Evidence for constitutive neural cell proliferation in the adult murine hypothalamus

Compelling evidence suggests that the mammalian brain is capable of generating new neurons throughout adult life. While neurogenesis can be induced at various brain sites by exogenous cues, constitutive birth of new neurons has been unambiguously demonstrated within the subventricular zone (SVZ) of the lateral ventricles and the subgranular zone (SGZ) of the dentate gyrus. The lack of strong evidence that constitutive neurogenesis occurs elsewhere in the adult brain could be due to its exclusive restriction to the SVZ and SGZ or, for instance, to the inadequacy of the methods used to reveal new-born neurons at other brain sites. By using intracerebroventricular (icv) delivery of the mitotic marker bromodeoxyuridine (BrdU) we demonstrate that new cells are born continuously and in substantial numbers in the adult murine hypothalamus and that many of these cells appear to differentiate into neurons as assessed by the expression of doublecortin (Dcx) and other neuronal fate markers. As compared to intraperitoneal (ip) BrdU injections, central BrdU infusion also uncovers a higher-fold induction of hypothalamic cell proliferation by ciliary neurotrophic factor (CNTF). It appears that new cells are born throughout the hypothalamic parenchyma without an apparent restriction to a specific neurogenic layer, as seen in the SVZ. Thus, we provide evidence that the adult hypothalamus is constitutively neurogenic and that hypothalamic cell proliferation is highly responsive to mitogen action.