Gabapentin has been approved for the treatment of neuropathic pain in six European countries, New Zealand and Australia, and numerous countries in Latin America. By January 2001, Pfizer was preparing to file an NDA in the US for this indication; by October 2001, this NDA had been filed with the FDA. The drug is a GABA analog, but is not a GABA mimetic, although some neurons that respond to gabapentin are GABAergic. Gabapentin, at relevant concentrations, binds to an auxiliary protein of voltage-gated calcium channels (a2/3) and apparently, as a result, modulates the action of calcium channels and neurotransmitter release. This may account for many of its pharmacological actions. Gabapentin is also a substrate for the large neutral amino acid transporter, and this may be the major route allowing gabapentin access to the CNS. Modulation of synaptic transmission between primary afferents and substantia gelatinosa neurons, and blockade of signal transduction, are two potential mechanisms of action, in addition to inhibition of glutamate release by voltage-sensitive calcium channels. In September 2001, Morgan Stanley predicted sales of US $1871 million in 2002 falling to US $413 million in 2006.