Chemokines and chemokine receptors are widely expressed by cells of the immune and nervous systems. This review focuses on our current knowledge concerning the role of chemokines in the pathophysiology of chronic pain syndromes. Injury-or disease-induced changes in the expression of diverse chemokines and their receptors have been demonstrated in the neural and nonneural elements of pain pathways. Under these circumstances, chemokines have been shown to modulate the electrical activity of neurons by multiple regulatory pathways including increases in neurotransmitter release through Ca-dependent mechanisms and transactivation of transient receptor channels. Either of these mechanisms alone, or in combination, may contribute to sustained excitability of primary afferent and secondary neurons within spinal pain pathways. Another manner in which chemokines may influence sustained neuronal excitability may be their ability to function as excitatory neurotransmitters within the peripheral and central nervous system. As is the case for traditional neurotransmitters, injury-induced up-regulated chemokines are found within synaptic vesicles. Chemokines released after depolarization of the cell membrane can then act on other chemokine receptor-bearing neurons, glia, or immune cells. Because up-regulation of chemokines and their receptors may be one of the mechanisms that directly or indirectly contribute to the development and maintenance of chronic pain, these molecules may then represent novel targets for therapeutic intervention in chronic pain states.
In his landmark treatise Sir Charles Scott Sherrington proposed the concept that pain is the evolved response to a potentially harmful, noxious stimulus (1). An example would be placing your hand on a hot iron and the resulting sensation of pain, which produces almost immediate withdrawal, thereby preventing tissue damage. This conscious perception of pain is the result of activity in a set of well defined neural pathways that start with the primary afferent neurons. These pseudounipolar neurons are responsible for the transmission of pain (''nociceptors'') and have unmyelinated or lightly myelinated axons. The cell bodies of nociceptors are found in the dorsal root ganglia (DRG). Nociceptors have both a peripheral connection that innervates potentially diseased or traumatized nerves, muscles, tendons, organs, and epithelia, and a centrally projecting axon that enters the central nervous system. This central axon conveys ''nociceptive'' information to second-order neurons in the dorsal horn of the spinal cord. Neural connections from the dorsal horn to the thalamus, and from there to the cortex, relay this noxious information to higher centers of conscious experience (Fig. 1). The central axons of primary afferent nociceptive neurons also provide information to polysynaptic spinal cord interneurons, which are essential for the initiation of the nociceptive withdrawal reflex. These neurons trigger motor reflexes that are important in the avoidance of potentially harmful ...