Steroids in the avian brain: heterogeneity across space and time

Abstract

Sex steroids influence a diversity of neural and behavioral endpoints in birds, including some that have little to do with reproduction per se. Recent advances in neurochemistry and molecular biology further indicate that the avian brain is comprised of a network of unique sex steroid microenvironments. Factors involved in steroid synthesis and metabolism are present in the avian brain with expression levels that vary from region to region and with activities that are, in some cases, subject to regulation over relatively slow or rapid time intervals. Advances in our ability to a) isolate steroids from brain tissue and b) precisely measure their concentrations reveal how steroid levels vary spatially and temporally. A full appreciation of sex steroid effects on the avian brain require not only measures of hormones in blood but also an understanding of the numerous and varied mechanisms whereby the brain creates such a heterogeneous steroidal environment.

Profiling of systemic and brain steroids in male songbirds: Seasonal changes in neurosteroids

“…The songbird brain is capable of synthesizing sex steroids either de novo from cholesterol or from circulating precursors 7,60 . In particular, the male song sparrow brain expresses 3β‐HSD, aromatase, 17β‐HSD and 5α‐reductase 19,25 .…”

“…1 Steroids synthesised locally within the brain are known as neurosteroids, 2 and they can be produced either by the local conversion of circulating prohormones or de novo from cholesterol (Figure 1). Neurosteroids are present in all classes of vertebrates, including fish, 3 amphibians, 4 reptiles, 5 birds 6,7 and mammals. 8,9 Because steroids can be synthesised locally in the brain, steroid levels in the blood often do not reflect steroid levels in specific brain regions.…”

Brain-Derived Steroids, Behavior and Endocrine Conflicts Across Life History Stages in Birds: A Perspective

“…Although male territorial aggression was classically thought to be associated with circulating testosterone, work over the last 30 years suggests that the neural sensitivity to sex steroids, and the synthesis and action of both androgens and estrogens can modulate such behavior ( 32 , 38 , 42 , 43 ). These are particularly important findings with regards to seasonal animals that express aggression during non-breeding life history stages when high circulating testosterone levels would likely lead to inappropriately timed reproductive behavior.…”

“…Secretion and production of sex steroids that modulate territorial aggression in the song sparrow change seasonally. For example, there is evidence for local production of testosterone and 17β-estradiol in the brain, both de novo from cholesterol and from circulating precursors ( 32 , 33 , 39 ). An example of the latter is dehydroepiandrosterone (DHEA), which is released by the adrenal glands, liver, and other peripheral organs, taken up by brain regions, and converted locally to more bioactive steroids ( 39 ).…”

“…( 5 , 9 , 25 – 31 )]. In seasonally breeding vertebrates, neurally derived sex steroids produced in the non-breeding season do not influence reproductive traits per se , as very little sex steroid from this source reaches the blood stream and such neurosteroids are synthesized and act in a paracrine fashion in localized areas of the brain ( 32 ). Still, neurosteroids can have numerous effects on behavioral traits such as locomotor activity, aggression, reproduction, body temperature, and blood pressure ( 9 , 29 , 33 – 38 ).…”