Increased Response to Glutamate in Small Diameter Dorsal Root Ganglion Neurons after Sciatic Nerve Injury

Abstract

Glutamate in the peripheral nervous system is involved in neuropathic pain, yet we know little how nerve injury alters responses to this neurotransmitter in primary sensory neurons. We recorded neuronal responses from the ex-vivo preparations of the dorsal root ganglia (DRG) one week following a chronic constriction injury (CCI) of the sciatic nerve in adult rats. We found that small diameter DRG neurons (<30 µm) exhibited increased excitability that was associated with decreased membrane threshold and rheobase, whereas responses in large diameter neurons (>30 µm) were unaffected. Puff application of either glutamate, or the selective ionotropic glutamate receptor agonists alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainic acid (KA), or the group I metabotropic receptor (mGluR) agonist (S)-3,5-dihydroxyphenylglycine (DHPG), induced larger inward currents in CCI DRGs compared to those from uninjured rats. N-methyl-D-aspartate (NMDA)-induced currents were unchanged. In addition to larger inward currents following CCI, a greater number of neurons responded to glutamate, AMPA, NMDA, and DHPG, but not to KA. Western blot analysis of the DRGs revealed that CCI resulted in a 35% increase in GluA1 and a 60% decrease in GluA2, the AMPA receptor subunits, compared to uninjured controls. mGluR1 receptor expression increased by 60% in the membrane fraction, whereas mGluR5 receptor subunit expression remained unchanged after CCI. These results show that following nerve injury, small diameter DRG neurons, many of which are nociceptive, have increased excitability and an increased response to glutamate that is associated with changes in receptor expression at the neuronal membrane. Our findings provide further evidence that glutamatergic transmission in the periphery plays a role in nociception.

GluN2B N-methyl-D-aspartate receptor and excitatory amino acid transporter 3 are upregulated in primary sensory neurons after 7 days of morphine administration in rats

“…Recent studies indicate that glutamatergic transmission occurs in sensory ganglions and that intraganglionic glutamate receptors are involved in certain forms of pain such as neuropathic pain [22; 42; 44]. We therefore investigated glutamate receptor responses in primary sensory neurons of rats after OIH was induced by sustained morphine administration for 7 days.…”

“…Glutamate currents generated in DRG neurons are mainly mediated by AMPA receptors, NMDA receptors, KA receptors and group I mGlu receptors [22; 42]. Accordingly we used selective antagonist CNQX (10 μM), APV (50 μM), and DL-AP3 (60 μM) to block all glutamate receptor-mediated currents.…”

“…Intact lumbar DRGs were prepared as previously described [22]. Briefly, after inducing anesthesia with sodium pentobarbital (50 mg/kg, i.p.…”

“…Peripheral NMDARs, however, might also have been involved and could constitute better therapeutic targets because of the fewer side effects of NMDAR antagonists that do not cross the blood-brain-barrier [57]. For this reason, we chose to study OIH associated changes in the DRG since in this peripheral location opiate and glutamate receptors are abundant and participate in nociceptive transmission [21; 22; 36; 42; 44]. We investigated changes in glutamate receptors and transporters expressed on the neurons of the lumbar DRGs of rats that were administered morphine for 7 days and showed clear signs of OIH.…”

Ameliorative potential of standardized fruit extract of Pterodon pubescens Benth on neuropathic pain in mice: Evidence for the mechanisms of action

“…It is known that there is an increased response to glutamate in small diameter dorsal root ganglion neurons after sciatic nerve injury (Gong et al, 2014) and cytokine induction via NMDA-mediated signaling after sciatic nerve crush and chronic constriction injury (Kleinschnitz et al, 2004) (Fig. 9A).…”