In the adult hippocampus and olfactory bulb, neural progenitor cells generate neurons that functionally integrate into the existing circuits. To understand how neuronal differentiation occurs in the adult hippocampus, we labeled dividing progenitor cells with a retrovirus expressing green fluorescent protein and studied the morphological and functional properties of their neuronal progeny over the following weeks. During the first week neurons had an irregular shape and immature spikes and were synaptically silent. Slow GABAergic synaptic inputs first appeared during the second week, when neurons exhibited spineless dendrites and migrated into the granule cell layer. In contrast, glutamatergic afferents were detected by the fourth week in neurons displaying mature excitability and morphology. Interestingly, fast GABAergic responses were the latest to appear. It is striking that neuronal maturation in the adult hippocampus follows a precise sequence of connectivity (silent 3 slow GABA 3 glutamate 3 fast GABA) that resembles hippocampal development. We conclude that, unlike what is observed in the olfactory bulb, the hippocampus maintains the same developmental rules for neuronal integration through adulthood.

Adult neural stem cells (NSC) proliferate and differentiate depending on the composition of the cellular and molecular niche in which they are immersed. Until recently, microglial cells have been ignored as part of the neurogenic niche. We studied the dynamics of NSC proliferation and differentiation in the dentate gyrus of the hippocampus (DG) and characterized the changes of the neurogenic niche in adrenalectomized animals (ADX). At the cellular level, we found increased NSC proliferation and neurogenesis in the ADX animals. In addition, a morphologically distinct subpopulation of NSC (Nestin+/GFAP-) with increased proliferating profile was detected. Interestingly, the number of microglial cells at stages 2 and 3 of activation correlated with increased neurogenesis (r2 = 0.999) and the number of Nestin-positive cells (r2 = 0.96). At the molecular level, transforming growth factor beta (TGF-beta) mRNA levels were increased 10-fold in ADX animals. Interestingly, TGF-beta levels correlated with the amount of neurogenesis detected (r2 = 0.99) and the number of stage 2 and 3 microglial cells (r2 = 0.94). Furthermore, blockade of TGF-beta biological activity by administration of an anti-TGF-beta type II receptor antibody diminished the percentage of 5-bromo-2'-deoxyuridine (BrdU)/PSA-NCAM-positive cells in vivo. Moreover, TGF-beta was able to promote neurogenesis in NSC primary cultures. This work supports the idea that activated microglial cells are not pro- or anti-neurogenic per se, but the balance between pro- and anti-inflammatory secreted molecules influences the final effect of this activation. Importantly, we identified an anti-inflammatory cytokine, TGF-beta, with neurogenic potential in the adult brain.

Parkinson's disease is a neurodegenerative disorder with uncertain aetiology and ill-defined pathophysiology. Activated microglial cells in the substantia nigra (SN) are found in all animal models of Parkinson's disease and patients with the illness. Microglia may, however, have detrimental and protective functions in this disease. In this study, we tested the hypothesis that a sub-toxic dose of an inflammogen (lipopolysaccharide) can shift microglia to a pro-inflammatory state and exacerbate disease progression in an animal model of Parkinson's disease. Central lipopolysaccharide injection in a degenerating SN exacerbated neurodegeneration, accelerated and increased motor signs and shifted microglial activation towards a pro-inflammatory phenotype with increased interleukin-1β (IL-1β) secretion. Glucocorticoid treatment and specific IL-1 inhibition reversed these effects. Importantly, chronic systemic expression of IL-1 also exacerbated neurodegeneration and microglial activation in the SN. In vitro, IL-1 directly exacerbated 6-OHDA-triggered dopaminergic toxicity. In vivo, we found that nitric oxide was a downstream molecule of IL-1 action and partially responsible for the exacerbation of neurodegeneration observed. Thus, IL-1 exerts its exacerbating effect on degenerating dopaminergic neurons by direct and indirect mechanisms. This work demonstrates an unequivocal association between IL-1 overproduction and increased disease progression, pointing to inflammation as a risk factor for Parkinson's disease and suggesting that inflammation should be efficiently handled in patients to slow disease progression.