Actions of opioids on primate spinothalamic tract neurons

The effects of iontophoretically applied opiates were tested on 24 spinothalamic tract cells in 12 anesthetized monkeys. The drugs used were chosen because of their agonist actions on different classes of opiate receptors (mu, morphine; kappa, dynorphin; delta, methionine enkephalinamide; sigma, N-allylnormetazocine or SKF 10047 and phencyclidine). The actions of the opiate drugs were generally inhibitory, although excitatory or mixed effects were sometimes seen, especially with morphine and dynorphin. Drug effects could change, depending on the position of the iontophoretic electrode array or on the current employed. Naloxone sometimes antagonized the action of the opiate drugs used, but naloxone did not seem to be a drug suited for iontophoretic application. A number of explanations are discussed to explain the variable actions of the opiate drugs.