A. Illustration of implantation of the injection cannula into the Vi/Vc region. Left: Horizontal view of a rat skull diagram illustrating the site of cannula implantation (burr hole: 4.7-5.2 mm posterior to lambda) . Note that two screws and a patch of cranioplastic cement were used to help secure the cannula. Right: Coronal brain section. The guide cannula was inserted into the brainstem site at a 15.75° angle. The tip of the cannula was located in the ventral Vi/Vc transition zone at a horizontal plane 9.1 mm ventral to the surface of the skull. B, C. Cresyl violet stained brainstem sections illustrating the site of microinjection in the Vi/Vc transition zone (B) and Vc (C) (arrows). Scale bars in B, C = 0.5 mm.