Photoperiod and Brain Blood Flow: Changing Day Length to Alter Cerebral Perfusion
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Material below summarizes the article Photoperiodic Regulation of Cerebral Blood Flow in White-footed Mice (Peromyscus leucopus), published on July 13, 2016, in eNeuro and authored by Jeremy C. Borniger, Seth Teplitsky, Surya Gnyawali, Randy J Nelson, and Cameron Rink.
Animals that live in nontropical climates need to adjust their behavior and physiology throughout the year to survive in the face of a cyclically changing environment. To save energy, organisms reroute resources to necessary functions in anticipation of unfavorable conditions. Many small animals, for example, turn off reproduction in favor of thermogenesis during the harsh winter months to ensure survival into the next breeding season. Because the brain is an energetically expensive organ, any reduction in size or blood flow to this organ would confer significant energy savings, allowing individuals to invest resources into vital functions such as immunity or keeping warm.
How do animals tell the time of year, and how do they ‘know’ when winter is coming? Ambient temperature, rainfall, barometric pressure, food availability, and other important environmental cues can vary widely over short time spans making them relatively useless for predicting time of year. Therefore, individuals of many species track seasons by monitoring a stable signal: day length (i.e., photoperiod).
Two bits of information, the absolute day length, and whether day lengths are increasing or decreasing, signal time of year and whether winter is approaching or receding. This information is coded in the body by the hormonal melatonin signal, which is suppressed by light and secreted exclusively at night. Using the information from duration of melatonin secretion animals can measure day length (an ability called photoperiodism) and respond to the shortening days of fall by reorganizing their physiology and behavior. The reproductive systems of some individuals do not respond to short days, and continue to be capable of breeding, but maintain other winter adaptations. White-footed mice (Peromyscus leucopus) are photoperiodic rodents that, in response to short days, alter their metabolism, immune capacity, and cognitive function. Specifically, short days are associated with reduced learning and memory, hippocampal neurogenesis, and impaired long-term potentiation (LTP) along the Shaffer-collateral-CA1 pathway.
What could be causing these deficits in cognitive capacity in response to short days? We hypothesized that changes in learning and memory would be preceded by a reduction in blood flow to the hippocampus, a primary brain structure that controls spatial memory formation. To assess this, we kept adult male white-footed mice in short (8 hours light: 16 hours dark) or long (16:8) day conditions for 8 weeks, and then assessed blood vessel distribution, flow, and gene expression in the hippocampus. We used fluorescently-tagged (FITC) lectin to label active vasculature by injecting it into live mice. After allowing some time for it to circulate, we collected brain tissue onto slides treated with RNAse inhibitors to prevent RNA degradation. Under a microscope, we were able to visualize where the lectin had circulated (it reaches all active capillaries), and use a laser to cut out specific blood vessels within the hippocampus.
We then extracted RNA from these vessels, used it to synthesize cDNA and examine the expression of several genes involved in angiogenesis and blood vessel remodeling. Within the hippocampus, short-day exposed mice showed a reduction in blood vessel density, and isolated capillaries expressed high levels of matrix metalloproteinase 2 (MMP2). This is a critical enzyme for the breakdown of type IV collagen, which is a component of blood vessel basement membranes. To examine whether this mRNA expression translated into increased protein, we used immunohistochemistry to label MMP2 in situ and saw elevated immunoreactivity in the hippocampus of short-day mice. This strongly indicated that capillary flow was reduced and blood vessels were being remodeled in the hippocampus in response to short day lengths.
To assess whether this change at the microvessel level translated into whole-brain reductions in flow, we used a live imaging technique (laser speckle flowmetry) to examine whole brain blood flow in real time in living mice. Mice were anesthetized and positioned under a camera equipped with a laser (785 nm, 80 mW), the braincase was exposed, and images were taken through the intact skull. Short-day mice showed a ~15% reduction in cortical blood flow compared to their long-day counterparts.
Importantly, these changes in blood flow occurred without changes in whole body, brain, or reproductive tissue masses, suggesting that short-day reductions in brain perfusion were independent of simple allometric scaling. Normally, blood vessel remodeling is associated with injury or disease, but exercise training or exposure to a novel environment can also enhance blood flow in the brain.
Our study demonstrated that in addition to these factors, day length can contribute to seasonal plasticity in blood flow. These findings may have implications for seasonal changes in cognitive functions in humans, as recent studies report that short photoperiods are associated with reduced hippocampal volumes in a large community sample. Short-day reductions in blood flow may have additional implications for seasonal changes in cardiovascular disease, as the incidence of stroke is increased during the short days of winter.
In sum, we have demonstrated that photoperiod is a modulating factor for cerebral blood flow in an adult mammal. Further studies are required to tease apart the molecular mechanisms that contribute to short-day reductions in brain blood perfusion. A likely candidate molecule is pineal melatonin, which varies seasonally and has been demonstrated to reduce cerebral blood flow in rats independent of photoperiod. Additionally, the time course of blood vessel remodeling needs to be examined in greater detail to understand how fast these changes occur and how persistent they remain after return to long or intermediate day lengths.
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Photoperiodic Regulation of Cerebral Blood Flow in White-footed Mice (Peromyscus leucopus). Jeremy C Borniger, Seth Teplitsky, Surya Gnyawali, Randy J Nelson, Cameron Rink. eNeuro Jul 2016 DOI: 10.1523/ENEURO.0058-16.2016