The effect of unilateral dorsal root ganglionectomies or ventral rhizotomies on alpha 2-adrenoceptor binding to, and the substance P, enkephalin, and neurotensin content of, the cat lumbar spinal cord.

The density of alpha 2-adrenoceptor binding sites and the content of substance P, enkephalins, and neurotensin were determined in quadrants of the lumbar spinal enlargement of control cats and of cats upon which either unilateral dorsal root ganglionectomies or unilateral ventral rhizotomies had been performed. The performance of unilateral dorsal root ganglionectomies resulted in a significant decrease (45-55%) of substance P content in the ipsilateral dorsal horn 7 and 21 days postoperatively. The concentration of alpha 2-adrenoceptor binding sites ([3H]rauwolscine Bmax) in the ipsilateral dorsal horn was consistently and significantly decreased at these same postganglionectomy times (20% reduced relative to the contralateral dorsal horn). Enkephalin content 7 and 21 days after ganglionectomies was not significantly different from control, whereas the neurotensin content of the ipsilateral dorsal horn was significantly increased in the 21-day survival cats. The performance of unilateral ventral rhizotomies did not produce any statistically significant changes in the density of alpha 2-adrenoceptor binding sites or in the substance P or enkephalin content of any spinal quadrant. The neurotensin content of both the ipsilateral dorsal and ipsilateral ventral quadrants of the ventral rhizotomized cats was significantly increased. The significant decrease of alpha 2-adrenoceptor binding site concentration in the ipsilateral dorsal horn after unilateral dorsal root ganglionectomies suggests that approximately 20% of the alpha 2-adrenoceptors present within the cat lumbar spinal dorsal gray are located on the axons or terminals of primary sensory afferents. Consistent with this interpretation of the ganglionectomy results, we found significant levels of saturable [3H]rauwolscine binding to homogenates of the cat L4-Sl spinal dorsal root ganglia. Because alpha 2-adrenoceptor binding sites in the ipsilateral ventral lumbar spinal gray were not significantly reduced after unilateral ventral rhizotomies, our results provide no evidence for the location of alpha 2-adrenoceptor on lumbar spinal motoneurons.