Monoaminergic integration of diet and social signals in the brains of juvenile spadefoot toads

Abstract

Social behavior often includes the production of species-specific signals (e.g., mating calls or visual displays) that evoke context-dependent behavioral responses from conspecifics. Monoamines are important neuromodulators that have been implicated in context-dependent social behavior yet we know little about development of the monoaminergic systems and whether they mediate the effects of early life experiences on adult behavior. We examined the effects of diet and social signals on monoamines early in development in the Plains spadefoot toad (Spea bombifrons), a species in which diet affects the developmental emergence of species recognition and body condition affects the expression of adult mating preferences. To do so, we manipulated the diet of juveniles for six weeks following metamorphosis and collected their brains 40 minutes following the presentation of either a conspecific or heterospecific call. We measured levels of monoamines and their metabolites using high pressure liquid chromatography from tissue punches of the auditory midbrain (i.e., torus semicircularis), hypothalamus, and preoptic area. We found that call type affected dopamine and norepinephrine signaling in the auditory midbrain and that diet affected dopamine and serotonin in the hypothalamus. In the preoptic area, we detected an interaction between diet and call type, indicating that diet modulates how the preoptic area integrates social information. Our results suggest that the responsiveness of monoamine systems varies across the brain and highlight preoptic dopamine and norepinephrine as candidates for mediating effects of early diet experience on later expression of social preferences.