Neurosteroid production in the songbird brain: a re-evaluation of core principles

Abstract

Concepts of brain-steroid signaling have traditionally placed emphasis on the gonads and adrenals as the source of steroids, the strict dichotomy of early developmental ("organizational") and mature ("activational") effects, and a relatively slow mechanism of signaling through intranuclear receptors. Continuing research shows that these concepts are not inaccurate, but they are certainly incomplete. In this review, we focus on the song control circuit of songbird species to demonstrate how each of these concepts is limited. We discuss the solid evidence for steroid synthesis within the brain ("neurosteroidogenesis"), the role of neurosteroids in organizational events that occur both early in development and later in life, and how neurosteroids can act in acute and non-traditional ways. The songbird model therefore illustrates how neurosteroids can dramatically increase the diversity of steroid-sensitive brain functions in a behaviorally-relevant system. We hope this inspires further research and thought into neurosteroid signaling in songbirds and other animals.

Figure 1

A schematic sagittal drawing of the songbird brain showing projections of major nuclei in the song system, and the distribution of steroid receptors. The descending motor pathway (black and dark gray arrows) controls the production of song. The dark gray arrows indicate inputs to HVC from the thalamic nucleus Uva and the nidopallial nucleus Nif. The black arrows indicate the descending projections from HVC in the nidopallium to RA in the arcopallium and thence to the vocal nucleus nXIIts, the respiratory nucleus RAm, and the laryngeal nucleus Am in the medulla. The white arrows indicate the anterior forebrain pathway that is essential for song learning. It indirectly links HVC to RA, via area X in the medial striatum, DLM in the thalamus, and LMAN in the nidopallium. LMAN also projects to area X. Field L is an auditory region in the nidopallium that projects to HVC (light grey arrow). Abbreviations: Am, nucleus ambiguous; DLM, medial portion of the dorsolateral nucleus of the thalamus; LMAN, lateral portion of the magnocellular nucleus of the anterior nidopallium; NIf, nucleus interface of the nidopallium; RA, robust nucleus of the arcopallium; RAm, nucleus retroambigualis; Uva, nucleus uvaeformis; V, ventricle; X, area X; nXIIts, tracheosyringeal nucleus of the hypoglossal nerve. Taken from [12].

Figure 2

Steroidogenesis pathway: Steroids are in bold; enzymes and cholesterol transport proteins are in italics. Enzyme abbreviations: cytochrome P450 side-chain cleavage=CYP11A1; cytochrome P450 17α-hydroxylase/C17,20 lyase=CYP17; 3β–hydroxysteroid dehydrogenase/Δ5-Δ4 isomerase=3β-HSD; 17β-hydroxysteroid dehydrogenase=17β-HSD; cytochrome P450 19A1 =CYP19 (“aromatase”). Cholesterol transport protein abbreviations: translocator protein (previously known as the peripheral-type benzodiazepine receptor)=TSPO; steroidogenic acute regulatory protein = StAR.

Figure 3

StAR, CYP11A1, 3β-HSD, and CYP17 expression along the lateral ventricles in birds at posthatch day 1. Film images of coronal sections illustrate the intense hybridization of all four genes overlapping with the major proliferative zone. StAR (A), CYP11A1 (C), 3β-HSD (E), and CYP17 (G) hybridized along the lateral ventricle only with antisense (A,C,E,G) but not sense (B,D,F,H) configured riboprobes. Arrows point to the intermediate portion of right hemisphere lateral ventricle. Quantitation of hybridization patterns for all four genes showed that StAR and CYP11A1 had the most precisely overlapping expression, likely because CYP11A1 activity is dependent upon cholesterol transporters. Otherwise, each gene had some unique patterns of hybridization, suggesting the potential for steroid microenvironments along the major cell proliferative zone. Scale bar = 1mm. Figure from [108].

Figure 4

In the adult brain, BrdU-labeled cells can be observed in close juxtaposition with aromatase-positive cells along the lateral ventricle after injury. The newly-divided cells can be seen in the same ventricular zone as the aromatase positive (aromatase represented by DAB-brown stain) cell bodies in (A). Aromatase positive cells morphologically are radial glia. The potential for the estrogen-producing radial glia to guide newly divided cells from the ventricular zone is demonstrated in (B–D), where BrdU-labeled cells (black chromagen-stained cell bodies) appear to migrate along the processes of the glia towards the lesion site (E, F) show aromatase-and BrdU-immunoreactivity colocalized in cells of undetermined types, possibly in the process of differentiation. The lateral ventricle is out of view at the bottom of images A–D. Position of lesion is out of view at top of images A–D; radial glia are extending towards the site of lesion, and restricted to the portion of the lateral ventricle proximal to the lesion site. Scale bar = 20 μm. Figure from [153].

Figure 5

Schematic showing StAR, CYP11A1, 3β-HSD, CYP17, 17βHSD, and aromatase expression patterns in and around the adult male zebra finch song control circuit, based on results of examining somata only (not including projections). Most steroidogenic enzymes and StAR are expressed within the major telencephalic song nuclei; aromatase is largely expressed in cell bodies located outside of these areas, but is found within synaptic terminals contained with song nuclei. These data suggest that neurosteroids could continue to influence the function of the song control circuit into adulthood, and that each brain area could be impacted by a distinct mixture of neurosteroids.

Figure 6

Synaptosomal aromatase expression and activity in adult zebra finches. Ultraphotomicrograph in (A) shows aromatase immune-product (arrowhead) expressed in presynaptic terminals (tm; arrows) of the zebra finch hippocampus (HP). Similar results were obtained in song nuclei, such as HVC (picture from [92]). Scale bar = 1 μm. The biochemical activity of aromatase (B) specifically within synaptosomes is elevated in males that have been singing vs. nonsingers after 30 min. Singers have higher synaptic aromatase activity is only within the posterior telencephalon (containing song and auditory nuclei, see Fig. 5), and not within the anterior telencephalon (* p = 0.008). Data from [166].