Presynaptic Control of Rapid Estrogen Fluctuations in the Songbird Auditory Forebrain

Abstract

Within the CNS of vertebrates, estrogens can directly modulate neural circuits that govern a wide range of behaviors, including feeding, spatial navigation, reproduction and auditory processing. The rapid actions of estrogens in brain (secs-mins) have become well-established, but it is unclear how estrogens are synthesized and released within restricted temporal and spatial domains in neural circuits. Anatomical localization of the estrogen-synthesis enzyme (aromatase) within presynaptic terminals suggests that neuroestrogens can be synthesized directly at the neuronal synapse. A consequent prediction follows that synaptic estrogen production is controlled via classical electrochemical events in neurons. Here, we present evidence that acute fluctuations in local neuroestrogen levels in the forebrain of the zebra finch depend on calcium influx within presynaptic terminals. In vivo experiments using microdialysis linked to a sensitive estrogen ELISA showed that local forebrain neuroestrogens were both suppressed by potassium-evoked excitation and upregulated during 30-min periods of extracellular calcium depletion in a region enriched with presynaptic aromatase. Furthermore, potassium-evoked changes in local neuroestrogens were blocked by targeted delivery of the voltage-gated calcium channel blocker omega-conotoxin GVIA. Together, these experiments indicate that neuroestrogens are controlled by specific, depolarization-sensitive, calcium-dependent events within forebrain presynaptic terminals.

A Quantification of the Injury‐Induced Changes in Central Aromatase, Oestrogenic Milieu and Steroid Receptor Expression in the Zebra Finch

“…Thus, although the location of the injury was slightly different, the nature of the injury and the time between trauma and tissue collection was identical. The methods utilised to analyse brain E 2 levels have been validated in several recent songbird models, including a demonstration of the combined liquid-liquid and solid-phase extraction providing sufficient purification for an accurate analysis of low tissue levels of E 2 and, importantly, validation with results obtained by mass spectrometry (29)(30)(31)(32).…”

“…Samples were assayed in triplicates using an E 2 Enzyme Immunoassay kit (Cayman Chemical) previously validated for use with zebra finch brain tissue in accordance with the manufacturer's instructions (30)(31)(32). Prior to ether and solid-phase extraction, an additional sample was spiked with radioinert E 2 to the concentration of 256 pg/ml, processed in an identical manner, and assayed in triplicate alongside the experimental samples to estimate recovery.…”

Insights into Rapid Modulation of Neuroplasticity by Brain Estrogens

“…In the hypothalamus, electron microscopy imaging has demonstrated a synaptic localization for aromatase in avian, mammalian, and human tissue (Naftolin et al, 1971a,b;Callard et al, 1978;MacLusky et al, 1986;Naftolin et al, 1996;Yague et al, 2006Yague et al, , 2008Srivastava et al, 2010;Remage-Healey et al, 2011). Biochemical studies using subcellular preparations of brain tissue have detected high levels of aromatase activity in isolated presynaptic tissue preparations (Mak et al, 1985;Schlinger and Callard, 1989;Peterson et al, 2005;Remage-Healey et al, 2011), indicating a potential role of estrogens at synapses. In mature cultured cortical neurons derived from embryonic rats, we found aromatase immunoreactivity to be Rapid Modulation of Neuroplasticity by Brain Estrogens present at synapses (Srivastava et al, 2010).…”

Rapid Control of Reproductive Behaviour by Locally Synthesised Oestrogens: Focus on Aromatase

“…This enzymatic down-regulation is prevented by blocking the activity of a variety of kinases or by chelating calcium (43,47,48,50), supporting the notion that the rapid control of aromatase activity depends on calcium-depend phosphorylations. Interestingly, data collected from microsomes and synatosomes isolated from the zebra finch telencephalon suggest that this acute modulation of AA by phosphorylating conditions primarily concerns synaptic aromatase rather than microsomal aromatase (mostly located in the cell bodies) (48,51).…”