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Effects of iontophoretically applied naloxone, picrotoxin and strychnine on dorsal horn neuron activities treated with high frequency conditioning stimulation in cats.

Transcutaneous electrical nerve stimulation(TENS), acupuncture-needling, and electroacupuncture are useful non-ablative methods in medical practice for relief of pain. These procedures appear to work by causing an increased discharge in afferent nerve fibers which in turn modifies the transmission of impulses in pain pathways. It is known that the mechanism of analagesic effect via these maneuvers are variable depending on the stimulating parameters. For example, the endogenous opioid system is profoundly related to the mechanism when a peripheral nerve stimulation is applied with parameters of low frequency and high intensity. However, when stimulated with parameters of high frequency and high intensity, the reduced activity of dorsal horn neurons is only slightly reversed by a systemic administration of naloxone, a specific opiate antagonist. Thus, the present study was performed to investigate the neurotransmitter that concerns the mechanism of peripheral nerve stimulation with parameters of high frequency and high intensity. We used an iontophoretic application of antagonists of possible related neurotransmitters. The dorsal horn neuron activity which was evoked by squeezing the peripheral cutaneous receptive field, was recorded as an index of pain with a microelectrode at the lumbo-sacral spinal cord. Naloxone, picrotoxin and strychnine were applied at 200nA during a period of conditioning nerve stimulation. We observed the effects of these drugs on the change of dorsal horn neuron activities. The main results of the experiment can be summarized as follows. The spontaneous activity of dorsal horn neurons increased in the presence of glutamate and decreased with GABA. It did not change with naloxone, picrotoxin or strychnine. When naloxone was applied iontophoretically during peripheral nerve stimulation, there was no statistically significant analgesic effect compared with that of the control group. When picrotoxin was applied iontophoretically during peripheral nerve stimulation, the analgesic effect was reduced. When strychnine was applied, the analgesic effect was reduced but did not show a statistically significant difference with the control group. These results suggested that the GABAergic system may have been partially related in the analgesic action of peripheral nerve stimulation with parameters of high frequency and high intensity.