Organization of the gymnotiform fish pallium in relation to learning and memory: I. Cytoarchitectonics and cellular morphology

The present article examines the anatomical organization of the dorsal telencephalon of two gymnotiform fish: Gymnotus sp. and Apteronotus leptorhynchus. These electric fish use elaborate electrical displays for agonistic and sexual communication. Our study emphasizes mainly pallial divisions: dorsolateral (DL), dorsodorsal (DD), and dorsocentral (DC), previously implicated in social learning dependent on electric signals. We found that the pallial cytoarchitectonics of gymnotiformes are similar to those reported for the commonly studied goldfish, except that DC is larger and better differentiated in gymnotiformes. We identified a new telencephalic region (Dx), located between DL and DC, and describe the morphological and some biochemical properties of its neurons. Most neurons in DL, DD, and DC are glutamatergic with spiny dendrites. However, the size of these cells as well as the orientation and extent of their dendrites vary systematically across these regions. In addition, both DD and DL contained numerous small GABAergic interneurons as well as well-developed GABAergic plexuses. One important and novel observation is that the dendrites of the spiny neurons within all three regions remain confined to their respective territories. We confirm that DL and DC express very high levels of NMDA receptor subunits as well as CaMKIIα, a key downstream effector of this receptor. In contrast, this enzyme is nearly absent in DD, while NMDA receptors are robustly expressed, suggesting different rules for synaptic plasticity across these regions. Remarkably, GABAergic pallial neurons do not express CaMKIIα, in agreement with previously reported results in the cortex of rats.