Central nuclei and spinal pathways in feedback inhibitory spinal cord potentials in ketamine-anaesthetized rats

Ketamine pharmacology: an update (pharmacodynamics and molecular aspects, recent findings)

For more than 50 years, ketamine has proven to be a safe anesthetic drug with potent analgesic properties. The active enantiomer is S(+)-ketamine. Ketamine is mostly metabolized in norketamine, an active metabolite. During "dissociative anesthesia", sensory inputs may reach cortical receiving areas, but fail to be perceived in some association areas. Ketamine also enhances the descending inhibiting serotoninergic pathway and exerts antidepressive effects. Analgesic effects persist for plasma concentrations ten times lower than hypnotic concentrations. Activation of the (N-Methyl-D-Aspartate [NMDA]) receptor plays a fundamental role in long-term potentiation but also in hyperalgesia and opioid-induced hyperalgesia. The antagonism of NMDA receptor is responsible for ketamine's more specific properties. Ketamine decreases the "wind up" phenomenon, and the antagonism is more important if the NMDA channel has been previously opened by the glutamate binding ("use dependence"). Experimentally, ketamine may promote neuronal apoptotic lesions but, in usual clinical practice, it does not induce neurotoxicity. The consequences of high doses, repeatedly administered, are not known. Cognitive disturbances are frequent in chronic users of ketamine, as well as frontal white matter abnormalities. Animal studies suggest that neurodegeneration is a potential long-term risk of anesthetics in neonatal and young pediatric patients.

Pain relief by transcutaneous electric nerve stimulation with bidirectional modulated sine waves in patients with chronic back pain: a randomized, double-blind, sham-controlled study

Objectives.  Newly developed bidirectional modulated sine waves (BMW) might provide some derived benefit to patients with low back pain. Pain relief by transcutaneous electric nerve stimulation (TENS) with BMWs was tested. Materials and Methods.  Analgesic effects of BMWs and conventional bidirectional pulsed waves on chronic back pain in 28 patients were compared, and effects of repeated TENS using BMWs on chronic back pain were investigated in 21 patients by means of a randomized double-blind, sham-controlled, parallel-group method. Pain intensity was assessed using numerical rating scale (NRS). Results.  There was significant immediate reduction in NRS in patients receiving BMWs, and 60 min after treatment compared to sham TENS. Weekly repeated treatments using massage and TENS with BMWs for 5 weeks resulted in a decrease of NRS, but there were no significant differences between the TENS plus massage and sham TENS plus massage groups. Conclusions.  This study shows that TENS with BMWs significantly inhibits chronic back pain, and treatment effects are attained within a day. The results also suggest that there were no statistically significant long-term effects of TENS with BMW in the repeated treatment.

Transcranial focused ultrasound to the thalamus alters anesthesia time in rats

A pulsed application of focused ultrasound (FUS) to the regional brain tissue alters the state of tissue excitability and thus provides the means for noninvasive functional neuromodulation. We report that the application of transcranial FUS to the thalamus of anesthetized rats reduced the time to emergence of voluntary movement from intraperitoneal ketamine/xylazine anesthesia. Low-intensity FUS was applied to the thalamus of anesthetized animals. The times required for the animals to show distinct physiological/behavioral changes were measured and compared with those times required in a control session without sonication. The sonication significantly reduced the time to show pinch response and voluntary movement. The modulatory effects of FUS on anesthesia suggest potential therapeutic applications for disorders of consciousness such as minimally consciousness states.

Effects of ketamine anesthesia on central nociceptive processing in the rat: a 2-deoxyglucose study

Ketamine is a dissociative anesthetic with complex actions on the CNS. We investigated here the effects of ketamine anesthesia on somatosensory processing in the rat spinal cord, thalamus, and cerebral cortex, using the quantitative 2-deoxyglucose mapping technique. Unanesthetized or ketamine-anesthetized male Sprague-Dawley rats received a s.c. injection of a dilute formaldehyde solution (5%, 0.08 ml) into a forepaw, inducing prolonged noxious afferent input, or an equal volume of isotonic saline as a control stimulus. The 2-deoxyglucose experiments started 30 min after the injection. In the cervical enlargement of the spinal cord, ketamine had no significant effect on glucose metabolic rates in saline-injected animals, whereas it prevented the metabolic increases elicited by prolonged noxious stimulation in unanesthetized animals. At the thalamic level, ketamine increased glucose uptake in both saline- and formalin-injected rats in the lateral posterior, lateral dorsal, medial dorsal, gelatinosus, antero-ventral and antero-medial thalamic nuclei, whereas it decreased metabolic activity in the ventro-basal complex. At the cortical level, the drug increased metabolic activity in both control and formalin groups in the lacunosus-molecularis layer of the dorsal hippocampus, posterior parietal, retrosplenial, cingulate and frontal cortex; significant metabolic decreases were found in the CA1 region of the dorsal hippocampus and in the parietal 1 and 2 cortical areas. In the investigated brain regions, ketamine did not abolish noxious-evoked increases in glucose uptake, which were in fact enhanced in the forelimb cortex and in the lacunosus-molecularis layer of the hippocampus. The dissociation between the spinal and supraspinal effects of ketamine suggests a specific antinociceptive action on spinal circuits, in parallel with complex changes of the activity of brain circuits involved in somatosensory processing. More generally, this study shows that functional imaging techniques are able to quantitatively assess the effects of anesthetic drugs on nociceptive processing at different levels of the neuraxis.

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.

Contralaterally induced dorsal root reflex shares common final pathway with ipsilaterally elicited ones in Wistar rats

The dorsal root reflex (DRR) in single fibers, recorded in the severed L5 dorsal root central stump, initiated by sciatic nerve (SN) stimulation (St), was studied in anesthetized rats. Both ipsi- and contralateral Sts elicited DRR, with central delays of 4.6 +/- 0.3 (n = 10) and 27.3 +/- 2.5 (n = 5) ms, respectively. Ipsilaterally induced DRRs showed repetitive firing (mean = 3 +/- 0.5 spikes, n = 5), while contralaterally induced ones were abolished after spinalization (T13). When the time interval between two ipsilateral suprathreshold Sts was less than 12 +/- 2.4 ms (n = 12), the second DRR was abolished. On the other hand, when it between two ipsilateral subthreshold Sts was less than 4.8 +/- 0.3 ms (n = 7), DRR was activated. Contralaterally induced DRR also showed temporal inhibition and facilitation with time intervals similar to that in ipsilateral cases. Furthermore, when the latency between a contralaterally and a ipsilaterally induced DRR was less than 10.8 +/- 1.8 ms (n = 5), the later DRR was blocked by the former, and vice versa. Facilitation can be also shown by a contralateral and a ipsilateral subthreshold St. These results suggested that contralaterally induced DRRs may share part of final common pathway with ipsilaterally induced ones.