Abstract
Summary Statement
TRPA1 antagonist reduced spontaneous excitatory postsynaptic currents of substantia gelatinosa neuron in spinal cord dorsal horn by in vivo patch-clamp analysis. TRPA1 may act as a mediator of excitatory synaptic transmission.
Introduction
Little is known about the pathophysiological mechanisms of radicular pain. The substantia gelatinosa (SG) in the spinal cord dorsal horn receives primary afferent inputs, which predominantly convey nociceptive sensations. Nociceptive information is modified and integrated in the SG, suggesting that the SG may be a therapeutic target for treating radicular pain. Electrophysiological study using in vivo patch-clamp recording from SG neurons is a useful method to analyze functional properties in synaptic transmission. Transient receptor potential ankyrin 1 (TRPA1) has been widely identified in the central and peripheral nervous system such as peripheral nociceptor, dorsal root ganglion (DRG), and spinal cord dorsal horn, and is considered that they are involved in synaptic transmission of pain. However, it is still unknown about its functional role and mechanism of pain transmission in spinal cord dorsal horn. The purpose of this study is to investigate changes in excitatory synaptic transmission of SG neurons with TRPA1 antagonist and to clarify the potential role of TRPA1 in the rat spinal cord dorsal horn using in vivo patch-clamp analysis.
Methods
Male Sprague-Dawley rat were divided into three experimental groups. In the root constriction (RC) group, the right L5 spinal root was ligated proximal to the DRG. The root was exposed only in the sham operation group, and no procedure was performed in the control group. In order to evaluate the excitability of the substantia gelatinosa neuron in the dorsal horn, we recorded excitatory postsynaptic currents (EPSCs) using in vivo whole-cell patch-clamp methods in each groups. Also, to clarify the function of TRPA1, we observed the change of EPSCs with application of TRPA1 antagonist (HC030031). Statistical significance was determined as P < 0.05 using Student's paired t test and one-way analysis of variance (ANOVA) followed by a Tukey–Kramer test.
Results
Spontaneous EPSCs (sEPSCs) were increased in the RC group more than in the sham and control group. With application of HC030031, the frequency and amplitude of sEPSCs were significantly reduced in all three groups. The relative frequency and the relative amplitude were 81% and 89% in the RC group, 81% and 94% in the control group, 70% and 88% in the sham group, respectively. There was no statistical significant difference among the three groups.
Discussion
The mechanism of synaptic transmission via TRPA1 in the spinal cord dorsal horn is considered that activated TRPA1 cause Ca2+ influx into presynaptic terminal and glutamate release from synaptic vesicle onto SG neuron. In the present study, sEPSCs were significantly reduced by TRPA1 antagonist not only in the RC group but also in the control group and sham group, which indicating that some TRPA1 were activated consistently in the rat spinal cord dorsal horn. It is considered that TRPA1 act as a mediator of excitatory transmission, thus, suppressing the activity of TRPA1 may lead to pain relief.
