Laminar distribution of the sources of ascending spino-supraspinal pathways involved in nociceptive transmission and pain modulation

In the present study, we made an attempt to detect more exactly the laminar distribution of spinomesencephalic, spinothalamic and spinotelencephalic tract neurons and to estimate the character of axonal collateralization in these spino-supraspinal pathways in the rat by means of retrograde double-labelling of cells with Fluoro-Gold (FG) and Primuline O (Pr). We found that sources of spino-supraspinal pathways to the upper centers of the brain stem, nucleus ac-cumbens and septal nuclei were mixed together in the marginal zone, deep dorsal horn, reticulated area, lamina X and the lateral spinal nucleus, but have as a rule their own discrete projection fields. Thus, spino-supraspinal projections could be characterized as extensive but not diffuse according to the current concept of structural and functional organization of the ascending spinal systems related with pain or motivational-affective reactions connected with pain. It was shown recently that suppression of tonic activity in an ascending spino-supraspinal pathway activates a descending supraspinal antinociceptive circuit with an opioid link in nucleus accumbens. We propose that the main effect of the straight spinal input into the limbic system is to facilitate nociceptive sensitivity, but not to suppress nociceptive transmission on the level of the spinal cord.In the present study, we made an attempt to detect more exactly the laminar distribution of spinomesencephalic, spinothalamic and spinotelencephalic tract neurons and to estimate the character of axonal collateralization in these spino-supraspinal pathways in the rat by means of retrograde double-labelling of cells with Fluoro-Gold (FG) and Primuline O (Pr). We found that sources of spino-supraspinal pathways to the upper centers of the brain stem, nucleus ac-cumbens and septal nuclei were mixed together in the marginal zone, deep dorsal horn, reticulated area, lamina X and the lateral spinal nucleus, but have as a rule their own discrete projection fields. Thus, spino-supraspinal projections could be characterized as extensive but not diffuse according to the current concept of structural and functional organization of the ascending spinal systems related with pain or motivational-affective reactions connected with pain. It was shown recently that suppression of tonic activity in an ascending spino-supraspinal pathway activates a descending supraspinal antinociceptive circuit with an opioid link in nucleus accumbens. We propose that the main effect of the straight spinal input into the limbic system is to facilitate nociceptive sensitivity, but not to suppress nociceptive transmission on the level of the spinal cord