Distribution and quantitative characterization of NADPH-diaphorase-reactive neurons in analgesic zones of the rat midbrain

The distribution of nitric oxide synthase (nicotinamide adenine dinucleotide phosphate diaphorase, NADPH-d)-containing neurons In the rat midbrain was studied. We found that NADPH-d-reactive neurons were predominantly concentrated in the dorsolateral part of the periaqueductal gray (FAG) and the dorsal raphe nucleus, which are implicated in the control of nociceptive transmission. Such neurons were also present in the supraoculomotor cap and laterodorsal tegmental nuclei. In the dorsolateral part of FAG, the moderately stained small fusiform cells were revealed. In the dorsal raphe nucleus and laterodorsal tegmental nuclei, the densely stained multipolar or oval cells of larger size dominated. The NADPH-d-reactive cells were not found in the ventrolateral part of central gray, which is considered the main source of antinociceptive descending influences. Quantitative analysis of histochemically revealed neurons showed that their number is somewhat higher in the caudal parts of dorsolateral central gray and considerably higher in the rostral regions of some dorsal raphe subnuclei. This peculiarity was expressed in significant accumulation of the NADPH-d-reactive neurons at the midbrain levels from Fr -7.6 to -8.0. The possible involvement of the NO-synthase-containing class of neurons in the functional organization of analgesic zones and formation of FAG antinociceptive output signals is discussed.