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Background  Fibroblast growth factors (FGFs) are crucial signaling molecules that direct the development of the vertebrate brain. FGF8 gene signaling in particular, may be important for the development of the hypothalamus–pituitary–adrenal (HPA)-axis. Indeed, newborn  Fgf8  hypomorphic mice harbor a major reduction in the number of vasopressin (VP) neurons in the paraventricular nucleus (PVN), the central output component of the HPA-axis. Additionally, recent studies indicated that adult heterozygous ( +/neo )  Fgf8  hypomorphic mice exhibit more anxiety-like behaviors than wildtype (WT) mice. These studies led us to investigate whether  Fgf8  hypomorphy abrogated VP and/or corticotropin-releasing hormone (CRH) neuronal development in the postnatal day (PN) 21 and adult mouse PVN. Furthermore, we studied whether  Fgf8  hypomorphy disrupted HPA responsiveness in these mice.    Methods  Using immunohistochemistry, we examined the development of VP and CRH neurons located in the PVN of PN 21 and adult  Fgf8  +/neo  mice. Moreover, we used a restraint stress (RS) paradigm and measured corticosterone levels with enzyme immunoassays in order to assess HPA axis activation.    Results  The number of VP neurons in the PVN did not differ between WT and  Fgf8  +/neo  mice on PN 21 and in adulthood. In contrast, CRH immunoreactivity was much higher in  Fgf8  +/neo  mice than in WT mice on PN 21, this difference was no longer shown in adult mice. RS caused a higher increase in corticosterone levels in adult  Fgf8  +/neo  mice than in WT mice after 15 min, but no difference was seen after 45 min.    Conclusions  First,  Fgf8  hypomorphy did not eliminate VP and CRH neurons in the mouse PVN, but rather disrupted the postnatal timing of neuropeptide expression onset in PVN neurons. Second,  Fgf8  hypomorphy may, in part, be an explanation for affective disorders involving hyperactivity of the HPA axis, such as anxiety.

Fibroblast growth factor 8 regulates postnatal development of paraventricular nucleus neuroendocrine cells