Peptidergic delineations of nucleus interface reveal a sex difference in volume.

Nucleus interface (Nif) is a song bird vocal control nucleus located in the neostriatum. In zebra finches (Taeniopygia guttata), high densities of fibers immunoreactive for the vasoactive intestinal polypeptide (VIP) and methionine enkephalin (ENK) define the boundaries of this nucleus. The volume of Nif measured using this pattern of peptidergic immunoreactivity was greater in males than in females. This difference was also found in gonadectomized, testosterone-treated birds that had similar levels of circulating steroids, suggesting that it is organizational in nature. These data show that Nif may form part of the neural substrate underlying sex differences in song behavior. They also provide further evidence that VIP and ENK may play a role in the activation of bird song.