SUPRASPINAL INHIBITION OF NOCICEPTIVE DORSAL HORN NEURONS IN THE ANESTHETIZED RAT - TONIC OR DYNAMIC

1. Tonic inhibition of sensory spinal neurones is well known to descen d from the rostroventral medulla. It is not clear if this inhibition i s dynamically activated by peripheral noxious stimuli. 2. Transection of the ipsilateral dorsolateral funiculus (DLF) removed a descending i nhibition of multireceptive spinal neurones and disproportionally prol onged the after-discharge component of their response to a noxious cut aneous stimulus. 3. Microinjection of GABA or tetracaine into the medu llary nucleus gigantocellularis pars alpha (GiA) similarly prolonged t he after-discharge in response to noxious stimuli. 4. Recordings of Gi A cells, initially using minimal surgery, revealed that many had low l evels of spontaneous activity and responded vigorously to noxious stim uli applied to any part of the body surface. One hour after the surger y necessary to expose the spinal cord, GiA cells had a high firing rat e but responded weakly to noxious stimuli. 5. The response of GiA cell s to noxious stimuli was abolished by transection of only the DLF cont ralateral to the stimulus. 6. It is concluded that the inhibition of m ultireceptive dorsal horn neurones from GiA is dynamically activated b y noxious cutaneous stimuli via a projection in the contralateral DLF: Surgical exposure of the spinal cord tonically activates this inhibit ion and masks the dynamic component.