Peptidergic transmission in the brain. IV. Sex hormone dependence in the vasopressin/oxytocin system.

The medial amygdaloid nucleus (AME) synthesizes a peptide similar to arginine vasopressin and projects peptidergic fibers to the ipsilateral hippocampus in the male rat. In previous studies, we have shown that the peptide acts as a transmitter and, using in vitro and in vivo electrophysiology, we have characterized its mechanism of action. Previous anatomical work has shown that the peptidergic fibers are more dense in male rats than females and are obliterated entirely by castration. Here we report the results of our attempt to find an electrophysiological correlate of these anatomical findings. First, we show specific mediation by a vasopressin- or oxytocin-like peptide by use of a structural vasotocin antagonist. Then we show that castration obliterates the peptidergic signal in males. However, we were unable to find any sex difference that corresponded to the male/female disparity noted in the density of the peptidergic fibers. The strength, nature and stimulus-response characteristics were the same between males and females. Apart from a very subtle difference in the duration of the signal, no physiological correlate of the sexual dimorphism could be found with our techniques. We conclude that the neurophysiology partly complements the anatomy and biochemistry of this system.