Research ReportAcute PAR2 activation reduces GABAergic inhibition in the spinal dorsal horn☆
Highlights
► PAR2 agonist applied intrathecally produced mechanical hyperalgesia. ► Acute PAR2 activation decreased spontaneous IPSCs in substantia gelatinosa. ► Acute PAR2 activation reduced GABA-induced currents in substantia gelatinosa. ► PAR2 agonist had no effect on glycine-induced currents in substantia gelatinosa.
Introduction
Protease-activated receptors (PARs) are a novel family of G-protein-coupled receptors that are found in various tissues throughout the body. They have recently attracted the attention of neuroscientists owing to their profound effects in the central and peripheral nervous systems (Noorbakhsh et al., 2003). The activation of protease-activated receptor-2 (PAR2) potentially regulates cell membrane ion channel through a series of signaling molecules, thus enhancing the neuronal excitability and causing pain (Lu et al., 2010). Recently, numerous researches have explored the role of PAR2 in peripheral inflammatory pain. For example, intraplantar injection of PAR2 agonist in rats causes marked and sustained hyperalgesia through its interaction with TRPA1 channels (Dai et al., 2007), TRPV4 channels (Grant et al., 2007), and TRPV1 channels (Dai et al., 2004). However, few studies have focused on the possibility that PAR2 agonist acts directly within the spinal cord to produce pronociceptive effects.
Alier et al. (2008) first reported that intrathecally applied PAR2 agonist produces mechanical and thermal hyperalgesia. And moreover, it augments the thermal and mechanical hyperalgesia produced in an inflammatory pain model. Surprisingly, their electrophysiological studies failed to demonstrate an increase in excitatory transmission or neuronal excitability in spinal cord dorsal horn following application of PAR2 agonist, and they did not report an analysis regarding PAR2 agonist's effect on inhibitory neurotransmission. As modulation of the inhibitory tone at the dorsal horn level maintains an important role in processing nociceptive information relayed to supraspinal centers (Drew et al., 2004, Malan et al., 2002), it is possible that PAR2 agonist acts directly within the spinal cord through modulating the inhibitory neurotransmission. Hence, in this study we investigated the effect of PAR2 activation on spontaneous inhibitory post-synaptic currents (sIPSCs) recorded from lamina II neurons. If there is evidence supporting our hypothesis of direct activation, we aim to explore the effect on GABAergic and glycinergic neurotransmission further.
Section snippets
Results
In the behavioral study, we observed a significant decrease in the nociceptive threshold in rats after intrathecal injection of the PAR2 agonist SL-NH2. Interestingly, this effect mostly disappeared when the PAR2 agonist SL-NH2 and PAR2 antagonist FS-NH2 were administered together (Fig. 1).
We next recorded sIPSCs in lamina II neurons by maintaining the holding potential at 0 mV. The PAR2 agonist SL-NH2 was perfused at the concentration of 100 uM for 2 min. Most neurons (7–9 of 15) responded to
Discussion
Previous research has demonstrated that intrathecal injection of PAR2 agonist induces and augments mechanical allodynia and thermal hyperalgesia in an inflammatory pain model (Alier et al., 2008). In this study, we further demonstrated that intrathecal administration of a certain concentration of PAR2 agonist can also induce mechanical hyperalgesia in normal rats, indicating that PAR2 receptors are not only involved in the process of maintaining chronic pathological pain but also play a role in
Statement of ethical considerations
Adult and 2–3 weeks old male Sprague Dawley rats were purchased from the Shanghai Experimental Animal Center at the Chinese Academy of Sciences. Animal experiments were approved by the Administrative Committee of Experimental Animal Care and Use, Second Military Medical University.
Intrathecal catheter placement
Male Sprague–Dawley rats with a body weight of approximately 200 g were used in this study. Animals were placed under general anesthesia using 2% halothane. PE10 tubing (Becton, Dickson and Company, Sparks, MD) was
Acknowledgments
This research was supported by the National Natural Science Foundation of China (No. 30801071) and the Shanghai Municipal Science and Technology Commission (No. 074119609). This research has no conflict of interest.
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Pain responses to protease-activated receptor-2 stimulation in the spinal cord of naïve and arthritic rats
2020, Neuroscience LettersCitation Excerpt :In naïve animals, intrathecal injection of a PAR-2 activating peptide (PAR-2-AP) caused hypersensitivity to both mechanical and thermal stimuli and significantly enhanced nociceptive responses in the formalin test [15,16]. Intrathecal administration of a PAR-2-AP has also been found to decrease GABAergic inhibitory synapse firing in laminae I and II of the spinal cord which enhances the neurotransmission of pain signals to the brain [17]. Patch-clamp recording of dorsal horn neurones revealed that a PAR-2-AP evoked excitatory currents via a TRPV1 and PKC pathway [18].
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This research was supported by the National Natural Science Foundation of China (No. 30801071) and the Shanghai Municipal Science and Technology Commission (No. 074119609). This research has no conflict of interest.
- 1
Contributed equally to this research work.