Plasma membrane poration by opioid neuropeptides: a possible mechanism of pathological signal transduction.

Neuropeptides induce signal transduction across the plasma membrane by acting through cell-surface receptors. The dynorphins, endogenous ligands for opioid receptors, are an exception; they also produce non-receptor-mediated effects causing pain and neurodegeneration. To understand non-receptor mechanism(s), we examined interactions of dynorphins with plasma membrane. Using fluorescence correlation spectroscopy and patch-clamp electrophysiology, we demonstrate that dynorphins accumulate in the membrane and induce a continuum of transient increases in ionic conductance. This phenomenon is consistent with stochastic formation of giant (~2.7 nm estimated diameter) unstructured non-ion-selective membrane pores. The potency of dynorphins to porate the plasma membrane correlates with their pathogenic effects in cellular and animal models. Membrane poration by dynorphins may represent a mechanism of pathological signal transduction. Persistent neuronal excitation by this mechanism may lead to profound neuropathological alterations, including neurodegeneration and cell death.

P2X3 receptor gating near normal body temperature.

P2X3 purinoreceptors expressed in mammalian sensory neurons are involved in nociception, mechanosensory transduction, and temperature sensation. Homomeric P2X3 receptors desensitize rapidly (<500 ms after activation by an agonist) and recover from desensitization very slowly (20-25 min at room temperature). They are susceptible to use-dependent inhibition by low nanomolar concentrations of ATP through developing the "high-affinity binding site" (HABS), which traps ATP molecules, thus keeping receptors in a desensitized state (Pratt et al., J Neurosci 25:7359-7365, 2005). Indeed, here we demonstrated directly that the desensitization of the receptor, after being activated by ATP, proceeds independently of the presence of agonist. We found that the temperature sensitivity of P2X3 receptors is abnormal: development of desensitization does not depend on temperature within the range between 25 and 40 degrees C, whereas the recovery from desensitization is greatly \accelerated with temperature increase (Q10 approximately 10). The sensitivity of HABS to low nanomolar ATP near normal body temperature (35 degrees C) is substantially lower than at 25 degrees C (IC50 is 3.2+/-0.3 nM at 35 degrees C and 0.79+/-0.09 nM at 25 degrees C). HABS itself is subjected to slow desensitization partially loosing its sensitivity to ATP: at 35 degrees C the response completely recovers in 10 min in the presence of 3 nM ATP, making the receptor operational in the presence of up to 30 nM ATP. Unusual combination of temperature sensitivity/insensitivity of P2X3 receptors may be related to their pivotal role in the processing of thermal sensitivity as revealed by recent knockout experiments.