Neurotransmitter serotonin (5-HT) released from descending
pain modulation pathways to the dorsal horn is crucial to spinal nociception processing. This study sought to gain insight into the modulatory roles of specific
serotonin receptor subtypes in experimentally induced
neuropathic pain. In rats subjected to spinal nerve
ligation (SNL) surgery, we recorded field potentials evoked in the spinal dorsal horn by C fibre-input, during spinal superfusion with subtype-selective drugs. In neuropathic rats, subtype
5-HT1A agonist 8-OH-DPAT (100 nM) was found to potently depress evoked field potentials, as opposed to 5-HT2A or 5-HT2B subtype agonists TCB-2 (100 nM) or
BW 723C86 (1 microM), respectively, which consistently enhanced evoked potentials. All three failed to alter spinal field potentials in
sham operated rats.
CP 94253 (1 microM),
WAY 161503 (1 mM) or SR 57227 (at 1 microM in SNL rats, and 100 microM in
sham rats), selective agonists for 5-HT1B, 5-HT2C and 5-HT3 receptors, respectively, significantly depressed evoked field potentials in both animal groups. The
5-HT4 agonist RS 67333 (1 microM) was depressant only in
sham operated animals. Only after SNL, spinal superfusion with 5-HT1A- or
5-HT1B receptor-antagonists (S)-
WAY 100135 (100 microM) or
SB 224289 (100 microM), respectively, disinhibited C fibre-evoked potentials, whereas 5-HT2A or
5-HT2B receptor-antagonists 4F 4PP (100 microM) or SB 204741 (100 microM) depressed evoked potentials, suggesting tonic activity of all four subtypes as a consequence of experimental nerve injury. The present findings reveal profound subtype-specific changes in the functional modulatory activities of spinal
serotonin receptors following
peripheral nerve injury. In particular, spinal hyperexcitation promoted by receptors 5-HT2A and 5-HT2B is suggested as a novel pathogenic pathway contributing to
neuropathic pain.