COLOCALIZATION OF NEURONAL NITRIC-OXIDE SYNTHASE WITH GABA IN RAT CUNEATE NUCLEUS

Pre- and post-embedding immunocytochemistry were employed in this elec tron microscopic investigation of cuneate neurons that are enriched in GABA and in nitric oxide synthase, the enzyme responsible for the syn thesis of nitric oxide. GABAergic neurons are local circuit interneuro ns; 10-20% of them also contain nitric oxide synthase. These are among the smallest GABA-positive perikarya. We describe a network of proces ses in the rat cuneate nucleus that are immunopositive for nitric oxid e synthase. Axon terminals positive for nitric oxide synthase are smal l and make synapses mainly onto dendrites; they make only occasional a xo-axonic contacts. Double-labelling immunocytochemistry verified that the large majority of terminals positive for nitric oxide synthase al so contained GABA. However, most GABA-positive profiles were negative for nitric oxide synthase and GABA-positive terminals that are negativ e for nitric oxide synthase frequently made axo-axonic contacts. These results suggest that nitric oxide synthase is within a specialized su bpopulation of interneurons in the cuneate nucleus.