Monoaminergic activities of limbic regions are elevated during aggression: Influence of sympathetic social signaling

Abstract

A visual social signal inhibiting aggression is coincident with limiting serotonergic and noradrenergic activity in subiculum, hippocampus, nucleus accumbens, medial amygdala, but not lateral amygdala, septum, and hypothalamus. Darkening of postorbital skin in the lizard Anolis carolinensis is stimulated by sympathetic activation of β 2-adrenergic receptors via adrenal catecholamines, and occurs more rapidly in dominant males during social interaction. Eyespot darkening functions as a social signal limiting aggressive interaction. To assess the effect of this social signal on telencephalic activity of monoamines, males were painted postorbitally with green or black paint, and exposed to a mirror. Serotonergic and noradrenergic turnover, as estimated by ratios of catabolite to transmitter, were elevated in the subiculum, hippocampus, nucleus accumbens, and medial amygdala of animals in which the eyespots were masked by green paint. Conversely, dopaminergic activity in these brain regions was lower in males with hidden eyespots (painted green). Hiding the eyespot evoked significantly increased aggressive activity toward the mirror image. Furthermore, changes in monoaminergic turnover were coincident with altered aggressive behavior, suggesting a relationship between them. Changes of monoaminergic activity were not observed in the septum, lateral amygdala, or hypothalamus, when males with eyespots permanently marked (black) were compared with those with eyespots hidden (painted green). Stimulated (serotonergic and noradrenergic) or inhibited (dopaminergic) activity due to social signal and aggression are confined to regions of the brain similarly activated during social stress, and do not constitute a generalized activation of monoaminergic systems.