Regulation of feeding behavior and plasma testosterone in response to central neuropeptide Y administration in a songbird

Neuroendocrine Mechanisms Underlying Non-breeding Aggression: Common Strategies Between Birds and Fish

Aggression is an adaptive behavior that plays an important role in gaining access to limited resources. Aggression may occur uncoupled from reproduction, thus offering a valuable context to further understand its neural and hormonal regulation. This review focuses on the contributions from song sparrows (Melospiza melodia) and the weakly electric banded knifefish (Gymnotus omarorum). Together, these models offer clues about the underlying mechanisms of non-breeding aggression, especially the potential roles of neuropeptide Y (NPY) and brain-derived estrogens. The orexigenic NPY is well-conserved between birds and teleost fish, increases in response to low food intake, and influences sex steroid synthesis. In non-breeding M. melodia, NPY increases in the social behavior network, and NPY-Y1 receptor expression is upregulated in response to a territorial challenge. In G. omarorum, NPY is upregulated in the preoptic area of dominant, but not subordinate, individuals. We hypothesize that NPY may signal a seasonal decrease in food availability and promote non-breeding aggression. In both animal models, non-breeding aggression is estrogen-dependent but gonad-independent. In non-breeding M. melodia, neurosteroid synthesis rapidly increases in response to a territorial challenge. In G. omarorum, brain aromatase is upregulated in dominant but not subordinate fish. In both species, the dramatic decrease in food availability in the non-breeding season may promote non-breeding aggression, via changes in NPY and/or neurosteroid signaling.

Negative energy balance in a male songbird, the Abert's towhee, constrains the testicular endocrine response to luteinizing hormone stimulation

Energy deficiency can suppress reproductive function in vertebrates. As the orchestrator of reproductive function, endocrine activity of the hypothalamo-pituitary-gonadal (HPG) axis is potentially an important mechanism mediating such effects. Previous experiments in wild-caught birds found inconsistent relationships between energy deficiency and seasonal reproductive function, but these experiments focused on baseline HPG axis activity and none have investigated the responsiveness of this axis to endocrine stimulation. Here, we present data from an experiment in Abert's towhees, Melozone aberti, using gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) challenges to investigate whether energy deficiency modulates the plasma testosterone responsiveness of the HPG axis. Wild-caught birds were either ad libitum fed or energetically constrained via chronic food restriction during photoinduced reproductive development. Energy deficiency did not significantly affect the development of reproductive morphology, the baseline endocrine activity of the HPG axis, or the plasma testosterone response to GnRH challenge. Energy deficiency did, however, decrease the plasma testosterone responsiveness to LH challenge. Collectively, these observations suggest that energy deficiency has direct gonadal effects consisting of a decreased responsiveness to LH stimulation. Our study, therefore, reveals a mechanism by which energy deficiency modulates reproductive function in wild birds in the absence of detectable effects on baseline HPG axis activity.

Innate immunity and testosterone rapidly respond to acute stress, but is corticosterone at the helm?

When faced with a stressor, vertebrates can rapidly increase the secretion of glucocorticoids, which is thought to improve the chances of survival. Concurrent changes in other physiological systems, such as the reproductive endocrine or innate immune systems, have received less attention, particularly in wild vertebrates. It is often thought that glucocorticoids directly modulate immune performance during a stress response, but, in many species, androgens also rapidly respond to stress. However, to our knowledge, no study has simultaneously examined the interactions between the glucocorticoid, androgen, and innate immune responses to stress in a wild vertebrate. To address this issue, we tested the hypothesis that the change in plasma corticosterone (CORT) in response to the acute stress of capture and restraint is correlated with the concurrent changes in plasma testosterone (T) and innate immune performance (estimated by the capacity of plasma to agglutinate and lyse foreign cells) in the Abert's Towhee (Melozone aberti). Furthermore, to broaden the generality of the findings, we compared male and female towhees, as well as males from urban and non-urban populations. Acute stress increased plasma CORT, decreased plasma T in males, and decreased innate immune performance, but the increase in CORT during stress was not correlated with the corresponding decreases in either plasma T or innate immunity. By contrast, the plasma T stress response was positively correlated with the innate immune stress response. Collectively, our results challenge the proposition that the glucocorticoid stress response is correlated with the concurrent changes in plasma T, a key reproductive hormone, and innate immunity, as estimated by agglutination and lysis.

Food restriction negatively affects multiple levels of the reproductive axis in male house finches, Haemorhous mexicanus

Nutrition influences reproductive functions across vertebrates, but the effects of food availability on the functioning of the hypothalamic-pituitary-gonadal (HPG) axis in wild birds and the mechanisms mediating these effects remain unclear. We investigated the influence of chronic food restriction on the HPG axis of photostimulated House Finches, Haemorhous mexicanus. Food-restricted birds had underdeveloped testes with smaller seminiferous tubules than ad libitum-fed birds. Baseline plasma testosterone (T) increased in response to photostimulation in ad libitum-fed but not in food-restricted birds. Food availability did not, however, affect the plasma T increase resulting from a gonadotropin-releasing hormone (GnRH) or a luteinizing hormone (LH) challenge. The number of hypothalamic GnRH-I immunoreactive (ir) but not proGnRH-ir perikarya was higher in food-restricted than ad libitum-fed finches, suggesting inhibited secretion of GnRH. Hypothalamic gonadotropin-inhibitory hormone (GnIH)-ir and neuropeptide Y (NPY)-ir were not affected by food availability. Plasma corticosterone (CORT) was also not affected by food availability, indicating that the observed HPG axis inhibition did not result from increased activity of the hypothalamic-pituitary-adrenal (HPA) axis. This study is among the first to examine multilevel functional changes in the HPG axis in response to food restriction in a wild bird. The results indicate that food availability affects both hypothalamic and gonadal function, but further investigations are needed to clarify the mechanisms by which nutritional signals mediate these effects.