Effects of pentobarbital on heterosegmentally activated dorsal root depolarization in the rat. Investigation by sucrose-gap technique in vivo

The effects of isoflurane on conditioned inhibition by dorsal column stimulation

Spinal dorsal column stimulation (DCS) modulates sensory transmission, including pain, at the dorsal horn of the cord. However, the mechanisms of DCS modulatory actions and the effects of anesthetics on these mechanisms remain to be investigated. We studied the effects of isoflurane (1.0% and 2.0%) on conditioned inhibition, the amplitude decrease of the spinal cord potentials (SCPs) after a conditioning volley (DCS), in the ketamine-anesthetized rat by recording the sharp negative (N) and slow positive (P) waves of the SCPs evoked by conditioning dorsal column (DC) and testing segmental stimulations. The N wave is believed to be the synchronized activity of the dorsal horn neurons, and the P wave, primary afferent depolarization (PAD), reflecting presynaptic inhibition. The P potentials evoked by either DC or segmental stimulation were depressed by isoflurane, whereas the N waves remained unchanged, indicating that the pharmacological characteristics of these N and P waves are similar between DC-evoked and segmentally evoked SCPs. The conditioned inhibition of segmental N and P waves by DC stimulation was almost completely suppressed by 2.0% isoflurane. The conditioned inhibition of the segmental N wave was not changed by spinal cord transection, whereas the conditioned inhibition of the segmental P wave was decreased. The results indicate that isoflurane depresses presynaptic inhibition without affecting the synchronized activity of dorsal horn neurons and, most profoundly, depresses the conditioned inhibition by DC stimulation of the dorsal horn neurons and PAD. Further, the results indicate that conditioned inhibition by DC stimulation of PAD receives a facilitatory influence from the supraspinal structures, whereas that of the synchronized activity of the dorsal horn neurons does not.

IMPLICATIONS

To investigate how anesthetics affect supraspinal modulation of sensory transmission in the spinal cord, the spinal cord potential (SCP) evoked by dorsal cord stimulation (DCS) and segmentally evoked SCP conditioned by DCS were recorded in intact and spinal cord-transected rats during isoflurane anesthesia.

Neurophysiological assessment of the feasibility and safety of neural tissue transplantation in patients with syringomyelia

The feasibility and safety of a procedure involving fetal spinal cord tissue transplantation in patients with syringomyelia was assessed using a neurophysiological protocol designed to quantitate peripheral nerve function, spinal cord reflex excitability, and spinal cord conduction pathways essential for somatosensory evoked potentials. We report here data obtained before and for 18 months following the transplantation procedure performed on the first two patients in this study. The neurophysiological assessment protocols included measures of cortical and spinal cord evoked potentials, H-reflex excitability, and peripheral nerve conduction. Prior to the procedure, both patients had significant deficits on some of the neurophysiological measures, for example, lower extremity cortical evoked potentials. However, robust measures of intact pathways, such as upper extremity cortical evoked potentials, were also observed preoperatively in both patients. Thus, it was anticipated that conduction in these intact pathways could be at risk either from complications from the transplantation procedure and/or from continued expansion of the syrinx. Following the transplantation procedure, no negative changes were observed in any of the neurophysiological measures in either patient. In addition, patient 1 showed a decrease in the rate potentiation of tibial H-reflexes on the right side and an increase in the response probability of left tibial H-reflexes. The results of this postoperative longitudinal assessment provide a first-level demonstration of the safety of the intraspinal neural tissue transplantation procedure. However, the consideration of safety is currently limited to the grafting procedure itself, since the long-term fates of the donor tissue in these two patients remain to be shown more definitively.