Photoperiodic Expression of Two RALDH Enzymes and the Regulation of Cell Proliferation by Retinoic Acid in the Rat Hypothalamus

Retinoic acid has been found to regulate hypothalamic function but precisely where it acts is unknown. This study shows expression of RALDH enzymes in tanycytes that line the third ventricle in an area overlapping with the site of hypothalamic neural stem cells. The influence of retinoic acid was examined on the proliferation of progenitors lining the third ventricle using organotypic slice cultures. As has been shown in other regions of neurogenesis, retinoic acid was found to inhibit proliferation. Investigations of the dynamics of RALDH1 expression in the rat hypothalamus have shown that this enzyme is under photoperiodic control with highest levels during long versus short days. In parallel to this shift in RA synthesis cell proliferation in the third ventricle was found to be lowest during long days, when retinoic acid was highest, implying that RALDH1 synthesized retinoic acid may regulate neural stem cell proliferation. A second RA synthesizing enzyme, RALDH2 was present in a2 tanycytes lining the third ventricle. In contrast to RALDH1, RALDH2 showed little change with photoperiodicity, but surprisingly the protein was present in the apparent absence of mRNA transcript and it is hypothesized that the endocytic tanycytes may take this enzyme up from the cerebrospinal fluid. © 2012 The Authors Journal of Neurochemistry © 2012 International Society for Neurochemistry.