Electron microscopic study of GABAergic synaptic innervation of nitric oxide synthase immunoreactive neurons in the dorsal raphe nucleus in the rat.

A double immunocytochemical method combining the preembedding avidin biotin peroxidase complex technique and the postembedding immunogold technique was used to examine synaptic interactions between GABAergic and nitric oxide synthase containing neurons in the same tissue sections of the dorsal raphe nucleus of the Wistar white rat. Although a large number of immunogold stained GABAergic axon terminals were found to be presynaptic to dendrites containing nitric oxide synthase-like immunoreaction product, synapses between GABA-like immunoreactive axon terminals and nitric oxide synthase-like immunoreactive perikarya were rare. The labeled boutons were found to make symmetrical and asymmetrical synapses. No axo-axonic synapse was found. These results suggest that GABAergic neurons could modulate nitric oxide producing neurons in the dorsal raphe nucleus through direct synaptic relations.