Time-dependent cross talk between spinal serotonin 5-HT2A receptor and mGluR1 subserves spinal hyperexcitability and neuropathic pain after nerve injury

Abstract

Emerging evidence implicates serotonergic descending facilitatory pathways from the brainstem to the spinal cord in the maintenance of pathologic pain. Upregulation of the serotonin receptor 2A (5-HT2AR) in dorsal horn neurons promotes spinal hyperexcitation and impairs spinal μ-opioid mechanisms during neuropathic pain. Weinvestigated the involvement of spinal glutamate receptors, including metabotropic receptors (mGluRs) andNMDA,in 5-HT2AR-induced hyperexcitability after spinal nerve ligation (SNL) in rat. High-affinity 5-HT2AR agonist (4-bromo-3,6-dimethoxybenzocyclobuten-1-γl)methylamine hydrobromide (TCB-2) enhanced C-fiber-evoked dorsal horn potentials after SNL, which was prevented bymGluR1antagonistAIDA[(RS)-1-aminoindan-1,5-dicarboxylic acid] but not by group II mGluR antagonist LY 341495 [(2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl)propanoic acid] orNMDAantagonist D-AP5 [D-(-)-2-amino-5-phosphonopentanoic acid]. 5-HT2AR and mGluR1 were found to be coexpressed in postsynaptic densities in dorsal horn neurons. In the absence of SNL, pharmacological stimulation of 5-HT2AR with TCB-2 both induced rapid bilateral upregulation of mGluR1 expression in cytoplasmic and synaptic fractions of spinal cord homogenates, which was attenuated by PKC inhibitor chelerythrine, and enhanced evoked potentials during costimulation of mGluR1 with 3,5-DHPG [(RS)-3,5-dihydroxyphenylglycine]. SNL was followed by bilateral upregulation of mGluR1 in 5-HT2AR-containing postsynaptic densities. Upregulation of mGluR1 in synaptic compartments was partially prevented by chronic administration of selective 5-HT2AR antagonist M100907 [(R)-(+)-α-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenyl)ethyl]-4-pipidinemethanol], confirming 5-HT2AR-mediated control of mGluR1 upregulation triggered by SNL. Changes in thermal and mechanical pain thresholds following SNL were increasingly reversed over the days after injury by chronic 5-HT2AR blockade. These results emphasize a role for 5-HT2AR in hyperexcitation and pain after nerve injury and support mGluR1 upregulation as a novel feedforward activation mechanism contributing to 5-HT2AR-mediated facilitation. © 2012 the authors.