The effect of low frequency peripheral stimulation on the tail-flick reflex and evoked cortical potentials in the...

The effects of low frequency peripheral electrical stimulation of innocuous intensity (LES) on the tail-flick latency and the amplitude of the late component of the cortical potentials evoked by single electrical stimuli (20-40 ms latency) were studied in 59 awake and restrained rabbits. Increase of the tail-flick latency and profound decrease of the amplitude of the late positive component of evoked cortical potentials was observed in 50 animals. Five animals of this group and five animals from a group of 9, resistant to electrostimulation, were tested to the analgesic effects of morphine using the same criteria. Positive effects were found in the former but not in the latter group. Naloxone (0.1 mg/kg) profoundly decreased the effects of LES which suggests that an opioid mechanism is involved in the action of conditioning electrostimulation. Pretreatment of the animals with serotonin depletor p-CPA resulted in partial blockade of the effects of LES. The blockade was removed by administration of serotonin precursor 5-HTP. This is in agreement with the hypothesis that the opioid effects are partially exerted through serotoninergic descending pathways.