Spinal pathways mediating tonic or stimulation-produced descending inhibition from the periaqueductal gray or nucleus raphe magnus are separate in the cat.

1. The spinal pathways for tonic and stimulation-produced descending inhibition of spinal nociceptive neurons were investigated in anesthetized paralyzed cats. Reversible circumscribed blocks were produced at various depths in the lateral funiculi (LF) at L1-L2 using the microinjection of the local anesthetic lidocaine. The total amount of tonic descending inhibition in the absence of LF blocks was evaluated by monitoring the spinal neuronal activity during reversible spinalization by cold block and compared with the activity of the same neuron during LF blocks. Stimulation-induced descending inhibition of neuronal responses to noxious skin heating was produced by bipolar focal electrical stimulation in the periaqueductal gray (PAG) or nucleus raphe magnus (NRM) and compared with the inhibition of the same neurons during LF blocks. The relative significance of ipsi- and contralateral pathways in the dorsal, medial, or ventral aspects of the lateral funiculi for these types of descending inhibition are quantitatively described. 2. All 35 lumbar spinal dorsal horn neurons studied responded to noxious and innocuous mechanical and noxious thermal stimuli applied within the receptive fields on the glabrous skin of the hindlimb. Responses to noxious skin stimuli (50 degrees C, 10 s at 3-min intervals) were constant over time and served as a parameter to evaluate tonic and stimulation-produced descending inhibition. All neurons also responded to electrical stimulation of hindlimb cutaneous nerves supramaximal for the activation of A-beta-, delta-, and C-fibers. Neurons were located in laminae I-VI of the dorsal horn at L5-L7 levels. LF blocks were produced by the microinjection of 1 microliter lidocaine at each of one to six sites in the ipsilateral and/or contralateral LF 500, 1,500, and/or 2,500 microns below cord surface. 3. LF blocks ipsilateral to the recording sites in the cord significantly reduced tonic inhibition, with blocks in the dorsal part of the LF [i.e., the dorsolateral funiculus (DLF)] being equally effective to complete LF blocks. Stimulation-produced inhibition from PAG or NRM was, however, not significantly affected by ipsilateral LF blocks. 4. Contralateral LF blocks significantly reduced stimulation-produced descending inhibition and failed to affect tonic descending inhibition. Ventral LF blocks attenuated inhibition from the PAG but not from NRM, whereas DLF blocks were more effective on inhibition from the NRM. 5. Bilateral LF blocks significantly reduced tonic as well as stimulation-produced descending inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)