Photoperiod-testicular-immune interaction in a seasonal breeder Indian palm squirrel Funambulus pennanti during the reproductively inactive and active phases

Annual rhythm in immune functions of blood leucocytes in an ophidian, Natrix piscator

Annual variations in animal's physiological functions are an essential strategy to deal with seasonal challenges which also vary according to the time of year. Information regarding annual adaptations in the immune-competence to cope with seasonal stressors in reptiles is scarce. The present research plan was designed to analyze the presence of circannual immune rhythms in defense responses of the leucocytes in an ophidian, Natrix piscator. Peripheral blood leucocytes were obtained, counted, and superoxide anion production, neutrophil phagocytosis, and nitrite release were tested to assess the innate immune functions. Peripheral blood lymphocytes were separated by centrifugation (utilizing density gradient) and the cell proliferation was measured. The Cosinor rhythmometry disclosed the presence of significant annual rhythms in the number of leucocytes, superoxide anion production, nitric oxide production, and proliferation of stimulated lymphocytes. The authors found that respiratory burst activity and proliferative responses of lymphocytes were crucial immune responses that showed the annual rhythm. It was summarized that the immune function of the N. piscator is a labile attribute that makes the animal competent to cope with the seasonal stressor by adjustment in the potency of response.

Photoperiodic modulation of melatonin receptor and immune genes in migratory redheaded bunting

The transcriptional regulation of innate immune function across annual life history states (LHS) remains obscure in avian migrants. We, therefore, investigated this in a migratory passerine songbird, redheaded bunting (Emberiza bruniceps), which exhibits long-distance vernal migration from India to Central Asia. We exposed the birds (N = 10) to differential photoperiodic conditions to induce a non-migratory (NM), pre-migratory (PM), migratory (MIG), and refractory (REF) state, and performed gene expression assays of melatonin receptors (MEL1A and MEL1B), and innate immunity-linked genes (IL1B, IL6, TLR4, and NFKB) in spleen and blood. We found a significant reduction in splenic mass and volume, and a parallel increase in fat accumulation, and testicular growth in birds under migratory state. The gene expression assay revealed an upregulation of MEL1A and MEL1B mRNA levels in both the tissues in MIG. Additionally, we found a nocturnal increase of splenic IL1B expression, and IL1B, IL6, and TLR4 expression in the blood. The mRNA expression of melatonin receptors and proinflammatory cytokine showed a positive correlation. These results suggest that melatonin relays the photoperiodic signal to peripheral immune organs, which shows LHS-dependent changes in mRNA expression of immune genes.

Seasonal immune rhythm of leukocytes in the freshwater snakehead fish, Channa punctatus

Annual rhythms are observed in many physiological processes and are an important approach to cope with seasonal stressors. The use of lower vertebrates as an experimental model is crucial to understand the evolution of this biological clock. This study aims to characterize the seasonal variability in the leukocyte immune responses in Channa punctatus. Leukocytes were harvested from peripheral blood and respiratory burst activity, leukocyte phagocytosis, and nitrite production were assessed to study innate immunity. Peripheral blood lymphocytes were segregated by centrifugation (density gradient) and proliferative responses of lymphocytes, in the presence of mitogens, were used to study cell-mediated immunity. Annual rhythms were validated in superoxide anion production, nitrite release and phagocytosis. Cosinor analysis revealed a differential pattern of lymphocyte proliferation which was dependent upon season and mitogen used. It was concluded that seasonal variation in immune activity might be associated with annual adaptation against diseases and the optimum immune status of seasonal breeders like fish helps them fight seasonal changes.

Involvement of steroid and antioxidant pathways in spleen-mediated immunity in migratory birds

The molecular underpinnings of the spleen-mediated immune functions during the period of heightened energetic needs in the year are not known in avian migrants. We investigated this, in Palearctic-Indian migratory male redheaded buntings, which exhibited vernal (spring) premigratory / early testicular maturation states under artificial long days. This was evidenced by increased dio2 and decreased dio3 mRNA expression in the hypothalamus, elevated levels of circulating corticosterone and testosterone, and enlarged testes in long-day-photostimulated birds, as compared to unstimulated controls under short days. The concomitant decrease in both mass and volume of the spleen, and increase in the heterophil/ lymphocyte ratio suggested the parallel innate immunity effects in photostimulated buntings. Importantly, we found increased mRNA expression of genes coding for the cytokines (il15 and il34), steroid receptors (nr3c2) and oxidative stress marker enzymes (gpx1 and sod1) in the spleen, suggesting the activation of both immune and antioxidant molecular pathways during the early photostimulated state. However, the splenic expressions of il1β, il6, tgfβ, ar and nos2 genes were not significantly different between long-day stimulated and short-day unstimulated birds. The negative correlation of plasma corticosterone levels with spleen mass further indicated a role of corticosterone in the modulation of the spleen function, probably via nr3c2 gene encoded mineralocorticoid receptors. These results suggest the activation of the spleen-mediated innate immunity in anticipation of the heightened energetic stress state of the photostimulated spring migratory/breeding period in migratory songbirds.

Food restriction, but not seasonality, modulates the acute phase response of a Neotropical bat

Season and food intake are known to affect immune response of vertebrates yet their effects on metabolic rate have been rarely explored. We tested the effect of season and acute food restriction and their interaction on the energetic cost of immune response activation of a tropical vertebrate, the Seba's short-tailed fruit bat (Carollia perspicillata). We specifically stimulated the acute phase response (APR) with bacterial lipopolysaccharide (LPS) to measure metabolic changes along with changes in body temperature (T_b), body mass (M_b), white blood cell counts and the Neutrophil/Lymphocyte ratio (N/L). We found no effect of season on the different factors associated to the activation of the APR. In contrast to our expectations, unfed bats reached similar T_b increments and RMR peak values and had higher RMR scope values and higher caloric costs than fed bats after LPS injection. However, food deprivation led to delayed metabolic response indicated by longer time required to reach peak RMR values in unfed bats. Both food-deprived and fed bats did not present leukocytosis after APR activation and their WBC counts were similar, but unfed bats had a significant increase of N/L. APR activation represented a small fraction of the bat daily energy requirements which might explain why unfed bats were not limited to mount a metabolic response. Our study adds to recent evidence showing that activating the innate immune system is not an energetically expensive process for plant-eating bats.

Short day length enhances physiological resilience of the immune system against 2-deoxy-d-glucose-induced metabolic stress in a tropical seasonal breeder Funambulus pennanti

Studies demonstrate the importance of metabolic resources in the regulation of reproduction and immune functions in seasonal breeders. In this regard, the restricted energy availability can be considered as an environmental variable that may act as a seasonal stressor and can lead to compromised immune functions. The present study explored the effect of photoperiodic variation in the regulation of immune function under metabolic stress condition. The T-cell-dependent immune response in a tropical seasonal breeder Funambulus pennanti was studied following the inhibition of cellular glucose utilization with 2-deoxy-d-glucose (2-DG). 2-DG treatment resulted in the suppression of general (e.g., proliferative response of lymphocytes) and antigen-specific [anti-keyhole limpet hemocyanin IgG titer and delayed-type hypersensitivity response] T-cell responses with an activation of the hypothalamic-pituitary-adrenal axis, which was evident from the increased levels of plasma corticosterone. 2-DG administration increased the production of inflammatory cytokines [interleukin (IL)-1β and tumor necrosis factor (TNF)-α] and decreased the autocrine T-cell growth factor IL-2. The immunocompromising effect of 2-DG administration was retarded in animals exposed to short photoperiods compared with the control and long photoperiod-exposed groups. This finding suggested that short photoperiodic conditions enhanced the resilience of the immune system, possibly by diverting metabolic resources from the reproductive organs toward the immune system. In addition, melatonin may have facilitated the energy "trade-off" between reproductive and immune mechanisms, thereby providing an advantage to the seasonal breeders for their survival during stressful environmental conditions.

Photoperiodic modulation of local melatonin synthesis and its role in regulation of thymic homeostasis in Funambulus pennanti

The effect of photo-neuroendocrine system on the thymic (immune) functions is mediated by gonadal steroid and the pineal hormone melatonin. The present study explored the effect of photoperiod on the thymic melatonergic system and its role in protection of thymic T-cells from the testosterone induced seasonal oxidative stress and apoptosis. Exposure to long day-length (LD) was noted to decrease local (thymic) melatonin content and induce oxidative stress and apoptosis in the thymus. Increased peripheral level of testosterone upregulated the androgen receptor expression and, consequently reduced proliferation response of the thymocytes. Short day conditions (SD) however, reversed the effect of LD on the thymic physiology. Low level of testosterone was concomitant with diminished nitro-oxidative stress and decreased expression of redox sensitive factors (NF-κB, p53 and Bax/Bcl-2 ratio) in the thymus. SD retarded activation of caspase-3 resulting in procaspase-3 accumulation. Further, in vitro treatment of thymocytes with AR antagonist flutamide impaired the sensitivity of thymocytes to androgen and reversed the deleterious effects of testosterone on the proliferative and apoptotic responses of thymocytes. Therefore, it can be suggested that thymus derived melatonin protects thymic T-cells from testosterone induced seasonal oxidative stress, apoptosis and also acts as a potent paracrine factor for maintenance of redox status to ensure thymocyte survival.

Reciprocal interaction between melatonin receptors (Mel(1a), Mel(1b), and Mel(1c)) and androgen receptor (AR) expression in immunoregulation of a seasonally breeding bird, Perdicula asiatica: role of photoperiod

Light is the major environmental stimulus affecting behaviour and physiology of avian species. Our study elaborates the photoperiodic regulation of melatonin (Mel1a, Mel1b, and Mel1c) and androgen receptor (AR) to elucidate its reciprocal interaction in regulation of general immunity in tropical wild bird, Perdicula asiatica. Effect of different photoperiodic exposures such as continuous light (LL), continuous dark (DD), long days (LD; 16 h light/day), short days (SD; 10h light/day) and normal day length (NDL) was accessed both on cellular and humoral immune parameters like per cent stimulation ratio (%SR), total leukocyte count (TLC), leukocyte count (LC), plasma interleukin-2 (IL-2), tumor necrosis factor-alpha (TNF-α), spleen and gonad weight, plasma melatonin, and testosterone level as well as their receptor expression on spleen and testis. Expression of melatonin receptor, Mel1a and Mel1b in spleen was high in SD experiencing bird as compared to LD birds. In all photoperiodic groups, AR expression was upregulated in spleen. In addition, our reverse transcription polymerase chain reaction (RT-PCR) results support differentially localized mRNA Mel1b and Mel1c expression in spleen and testis. In sum, photoperiodically modulated level of melatonin via reciprocal regulation of Mel1a, Mel1b, and Mel1c, and AR in spleen as well as in testis modulates immunity, suggesting a compensatory mechanism between reproduction and immunity in a seasonally breeding bird, P. asiatica.

Physiological crosstalk between melatonin and glucocorticoid receptor modulates T-cell mediated immune responses in a wild tropical rodent, Funambulus pennanti

Immunoenhancing attributes of melatonin (Mel) on the immunocompromised state induced by glucocorticoid is well known, but the involvement of their receptors in the modulation of immunity has never been studied in any rodent. The present study explores the role of Mel and its receptors (MT1 and MT2) in amelioration of immunocompromised state induced by a synthetic glucocorticoid, dexamethasone (Dex) in a tropical rodent Funambulus pennanti. Immune parameters viz. DTH response, Lymphocyte proliferation, cytokine (IL-2) and antibody production were assessed following pretreatment of Mel and Dex alone or in combination. Mel enhanced the IL-2 production, thymic and splenic lymphocyte proliferation thereby increasing T helper cell associated immune responses and anti-KLH-IgG production. MT1 and MT2 receptor expression was downregulated following Dex treatment while glucocorticoid receptors (GR) expression was downregulated in Mel treated groups suggesting that the immunomodulatory effects of glucocorticoids and Mel are mediated via their receptors. To gain further insights on the role of Mel receptors, we used nonselective melatonin receptor antagonist luzindole which resulted in reversal of most of the immunomodulatory actions of Mel. Therefore, it may be suggested that a physiological cross talk exist between Mel and GR which is of high adaptive significance in wild animals for balancing the immunity during ecologically stressful conditions.

Immune responses to lipopolysaccharide challenge in a tropical rodent (Funambulus pennanti): photoperiod entrainment and sex differences

Annual variation in day length (photoperiod) triggers changes in the immune system of seasonal breeders. The rationale behind this study was to delineate any sex differences in immune responses of photoperiodically entrained animals challenged against lipopolysaccharide (LPS)-induced inflammatory stress. We observed that photoperiodically entrained [short day, SD, 10 h light (L):14 h dark (D); long day, LD, 16 h L:8 h D; and natural day length, NDL, 12 h L:12 h D] male and female Indian palm squirrels, Funambulus pennanti, presented sexual dimorphism in immune status after LPS-induced stress. Females presented high humoral (anti-keyhole limpet hemocyanin immunoglobulin) and cellular immunity (lymphocyte proliferation) compared with the males of all photoperiodic conditions. Female squirrels showed reduced pro-inflammatory cytokine levels (interleukin-1β, interleukin-6, and tumor necrosis factor-α) than the males suggesting their high efficiency to recover from LPS-induced inflammatory stress. Increased duration of melatonin secretion and corticosterone concentration in squirrels experiencing SD evidently supported survival of squirrels as compared with control (NDL) and LD squirrels of both sexes. Decreased immune status in both sexes under LD condition might be due to a short melatonin signal mimicking the LDs of summer. Thus, we infer that photoperiodic entrainment via the levels of melatonin and corticosterone synergistically supported more the survival of female squirrels under LPS-induced stress.

Photoperiodic regulation of MT1 and MT2 melatonin receptor expression in spleen and thymus of a tropical rodent Funambulus pennanti during reproductively active and inactive phases

Photoperiodic regulation of melatonin receptor types on target tissues, such as lymphatic organs, has never been explored for any seasonal breeder. In the present study, we accessed the high affinity membrane melatonin receptors MT1 and MT2 expression dynamics in lymphoid organs (i.e., spleen and thymus) of a seasonally breeding rodent Funambulus pennanti during two major reproductive phases (i.e., active and inactive), when the internal hormonal (melatonin and gonadal steroid) as well as the ecological conditions were entirely different. Photoperiod regulates circulatory melatonin level; hence, we noted the effect of different photoperiodic regimes (long; 16L:8D and short; 10L:14D photoperiod) equivalent to summer and winter daylength on membrane melatonin receptor MT1 and MT2 expression in spleen and thymus. We have correlated the melatonin receptor expression with two major hormones varying seasonally (i.e., melatonin and testosterone) also being responsible for modulation of immunity of a seasonal breeder. Differential immunoreactivity of MT1 and MT2 receptor in spleen and thymus of F. pennanti suggests an involvement of both the receptor types in signal transduction of photoperiod for seasonal immunomodulation, because in the tropical zone, a slight difference (1:45-2 h) in daylength may change reproductive physiology and immunity of animals for adaptation. Our above suggestion receives strong support from the experiment of photoperiodic exposure on MT1 and MT2 expression at the translational level, where long daylength decreased the circulatory melatonin level and melatonin receptor expression in both lymphatic tissues. On the other hand, under short daylength, expression of MT1 and MT2 receptor increased in both spleen and thymus along with concomitant increase in circulatory melatonin level. Differential hormonal level of melatonin and gonadal hormones during reproductively active and inactive phase and its direct relation with melatonin receptor expression dynamics in lymphoid organs could be responsible for seasonal adjustment of immunity and reproduction.

Effect of intra-testicular melatonin injection on testicular functions, local and general immunity of a tropical rodent Funambulus pennanti

Local antigonadotrophic action of melatonin in testes has never been correlated with local and general immune status of any rodent. Intra-testicular injection of melatonin (2.5 μg/50 μl) for 10 days (MI-10D) and 20 days (MI-20D) was given to young adult male of Funambulus pennanti and testicular androgen receptor (AR), androgen binding protein (ABP) expression, 3β-hydroxysteroid dehydrogenase (3β-HSD) activity, and Mel1aR expression in thymus was checked along with general immune parameters. Further, immunohistochemical localization of Mel1aR in testes was done. Decreased AR, ABP expression, testes weight, 3β-HSD activity, testosterone level, and spermatogenesis but increased Mel1aR expression in thymus, immunoreactivity in testes, and testicular macrophages following injection was noted. Lymphatic tissue weight, leukocyte, lymphocyte count, lymphocyte proliferation in spleen, thymus, plasma melatonin, and IL-2 level increased in a duration-dependent manner following intra-testicular injection. Intra-testicular injection of melatonin decreased steroidogenesis by enhancing the primary effect of melatonin on Leydig cell endocrine function. Along with reduced circulatory testosterone production, an increase in testicular as well as general immunity was observed in a duration-dependent manner. Therefore, a local participation of melatonin in testes of F. pennanti to control testicular androgen production is suggested.

Response of melatonin receptor MT1 in spleen of a tropical Indian rodent, Funambulus pennanti, to natural solar insolation and different photoperiodic conditions

We analyzed the effect of natural solar insolation and artificial photoperiodic conditions on melatonin MT1 receptor expression of a tropical rodent, Funambulus pennanti. Melatonin mediates reproductive and circadian responses and regulates the production of a large number of cytokines, including interleukin-2 (IL-2), via modulation of MT1 receptor expression. Maximum pineal activity, resulting in high melatonin level, low melatonin receptor expression, and increased splenic mass, was noted in the winter months, while an opposite effect was noted during the summer months. Further, constant light exposure mimicked an "enhanced summer"-like condition with significant hyposplenia, and an opposite effect was observed with constant dark exposure with significant splenomegaly in F. pennanti. In the annual study, a slight increase in melatonin level was noted during the monsoon period, when the duration of photoperiod was the same but the amount of solar insolation and direct radiation decreased. The present study found that not only the duration of natural sunlight (i.e., photoperiod) but the intensity of sunlight expressed by solar insolation affects the circulatory level of melatonin and melatonin receptor expression in this wild tropical rodent. An increase in the circulatory level of melatonin induced a decrease in its receptor subtype MT1 expression in splenic cells, both at the transcriptional and translational levels, thus reflecting autoregulatory down-regulation of melatonin receptors. Therefore, in our animal model, F. pennanti melatonin may be suggested as a molecular messenger of photoperiodic signals (duration and intensity) directly acting via MT1 receptor regulation to adapt the immune system of animals residing in the tropical zone.