NADPH-diaphorase-containing neurons (it is supposed that this enzyme is a form of NO-synthase, NOS) were histochemically identified in the spinal cord of rats. In another set of the experiments, we identified neurons w sources of spinothalamic and spinomesencephalic pathways m by their retrograde labelling with Fluoro-Gold (FG) injected into the thalamus and periaqueductal gray substance. It was shown that NOS-containing spinal cells are, as a rule, propriospinal intersegmental or intrasegmental interneurons. We discuss the possible involvement of these cells in the "inhibition-of-inhibition" processes and in potentiation of the synaptic transmission in spinal neurons under conditions of the development of tonic or chronic pain. In rats, the number of NOS-containing neurons, which are also retrogradely labelled with FG after dye injection into the thalamus and midbrain, is rather limited.

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A comparative study of mmunoreactivity with respect to c-Fos protein in the motor (zones М1 and М2), medial prefrontal (PrL and IL), and cingular (Cg1 and Cg2) cortices allowed us to find significant differences between the intensities of expression of gene c-fos in these cortical regions in control rats (group 1) and animals trained to perform catching of food globules by the forelimb (i.e., realizing an operant food-procuring reflex, group 2). The density of distribution of Fos-immunoreactive (Fos-ir) neurons in rats of group 2 in motor and limbic cortical zones at +2.2 to +0.2 levels rostrally from the bregma were significantly lower than in control rats (Р < 0.05). In animals of group 2, we also found significantly greater numbers of Fos-ir neurons in the contralateral (with respect to the active extremity) zones of the cortex at all examined levels. These changes are probably related to functional changes in the cortex resulting from learning of motor habits in the course of training sessions for stabilization of the operant reflex. Histochemical estimation of the NADPH-diaphorase (NADPH-d) activity in the motor and limbic cortex showed that, in rats of both groups, the maximum number of labeled interneurons per slice in the М1 zone were observed in layers II/III, V, and VI (5.6 ± 0.4, 6.4 ± 0.5, and 14.0 ± 0.8, respectively, within 200 × 200 μm 2 areas). In the limbic cortex, NADPH-d-reactive (NADPH-d-r) interneurons were also met in layers II/III, V, and VI. Cortical NADPHd-r neurons with the Fos-ir nuclei were not found. The presence of spatial associations of the somata or processes of NADPH-d-r neurons with intraparenchimal arterioles and microvessels was a typical feature of the distribution of NADPH-d-reactivity in the М1 and М2 zones, as well as in Cg1, Cg2, PrL, and IL. The following succession of the density of neurovascular associations was observed: Cg1 > Cg2 > М1 > М2 > > PrL. As is supposed, NADPH-d-r neurons (i.e., cells generating NO) are involved in the control of regional blood flow in the studied cortical regions.