A synthetic kainoid, (2S,3R,4R)-3-carboxymethyl-4-(phenylthio)pyrrolidine-2-carboxylic acid (PSPA-1) serves as a novel anti-allodynic agent for neuropathic pain

In spite of prominent progress in basic pain research, neuropathic pain remains a significant medical problem, because it is often poorly relieved by conventional analgesics. Thus this situation encourages us to make more sophisticated efforts toward the discovery of new analgesics. We previously showed that i.t. administration of acromelic acid-A (ACRO-A), a Japanese mushroom poison, provoked prominent tactile pain (allodynia) at an extremely low dose of 1 fg/mouse. In the present study we synthesized ACRO-A analogues (2S,3R,4R)-3-carboxymethyl-4-phenoxypyrrolidine-2-carboxylic acid (POPA-2) and (2S,3R,4R)-3-carboxymethyl-4-(phenylthio)pyrrolidine-2-carboxylic acid (PSPA-1) chemically and examined their ability to induce allodynia in conscious mice. Whereas POPA-2 induced allodynia at extremely low doses from 1 to 100 fg/mouse, similar to ACRO-A, PSPA-1 did not induce allodynia; rather, it inhibited the ACRO-A-induced allodynia with an ID50 value (95% confidence limits) of 2.19 fg/mouse (0.04–31.8 fg/mouse). Furthermore, PSPA-1 relieved neuropathic pain produced by L5 spinal nerve transection on day 7 after the operation in a dose-dependent manner from 1 to 100 pg/mouse. In contrast, it did not affect thermal or mechanical nociception or inflammatory pain. PSPA-1 reduced the increase in neuronal nitric oxide synthase activity in the spinal cord of neuropathic pain mice assessed by NADPH-diaphorase histochemistry and blocked the allodynia induced by N-methyl-d-aspartate. These results demonstrate that PSPA-1 may represent a novel class of anti-allodynic agents for neuropathic pain acting by blocking the glutamate-nitric oxide pathway.