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

The differences between male and female behaviors are reflected in sexual d imorphism of brain structures and are found throughout the nervous system i n a variety of vertebrates. The present study examined neurons immunolabele d 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 neuro ns. Similarly. gonadotropin-releasing hormone neurons of the terminal nerve and midbrain tegmentum did not differ between sexually mature males, femal es and maturing females replaced with sex steroids with respect to distribu tion, numbers, optical density of staining, or gross morphology. In maturin g females, testosterone specifically recruited additional preoptic gonadotr opin-releasing hormone neurons to equal those in sexually mature individual s. Since estrogen had no effect, the influence of testosterone on gonadotro pin-releasing hormone neuronal numbers appears to be independent of aromati zation. Specifically. the preoptic gonadotropin-releasing hormone neuronal size was significantly larger in sexually mature males than females. 11-Ket o-testosterone-replacement to ovariectomized maturing females induced male- typical secondary characters and male-type courtship behavior but did not m asculinize the preoptic gonadotropin-releasing hormone neuronal size. Our results show that the sexually dimorphic preoptic gonadotropin-releasin g hormone neuronal size is determined by factors (genetic) other than gonad al steroids. Further. we propose the hypothesis that phenotypic and behavio ral sex differences need not be accompanied by structural differences in go nadotropin-releasing hormone and arginine vasotocin in the brain. (C) 2001 IBRO. Published by Elsevier Science Ltd. All rights reserved.