Our previous study have shown that ketamine induced peripheral
analgesia was coupled with opioid receptors. The present
work was to examine their relationship by means of rat acute morphine tolerance model. The results showed that intraplantar
injection of morphine (10μg/μl, 5μl) in the region of peripheral receptive field significantly inhibited nociceptive firing of electromyogram in the posterior biceps semitendinous muscle. The morphine induced inhibition was gradually reduced following the repetitive injection of morphine. Acute tolerance to morphine induced peripheral
analgesia was produced generally after 5th repeated administration of morphine. In rats suffering from tolerance to morphine, intraplantar injection of ketamine (50μg/μl, 20μl) in the same region still elicited potent inhibition of nociceptive responses. The present results showed: 1. Acute tolerance to morphine induced peripheral analgesia in the rat could be used as a sample model for studying opioid tolerance. 2. In addition to opioid mechanism, other mechanisms underlying non tolerarable to ketamine may be involved in ketamine induced peripheral analgesia.