Succinic dehydrogenase histochemistry reveals the location of the putativeprimary visual and auditory areas within the dorsal ventricular ridge of Sphenodon punctatus

In turtles, crocodilians, lizards and snakes, the dorsal ventricular ridge (DVR) is a nuclear cell mass that contains distinct visual and auditory tha lamorecipient cell groups, In the tuatara (Sphenodon punctatus), the DVR is not organized into diverse cell groups but instead possesses a trilaminar cytoarchitecture resembling that characteristic of the telencephalic cortex in reptiles. To determine if visual and auditory fields might also be pres ent in the DVR of Sphenodon punctatus, we used succinic dehydrogenase (SDH) histochemistry, which has been shown to delineate the visual and auditory fields of the DVR in turtles, crocodilians and lizards. We a Iso used acety lcholinesterase (AChE) histochemistry to determine the boundary between the DVR and the basal ganglia in Sphenodon. We found an SDH-rich region in the neuropil ventral to the cell plate of the rostrolateral DVR and a slightly less intense SDH-rich zone in the neuropil deep to the cell plate of the v entromedial DVR. These SDH-rich zones appear to be located at the apical de ndrites of the neurons of the adjacent cell plate. These SDH-rich zones wer e clearly located within the DVR and were distinct from the AChE-rich stria tal part of the basal ganglia, which occupied the ventrolateral wall of the telencephalon. Based on findings in other reptiles, it seems likely that t he SDH-rich zone in rostrolateral DVR represents the zone of termination of nucleus rotundus visual input to the DVR, whereas the zone in ventromedial DVR represents the zone of termination of nucleus reuniens auditory input. Because a trilaminar DVR such as that in Sphenodon might be the primitive DVR condition for reptiles, our results suggest that the cytoarchitecture o f the DVR and the synaptic organization of its thalamic sensory input in th e common ancestor of living reptiles might have been much like of the dorsa l cortex. Copyright (C) 2000 S. Karger AG, Basel.