Sexual dimorphisms in swimming behavior, cerebral metabolic activity and adrenoceptors in adult zebrafish (Danio rerio).

Sexually dimorphic behaviors and brain sex differences, not only restricted to reproduction, are considered to be evolutionary preserved. Specifically, anxiety related behavioral repertoire is suggested to exhibit sex-specific characteristics in rodents and primates. The present study investigated whether behavioral responses to novelty, have sex-specific characteristics in the neurogenetic model organism zebrafish (Danio rerio), lacking chromosomal sex determination. For this, aspects of anxiety-like behavior (including reduced exploration, increased freezing behavior and erratic movement) of male and female adult zebrafish were tested in a novel tank paradigm and after habituation. Male and female zebrafish showed significant differences in their swimming activity in response to novelty, with females showing less anxiety spending more time in the upper tank level. When fish have habituated, regional cerebral glucose uptake, an index of neuronal activity, and brain adrenoceptors' (ARs) expression (α2-ARs and β-ARs) were determined using in vivo 2-[(14)C]-deoxyglucose methodology and in vitro neurotransmitter receptors quantitative autoradiography, respectively. Intriguingly, females exhibited higher glucose utilization than males in hypothalamic brain areas. Adrenoceptor's expression pattern was dimorphic in zebrafish telencephalic, preoptic, hypothalamic nuclei, central gray, and cerebellum, similarly to birds and mammals. Specifically, the lateral zone of dorsal telencephalon (Dl), an area related to spatial cognition, homologous to the mammalian hippocampus, showed higher α2-AR densities in females. In contrast, male cerebellum included higher densities of β-ARs in comparison to female. Taken together, our data demonstrate a well-defined sex discriminant cerebral metabolic activity and ARs' pattern in zebrafish, possibly contributing to male-female differences in the swimming behavior.