Distribution of NADPH-diaphorase-positive ascending interneurones in the crayfish terminal abdominal ganglion

Previous neuropharmacological studies have described the presence of a nitr ic oxide-cGMP signalling pathway in the crayfish abdominal nervous system. In this study we have analysed the distribution of putative nitric oxide sy nthase (NOS)-containing ascending interneurones in the crayfish terminal ab dominal ganglion using NADPH-diaphorase (NADPHd) histochemistry. Ascending intersegmental interneurones were stained intra-cellularly using the fluore scent dye Lucifer yellow and the ganglia containing the stained interneuron es subsequently processed for NADPHd activity. Fluorescence persisted throu ghout histochemical processing. These double-labelling experiments showed t hat 12 of 18 identified ascending interneurones were NADPHd positive. Thus many ascending interneurones that process mechanosensory signals in the ter minal ganglion may contain NOS, and are themselves likely sources of NO whi ch is known to modulate their synaptic inputs. Three clear relationships em erged from our analysis between the effects of NO on the synaptic inputs of interneurones, their output properties and their staining for NADPH-diapho rase. First were class I interneurones with no local outputs in the termina l ganglion, the NE type interneurones, which had sensory inputs that were e nhanced by NO and were NADPHd positive. Second were class I interneurones w ith local and intersegmental output effects that had sensory inputs that we re also enhanced by NO but were NADPHd negative. Third were class 2 interne urones with local and intersegmental outputs that had synaptic inputs that were depressed by the action of NO but were NADPHd positive. These results suggest that NO could selectively enhance specific synaptic connections and sensory processing pathways in local circuits.