Aggressive interactions rapidly increase androgen synthesis in the brain during the non-breeding season

Abstract

In male song sparrows (Melospiza melodia), territorial challenges during the breeding season can rapidly increase circulating levels of testosterone (T). During the non-breeding season, male song sparrows are highly aggressive, but the gonads are regressed and plasma T levels are non-detectable and unaffected by territorial challenges. The pro-hormone dehydroepiandrosterone (DHEA) is elevated in song sparrow plasma and brain during the non-breeding season and may be locally converted to sex steroids in the brain to regulate aggression. The enzyme 3beta-hydroxysteroid dehydrogenase/Delta5-Delta4 isomerase (3beta-HSD) converts DHEA to androstenedione (AE) using the cofactor NAD(+), and this is a critical rate-limiting step. We predicted that brain 3beta-HSD activity varies seasonally and is rapidly modulated by aggressive challenges. In the first study, brain 3beta-HSD activity was highest in the non-breeding season in specific regions. In the second study, a simulated territorial challenge rapidly increased aggressive behavior in non-breeding song sparrows. Brain 3beta-HSD activity, when measured without exogenous NAD(+), increased by approximately 250 to 500% in telencephalic regions of challenged subjects. When brain 3beta-HSD activity was measured with exogenous NAD(+), these effects of territorial challenges were not observed. These data suggest that territorial challenges rapidly increase endogenous NAD(+) levels or increase 3beta-HSD activity specifically within a NAD-rich subcellular compartment. Together, these two studies suggest a shift from systemic to local sex steroid signaling in the non-breeding season. Local steroid signaling produces high spatial and temporal specificity of steroid signals and avoids the costs of high systemic T levels during the non-breeding season.

Figure 1

Timecourse of 3β-HSD activity using 200 nM [³H]DHEA as substrate and brain supernatants from non-breeding adult male song sparrows. (A) Formed tritiated androgens measured with the addition of 1 mM NAD⁺, from 2.5 to 60 min. n = 3 replicates per timepoint. (B) Formed tritiated androgens and estrogens measured without the addition of NAD⁺, from 10 to 360 min. n = 2 replicates per timepoint.

Figure 2

Representative HPLC chromatograph illustrating the peak and retention time of [³H]AE produced in vitro from brain tissue of non-breeding adult male song sparrows. Substrate was 200 nM [³H]DHEA. Steroids were separated using TLC and then injected through the HPLC coupled to a flow radiodetector.

Figure 3

Seasonal changes in baseline brain 3β-HSD activity in wild male song sparrows (n=6 per season). [³H]DHEA was converted to [³H]AE (major product) and [³H]5β-A (minor product). *p<0.05.

Figure 4

Effect of a simulated territorial intrusion (30 min) on brain 3β-HSD activity in wild male song sparrows during the non-breeding season. NAD⁺ was not added in this experiment (Expt. 2B). The aggressive challenge rapidly increased 3β-HSD activity in the central medial telencephalon (n=9 control and 8 STI) and caudal telencephalon (n=7 control and 8 STI). *p<0.05, **p<0.01

Figure 5

Correlation between time spent by the resident within 1 m of the intruder and brain 3β-HSD activity. NAD⁺ was not added in this experiment (Expt. 2B). Time spent within 1m was positively correlated with 3β-HSD activity in (A) central medial telencephalon (n=9 control and 8 STI) and (B) caudal telencephalon (n=7 control and 8 STI).

Figure 6

Hypothesized patterns of gonadal androgen synthesis (solid line) and neural androgen synthesis (broken line) in male song sparrows. Level A, constitutive levels of androgen synthesis for homeostatic functions; Level B, seasonal increase in androgen synthesis (regulated by predictable environmental cues); Level C, facultative and transient increase in androgen synthesis (regulated by unpredictable social cues).