SUBPOPULATIONS OF GABAERGIC NEURONS IN LAMINAE I-III OF RAT SPINAL DORSAL HORN DEFINED BY COEXISTENCE WITH CLASSICAL TRANSMITTERS, PEPTIDES, NITRIC-OXIDE SYNTHASE OR PARVALBUMIN

GABAergic neurons in laminae I-III of the spinal dorsal horn may conta in one or more of the following compounds: glycine, acetylcholine, neu ropeptide Y, enkephalin, nitric oxide synthase or parvalbumin. Althoug h the pattern of co-localization of some of these compounds is underst ood, it is not known which types of GABAergic neurons contain parvalbu min, or whether nitric oxide synthase coexists with peptides, acetylch oline or parvalbumin in any of these neurons, and in this study we hav e used immunocytochemistry and enzyme histochemistry to resolve these issues. Parvalbumin-immunoreactivity was restricted to those GABA-immu noreactive neurons that also showed glycine-immunoreactivity and was n ot co-localized with neuropeptide Y-immunoreactivity or NADPH diaphora se activity. By combining NADPH diaphorase histochemistry with immunoc ytochemistry with an antiserum to nitric oxide synthase, we were able to show that NADPH diaphorase activity was a reliable marker for nitri c oxide synthase in the spinal cord. Neurons that possess GABA- but no t glycine-immunoreactivity may contain neuropeptide Y, enkephalin, ace tylcholine or NADPH diaphorase, and all of the cholinergic neurons app ear to contain NADPH diaphorase. By combining immunofluorescent detect ion of neuropeptide Y or enkephalin with NADPH diaphorase histochemist ry, we showed that peptide-immunoreactivity did not coexist with NADPH diaphorase. This suggests that neither of these peptides coexists wit h nitric oxide synthase or with acetylcholine in neurons in the superf icial dorsal horn. Several phenotypically distinct groups of GABA-immu noreactive neuron can therefore be identified in laminae I-III of the dorsal horn, and these may represent different functional types of inh ibitory neuron.