Trem2 expression in microglia is required to maintain normal neuronal bioenergetics during development.

Triggering receptor expressed on myeloid cells 2 (Trem2) is a myeloid cell-specific gene expressed in brain microglia, with variants that are associated with neurodegenerative diseases, including Alzheimer's disease. Trem2 is essential for microglia-mediated synaptic refinement, but whether Trem2 contributes to shaping neuronal development remains unclear. Here, we demonstrate that Trem2 plays a key role in controlling the bioenergetic profile of pyramidal neurons during development. In the absence of Trem2, developing neurons in the hippocampal cornus ammonis (CA)1 but not in CA3 subfield displayed compromised energetic metabolism, accompanied by reduced mitochondrial mass and abnormal organelle ultrastructure. This was paralleled by the transcriptional rearrangement of hippocampal pyramidal neurons at birth, with a pervasive alteration of metabolic, oxidative phosphorylation, and mitochondrial gene signatures, accompanied by a delay in the maturation of CA1 neurons. Our results unveil a role of Trem2 in controlling neuronal development by regulating the metabolic fitness of neurons in a region-specific manner.

Main olfactory bulb reconfiguration by prolonged passive olfactory experience correlates with increased brain-derived neurotrophic factor and improved innate olfaction.

The main olfactory bulb (MOB) is highly plastic and constantly reconfiguring its function and structure depending on sensory experience. Despite the extensive evidence of anatomical, functional and behavioural changes in the olfactory system induced by highly variable olfactory experiences, it is still unknown whether prolonged passive odour experience could reconfigure the MOB at its input and network activity levels and whether these changes impact innate olfaction. Here, by measuring odour-induced glomerular activation, MOB network activity and innate olfactory behaviours, we described a profound MOB reconfiguration induced by prolonged passive olfactory experience in adult animals that impacts MOB input integration at the glomerular layer including an increase in the activated glomerular area and signal intensity, which is combined with a refinement in the number of activated glomeruli and less-overlapped glomerular maps. We also found that prolonged passive olfactory experience dramatically changes MOB population activity in the presence and absence of odours, which is reflected as a decrease in slow oscillations (<12 Hz) and an increase in fast oscillations (>12 Hz). All these functional changes in awake and anaesthetized mice correlate with an increase in brain-derived neurotrophic factor (BDNF) and with improved innate olfactory responses such as habituation/dishabituation and innate preference/avoidance. Our study shows that prolonged passive olfactory experience in adult animals produces a dramatic reconfiguration of the MOB network, possibly driven by BDNF, that improves innate olfactory responses.

Sudden Intrabulbar Amyloid Increase Simultaneously Disrupts Olfactory Bulb Oscillations and Odor Detection.

There seems to be a correlation between soluble amyloid beta protein (Aß) accumulation in the main olfactory bulb (OB) and smell deterioration in both Alzheimer's disease (AD) patients and animal models. Moreover, this loss of smell appears to be related to alterations in neural network activity in several olfactory-related circuits, including the OB, as has been observed in anesthetized animals and brain slices. It is possible that there is a correlation between these two pathological phenomena, but a direct and simultaneous evaluation of the acute and direct effect of Aß on OB activity while animals are actually smelling has not been performed. Thus, here, we tested the effects of acute intrabulbar injection of Aß at a low dose (200 pmol) on the OB local field potential before and during the presence of a hidden piece of smelly food. Our results show that Aß decreases the power of OB network activity while impairing the animal's ability to reach the hidden food. We found a strong relationship between the power of the OB oscillations and the correlation between OBs and the olfactory detection test scores. These findings provide a direct link between Aß-induced OB network dysfunction and smell loss in rodents, which could be extrapolated to AD patients.

Chronic intermittent hypoxia alters main olfactory bulb activity and olfaction.

Olfactory dysfunction is commonly observed in patients with obstructive sleep apnea (OSA), which is related to chronic intermittent hypoxia (CIH). OSA patients exhibit alterations in discrimination, identification and odor detection threshold. These olfactory functions strongly rely on neuronal processing within the main olfactory bulb (MOB). However, a direct evaluation of the effects of controlled CIH on olfaction and MOB network activity has not been performed. Here, we used electrophysiological field recordings in vivo to evaluate the effects of 21-day-long CIH on MOB network activity and its response to odors. In addition, we assessed animals´ olfaction with the buried food and habituation/dishabituation tests. We found that mice exposed to CIH show alterations in MOB spontaneous activity in vivo, consisting of a reduction in beta and gamma frequency bands power along with an increase in the theta band power. Likewise, the MOB was less responsive to odor stimulation, since the proportional increase of the power of its population activity in response to four different odorants was smaller than the one observed in control animals. These CIH-induced MOB functional alterations correlate with a reduction in the ability to detect, habituate and discriminate olfactory stimuli. Our findings indicate that CIH generates alterations in the MOB neural network, which could be involved in the olfactory deterioration in patients with OSA.

Alterations in Piriform and Bulbar Activity/Excitability/Coupling Upon Amyloid-ß Administration in vivo Related to Olfactory Dysfunction.

BACKGROUND: Deficits in odor detection and discrimination are premature symptoms of Alzheimer's disease (AD) that correlate with pathological signs in the olfactory bulb (OB) and piriform cortex (PCx). Similar olfactory dysfunction has been characterized in AD transgenic mice that overproduce amyloid-ß peptide (Aß), which can be prevented by reducing Aß levels by immunological and pharmacological means, suggesting that olfactory dysfunction depends on Aß accumulation and Aß-driven alterations in the OB and/or PCx, as well as on their activation. However, this possibility needs further exploration. OBJECTIVE: To characterize the effects of Aß on OB and PCx excitability/coupling and on olfaction. METHODS: Aß oligomerized solution (containing oligomers, monomers, and protofibrils) or its vehicle were intracerebroventricularlly injected two weeks before OB and PCx excitability and synchrony were evaluated through field recordings in vivo and in brain slices. Synaptic transmission from the OB to the PCx was also evaluated in slices. Olfaction was assessed through the habituation/dishabituation test. RESULTS: Aß did not affect lateral olfactory tract transmission into the PCx but reduced odor habituation and cross-habituation. This olfactory dysfunction was related to a reduction of PCx and OB network activity power in vivo. Moreover, the coherence between PCx-OB activities was also reduced by Aß. Finally, Aß treatment exacerbated the 4-aminopyridine-induced excitation in the PCx in slices. CONCLUSION: Our results show that Aß-induced olfactory dysfunction involves a complex set of pathological changes at different levels of the olfactory pathway including alterations in PCx excitability and its coupling with the OB. These pathological changes might contribute to hyposmia in AD.

Chronic intermittent hypoxia transiently increases hippocampal network activity in the gamma frequency band and 4-Aminopyridine-induced hyperexcitability in vitro.

Chronic intermittent hypoxia (CIH) is the most distinct feature of obstructive sleep apnea (OSA), a common breathing and sleep disorder that leads to several neuropathological consequences, including alterations in the hippocampal network and in seizure susceptibility. However, it is currently unknown whether these alterations are permanent or remit upon normal oxygenation. Here, we investigated the effects of CIH on hippocampal spontaneous network activity and hyperexcitability in vitro and explored whether these alterations endure or fade after normal oxygenation. Results showed that applying CIH for 21 days to adult rats increases gamma-band hippocampal network activity and aggravates 4-Aminopyridine-induced epileptiform activity in vitro. Interestingly, these CIH-induced alterations remit after 30 days of normal oxygenation. Our findings indicate that hippocampal network alterations and increased seizure susceptibility induced by CIH are not permanent and can be spontaneously reverted, suggesting that therapeutic interventions against OSA in patients with epilepsy, such as surgery or continuous positive airway pressure (CPAP), could be favorable for seizure control.

Tolfenamic Acid Prevents Amyloid ß-induced Olfactory Bulb Dysfunction In Vivo.

BACKGROUND: Amyloid beta inhibits olfactory bulb function. The mechanisms involved in this effect must include alterations in network excitability, inflammation and the activation of different transduction pathways. Thus, here we tested whether tolfenamic acid, a drug that modulates several of these pathological processes, could prevent amyloid beta-induced olfactory bulb dysfunction. OBJECTIVE: To test whether tolfenamic acid prevents amyloid beta-induced alterations in olfactory bulb network function, olfaction and GSK3ß activity. METHOD: The protective effects of tolfenamic acid against amyloid beta-induced population activity inhibition were tested in olfactory bulb slices from adult mice, while tolfenamic acid and amyloid beta were bath-applied. We also tested the effects of amyloid-beta in slices obtained from animals pre-treated chronically (21 days) with tolfenamic acid. The effects of amyloid beta micro-injected into the olfactory bulbs were also tested, after two weeks, on olfactory bulb population activity and olfaction in control and tolfenamic acid chronically treated animals. Olfaction was assessed with the odor-avoidance and the habituation/cross-habituation tests. GSK3ß activation was evaluated with Western-blot. RESULTS: Acute bath application of tolfenamic acid does not prevent amyloid beta-induced inhibition of olfactory bulb network activity in vitro. In contrast, chronic treatment with tolfenamic acid renders the olfactory bulb resistant to amyloid beta-induced network activity inhibition in vitro and in vivo, which correlates with the inhibition of GSK3ß activation and the protection against amyloid beta-induced olfactory dysfunction. CONCLUSION: Our data further support the use of tolfenamic acid to prevent amyloid beta-induced pathology and the early symptoms of Alzheimer Disease.