Biomedical subjects

G E Demas

Publications and source records attributed to G E Demas.

At least 19 recordsLinked to original sources

PubMed2004

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.

Aggression↗

PubMed2003

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.

Animals↗

PubMed2002

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.

Adrenal Glands↗

PubMed2001

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.

Adipose Tissue↗

PubMed2001

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.

Acute Disease↗

PubMed2001

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.

Adipose Tissue↗

PubMed2001

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.

Animals↗

PubMed2001

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.

Animals↗

PubMed2000

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.

Aggression↗

PubMed2000

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.

Animals↗

PubMed1999

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.

Aggression↗

PubMed1999

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.

Aggression↗

PubMed1999

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.

Analysis of Variance↗

PubMed1999

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.

Animals↗

PubMed1999

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.

Animals↗

PubMed1999

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.

Animals↗

PubMed1998

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.

Animals↗

PubMed1998

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.

Animals↗