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

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.

Early life experiences affect adult delayed-type hypersensitivity in short and long photoperiods

Environmental experiences during development provide animals with important information about future conditions. Siberian hamsters are photoperiodic rodents that dramatically adjust their physiology and behavior to adapt to seasonal changes. For example, during short winter-like days, hamsters enhance some components of immune function putatively to cope with increasing environmental challenges. Furthermore, early life stress alters the developmental course of the immune system. Overall, immune function is typically suppressed in response to chronic stress, but responses vary depending on the type of stress and components of immune function assessed. This led us to hypothesize that delayed-type hypersensitivity (DTH), an antigen-specific, cell-mediated immune response, would be differentially modulated in hamsters that underwent early life maternal separation (MS) in either short or long photoperiods. At birth, hamsters were assigned to either short (SD; 8 h light/day) or long (LD; 16 h light/day) photoperiods and either daily 3 h MS, daily 15-min brief maternal separation (BMS), or no manipulation from postnatal day 2 through 14. In adulthood DTH was assessed. Hamsters reared in short days enhanced DTH responses. MS and BMS attenuated DTH responses in both short and long days. However, BMS long-day female hamsters did not suppress pinna swelling, suggesting a protective effect of female sex steroids on immune function. As is typical in short days, reproductive tissue was regressed. Reproductive tissue mass was also decreased in long-day MS female hamsters. Furthermore, MS altered photoperiod-induced changes in body mass. Taken together, these findings suggest that manipulations of early life mother-pup interactions in Siberian hamsters result in physiological changes and suppressed cell-mediated immunity.

The effects of the intraamygdalar melatonin injections on the anxiety like behavior and the spatial memory performance in male Wistar rats

In the present study, the effects of intraamygdalar administrations of melatonin (1 and 100μg/kg), saline and diazepam on the anxiety-like behavior and spatial memory performance in pinealectomized and sham-pinealectomized Wistar rats were investigated. The animals were tested by open field and elevated plus maze tests for anxiety-like behavior, and Morris water maze test for spatial memory. In open field, (a) diazepam was more effective in reducing the anxiety, (b) control subjects were more mobile than pinealectomized subjects and (c) 100μg/kg melatonin administrations reduced the velocity of the animals. In elevated plus maze, (a) 100μg/kg melatonin administrations increased the distance totally travelled and (b) enhanced the time spent in open arms, however, after the pinealectomy, 1μg/kg melatonin administrations decreased it and (c) control animals were less mobile than pinealectomized ones. In Morris water maze, (a) diazepam group travelled more distance than the others in control condition whereas, in pinealectomy condition high dose of melatonin and saline groups travelled more distance than the others, (b) in pinealectomy condition subjects who received 100μg/kg melatonin also travelled more distance than those who received 1μg/kg melatonin and diazepam, (c) the subjects who received 1μg/kg spent less time than those who received other treatments, and (d) in control condition subjects who received 100μg/kg melatonin were slower than those who received the other treatments. In conclusion, melatonin administration to amygdala decreased the anxiety; however, spatial memory performance of the rats was impaired by the pinealectomy and melatonin administrations.

Perinatal photoperiod imprints the circadian clock

Using real-time gene expression imaging and behavioral analysis, we found that perinatal photoperiod has lasting effects on the circadian rhythms expressed by clock neurons as well as on mouse behavior, and sets the responsiveness of the biological clock to subsequent changes in photoperiod. These developmental gene x environment interactions tune circadian clock responses to subsequent seasonal photoperiods and may contribute to the influence of season on neurobehavioral disorders in humans.

Potential animal models of seasonal affective disorder

Seasonal affective disorder (SAD) is characterized by depressive episodes during winter that are alleviated during summer and by morning bright light treatment. Currently, there is no animal model of SAD. However, it may be possible to use rodents that respond to day length (photoperiod) to understand how photoperiod can shape the brain and behavior in humans. As nights lengthen in the autumn, the duration of the nightly elevation of melatonin increase; seasonally breeding animals use this information to orchestrate seasonal changes in physiology and behavior. SAD may originate from the extended duration of nightly melatonin secretion during fall and winter. These similarities between humans and rodents in melatonin secretion allows for comparisons with rodents that express more depressive-like responses when exposed to short day lengths. For instance, Siberian hamsters, fat sand rats, Nile grass rats, and Wistar rats display a depressive-like phenotype when exposed to short days. Current research in depression and animal models of depression suggests that hippocampal plasticity may underlie the symptoms of depression and depressive-like behaviors, respectively. It is also possible that day length induces structural changes in human brains. Many seasonally breeding rodents undergo changes in whole brain and hippocampal volume in short days. Based on strict validity criteria, there is no animal model of SAD, but rodents that respond to reduced day lengths may be useful to approximate the neurobiological phenomena that occur in people with SAD, leading to greater understanding of the etiology of the disorder as well as novel therapeutic interventions.

Photoperiod alters pain responsiveness via changes in pelage characteristics

Small mammals use day length to adjust morphology and physiology to anticipate seasonal changes in environmental conditions. The canonical photoperiod-mediated annual adaptation is seasonal breeding. However, increasing evidence suggests that day-length information can induce plasticity in the nervous system, and thus provoke behavioral plasticity that can aid in winter survival. We hypothesized that low temperatures and reduced food availability in the winter would necessitate the evolution of increased pain tolerance mediated by short day lengths. Siberian hamsters ( Phodopus sungorus (Pallas, 1773)) housed in short days regressed their reproductive tracts and molted to winter pelage. Short-day hamsters also displayed elevated latencies of nociceptive responses in the hot-plate test, suggesting reduced pain responsivity. Prior to assessing potential neuronal or neuroendocrine mediators of altered pain responses, however, we investigated the possibility that changes in fur characteristics mediated photoperiod differences in pain responsivity. Removal of fur with a depilatory cream eliminated photoperiod differences in pain responsivity. Taken together, these data indicate that day length regulates thermal pain responses via changes in fur properties; also, changes in pelage properties have both thermoregulatory and thermal insulatory properties.

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.

Affective and adrenocorticotrophic responses to photoperiod in Wistar rats

The present study tested the hypothesis that seasonal intervals of exposure to modest changes in photoperiod, typical of those experienced by humans living in temperate latitudes (10-14 h light/day), engage changes in emotional behaviour of Wistar rats, a commonly-used animal model for investigations of affective physiology. Short day lengths (<or= 12 h light/day) induced behavioural despair in a forced-swim test, exploratory anxiety in an open field arena, and anhedonia in a two-bottle sucrose preference task, relative to longer day lengths. Plasma adrenocorticotrophic hormone was lower in short-day relative to long-day rats, but testosterone and corticosterone concentrations were comparable across treatments. In common with animals that engage reproductive responses to day length, reproductively nonresponsive mammals such as Wistar rats exhibit changes in affective state following small changes in day length. Wistar rats may provide an animal model for the study of seasonal mood regulation because the neuroendocrine, depressive, anxious and anhedonic responses of Wistar rats to short days bear similarities to those observed in some human populations. Standard laboratory husbandry practices (exposure to a 12 : 12 h light/dark cycle) may inadvertently deliver a chronic background depressive and anxiogenic stimulus.

Photoperiod alters central distribution of estrogen receptor alpha in brain regions that regulate aggression

Testosterone or its metabolite, estrogen, regulates aggression in males of many mammalian species. Because plasma testosterone levels are typically positively correlated with both aggression and reproduction, aggression is expected to be higher when males are in reproductive condition. However, in some photoperiodic species such as Siberian hamsters (Phodopus sungorus), males are significantly more aggressive in short day lengths when the testes are regressed and circulating testosterone concentrations are reduced. These results led to the formation of the hypothesis that aggression is modulated independently of circulating steroids in Siberian hamsters. Thus, recent studies have been designed to characterize the role of other neuroendocrine factors in modulating aggression. However, aggression may be mediated by testosterone or estrogen despite basal concentrations of these steroids by increasing sensitivity to steroids in specific brain regions. Consistent with this hypothesis, we found that males housed under short days have increased expression of estrogen receptor alpha in the bed nucleus of the stria terminalis, medial amygdala, and central amygdala. Neural activation in response to an aggressive encounter was also examined across photoperiod.

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.

Housing condition alters immunological and reproductive responses to day length in Siberian hamsters (Phodopus sungorus)

During winter, increased thermoregulatory demands coincide with limited food availability necessitating physiological tradeoffs among expensive physiological processes resulting in seasonal breeding among small mammals. In the laboratory, short winter-like day lengths induce regression of the reproductive tract, but also enhance many aspects of immune function. It remains unspecified the extent to which bolstered immune responses in short days represent enhanced immune function per se compared to long days or represents energetic disinhibition mediated by the regression of the reproductive tract. Cohabitation of male Siberian hamsters with intact female conspecifics can block short-day reproductive regression. We sought to determine whether female cohabitation could also block the enhanced immune function associated with short days. Adult male Siberian hamsters were housed in long or short day lengths in one of three housing conditions: (1) single-housed, (2) housed with a same sex littermate, or (3) housed with an ovariectomized female. Delayed-type hypersensitivity (DTH) responses were assessed after 8 weeks of photoperiod treatment. Housing with an ovariectomized female was not sufficient to block short-day reproductive regression, but prevented short-day enhancement of DTH responses. Housing with a male littermate did not alter reproductive or immune responses in either photoperiod. These data suggest that short day enhancement of immune function is independent of photoperiod-mediated changes in the reproductive system.

HPA activity and neotic and anxiety-like behavior vary among Peromyscus species

Behaviorally plastic species are more likely to invade and endure in new areas, and behaviorally plastic individuals tend to be attracted to novelty (i.e., neophilic). Furthermore, neophilic behaviors are often influenced by glucocorticoids. Thus in addition to environmental conditions and vicariant events, behavioral plasticity and its endocrinological mediators may influence the extent of vertebrate geographic distributions. Some species of mice in the genus, Peromyscus, occupy most of North America whereas others are restricted to small areas. We predicted that one widespread species (Peromyscus maniculatus) would interact more with novel objects, more readily explore novel environments, and possess hypo-responsive HPA axes compared to species with small ranges. Our hypothesis was not supported, but given the small number of species in this study and the high anxiety-like behavior in captive P. maniculatus, it is premature to reject the hypothesis that behavioral flexibility affects geographic distribution in Peromyscus. Indeed, behavioral and HPA axis variation was complementary among species, which is opposite of the pattern typically detected within species, suggesting that future studies of glucocorticoid mediation of neotic and anxiety-like behaviors in Peromyscus would be valuable.

Photoperiod alters affective responses in collared lemmings

This study examined photoperiodic regulation of affective behaviours in collared lemmings (Dicrostonyx groenlandicus). Male and female lemmings were housed in either long (LD 22:2), intermediate (LD 16:8), or short days (LD 8:16) for 9 weeks. Exposure to short days induced moult to a winter pelage and gonadal regression in male, but not female, lemmings. Lemmings housed in long days reduced anxiety-like responses in the elevated plus maze. Depressive-like behaviours were decreased in the intermediate photoperiod relative to other photoperiod groups.

Perinatal photoperiod organizes adult immune responses in Siberian hamsters (Phodopus sungorus)

Individuals of many nontropical rodent species display reproductive, immunological, and somatic responses to day length. In general, short day (SD) lengths inhibit reproduction and enhance immune function in the laboratory when all other conditions are held constant. Most studies to date have focused on seasonal variation in immune function in adulthood. However, perinatal photoperiods also communicate critical day length information and serve to establish a developmental trajectory appropriate for the time of year. Nontropical rodents born early in the breeding season undergo rapid reproductive development, presumably to promote mating success during their first reproductive season. Rodents born late in the breeding season suspend somatic growth and puberty until the following vernal breeding season. We tested the hypothesis that perinatal day lengths have similar enduring effects on the immune system of rodents. Siberian hamsters (Phodopus sungorus) were maintained prenatally and until weaning (21 days) in either SDs (8 h light:16 h dark) or long days (LD) (16 h light:8 h dark), then they were weaned into either the opposite photoperiod or maintained in their natal photoperiod, forming four groups (LD-LD, LD-SD, SD-LD, and SD-SD). After 8-wk in these conditions, cell-mediated immune activity was compared among groups. SD-SD hamsters of both sexes enhanced immune function relative to all other groups. The reproductive effects of perinatal photoperiod were not evident by the end of the experiment; circulating testosterone and cortisol sampled at the end of the experiment reflected the postweaning, but not the perinatal photoperiod. This experiment demonstrates long-lasting organizational effects of perinatal photoperiod on the rodent immune system and indicates that photoperiod-induced changes in the immune system are dissociable from changes in the reproductive system.