Der Schmerz
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alpha(2)-Adrenoceptor agonists like clonidine, dexmedetomidine, and ST-91, inhibit nociceptive reflex activity predominantly by a spinal mode of action. They mimic the action of the inhibitory transmitter noradrenaline, which is released from the terminals of bulbospinal monoaminergic pathways. The inhibition by noradrenaline is due partly to hyperpolarization of the postsynaptic neuronal membrane; however, the selective antinociceptive effect of the alpha(2)-adrenoceptor agonists results from reduction of the release of the excitatory transmitters such as glutamate and substance P, blockade of the binding of substance P to spinal neurones, and enhancement of the action of the inhibitory transmitter, 5-hydroxytryptamine. ⋯ Moreover, impulse conduction in C fibres of peripheral nerves is far more reduced by these compounds than that in A fibres. Antinociceptive effects are reported to occur in various models of clinical pain, e.g. the formalin test, adjuvans-induced arthritis, autotomy following deafferentation, and "hyperalgesia" after nerve ligation. Therefore, the mechanisms involved in antinociception may also be responsible for the analgesia produced by alpha(2)-adrenoceptor agonists.