Sex steroid-independent effects of photostimulation on the song-control system of white-throated sparrows (Zonotrichia albicollis)

Behavioral evidence for sex steroids hypersensitivity in castrated male canaries

In seasonally breeding songbirds such as canaries, singing behavior is predominantly under the control of testosterone and its metabolites. Short daylengths in the fall that break photorefractoriness are followed by increasing daylengths in spring that activate singing via both photoperiodic and hormonal mechanisms. However, we observed in a group of castrated male Fife fancy canaries maintained for a long duration under a short day photoperiod a large proportion of subjects that sang at high rates. This singing rate was not correlated with variation in the low circulating concentrations of testosterone. Treatment of these actively singing castrated male canaries with a combination of an aromatase inhibitor (ATD) and an androgen receptor blocker (flutamide) only marginally decreased this singing activity as compared to control untreated birds and did not affect various measures of song quality. The volumes of HVC and of the medial preoptic nucleus (POM) were also unaffected by these treatments but were relatively large and similar to volumes in testosterone-treated males. In contrast, peripheral androgen-sensitive structures such as the cloacal protuberance and syrinx mass were small, similar to what is observed in castrates. Together these data suggest that after a long-term steroid deprivation singing behavior can be activated by very low concentrations of testosterone. Singing normally depends on the activation by testosterone and its metabolites of multiple downstream neurochemical systems such as catecholamines, nonapeptides or opioids. These transmitter systems might become hypersensitive to steroid action after long term castration as they probably are at the end of winter during the annual cycle in seasonally breeding temperate zone species.

Sex and seasonal differences in neurogenesis and volume of the song-control system are associated with song in brood-parasitic and non-brood-parasitic icterid songbirds

The song-control system in the brain of songbirds is important for the production and acquisition of song and exhibits both remarkable seasonal plasticity and some of the largest neural sex differences observed in vertebrates. We measured sex and seasonal differences in two nuclei of the song-control system of brood-parasitic brown-headed cowbirds (Molothrus ater) and closely-related non-parasitic red-winged blackbirds (Agelaius phoeniceus). These species differ in both the development and function of song. Brown-headed cowbirds have a larger sex difference in song than red-winged blackbirds. Female cowbirds never sing, whereas female blackbirds do though much less than males. In cowbirds, song primarily functions in mate choice and males modify their song as they approach sexual maturity and interact with females. In red-winged blackbirds, song is used primarily in territorial defence and is crystalized earlier in life. We found that the HVC was more likely to be discernable in breeding female blackbirds than in breeding female cowbirds. Compared to males, females had a smaller HVC and a smaller robust nucleus of the arcopallium (RA). However, females had higher doublecortin immunoreactivity (DCX+) in HVC, a measure of neurogenesis. Consistent with sex differences in song, the sex difference in RA volume was greater in cowbirds than in blackbirds. Males of both species had a smaller HVC with higher DCX+ in post-breeding condition than in breeding condition when song is more plastic. Sex and seasonal differences in the song-control system were closely related to variation in song in these two icterid songbirds. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1226-1240, 2016.

Testosterone Mediates Seasonal Growth of the Song Control Nuclei in a Tropical Bird

In mid- to high-latitude songbirds, seasonal reproduction is stimulated by increasing day length accompanied by elevated plasma sex steroid levels, increased singing, and growth of the song control nuclei (SCN). Plasticity of the SCN and song behavior are primarily mediated by testosterone (T) and its metabolites in most species studied thus far. However, the majority of bird species are tropical and have less pronounced seasonal reproductive cycles. We have previously documented that equatorial rufous-collared sparrows (Zonotrichia capensis) exhibit seasonal neuroplasticity in the SCN. Manipulating T in these birds, however, did not alter singing behavior. In the current study, we investigated whether T mediates plasticity of the SCN in a similar manner to temperate songbirds. In the first experiment, we treated captive male birds with T or blank implants during the nonbreeding season. In a second experiment, we treated captive male birds with either blank implants, T-filled implants, T with flutamide (FLU; an androgen receptor antagonist) or T with FLU and 1,4,6-androstatriene-3,17-dione (ATD; an estrogen synthesis inhibitor) during the breeding season. In both experiments, the volumes of the brain areas high vocal center (HVC), Area X, and robust nucleus of the arcopallium (RA) were measured along with singing behavior. In summary, T stimulated growth of HVC and RA, and the combined effect of FLU and ATD reversed this effect in HVC. Area X was not affected by T treatment in either experiment. Neither T-treated birds nor controls sang in captivity during either experiment. Together, these data indicate that T mediates seasonal changes in the HVC and RA of both tropical and higher- latitude bird species even if the environmental signals differ. However, unlike most higher-latitude songbirds, we found no evidence that motivation to sing or growth of Area X are stimulated by T under captive conditions.

"Seasonal changes in the neuroendocrine system": some reflections

This perspective considers first the general issue of seasonality and how it is shaped ecologically. It asks what is the relative importance of "strategic" (photoperiod-dependent) versus "tactical" (supplemental) cues in seasonality and what neural circuits are involved? It then considers recent developments as reflected in the Special Issue. What don't we understand about the photoperiodic clock and also the long-term timing mechanisms underlying refractoriness? Are these latter related to the endogenous annual rhythms? Can we finally identify the opsins involved in photodetection? What is the present position with regard to melatonin as "the" annual calendar? An exciting development has been the recognition of the involvement of thyroid hormones in seasonality but how does the Dio/TSH/thyroid hormone pathway integrate with downstream components of the photoperiodic response system? Finally, there are the seasonal changes within the central nervous system itself--perhaps the most exciting aspect of all.

Seasonal change in the avian hippocampus

The hippocampus plays an important role in cognitive processes, including memory and spatial orientation, in birds. The hippocampus undergoes seasonal change in food-storing birds and brood parasites, there are changes in the hippocampus during breeding, and further changes occur in some species in association with migration. In food-storing birds, seasonal change in the hippocampus occurs in fall and winter when the cognitively demanding behaviour of caching and retrieving food occurs. The timing of annual change in the hippocampus of food-storing birds is quite variable, however, and appears not to be under photoperiod control. A variety of factors, including cognitive performance, exercise, and stress may all influence seasonal change in the avian hippocampus. The causal processes underlying seasonal change in the avian hippocampus have not been extensively examined and the more fully described hormonal influences on the mammalian hippocampus may provide hypotheses for investigating the control of hippocampal seasonality in birds.