Since substance P plays a role in the early phase of nociceptive signal transmission (Greco et al., 2008; Sahbaie et al., 2009), it is possible that nerve injury-induced increased release of substance P inhibits the conversion of progesterone into allopregnanolone, which is initiated by 5α-reductase, resulting in a reduction of GABAergic transmission during the early phase of peripheral neuropathy. This evidence concerns the gene TAC1 and peripheral neuropathy.