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 the development of 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 6 weeks following metamorphosis and collected their brains 40 min following the presentation of either a conspecific or a 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 noradrenaline 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 noradrenaline as candidates for mediating effects of early diet experience on later expression of social preferences.

Keywords: Anurans; Behavioral plasticity; Body condition; Monoamines; Social preferences; Species recognition.

Fig. 1.

Schema of the juvenile spadefoot toad brain and diagrams of transverse sections illustrating the brain regions studied. Vertical bars indicate the starting point of each 300 μm transverse section from which we micropunched the brain regions of interest, shown in corresponding rows (i, auditory midbrain; ii, hypothalamus; iii, preoptic area). The left side of each row shows schematic diagrams of the caudal position of each micropunch (indicated by circles), and the photomicrographs of the Thionine-stained sections represent the ‘before’ (center) and ‘after’ (right) sections of each punch. Scale bar, 300 µm.

Fig. 2.

Diet manipulation affects growth and body condition. Toads were fed a high- or low-food diet. Growth was assessed by final length; body condition is represented by residuals of mass regressed on length. Data are means±s.e.m. There were 15 animals in each diet treatment; P-values correspond to two-tailed t-tests.

Fig. 3.

The type of mating call heard affects noradrenergic and dopaminergic signaling in the auditory midbrain, while diet manipulation has no detectable effect. NA, noradrenaline; DA, dopamine; 5-HT, serotonin; MHPG, 3-methoxy-4-hydroxyphenylglycol (NA metabolite); DOPAC, 3,4-dihydroxyphenylacetic acid (DA metabolite); 5HIAA, 5-hydroxyindoleacetic acid (5-HT metabolite); con, conspecific; het, heterospecific. Data are means±s.e.m. The number of animals in each group is indicated parenthetically; P-values correspond to two-way ANOVA.

Fig. 4.

Diet manipulation affects concentrations of DA, 5-HT and 5HIAA in the hypothalamus, while mating call type has no effect. Data are means±s.e.m. The number of animals in each group is indicated parenthetically; P-values correspond to two-way ANOVA.

Fig. 5.

In the preoptic area, diet manipulation reverses the relative response of NA and DA to mating call type. Data are means±s.e.m. The number of animals in each group is indicated parenthetically; P-values correspond to two-way ANOVA.