Amygdalostriatal projections in reptiles: a tract-tracing study in the lizard Podarcis hispanica

Abstract

Whereas the lacertilian anterior dorsal ventricular ridge contains unimodal sensory areas, its posterior part (PDVR) is an associative center that projects to the hypothalamus, thus being comparable to the amygdaloid formation. To further understand the organization of the reptilian cerebral hemispheres, we have used anterograde and retrograde tracing techniques to study the projections from the PDVR and adjoining areas (dorsolateral amygdala, DLA; deep lateral cortex, dLC; nucleus sphericus, NS) to the striatum in the lizard Podarcis hispanica. This information is complemented with a detailed description of the organization of the basal telencephalon of Podarcis. The caudal aspect of the dorsal ventricular ridge projects nontopographically mainly (but not exclusively) to the ventral striatum. The NS projects bilaterally (with strong ipsilateral dominance) to the nucleus accumbens, thus recalling the posteromedial cortical amygdala of mammals. The PDVR (especially its lateral aspect) and the dLC project massively to a continuum of structures connecting the striatoamygdaloid transition area (SAT) and the nucleus accumbens (rostrally), the projection arising from the dLC being probably bilateral. Finally, the DLA projects massively and bilaterally to both the ventral and dorsal striatum, including the SAT. Our findings lend further support to the view that the PDVR and neighboring structures constitute the reptilian basolateral amygdala and indicate that an emotional brain was already present in the ancestral amniote. These results are important to understand the comparative significance of the caudal aspect of the amniote cerebral hemispheres, and specifically challenge current views on the nature of the avian caudal neostriatum.