2013 Chronobiology International, 2013; 30(9): 1089–1100 ! Informa Healthcare USA, Inc. ISSN: 0742-0528 print / 1525-6073 online DOI: 10.3109/07420528.2013.800090 Photoperiod affects the diurnal rhythm of hippocampal neuronal morphology of siberian hamsters Tomoko Ikeno, Zachary M. Weil, and Randy J. Nelson Department of Neuroscience, The Ohio State University Wexner Medical Center, Columbus, Ohio, USA Individuals of many species can regulate their physiology, morphology, and behavior in response to annual changes of day length (photoperiod). In mammals, the photoperiodic signal is mediated by a change in the duration of melatonin, leading to alterations in gene expressions, neuronal circuits, and hormonal secretion. The hippocampus is one of the most plastic structures in the adult brain and hippocampal neuronal morphology displays photoperiod- induced differences. Because the hippocampus is important for emotional and cognitive behaviors, photoperiod- driven remodeling of hippocampal neurons is implicated in seasonal differences of affect, including seasonal affective disorder (SAD) in humans. Because neuronal architecture is also affected by the day-night cycle in several brain areas, we hypothesized that hippocampal neuronal morphology would display a diurnal rhythm and that day length would influence that rhythm. In the present study, we examined diurnal and seasonal differences in hippocampal neuronal morphology, as well as mRNA expression of the neurotrophic factors (i.e., brain-derived neurotrophic factor [Bdnf], tropomyosin receptor kinase B [trkB; a receptor for BDNF], and vascular endothelial growth factor [Vegf]) and a circadian clock gene, Bmal1, in the hippocampus of Siberian hamsters. Diurnal rhythms in total length of dendrites, the number of primary dendrites, dendritic complexity, and distance of the furthest intersection from the cell body were observed only in long-day animals; however, diurnal rhythms in the number of branch points and mean length of segments were observed only in short-day animals. Spine density of dendrites displayed diurnal rhythmicity with different peak times between the CA1 and DG subregions and between long and short days. These results indicate that photoperiod affects daily morphological changes of hippocampal neurons and the daily rhythm of spine density, suggesting the possibility that photoperiod-induced adjustments of hippocampal neuronal dynamics might underlie seasonal difference of affective responses. Bmal1 mRNA showed a diurnal rhythm and different expression levels between long and short days were observed. However, there were no strong effects of day length on Bdnf, trkb, and Vegf gene expression, suggesting that these genes are not involved in the photoperiodic effects on hippocampal neurons. Keywords: Animal models, BDNF, circadian clock, day length, diurnal rhythm, seasonal affective disorder, seasonality, Siberian hamsters, trkb, VEGF INTRODUCTION Many organisms are able to respond to annual cycles of day length (photoperiod) by physiological, morpho- logical, and behavioral changes in anticipation of approaching seasons (Goldman, 2001). In response to short day lengths, many small rodents cease reproduct- ive activities to avoid producing offspring during the winter when temperatures and food availability are challenging. The photoperiodic response is mediated by a change in the duration of nocturnal hormone, mela- tonin (Pe ´vet, 1988). Melatonin secretion from the pineal gland is regulated by a circadian clock, which is an internal daily timekeeping system, and is directly sup- pressed by light (Goldman, 2001). In addition to the reproductive traits, seasonal changes of brain morphology (such as brain volume and neuronal architecture) have also been reported for individuals of some vertebrate species. In songbirds, for example, the volumes of brain regions that control song production increase dramatically in anticipation of the breeding season (Brenowitz et al., 1997; Smith et al., 1997; Tramontin et al., 2001). These volumetric changes are mediated by increases in neuronal size, number, and spacing (reviewed in Tramontin & Brenowitz, 2000). In mammals, seasonal differences have been observed in various brain regions, especially in the hippocampus, which is one of the most plastic sites in adult mamma- lian brains (Breedlove & Jordan, 2001). Wild-captured rodents of many species display seasonal variation in Correspondence: Tomoko Ikeno, Department of Neuroscience, The Ohio State University, Columbus, OH 43210, USA. Tel. +1 614 688 4674; Fax: +1 614 292 3464; E-mail: tomoko.ikeno@ousmc.edu Submitted March 6, 2013, Returned for revision April 12, 2013, Accepted April 24, 2013 1089 Chronobiol Int Downloaded from informahealthcare.com by Mr. Mitchell Gang on 01/06/14 For personal use only.