Inhibition of Glutamatergic Synaptic Input to Spinal Lamina II_oNeurons by Presynaptic α₂-Adrenergic Receptors

Abstract

Activation of spinal alpha(2)-adrenergic receptors by the descending noradrenergic system and alpha(2)-adrenergic agonists produces analgesia. However, the sites and mechanisms of the analgesic action of spinally administered alpha(2)-adrenergic receptor agonists such as clonidine are not fully known. The dorsal horn neurons in the outer zone of lamina II (lamina II(o)) are important for processing nociceptive information from C-fiber primary afferents. In the present study, we tested a hypothesis that activation of presynaptic alpha(2)-adrenergic receptors by clonidine inhibits the excitatory synaptic input to lamina II(o) neurons. Whole cell voltage-clamp recordings were performed on visualized lamina II(o) neurons in the spinal cord slice of rats. The miniature excitatory postsynaptic currents (mEPSCs) were recorded in the presence of tetrodotoxin, bicuculline, and strychnine. The evoked EPSCs were obtained by electrical stimulation of the dorsal root entry zone or the attached dorsal root. Both mEPSCs and evoked EPSCs were abolished by application of 6-cyano-7-nitroquinoxaline-2,3-dione. Clonidine (10 microM) significantly decreased the frequency of mEPSCs from 5.8 +/- 0.9 to 2.7 +/- 0.6 Hz (means +/- SE) without altering the amplitude and the decay time constant of mEPSCs in 25 of 27 lamina II(o) neurons. Yohimbine (2 microM, an alpha(2)-adrenergic receptor antagonist), but not prazosin (2 microM, an alpha(1)-adrenergic receptor antagonist), blocked the inhibitory effect of clonidine on the mEPSCs. Clonidine (1-20 microM, n = 8) also significantly attenuated the peak amplitude of evoked EPSCs in a concentration-dependent manner. The effect of clonidine on evoked EPSCs was abolished in the presence of yohimbine (n = 5). These data suggest that clonidine inhibits the excitatory synaptic input to lamina II(o) neurons through activation of alpha(2)-adrenergic receptors located on the glutamatergic afferent terminals. Presynaptic inhibition of glutamate release from primary afferents onto lamina II(o) neurons likely plays an important role in the analgesic action produced by activation of the descending noradrenergic system and alpha(2)-adrenergic agonists.

Modulation of pain transmission by G-protein-coupled receptors

“…Clonidine also significantly reduces capsaicin-evoked glutamate release from the primary afferent nerves (Ueda et al, 1995). This evidence supports the notion that presynaptic α 2 -adrenergic receptors play an important role in the regulation of the glutamatergic synaptic input to spinal dorsal horn neurons, which could contribute to the analgesic actions produced by α 2 -adrenergic receptor agonists (Pan et al, 2002b). Interestingly, removal of α 2A -adrenergic receptors on TRPV1-expressing afferent neurons paradoxically potentiates the antinociceptive effect produced by intrathecal injection of clonidine in rats (Chen et al, 2007a).…”

“…Electrophysiological studies using rat spinal cord slices have demonstrated that clonidine can inhibit synaptic glutamate release from primary afferent nerves to spinal dorsal horn neurons (Pan et al, 2002b). Clonidine also significantly reduces capsaicin-evoked glutamate release from the primary afferent nerves (Ueda et al, 1995).…”

Primary Afferent Stimulation Differentially Potentiates Excitatory and Inhibitory Inputs to Spinal Lamina II Outer and Inner Neurons

“…The spinal cord transverse slices (300 m in thickness) with an attached dorsal root were prepared as described previously Pan et al 2002). Briefly, the lumbosacral segment of the spinal cord of Sprague-Dawley rats (5-6 wk old, Harlan, Indianapolis, IN) was rapidly removed using halothane anesthesia and cut in a preoxygenated ice-cold sucrose artificial cerebrospinal fluid (ACSF) using a vibratome (Technical Product International, St. Louis, MO).…”

“…Lamina II is an important region for modulation of nociceptive information and regulation of the firing activity of dorsal horn projection neurons (Cervero and Iggo 1980;Li et al 2002;Lu and Perl 2003;Yoshimura and Jessell 1989;Yoshimura and Nishi 1995). Recent neuroanatomical studies have shown that the isolectin-B4-positive C-fiber afferents predominantly terminate at lamina II o (Pan et al 2003;Woodbury et al 2000). Furthermore, nociceptive neurons are mostly found in lamina I and II o and innocuous mechanoreceptive cells are mostly located in lamina II i (Light and Willcockson 1999).…”

Current and Future Issues in the development of spinal agents for the management of pain

“…Interestingly, discrepancies are noted between the α 2 subtypes as defined by message and immunohistochemistry (see [91]). Spinal α 2 adrenergic receptors display several characteristics: i) binding is present presynaptically on C-fibers and postsynaptically on spinal neurons; ii) agonists are believed to alter spinal nociceptive processing by preventing opening of voltage-sensitive Ca channels (blocking release) [92,93] and increasing potassium conductance leading to hyperpolarization of dorsal horn neurons through increased pK conductance [87,88,94,95]. Consistent with their pharmacology, the analgesic activity is reversed by α 2 antagonists such as atipamezole [96][97][98].…”