Interhemispheric olfactory circuit and the memory beyond

Laminar organization of the anterior olfactory nucleus-the interplay between neurogenesis timing and neuroblast migration

Introduction

The anterior olfactory nucleus (AON) is a laminar structure embedded within the olfactory peduncle which serves as the conduit for connectivity between the olfactory bulb (OB) and the central processing centers of the brain. The largest portion of the AON is a ring of neurons and fibers that surround the core of the peduncle, the pars principalis (AONpP). The AONpP is further subdivided into an outer plexiform layer, or layer 1 (L1), that contains axons and dendrites, and an inner cell zone, or layer 2 (L2), formed by densely packed pyramidal cells. Relative to other regions of the olfactory system, the development of the AON remains poorly understood.

Methods

We performed injections of thymidine analogs in pregnant mice from E10 to E18 to determine the timeline of AON neurogenesis and used immunohistochemistry to study neuronal phenotypes both at adult and embryonic stages. To better understand migration and differentiation of the AON neurons, we labeled AON precursors using in utero electroporations with the piggyBac transposon into the rostral lateral ganglionic eminence, the embryonic source of AON neurons.

Results

Our analyses established that the earliest neurons targeted to the AON laminae arose at E10 with neurogenesis peaking at E13. In L1, we found a caudal-to-rostral neurogenic gradient not detected in L2. Quantification across the cardinal axes showed no gradients in L2 and a medial-to-lateral gradient for L1. Using immunohistochemistry, we found that AON neurons express the most common cortical markers Tbr1, Ctip2, NeuroD1, Sox5 and Cux1+2 at adult stages without laminar distinction. Tbr1 and NeuroD1 first appeared embryonically at E12, while Ctip2 and Sox5 were present at E13, following a dorsal-ventral pattern. Cux1+2 was not detected embryonically. Embryonically, our data on neuroblasts migration revealed that AON neuroblasts use a scaffold of radial glia to migrate to their final destinations in both L1 and L2 through a caudal-to-rostral migratory gradient.

Conclusion

For the first time, our data show a comprehensive timeline for the AON neurogenesis across the anatomical axes, and a detailed analysis on neuroblast migration in the mouse embryo. These data are crucial to understanding the embryonic formation and relationship of relay stations along the olfactory pathway.

Interhemispheric asymmetry of c-Fos expression in glomeruli and the olfactory tubercle following repeated odor stimulation

Odor adaptation allows the olfactory system to regulate sensitivity to different stimulus intensities, which is essential for preventing saturation of the cell‐transducing machinery and maintaining high sensitivity to persistent and repetitive odor stimuli. Although many studies have investigated the structure and mechanisms of the mammalian olfactory system that responds to chemical sensation, few studies have considered differences in neuronal activation that depend on the manner in which the olfactory system is exposed to odorants, or examined activity patterns of olfactory‐related regions in the brain under different odor exposure conditions. To address these questions, we designed three different odor exposure conditions that mimicked diverse odor environments and analyzed c‐Fos‐expressing cells (c‐Fos+ cells) in the odor columns of the olfactory bulb (OB). We then measured differences in the proportions of c‐Fos‐expressing cell types depending on the odor exposure condition. Surprisingly, under the specific odor condition in which the olfactory system was repeatedly exposed to the odorant for 1 min at 5‐min intervals, one of the lateral odor columns and the ipsilateral hemisphere of the olfactory tubercle had more c‐Fos+ cells than the other three odor columns and the contralateral hemisphere of the olfactory tubercle. However, this interhemispheric asymmetry of c‐Fos expression was not observed in the anterior piriform cortex. To confirm whether the anterior olfactory nucleus pars externa (AONpE), which connects the left and right OB, contributes to this asymmetry, AONpE‐lesioned mice were analyzed under the specific odor exposure condition. Asymmetric c‐Fos expression was not observed in the OB or the olfactory tubercle. These data indicate that the c‐Fos expression patterns of the olfactory‐related regions in the brain are influenced by the odor exposure condition and that asymmetric c‐Fos expression in these regions was observed under a specific odor exposure condition due to synaptic linkage via the AONpE.

Unilateral olfactory deficit in a hemiparkinson's disease mouse model

Parkinson's disease is a neurodegenerative disease which often presents hyposmia (80-90% of the cases). We characterized the olfactory behavior in the model of 6-hydroxydopamine of Parkinson's disease. Mice were trained to discriminate between two odorants in a radial maze. One of the odorants was associated with water as a reward. 6-hydroxydopamine was injected directly into the dorsal striatum; after complete striatal denervation, olfactory performance was evaluated in a radial maze. In the first evaluation, experimental mice performed as control mice. After the first evaluation, the narine of the contralateral side to the striatal injection was closed and mice were evaluated again. The experimental group completely lost the capacity to discriminate between the odorant associated with the reward (heptaldehyde) and the unconditioned odorant (2-heptanone). We propose that the olfactory deficit was caused by dopaminergic denervation to the olfactory tubercle and nucleus accumbens.

Construction of functional neuronal circuitry in the olfactory bulb

Recent studies using molecular genetics, electrophysiology, in vivo imaging, and behavioral analyses have elucidated detailed connectivity and function of the mammalian olfactory circuits. The olfactory bulb is the first relay station of olfactory perception in the brain, but it is more than a simple relay: olfactory information is dynamically tuned by local olfactory bulb circuits and converted to spatiotemporal neural code for higher-order information processing. Because the olfactory bulb processes ∼1000 discrete input channels from different odorant receptors, it serves as a good model to study neuronal wiring specificity, from both functional and developmental aspects. This review summarizes our current understanding of the olfactory bulb circuitry from functional standpoint and discusses important future studies with particular focus on its development and plasticity.

Development of a MR-visible compound for tracing neuroanatomical connections in vivo

Traditional studies of neuroanatomical connections require injection of tracer compounds into living brains, then histology of the postmortem tissue. Here, we describe and validate a compound that reveals neuronal connections in vivo, using MRI. The classic anatomical tracer CTB (cholera-toxin subunit-B) was conjugated with a gadolinium-chelate to form GdDOTA-CTB. GdDOTA-CTB was injected into the primary somatosensory cortex (S1) or the olfactory pathway of rats. High-resolution MR images were collected at a range of time points at 11.7T and 7T. The transported GdDOTA-CTB was visible for at least 1 month post-injection, clearing within 2 months. Control injections of non-conjugated GdDOTA into S1 were not transported and cleared within 1-2 days. Control injections of Gd-Albumin were not transported either, clearing within 7 days. These MR results were verified by classic immunohistochemical staining for CTB, in the same animals. The GdDOTA-CTB neuronal transport was target specific, monosynaptic, stable for several weeks, and reproducible.

State-dependent coherences between the olfactory bulbs for delta and theta oscillations

Correlations of neuronal oscillations in different brain regions are closely related to a variety of brain functions. The delta and theta oscillations in the olfactory bulb (OB), coupled with respiration rhythm, could play important roles in olfactory tasks. The correlations between the two OBs, however, are largely unknown. By simultaneously recording local field potentials from the OBs, we found that for these oscillations, the intrabulbar coherences were high and state-independent, while the interbulbar coherences were also high but state-dependent. Higher activity states, generated by lighter depth of anesthesia or peripheral odor stimulation, have lower coherence, suggesting that the high interbulbar correlations can be modulated by both internal and external factors.

Achieving singularity in mammalian odorant receptor gene choice

Odor discrimination requires differential expression of odor detectors. In fact, olfactory input to the brain is organized in units (glomeruli) innervated only by olfactory sensory neurons that express the same odorant receptor (OR). Therefore, discriminatory capacity is maximized if each sensory neuron expresses only one allele of a single OR gene, a postulate sometimes canonized as the "one neuron-one receptor rule." OR gene choice appears to result from a hierarchy of processes: differential availability of the alleles of each OR gene, zonal exclusion (or selection), OR gene switching during the initiation of OR gene transcription, and OR-dependent feedback to solidify the choice of one OR gene. The mechanisms underlying these processes are poorly understood, though a few elements are known or suspected. For example, the mechanism of activation of OR gene transcription appears to work in part through a few homeobox transcription factors (Emx2, and perhaps Lhx2) and the Ebf family of transcription factors. Further insights will probably come from several directions, but a promising hypothesis is that epigenetic mechanisms contribute to all levels of the hierarchical control of OR gene expression, especially the repressive events that seem to be necessary to achieve the singularity of OR gene choice.

Wiring Olfaction: The Cellular and Molecular Mechanisms that Guide the Development of Synaptic Connections from the Nose to the Cortex

Within the central nervous system, the olfactory system fascinates by its developmental and physiological particularities, and is one of the most studied models to understand the mechanisms underlying the guidance of growing axons to their appropriate targets. A constellation of contact-mediated (laminins, CAMs, ephrins, etc.) and secreted mechanisms (semaphorins, slits, growth factors, etc.) are known to play different roles in the establishment of synaptic interactions between the olfactory epithelium, olfactory bulb (OB) and olfactory cortex. Specific mechanisms of this system (including the amazing family of about 1000 different olfactory receptors) have been also proposed. In the last years, different reviews have focused in partial sights, specially in the mechanisms involved in the formation of the olfactory nerve, but a detailed review of the mechanisms implicated in the development of the connections among the different olfactory structures (olfactory epithelium, OB, olfactory cortex) remains to be written. In the present work, we afford this systematic review: the different cellular and molecular mechanisms which rule the formation of the olfactory nerve, the lateral olfactory tract and the intracortical connections, as well as the few data available regarding the accessory olfactory system. These mechanisms are compared, and the implications of the differences and similarities discussed in this fundamental scenario of ontogeny.