Estradiol interacts with an opioidergic network to achieve rapid modulation of a vocal pattern generator

Estrogens rapidly regulate neuronal activity within seconds-to-minutes, yet it is unclear how estrogens interact with neural circuits to rapidly coordinate behavior. This study examines whether 17-beta-estradiol interacts with an opioidergic network to achieve rapid modulation of a vocal control circuit. Adult plainfin midshipman fish emit vocalizations that mainly differ in duration, and rhythmic activity of a hindbrain–spinal vocal pattern generator (VPG) directly establishes the temporal features of midshipman vocalizations. VPG activity is therefore predictive of natural calls, and ‘fictive calls’ can be elicited by electrical microstimulation of the VPG. Prior studies show that intramuscular estradiol injection rapidly (within 5 min) increases fictive call duration in midshipman. Here, we delivered opioid antagonists near the VPG prior to estradiol injection. Rapid estradiol actions on fictive calling were completely suppressed by the broad-spectrum opioid antagonist naloxone and the mu-opioid antagonist beta-funaltrexamine, but were unaffected by the kappa-opioid antagonist nor-binaltorphimine. Unexpectedly, prior to estradiol administration, all three opioid antagonists caused immediate, transient reductions in fictive call duration. Together, our results indicate that: (1) vocal activity is modulated by opioidergic networks, confirming hypotheses from birds and mammals, and (2) the rapid actions of estradiol on vocal patterning depend on interactions with a mu-opioid modulatory network

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

Testosterone synthesis in the female brain during auditory processing.

Steroids are synthesized in the brain and act as neuromodulators to regulate important biological functions such as reproduction, cognition or sensory processing. Songbirds are a tractable model to study the cellular/molecular mechanisms that regulate the production and the sensory coding of learned vocalizations. In zebra finches, estradiol is rapidly synthesized in the auditory cortex (caudomedial nidopallium; NCM) in response to conspecific songs. In both males and females, estradiol is produced in the brain via aromatization of androgens like testosterone, and the aromatase enzyme is densely expressed in NCM. Local estradiol levels within NCM are rapidly elevated when males and females hear song, and local increases in estradiol rapidly enhance the auditory responses of NCM neurons. However, the mechanisms by which estradiol elevations occur within NCM remain unknown. Here, we hypothesize that local estradiol rises during song exposure via an increase in local levels of the androgen substrate, testosterone. The enzyme that regulates testosterone synthesis, 3β-HSD, is present and active in the zebra finch NCM and can be acutely regulated by environmental stimuli. In this study, we assessed testosterone fluctuations within NCM during song exposure using in vivo microdialysis. These fluctuations were assessed in females because of their lower peripheral testosterone levels and higher 3β-HSD activity in NCM as compared to males. In adult females, we observe that local levels of estradiol, but not testosterone, were significantly elevated in the NCM during song playback. By contrast, when the local production of estradiol was concurrently blocked by an aromatase inhibitor, we then observe a massive (>500%) song-evoked increase in testosterone levels within NCM. Furthermore, no significant changes in plasma testosterone were observed in females during song exposure, indicating that the song-evoked peak in NCM testosterone resulted from local brain synthesis and not peripheral fluctuations. Ongoing experiments using an androgen synthesis inhibitor will further test this mechanism of local testosterone production in the brain. Together, these results strongly suggest that elevations in NCM estradiol levels associated with song processing are mediated by a local increase in testosterone production. To our knowledge, this is the first direct evidence showing that testosterone can be synthesized within the brain. As testosterone is currently viewed as a male-typical hormone, these results suggest that its production in the female brain could be critical for fundamental brain functions and behaviors

Establishing regional specificity of neuroestrogen action

The specificity of estrogen signaling in brain is defined at one level by the types and distributions of receptor molecules that are activated by estrogens. At another level, as our understanding of the neurobiology of the estrogen synthetic enzyme aromatase has grown, questions have emerged as to how neuroactive estrogens reach specific target receptors in functionally relevant concentrations. Here we explore the spatial specificity of neuroestrogen signaling with a focus on studies of songbirds to provide perspective on some as-yet unresolved questions. Studies conducted in both male and female songbirds have helped to clarify these interesting facets of neuroestrogen physiology. © 2014 Elsevier Inc. All rights reserved

Testosterone Synthesis in the Female Songbird

Decades of work have established the brain as a source of steroid hormones, termed ‘neurosteroids’. The neurosteroid neuroestradiol is produced in discrete brain areas and influences cognition, sensory processing, reproduction, neurotransmission, and disease. A prevailing research focus on neuroestradiol has essentially ignored whether its immediate synthesis precursor - the androgen testosterone - is also dynamically regulated within the brain. Testosterone itself can rapidly influence neurophysiology and behavior, and there is indirect evidence that the female brain may synthesize significant quantities of testosterone to regulate cognition, reproduction, and behavior. In songbirds, acoustic communication is regulated by neuroestrogens. Neuroestrogens are rapidly synthetized in the caudomedial nidopallium (NCM) of the auditory cortex of zebra finches in response to song and can influence auditory processing and song discrimination. Here, we examined the in vivo dynamics of NCM levels of the neuroestrogen synthesis precursor, testosterone. Unlike estradiol, testosterone did not appear to fluctuate in the female NCM during song exposure. However, a substantial song-induced elevation of testosterone was revealed in the left hemisphere NCM of females when local aromatization (i.e., conversion to estrogens) was locally blocked. This elevation was eliminated when local androgen synthesis was concomitantly blocked. Further, no parallel elevation was observed in the circulation in response to song playback, consistent with a local, neural origin of testosterone synthesis. To our knowledge, this study provides the first direct demonstration that testosterone fluctuates rapidly in the brain in response to socially-relevant environmentalstimuli. Our findings suggest therefore that locally-derived ‘neuroandrogens’ can dynamically influence brain function and behavior