Prolonged inhibition of primate spinothalamic tract cells by peripheral nerve stimulation

Inhibition of spinothalamic tract (STT) cells was produced by repetitive peripheral nerve conditioning stimulation with high intensity and low frequency pulses. Identified STT cells were recorded from the lumbosacral spinal cord of intact, anesthetized monkeys. In addition, presumed STT cells were recorded from both unanesthetized, decerebrated and decerebrated, spinalized monkeys. These cells were identified by antidromically activating them from the contralateral ventral lateral funiculus of the cervical spinal cord. Both C fiber activity evoked by electrical stimulation of the sural nerve and activity evoked by noxious heat were greatly inhibited by repetitive conditioning stimuli applied either to the common peroneal or tibial nerve with a strong enough intensity for activation of C fibers at 2 Hz for 15 min. The inhibition was maintained during the period of conditioning stimulation and often outlasted stimulation by 20-30 min. The inhibition of cells produced by peripheral nerve stimulation was observed in decerebrate and spinalized animals as well as in intact anesthetized monkeys, although the mean recovery time in the decerebrate group was faster. This indicates that anesthetics did not interfere with the inhibitory mechanism. Furthermore, the presence of inhibition in spinalized animals means the inhibition must depend in part on spinal cord neuronal circuitry. Intravenous injection of naloxone produced a significant but small reduction of the recovery phase of the inhibition. No pharmacological agent was found that substantially interfered with the powerful inhibition produced during peripheral conditioning stimuli. The experimental animal model used in these experiments seems appropriate for studying the mechanisms of analgesia produced by peripheral nerve stimulation.