Photoperiod affects neuronal nitric oxide synthase and aggressive behaviour in male Siberian hamsters (Phodopus sungorus)

Amygdala and subregion volumes are associated with photoperiod and seasonal depressive symptoms: A cross-sectional study in the UK Biobank cohort

Although seasonal changes in amygdala volume have been demonstrated in animals, seasonal differences in human amygdala subregion volumes have yet to be investigated. Amygdala volume has also been linked to depressed mood. Therefore, we hypothesised that differences in photoperiod would predict differences in amygdala or subregion volumes and that this association would be linked to depressed mood. 10,033 participants ranging in age from 45 to 79 years were scanned by MRI in a single location. Amygdala subregion volumes were obtained using automated processing and segmentation algorithms. A mediation analysis tested whether amygdala volume mediated the relationship between photoperiod and mood. Photoperiod was positively associated with total amygdala volume (p < .001). Multivariate (GLM) analyses revealed significant effects of photoperiod across all amygdala subregion volumes for both hemispheres (p < .001). Post hoc univariate regression analyses revealed significant associations of photoperiod with each amygdala subregion volume (p < .001). PLS showed the highest loadings of amygdala subregions in lateral nucleus, ABN, basal nucleus, CAT, PLN, AAA, central nucleus, cortical nucleus and medial nucleus for left hemisphere and ABN, lateral nucleus, CAT, PLN, cortical nucleus, AAA, central nucleus and medial nucleus for right hemisphere. There were no significant associations between photoperiod and mood nor between mood scores and amygdala volumes, and due to the lack of these associations, the mediation hypothesis was not supported. This study is the first to demonstrate an association between photoperiod and amygdala volume. These findings add to the evidence supporting the role of photoperiod on brain structural plasticity.

Evolutionary diversity as a catalyst for biological discovery

The tremendous diversity of animal behaviors has inspired generations of scientists from an array of biological disciplines. To complement investigations of ecological and evolutionary factors contributing to behavioral evolution, modern sequencing, gene editing, computational and neuroscience tools now provide a means to discover the proximate mechanisms upon which natural selection acts to generate behavioral diversity. Social behaviors are motivated behaviors that can differ tremendously between closely related species, suggesting phylogenetic plasticity in their underlying biological mechanisms. In addition, convergent evolution has repeatedly given rise to similar forms of social behavior and mating systems in distantly related species. Social behavioral divergence and convergence provides an entry point for understanding the neurogenetic mechanisms contributing to behavioral diversity. We argue that the greatest strides in discovering mechanisms contributing to social behavioral diversity will be achieved through integration of interdisciplinary comparative approaches with modern tools in diverse species systems. We review recent advances and future potential for discovering mechanisms underlying social behavioral variation; highlighting patterns of social behavioral evolution, oxytocin and vasopressin neuropeptide systems, genetic/transcriptional "toolkits," modern experimental tools, and alternative species systems, with particular emphasis on Microtine rodents and Lake Malawi cichlid fishes.

DHEA effects on brain and behavior: insights from comparative studies of aggression

Historically, research on the neuroendocrinology of aggression has been dominated by the paradigm that the brain receives sex steroid hormones, such as testosterone (T), from the gonads, and then these gonadal hormones modulate behaviorally relevant neural circuits. While this paradigm has been extremely useful for advancing the field, recent studies reveal important alternatives. For example, most vertebrate species are seasonal breeders, and many species show aggression outside of the breeding season, when the gonads are regressed and circulating levels of gonadal steroids are relatively low. Studies in diverse avian and mammalian species suggest that adrenal dehydroepiandrosterone (DHEA), an androgen precursor and prohormone, is important for the expression of aggression when gonadal T synthesis is low. Circulating DHEA can be converted into active sex steroids within the brain. In addition, the brain can synthesize sex steroids de novo from cholesterol, thereby uncoupling brain steroid levels from circulating steroid levels. These alternative mechanisms to provide sex steroids to specific neural circuits may have evolved to avoid the costs of high circulating T levels during the non-breeding season. Physiological indicators of season (e.g., melatonin) may allow animals to switch from one neuroendocrine mechanism to another across the year. DHEA and neurosteroids are likely to be important for the control of multiple behaviors in many species, including humans. These studies yield fundamental insights into the regulation of DHEA secretion, the mechanisms by which DHEA affects behavior, and the brain regions and neural processes that are modulated by DHEA. It is clear that the brain is an important site of DHEA synthesis and action. This article is part of a Special Issue entitled 'Essential role of DHEA'.

Photoperiod modulation of aggressive behavior is independent of androgens in a tropical cichlid fish

Photoperiod is a major environmental cue that signals breeding conditions in animals living in temperate climates. Therefore, the activity of the reproductive (i.e. hypothalamic-pituitary-gonadal, HPG) axis and of the expression of reproductive behaviors, including territoriality, is responsive to changes in day length. However, at low latitudes the seasonal variation in day length decreases dramatically and photoperiod becomes less reliable as a breeding entraining cue in tropical species. In spite of this, some tropical mammals and birds have been found to still respond to small amplitude changes in photoperiod (e.g. 17min). Here we tested the effect of 2 photoperiod regimes, referred to as long-day (LD: 16L:08D) and short-day (SD: 08L:16D), on the activity of the HPG axis, on aggressive behavior and in the androgen response to social challenges in males of the tropical cichlid fish Tilapia rendalli. For each treatment, fish were transferred from a pre-treatment photoperiod of 12L:12D to their treatment photoperiod (either LD or SD) in which they were kept for 20days on stock tanks. Afterwards, males were isolated for 4days in glass aquaria in order to establish territories and initial androgen levels (testosterone, T; 11-ketotestosterone, KT) were assessed. On the 4th day, territorial intrusions were promoted such that 1/3 of the isolated males acted as residents and another 1/3 as intruders. Territorial intrusions lasted for 1h to test the effects of a social challenge under different photoperiod regimes. Photoperiod treatment (either SD or LD) failed to induce significant changes in the HPG activity, as measured by androgen levels and gonadosomatic index. However, SD increased the intensity of aggressive behaviors and shortened the time to settle a dominance hierarchy in an androgen-independent manner. The androgen responsiveness to the simulated territorial intrusion was only present in KT but not for T. The percent change in KT levels in response to the social challenge was different between treatments (SD>LD) and between male types (resident>intruder). The higher androgen response to a social challenge in residents under SD may be explained by the time course of the androgen response that due to the long time it takes to fight resolution under LD, might have been delayed. This result illustrates the importance of incorporating time response data in social endocrinology studies.

Rapid effects of estrogens on behavior: environmental modulation and molecular mechanisms

Estradiol can modulate neural activity and behavior via both genomic and nongenomic mechanisms. Environmental cues have a major impact on the relative importance of these signaling pathways with significant consequences for behavior. First we consider how photoperiod modulates nongenomic estrogen signaling on behavior. Intriguingly, short days permit rapid effects of estrogens on aggression in both rodents and song sparrows. This highlights the importance of considering photoperiod as a variable in laboratory research. Next we review evidence for rapid effects of estradiol on ecologically-relevant behaviors including aggression, copulation, communication, and learning. We also address the impact of endocrine disruptors on estrogen signaling, such as those found in corncob bedding used in rodent research. Finally, we examine the biochemical mechanisms that may mediate rapid estrogen action on behavior in males and females. A common theme across these topics is that the effects of estrogens on social behaviors vary across different environmental conditions.

The effects of exogenous melatonin and melatonin receptor blockade on aggression and estrogen-dependent gene expression in male California mice (Peromyscus californicus)

Photoperiodic regulation of aggression has been well established in several vertebrate species, with rodents demonstrating increased aggression in short day photoperiods as compared to long day photoperiods. Previous work suggests that estrogens regulate aggression via rapid nongenomic pathways in short days and act more slowly in long days, most likely via genomic pathways. The current study therefore examines the role of melatonin in mediating aggression and estrogen-dependent gene transcription. In Experiment 1, male California mice were housed under long day photoperiods and were treated with either 0.3 μg/g of melatonin, 40 mg/kg of the melatonin receptor antagonist luzindole, or vehicle for 10 days. We found that melatonin administration significantly increased aggression as compared to mice receiving vehicle, but this phenotype was not completely ameliorated by luzindole. In Experiment 2, male California mice were injected with either 1mg/kg of the aromatase inhibitor letrozole or vehicle, and oxytocin receptor (OTR), estrogen receptor alpha (ERα), and c-fos gene expression was examined in the bed nucleus of the stria terminalis (BNST) and medial preoptic area (MPOA). In the BNST, but not MPOA, OTR mRNA was significantly downregulated following letrozole administration, indicating that OTR is an estrogen-dependent gene in the BNST. In contrast, ERα was not estrogen dependent in either brain region. In the MPOA, OTR mRNA was inhibited by melatonin, and luzindole suppressed this effect. C-fos and ERα did not differ between treatments in any brain region examined. These results suggest that it is unlikely that melatonin facilitates aggression via broad spectrum regulation of estrogen-dependent gene expression. Instead, melatonin may act via regulation of other transcription factors such as extracellular signal regulated kinase.

Nitric oxide and serotonin interactions in aggression

Violence is a significant public health problem worldwide. Neurobiological research on violence and aggression attempts to elucidate the cellular and molecular pathways that increase the propensity toward this behavior. Research over the past 40 years has implicated several brain regions and neurotransmitters in aggression, mainly using rodent models. Perhaps the strongest association is the link between serotonin and aggression, which has compelling interactions with the nitric oxide system. Recently, new insights into these relationships have been added as modern techniques allow more sophisticated analyses. This chapter will discuss current developments implicating serotonin and nitric oxide in aggressive behavior. Recently developed high-resolution methods for examining the neurobiological basis of aggression will be considered, with emphasis on future directions for the field.

Photoperiod-dependent effects of neuronal nitric oxide synthase inhibition on aggression in Siberian hamsters

Many nontropical species undergo physiological and behavioral adaptations in response to seasonal changes in photoperiod, or day length. In most rodent species, short winter photoperiods reduce testosterone concentrations, which provoke gonadal regression and reduce testosterone-dependent behaviors such as mating and aggression. Seasonally-breeding Siberian hamsters, however, are paradoxically more aggressive in short-days, despite much reduced reproductive activity and testosterone concentrations. Nitric oxide (NO) signaling has been proposed as part of an alternate mechanism underlying this phenomenon. A reduction in neuronal nitric oxide synthase (nNOS), the enzyme responsible for synthesizing NO in the brain, is associated with increased aggression in male short-day hamsters. In the present study, we hypothesized that pharmacological inhibition of nNOS would increase aggressive behavior in long days, but not in short days because nNOS is already reduced. Adult male Siberian hamsters were housed in either long (LD 16:8h) or short (LD 8:16h) photoperiods for 8weeks, then treated with either the selective nNOS inhibitor, 3-bromo-7-nitroindazole (3BrN) or oil vehicle, and subsequently tested for aggression in a resident-intruder test. Treatment with 3BrN increased attack frequency and duration in long days, but had no effect in short days. Short days also reduced testosterone concentrations, without any effect of treatment. These data provide further evidence linking reduced nNOS to elevated short-day aggression and support a role for NO signaling in this phenomenon.

Influence of photoperiod on hormones, behavior, and immune function

Photoperiodism is the ability of plants and animals to measure environmental day length to ascertain time of year. Central to the evolution of photoperiodism in animals is the adaptive distribution of energetically challenging activities across the year to optimize reproductive fitness while balancing the energetic tradeoffs necessary for seasonally-appropriate survival strategies. The ability to accurately predict future events requires endogenous mechanisms to permit physiological anticipation of annual conditions. Day length provides a virtually noise free environmental signal to monitor and accurately predict time of the year. In mammals, melatonin provides the hormonal signal transducing day length. Duration of pineal melatonin is inversely related to day length and its secretion drives enduring changes in many physiological systems, including the HPA, HPG, and brain-gut axes, the autonomic nervous system, and the immune system. Thus, melatonin is the fulcrum mediating redistribution of energetic investment among physiological processes to maximize fitness and survival.

Activation of extracellular signal-regulated kinases in social behavior circuits during resident-intruder aggression tests

Using a variety of experimental methods, a network of brain areas regulating aggressive behaviors has been identified in several groups of vertebrates. However, aggressive behavior expressed in different contexts is associated with different patterns of activity across hypothalamic and limbic brain regions. Previous studies in rodents demonstrated that short day photoperiods reliably increase both male and female aggression versus long day photoperiods. Here we used immunohistochemistry and western blots to examine the effect of photoperiod on phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK) in male California mice (Peromyscus californicus) during resident-intruder tests. Phosphorylated ERK (pERK) can alter neuronal activity in the short term and in the long term acts as a transcription factor. In the posterior bed nucleus of the stria terminalis (BNST) males tested in aggression tests had more pERK positive cells when housed in short days but not long days. This result was replicated in western blot analyses from microdissected BNST samples. In the medial amygdala (MEA), immunostaining and western analyses showed that pERK expression also was generally increased in short days. Immunostaining was also used to examine phosphorylation of cyclic AMP response element binding protein (CREB). CREB can be phosphorylated by pERK as well as other kinases and functions primarily as a transcription factor. Intriguingly, aggressive interactions reduced the number of cells stained positive for phosphorylated CREB in the infralimbic cortex, ventral lateral septum and MEA. This effect was observed in mice housed in long days but not short days. Overall, these data suggest that different (but overlapping) networks of aggressive behavior operate under different environmental conditions.

Enrichment and photoperiod interact to affect spatial learning and hippocampal dendritic morphology in white-footed mice (Peromyscus leucopus)

In seasonally changing environments, individuals must coordinate endogenous processes with ambient conditions. Winter is a challenging time to survive and reproduce. In order to anticipate decreased food availability and low temperatures in winter, many rodents use decreasing day lengths as a precise temporal cue. Short day lengths alter several adaptations, including reproduction, immune function, aggressive behavior and spatial learning in non-tropical rodents. Specifically, short days impair spatial learning in white-footed mice (Peromyscus leucopus) and alter dendritic complexity in the hippocampus. The goal of the current study was to determine whether short days constrain neural plasticity. If short days limit the capacity for plasticity, then environmental enrichment, a manipulation that induces morphological changes, should alter dendritic morphology in long, but not short, days. Male white-footed mice were assigned to long (16 : 8 LD) or short (8 : 16 LD) photoperiod in either enriched or standard cages. Enrichment consisted of a large cage, cage mates, Habitrail tubes, a nest box and a running wheel. Mice were tested in the Morris water maze. Reproductive tissues were collected and weighed; brains were processed for dendritic morphology. Short days impaired spatial learning. Short days also reduced spine density on apical dendrites within the CA3 region of the hippocampus. However, enrichment prevented short-day-induced deficits in learning and also increased hippocampal spine density in the CA1 and CA3 regions in short-day mice. These results suggest that day length and other non-photic environmental factors interact to regulate dendritic morphology, and that short photoperiods do not constrain the capacity for functional neural plasticity.

Novel mechanisms for neuroendocrine regulation of aggression

In 1849, Berthold demonstrated that testicular secretions are necessary for aggressive behavior in roosters. Since then, research on the neuroendocrinology of aggression has been dominated by the paradigm that the brain receives gonadal hormones, primarily testosterone, which modulate relevant neural circuits. While this paradigm has been extremely useful, recent studies reveal important alternatives. For example, most vertebrate species are seasonal breeders, and many species show aggression outside of the breeding season, when gonads are regressed and circulating testosterone levels are typically low. Studies in birds and mammals suggest that an adrenal androgen precursor-dehydroepiandrosterone (DHEA)-may be important for the expression of aggression when gonadal testosterone synthesis is low. Circulating DHEA can be metabolized into active sex steroids within the brain. Another possibility is that the brain can autonomously synthesize sex steroids de novo from cholesterol, thereby uncoupling brain steroid levels from circulating steroid levels. These alternative neuroendocrine mechanisms to provide sex steroids to specific neural circuits may have evolved to avoid the "costs" of high circulating testosterone during particular seasons. Physiological indicators of season (e.g., melatonin) may allow animals to switch from one neuroendocrine mechanism to another across the year. Such mechanisms may be important for the control of aggression in many vertebrate species, including humans.

Maternal pinealectomy increases depressive-like responses in Siberian hamster offspring

This study investigated the effect of maternal pinealectomy and postnatal pinealectomy on affective responses. Siberian hamsters were born to either pinealectomized or sham-operated dams and then underwent pinealectomy or a sham operation. Maternal pinealectomy increased depressive-like responses of offspring in the forced swim test. Maternal pinealectomy increased rearing behaviour and postnatal pinealectomy increased locomotor behaviour in the open field test. These results suggest that prenatal melatonin organizes adult affective responses.

Rapid effects of estradiol on male aggression depend on photoperiod in reproductively non-responsive mice

In three genuses and four species of rodents, housing in winter-like short days (8L:16D) increases male aggressive behavior. In all of these species, males undergo short-day induced regression of the reproductive system. Some studies, however, suggest that the effect of photoperiod on aggression may be independent of reproductive responses. We examined the effects of photoperiod on aggressive behavior in California mice (Peromyscus californicus), which do not display reproductive responsiveness to short days. As expected, short days had no effect on plasma testosterone. Estrogen receptor alpha and estrogen receptor beta immunostaining did not differ in the lateral septum, medial preoptic area, bed nucleus of the stria terminalis, or medial amygdala. However, males housed in short days were significantly more aggressive than males housed in long days. Similar to previous work in beach mice (Peromyscus polionotus), estradiol rapidly increased aggression when male California mice were housed in short days but not when housed in long days. These data suggest that the effects of photoperiod on aggression and estrogen signaling are independent of reproductive responses. The rapid action of estradiol on aggression in short-day mice also suggests that nongenomic mechanisms mediate the effects of estrogens in short days.

Photoperiod affects estrogen receptor alpha, estrogen receptor beta and aggressive behavior

Estrogens have important effects on male and female social behavior. Despite growing knowledge of the anatomy and behavioral effects of the two predominant estrogen receptor subtypes in mammals (ERalpha and ERbeta), relatively little is known about how these receptors respond to salient environmental stimuli. Many seasonally breeding species respond to changing photoperiods that predict seasonal changes in resource availability. We characterized the effects of photoperiod on aggressive behavior in two species of Peromyscus that exhibit gonadal regression in short days. P. polionotus (old field mice) were more aggressive than P. maniculatus (deer mice) and both species were more aggressive in short days. We used immunocytochemistry and real-time polymerase chain reaction to characterize the effects of photoperiod on ERalpha and ERbeta expression. In both species ERalpha-immunoreactive staining in the posterior bed nucleus of the stria terminalis (BNST) was increased in short vs. long days. Both species had reduced ERbeta-immunoreactive expression in the posterior BNST in short days. In the medial amygdala ERbeta immunoreactivity was increased in long days for both species. Using real-time polymerase chain reaction on punch samples that included the BNST, we observed that ERalpha mRNA was increased and ERbeta mRNA was decreased in short days. These data suggest that the effects of photoperiod on ERalpha and ERbeta expression may thus have important behavioral consequences.

Enduring effects of photoperiod on affective behaviors in Siberian hamsters (Phodopus sungorus)

The effects of perinatal and postweaning photoperiods on subsequent affective behaviors were examined in adult Siberian hamsters (Phodopus sungorus). Hamsters exposed perinatally to short days (8 hr light/day) exhibited mixed results for adult anxiety-like behaviors and increased some depressive-like behaviors compared with hamsters exposed to long days (16 hr light/day). Postweaning exposure to short days increased depressive- and anxiety-like behaviors compared with long days. Sex differences in affective behaviors were observed. These results suggest that anxiety-like behaviors are organized early in life and endure throughout adulthood, and anxiety- and depressive-like behaviors are modified by postweaning photoperiod. The persistence of photoperiod-induced affective behaviors in rodents supports the hypothesis that symptoms of human affective disorders may reflect ancestral adaptations to seasonal environments.

Pleiotropic contributions of nitric oxide to aggressive behavior

Male mice with targeted deletion of the genes encoding the neuronal (NOS-1-/- or nNOS-/-) isoform of nitric oxide synthase display altered aggressive behaviors. Male nNOS-1-/- mice are more aggressive than wild-type (WT) mice in all testing paradigms. Testosterone is necessary, but not sufficient, for evoking the persistent aggression, and that serotonin (5-HT) metabolism is altered in male nNOS-1-/- mice. The specific deletion of the nNOS-1 gene not only results in a lack of nNOS-1 protein, but in common with many genes, affects several 'down-stream' processes. In this review, we address whether the elevated aggression in male nNOS-1-/- mice reflects pleiotropic effects of the nNOS-1 gene on pain sensitivity, 'anxiety-like', or 'depressive-like' behaviors. For example, male nNOS-1-/- mice display increased sensitivity to painful stimuli, which may prolong aggressive interactions. Despite elevated corticosterone concentrations, nNOS-1 knockout mice appear to be less 'anxious' or fearful than WT mice. Male nNOS-1-/- mice display longer latencies to right themselves on an inverted platform and spend more time in the center of an open field than WT mice. Because of reduced serotonin turnover, the excessive aggressiveness displayed by nNOS-1-/- mice may be symptomatic of a depressive-like syndrome. However, nNOS-1-/- mice rarely display behavioral 'despair' when assessed with the Porsolt forced swim test; rather, nNOS-1-/- mice show vigorous swimming throughout the assessment suggesting that the aggressive behavior does not represent depressive-like behavior. Importantly, aggressive behavior is not a unitary process, but is the result of complex interactions among several physiological, motivational, and behavioral systems, with contributions from the social as well as the physical environment. Lastly, the multiple, and often unanticipated, effects of targeted gene disruption on aggressive behavior are considered.