Long-Term Activation of Group I Metabotropic Glutamate Receptors Increases Functional TRPV1-Expressing Neurons in Mouse Dorsal Root Ganglia

Abstract

Damaged tissues release glutamate and other chemical mediators for several hours. These chemical mediators contribute to modulation of pruritus and pain. Herein, we investigated the effects of long-term activation of excitatory glutamate receptors on functional expression of transient receptor potential vaniloid type 1 (TRPV1) in dorsal root ganglion (DRG) neurons and then on thermal pain behavior. In order to detect the TRPV1-mediated responses in cultured DRG neurons, we monitored intracellular calcium responses to capsaicin, a TRPV1 agonist, with Fura-2. Long-term (4 h) treatment with glutamate receptor agonists (glutamate, quisqualate or DHPG) increased the proportion of neurons responding to capsaicin through activation of metabotropic glutamate receptor mGluR1, and only partially through the activation of mGluR5; engagement of these receptors was evident in neurons responding to allylisothiocyanate (AITC), a transient receptor potential ankyrin type 1 (TRPA1) agonist. Increase in the proportion was suppressed by phospholipase C (PLC), protein kinase C, mitogen/extracellular signal-regulated kinase, p38 mitogen-activated protein kinase or transcription inhibitors. Whole-cell recording was performed to record TRPV1-mediated membrane current; TRPV1 current density significantly increased in the AITC-sensitive neurons after the quisqualate treatment. To elucidate the physiological significance of this phenomenon, a hot plate test was performed. Intraplantar injection of quisqualate or DHPG induced heat hyperalgesia that lasted for 4 h post injection. This chronic hyperalgesia was attenuated by treatment with either mGluR1 or mGluR5 antagonists. These results suggest that long-term activation of mGluR1/5 by peripherally released glutamate may increase the number of neurons expressing functional TRPV1 in DRG, which may be strongly associated with chronic hyperalgesia.

Sensitization of glutamate receptor‐mediated pain behaviour via nerve growth factor‐dependent phosphorylation of transient receptor potential V1 under inflammatory conditions

“…Changes in intracellular calcium were measured with a fluorescent calcium indicator, as described previously (Masuoka et al, 2015 and 2016). For microscopic fluorometric measurement, cultured DRG neurons were washed with ACSF (138.6‐mM NaCl, 3.35‐mM KCl, 21‐mM NaHCO 3 , 9.9‐mM glucose, 0.6‐mM NaH 2 PO 4 , 2.5‐mM CaCl 2 , and 1‐mM MgCl 2 ) and incubated for 45 min in the CO 2 incubator (37 ± 2°C) in a solution containing 3‐μM Fura‐2‐acetoxymethyl ester (Fura‐2 AM; Dojindo Laboratories, Kumamoto, Japan) and 0.005% Cremophor® EL (Sigma‐Aldrich).…”

“…Cultures were prepared as described previously (Masuoka et al, 2015(Masuoka et al, , 2016(Masuoka et al, and 2017. Six-to 14-day-old mice were anaesthetized with isoflurane.…”

“…We previously showed that transient activation of mGlu 5 receptors potentiates TRPV1 channel‐mediated currents and persistently inhibits voltage‐gated calcium currents, thereby modulating pain behaviours in response to noxious heat (Masuoka et al, 2015). Moreover, long‐term (several hours) activation of mGlu 1/5 receptors increases the number of DRG neurons expressing functional TRPV1 channels, especially those nociceptive neurons expressing TRPA1 channels (Masuoka et al, 2016). Therefore, mGlu 1/5 receptors in the primary afferent fibres are potential therapeutic targets for the prevention and treatment of inflammation‐associated pain disorders.…”

TRPA1 Channels Modify TRPV1-Mediated Current Responses in Dorsal Root Ganglion Neurons

“…Culture preparation was conducted as previously described (Masuoka et al, 2015 , 2016 ). The C57BL/6J male and female mice (6- to 14-day-old) were anesthetized by the inhalation of isoflurane (Escain®; Mylan Inc., Cecil Township, PA, USA).…”

<p>Electroacupuncture Regulates Pain Transition by Inhibiting the mGluR5-PKCϵ Signaling Pathway in the Dorsal Root Ganglia</p>

“…19,20 Previous studies have found that damaged peripheral tissue can release glutamate 21,22 and activate primary afferent neurons that express mGluR5, causing hyperalgesia. 23,24 The use of selective inhibitors of mGluR5 can significantly reduce hyperalgesia. 25,26 In addition, the process of hyperalgesia induced by mGluR5 may be achieved through PKCε.…”