Impact of melatonin receptor‐signaling on Zeitgeber time‐dependent changes in cell proliferation and apoptosis in the adult murine hippocampus

Abstract

The hippocampus is subjected to diurnal/circadian rhythms on both the morphological and molecular levels. Certain aspects of cell proliferation in the adult hippocampus are regulated by melatonin and accompanied by apoptosis to ensure proper tissue maintenance and function. The present study investigated Zeitgeber time (ZT)-dependent changes in cell proliferation and apoptosis in the adult murine hippocampus and their regulation by melatonin receptor type1 and type2 (MT1/2)-mediated signaling. Adult melatonin-proficient C3H/HeN mice and melatonin-proficient (C3H/HeN) mice with targeted deletion of MT1/2 were adapted to a 12-h light, 12-h dark photoperiod and were sacrificed at ZT00, ZT06, ZT12, and ZT18. Immunohistochemistry for Ki67 and activated caspase-3 in combination with different markers for the diverse cell types residing in the hippocampus served to identify and quantify proliferating and apoptotic cells in the hippocampal subregions. ZT-dependent changes in cell proliferation and apoptosis were found exclusively in the subgranular zone (SGZ) and granule cell layer (GCL) of melatonin-proficient mice with functional MT1/2. Cell proliferation in the SGZ showed ZT-dependent changes indicated by an increase of proliferating immature neurons during the dark phase of the 24-h light-dark cycle. Apoptosis showed ZT-dependent changes in the SGZ and GCL indicated by an increase of apoptotic immature neurons at ZT06 (SGZ) and a decrease of immature and mature neurons at ZT18 (GCL). Our results indicate that ZT-dependent changes in proliferation of immature neurons in the SGZ are counterbalanced by ZT-dependent changes in apoptosis of immature and mature neurons in the SGZ and GCL exclusively in mice with functional MT1/2. Therefore, MT1/2-mediated signaling appears to be crucial for generation and timing of ZT-dependent changes in cell proliferation and apoptosis and for differentiation of proliferating cells into neurons in the SGZ. © 2017 Wiley Periodicals, Inc.

Differential Regulation of Cell Proliferation and Apoptosis by Melatonin Receptor Subtype-Signaling in the Adult Murine Brain

“…MT1 KO mice displayed ZT-dependent changes in cell proliferation in the SGZ and ME similar to those previously found in C3H mice with functional MT1/2 [13,14] (Fig. 1a, c).…”

“…As previously shown for C3H and MT1/2 KO mice [13,14], cell proliferation in the GCL and PT did not change significantly between the different ZTs, neither in MT1 KO mice nor in MT2 KO mice (Fig. 1b, d).…”

“…Melatonin and its receptors exert distinct effects on adult cell proliferation, differentiation, and survival in the hippocampus [20,[53][54][55][56][57] but also on apoptosis [58][59][60]. In two studies preceding and complementing the present study, we showed that Zeitgeber time (ZT)-dependent changes in cell proliferation and apoptosis occur exclusively in melatonin-proficient C3H/Hen (C3H) mice with functional MT1/2s, while C3H mice lacking both functional melatonin receptor subtypes (MT1/2 KO) showed no such changes in the hippocampus and hypothalamic-hypophyseal system of adult mice [13,14]. Therefore, the goal of the present study was to unravel which MT subtype is required or sufficient for ZT-dependent changes in cell proliferation and apoptosis in the adult hippocampus and hypothalamic-hypophyseal system.…”

“…By now, it is well known that the subventricular zone of the lateral ventricle and the subgranular zone (SGZ) of the hippocampal dentate gyrus generate new cells not only during ontogeny but also in the adult brain [2,3]. However, constitutive cell proliferation and apoptosis have also been found in the adult hypothalamic-hypophyseal system, namely the neuronfree median eminence (ME) at the base of the mediobasal hypothalamus, and the pars tuberalis (PT) and pars distalis of the pituitary [4][5][6][7][8][9][10][11][12][13][14].…”

“…Therefore, the goal of the present study was to unravel which MT subtype is required or sufficient for ZT-dependent changes in cell proliferation and apoptosis in the adult hippocampus and hypothalamic-hypophyseal system. To this end, cell proliferation and apoptosis were analyzed in the brain and pituitary of adult melatonin-proficient MT1 KO and MT2 KO mice and compared with those of adult melatonin-proficient C3H mice with functional MTs and MT1/2 KO mice [13,14].…”