Interactions between environmental changes and brain plasticity in birds

Characterization of cell proliferation throughout the brain of the African cichlid fish Astatotilapia burtoni and its regulation by social status

New cells are added in the brains of all adult vertebrates, but fishes have some of the greatest potential for neurogenesis and gliogenesis among all taxa, partly due to their indeterminate growth. Little is known, however, about how social interactions influence cell proliferation in the brain of these fishes that comprise the largest group of vertebrates. We used 5-bromo-2'-deoxyuridine (BrdU) to identify and localize proliferation zones in the telencephalon, diencephalon, mesencephalon, and rhombencephalon that were primarily associated with ventricular surfaces in the brain of the African cichlid fish Astatotilapia burtoni. Cell migration was evident in some regions by 1 day post injection, and many newborn cells coexpressed the neuronal marker HuC/D at 30 days, suggesting they had differentiated into neurons. To test the hypothesis that social status and perception of an opportunity to rise in rank influenced cell proliferation, we compared numbers of BrdU-labeled cells in multiple brain nuclei among fish of different social status. Socially suppressed subordinate males had the lowest numbers of proliferating cells in all brain regions examined, but males that were given an opportunity to rise in status had higher cell proliferation rates within 1 day, suggesting rapid upregulation of brain mitotic activity associated with this social transition. Furthermore, socially isolated dominant males had similar numbers of BrdU-labeled cells compared with dominant males that were housed in a socially rich environment, suggesting that isolation has little effect on proliferation and that reduced proliferation in subordinates is a result of the social subordination. These results suggest that A. burtoni will be a useful model to analyze the mechanisms of socially induced neurogenesis in vertebrates.

Birds as a model to study adult neurogenesis: bridging evolutionary, comparative and neuroethological approaches

During the last few decades, evidence has demonstrated that adult neurogenesis is a well-preserved feature throughout the animal kingdom. In birds, ongoing neuronal addition occurs rather broadly, to a number of brain regions. This review describes adult avian neurogenesis and neuronal recruitment, discusses factors that regulate these processes, and touches upon the question of their genetic control. Several attributes make birds an extremely advantageous model to study neurogenesis. First, song learning exhibits seasonal variation that is associated with seasonal variation in neuronal turnover in some song control brain nuclei, which seems to be regulated via adult neurogenesis. Second, food-caching birds naturally use memory-dependent behavior in learning the locations of thousands of food caches scattered over their home ranges. In comparison with other birds, food-caching species have relatively enlarged hippocampi with more neurons and intense neurogenesis, which appears to be related to spatial learning. Finally, migratory behavior and naturally occurring social systems in birds also provide opportunities to investigate neurogenesis. This diversity of naturally occurring memory-based behaviors, combined with the fact that birds can be studied both in the wild and in the laboratory, make them ideal for investigation of neural processes underlying learning. This can be done by using various approaches, from evolutionary and comparative to neuroethological and molecular. Finally, we connect the avian arena to a broader view by providing a brief comparative and evolutionary overview of adult neurogenesis and by discussing the possible functional role of the new neurons. We conclude by indicating future directions and possible medical applications.

Song exposure regulates known and novel microRNAs in the zebra finch auditory forebrain

BACKGROUND

In an important model for neuroscience, songbirds learn to discriminate songs they hear during tape-recorded playbacks, as demonstrated by song-specific habituation of both behavioral and neurogenomic responses in the auditory forebrain. We hypothesized that microRNAs (miRNAs or miRs) may participate in the changing pattern of gene expression induced by song exposure. To test this, we used massively parallel Illumina sequencing to analyse small RNAs from auditory forebrain of adult zebra finches exposed to tape-recorded birdsong or silence.

RESULTS

In the auditory forebrain, we identified 121 known miRNAs conserved in other vertebrates. We also identified 34 novel miRNAs that do not align to human or chicken genomes. Five conserved miRNAs showed significant and consistent changes in copy number after song exposure across three biological replications of the song-silence comparison, with two increasing (tgu-miR-25, tgu-miR-192) and three decreasing (tgu-miR-92, tgu-miR-124, tgu-miR-129-5p). We also detected a locus on the Z sex chromosome that produces three different novel miRNAs, with supporting evidence from Northern blot and TaqMan qPCR assays for differential expression in males and females and in response to song playbacks. One of these, tgu-miR-2954-3p, is predicted (by TargetScan) to regulate eight song-responsive mRNAs that all have functions in cellular proliferation and neuronal differentiation.

CONCLUSIONS

The experience of hearing another bird singing alters the profile of miRNAs in the auditory forebrain of zebra finches. The response involves both known conserved miRNAs and novel miRNAs described so far only in the zebra finch, including a novel sex-linked, song-responsive miRNA. These results indicate that miRNAs are likely to contribute to the unique behavioural biology of learned song communication in songbirds.

Steroidal and gonadal effects on neural cell proliferation in vitro in an adult songbird

Neurogenesis in the adult songbird brain occurs along the ventricular zone (VZ), a specialized cell layer surrounding the lateral ventricles. To examine the acute effects of sex steroids on VZ cell proliferation, male and female adult zebra finch brain slices containing the VZ were exposed to 5-bromo-2'-deoxyuridine-5'-monophosphate (BrdU) in vitro. Slices from one hemisphere served as the control, while contralateral slices were treated with steroids, steroidogenic enzyme inhibitors or gonadal tissue itself. There were no significant effects on VZ cell proliferation in either sexes by acute exposure to 17beta-estradiol (E2), dihydrotestosterone (DHT), a cocktail of four sex steroids, and inhibitors of sex steroid synthesis (aminoglutethimide, ketoconazole, and fadrozole), or by activation of a mitochondrial cholesterol transporter. By contrast, dehydroepiandrosterone (DHEA) suppressed VZ cell proliferation in males, but not females, replicating previous observations involving treatments with corticosterone and RU-486. This suggests that DHEA suppresses proliferation in males via a glucocorticoid receptor-related mechanism. These results suggest that neurosteroidogenesis per se has little effect on acute VZ cell proliferation. Co-incubation with an ovary of female, but not male, slices significantly increased VZ cell proliferation; testicular tissue had no impact on proliferation in males or females. This suggests a role for a non-steroidal ovarian factor on adult female VZ cell proliferation. We also have evidence that previously reported sex-differences in BrdU-labeling along the adult VZ (males>females) result from a more rapid loss of cells in females. Sex differences in steroid action and cell death along the VZ may contribute to the maintenance of the sexually dimorphic song system.