From invertebrates to humans, it has been demonstrated that new neurons are added to specific brain structures throughout adult life. In the house cricket, adult neurogenesis occurs in the mushroom bodies, the main sensory integrative center of the brain, often considered an analogue of vertebrate hippocampus. We have previously shown that this neurogenesis can be modulated by hormones through the polyamine pathway and by environmental conditions through sensory inputs and the nitric oxide pathway. Environment-induced neurogenesis is independent of juvenile hormone levels, so we addressed the roles of sensory inputs and hormones in the control of neuroblast proliferation. Here, by using double labelling of cells specifically in S phase (5-bromo-2'-deoxyuridine) together with labelling of mitotically active cells in any phase (proliferating cell nuclear antigen), we show that juvenile hormone acts on progenitor cell proliferation by inducing quiescent neuroblasts to enter the cell cycle, whereas sensory inputs act by shortening the cell cycle. Thus, in the adult house cricket, regulation of neuroblast proliferation by hormonal and environmental cues occurs through two independent modes of action.
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