Interleukin-2 regulates membrane potentials and calcium channels via μ opioid receptors in rat dorsal root ganglion neurons
Introduction
Interleukin-2 (IL-2), the actively studied cytokine known as T-cell growth factor, is a glycoprotein mainly released from activated CD4+ T-lymphocytes and capable of stimulating proliferation of CD8+ T-lymphocytes. Rapidly accumulating evidence has strongly supported the notion that IL-2 plays an important role in the modulation of neural and neuroendocrine functions (Hanisch et al., 1993). IL-2 and morphine exert similar inhibitory effects in various aspects by decreasing intracellular cyclic AMP (cAMP) content and modulating neuroendocrine activity (Bindoni et al., 1988, Tancredi et al., 1990, Yao et al., 1994). Both IL-2 receptors and μ opioid receptors are expressed on small-sized dorsal root ganglion (DRG) neurons (Song et al., 2000, Wang and Wessendorf, 2001). Our previous studies additionally reported IL-2-induced peripheral antinociception that was partially mediated by μ opioid receptors (Song and Zhao, 2000). No ionic mechanisms underlying this effect have yet been reported.
Capsaicin-sensitive small DRG neurons transmit nociceptive information from peripheral tissues to the spinal cord (Harper and Lawson, 1985a, Harper and Lawson, 1985b, Heyman and Rang, 1985). It is also well known that free intracellular calcium plays a critical role in neurotransmitter release and signal transduction (DeLorenzo, 1981, Wollheim and Sharp, 1981, Wheeler et al., 1990, Zhang and Zhou, 2002). The present study was designed to investigate the effects of IL-2 on membrane potential, Ca2+ currents, and intracellular calcium concentration ([Ca2+]i) in DRG neurons and to find whether μ opioid receptors were involved in these effects. Understanding of these processes will enrich our knowledge of the physiological functions of IL-2 and ultimately extend the clinical application of this cytokine.
Section snippets
In vitro DRG/peripheral nerve preparation and intracellular recordings
Male Sprague–Dawley rats (150–200 g body weight) were anesthetized with urethane (1.25 g/kg). A dorsal laminectomy was performed to expose the spinal cord, which was carefully dissected along with DRGs (L4–L6) attached sciatic nerve. Free of surrounding tissue, the spinal cord, DRGs, and sciatic nerve were removed into oxygenated (95% O2–5% CO2) balanced salt solution (BSS) at 4°C. The solution contained (in mM): NaCl 124, KCl 2.5, CaCl2 2, MgSO4 2, KH2PO4 1.25, NaHCO3 25, and glucose 11. The
IL-2 and DAMGO exerted similar effects in DRG/peripheral nerve preparation
Intracellualr recordings of small-sized DRG neurons (<25 μm in diameter) were made in DRG/peripheral nerve preparation in vitro. Bath application of IL-2 (103 U/ml) led to membrane hyperpolarization in 62% of DRG neurons (–9.4 ± 3.0 mV, 29/47) and depolarization in 11% of DRG neurons (6.5 ± 1.5 mV, 5/47). IL-2-induced changes in membrane potential were blocked by the μ opioid receptor antagonist β-FNA (5 μM) (Fig. 1A). Consistent with previous studies, selective μ opioid receptor agonist DAMGO
Discussion
In the present study, we showed that IL-2 and selective μ opioid receptor agonist DAMGO shared some physiological functions. IL-2 produced membrane hyperpolarization in DRG neurons in DRG/peripheral nerve preparation in vitro. IL-2 inhibited transient high-threshold Ca2+ currents in acutely dissociated small DRG neurons. Simultaneous calcium imaging further revealed that IL-2 reduced depolarization-evoked Ca2+ entry through voltage-gated Ca2+ channels. All these effects of IL-2 can be blocked
Conclusions
The present study showed that, similarly to DAMGO, IL-2 significantly produced membrane hyperpolarization, suppressed transient high-threshold Ca2+ currents and decreased intracellular calcium concentration, which may contribute to IL-2-induced peripheral analgesia. All these effects were blocked by μ opioid antagonists, suggesting the interplay between IL-2 and μ opioid receptors.
Acknowledgements
Supported by grants from National Program of Basic Research of China (G1999054000) (ZQZ), (G2000077800) and NSFC to ZZ.
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These authors contributed equally to this work.