Organization of the gymnotiform fish pallium in relation to learning and memory: III. Intrinsic connections.

The present article reports on the telencephalic connections of regions of the dorsal telencephalon of the weakly electric fish Apteronotus leptorhynchus and Gymnotus sp. that are involved in learning and memory: the lateral (DL), central (DC), and dorsal (DD) regions of the pallium and the intermediate region between DL and DC (Dx). We find that the main route of transmission consists of diencephalic (preglomerular complex; PG) glutamatergic input to DL; glutamatergic projections from DL to DC and Dx; and glutamatergic output from DC/Dx to di-, mes-, and rhombencephalic nuclei. Although PG efferents to DL are spatially organized, the projection from DL to DC appears to be diffuse. The connections of DD are entirely intrinsic to the pallium: DL projects to DD (glutamatergic) and DD feeds back to DL (glutamatergic); DD also projects to DC and has strong contralateral connections. In addition, DL and DD receive input from subpallial regions; we suggest that these are associated with the previously identified γ-aminobutyric acid (GABA)-ergic, dopaminergic, and somatostatin-positive input to these regions. The DL/DD connections are very complex, because DL projects to and receives input from different subdivisions of DD. These subdivisions are linked by circuitry intrinsic to DD itself. DL and DD both contain recurrent putatively excitatory (glutamatergic) connections as well as local putatively inhibitory (GABAergic) interneurons. In contrast, recurrent excitatory connections appears to be absent in DC, and local inhibition is also barely present. Finally, we speculate on the implications of this pattern of connectivity for theories of short-term memory and long-term associative memory.