Visible sympathetic activity as a social signal in Anolis carolinensis: changes in aggression and plasma catecholamines

Serotonin reverses dominant social status

Social stress from aggressive interaction is expressed differently in specific brain regions of dominant and subordinate male Anolis carolinensis. Prior to aggressive behavior, the outcome is predictable via the celerity of postorbital coloration: Dominant males exhibit more rapid eyespot darkening. Serotonergic activation is manifest rapidly (1 h) in hippocampus, nucleus accumbens and brainstem of subordinate males, and is expressed more rapidly in dominant males. Amygdalar serotonergic activation responds rapidly (1 h) in dominant males, but is expressed slowly (1 w) and chronically in subordinate males. We hypothesized that chronic (1 w) serotonin elevation, manipulated by the selective serotonin reuptake inhibitor sertraline, would decrease aggressiveness and result in subordinate status. Dominant status was established in pairs of male A. carolinensis. The pairs were separated and treated with sertraline or vehicle. Sertraline was given in food to either the dominant or the subordinate male, both males or neither male for 1 week. Pairs were reintroduced, and behavior and social status recorded. When both dominant and subordinate males were treated with sertraline (or vehicle), or when subordinate males alone were treated with sertraline, previously established social relationships remained unchanged or became associative. However, when dominant males alone were treated with sertraline, their social status was reversed (43%) or negated (57%). Latency to eyespot darkening was significantly retarded in dominant males treated with sertraline, and aggressive displays and attacks were reduced. Chronic 5-HT elevation is consistent with subordinate status. Social status and aggressive disposition do not appear to be immutable, but may be changed by neuroendocrine mechanisms that mediate adaptation to environmental conditions like stress.

The role of monoaminergic nuclei during aggression and sympathetic social signaling

A social sign stimulus that is sympathetically induced affects aggressive approaches and influences serotonergic, dopaminergic and noradrenergic activity in the brainstem nuclei of Anolis carolinensis. Darkening of postorbital skin via sympathetic activation of adrenal catecholamines and beta(2)-adrenergic receptors provides a visual signal that forms more rapidly in dominant than subordinate males during social interactions. This signal limits aggressive interactions. Males were painted postorbitally with green or black paint and then exposed to a mirror. Aggressive approaches to the mirror were inhibited in males viewing a reflection with darkened eyespots, and increased in males viewing a reflection without eyespots (hidden). Noradrenergic turnover in the raphe and locus ceruleus were greatest in test subjects that viewed a reflection with eyespots hidden by green paint. Perception of darkened eyespots stimulated greater serotonergic turnover in raphe, locus ceruleus and substantia nigra/ventral tegmental area (SN/VTA). Dopaminergic turnover was higher in the raphe and SN/VTA of Anolis that viewed a reflection with darkened eyespots. However, these animals had lower dopamine turnover in the locus ceruleus than isolated and hidden eyespot groups. Of the possible roles of perikarya on central function and behavior, our results suggest feedback, cross-nuclear regulation, and some independence of function between nuclei and the forebrain terminal fields. Decreased serotonergic activity corresponds with increased aggression only in the raphe, suggesting that the raphe nuclei might be important for this behavioral trait. Increased serotonergic, noradrenergic and dopaminergic activities in SN/VTA in Anolis that view a reflected opponent with dark eyespots suggests that the SN/VTA might be directly involved in recognition of this social sign stimulus and the resulting inhibition of aggression.

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

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 beta(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.