Mechanisms and methods in ecoimmunology: integrating within-organism and between-organism processes

Ecoimmunology utilizes techniques from traditionally laboratory-based disciplines--for example, immunology, genomics, proteomics, neuroendocrinology, and cell biology--to reveal how the immune systems of wild organisms both shape and respond to ecological and evolutionary pressures. Immunological phenotypes are embedded within a mechanistic pathway leading from genotype through physiology to shape higher-order biological phenomena. As such, "mechanisms" in ecoimmunology can refer to both the within-host processes that shape immunological phenotypes, or it can refer the ways in which different immunological phenotypes alter between-organism processes at ecological and evolutionary scales. The mechanistic questions ecoimmunologists can ask, both within-organisms and between-organisms, however, often have been limited by techniques that do not easily transfer to wild, non-model systems. Thus, a major focus in ecoimmunology has been developing and refining the available toolkit. Recently, this toolkit has been expanding at an unprecedented rate, bringing new challenges to choosing techniques and standardizing protocols across studies. By confronting these challenges, we will be able to enhance ecoimmunological inquiries into the physiological basis of life-history trade-offs; the development of low-cost biomarkers for susceptibility to disease; and the investigation of the ecophysiological underpinnings of disease ecology, behavior, and the coevolution of host-parasite systems. The technical advances in, and crossover technologies from, disciplines associated with ecoimmunology and how these advances can help us understand the mechanistic basis of immunological variability in wild species were the focus of the symposium, Methods and Mechanisms in Ecoimmunology.

Photoperiodic changes in endocannabinoid levels and energetic responses to altered signalling at CB1 receptors in Siberian hamsters

Siberian hamsters (Phodopus sungorus) adapt to seasonal environmental conditions with marked changes in body mass, primarily in the form of adiposity. Winter-like conditions (e.g. short days) are sufficient to decrease body mass by approximately 30% in part via reductions in food intake. The neuroendocrine mechanisms responsible for these changes are not well understood, and homeostatic orexigenic/anorexigenic systems of the hypothalamus provide little explanation. We investigated the potential role of endocannabinoids, which are known modulators of appetite and metabolism, in mediating seasonal changes in energy balance. Specifically, we housed hamsters in long or short days for 0, 3, or 9 weeks and measured endocannabinoid levels in the hypothalamus, brainstem, liver and retroperitoneal white adipose tissue (RWAT). An additional group of males housed in short days for 25 weeks were also compared with long-day controls. Following 9 weeks in short days, levels of the endocannabinoid 2-arachidonoylglycerol (2-AG) were significantly elevated in RWAT and reduced in brainstem, although they returned to long-day levels by week 25 in short-day males that had cycled back to summer-like energy balance. Endocannabinoid levels in these tissues correlated significantly with adiposity and change in body mass. No photoperiodic changes were observed in the hypothalamus or liver; however, sex differences in 2-AG levels were found in the liver (males > females). We further tested the effects of CB(1) receptor signalling on ingestive behaviour. Five daily injections of CB(1) antagonist SR141716 significantly reduced food intake and body mass but not food hoarding. Although the CB(1) agonist arachidonyl-2-chloroethylamide did not appreciably affect either ingestive behaviour, body mass was significantly elevated following 2 days of injections. Taken altogether, these findings demonstrate that endocannabinoid levels vary with sex and photoperiod in a site-specific manner, and that altered signalling at CB(1) receptors affects energy balance in Siberian hamsters.

Maternal contact differentially modulates central and peripheral oxytocin in rat pups during a brief regime of mother-pup interaction that induces a filial huddling preference

Central oxytocin mediates the acquisition of a filial preference for maternal odour in rat pups, manifested by their huddling preferences. The present study was designed to examine whether maternal care modulates oxytocin concentrations in rat pups and, if so, how different types of maternal contact are associated with the pups' oxytocin concentrations. Pairs of 14-day-old littermates were removed from their home cage for 1 h and then placed with a lactating foster mother for 2 h, or they remained isolated at room temperature. Enzyme immunoassays revealed that maternal care and maternal separation can differentially modulate pups' oxytocin concentrations. Both hypothalamic and serum oxytocin increased during the 1-h separation. Pups placed with a foster mother after the separation maintained the same concentrations in the hypothalamus and serum through the fostering period. By contrast, pups placed with no mother showed a further increase in hypothalamic oxytocin but serum oxytocin decreased. Behavioural analyses revealed that skin-to-skin contact with the mother, but not simple physical contact or maternal licking/grooming, was positively correlated with the pups' hypothalamic oxytocin concentrations. These neuroendocrine data match previous findings showing that skin-to-skin contact with mother facilitates the acquisition of the pups' huddling preference for a maternally-associated odour. Taken together, the present study suggests that maternal skin-to-skin contact stimulates pups' central oxytocin, at the same time as creating the conditions for inducing a preference for maternal odour and establishing a social affiliation in rat pups; the natural schedule of maternal separation and reunion may modulate pups' oxytocin concentrations, providing scaffolding for the acquisition of their filial huddling preference.

Photoperiod and testosterone interact to drive seasonal changes in kisspeptin expression in Siberian hamsters (Phodopus sungorus)

Kisspeptin, a neuropeptide product of the KiSS-1 gene, has recently been implicated in the regulation of seasonal breeding in a number of species, including Siberian hamsters. In this species, kisspeptin expression is reduced in the anteroventral periventricular nucleus (AVPV) following exposure to inhibitory day lengths, and exogenous kisspeptin activates the reproductive neuroendocrine axis of reproductively quiescent animals. Because sex steroids can impact kisspeptin expression, it is unclear whether changes in kisspeptin occur in direct response to photoperiodic cues or secondarily in response to changes in sex steroid concentrations resulting from the transition to reproductive quiescence. The present study aimed to assess the relative contributions of photoperiod and testosterone in regulating kisspeptin expression in Siberian hamsters. Animals housed in long or short day lengths for 8 weeks were either castrated or received sham surgeries. Half of the hamsters in each photoperiod were given testosterone to mimic long-day sex steroid concentrations. The results obtained indicate that kisspeptin neurones in the AVPV and arcuate nuclei were influenced by both photoperiod and testosterone. In the AVPV, removal of testosterone or exposure to inhibitory day lengths led to a marked reduction in kisspeptin-immunoreactive cells, and testosterone treatment increased cell numbers across conditions. Importantly, long-day castrates exhibited significantly more kisspeptin cells than short-day castrates or intact short-day animals with empty capsules, suggesting the influences of photoperiod, independent of gonadal steroids. In general, the opposite pattern emerged for the arcuate nuclei. Collectively, these data suggest a role for both gonadal-dependent and independent (i.e. photoperiodic) mechanisms regulating seasonal changes in kisspeptin expression in Siberian hamsters.

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

Many nontropical animals display physiological and behavioural changes in response to seasonal environmental cues including photoperiod (day length). Male Siberian hamsters (Phodopus sungorus) housed in short photoperiod undergo testicular regression accompanied by reduced circulating testosterone and decreased reproductive behaviour. By contrast to the majority of small mammals studied, aggressive behaviour is elevated in short-day Siberian hamsters when blood testosterone concentrations are not detectable. Because gonadal steroid hormones influence neuronal nitric oxide synthase (nNOS), and this enzyme has been implicated in aggressive behaviour, we hypothesized that nNOS expression would be decreased in short-day male Siberian hamsters and negatively correlated with the display of territorial aggression. Adult male Siberian hamsters were individually housed in either long (LD 16:8 h) or short (LD 8:16 h) photoperiods for 10 weeks. Hamsters were assigned to one of two categories by assessing testicular volume and plasma testosterone values: (i) photoperiodic responsive (i.e. regressed testes and low testosterone concentrations) or (ii) photoperiodic nonresponsive (i.e. testes size and circulating testosterone concentrations equivalent to hamsters maintained in long days). At week 10, aggression was assessed using a resident-intruder test. Latency to initial attack, frequency of attacks and duration of total attacks were recorded during a 10-min aggression trial. Brains were collected immediately after behavioural testing and stained for nNOS expression using immunohistochemistry. All short day-housed hamsters were significantly more aggressive than long-day animals, regardless of gonadal size or testosterone concentrations. Short-day animals, both reproductively responsive and nonresponsive morphs, also had significantly less nNOS-immunoreactive cells in the anterior and basolateral amygdaloid areas and paraventricular nuclei compared to long-day hamsters. Together, these results suggest that seasonal aggression in male Siberian hamsters is regulated by photoperiod, through mechanisms that are likely independent from gonadal steroid hormones.

Lack of immunological responsiveness to photoperiod in a tropical rodent, Peromyscus aztecus hylocetes

Non-tropical rodents undergo seasonal changes in immune function and disease. It has been hypothesized that seasonal fluctuations in immunity of non-tropical rodents are due to suppressed immune function during harsh winter conditions. A logical extension of this hypothesis is that seasonal changes in immunity should be reduced or absent in tropical rodents that do not experience marked seasonal fluctuations in environmental conditions; however this hypothesis remains to be tested. The present study tested the effects of photoperiod on humoral and cell-mediated immune function of male Aztec mice ( Peromyscus aztecus hylocetes). P. a. hylocetes were housed in long (L:D 16:8) or short days (L:D 8:16) for 10 weeks. Animals were then immunized with the antigen keyhole limpet hemocyanin (KLH). Serum anti-KLH immunoglobulin G (IgG) concentrations and splenocyte proliferation in response to the T-cell mitogen Concanavalin A were assessed. Short-day P. a. hylocetes did not display differences in reproductive or immune measures compared with long-day mice. Collectively, these results suggest that P. a. hylocetes are reproductively and immunologically non-responsive to photoperiod. This lack of immunological responsiveness is likely due to the relative seasonal stability of their environment compared with temperate zone species.

Sympathoadrenal system differentially affects photoperiodic changes in humoral immunity of Siberian hamsters (Phodopus sungorus)

Siberian hamsters (Phodopus sungorus) rely on photoperiod as a primary cue to coordinate seasonally appropriate changes in physiology and behaviour. Among these seasonal changes is reduced immune function in short 'winter-like' days, compared to long 'summer-like' days. Previous evidence suggests that immune function is regulated, in part, by the sympathoadrenal system. The precise role of the sympathoadrenal system in regulating photoperiodic changes in immune function, however, remains unspecified. The goal of the present study was to examine the differential contributions of direct sympathetic innervation of immune target tissue, as well as adrenal medullary catecholamines, to photoperiodic changes in immune function in male Siberian hamsters. In Experiment 1, hamsters underwent either bilateral surgical removal of the adrenal medulla (ADMEDx), or sham surgeries, and were maintained in long (LD 16 : 8) or short days (LD 8 : 16). In Experiment 2, hamsters received either surgical denervation of the spleen, or sham surgeries, and were then housed in long or short days. Serum anti-KLH IgG concentrations and splenic norepinephrine (NE) content were determined in both experiments. Short-day hamsters had reduced humoral immunity compared to long-day hamsters. ADMEDx reduced immune function, but only in long-day hamsters. In contrast, splenic denervation reduced humoral immunity, but only in short-day hamsters. Splenic NE content was increased in short days and by ADMEDx. NE content was markedly reduced in denervated hamsters compared to sham-operated hamsters. Collectively, these results suggest that the sympathoadrenal system is associated with photoperiodic changes in immune function.

Novel method for localized, functional sympathetic nervous system denervation of peripheral tissue using guanethidine

A simple technique for local chemical sympathectomy of peripheral tissues is described using guanethidine. Multiple microinjections of guanethidine were made into inguinal or epididymal white adipose tissue (IWAT and EWAT) pads or spleens of hamsters. Guanethidine virtually abolished the sympathetic innervation of both EWAT and IWAT, as measured by the absence of significant norepinephrine (NE) tissue content two weeks later and as suggested by the two-fold increase in IWAT mass characteristic of surgically induced WAT denervation. These measures were not affected in the contralateral pads given equivolumetric injections of saline. Guanethidine injections into the spleen lead to a functional sympathectomy, as indicated by significant depletions of NE content. Because guanethidine treatment did not decrease body mass, induce ptosis, or spread to closely associated adjacent tissue (contralateral EWAT pad), no chemical-induced malaise or global sympathetic denervation was suggested. Guanethidine was more effective than two other local sympathectomy treatments, injections of the sympathetic neurotoxin anti-dopamine-beta-hydroxylase saporin or surgical denervation, in decreasing IWAT NE content and increasing IWAT pad mass. Collectively, these results suggest that locally applied, chemical sympathectomy with guanethidine provides an effective, restricted method for sympathectomizing WAT, spleen and likely other peripheral tissues.

Direct innervation of white fat and adrenal medullary catecholamines mediate photoperiodic changes in body fat

Seasonal adjustments in Siberian hamster adiposity are triggered by day length changes [i.e., short "winter-like" days (SDs) elicit body fat decreases vs. long "summer-like" days (LDs)]. These and other white adipose tissue (WAT) mass decreases traditionally have been ascribed to lipolysis triggered by sympathetically mediated, adrenal medullary released epinephrine; however, recent evidence suggests that direct sympathetic innervation of WAT also is important. Therefore, the contributions of WAT sympathetic innervation and adrenal medullary catecholamines to SD-induced decreases in adiposity were tested. Siberian hamsters were surgically bilaterally adrenal demedullated (ADMEDx) or sham ADMEDx, and all had one inguinal WAT (IWAT) pad sympathectomized via locally injected guanethidine, with the contralateral pad serving as a within-animal innervated control. One-half of the hamsters remained in LDs; the remainder was transferred to SDs. Guanethidine and ADMEDx abolished IWAT norepinephrine and adrenal epinephrine contents, respectively. Although sympathetic denervation or ADMEDx alone did not block SD-induced decreases in IWAT mass, their combination did. These results suggest that both adrenal catecholamines and the sympathetic innervation of WAT interact to decrease SD-induced decreased adiposity.

Acute and chronic social defeat suppresses humoral immunity of male Syrian hamsters (Mesocricetus auratus)

Stressors, both physical and psychological, can activate the hypothalamic-pituitary-adrenal (HPA) axis, leading to a wide range of physiological responses including increased glucocorticoid release and suppression of immune function. The majority of studies published to date have focused on the effects of physical stressors (e.g., cold exposure, electric shock) on immunity. The present study examined the role of a stressor, social defeat, on humoral immune function of Syrian hamsters (Mesocricetus auratus). Specifically, adult male Syrian hamsters experienced social defeat (i.e., exposure to a dominant animal in that animal's home cage) that was either acute (i.e., a single exposure) or chronic (i.e., daily exposures across 5 days). A control group of animals was placed in a resident's home cage without the resident animal present and did not experience defeat. After the last encounter, blood samples were drawn and animals were subsequently injected with keyhole limpet hemocyanin (KLH). Blood samples were again taken 5 and 10 days postimmunization and serum was analyzed to determine serum cortisol and anti-KLH immunoglobulin G (IgG) concentrations. Cortisol concentrations were elevated in both acutely and chronically defeated hamsters compared with control animals. In contrast, serum IgG concentrations were significantly reduced in both groups of defeated hamsters compared with control animals. Collectively, these results demonstrate that both acute social defeat and chronic social defeat lead to activation of the HPA axis and suppression of humoral immune function. These data suggest that social defeat is an important, ecologically relevant model with which to examine stress-induced immune suppression in rodents.

Leptin effects on immune function and energy balance are photoperiod dependent in Siberian hamsters (Phodopus sungorus)

Many adaptations have evolved in small mammals to maximize survival during winter. One such coping tactic in many species is an alteration of immune function in advance of the stressful conditions of winter. Leptin is a hormone produced by adipose tissue, and in addition to its central role in energy metabolism, leptin mediates the interactions among energy allocation, immune function, and reproduction. To examine this interaction further, exogenous leptin was administered for 2 weeks via osmotic minipumps to Siberian hamsters (Phodopus sungorus) housed in long or short days for a total of 12 weeks. Short-day hamsters displayed the expected reductions in humoral immune function, body mass, fat mass, and food intake. In Exp 1, exogenous leptin counteracted the reduction in food intake and the suppression of immune function in short days. In Exp 2, when the leptin-induced increase in food intake in short-day hamsters was prevented, leptin did not enhance immune function. In most of the measured fat pads and body mass, leptin had no effect in long days. In sum, leptin administered to short-day animals caused them to respond, in many cases, like long-day animals. Taken together, these data suggest that leptin acts indirectly to mediate energy allocation to humoral immune function. Additionally, leptin appears to act differentially, according to photoperiod, to regulate both immune and energetic parameters.

SCN efferents to peripheral tissues: implications for biological rhythms

The suprachiasmatic nucleus (SCN) is the principal generator of circadian rhythms and is part of an entrainment system that synchronizes the animal with its environment. Here, we review the possible communication of timing information from the SCN to peripheral tissues involved in regulating fundamental physiological functions as revealed using a viral, transneuronal tract tracer, the pseudorabies virus (PRV). The sympathetic nervous system innervation of the pineal gland and the sympathetic outflow from brain to white adipose tissue were the first demonstrations of SCN-peripheral tissue connections. The inclusion of the SCN as part of these and other circuits was the result of lengthened postviral injection times compared with those used previously. Subsequently, the SCN has been found to be part of the sympathetic outflow from the brain to brown adipose tissue, thyroid gland, kidney, bladder, spleen, adrenal medulla, and perhaps the adrenal cortex. The SCN also is involved in the parasympathetic nervous system innervation of the thyroid, liver, pancreas, and submandibular gland. Individual SCN neurons appear connected to more than one autonomic circuit involving both sympathetic and parasympathetic innervation of a single tissue, or sympathetic innervation of two different peripheral tissues. Collectively, the results of these PRV studies require an expansion of the traditional roles of the SCN to include the autonomic innervation of peripheral tissues and perhaps the modulation of neuroendocrine systems traditionally thought to be controlled solely by hypothalamic stimulating/inhibiting factors.

Short-day increases in aggression are inversely related to circulating testosterone concentrations in male Siberian hamsters (Phodopus sungorus)

Many nontropical rodent species display seasonal changes in both physiology and behavior that occur primarily in response to changes in photoperiod. Short-day reductions in reproduction are due, in part, to reductions in gonadal steroid hormones. In addition, gonadal steroids, primarily testosterone (T), have been implicated in aggression in many mammalian species. Some species, however, display increased aggression in short days despite basal circulating concentrations of T. The goal of the present studies was to test the effects of photoperiod on aggression in male Siberian hamsters (Phodopus sungorus) and to determine the role of T in mediating photoperiodic changes in aggression. In Experiment 1, hamsters were housed in long and short days for either 10 or 20 weeks and aggression was determined using a resident-intruder model. Hamsters housed in short days for 10 weeks underwent gonadal regression and displayed increased aggression compared to long-day-housed animals. Prolonged maintenance in short days (i.e., 20 weeks), however, led to gonadal recrudescence and reduced aggression. In Experiment 2, hamsters were housed in long and short days for 10 weeks. Half of the short-day-housed animals were implanted with capsules containing T whereas the remaining animals received empty capsules. In addition, half of the long-day-housed animals were castrated whereas the remaining animals received sham surgeries. Short-day control hamsters displayed increased aggression compared to either castrated or intact long-day-housed animals. Short-day-housed T treated hamsters, however, did not differ in aggression from long-day-housed animals. Collectively, these results confirm previous findings of increased aggression in short-day-housed hamsters and suggest that short-day-induced increases in aggression are inversely related to gonadal steroid hormones.

Stroke in estrogen receptor-alpha-deficient mice

BACKGROUND AND PURPOSE

Recent evidence suggests that endogenous estrogens or hormone replacement therapy can ameliorate brain damage from experimental stroke. Protective mechanisms involve enhanced cerebral vasodilation during ischemic stress as well as direct preservation of neuronal viability. We hypothesized that if the intracellular estrogen receptor subtype-alpha (ERalpha) is important to estrogen's signaling in the ischemic brain, then ERalpha-deficient (knockout) (ERalphaKO) female mice would sustain exaggerated cerebral infarction damage after middle cerebral artery occlusion.

METHODS

The histopathology of cresyl violet-stained tissues was evaluated after reversible middle cerebral artery occlusion (2 hours, followed by 22 hours of reperfusion) in ERalphaKO transgenic and wild-type (WT) mice (C57BL/6J background strain). End-ischemic cerebral blood flow mapping was obtained from additional female murine cohorts by using [(14)C]iodoantipyrine autoradiography.

RESULTS

Total hemispheric tissue damage was not altered by ERalpha deficiency in female mice: 51.9+/-10.6 mm(3) in ERalphaKO versus 60.5+/-5.0 mm(3) in WT. Striatal infarction was equivalent, 12.2+/-1.7 mm(3) in ERalphaKO and 13.4+/-1.0 mm(3) in WT mice, but cortical infarction was paradoxically smaller relative to that of the WT (20.7+/-4.5 mm(3) in ERalphaKO versus 30.6+/-4.1 mm(3) in WT). Intraocclusion blood flow to the parietal cortex was higher in ERalphaKO than in WT mice, likely accounting for the reduced infarction in this anatomic area. There were no differences in stroke outcomes by region or genotype in male animals.

CONCLUSIONS

Loss of ERalpha does not enhance tissue damage in the female animal, suggesting that estrogen inhibits brain injury by mechanisms that do not depend on activation of this receptor subtype.

Ejaculatory abnormalities in mice lacking the gene for endothelial nitric oxide synthase (eNOS-/-)

Nitric oxide (NO) has been established as a neurotransmitter in both the central and peripheral nervous systems. Three isoforms of its synthetic enzyme, NO synthase (NOS), have been identified: 1) in the endothelial lining of blood vessels (eNOS), 2) an inducible form found in macrophages (iNOS), and 3) in neurons (nNOS). Previous studies using pharmacological agents that block all three isoforms of NOS have revealed that NO mediates several aspects of reproductive physiology and behavior, including anomalies in male sexual behavior and erectile function. To determine the specific contribution of the endothelial isoform of NOS in male reproductive behavior, we studied mice missing the gene for only eNOS (eNOS-/-). Wild-type (WT) and eNOS-/- animals were placed with an estrous WT female and observed for 45 min. Both WT and eNOS-/- mice displayed equivalent motivation to mount the stimulus female. However, eNOS-/- mice exhibited striking anomalies in ejaculatory function. A higher percentage of eNOS-/- than WT mice ejaculated during the testing period (p < 0.001). This increased propensity to ejaculate was apparently due to reduced stimulation required to elicit ejaculation; eNOS-/- mice required significantly fewer mounts (p < 0.003) and intromissions (p < 0.001) to ejaculate compared to WT mice. Taken together, these results suggest that NO synthesized by eNOS may be involved in ejaculatory physiology, but not sexual motivation.

Elimination of aggressive behavior in male mice lacking endothelial nitric oxide synthase

Male mice with targeted deletion of the gene encoding the neuronal isoform of nitric oxide synthase (nNOS(-/-)) display increased aggressive behavior compared with wild-type (WT) mice. Specific pharmacological inhibition of nNOS with 7-nitroindazole also augments aggressive behavior. We report here that male mice with targeted deletion of the gene encoding endothelial NOS (eNOS(-/-)) display dramatic reductions in aggression. The effects are selective, because an extensive battery of behavioral tests reveals no other deficits. In the resident-intruder model of aggression, resident eNOS(-/-) males show virtually no aggression. Latency for aggression onset is 25-30 times longer in eNOS(-/-) males compared with WT males in the rare instances of aggressive behaviors. Similarly, a striking lack of aggression is noted in tests of aggression among groups of four mice monitored in neutral cages. Although eNOS(-/-) mice are hypertensive ( approximately 14 mmHg blood pressure elevation), hypertension does not appear responsible for the diminished aggression. Reduction of hypertension with hydralazine does not change the prevalence of aggression in eNOS(-/-) mice. Extensive examination of brains from eNOS(-/-) male mice reveals no obvious neural damage from chronic hypertension. In situ hybridization in WT animals reveals eNOS mRNA in the brain associated exclusively with blood vessels and no neuronal localizations. Accordingly, vascular eNOS in the brain appears capable of influencing behavior with considerable selectivity.

Effects of food deprivation and metabolic fuel utilization on food hoarding by jirds (Meriones shawi)

Food hoarding plays an important role in the energetic repertoire of a variety of mammalian species. Both food hoarding and food intake have been examined in rodents using several energetic challenges including food deprivation, treatment with metabolic fuel blockers, and enhancement of fuel storage. In the present experiment, we examined food hoarding by female jirds (Meriones shawi), a desert rodent species occupying the arid steppes and desert regions of Egypt. Jirds are prodigious hoarders in the field; however, virtually nothing is known about their hoarding within controlled laboratory settings. In the present study, the effects of food deprivation as well as alterations in metabolic fuel utilization (i.e., 2-deoxy-D-glucose and isophane insulin) on food hoarding and food intake were tested in female jirds using a simulated burrow system. Jirds decreased body mass and increased food consumption following either 32 or 56-h food deprivation. Food hoarding, however, was virtually abolished after food deprivation and treatment with 2-DG. In contrast, isophane insulin treatment had no effect on food consumption or hoarding in this species. Taken together, the present results suggest that total body mass (fat), rather than short-term metabolic fuel utilization, regulates both food consumption and hoarding in female jirds. In addition, these results provide a novel set of appetitive responses to these energetic challenges in small mammals.

Nocturnal motor coordination deficits in neuronal nitric oxide synthase knock-out mice

Nitric oxide is formed in the brain primarily by neurons containing neuronal nitric oxide synthase (nNOS), though some neurons may express endothelial NOS (eNOS), and inducible NOS (iNOS) only occurs in neurons following toxic stimuli. Mice with targeted disruption of nNOS (nNOS-) display distended stomachs with hypertrophied pyloric sphincters reflecting loss of nNOS in myenteric plexus neurons. nNOS- animals resist brain damage following middle cerebral artery occlusions consistent with evidence that excess release of nitric oxide mediates neurotoxicity in ischemic stroke. Neuronal NOS- mice have no grossly evident defects in locomotor activity, breeding long-term depression in the cerebellum, long-term potentiation in the hippocampus, and overall sensorimotor function. However, nNOS- animals display excessive, inappropriate sexual behavior and dramatic increases in aggression. Because the cerebellum possesses the greatest levels of nNOS neurons in the brain, it was surprising that presumed cerebellar functions such as balance and coordination were grossly normal in nNOS- mice. These previous studies were all conducted during the day (between 1400 and 1600, lights on at 0700). We now report striking, discrete abnormalities in balance and motor coordination in nNOS-mice reflected selectively at night.

Castration does not inhibit aggressive behavior in adult male prairie voles (Microtus ochrogaster)

The relationship between castration and reduced male aggression is well established. However, anecdotal observations of male prairie voles (Microtus ochrogaster) suggest that castration does not reduce aggressive behavior. To investigate the role of testicular androgens on aggressive behavior, castrated or gonadally intact male prairie voles were paired in a neutral arena with a gonadally intact vole. Castration did not reduce the frequency of intermale aggression. In Experiment 2, aggressive behavior was examined further using resident-intruder, grouped aggression, and aggression against a lactating female models. Again, castration did not affect the frequency of aggression in male prairie voles. Taken together, the results of this study suggest that aggressive behavior may be independent of gonadal steroid hormones in adult male prairie voles.

Circadian locomotor analysis of male mice lacking the gene for neuronal nitric oxide synthase (nNOS-/-)

Nitric oxide (NO) is an endogenous gas that functions as a neurotransmitter. Because NO is very labile with a half-life of less than 5 sec, most functional studies of NO have manipulated its synthetic enzyme, NO synthase (NOS). Three isoforms of NOS have been identified: (1) in the endothelial lining of blood vessels (eNOS), (2) an inducible form found in macrophages (iNOS), and (3) in neurons (nNOS). Most pharmacological studies to date have blocked all three isoforms of NOS. Previous studies using such agents have revealed that NO might be necessary for photic entrainment of circadian rhythms; general NOS inhibitors attenuate phase shifts of free-running behavior, light-induced c-fos expression in the suprachiasmatic nucleus (SCN), and phase shifts of neural firing activity in SCN maintained in vitro. To assess the specific role of nNOS in mediating entrainment of circadian rhythms, mice with targeted deletion of the gene encoding the neuronal isoform of NOS (nNOS-/-) were used. Wild-type (WT) and nNOS-/- mice initially were entrained to a 14:10 light:dark (LD) cycle. After 3 weeks, the LD cycle was either phase advanced or phase delayed. After an additional 3 weeks, animals were held in either constant dim light or constant dark. WT and nNOS-/- animals did not differ in their ability to entrain to the LD cycle, phase shift locomotor activity, or free run in constant conditions. Animals held in constant dark were killed after light exposure during either the subjective day or subjective night to assess c-fos induction in the SCN. Light exposure during the subjective night increased c-fos expression in the SCN of both WT and nNOS-/- mice relative to animals killed after light exposure during the subjective day. Taken together, these findings suggest that NO from neurons might not be necessary for photic entrainment.

Neurobehavioral deficits in mice lacking the erythrocyte membrane cytoskeletal protein 4.1

The erythrocyte membrane cytoskeletal protein 4.1 (4.1R) is a structural protein that confers stability and flexibility to erythrocytes via interactions with the cytoskeletal proteins spectrin and F-actin and with the band 3 and glycophorin C membrane proteins. Mutations in 4.1R can cause hereditary elliptocytosis, a disease characterized by a loss of the normal discoid morphology of erythrocytes, resulting in hemolytic anemia [1]. Different isoforms of the 4.1 protein have been identified in a wide variety of nonerythroid tissues by immunological methods [2-5]. The variation in molecular weight of these different 4.1 isoforms, which range from 30 to 210 kDa [6], has been attributed to complex alternative splicing of the 4.1R gene [7]. We recently identified two new 4.1 genes: one is generally expressed throughout the body (4. 1G) [8] and the other is expressed in central and peripheral neurons (4.1N) [9]. Here, we examined 4.1R expression by in situ hybridization analysis and found that 4.1R was selectively expressed in hematopoietic tissues and in specific neuronal populations. In the brain, high levels of 4.1R were discretely localized to granule cells in the cerebellum and dentate gyrus. We generated mice that lacked 4.1R expression; these mice had deficits in movement, coordination, balance and learning, in addition to the predicted hematological abnormalities. The neurobehavioral findings are consistent with the distribution of 4.1R in the brain, suggesting that 4.1R performs specific functions in the central nervous system.

Short-day enhancement of immune function is independent of steroid hormones in deer mice (Peromyscus maniculatus)

The effects of photoperiod and steroid hormones on immune function were assessed in male and female deer mice (Peromyscus maniculatus). In experiment 1, male deer mice were castrated, castrated and given testosterone replacement, or sham-operated. Half of each experimental group were subsequently housed in either long (LD 16:8) or short days (LD 8:16) for 10 weeks. Short-day deer mice underwent reproductive regression and displayed elevated lymphocyte proliferation in response to the T-cell mitogen concanavalin A, as compared to long-day mice. In experiment 2, female deer mice were ovariectomized, ovariectomized and given estrogen replacement, or sham-operated. Animals from each of these experimental groups were subsequently housed in either LD 16:8 or LD 8:16 for 10 weeks. Short-day deer mice underwent reproductive regression and displayed reduced serum estradiol concentrations and elevated lymphocyte proliferation in response to concanavalin A, as compared to long-day mice. Surgical manipulation had no effect on lymphocyte proliferation in either male or female deer mice. Neither photoperiod nor surgical manipulation affected serum corticosterone concentrations. These results confirm that both male and female deer mice housed in short days enhance immune function relative to long-day animals. Additionally, short-day elevation in splenocyte proliferation appears to be independent of the influence of steroid hormones in this species.

Melatonin, immunity and cost of reproductive state in male European starlings

The effects of reproductive condition and exogenous melatonin on immune function were investigated in castrated European starlings, Sturnus vulgaris. Photorefractory and photostimulated starlings exposed to long days were implanted with melatonin or with blank capsules. Photostimulated starlings with blank capsules exhibited reduced splenocyte proliferation in response to the T-cell mitogen, concanavalin A, compared with the other long-day birds. Exogenous melatonin prevented the suppression of immune function by photostimulation. Photorefractory starlings, with or without melatonin implants, exhibited enhanced immune function compared with photostimulated starlings implanted with blanks. This enhancement was not mediated by endogenous melatonin, but appeared to be related to changes in reproductive state. In addition to the traditional costs of reproduction in birds (e.g. raising of young), there may be a cost of the reproductive state of starlings (i.e. whether they are photorefractory or photostimulated). These data are, we believe, the first to indicate a direct effect of reproductive state on immune function that is independent of both photoperiod (i.e., changes in the duration of melatonin secretion) and gonadal steroids.

Photoperiod, ambient temperature, and food availability interact to affect reproductive and immune function in adult male deer mice (Peromyscus maniculatus)

Winter is often stressful. Increased energetic demands in winter and concurrent reductions in energy availability can lead to an energetic imbalance and compromise survival. To increase the odds of surviving winter, individuals of some nontropical rodent species have evolved mechanisms to enhance immune function in advance of harsh winter conditions. Short day lengths provide a proximate cue for enhancement of immune function, an adaptive functional response to counter environmental stress-induced reduction in immune function. In the present study, photoperiod, ambient temperature, and food availability were manipulated and reproductive function and cell-mediated immunity were assessed in adult male deer mice (Peromyscus maniculatus). Mice maintained in short days regressed their reproductive systems and displayed enhanced immune function compared to long-day animals. Reduced food availability elevated corticosterone concentrations and suppressed reproductive and immune function, whereas ambient temperature alone had no effect on cell-mediated immunity. The suppressive effect of food restriction on reproductive and immune function was overcome by maintaining animals in short days. However, short-day, food-restricted mice maintained at low ambient temperatures displayed reduced reproductive and immune function compared to animals maintained at mild temperatures. Taken together, these results suggest that short-day enhancement of immune function can counteract some, but not all, of the immunosuppressive effects of winter stressors. These data are consistent with the hypothesis that immune function is enhanced in short days to counteract stress-mediated immune suppression occurring during winter.

Exogenous melatonin enhances cell-mediated, but not humoral, immune function in adult male deer mice (Peromyscus maniculatus)

Many nontropical rodent species display seasonal changes in reproductive physiology and metabolism, as well as in immune function. Field studies of seasonal changes in immune function typically report decreased immune function in the short days of winter compared to summer; presumably, reduced immunity in winter reflects increased glucocorticoid secretion in response to environmental stressors. In contrast, laboratory studies of photoperiodic changes in immunity invariably demonstrate increased immune function in short compared to long days. Although the precise mechanisms regulating short-day enhancement of immune function are not known, it is hypothesized that increased immunity is due to the increased duration of melatonin secretion in short compared to long days. However, melatonin can act both directly (i.e, via melatonin receptors located on lymphatic tissue) and indirectly (i.e., via alterations in gonadal steroids) to affect immune function. The present study examined the effects of exogenous melatonin administration on both cell-mediated and humoral immune function in adult male deer mice (Peromyscus maniculatus), as well as the role of gonadal steroid hormones in mediating these effects. Mice either were castrated to remove circulating androgens or received sham operations and were implanted with empty capsules or capsules containing melatonin. Individual mice implanted with melatonin underwent reproductive regression and displayed enhanced splenocyte proliferation to the T-cell mitogen concanavalin A; antigen-specific serum immunoglobulin M production was unaffected by melatonin treatment. Castration had no effect on either cell-mediated or humoral immune function. Taken together, these results suggest that exogenous melatonin enhances cell-mediated, but not humoral, immune function in adult male deer mice and that this effect is independent of gonadal steroid hormones. These results are consistent with a direct effect of melatonin on immunity.

Social, but not photoperiodic, influences on reproductive function in male Peromyscus aztecus

Nontropical rodents rely on environmental factors to restrict breeding to a specific time of the year. Among these factors, photoperiod appears to be the primary environmental cue used for predicting optimal breeding conditions. The purpose of the present study was to characterize reproductive function, as well as photoperiodic and social responsiveness in male Peromyscus aztecus, which occupy low-latitude, high-altitude habitats. In experiment 1, adult male P. aztecus were individually housed in either long (16L:8D) or short days (8L:16D) for 10 wk. Short-day mice did not differ from long-day mice on any reproductive or nonreproductive parameter. Comparisons to related Peromyscus species suggested that relative reproductive organ size and function were reduced in both long- and short-day males. Because ad libitum food and water were available, we reasoned that males in both photoperiodic conditions lacked social stimuli. To test this hypothesis, adult male P. aztecus were housed in long days either individually or with a female conspecific in experiment 2. Mice housed with females had significantly larger relative paired testes and epididymal masses, and higher testicular sperm counts and serum testosterone levels compared to those of individually housed mice. Taken together, these results suggest that social factors may play a more prominent role than photoperiod in stimulating reproductive development in laboratory-housed P. aztecus. These results are consistent with the results found for other low-latitude rodent species and suggest that P. aztecus uses a flexible rather than obligatory breeding strategy.

Impaired spatial working and reference memory in segmental trisomy (Ts65Dn) mice

To evaluate the cognitive phenotype of the segmental trisomy 16 (Ts65Dn) mouse, a model of Down Syndrome (DS, trisomy 21), we assessed spatial working and reference memory using a 12-arm radial maze (RAM). Ts65Dn mice made a greater number of reference memory errors across trials compared to control mice. Both genotypes showed improvement across trials, although improvement was slower in Ts65Dn mice. Ts65Dn mice also made a greater number of working memory errors on the RAM, and in contrast to control mice, did not improve across trials, always performing at near-chance levels. These results provide evidence for both spatial working and reference memory deficits in Ts65Dn mice, characteristics of cognitive dysfunction.

Ejaculatory abnormalities in mice with targeted disruption of the gene for heme oxygenase-2

Nitric oxide (NO) is well established as a neurotransmitter in the central and peripheral nervous systems. More recently, another gas, carbon monoxide (CO) has also been implicated in neurotransmission. In the nervous system CO is formed by a subtype of heme oxygenase (HO) designated HO2. HO2 is localized to discrete neuronal populations in the brain resembling localizations of soluble guanylyl cyclase, which is activated by CO. CO may also function in the peripheral autonomic nervous system, in conjunction with NO. The majority of ganglia in the myenteric plexus possess both HO2 and neuronal NO synthase (NOS). Defects in myenteric plexus neurotransmission occur both in mice with targeted deletion of genes for HO2 and neuronal NOS. HO2 also occurs in other autonomic ganglia including the petrosal, superior cervical and nodose ganglia. Neuronal NOS is localized to neurons regulating male reproductive behavior, such as penile erection, and NOS inhibitors prevent erection. Because of the other parallels between NO and CO, we speculated that CO may play a role in male reproductive behavior. In the present study we describe HO2 localization in neuronal structures regulating copulatory reflexes. Reflex activity of the bulbospongiosus muscle, which mediates ejaculation and ejaculatory behavior, is markedly diminished in mice with targeted deletion of the gene for HO2 (HO2-).

Role of melatonin in mediating seasonal energetic and immunologic adaptations

Winter is energetically demanding and stressful; thermoregulatory demands increase when food availability usually decreases. Physiological and behavioral adaptations, including termination of breeding, have evolved among nontropical animals to cope with the energy shortages during winter. Presumably, selection for the mechanisms that permit physiological and behavioral anticipation of seasonal ambient changes have led to current seasonal breeding patterns for many populations. In addition to the well-studied seasonal cycles of mating and birth, there are also significant seasonal cycles of illness and death among field populations of mammals and birds. Energetically challenging winter conditions can directly induce death via hypothermia, starvation, or shock; surviving these demanding conditions likely puts individuals under great physiological stress. The stress of coping with energetically demanding conditions may increase adrenocortical steroid levels that could indirectly cause illness and death by compromising immune function. Individuals would enjoy a survival advantage if seasonally recurring stressors could be anticipated and countered by bolstering immune function. The primary environmental cue that permits physiological anticipation of season is daily photoperiod, a cue that is mediated by melatonin. However, other environmental factors may interact with photoperiod to affect immune function and disease processes. Immune function is compromised during the winter in field studies of birds and mammals. However, laboratory studies of seasonal changes in mammalian immunity consistently report that immune function is enhanced in short day lengths. To resolve this apparent discrepancy, we hypothesize that winter stressors present in field studies counteract short-day enhancement of immune function. Prolonged melatonin treatment mimics short days, and also enhances rodent immune function. Reproductive responsiveness to melatonin appears to affect immune function. In sum, melatonin may be part of an integrative system to coordinate reproductive, immunologic, and other physiological processes to cope successfully with energetic stressors during winter.

Amygdala but not hippocampal lesions impair olfactory memory for mate in prairie voles (Microtus ochrogaster)

Exposure to an unfamiliar male conspecific results in pregnancy interruption (i.e., the Bruce effect) in rodents. Unlike most laboratory rodents, female prairie voles (Microtus ochrogaster) are induced into estrus by chemosensory stimuli contained in the urine of male conspecifics while grooming the anogenital (A-G) region of unfamiliar males. Female prairie voles maintain a brief "memory" for the stud male for 8-10 days after mating. Subsequent exposure to the same mate within this 8- to 10-day window does not elicit A-G investigation by the female and pregnancy block does not result. However, exposure to the original male after 10 days evokes A-G investigation and pregnancy block. To determine the neuroanatomic area(s) involved in olfactory memory for mate, female voles received bilateral electrolytic lesions of either the amygdala or hippocampus. Females were subsequently exposed to males for 48 h, separated for 3 days, then reintroduced to their original mate for 24 h. Although pregnancy rate did not differ among the experimental groups, a greater proportion of amygdala-lesioned females displayed pregnancy block when reexposed to their previous mates compared with hippocampal- or sham-lesioned voles. Amygdala-lesioned voles also displayed a greater number of A-G investigations compared with the other groups. Performance on olfactory tests was not impaired. Taken together, these results suggest that the amygdala plays an important role in olfactory memory for mate in prairie voles.

Metabolic costs of mounting an antigen-stimulated immune response in adult and aged C57BL/6J mice

Animals must balance their energy budget despite seasonal changes in both energy availability and physiological expenditures. Immunity, in addition to growth, thermoregulation, and cellular maintenance, requires substantial energy to maintain function, although few studies have directly tested the energetic cost of immunity. The present study assessed the metabolic costs of an antibody response. Adult and aged male C5BL/6J mice were implanted with either empty Silastic capsules or capsules filled with melatonin and injected with either saline or keyhole limpet hemocyanin (KLH). O2 consumption was monitored periodically throughout antibody production using indirect calorimetry. KLH-injected mice mounted significant immunoglobulin G (IgG) responses and consumed more O2 compared with animals injected with saline. Melatonin treatment increased O2 consumption in mice injected with saline but suppressed the increased metabolic rate associated with an immune response in KLH-injected animals. Melatonin had no effect on immune response to KLH. Adult and aged mice did not differ in antibody response or metabolic activity. Aged mice appear unable to maintain sufficient heat production despite comparable O2 production to adult mice. These results suggest that mounting an immune response requires significant energy and therefore requires using resources that could otherwise be allocated to other physiological processes. Energetic trade-offs are likely when energy demands are high (e.g., during winter, pregnancy, or lactation). Melatonin appears to play an adaptive role in coordinating reproductive, immunologic, and energetic processes.

Stress affects corticosteroid and immunoglobulin concentrations in male house mice (Mus musculus) and prairie voles (Microtus ochrogaster)

Glucocorticoids, secreted in response to perceived stress, can suppress immunoglobulin (Ig) levels and compromise immune function in mice and rats. Prairie voles (Microtus ochrogaster) have been reported to exhibit basal corticosterone concentrations that would cause pathological changes in the immune function of most other rodents. The goals of the present study were to verify that serum corticosterone concentrations are high in prairie voles, as compared with house mice (Mus musculus), by measuring serum corticosterone with the same RIA; to examine the effects of mild stressors on corticosterone response in both species and to examine the effects of elevated corticosterone levels on IgM and IgG levels in prairie voles and house mice. After 2 weeks of randomly timed 15-min daily restraint or cold-water swim sessions, animals were injected with sheep red blood cells. The data confirmed that basal blood concentrations of corticosterone were higher in prairie voles than house mice, but these high levels doubled after the first swim session in prairie voles, indicating that the adrenals can respond to stressors by producing increased corticosterone. After stress, antibody production (both IgM and IgG) was reduced in house mice but not in prairie voles, despite higher blood concentrations of glucocorticoids in prairie voles. Although body mass was statistically equivalent between species, prairie voles and mice differed dramatically in adrenal and splenic masses. Average adrenal mass of prairie voles was approximately three times the average mass of these organs in house mice; in contrast, the average splenic mass of house mice was approximately three times that of prairie voles. These data may be relevant to seasonal changes in immune function and survival.

Inhibition of neuronal nitric oxide synthase increases aggressive behavior in mice

BACKGROUND

Mice with targeted disruption of the gene for the neuronal isoform of nitric oxide synthase (nNOS) display exaggerated aggression. Behavioral studies of mice with targeted gene deletions suffer from the criticism that the gene product is missing not only during the assessment period but also throughout development when critical processes, including activation of compensatory mechanisms, may be affected. To address this criticism, we have assessed aggressive behavior in mice treated with a specific pharmacological inhibitor of nNOS.

MATERIALS AND METHODS

Aggressive behavior, as well as brain citrulline levels, were monitored in adult male mice after treatment with a specific nNOS inhibitor, 7-nitroindazole (7-NI) (50 mg/kg i.p.), which is known to reduce NOS activity in brain homogenates by > 90%. As controls, animals were treated with a related indazole, 3-indazolinone (3-I) (50 mg/kg i.p.) that does not affect nNOS or with on oil vehicle.

RESULTS

Mice treated with 7-NI displayed substantially increased aggression as compared with oil- or 3-I-injected animals when tested in two different models of aggression. Drug treatment did not affect nonspecific locomotor activities or body temperature. Immunohistochemical staining for citrulline in the brain revealed a dramatic reduction in 7-NI-treated animals.

CONCLUSIONS

7-NI augmented aggression in WT mice to levels displayed by nNOS- mice, strongly implying that nNOS is a major mediator of aggression. NOS inhibitors may have therapeutic roles in inflammatory, cardiovascular, and neurologic diseases. The substantial aggressive behavior soon after administration of an nNOS inhibitor raises concerns about adverse behavioral sequelae of such pharmacological agents.

Effects of photoperiod and 2-deoxy-D-glucose-induced metabolic stress on immune function in female deer mice

Nontropical rodents may experience large fluctuations in both food availability and energetic demands. The energy required for thermoregulation is high during the winter when energy availability is usually low. Winter conditions can induce a state of energetic stress that elevates circulating glucocorticoid levels and compromises immune function. Exposure to short days enhances immune function; the adaptive function of short-day enhancement of immune function may be to counteract the effects of stress-induced immunocompromise. To examine the role of energy availability in immune function, female deer mice were housed in either long (16:8-h light-dark cycle) or short (8:16-h light-dark cycle) days for 8 wk and then injected with either saline or 2-deoxy-D-glucose (2-DG), a glucose analog that inhibits cellular utilization of glucose and induces energetic stress. Long-day mice injected with 2-DG exhibited elevated corticosterone levels and reduced splenocyte proliferation compared with control mice. Short days buffered the animals against glucoprivation stress. Neither corticosterone levels nor splenocyte proliferation differed between 2-DG injected and control mice housed in short days. These data are consistent with the hypothesis that short days provide a buffer against metabolic stress.

Reproductive and immune responses to photoperiod and melatonin are linked in Peromyscus subspecies

The effects of photoperiod and melatonin treatment on reproductive and immune function were assessed in two subspecies of Peromyscus maniculatus from different latitudes of origin. In experiment 1, P. m. bairidii (latitude = 42 degrees 51' N) and P. m. luteus (latitude = 30 degrees 37' N) were housed in either long (LD 16:8) or short days (LD 8:16) for 8 weeks. Short-day P. m. bairdii displayed reproductive regression and elevated splenocyte proliferation in response to the T-cell mitogen concanavalin A, as compared to long-day mice. In contrast, P. m. luteus did not undergo reproductive regression or exhibit any increase in lymphocyte proliferation in short days. In experiment 2, individuals of both P. m. bairdii and P. m. luteus were implanted with empty capsules or capsules that contained melatonin. Individual P. m. bairdii implanted with melatonin underwent reproductive regression. Individuals of this subspecies also displayed elevated lymphocyte proliferation compared to control mice. Conversely, P. m. luteus implanted with melatonin did not undergo reproductive regression and displayed no significant changes in lymphocyte proliferation. These results suggest that reproductive responsiveness to melatonin mediates short-day enhancement of immune function in deer mice. These data also imply that melatonin may not possess universal immunoenhancing properties. Rather, the effectiveness of melatonin to influence immune responses may be constrained by reproductive responsiveness to this indole-amine.

Seasonal changes in immune function

Winter is energetically demanding. Physiological and behavioral adaptations have evolved among nontropical animals to cope with winter because thermoregulatory demands increase when food availability decreases. Seasonal breeding is central within the suite of winter adaptations among small animals. Presumably, reproductive inhibition during winter conserves energy at a time when the adds of producing viable young are low. In addition to the well-studied seasonal cycles of mating and birth, there are also significant seasonal cycles of illness and death among many populations of mammals and birds in the field. Challenging winter conditions, such as low ambient temperatures and decreased food availability, can directly induce death via hypothermia, starvation or shock. In some cases, survival in demanding winter conditions puts individuals under great physiological stress, defined here as an adaptive process that results in elevated blood levels of glucocorticoids. The stress of coping with energetically demanding conditions can also indirectly cause illness and death by compromising immune function. Presumably, the increased blood concentrations of adrenocortical steroids in response to winter stressors compromise immune function and accelerate catabolic mechanisms in the field, although the physiological effects of elevated glucocorticoids induced by artificial stressors have been investigated primarily in the laboratory. However, recurrent environmental stressors could reduce survival if they evoke persistent glucocorticoid secretion. The working hypothesis of this article is that mechanisms have evolved in some animals to combat seasonal stress-induced immunocompromise as a temporal adaptation to promote survival. Furthermore, we hypothesize that mechanisms have evolved that allow individuals to anticipate periods of immunologically challenging conditions, and to cope with these seasonal health-threatening conditions. The primary environmental cue that permits physiological anticipation of season is the daily photoperiod; however, other environmental factors may interact with photoperiod to affect immune function and disease processes. The evidence for seasonal fluctuations in lymphatic organ size, structure, immune function, and disease processes, and their possible interactions with recurrent environmental stressors, is reviewed. Seasonal peaks of lymphatic organ size and structure generally occur in late autumn or early winter and seasonal minima are observed prior to the onset of breeding. Although many of the field data suggest that immune function and disease processes are also enhanced during the winter, the opposite seasonal pattern is also observed in some studies. We propose that compromised immune function may be observed in some populations during particularly harsh winters when stressors override the enhancement of immune function evoked by short day lengths. Because so many factors covary in field studies, assessment of our proposal that photoperiod mediates seasonal changes in immune function requires laboratory studies in which only photoperiod is varied. A review of the effects of photoperiod on immune function in laboratory studies reveals that exposure to short day lengths enhances immune function in every species examined. Short day exposure in small mammals causes reproductive inhibition and concomitant reduction in plasma levels of prolactin and steroid hormones, as well as alterations in the temporal pattern of pineal melatonin secretion. These hormones affect immune function, and influence the development of opportunistic diseases, including cancer: however, it appears that either prolactin or melatonin secretion is responsible for mediating the effects of photoperiod on immune function. Taken together, day length appears to affect immune function in many species, including animals that typically do not exhibit reproductive responsiveness to day length.

Seasonal changes in immune function

Winter is energetically demanding. Physiological and behavioral adaptations have evolved among nontropical animals to cope with winter because thermoregulatory demands increase when food availability decreases. Seasonal breeding is central within the suite of winter adaptations among small animals. Presumably, reproductive inhibition during winter conserves energy at a time when the adds of producing viable young are low. In addition to the well-studied seasonal cycles of mating and birth, there are also significant seasonal cycles of illness and death among many populations of mammals and birds in the field. Challenging winter conditions, such as low ambient temperatures and decreased food availability, can directly induce death via hypothermia, starvation or shock. In some cases, survival in demanding winter conditions puts individuals under great physiological stress, defined here as an adaptive process that results in elevated blood levels of glucocorticoids. The stress of coping with energetically demanding conditions can also indirectly cause illness and death by compromising immune function. Presumably, the increased blood concentrations of adrenocortical steroids in response to winter stressors compromise immune function and accelerate catabolic mechanisms in the field, although the physiological effects of elevated glucocorticoids induced by artificial stressors have been investigated primarily in the laboratory. However, recurrent environmental stressors could reduce survival if they evoke persistent glucocorticoid secretion. The working hypothesis of this article is that mechanisms have evolved in some animals to combat seasonal stress-induced immunocompromise as a temporal adaptation to promote survival. Furthermore, we hypothesize that mechanisms have evolved that allow individuals to anticipate periods of immunologically challenging conditions, and to cope with these seasonal health-threatening conditions. The primary environmental cue that permits physiological anticipation of season is the daily photoperiod; however, other environmental factors may interact with photoperiod to affect immune function and disease processes. The evidence for seasonal fluctuations in lymphatic organ size, structure, immune function, and disease processes, and their possible interactions with recurrent environmental stressors, is reviewed. Seasonal peaks of lymphatic organ size and structure generally occur in late autumn or early winter and seasonal minima are observed prior to the onset of breeding. Although many of the field data suggest that immune function and disease processes are also enhanced during the winter, the opposite seasonal pattern is also observed in some studies. We propose that compromised immune function may be observed in some populations during particularly harsh winters when stressors override the enhancement of immune function evoked by short day lengths. Because so many factors covary in field studies, assessment of our proposal that photoperiod mediates seasonal changes in immune function requires laboratory studies in which only photoperiod is varied. A review of the effects of photoperiod on immune function in laboratory studies reveals that exposure to short day lengths enhances immune function in every species examined. Short day exposure in small mammals causes reproductive inhibition and concomitant reduction in plasma levels of prolactin and steroid hormones, as well as alterations in the temporal pattern of pineal melatonin secretion. These hormones affect immune function, and influence the development of opportunistic diseases, including cancer: however, it appears that either prolactin or melatonin secretion is responsible for mediating the effects of photoperiod on immune function. Taken together, day length appears to affect immune function in many species, including animals that typically do not exhibit reproductive responsiveness to day length.

Spatial memory deficits in segmental trisomic Ts65Dn mice

Spatial memory was assessed in the segmental trisomic 16 mouse (Ts65Dn), a potential model for Down syndrome (DS), using the 12-arm radial maze (RAM). Ts65Dn mice have a portion of mouse chromosome 16 syntenic to the distal end of human chromosome 21 triplicated. On each of 8 daily trials of the RAM, Ts65Dn mice made fewer correct choices than control mice and performed at or near chance levels, indicating a deficit in spatial working memory. On trials 9 and 10, Ts65Dn mice performed as well as control mice on the initial 12 choices, but required a greater number of choices to complete the RAM. The improved performance of Ts65Dn mice on trials 9 and 10 was lost when the animals were retested after a 50-day retention period, suggesting that long-term memory is also defective. These results are not likely explained by differences in either response bias or perceptual discrimination. Ts65Dn and control mice displayed comparable levels of performance in spontaneous alternation in a T-maze, demonstrating that simple spatial memory was not impaired. In the elevated plus maze, Ts65Dn mice did not display higher anxiety levels which could affect their performance in the RAM. In fact, Ts65Dn mice visited open arms on the elevated plus maze more frequently and spent more time on open arms than did control mice. Taken together, these results provide evidence for short- and long-term spatial memory deficits in Ts65Dn mice.

Photoperiod and temperature interact to affect immune parameters in adult male deer mice (Peromyscus maniculatus)

Nontropical rodents often experience large seasonal fluctuations in both food availability and energy demands. The energy required for thermoregulation is highest during the winter when food availability may be at an annual minimum. Failure to cope with winter probably accounts, in part, for the increased prevalence of disease and death relative to that in summer. Winter conditions may elevate circulating glucocorticosteroid levels, which can compromise immune function. To increase the odds of surviving the energetic demands of winter, individuals of some rodent species appear to enhance immune function before conditions deteriorate. Previous laboratory studies suggest that immune enhancement can be induced by short days. These findings contrast with the results of several field studies reporting suppressed immune function during the winter. To resolve this conflict, the authors hypothesized that winter stressors present in field studies counteracted the short-day enhancement of immune function reported in laboratory studies. If true, then immune function of captive mice in short days should be compromised by low temperature or reduced food availability. Both ambient temperature and photoperiod were manipulated in the present study to assess their effects on immune parameters in male deer mice (Peromyscus maniculatus). Animals in short days regressed their reproductive systems and also displayed significantly higher immunoglobulin G (IgG) levels than did those in long days. Deer mice maintained in low temperatures had significantly reduced splenic masses and basal IgG levels independent of day length. Animals maintained in both short days and low temperatures displayed IgG levels comparable to those of mice in long-day/mild-temperature conditions. Animals maintained in long days and low temperatures had significantly higher serum corticosterone levels than did animals maintained in long days at mild temperatures. These data are consistent with the hypothesis that immune parameters are enhanced in short days to counteract stress-mediated immune suppression occurring during the winter.

Nitric oxide-dependent penile erection in mice lacking neuronal nitric oxide synthase

BACKGROUND

Nitric oxide (NO) has been implicated as a mediator of penile erection, because the neuronal isoform of NO synthase (NOS) is localized to the penile innervation and NOS inhibitors selectively block erections. NO can also be formed by two other NOS isoforms derived from distinct genes, inducible NOS (iNOS) and endothelial NOS (eNOS). To clarify the source of NO in penile function, we have examined mice with targeted deletion of the nNOS gene (nNOS- mice).

MATERIALS AND METHODS

Mating behavior, electrophysiologically induced penile erection, isolated erectile tissue isometric tension, and eNOS localization by immunohistochemistry and Western blot were performed on nNOS- mice and wild-type controls.

RESULTS

Both intact animal penile erections and isolated erectile tissue function are maintained in nNOS mice, in agreement with demonstrated normal sexual behaviors, but is stereospecifically blocked by the NOS inhibitor, L-nitroarginine methyl ester (L-NAME). eNOS is abundantly present in endothelium of penile vasculature and sinusoidal endothelium within the corpora cavemosa, with levels that are significantly higher in nNOS- mice than in wild-type controls.

CONCLUSIONS

eNOS mediates NO-dependent penile erection in nNOS- animals and normal penile erection. These data clarify the role of nitric oxide in penile erection and may have implications for therapeutic agents with selective effects on NOS isoforms.

Photoperiod and population density interact to affect reproductive and immune function in male prairie voles

Seasonal breeding of rodents is often associated with changes in adrenal function; altered adrenal function could account, in part, for seasonal changes in immune function and, ultimately, influence seasonal fluctuations in survival. Animals commonly monitor the annual change in photoperiod to ascertain the time of year and to make appropriate seasonal adjustments in physiology and behavior. Several extrinsic factors affect reproductive responsiveness to photoperiod. The interaction between population density and reproductive and adrenal responsiveness to photoperiod was assessed in the present experiment. Adult male prairie voles (Microtus ochrogaster) were maintained individually for 10 wk in long [light:dark (LD) 16:8] or short (LD 8:16) photoperiods in rooms with either high (10.96 animals/m3) or low (0.18 animals/m3) population densities. Regardless of population density, short-day voles regressed the size of their reproductive organs; reproductive organ masses were higher in long-day voles housed in high-density compared with low-density rooms. Paired adrenal masses were reduced in short-day voles, but were unaffected by population density; serum corticosterone concentrations were significantly elevated in short-day compared with long-day animals. In both photoperiods, basal blood corticosterone levels were higher in voles from low-density compared with high-density rooms. Splenic masses were unaffected by day length, but were elevated among high-density animals. Similarly, serum immunoglobulin (IgG) levels were elevated among high-density animals. These results suggest that population density per se, in the absence of behavioral interactions, can affect reproductive size, and possibly function, in long-day conditions, and that prairie voles, which are highly social, exhibit higher corticosterone and lower IgG levels in low compared with high densities. These results may be important in understanding arvicoline population fluctuations, as well as improving animal husbandry practices in the lab.

Behavioural abnormalities in male mice lacking neuronal nitric oxide synthase

In addition to its role in blood vessel and macrophage function, nitric oxide (NO) is a neurotransmitter found in high densities in emotion-regulating brain regions. Mice with targeted disruption of neuronal NO synthase (nNOS) display grossly normal appearance, locomotor activity, breeding, long-term potentiation and long-term depression. The nNOS- mice are resistant to neural stroke damage following middle cerebral artery ligation. Although CO2-induced cerebral vasodilatation in wild-type mice is NO-dependent, in nNOS- mice this vasodilation is unaffected by NOS inhibitors. Establishing a behavioural role for NO has, until now, not been feasible, as NOS inhibitor drugs can only be administered acutely and because their pronounced effects on blood pressure and other body functions obfuscate behavioural interpretations. We now report a large increase in aggressive behaviour and excess, inappropriate sexual behaviour in nNOS- mice.

The influence of season, photoperiod, and pineal melatonin on immune function

In addition to the well-documented seasonal cycles of mating and birth, there are also significant seasonal cycles of illness and death among many animal populations. Challenging winter conditions (i.e., low ambient temperature and decreased food availability) can directly induce death via hypothermia, starvation, or shock. Coping with these challenges can also indirectly increase morbidity and mortality by increasing glucocorticoid secretion, which can compromise immune function. Many environmental challenges are recurrent and thus predictable; animals could enhance survival, and presumably increase fitness, if they could anticipate immunologically challenging conditions in order to cope with these seasonal threats to health. The annual cycle of changing photoperiod provides an accurate indicator of time of year and thus allows immunological adjustments prior to the deterioration of conditions. Pineal melatonin codes day length information. Short day lengths enhance several aspects of immune function in laboratory studies, and melatonin appears to mediate many of the enhanced immunological effects of photoperiod. Generally, field studies report compromised immune function during the short days of autumn and winter. The conflict between laboratory and field data is addressed with a multifactor approach. The evidence for seasonal fluctuations in lymphatic tissue size and structure, as well as immune function and disease processes, is reviewed. The role of pineal melatonin and the hormones regulated by melatonin is discussed from an evolutionary and adaptive functional perspective. Finally, the clinically significance of seasonal fluctuations in immune function is presented. Taken together, it appears that seasonal fluctuations in immune parameters, mediated by melatonin, could have profound effects on the etiology and progression of diseases in humans and nonhuman animals. An adaptive functional perspective is critical to gain insights into the interaction among melatonin, immune function, and disease processes.

Evidence for spatial working memory in honeybees (Apis mellifera)

Spatial working memory (the ability to represent multiple locations in a flexible, dynamic manner) has been studied in a range of vertebrate species. The results of 3 experiments indicate that this ability also exists in at least one invertebrate (honeybees; Apis mellifera). Individual honeybees collected sugar solution from a matrix of 6 locations. They avoided revisits to locations previously depleted of solution more accurately than expected by chance. The results rule out several nonmemorial explanations for this ability, and it is therefore best explained by a spatial working memory system that allows discrimination of previously visited locations from those not yet visited. These results substantially expand the range of species in which spatial working memory has been demonstrated.