A brain for all seasons: cellular and molecular mechanisms of photoperiodic plasticity

Multi-Modal Regulation of Circadian Physiology by Interactive Features of Biological Clocks

The circadian clock is a fundamental biological timing mechanism that generates nearly 24 h rhythms of physiology and behaviors, including sleep/wake cycles, hormone secretion, and metabolism. Evolutionarily, the endogenous clock is thought to confer living organisms, including humans, with survival benefits by adapting internal rhythms to the day and night cycles of the local environment. Mirroring the evolutionary fitness bestowed by the circadian clock, daily mismatches between the internal body clock and environmental cycles, such as irregular work (e.g., night shift work) and life schedules (e.g., jet lag, mistimed eating), have been recognized to increase the risk of cardiac, metabolic, and neurological diseases. Moreover, increasing numbers of studies with cellular and animal models have detected the presence of functional circadian oscillators at multiple levels, ranging from individual neurons and fibroblasts to brain and peripheral organs. These oscillators are tightly coupled to timely modulate cellular and bodily responses to physiological and metabolic cues. In this review, we will discuss the roles of central and peripheral clocks in physiology and diseases, highlighting the dynamic regulatory interactions between circadian timing systems and multiple metabolic factors.

Neurogenesis in the adult hypothalamus: A distinct form of structural plasticity involved in metabolic and circadian regulation, with potential relevance for human pathophysiology

The adult brain harbors specific niches where stem cells undergo substantial plasticity and, in some regions, generate new neurons throughout life. This phenomenon is well known in the subventricular zone of the lateral ventricles and the subgranular zone of the hippocampus and has recently also been described in the hypothalamus of several rodent and primate species. After a brief overview of preclinical studies illustrating the pathophysiologic significance of hypothalamic neurogenesis in the control of energy metabolism, reproduction, thermoregulation, sleep, and aging, we review current literature on the neurogenic niche of the human hypothalamus. A comparison of the organization of the niche between humans and rodents highlights some common features, but also substantial differences, e.g., in the distribution and extent of the hypothalamic neural stem cells. Exploring the full dynamics of hypothalamic neurogenesis in humans raises a formidable challenge however, given among others, inherent technical limitations. We close with discussing possible functional role(s) of the human hypothalamic niche, and how gaining more insights into this form of plasticity could be relevant for a better understanding of pathologies associated with disturbed hypothalamic function.

Profound seasonal changes in brain size and architecture in the common shrew

The seasonal changes in brain size of some shrews represent the most drastic reversible transformation in the mammalian central nervous system known to date. Brain mass decreases 10-26% from summer to winter and regrows 9-16% in spring, but the underlying structural changes at the cellular level are not yet understood. Here, we describe the volumetric differences in brain structures between seasons and sexes of the common shrew (Sorex araneus) in detail, confirming that changes in different brain regions vary in the magnitude of change. Notably, shrews show a decrease in hypothalamus, thalamus, and hippocampal volume and later regrowth in spring, whereas neocortex and striatum volumes decrease in winter and do not recover in size. For some regions, males and females showed different patterns of seasonal change from each other. We also analyzed the underlying changes in neuron morphology. We observed a general decrease in soma size and total dendrite volume in the caudoputamen and anterior cingulate cortex. This neuronal retraction may partially explain the overall tissue shrinkage in winter. While not sufficient to explain the entire seasonal process, it represents a first step toward understanding the mechanisms beneath this remarkable phenomenon.

Photoperiodic regulation of behavior: Peromyscus as a model system

Winter and summer present vastly different challenges to animals living outside of the tropics. To survive and reproduce, individuals must anticipate seasonal environmental changes and adjust physiology and behavior accordingly. Photoperiod (day length) offers a relatively 'noise free' environmental signal that non-tropical animals use to tell the time of year, and whether winter is approaching or receding. In some cases, photoperiodic signals may be fine-tuned by other proximate cues such as food availability or temperature. The pineal hormone, melatonin, is a primary physiological transducer of the photoperiodic signal. It tracks night length and provokes changes in physiology and behavior at appropriate times of the year. Because of their wide latitudinal distribution, Peromyscus has been well studied in the context of photoperiodic regulation of physiology and behavior. Here, we discuss how photoperiodic signals are transduced by pineal melatonin, how melatonin acts on target tissues, and subsequent consequences for behavior. Using a life-history paradigm involving trade-offs between the immune and reproductive systems, specific emphasis is placed on aggression, metabolism, and cognition. We discuss future directions including examining the effects of light pollution on photoperiodism, genetic manipulations to test the role of specific genes in the photoperiodic response, and using Peromyscus to test evolutionary theories of aging.

Experimental evidence for sex-specific plasticity in adult brain

Background

Plasticity in brain size and the size of different brain regions during early ontogeny is known from many vertebrate taxa, but less is known about plasticity in the brains of adults. In contrast to mammals and birds, most parts of a fish’s brain continue to undergo neurogenesis throughout adulthood, making lifelong plasticity in brain size possible. We tested whether maturing adult three-spined sticklebacks (Gasterosteus aculeatus) reared in a stimulus-poor environment exhibited brain plasticity in response to environmental enrichment, and whether these responses were sex-specific, thus altering the degree of sexual size dimorphism in the brain.

Results

Relative sizes of total brain and bulbus olfactorius showed sex-specific responses to treatment: males developed larger brains but smaller bulbi olfactorii than females in the enriched treatment. Hence, the degree of sexual size dimorphism (SSD) in relative brain size and the relative size of the bulbus olfactorius was found to be environment-dependent. Furthermore, the enriched treatment induced development of smaller tecta optica in both sexes.

Conclusions

These results demonstrate that adult fish can alter the size of their brain (or brain regions) in response to environmental stimuli, and these responses can be sex-specific. Hence, the degree of SSD in brain size can be environment-dependent, and our results hint at the possibility of a large plastic component to SSD in stickleback brains. Apart from contributing to our understanding of the processes shaping and explaining variation in brain size and the size of different brain regions in the wild, the results show that provision of structural complexity in captive environments can influence brain development. Assuming that the observed plasticity influences fish behaviour, these findings may also have relevance for fish stocking, both for economical and conservational purposes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12983-015-0130-0) contains supplementary material, which is available to authorized users.

Evolutionary ecology of intraspecific brain size variation: a review

The brain is a trait of central importance for organismal performance and fitness. To date, evolutionary studies of brain size variation have mainly utilized comparative methods applied at the level of species or higher taxa. However, these studies suffer from the difficulty of separating causality from correlation. In the other extreme, studies of brain plasticity have focused mainly on within-population patterns. Between these extremes lie interpopulational studies, focusing on brain size variation among populations of the same species that occupy different habitats or selective regimes. These studies form a rapidly growing field of investigations which can help us to understand brain evolution by providing a test bed for ideas born out of interspecific studies, as well as aid in uncovering the relative importance of genetic and environmental factors shaping variation in brain size and architecture. Aside from providing the first in depth review of published intraspecific studies of brain size variation, we discuss the prospects embedded with interpopulational studies of brain size variation. In particular, the following topics are identified as deserving further attention: (i) studies focusing on disentangling the contributions of genes, environment, and their interactions on brain variation within and among populations, (ii) studies applying quantitative genetic tools to evaluate the relative importance of genetic and environmental factors on brain features at different ontogenetic stages, (iii) apart from utilizing simple gross estimates of brain size, future studies could benefit from use of neuroanatomical, neurohistological, and/or molecular methods in characterizing variation in brain size and architecture. Evolution of brain size and architecture is a widely studied topic. However, the majority of studies are interspecific and comparative. Here we summarize the recently growing body of intraspecific studies based on population comparisons and outline the future potential in this approach.

Neuronal plasticity in hibernation and the proposed role of the microtubule-associated protein tau as a "master switch" regulating synaptic gain in neuronal networks

The present paper provides an overview of adaptive changes in brain structure and learning abilities during hibernation as a behavioral strategy used by several mammalian species to minimize energy expenditure under current or anticipated inhospitable environmental conditions. One cellular mechanism that contributes to the regulated suppression of metabolism and thermogenesis during hibernation is reversible phosphorylation of enzymes and proteins, which limits rates of flux through metabolic pathways. Reversible phosphorylation during hibernation also affects synaptic membrane proteins, a process known to be involved in synaptic plasticity. This mechanism of reversible protein phosphorylation also affects the microtubule-associated protein tau, thereby generating a condition that in the adult human brain is associated with aggregation of tau protein to paired helical filaments (PHFs), as observed in Alzheimer's disease. Here, we put forward the concept that phosphorylation of tau is a neuroprotective mechanism to escape NMDA-mediated hyperexcitability of neurons that would otherwise occur during slow gradual cooling of the brain. Phosphorylation of tau and its subsequent targeting to subsynaptic sites might, thus, work as a kind of "master switch," regulating NMDA receptor-mediated synaptic gain in a wide array of neuronal networks, thereby enabling entry into torpor. If this condition lasts too long, however, it may eventually turn into a pathological trigger, driving a cascade of events leading to neurodegeneration, as in Alzheimer's disease or other "tauopathies".

Photoperiod-mediated impairment of long-term potention and learning and memory in male white-footed mice

Adult mammalian brains are capable of some structural plasticity. Although the basic cellular mechanisms underlying learning and memory are being revealed, extrinsic factors contributing to this plasticity remain unspecified. White-footed mice (Peromyscus leucopus) are particularly well suited to investigate brain plasticity because they show marked seasonal changes in structure and function of the hippocampus induced by a distinct environmental signal, viz., photoperiod (i.e. the number of hours of light/day). Compared to animals maintained in 16 h of light/day, exposure to 8 h of light/day for 10 weeks induces several phenotypic changes in P. leucopus, including reduction in brain mass and hippocampal volume. To investigate the functional consequences of reduced hippocampal size, we examined the effects of photoperiod on spatial learning and memory in the Barnes maze, and on long-term potentiation (LTP) in the hippocampus, a leading candidate for a synaptic mechanism underlying spatial learning and memory in rodents. Exposure to short days for 10 weeks decreased LTP in the Schaffer collateral-CA1 pathway of the hippocampus and impaired spatial learning and memory ability in the Barnes maze. Taken together, these results demonstrate a functional change in the hippocampus in male white-footed mice induced by day length.

Stress-induced alterations in catecholamine enzymes gene expression in the hypothalamic dorsomedial nucleus are modulated by caudal brain and not hypothalamic paraventricular nucleus neurons

The hypothalamic dorsomedial nucleus (DMN) represents an important coordinate center for regulation of autonomic and neuroendocrine systems, especially during stress response. The present study was focused on the gene expression of catecholamine-synthesizing enzymes and the protein levels of tyrosine hydroxylase in DMN, both in control and stressed rats. Moreover, pathways modulating the gene expression of tyrosine hydroxylase in DMN during immobilization (IMO) stress were also investigated. Gene expressions of all catecholamine-synthesizing enzymes were detected in DMN samples. While the levels of tyrosine hydroxylase and phenylethanolamine N-methyltransferase mRNA were increased in IMO rats, aromatic L-amino acid decarboxylase and dopamine-beta-hydroxylase mRNA remained unchanged. Tyrosine hydroxylase protein levels were significantly elevated in the DMN only after repeated IMO stress. Postero-lateral deafferentations of the DMN, or transections of the ascending catecholaminergic pathways originating in the lower brainstem abolished the IMO-induced increase of tyrosine hydroxylase gene expression in the DMN. Nevertheless, postero-lateral deafferentations of the hypothalamic paraventricular nucleus (PVN), which separate the DMN from the PVN, had no effect on IMO-induced elevation of tyrosine hydroxylase mRNA in the DMN. The present data indicate that certain DMN neurons synthesize mRNA of catecholamine enzymes. The stress-induced increase of tyrosine hydroxylase and phenylethanolamine N-methyltransferase mRNA in DMN neurons indicates the involvement of these catecholaminergic neurons in stress response. The gene expression of tyrosine hydroxylase in DMN is modulated by lower brainstem and/or spinal cord, but not by PVN afferents.

Combined organizational and activational effects of short and long photoperiods on spatial and temporal memory in rats

The present study examined the effects of photoperiod on spatial and temporal memory in adult Sprague-Dawley rats that were conceived and reared in different day lengths, i.e., short day (SD-8:16 light/dark) and long day (LD-16:8 light/dark). Both male and female LD rats demonstrated increased spatial memory capacity as evidenced by a lower number of choices to criterion in a 12-arm radial maze task relative to the performance of SD rats. SD rats also demonstrated a distortion in the content of temporal memory as evidenced by a proportional rightward shift in the 20 and 60 s temporal criteria trained using the peak-interval procedure that is consistent with reduced cholinergic function. The conclusion is that both spatial and temporal memory are sensitive to photoperiod variation in laboratory rats in a manner similar to that previously observed for reproductive behaviour.

Short photoperiods impair spatial learning and alter hippocampal dendritic morphology in adult male white-footed mice (Peromyscus leucopus)

Although seasonal changes in brain morphology and function are well established in songbirds, seasonal plasticity of brain structure and function remain less well documented in mammals. Nontropical animals display many adaptations to reduce energy use to survive winter, including cessation of reproductive activities. Because of the high energetic costs of brain tissue, we hypothesized that male white-footed mice (Peromyscus leucopus) would reduce brain size in response to short days as well as regress their reproductive systems. Because short days may decrease hippocampal volume and impair spatial learning and memory in rodents and because of the potential for seasonal plasticity in the hippocampus, we hypothesized that photoperiod alters hippocampal morphology to affect spatial learning and memory. Mice housed in either long or short days for 10 weeks were examined for performance in a water maze; brains were then removed and weighed, and hippocampal volumes were determined. We also measured dendritic morphology and spine density in the CA1, CA3, and dentate gyrus. Short days decreased brain mass and hippocampal volume compared with long days. Short days also impaired long-term spatial learning and memory relative to long days but did not affect sensory discrimination or other types of memory. Short days decreased apical (stratum lacunosum-moleculare) CA1 spine density, as well as increased basilar (stratum oriens) CA3 spine density. Results from this study suggest that photoperiod alters brain size and morphology, as well as cognitive function. Understanding the mechanisms mediating these photoperiod-induced alterations may provide insight for treatment of seasonal cognitive and affective disorders.

Photoperiod influences the ontogenetic expression of aromatase and estrogen receptor alpha in the developing tilapia brain

Neural development is determined not only by genetic regulation, but also by environmental cues. Central estrogen-forming/estrogen-sensitive systems play an important role in the neural development of the brain. In the present study, the quantitative reverse transcription-polymerase chain reaction method was used to investigate the effects of photoperiod on the ontogenetic expression of aromatase and estrogen receptor alpha (ERalpha) in the developing tilapia brain. Before day 5 post-hatch, brain aromatase mRNA expression was significantly decreased by constant light but not influenced by constant darkness. During this period, brain ERalpha mRNA expression was significantly increased under both constant light and constant darkness. Between days 5 and 10, and between days 10 and 15, neither brain aromatase nor brain ERalpha expression was altered under constant darkness and constant light. These results indicate that the ontogenetic expression of brain aromatase and brain ERalpha is not via a light-inducing process but influenced by a light-entraining signal during the very early period of development.

Effects of photoperiod on the development of the central glutamate system in tilapia, Oreochromis mossambicus

The effect of photoperiod (light/dark cycle) on the development of the central glutamate system was investigated in tilapia, Oreochromis mossambicus. Tilapia, at 0, 5, and 10 days posthatching were respectively divided into three equal groups to be kept in different photoperiods: 12/12 h, 24/0 h (full day), and 0/24 h (full night). Neither the full-day nor the full-night photoperiod showed any influence on the development of the central glutamate system, including glutamate content and mRNA expression of glutamate receptor 3 alpha, in the developing tilapia brain. These results suggest that neither constant light nor dark photoperiods affected the influence of the central glutamate system on brain sex differentiation in tilapia during the early developing period.

The brain's calendar: neural mechanisms of seasonal timing

The suprachiasmatic nucleus (SCN) of the hypothalamus is the principal component of the mammalian biological clock, the neural timing system that generates and coordinates a broad spectrum of physiological, endocrine and behavioural circadian rhythms. The pacemaker of the SCN oscillates with a near 24 h period and is entrained to the diurnal light-dark cycle. Consistent with its role in circadian timing, investigations in rodents and non-human primates furthermore suggest that the SCN is the locus of the brain's endogenous calendar, enabling organisms to anticipate seasonal environmental changes. The present review focuses on the neuronal organization and dynamic properties of the biological clock and the means by which it is synchronized with the environmental lighting conditions. It is shown that the functional activity of the biological clock is entrained to the seasonal photic cycle and that photoperiod (day length) may act as an effective zeitgeber. Furthermore, new insights are presented, based on electrophysiological and molecular studies, that the mammalian circadian timing system consists of coupled oscillators and that the clock genes of these oscillators may also function as calendar genes. In summary, there are now strong indications that the neuronal changes and adaptations in mammals that occur in response to a seasonally changing environment are driven by an endogenous circadian clock located in the SCN, and that this neural calendar is reset by the seasonal fluctuations in photoperiod.

Photoperiod affects the development of central neurotransmitter systems of tilapia, Oreochromis mossambicus

The effects of photoperiod on the development of central neurotransmitters were investigated with tilapia, Oreochromis mossambicus. Zero-day-old (the hatching day) tilapia were raised in three different photoperiods (light/dark cycle): 12/1, 24/0, and 0/24 h, respectively. On the 5th day, brain serotonin (5-HT), norepinephrine (NE), gamma-aminobutyric acid (GABA), and glutamate (Glu) contents were quantified by a high-performance liquid chromatograph with electrochemical detection. Similar experiments were performed on the 5-, 10-, 15-, 20-, and 25-day-olds. These results showed that the photoperiod influenced both brain NE and GABA contents during its respective restricted period, before days 10 posthatching. Brain 5-HT content was influenced, either facilitated or suppressed according to the developing stage, whereas, brain Glu content was not altered by the different photoperiod exposure throughout the present studies.