Higher cognitive function depends on accurate detection and processing of subtle features of sensory stimuli. Such precise computations require neural circuits to be modulated over rapid timescales, yet this modulation is poorly understood. Brainderived steroids (neurosteroids) can act as fast signaling molecules in the vertebrate central nervous system and could therefore modulate sensory processing and guide behavior, but there is no empirical evidence for this possibility. Here we report that acute inhibition of estrogen production within a cortical-like region involved in complex auditory processing disrupts a songbird's ability to behaviorally respond to song stimuli. Identical manipulation of local estrogen levels rapidly changes burst firing of single auditory neurons. This acute estrogen-mediated modulation targets song and not other auditory stimuli, possibly enabling discrimination among species-specific signals. Our results demonstrate a crucial role for neuroestrogen synthesis among vertebrates for enhanced sensory encoding. Cognitive impairments associated with estrogen depletion, including verbal memory loss in humans, may therefore stem from compromised moment-by-moment estrogen actions in higher-order cortical circuits.I n vertebrates, steroid hormones can rapidly influence the activity of neurons and neural circuits (1-3), although the functional consequences for higher processing are unclear. The exquisite recognition of species-typical vocalizations (4, 5) and abundant production of neuroestrogens (6, 7) are both especially prominent in songbirds. A cortical-like auditory region (the caudomedial nidopallium, or NCM) of songbirds is a critical locus for song learning as well as auditory processing and song recognition (8-11). NCM exhibits rich expression of the estrogen-synthetic enzyme aromatase in both cell bodies and synaptic terminals (6, 12), and neuroestrogen levels within NCM fluctuate rapidly (<30 min) and independent of peripheral sex-steroid levels during social interactions, song playback, and after neurotransmitter activation (13). In songbirds, therefore, rapid changes in the local production of NCM neuroestrogens could in turn rapidly affect processing of complex auditory stimuli, such as song.Here, we test this hypothesis in male Australian zebra finches (Taeniopygia guttata). Our recent optimization of in vivo microdialysis in this species has enabled the measurement and manipulation of local estrogen production within discrete brain regions in awake, behaving males (13). We have further established that local estrogen levels are dependent on the activity of the enzyme aromatase within NCM, because retrodialysis (reverse delivery using microdialysis) of the aromatase inhibitor fadrozole (FAD) into NCM transiently suppresses local estradiol levels (13). ResultsIn Vivo Retrodialysis and Behavior. Adult zebra finches express a robust behavioral preference for acoustic playback of their tutor's song or bird's own song (BOS) compared with conspecific male song (CON) (4, 5, 8), and be...

The enzyme aromatase (which converts androgens into oestrogens) is expressed throughout the brain in zebra finches. Aromatase is enzymatically active in both cell bodies and synaptic terminals of neurones of the songbird brain, particularly within forebrain motor and auditory networks. Aromatisation within synaptic terminals could thus provide localised and acute modulatory oestrogens within the forebrain during singing and/or audition. In male zebra finches, we tested the hypothesis that forebrain aromatase activity is elevated during singing behaviour and/or hearing male song. This study reports that aromatase activity is elevated in males that were singing for 30 min as compared to non-singing males, and that this elevation occurs only within the cellular compartment that contains synaptic terminals. In a separate experiment, males that heard acoustic playback of song for 30 min exhibited no differences in aromatase activity or in aromatase mRNA levels as revealed by quantitative PCR analysis. Therefore, these results indicate that activation of the motor pathway for song production is linked to local elevations in synaptic aromatase activity within the forebrain of male zebra finches. Future experiments could assess whether elevated synaptic aromatase activity during song is dependent on acute regulation of the aromatase protein.

BackgroundSteroids affect many tissues, including the brain. In the zebra finch, the estrogenic steroid estradiol (E2) is especially effective at promoting growth of the neural circuit specialized for song. In this species, only the males sing and they have a much larger and more interconnected song circuit than females. Thus, it was surprising that the gene for 17β-hydroxysteroid dehydrogenase type 4 (HSD17B4), an enzyme that converts E2 to a less potent estrogen, had been mapped to the Z sex chromosome. As a consequence, it was likely that HSD17B4 was differentially expressed in males (ZZ) and females (ZW) because dosage compensation of Z chromosome genes is incomplete in birds. If a higher abundance of HSD17B4 mRNA in males than females was translated into functional enzyme in the brain, then contrary to expectation, males could produce less E2 in their brains than females.ResultsHere, we used molecular and biochemical techniques to confirm the HSD17B4 Z chromosome location in the zebra finch and to determine that HSD17B4 mRNA and activity were detectable in the early developing and adult brain. As expected, HSD17B4 mRNA expression levels were higher in males compared to females. This provides further evidence of the incomplete Z chromosome inactivation mechanisms in birds. We detected HSD17B4 mRNA in regions that suggested a role for this enzyme in the early organization and adult function of song nuclei. We did not, however, detect significant sex differences in HSD17B4 activity levels in the adult brain.ConclusionsOur results demonstrate that the HSD17B4 gene is expressed and active in the zebra finch brain as an E2 metabolizing enzyme, but that dosage compensation of this Z-linked gene may occur via post-transcriptional mechanisms.

A variety of genetic techniques have been devised to determine cell lineage relationships during tissue development. Some of these systems monitor cell lineages spatially and/or temporally without regard to gene expression by the cells, whereas others correlate gene expression with the lineage under study. The GAL4 Technique for Real-time and Clonal Expression (G-TRACE) system allows for rapid, fluorescent protein-based visualization of both current and past GAL4 expression patterns and is therefore amenable to genome-wide expression-based lineage screens. Here we describe the results from such a screen, performed by undergraduate students of the University of California, Los Angeles (UCLA) Undergraduate Research Consortium for Functional Genomics (URCFG) and high school summer scholars as part of a discovery-based education program. The results of the screen, which reveal novel expression-based lineage patterns within the brain, the imaginal disc epithelia, and the hematopoietic lymph gland, have been compiled into the G-TRACE Expression Database (GED), an online resource for use by the Drosophila research community. The impact of this discovery-based research experience on student learning gains was assessed independently and shown to be greater than that of similar programs conducted elsewhere. Furthermore, students participating in the URCFG showed considerably higher STEM retention rates than UCLA STEM students that did not participate in the URCFG, as well as STEM students nationwide.