Implications of intrathecal pertussis toxin animal model on the cellular mechanisms of neuropathic pain syndrome.

Like opioid tolerance, neuropathic pain syndrome manifested by hyperalgesia and allodynia responds poorly to opioids. Hitherto, its development is still not clear and its treatment and prevention are still disputable. Pertussis toxin (PTX) which ADP-ribosylates the alpha-subunit of inhibitory guanine nucleotide binding regulatory proteins (Gi/Go), is used to induce morphine tolerance through intrathecal (i.t.) injection. It decreases the antinociceptive effect of opioid receptor agonists, and produces a thermal hyperalgesia as well. With treatment of PTX the inhibitory Gi- and Go-proteins signal transduction is inactivated. Inhibition of the inhibitory system would likely lead to a predominance of the excitatory system. Intrathecal PTX administration has also been suggested as a model for study of the central mechanisms of neuropathic pain. In our previous studies, with intrathecal microdialysis and drug delivery techniques, we correlated the biochemical and pharmacological effects on the behavioral expressions of i.t. PTX-treated rats. Intrathecal PTX administration would induce thermal hyperalgesia in rats, with accompaniments of a prolonged increase in the concentrations of excitatory amino acids (EAAs), glutamate and aspartate, and a decrease in the concentration of the inhibitory amino acid (IAA) glycine in the spinal CSF dialysates. The PTX-induced thermal hyperalgesia peaked between day 2 and 4, but no cold allodynia is observed; i.t. administration of N-methyl-D-aspartate (NMDA) receptor antagonist, D-2-amino-5-phosponovaleric acid (D-AP5), glycine and protein kinase C (PKC) inhibitor chelerythrine attenuated the thermal hyperalgesia. The PKC gamma content of both synaptosomal and cytosolic fractions were significantly increased in PTX-treated rats. In contrast, the levels of PKC alpha, beta I, or beta II isozymes in these fractions were unaffected. Infusion of NMDA antagonist D-AP5 prevented both the thermal hyperalgesia and the increase in PKC gamma expression in PTX-treated rats. Similar to our previous report, i.t. PTX reduced morphine's analgesic effect. PKC inhibitor chelerythrine attenuated this reduction of morphine's analgesia, and an inhibition of the morphine-evoked EAAs release was observed in PTX-treated rats as well. Taken together, i.t. PTX-induced neuropathic pain syndrome is accompanied by increasing of EAAs, decreasing of IAA release, and a selective increasing of PKC gamma expression in the spinal cord. Inhibition of PKC not only blocked thermal hyperalgesia, but also reversed the reduction of morphine's analgesic effect in PTX-rats. These results suggest that PTX-induced neuropathic pain syndromes are involved in EAAs, IAAs and PKC alternations.