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Estrogens like estradiol (E2) via their receptors are pluripotent steroid hormones that exert a profound influence on the developing and adult brain in many vertebrates. In songbirds and mammals, acute brain injury resulting from mechanical damage, anoxia and ischemia rapidly upregulates aromatase and E2 synthesis. Interestingly, this E2 provision occurs due to the induction of aromatase expression in reactive astrocytes in areas surrounding brain injury. The resultant increase in E2 is neuroprotective with established influences on apoptosis, gliosis, cytogenesis, neurogenesis and neuroinflammation. Correspondingly, E2 decreases secondary damage following acute brain trauma and may improve recovery. Until very recently however, the signals responsible for the induction of astrocytic aromatase expression in reactive astrocytes were unknown. In the current review, we discuss what is known about the role of astrocytic E2 in neuroprotection with a particular emphasis on a recently discovered interaction between neuroinflammatory and steroidogenic signaling in the zebra finch. We first describe the role of acute inflammatory signaling in the regulation of astrocytic aromatase and central E2 levels. Next, we discuss the emerging role of central E2 in the control of chronic neuroinflammation. Finally, we provide a framework for further work investigating the important role of the interaction between inflammatory and steroidogenic signaling in the protection of neural circuits and behavior following traumatic brain injury (TBI). We also highlight dimorphisms that point to important aspects of sex-specific pathways that underlie the interactions of neuroinflammation and neurosteroidogenesis.

Estradiol provision via neural aromatization decreases neuro-inflammation and –degeneration, but almost nothing is known about the interactions between the peripheral immune system and brain aromatase. Given the vulnerability of the CNS we reasoned that brain aromatization may protect circuits from the threats of peripheral infection; perhaps shielding cells that are less resilient from the degeneration associated with peripheral infection or trauma. Lipopolysaccharide (LPS) or vehicle was administered peripherally to adult zebra finches and sickness behavior was recorded 2 or 24 hours later. The central transcription of cytokines and aromatase was measured, as were telencephalic aromatase activity and immunoreactive aromatase (24 hour time point only). Two hours post LPS, sickness-like behaviors increased, the transcription of IL-1β was higher in both sexes, and TNFα was elevated in females. 24 hours post-LPS, the behavior of LPS birds was similar to controls, and cytokines had returned to baseline, but aromatase mRNA and activity were elevated in both sexes. Immunocytochemistry revealed greater numbers of aromatase-expressing neurons in LPS birds. These data suggest that the activation of the immune system via peripheral endotoxin increases neuronal aromatase; a mechanism that may rapidly generate a potent anti-neuroinflammatory steroid in response to peripheral activation of the immune system.

Injury to the vertebrate brain causes neuroinflammation, characterized in part by increases in prostaglandins. In rodents and songbirds, brain injury also induces the transcription and translation of aromatase in reactive astrocytes around the site of damage. Interestingly, this induction is more rapid in female zebra finches relative to males. Induced aromatization is neuroprotective, as inhibition of aromatase and estrogen replacement, increases and decreases the extent of damage, respectively. Although the consequences of induced astrocytic aromatization are intensely studied, little is known about what factors induce aromatase. Inflammation is sufficient to induce astrocytic aromatase suggesting that the link between inflammation and aromatase expression may be causal. To test this hypothesis, adult male and female zebra finches received bilateral mechanical injuries through which either the cyclooxygenase (COX)-1/2 inhibitor indomethacin or vehicle was administered into contralateral hemispheres. Subjects were killed either 6 or 24 hours after injury. In both sexes, an enzyme immunoassay for prostaglandin E2 (PGE2) revealed that indomethacin decreased PGE2 relative to the contralateral hemisphere at both time points, suggesting that the dose and mode of administration used were successful in affecting neuroinflammation locally. Indomethacin reduced aromatase expression and 17β-estradiol (E2) content at 6 hours but not 24 hours following injury in females. However, in males, the inhibitory effect of indomethacin on aromatase and E2 was apparent at 24 but not 6 hours after treatment. These data suggest that COX activity, perhaps via consequent prostaglandin secretion, may induce aromatase expression and central E2, an effect that is detectable in temporally distinct patterns between sexes.

The estrogen-synthesizing enzyme aromatase is abundant at the synapse in the zebra finch hippocampus (HP), and its inhibition impairs spatial memory function. To more fully test the role of local estradiol (E2) synthesis in memory, the HP of adult male zebra finches was exposed to either control pellets or those containing the aromatase inhibitor 1,4,6-androstatriene-3,17-dione (ATD), ATD and E2, ATD and the G protein-coupled estrogen receptor (GPER) agonist G1, or the antagonist G15 alone. Birds were tested for spatial memory acquisition and performance, and HP levels of the postsynaptic protein PSD95 were measured. ATD-treated birds took longer to reach criterion than control birds, whereas acquisition in ATD+E2 and ATD+G1 birds was indistinguishable from control and ATD treatments. Interestingly, all G15 birds failed to acquire the task. Following a retention interval, ATD birds took the longest to reach the (formerly) baited cup and made the most mistakes. ATD+E2 animals displayed the lowest retention latencies and made fewer mistakes than ATD-treated birds, and ATD+G1 birds did not significantly differ from controls in retention latencies. The amount of PSD95 in the HP was lowest in ATD-treated animals compared with birds with silicone-only–implanted craniotomies, ATD+E2, and ATD+G1 birds, who did not differ in this expression. Thus, spatial memory acquisition and performance appear aromatase and E2 dependent, an effect more reliably revealed after consolidation and/or recall compared to acquisition. E2 may exert this effect via GPERs, resulting in an increase in PSD95 levels that may modify receptor activity or intracellular signaling pathways to increase synaptic strength.

Summary Developmentally programmed genome rearrangements are rare in vertebrates, but have been reported in scattered lineages including the bandicoot, hagfish, lamprey, and zebra finch ( Taeniopygia guttata ) [1]. In the finch, a well-studied animal model for neuroendocrinology and vocal learning [2], one such programmed genome rearrangement involves a germline-restricted chromosome, or GRC, which is found in germlines of both sexes but eliminated from mature sperm [3, 4]. Transmitted only through the oocyte, it displays uniparental female-driven inheritance, and early in embryonic development is apparently eliminated from all somatic tissue in both sexes [3, 4]. The GRC comprises the longest finch chromosome at over 120 million base pairs [3], and previously the only known GRC-derived sequence was repetitive and non-coding [5]. Because the zebra finch genome project was sourced from male muscle (somatic) tissue [6], the remaining genomic sequence and protein-coding content of the GRC remain unknown. Here we report the first protein-coding gene from the GRC: a member of the α-soluble N- ethylmaleimide sensitive fusion protein (NSF) attachment protein (α-SNAP) family hitherto missing from zebra finch gene annotations. In addition to the GRC-encoded α-SNAP, we find an additional paralogous α-SNAP residing in the somatic genome (a somatolog)—making the zebra finch the first example in which α-SNAP is not a single-copy gene. We show divergent, sex-biased expression for the paralogs and also that positive selection is detectable across the bird α-SNAP lineage, including the GRC-encoded α-SNAP. This study presents the identification and evolutionary characterization of the first protein-coding GRC gene in any organism.

Given the profound influence of steroids on the organization and activation of the vertebrate central nervous system (CNS), it is perhaps not surprising that these molecules are involved in processes that restructure the cytoarchitecture of the brain. This includes processes such as neurogenesis and the connectivity of neural circuits. In the last 30 years or so, we have learned that the adult vertebrate brain is far from static; it responds to changes in androgens and estrogens, with dramatic alterations in structure and function. Some of these changes have been directly linked to behavior, including sex, social dominance, communication, and memory. Perhaps the most dramatic levels of neuroplasticity are observed in teleosts, where circulating and centrally derived steroids can affect several end points, including cell proliferation, migration, and behavior. Similarly, in passerine songbirds and mammals, testosterone and estradiol are important modulators of adult neuroplasticity, with documented effects on areas of the brain necessary for complex behaviors, including social communication, reproduction, and learning. Given that many of the cellular processes that underlie neuroplasticity are often energetically demanding and temporally protracted, it is somewhat surprising that steroids can affect physiological and behavioral end points quite rapidly. This includes recent demonstrations of extremely rapid effects of estradiol on synaptic neurotransmission and behavior in songbirds and mammals. Indeed, we are only beginning to appreciate the role of temporally and spatially constrained neurosteroidogenesis, like estradiol and testosterone being made in the brain, on the rapid regulation of complex behaviors.

Estrogens affect the vertebrate brain throughout life and modulate a diverse set of physiological endpoints, including neuroprotection and repair. Here we narrate our journey through the discovery and continued interest in one particular form of estradiol provision during neurotrauma—the induced expression of aromatase (estrogen synthase) in astrocytes, and its effects on the protection and repair of neural circuits. We discuss what we have learned about brain injury and astrocytic aromatization in the songbird, a time-tested animal model in neuroendocrinology. We first discuss the discovery of aromatase expression in astroglia, and the molecular and physiological pathways potentially responsible for this induction. Finally, we focus attention on the effects of astrocytic estradiol provision on cell turnover. It is our intent to tell the reader a story about this relatively novel form of spatial and temporally discrete estrogen provision in a resilient vertebrate brain.