Sensory deafferentation transsynaptically alters neuronal GluR1 expression in the external plexiform layer of the adult mouse main olfactory bulb

Fast-spiking interneuron detonation drives high-fidelity inhibition in the olfactory bulb

Inhibitory circuits in the mammalian olfactory bulb (OB) dynamically reformat olfactory information as it propagates from peripheral receptors to downstream cortex. To gain mechanistic insight into how specific OB interneuron types support this sensory processing, we examine unitary synaptic interactions between excitatory mitral and tufted cells (MTCs), the OB projection cells, and a conserved population of anaxonic external plexiform layer interneurons (EPL-INs) using pair and quartet whole-cell recordings in acute mouse brain slices. Physiological, morphological, neurochemical, and synaptic analyses divide EPL-INs into distinct subtypes and reveal that parvalbumin-expressing fast-spiking EPL-INs (FSIs) perisomatically innervate MTCs with release-competent dendrites and synaptically detonate to mediate fast, short-latency recurrent and lateral inhibition. Sparse MTC synchronization supralinearly increases this high-fidelity inhibition, while sensory afferent activation combined with single-cell silencing reveals that individual FSIs account for a substantial fraction of total network-driven MTC lateral inhibition. OB output is thus powerfully shaped by detonation-driven high-fidelity perisomatic inhibition.

A novel hypothesis for COVID-19 pathogenesis: Retinol depletion and retinoid signaling disorder

The SARS-CoV-2 virus has caused a worldwide COVID-19 pandemic. In less than a year and a half, more than 200 million people have been infected and more than four million have died. Despite some improvement in the treatment strategies, no definitive treatment protocol has been developed. The pathogenesis of the disease has not been clearly elucidated yet. A clear understanding of its pathogenesis will help develop effective vaccines and drugs. The immunopathogenesis of COVID-19 is characteristic with acute respiratory distress syndrome and multiorgan involvement with impaired Type I interferon response and hyperinflammation. The destructive systemic effects of COVID-19 cannot be explained simply by the viral tropism through the ACE2 and TMPRSS2 receptors. In addition, the recently identified mutations cannot fully explain the defect in all cases of Type I interferon synthesis. We hypothesize that retinol depletion and resulting impaired retinoid signaling play a central role in the COVID-19 pathogenesis that is characteristic for dysregulated immune system, defect in Type I interferon synthesis, severe inflammatory process, and destructive systemic multiorgan involvement. Viral RNA recognition mechanism through RIG-I receptors can quickly consume a large amount of the body's retinoid reserve, which causes the retinol levels to fall below the normal serum levels. This causes retinoid insufficiency and impaired retinoid signaling, which leads to interruption in Type I interferon synthesis and an excessive inflammation. Therefore, reconstitution of the retinoid signaling may prove to be a valid strategy for management of COVID-19 as well for some other chronic, degenerative, inflammatory, and autoimmune diseases.

Ca2+-permeable AMPA receptors in mouse olfactory bulb astrocytes

Ca²⁺ signaling in astrocytes is considered to be mainly mediated by metabotropic receptors linked to intracellular Ca²⁺ release. However, recent studies demonstrate a significant contribution of Ca²⁺ influx to spontaneous and evoked Ca²⁺ signaling in astrocytes, suggesting that Ca²⁺ influx might account for astrocytic Ca²⁺ signaling to a greater extent than previously thought. Here, we investigated AMPA-evoked Ca²⁺ influx into olfactory bulb astrocytes in mouse brain slices using Fluo-4 and GCaMP6s, respectively. Bath application of AMPA evoked Ca²⁺ transients in periglomerular astrocytes that persisted after neuronal transmitter release was inhibited by tetrodotoxin and bafilomycin A1. Withdrawal of external Ca²⁺ suppressed AMPA-evoked Ca²⁺ transients, whereas depletion of Ca²⁺ stores had no effect. Both Ca²⁺ transients and inward currents induced by AMPA receptor activation were partly reduced by Naspm, a blocker of Ca²⁺-permeable AMPA receptors lacking the GluA2 subunit. Antibody staining revealed a strong expression of GluA1 and GluA4 and a weak expression of GluA2 in periglomerular astrocytes. Our results indicate that Naspm-sensitive, Ca²⁺-permeable AMPA receptors contribute to Ca²⁺ signaling in periglomerular astrocytes in the olfactory bulb.

Mature and precursor brain-derived neurotrophic factor have individual roles in the mouse olfactory bulb

Background

Sensory deprivation induces dramatic morphological and neurochemical changes in the olfactory bulb (OB) that are largely restricted to glomerular and granule layer interneurons. Mitral cells, pyramidal-like neurons, are resistant to sensory-deprivation-induced changes and are associated with the precursor to brain-derived neurotrophic factor (proBDNF); here, we investigate its unknown function in the adult mouse OB.

Principal Findings

As determined using brain-slice electrophysiology in a whole-cell configuration, brain-derived neurotrophic factor (BDNF), but not proBDNF, increased mitral cell excitability. BDNF increased mitral cell action potential firing frequency and decreased interspike interval in response to current injection. In a separate set of experiments, intranasal delivery of neurotrophic factors to awake, adult mice was performed to induce sustained interneuron neurochemical changes. ProBDNF, but not BDNF, increased activated-caspase 3 and reduced tyrosine hydroxylase immunoreactivity in OB glomerular interneurons. In a parallel set of experiments, short-term sensory deprivation produced by unilateral naris occlusion generated an identical phenotype.

Conclusions

Our results indicate that only mature BDNF increases mitral cell excitability whereas proBDNF remains ineffective. Our demonstration that proBDNF activates an apoptotic marker in vivo is the first for any proneurotrophin and establishes a role for proBDNF in a model of neuronal plasticity.

Subventricular zone cell migration: lessons from quantitative two-photon microscopy

Neuroblasts born in the adult subventricular zone (SVZ) migrate long distances in the rostral migratory stream (RMS) to the olfactory bulbs where they integrate into circuitry as functional interneurons. As very little was known about the dynamic parameters of SVZ neuroblast migration, we used two-photon time-lapse microscopy to analyze migration in acute slices. This involved analyzing 3D stacks of images over time and uncovered several novel aspects of SVZ migration: chains remain stable, cells can be immotile for extensive periods, morphology does not necessarily correlate with motility, neuroblasts exhibit local exploratory motility, dorsoventral migration occurs throughout the striatal SVZ, and neuroblasts turn at distinctive angles. We investigated these novel findings in the SVZ and RMS from the population to the single cell level. In this review we also discuss some technical considerations when setting up a two-photon microscope imaging system. Throughout the review we identify several unsolved questions about SVZ neuroblast migration that might be addressed with current or emerging techniques.

Olfactory sensory deprivation increases the number of proBDNF-immunoreactive mitral cells in the olfactory bulb of mice

In the olfactory bulb, apoptotic cell-death induced by sensory deprivation is restricted to interneurons in the glomerular and granule cell layers, and to a lesser extent in the external plexiform layer, whereas mitral cells do not typically undergo apoptosis. With the goal to understand whether brain-derived neurotrophic factor (BDNF) mediates mitral cell survival, we performed unilateral naris occlusion on mice at postnatal day one (P1) and examined the subsequent BDNF-immunoreactive (BDNF-ir) profile of the olfactory bulb at P20, P30, and P40. Ipsilateral to the naris occlusion, there was a significant increase in the number of BDNF-ir mitral cells per unit area that was independent of the duration of the sensory deprivation induced by occlusion. The number of BDNF-ir juxtaglomerular cells per unit area, however, was clearly diminished. Western blot analysis revealed the presence of primarily proBDNF in the olfactory bulb. These data provide evidence for a neurotrophic role of proBDNF in the olfactory system of mice and suggest that proBDNF may act to protect mitral cells from the effects of apoptotic changes induced by odor sensory deprivation.

Reassembling a system from the sensor to cerebral representation: the olfactory system in vitro

An odorant's code is represented by activity in a dispersed ensemble of olfactory sensory neurons in the nose, activation of a specific combination of groups of mitral cells in the olfactory bulb and is considered to be mapped at divergent locations in the olfactory cortex. We present here an in vitro model of the mammalian olfactory system developed to gain easy access to all stations of the olfactory pathway. Mouse olfactory epithelial explants are cocultured with a brain slice that includes the olfactory bulb and olfactory cortex areas and maintains the central olfactory pathway intact and functional. Organotypicity of bulb and cortex is preserved and mitral cell axons can be traced to their target areas. Calcium imaging shows propagation of mitral cell activity to the piriform cortex. Long term coculturing with postnatal olfactory epithelial explants restores the peripheral olfactory pathway. Olfactory receptor neurons renew and progressively acquire a mature phenotype. Axons of olfactory receptor neurons grow out of the explant and rewire into the olfactory bulb. The extent of reinnervation exhibits features of a postlesion recovery. Functional imaging confirms the recovery of part of the peripheral olfactory pathway and shows that activity elicited in olfactory receptor neurons or the olfactory nerves is synaptically propagated into olfactory cortex areas. This model is the first attempt to reassemble a sensory system in culture, from the peripheral sensor to the site of cortical representation. It will increase our knowledge on how neuronal circuits in the central olfactory areas integrate sensory input and counterbalance damage.