The rate limiting enzyme in tetrahydrobiopterin (BH4) synthesis, GTP cyclohydrolase, is upregulated in DRGs following peripheral nerve injury or inflammation. The excess tetrahydrobiopterin synthesis contributes to the manifestation and persistence of pain. Using this protocol we find increased BH4 levels in the L4/5 DRGs and spinal cord tissue. Normalization of excess BH4 production reduces the nociceptive behaviour in various rodent models of neuropathic and inflammatory pain.
In humans we identified single nucleotide polymorphisms (SNPs) in the gene encoding GTP Cyclohydrolase (GCH1) that are associated with low pain scores. Carriers of a specific pain protective haplotype show reduced upregulation of GTP Cyclohydrolase expression and activity. Using this protocol we showed that BH4 measured as its oxidation product biopterin, is reduced in forskolin-stimulated whole blood and white blood cells in carriers of the haplotype as compared to non-carriers. The "forskolin whole blood assay" for biopterin might be a useful diagnostic tool in combination with GCH1 genotyping to assess the individual patient's risk for chronic pain.