Season-related changes in circulating androgen, brain aromatase, and perch-calling in male ring doves

Estradiol concentration and the expression of estrogen receptors in the testes of the domestic goose (Anser anser f. domestica) during the annual reproductive cycle

Seasonal fluctuations in the activity of bird testes are regulated by a complex mechanism where androgens play a key role. Until recently, the role played by estrogens in males has been significantly underestimated. However, there is growing evidence that the proper functioning of the testes is associated with optimal estradiol (E2) concentration in both the plasma and testes of many mammalian species. Estrogens are gradually emerging as very important players in hormonal regulation of reproductive processes in male mammals. Despite the previously mentioned, it should be noted that estrogenic action is limited by the availability of specific receptors--estrogen receptor alpha (ERα) and estrogen receptor beta (ERβ). Interestingly, there is a general scarcity of information concerning the estrogen responsive system in the testes of male birds, which is of particular interest in exploring the phenomenon of seasonality of reproduction. To address this question, we have investigated for the first time the simultaneous expression of testicular ERα and ERβ genes and proteins with the accompanying plasma and testicular E2 concentrations during the annual reproductive cycle of male bird. The research model was the domestic goose (Anser anser f. domestica), a species whose annual reproductive cycle can be divided into 3 distinct phases characterized by changes in testicular activity. It has been revealed that the stable plasma E2 profile did not correspond to changing intratesticular E2 profile throughout the experiment. The expression of ERα and ERβ genes and proteins was detected in gander testes and it fluctuated on a seasonal basis with lower level in breeding and sexual reactivation stages and higher level during the nonbreeding stage. Our results demonstrated changes in testicular sensitivity to estrogens in male domestic goose during the annual reproductive cycle. The seasonal pattern of estrogen receptors (ERs) expression was analyzed against the hormonal background and a potential mechanism of ERs regulation in bird testes was proposed. The present study revealed seasonal variations in the estrogen responsive system, but further research is needed to fully explore the role of estrogens in the reproductive tract of male birds.

Using the canary genome to decipher the evolution of hormone-sensitive gene regulation in seasonal singing birds

BACKGROUND

While the song of all songbirds is controlled by the same neural circuit, the hormone dependence of singing behavior varies greatly between species. For this reason, songbirds are ideal organisms to study ultimate and proximate mechanisms of hormone-dependent behavior and neuronal plasticity.

RESULTS

We present the high quality assembly and annotation of a female 1.2-Gbp canary genome. Whole genome alignments between the canary and 13 genomes throughout the bird taxa show a much-conserved synteny, whereas at the single-base resolution there are considerable species differences. These differences impact small sequence motifs like transcription factor binding sites such as estrogen response elements and androgen response elements. To relate these species-specific response elements to the hormone-sensitivity of the canary singing behavior, we identify seasonal testosterone-sensitive transcriptomes of major song-related brain regions, HVC and RA, and find the seasonal gene networks related to neuronal differentiation only in the HVC. Testosterone-sensitive up-regulated gene networks of HVC of singing males concerned neuronal differentiation. Among the testosterone-regulated genes of canary HVC, 20% lack estrogen response elements and 4 to 8% lack androgen response elements in orthologous promoters in the zebra finch.

CONCLUSIONS

The canary genome sequence and complementary expression analysis reveal intra-regional evolutionary changes in a multi-regional neural circuit controlling seasonal singing behavior and identify gene evolution related to the hormone-sensitivity of this seasonal singing behavior. Such genes that are testosterone- and estrogen-sensitive specifically in the canary and that are involved in rewiring of neurons might be crucial for seasonal re-differentiation of HVC underlying seasonal song patterning.

Synaptocrine signaling: steroid synthesis and action at the synapse

Sex steroids have long been recognized for their dramatic impact on brain and behavior, including rapid modulation of membrane excitability. It is a widely held perception that these molecules are largely derived from peripheral sources and lack the spatial and temporal specificity ascribed to classical neuromodulatory systems. Neuromodulatory systems, in contrast, are defined by their regulated neuronal presynaptic secretion and by their functional modulation of perisynaptic events. Here we provide evidence for regulated presynaptic estrogen synthesis and functional postsynaptic actions. These results meet all the criteria for a neuromodulatory system and shift our perception of estrogens from that of peripheral signals exclusively to include that of a signaling system intrinsic to the brain itself. We apply the term synaptocrine to describe this form of neuromodulation.

Estradiol synthesis and action at the synapse: evidence for "synaptocrine" signaling

Classically, the modulation of brain function and behavior by steroid hormones was linked exclusively to secretion by peripheral endocrine glands. Subsequently, steroid actions within the brain were shown dependent upon either synthesis and secretion by peripheral organs or by production within the CNS itself using peripheral sources of precursors. Discovery of the estrogen-synthetic enzyme aromatase in brain further bolstered the latter view and served as a catalyst for expanding concepts of neurosteroidogenesis. In parallel research, several steroids, including estradiol, were found to have rapid effects on neuronal excitability, partially explained by novel actions at neuronal membranes. Recent findings from multiple levels of analysis and labs necessitate an updated view on how steroids are delivered to neural circuits. There is now considerable evidence for expression of the aromatase enzyme within synaptic boutons in the vertebrate CNS. Furthermore, additional work now directly couples rapid regulation of neuroestrogen synthesis with neurophysiological and behavioral outcomes. In this review we summarize evidence for targeted and acute synaptic estrogen synthesis and perisynaptic estrogen actions in the CNS of songbirds. We evaluate these findings in the context of criteria associated with classic neuromodulatory signaling. We term this novel form of signaling "synaptocrine," and discuss its implications.

Low sex steroids, high steroid receptors: Increasing the sensitivity of the nonreproductive brain

Male aggressive behavior is generally regulated by testosterone (T). In most temperate breeding males, aggressive behavior is only expressed during the reproductive period. At this time circulating T concentrations, brain steroid receptors, and steroid metabolic enzymes are elevated in many species relative to the nonreproductive period. Many tropical birds, however, display aggressive behavior both during the breeding and the nonbreeding season, but plasma levels of T can remain low throughout the year and show little seasonal fluctuation. Studies on the year-round territorial spotted antbird (Hylophylax n. naevioides) suggest that T nevertheless regulates aggressive behavior in both the breeding and nonbreeding season. We hypothesize that to regulate aggressive behaviors during the nonbreeding season, when T is at its minimum, male spotted antbirds increase brain sensitivity to steroids. This can be achieved by locally up-regulating androgen receptors (ARs), estrogen receptors (ERs), or the enzyme aromatase (AROM) that converts T into estradiol. We therefore compared mRNA expression of AR, ERalpha, and AROM in free- living male spotted antbirds across reproductive and nonreproductive seasons in two brain regions known to regulate both reproductive and aggressive behaviors. mRNA expression of ERalpha in the preoptic area and AR in the nucleus taeniae were elevated in male spotted antbirds during the nonbreeding season when circulating T concentrations were low. This unusual seasonal receptor regulation may represent a means for the year-round regulation of vertebrate aggressive behavior via steroids by increasing the brain's sensitivity to sex steroids during the nonbreeding season.