P2X receptors in sensory neurons co-cultured with cancer cells exhibit a decrease in opioid sensitivity

Opioids are known to control the activity of P2X receptors in the sensory neurons of rats. These receptors are important in persistent pain signaling. However, there are extremely severe pain states, such as those associated with metastatic diseases, that are refractory to opioid treatment. We have tested the possibility that cancer cells affect the sensitivity of P2X_2/3 and P2X₂ receptors to opiates. The sensitivity of ATP-activated currents to the selective μ-opioid receptor agonist endomorphin-1 was evaluated in rat nodose neurons co-cultured (on separate coverslips) with fibrosarcoma cells (NCTC 2472) using whole-cell patch-clamp recordings. Both in control and in co-cultured neurons, P2X-mediated responses exhibited highly variable biphasic desensitization kinetics with fast and slow components. However, ATP-activated currents in co-cultured neurons acquired a new feature: the degree of their inhibition by endomorphin-1 demonstrated strong dependence on their desensitization kinetics. The neurons with 'slower' responses were subject to a smaller inhibitory effect of the opioid. The 'ultra-slow' responses completely lost their sensitivity to the opioid. The occurrence of such responses, rarely observed in the control neurons, was considerably increased with the duration of co-culturing. Application of endomorphin-1 to nodose neurons, co-cultured with rapidly proliferating but non-malignant cells (fibroblasts), resulted in data similar to those for the control. In summary, fibrosarcoma cells release diffusible factors altering the properties of desensitization kinetics of P2X receptors and, in particular, decrease their sensitivity to opioid inhibitory control. These phenomena may increase neuronal excitability initiated by peripheral ATP release and thereby contribute to the decreased sensitivity of cancer pain to opioids.