Serotonin is a major component of the inflammatory chemical milieu and contributes to the pain of tissue injury via an action on multiple receptor subtypes. Here we studied mice after genetic or pharmacological disruption of the 5-HT 3 receptor, an excitatory serotonin-gated ion channel. We demonstrate that tissue injury-induced persistent, but not acute, nociception is significantly reduced after functional elimination of this receptor subtype. Specifically, in the setting of tissue injury, the 5-HT 3 receptor mediates activation of nociceptors but does not contribute to injury-associated edema. This result is explained by the localization of 5-HT 3 receptor transcripts to a previously uncharacterized subset of myelinated and unmyelinated afferents, few of which express the proinflammatory neuropeptide substance P. Finally, we provide evidence that central serotonergic circuits modulate nociceptive transmission via a facilitatory action at spinal 5-HT 3 receptors. We conclude that activation of both peripheral and central 5-HT 3 receptors is pronociceptive and that the contribution of peripheral 5-HT 3 receptors involves a novel complement of primary afferent nociceptors.

Activation of unmyelinated (C-fiber) nociceptors by noxious chemicals plays a critical role in the initiation and maintenance of injury-induced pain. C-fiber nociceptors can be divided into two groups in which one class depends on nerve growth factor during postnatal development and contains neuropeptides, and the second class depends on glial cell line-derived neurotrophic factor during postnatal development and contains few neuropeptides but binds isolectin B(4) (IB(4)). We determined the sensitivity of these two populations to protons and capsaicin using whole cell recordings of dorsal root ganglion neurons from adult mouse. IB(4)-negative unmyelinated neurons were significantly more responsive to protons than IB(4)-positive neurons in a concentration-dependent manner. Approximately 86% of IB(4)-negative neurons responded to pH 5.0 with an inward current compared with only 33% of IB(4)-positive neurons. The subtypes of proton-evoked currents in IB(4)-negative unmyelinated neurons were also more diverse. Many IB(4)-negative neurons exhibited transient, rapidly inactivating proton currents as well as sustained proton currents. In contrast, IB(4)-positive neurons never displayed transient proton currents and responded to protons only with sustained, slowly inactivating inward currents. The two classes of neurons also responded differently to capsaicin. Twice as many naïve IB(4)-negative unmyelinated neurons responded to 1 microM capsaicin as IB(4)-positive neurons, and the capsaicin-evoked currents in IB(4)-negative neurons were approximately fourfold larger than those in IB(4)-positive neurons. Interestingly, proton exposure altered the capsaicin responsiveness of the two classes of neurons in opposite ways. Brief preexposure to protons increased the number of capsaicin-responsive IB(4)-positive neurons by twofold and increased the capsaicin-evoked currents by threefold. Conversely, proton exposure decreased the number of capsaicin-responsive IB(4)-negative neurons by 50%. These data suggest that IB(4)-negative unmyelinated nociceptors are initially the primary responders to both protons and capsaicin, but IB(4)-positive nociceptors have a unique capacity to be sensitized by protons to capsaicin-receptor agonists.