Mechanism of spinal monocyte chemoattractant protein-1-mediated maintenance of chronic pathological pain in rats: the relationship with synaptic transmission in spinal dorsal horns

Abstract

Objective To evaluate the relationship between the mechanism of spinal monocyte chemoattractant protein-1 (MCP-1)-mediated maintenance of chronic pathological pain and synaptic transmission in spinal dorsal horns of rats. Methods Female Sprague-Dawley rats, aged 2-3 weeks after birth, weighing 150-210 g, were studied.The experiment was performed in 2 parts.Experiment Ⅰ Eighteen Sprague-Dawley rats were randomly divided into 2 groups (n=9 each) on 7 days after intrathecal catheters were inserted: phosphate buffer solution(PBS)group and MCP-1 group.PBS 10 μl was intrathecally injected in group PBS, and PBS 10 μl containing 100 ng MCP-1 was intrathecally injected in group MCP-1.The mechanical pain threshold was measured at 30 and 60 min before intrathecal injection, and 30, 60, 90, 120, 150 and 180 min and 1, 2 and 3 days after intrathecal injection.Experiment Ⅱ The transverse spinal cord slices were prepared, and substantia gelatinosa neurons were selected for whole-cell patch-clamp recording.Electrophysiological recording was performed at 1 h of incubation with artificial cerebrospinal fluid (ACSF) and immediately after adding MCP-1: for excitatory synaptic transmission recording, MCP-1 (final concentration 100 nmol/L), N-methyl-D-aspartate (NMDA, final concentration 100 μmol/L) and α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA, final concentration 20 μmol/L) were added to ACSF, and spontaneous excitatory postsynaptic currents (sEPSCs), AMPA receptors-mediated currents and NMDA receptors-mediated currents were recorded; for inhibitory synaptic transmission recording, MCP-1 (final concentration 100 nmol/L) and γ-aminobutyric acid (GABA, final concentration 1 mmol/L) were added to ACSF, and spontaneous inhibitory postsynaptic currents (sIPSCs) and GABA receptors-mediated currents were recorded. Results Compared with group PBS, the mechanical pain threshold was significantly decreased at 30 min-2 days after intrathecal injection in group MCP-1 (P<0.01). Compared with those at 1 h of incubation with ACSF, the frequency and amplitude of sEPSCs were significantly increased, the amplitude of NMDA receptors- and AMPA receptors-mediated currents were increased, the frequency and amplitude of sIPSCs were decreased, and the amplitude of GABA receptors-mediated currents was decreased immediately after adding MCP-1 (P<0.05). Conclusion MCP-1 enhances excitatory synaptic transmission through enhancing the function of NMDA and AMPA receptors in the posterior substantia gelatinosa neurons of the spinal cord; MCP-1 weakens inhibitory synaptic transmission through inhibiting GABA receptor function, which may be involved in MCP-1-mediated maintenance of chronic pathological pain in rats. Key words: Chemokine CCL2; Pain; Synaptic transmission; Spinal cord