Long-lasting changes in the anatomy of the olfactory bulb after ionizing irradiation and bone marrow transplantation.

The adult brain is considered to be a radioresistant organ since it is mainly composed of non-dividing cells. However, in adult animals there are a few neurogenic brain areas that are affected by ionizing radiation whose plasticity and capacity for recovery are still unclear. Here, mice were irradiated with a minimal lethal dose of radiation in order to determine its effects on the subventricular zone (SVZ), the rostral migratory stream (RMS), and the olfactory bulb (OB). These regions underwent a dramatic reduction in cell proliferation and ensuing morphological alterations, accompanied by a patent reactive gliosis. Bone marrow stem cell (BMSC) transplants were also performed after the radiation treatment to allow the mouse survival with a view to analyzing long-term effects. Normal proliferation rates were not recovered over time and although bone marrow-derived cells reached the brain, they were not incorporated into the SVZ-RMS-OB pathway in an attempt to rescue the damaged regions. Since neurogenesis produces new interneurones in the OB, thus feeding cell turnover, the volume and lamination of the OB were analyzed. The volume of the OB proved to be dramatically reduced at postnatal day 300 (P300), and this shrinkage affected the periependymal white matter, the granule cell layer, the external plexiform layer, and the glomerular layer. These results should be taken into account in cell therapies employing BMSC, since such cells reach the encephalon, although they cannot restore the damage produced in neurogenic areas. This study thus provides new insight into the long-term effects of ionizing radiation, widely employed in animal experimentation and even in clinical therapies for human beings.

Distribution of neurocalcin-containing neurons reveals sexual dimorphism in the mouse olfactory bulb.

Olfactory sexual dimorphism has mainly been described in the vomeronasal system, in relation to reproductive behavior, while evidence of sexual dimorphism in the main olfactory bulb (OB) remains scarce. There are no data indicating sex-related differences in the neurochemistry of intrinsic olfactory elements. Neurocalcin (NC) is a calcium-binding protein that is expressed in specific neuronal populations of the central nervous system. Here we analyzed by immunohistochemistry the NC-containing neurons in the mouse main OB, comparing both their quantities and their locations between male and female animals. NC cell density was higher in males than in females in specific locations of the glomerular layer, the external plexiform layer, the mitral cell layer, and the internal plexiform layer. This divergence in the numbers of NC cells was especially patent in central rostrocaudal levels. The NC-containing neurons exhibiting sexual divergence were identified as both juxtaglomerular and short-axon cells. This is the first description of sexual dimorphism regarding neurons belonging to the mouse main OB. According to their distribution in the OB, neurocalcin-immunoreactive interneurons could reflect a sexually dimorphic regulation of specific odorants.

Changes in cell migration and survival in the olfactory bulb of the pcd/pcd mouse.

Postnatally, the Purkinje cell degeneration mutant mice lose the main projecting neurons of the main olfactory bulb (OB): mitral cells (MC). In adult animals, progenitor cells from the rostral migratory stream (RMS) differentiate into bulbar interneurons that modulate MC activity. In the present work, we studied changes in proliferation, tangential migration, radial migration patterns, and the survival of these newly generated neurons in this neurodegeneration animal model. The animals were injected with bromodeoxyuridine 2 weeks or 2 months before killing in order to label neuroblast incorporation into the OB and to analyze the survival of these cells after differentiation, respectively. Both the organization and cellular composition of the RMS and the differentiation of the newly generated neurons in the OB were studied using specific markers of glial cells, neuroblasts, and mature neurons. No changes were observed in the cell proliferation rate nor in their tangential migration through the RMS, indicating that migrating neuroblasts are only weakly responsive to the alteration in their target region, the OB. However, the absence of MC does elicit differences in the final destination of the newly generated interneurons. Moreover, the loss of MC also produces changes in the survival of the newly generated interneurons, in accordance with the dramatic decrease in the number of synaptic targets available.

Sex differences in catechol contents in the olfactory bulb of control and unilaterally deprived rats.

The dopaminergic system plays important roles in the modulation of olfactory transmission. The present study examines the distribution of dopaminergic cells and the content of dopamine (DA) and its metabolites in control and deprived olfactory bulbs (OB), focusing on the differences between sexes. The content of DA and of its metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), were measured by HPLC. The morphology and distribution of dopaminergic neurons were studied using tyrosine hydroxylase (TH) immunohistochemistry. Cells were typified with TH-parvalbumin, TH-cholecystokinin or TH-neurocalcin double-immunofluorescence assays. Biochemical analyses revealed sex differences in the content of DA and of its metabolites. In normal conditions, the OBs of male rats had higher concentrations of DA, DOPAC and HVA than the OBs of females. The immunohistochemical data pointed to sex differences in the number of TH-immunopositive cells (higher in male than in female rats). Colocalization analyses revealed that dopaminergic cells constitute a different cell subpopulation from those labelled after parvalbumin, cholecystokinin or neurocalcin immunostaining. Unilateral olfactory deprivation caused dramatic alterations in the dopaminergic system. The DA content and the density of dopaminergic cells decreased, the contents of DA and DOPAC as well as TH immunoreactivity were similar in deprived males and females and, finally, the metabolite/neurotransmitter ratio increased. Our results show that the dopaminergic modulation of olfactory transmission seems to differ between males and females and that it is regulated by peripheral olfactory activity. A possible role of the dopaminergic system in the sexually different olfactory sensitivity, discrimination and memory is discussed.