Neurophy~tology, VoL 28, No. I, January-February, 1996 Distribution and Quantitative Characterization of NADPH-Diaphorase-Reactive Neurons in Analgesic Zones of the Rat Midbrain A. I. Pilyavskii, ! V. A. Maisldi, l N. I. Hariri, 2 G. O. Peker, 2 and N. V. Bulgakova ! Neirofiziologiya/Neuroph ysiotogy, Vol. 28, No. 1, pp. 36-46, January-February, 1996. Received June 20, 1996. The distribution of nitric oxide synthase (nicotinamide adenine dinucleotlde phosphate diaphorase, NADPH-d)-con- raining neurons in the rat midbrain was studied. We found that NADPH-d-reactive neurons were predominantly concentrated in the dorsolateral part of the pertaqueductal gray (PAG) 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 dorsotateral part of PAG, 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 domi~mnted. 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 exp~d 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 PAG antinociceptive output signals is discussed. INTRODUCTION Subpopulations of neurons containing a co-enzyme NADPH-d (nicotinamide adenine dinucleotide phos- phate diaphorase) were identified in many regions of the central and peripheral nervous systems [1-4 ]. The above substance is supposed as an enzyme providing synthesis of nitric oxide (NO-synthase). Co-localization of NO-synthase immunoreactivity and NADPH-d reac- tivity was demonstrated in the nociceptive subgroups of dorsal root ganglion neurons [5]. In the spinal cord, NADPH-d-reactive neurons are predominantly scat- tered within the substantia gelatinosa, a structure directly implicated in transmission of impulses evoked by nociceptive stimuli [6, 71. Recently, it was shown that NO plays a prominent role in the development of some nociceptive processes, such as thermal hyperal- gesia and hyperpathic pain behavior [8 ]. Modulation of I Bogomoiets Institute of Physiology, National Academy of Sciences of Ukraine, Kiev. Ukraine, 2 Ege University, School of Medicine, Izmir, Turkey. 27 NO-synthase activity in the spinal cord inhibits the processes of facilitation of afferent activity directly mediated by activation of NMDA receptors followed by NO production [9 ]. These findings indicate that NO- synthase-containing neurons may be involved in the development of antinociception. At a supraspinal level, there are NO-synthase-containing neurons in many structures which are relevant to descending control of nociceptive transmission in the spinal cord. In the midbrain, such neurons are located in several discrete regions, in particular, in the periaqueductal gray (PAG) and dorsal raphe nucleus (DR), i.e., in the centers involved in producing descending antinociceptive sig- nals. Our present study was aimed at detailed description of rostrocaudal distribution of NADPH-d-reactive neurons in the midbrain, at their quantitative analysis, and at establishing correlations between localization of NADPH-d activity and the borders of physiologically denoted analgesic and "pure analgesic" zones in the midbrain. Our results show that the ventral subdivision of ventrolateral PAG, regarded as a "pure analgesic" 009~2977196/280t-0027515.00at99. Plenum PublishingCorporation