and network activity in the lateral entorhinal cortex (LEC), as well as subsequently, the hippocampus (HP) and the prefrontal cortex (PFC) via long-range projections from mitral cells to HP-projecting LEC neurons. Inhibition of vesicle release on LEC targeting mitral cell axons reveals direct involvement of LEC in the olfactory bulb-driven oscillatory entrainment of the limbic circuitry.

Olfactory-auditory sensory integration in the lateral entorhinal cortex

Multisensory integration plays an important role in animal cognition. Although many studies have focused on visual-auditory integration, studies on olfactory-auditory integration are rare. Here, we investigated neural activity patterns and odor decoding in the lateral entorhinal cortex (LEC) under uni-sensory and multisensory stimuli in awake, head-fixed mice. Using specific retrograde tracing, we verified that the LEC receives direct inputs from the primary auditory cortex (AC) and the medial geniculate body (MGB). Strikingly, we found that mitral/tufted cells (M/Ts) in the olfactory bulb (OB) and neurons in the LEC respond to both olfactory and auditory stimuli. Sound decreased the neural responses evoked by odors in both the OB and LEC, for both excitatory and inhibitory responses. Interestingly, significant changes in odor decoding performance and modulation of odor-evoked local field potentials (LFPs) were observed only in the LEC. These data indicate that the LEC is a critical center for olfactory-auditory multisensory integration, with direct projections from both olfactory and auditory centers.

How the sense of smell influences cognition throughout life

Although mostly unaware, we constantly navigate a complex landscape of airborne molecules. The perception of these molecules helps us navigate, shapes our social life, and can trigger emotionally charged memories transporting us back to the past within a split second. While the processing of olfactory information in early sensory areas is well understood, how the sense of smell affects cognition only recently gained attention in the field of neuroscience. Here, we review links between olfaction and cognition and explore the idea that the activity in olfactory areas may be critical for coordinating cognitive networks. Further, we discuss how olfactory activity may shape the development of cognitive networks and associations between the decline of olfactory and cognitive abilities in aging. Olfaction provides a great tool to study large-scale networks underlying cognitive abilities and bears the potential for a better understanding of cognitive symptoms associated with many mental disorders.

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.