Profiles of cooperative brains: A discriminant analysis of cleaner and client fish monoaminergic responses to different social contexts

Vertebrate cognitive function requires a dynamic coordination of multiple specialized areas of the brain. The challenge here is to understand how these brain areas respond in dependence to the neurophysiological mechanisms in place, as to enable the successful processing of information. For instance, social and cooperative behaviour has been linked to the activation of some specific brain areas, mostly associated with reward processing, however mechanisms could in principle change, in accordance to species social demands and taxon involved. Here we chose a classic model system of cooperation between species of fish and compared the brain monoaminergic response of cleaners Labroides dimidiatus exposed to several social-related treatments to the response of one client species (Naso elegans) introduced to similar contexts. We demonstrate that the variable appraisal of each social challenge contributes to brain dopaminergic and serotonergic changes, in cleaners and clients, with both showing the diencephalon and optic tectum as main areas of metabolite response. However, the role of the serotoninergic system activation was mostly demonstrated at the diencephalon and cerebellum of cleaners, a response that was driven by the exposure to clients. Our current results are the first to jointly demonstrate the level of selective similarity in brain monoaminergic mechanisms that underlie fish mutualistic and social engagement, for both sides of these partnerships.