Elsevier

Neuropharmacology

Volume 43, Issue 8, December 2002, Pages 1324-1329
Neuropharmacology

Interleukin-2 regulates membrane potentials and calcium channels via μ opioid receptors in rat dorsal root ganglion neurons

https://doi.org/10.1016/S0028-3908(02)00298-8Get rights and content

Abstract

Our previous studies revealed that interleukin-2 (IL-2) exerted peripheral antinociception that was partially mediated by μ opioid receptors. No ionic explanations of this effect have yet been reported. The present study was designed to investigate effects of IL-2 on the physiological properties of capsaicin-sensitive small dorsal root ganglion (DRG) neurons, which are predominantly responsible for nociceptive transmission from the periphery to the spinal cord. Intracellualr recordings of DRG neurons were made in DRG/peripheral nerve preparation in vitro. IL-2 (103 U/ml) produced membrane hyperpolarization of –9.4 ± 3.0 mV and this effect was blocked by β-FNA (5 μM), a μ opioid receptor antagonist. Under whole-cell patch clamp recordings, transient high-threshold Ca2+ currents were inhibited by –56.6 ± 11.3% by IL-2. Simultaneous calcium imaging showed that this cytokine also inhibited depolarization-evoked increase in intracellular calcium concentration. All the effects of IL-2 were blocked by naloxone (1 μM). Consistent with previous studies, DAMGO, a selective μ opioid agonist, exerted similar inhibitory effects on membrane potentials and Ca2+ currents.

The present results indicated that μ opioid receptors were involved in the regulatory effects of IL-2 on membrane potentials and calcium channels in DRG neurons, which may contribute to IL-2-induced peripheral analgesia.

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.

References (37)

  • R.W. Tsien et al.

    Multiple types of neuronal calcium channels and their selective modulation

    Trends in Neuroscience

    (1988)
  • Y. Wang et al.

    The analgesic domain of IL-2

    Biochemical and Biophysical Research Communication

    (1997)
  • B.J. Allen et al.

    Noxious cutaneous thermal stimuli induce a graded release of endogenous substance P in the spinal cord: imaging peptide action in vivo

    Journal of Neuroscience

    (1997)
  • C.M. Armstrong et al.

    Two distinct populations of calcium channels in a clonal line of pituitary cells

    Science

    (1985)
  • G.J. Augustine et al.

    Calcium entry and transmitter release at voltage-clamped nerve terminals of squid

    Journal of Physiology

    (1985)
  • A. Bateman et al.

    The immune–hypothalamic–pituitary–adrenal axis

    Endocrine Review

    (1989)
  • B.P. Bean

    Classes of calcium channels in vertebrate cells

    Annual Review of Physiology

    (1989)
  • E. Carbone et al.

    A low-voltage-activated, fully inactivating Ca channel in vertebrate sensory neurones

    Nature

    (1984)
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    These authors contributed equally to this work.

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