The Role of 5–Hydroxytryptamine (5–HT) Receptor Subtypes and Plasticity in the 5–HT Systems in the Regulation of Nociceptive Sensitivity

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Abstract
This review shows that the role of 5–hydroxytryptamine (5–HT) in the regulation of nociception depends on the 5–HT receptor subtypes involved and on long-term functional changes in the 5–HT receptors. Stimulation of the 5–HT 1 receptors, as well as of the 5–HT 2 and 5–HT 3 receptors, may reduce nociceptive sensitivity. In addition, activation of 5–HT 2 and 5–HT 3 receptors may also enhance nociceptive sensitivity. Up- or down-regulation of the 5–HT receptors may result in long-lasting changes, plasticity, in the 5–HT systems. Lesioning of 5–HT neurons induces denervation supersensitivity to 5–HT, and prolonged stimulation of 5–HT receptors may produce subsensitivity to 5–HT. In the spinal cord denervation supersensitivity to 5–HT may depend on reduced release of substance P (SP). An increase in the release of SP, on the other hand, may reduce the effects of 5–HT receptor activation. Long-term treatment with antidepressants which are used in clinical pain therapy appears to up-regulate the 5–HT 1 receptors and to down-regulate the 5–HT 2 receptors.