Sex differences in the brain of goldfish: gonadotropin-releasing hormone and vasotocinergic neurons.

Abstract

The differences between male and female behaviors are reflected in sexual dimorphism of brain structures and are found throughout the nervous system in a variety of vertebrates. The present study examined neurons immunolabeled for gonadotropin-releasing hormone and arginine vasotocin in the brain of the goldfish Carassius auratus to determine if these neurons are sexually dimorphic. There was no sex difference or influence of sex steroids on the neuronal volume and optical density of staining of arginine vasotocin neurons. Similarly, gonadotropin-releasing hormone neurons of the terminal nerve and midbrain tegmentum did not differ between sexually mature males, females and maturing females replaced with sex steroids with respect to distribution, numbers, optical density of staining, or gross morphology. In maturing females, testosterone specifically recruited additional preoptic gonadotropin-releasing hormone neurons to equal those in sexually mature individuals. Since estrogen had no effect, the influence of testosterone on gonadotropin-releasing hormone neuronal numbers appears to be independent of aromatization. Specifically, the preoptic gonadotropin-releasing hormone neuronal size was significantly larger in sexually mature males than females. 11-Ketotestosterone-replacement to ovariectomized maturing females induced male-typical secondary characters and male-type courtship behavior but did not masculinize the preoptic gonadotropin-releasing hormone neuronal size. Our results show that the sexually dimorphic preoptic gonadotropin-releasing hormone neuronal size is determined by factors (genetic) other than gonadal steroids. Further, we propose the hypothesis that phenotypic and behavioral sex differences need not be accompanied by structural differences in gonadotropin-releasing hormone and arginine vasotocin in the brain.