Coexistence of enkephalin and dynorphin immunoreactivities in neurons in the dorsal gray commissure of the sixth lumbar and first sacral spinal cord segments in rat.

The dorsal gray commissure (DGC) of the sixth lumbar (L6) and first sacral (S1) spinal cord segments in the rat has been shown to contain several different peptides, including enkephalin (ENK) and dynorphin (DYN). The present study was conducted to determine if DYN and ENK immunoreactivities coexist in neurons in the DGC in L6 and S1. Coexistence was determined using the elution/restaining technique of Tramu in which sections were stained first with anti-DYN then with anti-ENK after elution of the DYN antiserum. The number and location of DYN-immunoreactive cells was compared to the number and location of ENK-immunoreactive cells using photographic negatives. Of 67 ENK-immunoreactive cells counted in 20 sections through L6, 54 also contained DYN immunoreactivity. Of 100 ENK-immunoreactive cells counted in 47 sections through S1, 95 also contained DYN immunoreactivity. No cells with only DYN immunoreactivity were seen. S1 was found to contain a significantly greater number (P less than 0.005) of ENK-immunoreactive cells that also contained DYN than L6. The results of these studies indicate that ENK and DYN immunoreactivities coexist in most, but not all, neurons in the DGC of L6 and S1 and that a difference exists between L6 and S1 in the extent of coexistence of these peptides. Further studies are necessary to determine if coexistence of opioids is limited only to autonomic regions of the nervous system and also to define the functional significance of this coexistence.