Sex Difference in Vasoactive Intestinal Polypeptide Neural Projection in the Limbic System of the Mouse Brain

The central subdivision of the bed nucleus of the stria terminalis (BNSTc) of humans is innervated by vasoactive intestinal polypeptide (VIP) neurons with a male-biased sex difference in VIP neuronal fibers. The oval nucleus of the bed nucleus of the stria terminalis (BNSTov) of mice is in the same anatomical position as the BNSTc, but no evidence for such a sex difference exists. Immunohistochemical analysis of the BNSTov in prepubertal, postpubertal, and adult mice revealed that a male-biased sex difference in the volume containing VIP neuronal fibers appears by the end of puberty, and this difference becomes significant in adulthood. Additionally, analysis of hormonally manipulated mice revealed that the volume of the BNSTov containing VIP neuronal fibers decreased in adult males with neonatal castration and increased in adult females with neonatal testosterone treatment. However, gonadectomy before and after puberty had no effect on the BNSTov volume that contained VIP neuronal fibers in both sexes. VIP neuron-specific retrograde tracing showed that VIP neurons innervating the BNSTov were localized in the basolateral amygdalar nucleus and basomedial amygdalar nucleus and that males had more VIP neurons in the basolateral amygdalar nucleus than did females. These findings suggest that the mouse BNSTov has the same traits regarding morphological sex differences as the human BNSTc. This study further showed that neonatal testicular androgens are required to establish the sex difference in VIP neuronal fibers, which may be attributed to a male-biased sex difference in VIP neurons projecting from the amygdala to the BNSTov.