Principles of Glomerular Organization in the Human Olfactory Bulb – Implications for Odor Processing

Olfactory sensory neurons (OSN) in mice express only 1 of a possible 1,100 odor receptors (OR) and axons from OSNs expressing the same odor receptor converge into ∼2 of the 1,800 glomeruli in each olfactory bulb (OB) in mice; this yields a convergence ratio that approximates 2∶1, 2 glomeruli/OR. Because humans express only 350 intact ORs, we examined human OBs to determine if the glomerular convergence ratio of 2∶1 established in mice was applicable to humans. Unexpectedly, the average number of human OB glomeruli is >5,500 yielding a convergence ratio of ∼16∶1. The data suggest that the initial coding of odor information in the human OB may differ from the models developed for rodents and that recruitment of additional glomeruli for subpopulations of ORs may contribute to more robust odor representation

Evaluation of Brain Nuclear Medicine Imaging Tracers in a Murine Model of Sepsis-Associated Encephalopathy

PURPOSE: The purpose of this study was to evaluate a set of widely used nuclear medicine imaging agents as possible methods to study the early effects of systemic inflammation on the living brain in a mouse model of sepsis-associated encephalopathy (SAE). The lipopolysaccharide (LPS)-induced murine systemic inflammation model was selected as a model of SAE. PROCEDURES: C57BL/6 mice were used. A multimodal imaging protocol was carried out on each animal 4 h following the intravenous administration of LPS using the following tracers: [(99m)Tc][2,2-dimethyl-3-[(3E)-3-oxidoiminobutan-2-yl]azanidylpropyl]-[(3E)-3-hyd roxyiminobutan-2-yl]azanide ([(99m)Tc]HMPAO) and ethyl-7-[(125)I]iodo-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4]benzodiazepine-3-carbox ylate ([(125)I]iomazenil) to measure brain perfusion and neuronal damage, respectively; 2-deoxy-2-[(18)F]fluoro-D-glucose ([(18)F]FDG) to measure cerebral glucose uptake. We assessed microglia activity on another group of mice using 2-[6-chloro-2-(4-[(125)I]iodophenyl)-imidazo[1,2-a]pyridin-3-yl]-N-ethyl-N-methyl -acetamide ([(125)I]CLINME). Radiotracer uptakes were measured in different brain regions and correlated. Microglia activity was also assessed using immunohistochemistry. Brain glutathione levels were measured to investigate oxidative stress. RESULTS: Significantly reduced perfusion values and significantly enhanced [(18)F]FDG and [(125)I]CLINME uptake was measured in the LPS-treated group. Following perfusion compensation, enhanced [(125)I]iomazenil uptake was measured in the LPS-treated group's hippocampus and cerebellum. In this group, both [(18)F]FDG and [(125)I]iomazenil uptake showed highly negative correlation to perfusion measured with ([(99m)Tc]HMPAO uptake in all brain regions. No significant differences were detected in brain glutathione levels between the groups. The CD45 and P2Y12 double-labeling immunohistochemistry showed widespread microglia activation in the LPS-treated group. CONCLUSIONS: Our results suggest that [(125)I]CLINME and [(99m)Tc]HMPAO SPECT can be used to detect microglia activation and brain hypoperfusion, respectively, in the early phase (4 h post injection) of systemic inflammation. We suspect that the enhancement of [(18)F]FDG and [(125)I]iomazenil uptake in the LPS-treated group does not necessarily reflect neural hypermetabolism and the lack of neuronal damage. They are most likely caused by processes emerging during neuroinflammation, e.g., microglia activation and/or immune cell infiltration

Triiodothyronine receptors in developing mouse neuronal and glial cell cultures and in chick-cultured neurones and astrocytes

The evolution of L-triiodothyronine (T3) receptors was studied in developing cultures of cells dissociated from cerebral hemispheres of 14-day-old mouse embryos, which present successive distinct periods of cell proliferation and/or maturation. These periods are characterized essentially as neuronal from 1 to 12 days in vitro (DIV) and glial between 12 and 60 DIV. Furthermore myelin-related membranes are produced in this culture system. Binding capacities of the T3 nuclear receptors increased from 1 to 6 DIV, when it reached a maximum (16 fmol/100 µg DNA). A similar increase of the DNA content of the cell was observed until 8 DIV. Thereafter a sharp fall of receptor concentration leading to a 5-fold decrease in the binding capacity occurred until day 15, a period at which neurones disappeared from the cultures. From 25 to 50 DIV (coinciding with the glial period), the concentration of receptor remained more or less constant (1–2 fmol/100 µg DNA). In parallel, the DNA content did not vary greatly between 30 and 50 DIV. Scatchard analysis revealed the presence of a single class of receptors at 6 and 20 DIV, representative of ''neuronal'' and ''glial'' periods, respectively. The equilibrium dissociation constant (Kd) of the nuclear receptor from cells at 6 DIV (2 x 10–10M) was similar to that found at 20 DIV. These results were confirmed using pure cultured neurones and astrocytes prepared from embryonic chick brain. The effect of T3 on the cellular gangliosides used as an index of neuronal cell maturation, and on cerebroside sulfotransferase (CST), an enzyme involved in the production of myelin sulfatides, was studied to determine a possible correlation between the binding capacity of the T3 nuclear receptor and the response of the cultured cells to thyroid hormone. Our data demonstrate that T3 had no significant effect either on the content of gangliosides or on their developmental pattern, while it increased the level of CST activity by 75% between 18 and 25 DIV. These results show that, although the concentration of T3 receptors per 100 µg DNA in glial cells was lower than that in neurones, it was nevertheless sufficient to elicit a response in oligodendrocytes.Peer reviewe

Nicotinic receptors regulate the survival of newborn neurons in the adult olfactory bulb

Cholinergic axons and nicotinic receptors are abundant in all layers of the olfactory bulb (OB), the main region of newborn neuron integration in the adult brain. Here, we report that the OB granule cell layer in mice lacking the predominant form of brain high-affinity nicotinic acetylcholine receptors (β(2)(-/-) mice) displayed nearly 50% more newborn neurons and significantly fewer apoptotic cells than did β(2)(+/+) mice. Conversely, in vivo chronic nicotine exposure significantly decreased the number of newborn granule cells in β(2)(+/+) but not β(2)(-/-) adult mice, confirming that the survival of newborn neurons can be controlled by the activation of β(2)-containing nicotinic acetylcholine receptors. Unexpectedly, investigating the behavioral consequence of an increased number of granule cells in β(2)(-/-) mice revealed that these animals have a less robust short-term olfactory memory than their wild-type counterparts. Taken together, these results provide evidence that high-affinity nicotinic receptors are involved in the maturation of adult OB local circuits. They also indicate that an increase in the number of granule cells does not necessarily correlate with better olfactory performance and further highlight the importance of cholinergic afferents for olfactory processing

Activation of adult-born neurons facilitates learning and memory

International audienceThousand of local interneurons reach the olfactory bulb of adult rodents every day, but the functional effect of this process remains elusive. By selectively expressing channelrhodopsin in postnatal-born mouse neurons, we found that their activation accelerated difficult odor discrimination learning and improved memory. This amelioration was seen when photoactivation occurred simultaneously with odor presentation, but not when odor delivery lagged by 500 ms. In addition, learning was facilitated when light flashes were delivered at 40 Hz, but not at 10 Hz. Both in vitroand in vivoelectrophysiological recordings of mitral cells revealed that 40-Hz stimuli produced enhanced GABAergic inhibition compared with 10-Hz stimulation. Facilitation of learning occurred specifically when photoactivated neurons were generated during adulthood. Taken together, our results demonstrate an immediate causal relationship between the activity of adult-born neurons and the function of the olfactory bulb circuit

Adult generation of glutamatergic olfactory bulb interneurons

The adult mouse subependymal zone (SEZ) harbors neural stem cells that are thought to exclusively generate GABAergic interneurons of the olfactory bulb. We examined the adult generation of glutamatergic juxtaglomerular neurons, which had dendritic arborizations that projected into adjacent glomeruli, identifying them as short-axon cells. Fate mapping revealed that these originate from Neurog2- and Tbr2-expressing progenitors located in the dorsal region of the SEZ. Examination of the progenitors of these glutamatergic interneurons allowed us to determine the sequential expression of transcription factors in these cells that are thought to be hallmarks of glutamatergic neurogenesis in the developing cerebral cortex and adult hippocampus. Indeed, the molecular specification of these SEZ progenitors allowed for their recruitment into the cerebral cortex after a lesion was induced. Taken together, our data indicate that SEZ progenitors not only produce a population of adult-born glutamatergic juxtaglomerular neurons, but may also provide a previously unknown source of progenitors for endogenous repair