Variation in the gonadotrophin-releasing hormone-1 and the song control system in the tropical breeding rufous-collared sparrow (Zonotrichia capensis) is dependent on sex and reproductive state

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.

Does a short-term increase in testosterone affect the intensity or persistence of territorial aggression? - An approach using an individual's hormonal reactive scope to study hormonal effects on behavior

In this study, we describe an approach based on an individual's hormonal reactive scope to study short-term effects of hormones on behavior. The control of territorial aggression has been traditionally linked to testosterone. Males of some vertebrate species show an increase in testosterone during territorial interactions and implantation studies suggest that such an increase in testosterone enhances the intensity and persistence of aggression. Here, we tested whether a short-term maximum release of testosterone - based on an individual's hormonal reactive scope - affects the intensity or persistence of territorial aggression in male black redstarts, a bird species in which testosterone does not increase during territorial encounters. An injection with gonadotropin-releasing-hormone (GnRH) induced a physiological peak in plasma testosterone that was specific for each individual (=individual reactive scope). However, such short-term surges in an individual's testosterone concentration did not affect the intensity or persistence of aggression. In conclusion, this study demonstrated (1) that a species that naturally does not increase testosterone during male-male encounters would not benefit from such an increase in terms of being more aggressive, (2) that behavioral studies using GnRH-injections represent a promising approach to study species differences in androgen responsiveness, and (3) that injections of releasing or tropic hormones in general may be a suitable approach to study short-term influences of hormones on behavior. These injections effectively mimic the potential short-term changes in hormones that can occur in the real life of individuals and enable us to study the effects of hormonal changes on behavior or other traits within an ecological and evolutionary framework.

New insights into the hormonal and behavioural correlates of polymorphism in white-throated sparrows, Zonotrichia albicollis

The white-throated sparrow is a promising model for behavioural neuroendocrinology and genetics because behaviour and endocrine function may be linked to a chromosomal rearrangement that determines plumage colour. The notion that the two colour morphs, tan-striped (TS) and white-striped (WS), differ predictably in aggression and parenting has been widely accepted, despite conflicting evidence. It is also hypothesized that morph-typic behaviour is hormone mediated, yet no field study has measured sex steroids and behaviour in the same birds. Here, we re-evaluate the TS and WS phenotypes, describe the conditions under which they differ and investigate relationships between sex steroids and behaviour. We report that (1) during territorial intrusions, WS males were more aggressive than TS birds, but this difference was restricted to singing; WS males sang more than TS males but showed identical levels of physical aggression. WS females sang more than TS females and were also more physically aggressive. (2) TS males provisioned young more frequently than did WS males, but only during first broods. The parental strategy of WS males was flexible, and during replacement broods, WS and TS males provisioned at equal rates. (3) Consistent with previous studies, we detected no morph difference in female provisioning. (4) Plasma testosterone and dihydrotestosterone were higher in WS males than in TS males during periods of peak territorial defence and during first broods; within breeding stages, male androgen levels were positively correlated with singing and negatively correlated with provisioning. Plasma oestradiol levels were higher in WS females than in TS females and higher during peak territorial defence; oestradiol levels tended to be positively correlated with singing. Overall, our results refine the TS and WS phenotypes, show that behavioural differences between them are restricted to periods with relatively high mating opportunity, and demonstrate an association between sex steroids and morph-typic behaviour. These results will inform future studies of this promising model.

Hormonal regulation of vasotocin receptor mRNA in a seasonally breeding songbird

Behaviors associated with breeding are seasonally modulated in a variety of species. These changes in behavior are mediated by sex steroids, levels of which likewise vary with season. The effects of androgens on behaviors associated with breeding may in turn be partly mediated by the nonapeptides vasopressin (VP) and oxytocin (OT) in mammals, and vasotocin (VT) in birds. The effects of testosterone (T) on production of these neuropeptides have been well-studied; however, the regulation of VT receptors by T is not well understood. In this study, we investigated steroid-dependent regulation of VT receptor (VTR) mRNA in a seasonally breeding songbird, the white-throated sparrow (Zonotrichia albicollis). We focused on VTR subtypes that have been most strongly implicated in social behavior: V1a and oxytocin-like receptor (OTR). Using in situ hybridization, we show that T-treatment of non-breeding males altered V1a and OTR mRNA expression in several regions associated with seasonal reproductive behaviors. For example, T-treatment increased V1a mRNA expression in the medial preoptic area, bed nucleus of the stria terminalis, and ventromedial hypothalamus. T-treatment also affected both V1a and OTR mRNA expression in nuclei of the song system; some of these effects depended on the presence or absence of a chromosomal rearrangement that affects singing behavior, plasma T, and VT immunolabeling in this species. Overall, our results strengthen evidence that VT helps mediate the behavioral effects of T in songbirds, and suggest that the chromosomal rearrangement in this species may affect the sensitivity of the VT system to seasonal changes in T.

Estrogen receptor α polymorphism in a species with alternative behavioral phenotypes

The evolution of behavior relies on changes at the level of the genome; yet the ability to attribute a behavioral change to a specific, naturally occurring genetic change is rare in vertebrates. In the white-throated sparrow (Zonotrichia albicollis), a chromosomal polymorphism (ZAL2/2(m)) is known to segregate with a behavioral phenotype. Individuals with the ZAL2(m) haplotype engage in more territorial aggression and less parental behavior than individuals without it. These behaviors are thought to be mediated by sensitivity to sex steroids, and the chromosomal rearrangement underlying the polymorphism has captured a prime candidate gene: estrogen receptor 1 (ESR1), which encodes estrogen receptor α (ERα). We therefore hypothesized that the behavioral effects of the ZAL2(m) rearrangement are mediated by polymorphism in ESR1. We report here that (i) the ESR1 promoter region contains fixed polymorphisms distinguishing the ZAL2(m) and ZAL2 alleles; (ii); those polymorphisms regulate transcription efficiency in vitro and therefore potentially do the same in vivo (iii); the local expression of ERα in the brain depends strongly on genotype in a free-living population; and (iv) ERα expression in the medial amygdala and medial preoptic area may fully mediate the effects of genotype on territorial aggression and parenting, respectively. Thus, our study provides a rare glimpse of how a chromosomal polymorphism has affected the brain and social behavior in a vertebrate. Our results suggest that in this species, differentiation of ESR1 has played a causal role in the evolution of phenotypes with alternative life-history strategies.

Photoperiod-dependent regulation of gonadotropin-releasing hormone 1 messenger ribonucleic acid levels in the songbird brain

Annual changes in day length induce marked changes in reproductive function in temperate zone vertebrates. In many avian species, in contrast to other seasonally breeding animals, plasticity in hypothalamic gonadotropin-releasing hormone - 1 (GnRH1) expression rather than (or in addition to) release governs changes in pituitary-gonadal activity. Investigations of the cellular and molecular mechanisms that govern GnRH1 plasticity were previously hindered by a collective inability of scientists in the field to characterize the gnrh1 cDNA in songbirds. We finally overcame this roadblock after data from the zebra finch (Taeniopygia guttata) genome project enabled us to rapidly clone the gnrh1 cDNA from hypothalamic RNA of zebra finches and European starlings (Sturnus vulgaris). Here, we review the original data that identified GnRH1 protein plasticity in the songbird brain and discuss earlier failed attempts to clone gnrh1 in these animals. Then, we present recent efforts, including our own, that successfully characterized gnrh1 in zebra finch and starling, and demonstrated dynamic regulation of gnrh1 mRNA expression, particularly in sub-populations of preoptic area neurons, in the latter. Overall, this paper highlights GnRH1 plasticity in the avian brain, and weaves into the narrative the previously untold story of the challenges to sequencing gnrh1 in songbirds.

Gonadotropin releasing hormone and brooding behavior in the native Thai hen

Changes in the number of hypothalamic gonadotropin releasing hormone-I (GnRH-I) neurons within the Nucleus commissurae pallii (nCPa) were associated with the reproductive cycle of native Thai chickens. In order to further understand the association of GnRH-I in the regulation of brooding behavior in this bird, the native Thai chickens were divided into two groups; chick-rearing (R) and non-chick-rearing (NR) hens. Numbers of visible of GnRH-I-immunoreactive (GnRH-I-ir) neurons in the hypothalamus of R and NR hens were compared utilizing immunohistochemistry. Numbers of visible GnRH-I-ir neurons within the Nucleus anterior medialis hypothalami, Nucleus suprachaiasmaticus, pars medialis, Nucleus septalis lateralis, Nucleus paraventricularis magnocellularis, and Regio lateralis hypothalami areas were observed in both groups, but no differences were seen between R and NR hens. The number of visible GnRH-I neurons in the nCPa was higher (P<0.05) in the NR than in R hens, and increased in NR hens by day 14 after chick removal. These findings suggest, for the first time, an association of the GnRH system with brooding behavior in continuously breeding birds. Furthermore, the expression of brooding behavior of native Thai chickens might be regulated, in part, by GnRH-I neurons in the nCPa.

Testosterone alters genomic responses to song and monoaminergic innervation of auditory areas in a seasonally breeding songbird

Behavioral responses to social stimuli often vary according to endocrine state. Our previous work has suggested that such changes in behavior may be due in part to hormone-dependent sensory processing. In the auditory forebrain of female white-throated sparrows, expression of the immediate early gene ZENK (egr-1) is higher in response to conspecific song than to a control sound only when plasma estradiol reaches breeding-typical levels. Estradiol also increases the number of detectable noradrenergic neurons in the locus coeruleus and the density of noradrenergic and serotonergic fibers innervating auditory areas. We hypothesize, therefore, that reproductive hormones alter auditory responses by acting on monoaminergic systems. This possibility has not been examined in males. Here, we treated non-breeding male white-throated sparrows with testosterone to mimic breeding-typical levels and then exposed them to conspecific male song or frequency-matched tones. We observed selective ZENK responses in the caudomedial nidopallium only in the testosterone-treated males. Responses in another auditory area, the caudomedial mesopallium, were selective regardless of hormone treatment. Testosterone treatment reduced serotonergic fiber density in the auditory forebrain, thalamus, and midbrain, and although it increased the number of noradrenergic neurons detected in the locus coeruleus, it reduced noradrenergic fiber density in the auditory midbrain. Thus, whereas we previously reported that estradiol enhances monoaminergic innervation of the auditory pathway in females, we show here that testosterone decreases it in males. Mechanisms underlying testosterone-dependent selectivity of the ZENK response may differ from estradiol-dependent ones

Behavioral characterization of a white-throated sparrow homozygous for the ZAL2(m) chromosomal rearrangement

The white-throated sparrow is rapidly becoming an important model in the genetics of social behavior because of a chromosomal rearrangement that segregates with a behavioral phenotype. Within a population, 50 % of individuals are heterozygous for a rearranged chromosome 2 (ZAL2(m)). These birds sing more and are more aggressive than the other 50 %, who lack the rearrangement. A disassortative mating system, in which heterozygotes almost never interbreed, ensures that ZAL2(m)/2(m) homozygotes are extremely rare. Here, we provide the first systematic characterization of such a homozygote, a hatch-year female. Her plumage was atypical of her age and sex, resembling that of an adult male. She was extremely vocal and aggressive, dominating her opponents in behavioral tests. Her phenotype was thus an exaggerated version of a typical ZAL2/2(m) heterozygote, supporting the hypothesis that alleles inside the ZAL2(m) rearrangement confer high aggression and further emphasizing this species' value as a model of social behavior.

Gonadotropin-releasing hormone plasticity: a comparative perspective

Gonadotropin-releasing hormone 1 (GnRH1) is a key regulator of the reproductive neuroendocrine system in vertebrates. Recent developments have suggested that GnRH1 neurons exhibit far greater plasticity at the cellular and molecular levels than previously thought. Furthermore, there is growing evidence that sub-populations of GnRH1 neurons in the preoptic area are highly responsive to specific environmental and hormonal conditions. In this paper we discuss findings that reveal large variation in GnRH1 mRNA and protein expression that are regulated by social cues, photoperiod, and hormonal feedback. We draw upon studies using histochemistry and immediate early genes (e.g., c-FOS/ZENK) to illustrate that specific groups of GnRH1 neurons are topographically organized. Based on data from diverse vertebrate species, we suggest that GnRH1 expression within individuals is temporally dynamic and this plasticity may be evolutionarily conserved. We suggest that the plasticity observed in other neuropeptide systems (i.e. kisspeptin) may have evolved in a similar manner.