Current Pharmaceutical Biotechnology - Volume 12, Issue 10, 2011

The development of new therapeutic approaches to the treatment of painful neuropathies requires a better understanding of the mechanisms that underlie chronic pain syndromes. There is increasing evidence that immune competent cells such as microglia contribute to the development of chronic pain states. Chemokines play a pivotal role in mediating neuronal-microglial communication which leads to increased nociception. Fractalkine (FKN) is structurally unique amongst the family of chemokines and their receptors and expressed both in the central nervous system and peripheral nerves, as well as in endothelial cells and lymphocytes. Signalling via the CX3CR1 receptor, FKN is able to mediate critical physiological functions necessary for immune regulation. In its soluble forms FKN mediates chemotaxis of immune cells whilst membrane bound FKN acts as an adhesion molecule mediating leukocyte capture and infiltration. As FKN/CX3CR1 is such a key signalling pair for homeostatic functions it is not surprising that it is implicated in a large number of diseases in which imbalance of the immune system is implied. Here we review the evidence that FKN/CX3CR1 mediates neuron-microglial communication in chronic pain states and is therefore key in the development of neuropathic pain. In addition, the contribution of FKN/CX3CR1 signalling to the pathogenesis and progression of two chronic inflammatory conditions, atherosclerosis and rheumatoid arthritis, are discussed.

Voltage-gated sodium channels (NaV) are well validated targets for treating pain based both on human genetics and clinical experience. Consequently, there is an extensive literature on sodium channels for the treatment of pain and a number of excellent and thorough reviews have recently appeared; a selection of these is provided. This review does not attempt to evaluate all aspects of the studies in this area, but rather will focuses on several key issues that are incompletely addressed in prior reviews or that represent very recent additions to the literature. Key questions that arise are: 1) How much channel block is required to observe efficacy against neuropathic or inflammatory pain? 2) How can one improve upon the therapeutic index of previously tested NaV blockers?

Bv8 is a small protein secreted by frog skin. Mammalian homologues of Bv8, the prokineticins PK1 and PK2, and their G-protein coupled receptors PKR1 and PKR2 have been identified and linked to several biological effects as gut motility, circadian rhythms, neurogenesis, angiogenesis and cancer progression, haematopoiesis and nociception. In rodents, administration of amphibian Bv8 lowers nociceptor thresholds to a broad spectrum of physical and chemical stimuli. The prokineticin receptors are present in regions of the nervous system associated with pain; primary sensitive neurons expressing PKRs also express the vanilloid receptor TRPV1, providing an anatomical basis for PKR1/TRPV1 cooperative interaction in nociceptor sensitization. Bv8/PK2, strongly up-regulated in neutrophils and other inflammatory cells, is a main pronociceptive mediator in inflamed tissues. Indeed Bv8/PK2 produced by inflammatory cells is released at the site of inflammation where it sensitizes peripheral nociceptors, stimulates chemotaxis and modulates the release of inflammatory and pronociceptive cytokines. Availability of a non-peptide PKR antagonist, leading to blockade the PK/PKR system, ameliorates pain arising from tissue injury and, additionally, reduces the time required for recovery from injury.

Understanding and consequently treating neuropathic pain effectively is a challenge for modern medicine, as unlike inflammation, which can be controlled relatively well, chronic pain due to nerve injury is refractory to most current therapeutics. Here we define a target pathway for a new class of analgesics, tetrahydrobiopterin (BH4) synthesis and metabolism. BH4 is an essential co-factor in the synthesis of serotonin, dopamine, epinephrine, norepinephrine and nitric oxide and as a result, its availability influences many systems, including neurons. Following peripheral nerve damage, levels of BH4 are dramatically increased in sensory neurons, consequently this has a profound effect on the physiology of these cells, causing increased activity and pain hypersensitivity. These changes are principally due to the upregulation of the rate limiting enzyme for BH4 synthesis GTP Cyclohydrolase 1 (GCH1). A GCH1 pain-protective haplotype which decreases pain levels in a variety of settings, by reducing the levels of endogenous activation of this enzyme, has been characterized in humans. Here we define the control of BH4 homeostasis and discuss the consequences of large perturbations within this system, both negatively via genetic mutations and after pathological increases in the production of this cofactor that result in chronic pain. We explain the nature of the GCH1 reduced-function haplotype and set out the potential for a ‘ BH4 blocking’ drug as a novel analgesic.