Ultrastructural features of the nitric oxide synthase-containing interneurons in the nucleus accumbens and their relationship with tyrosine hydroxylase-containing terminals

The ultrastructural features of neuronal nitric oxide synthase (NOS) -immun oreactive interneurons of rat nucleus accumbens shell and core were studied and compared. The NOS-containing subpopulation displayed characteristics s imilar to those previously described for nicotinamide adenine dinucleotide phosphate diaphorase-, neuropeptide Y, or somatostatin-containing striatal neurons, but also showed properties not previously associated with them, pa rticularly the formation of both asymmetric and symmetric synaptic junction s. Inputs derived mainly from unlabeled terminals, but some contacts were m ade by NOS-immunolabeled terminals, by means of asymmetric synapses. Immuno positive endings that formed symmetric synapses were mainly onto dendritic shafts, whereas those that formed asymmetric synapses targeted spine heads. Morphometric analysis revealed that the core and shell NOS-stained neurons had subtly different innervation patterns and that immunostained terminals were significantly larger in the shell. A parallel investigation explored synaptic associations with dopaminergic innervation identified by labeling with an antibody against tyrosine hydroxylase (TH). In both shell and core, TH-positive boutons formed symmetric synapses onto NOS-containing dendrite s, and in the core, TH- and NOS-immunolabeled terminals converged on both a single spiny dendrite and a spine. These results suggest that, in the rat nucleus accumbens, NOS-containing neurons may be further partitioned into s ubtypes, with differing connectivities in shell and core regions. These NOS -containing neurons may be influenced by a dopaminergic input. Recent studi es suggest that nitric oxide potentiates dopamine release and the current s tudy identifies the medium-sized, densely spiny neurons as a possible site of such an interaction. J. Comp. Neurol. 431:139-154, 2001. (C) 2001 Wiley- Liss, Inc.