Nucleus accumbens in the lizard Psammodromus algirus: Chemoarchitecture and cortical afferent connections

To better understand the organization and evolution of the basal ganglia of vertebrates, in the present study we have analyzed the chemoarchitecture a nd the cortical input to the nucleus accumbens in the lacertid lizard Psamm odromus algirus. The nucleus accumbens contains many gamma-aminobutyric aci d (GABA)-positive neurons and calbindin-positive neurons, the majority of w hich may be spiny projection neurons, and a few dispersed neuropeptide Y-po sitive neurons that likely represent aspiny interneurons. The nucleus accum bens contains two chemoarchitectonically different fields: a rostromedial f ield that stains heavily for substance P, dopamine, GABA(A) receptor, and a caudolateral field that stains only lightly to moderately for them, appear ing more similar to the adjacent striatum. Injections of biotinylated dextr an amine were placed in either the medial, dorsomedial, or dorsal cortices of Psammodromus. The medial and the dorsal cortices project heavily to the rostromedial field of the accumbens, whereas they project lightly to modera tely to the caudolateral field. Cortical terminals make asymmetric, presuma bly excitatory, synaptic contacts with distal dendrites and the head of spi nes. Our results indicate that the hippocampal-like projection to the nucle us accumbens is similar between mammals and reptiles in that cortical termi nals make mainly excitatory synapses on spiny, putatively projection neuron s. However, our results and results from previous investigations indicate t hat important differences exist between the nucleus accumbens of mammals an d reptiles regarding local modulatory interactions between cortical, dopami nergic, and cholinergic elements, which suggest that the reptilian nucleus accumbens may be as a whole comparable to the shell of the mammalian nucleu s accumbens. (C) 1999 Wiley-Liss, Inc.