Memory circuits for vocal imitation.

Many complex behaviors exhibited by social species are first learned by imitating the behavior of other more experienced individuals. Speech and language are the most widely appreciated behaviors learned in this way. Vocal imitation in songbirds is perhaps the best studied socially transmitted behavior, and research over the past few years has begun to crack the circuit mechanisms for how songbirds learn from vocal models. Studies in zebra finches are revealing an unexpected and essential role for premotor cortical circuits in forming the behavioral-goal memories used to guide song imitation, challenging the view that song memories used for imitation are stored in auditory circuits. Here, we provide a summary of this recent progress focusing on the What, Where, and How of tutor song memory, and propose a circuit hypothesis for song learning based on these recent findings.

Acute neuroestrogen blockade attenuates song-induced immediate early gene expression in auditory regions of male and female zebra finches.

Neuron-derived estrogens are synthesized by aromatase and act through membrane receptors to modulate neuronal physiology. In many systems, long-lasting hormone treatments can alter sensory-evoked neuronal activation. However, the significance of acute neuroestrogen production is less understood. Both sexes of zebra finches can synthesize estrogens rapidly in the auditory cortex, yet it is unclear how this modulates neuronal cell signaling. We examined whether acute estrogen synthesis blockade attenuates auditory-induced expression of early growth response 1 (Egr-1) in the auditory cortex of both sexes. cAMP response element-binding protein phosphorylation (pCREB) induction by song stimuli and acute estrogen synthesis was also examined. We administered the aromatase inhibitor fadrozole prior to song exposure and measured Egr-1 across several auditory regions. Fadrozole attenuated Egr-1 in the auditory cortex greater in males than females. Females had greater expression and clustering of aromatase cells than males in high vocal center (HVC) shelf. Auditory-induced Egr-1 expression exhibited a large sex difference following fadrozole treatment. We did not observe changes in pCREB expression with song presentation or aromatase blockade. These findings are consistent with the hypothesis that acute neuroestrogen synthesis can drive downstream transcriptional responses in several cortical auditory regions, and that this mechanism is more prominent in males.

A Membrane G-Protein-Coupled Estrogen Receptor Is Necessary but Not Sufficient for Sex Differences in Zebra Finch Auditory Coding.

Estradiol acts as a neuromodulator in brain regions important for cognition and sensory processing. Estradiol also shapes brain sex differences but rarely have these concepts been considered simultaneously. In male and female songbirds, estradiol rapidly increases within the auditory forebrain during song exposure and enhances local auditory processing. We tested whether G-protein-coupled estrogen receptor 1 (GPER1), a membrane-bound estrogen receptor, is necessary and sufficient for neuroestrogen regulation of forebrain auditory processing in male and female zebra finches (Taeniopygia guttata). At baseline, we observed that females had elevated single-neuron responses to songs vs males. In males, narrow-spiking (NS) neurons were more responsive to conspecific songs than broad-spiking (BS) neurons, yet cell types were similarly auditory responsive in females. Following acute inactivation of GPER1, auditory responsiveness and coding were suppressed in male NS yet unchanged in female NS and in BS of both sexes. By contrast, GPER1 activation did not mimic previously established estradiol actions in either sex. Lastly, the expression of GPER1 and its coexpression with an inhibitory neuron marker were similarly abundant in males and females, confirming anatomical similarity in the auditory forebrain. In this study, we found: (1) a role for GPER1 in regulating sensory processing and (2) a sex difference in auditory processing of complex vocalizations in a cell type-specific manner. These results reveal sex specificity of a rapid estrogen signaling mechanism in which neuromodulation accounts and/or compensates for brain sex differences, dependent on cell type, in brain regions that are anatomically similar in both sexes.

Clustered organization and region-specific identities of estrogen-producing neurons in the forebrain of Zebra Finches (Taeniopygia guttata).

A fast, neuromodulatory role for estrogen signaling has been reported in many regions of the vertebrate brain. Regional differences in the cellular distribution of aromatase (estrogen synthase) in several species suggest that mechanisms for neuroestrogen signaling differ between and even within brain regions. A more comprehensive understanding of neuroestrogen signaling depends on characterizing the cellular identities of neurons that express aromatase. Calcium-binding proteins such as parvalbumin and calbindin are molecular markers for interneuron subtypes, and are co-expressed with aromatase in human temporal cortex. Songbirds like the zebra finch have become important models to understand the brain synthesis of steroids like estrogens and the implications for neurobiology and behavior. Here, we investigated the regional differences in cytoarchitecture and cellular identities of aromatase-expressing neurons in the auditory and sensorimotor forebrain of zebra finches. Aromatase was co-expressed with parvalbumin in the caudomedial nidopallium (NCM) and HVC shelf (proper name) but not in the caudolateral nidopallium (NCL) or hippocampus. By contrast, calbindin was not co-expressed with aromatase in any region investigated. Notably, aromatase-expressing neurons were found in dense somato-somatic clusters, suggesting a coordinated release of local neuroestrogens from clustered neurons. Aromatase clusters were also more abundant and tightly packed in the NCM of males as compared to females. Overall, this study provides new insights into neuroestrogen regulation at the network level, and extends previous findings from human cortex by identifying a subset of aromatase neurons as putative inhibitory interneurons.

Norepinephrine Modulates Coding of Complex Vocalizations in the Songbird Auditory Cortex Independent of Local Neuroestrogen Synthesis.

The catecholamine norepinephrine plays a significant role in auditory processing. Most studies to date have examined the effects of norepinephrine on the neuronal response to relatively simple stimuli, such as tones and calls. It is less clear how norepinephrine shapes the detection of complex syntactical sounds, as well as the coding properties of sensory neurons. Songbirds provide an opportunity to understand how auditory neurons encode complex, learned vocalizations, and the potential role of norepinephrine in modulating the neuronal computations for acoustic communication. Here, we infused norepinephrine into the zebra finch auditory cortex and performed extracellular recordings to study the modulation of song representations in single neurons. Consistent with its proposed role in enhancing signal detection, norepinephrine decreased spontaneous activity and firing during stimuli, yet it significantly enhanced the auditory signal-to-noise ratio. These effects were all mimicked by clonidine, an α-2 receptor agonist. Moreover, a pattern classifier analysis indicated that norepinephrine enhanced the ability of single neurons to accurately encode complex auditory stimuli. Because neuroestrogens are also known to enhance auditory processing in the songbird brain, we tested the hypothesis that norepinephrine actions depend on local estrogen synthesis. Neither norepinephrine nor adrenergic receptor antagonist infusion into the auditory cortex had detectable effects on local estradiol levels. Moreover, pretreatment with fadrozole, a specific aromatase inhibitor, did not block norepinephrine's neuromodulatory effects. Together, these findings indicate that norepinephrine enhances signal detection and information encoding for complex auditory stimuli by suppressing spontaneous "noise" activity and that these actions are independent of local neuroestrogen synthesis.