We report that GTP cyclohydrolase (GCH1), the rate-limiting enzyme for tetrahydrobiopterin (BH4) synthesis, is a key modulator of peripheral neuropathic and inflammatory pain. BH4 is an essential cofactor for catecholamine, serotonin and nitric oxide production. After axonal injury, concentrations of BH4 rose in primary sensory neurons, owing to upregulation of GCH1. After peripheral inflammation, BH4 also increased in dorsal root ganglia (DRGs), owing to enhanced GCH1 enzyme activity. Inhibiting this de novo BH4 synthesis in rats attenuated neuropathic and inflammatory pain and prevented nerve injury-evoked excess nitric oxide production in the DRG, whereas administering BH4 intrathecally exacerbated pain. In humans, a haplotype of the GCH1 gene (population frequency 15.4%) was significantly associated with less pain following diskectomy for persistent radicular low back pain. Healthy individuals homozygous for this haplotype exhibited reduced experimental pain sensitivity, and forskolin-stimulated immortalized leukocytes from haplotype carriers upregulated GCH1 less than did controls. BH4 is therefore an intrinsic regulator of pain sensitivity and chronicity, and the GTP cyclohydrolase haplotype is a marker for these traits.

Peripheral nerve injuries that provoke neuropathic pain are associated with microglial activation in the spinal cord. We have investigated the characteristics of spinal microglial activation in three distinct models of peripheral neuropathic pain: spared nerve injury (SNI), chronic constriction injury, and spinal nerve ligation. In all models, dense clusters of cells immunoreactive for the microglial marker CD11b formed in the ipsilateral dorsal horn 7 days after injury. Microglial expression of ionized calcium binding adapter molecule 1 (Iba1) increased by up to 40% and phosphorylation of p38 mitogen-activated protein kinase, a marker of microglial activity, by 45%. Expression of the lysosomal ED1-antigen indicated phagocytic activity of the cells. Unlike the peripheral nerve lesions, rhizotomy produced only a weak microglial reaction within the spinal gray matter but a strong activation of microglia and phagocytes in the dorsal funiculus at lumbar and thoracic spinal cord levels. This suggests that although degeneration of central terminals is sufficient to elicit microglial activation, it does not account for the inflammatory response in the dorsal horn after peripheral nerve injury. Early intrathecal treatment with low-dose methotrexate, beginning at the time of injury, decreased microglial activation, reduced p38 phosphorylation, and attenuated pain-like behavior after SNI. In contrast, systemic or intrathecal delivery of the glucocorticoid dexamethasone did not inhibit the activation of microglia or reduce pain-like behavior. We confirm that microglial activation is crucial for the development of pain after nerve injury, and demonstrate that suppression of this cellular immune response is a promising approach for preventing neuropathic pain.

The presence of an efficient high affinity uptake system for L-CSA has been demonstrated in cultured neuronal and glial cells of various types. In neurons and most glial cells L-CSA uptake is inhibited by acidic amino acids, L-glutamate and L-aspartate and requires sodium ions; however the sodium dependence varies from one cell type to the other. The characteristics of the uptake system change during cell maturation, especially in astroblasts. The predominance of CSA uptake in glial cells as compared to neurons, the similarity of the kinetic parameters and of the structural specificity of L-glutamate uptake suggest that both excitatory amino acids are transported by a common system. In view of accumulating evidence, the present results are in agreement with a role of CSA as a neurotransmitter and as a precursor for taurine biosynthesis in the central nervous system.