GABAB receptor-mediated selective peripheral analgesia by the non-proteinogenic amino acid, isovaline

Peripherally restricted analgesics are desirable to avoid central nervous system (CNS) side effects of opioids. Nonsteroidal anti-inflammatory drugs produce peripheral analgesia but have significant toxicity. GABA(B) receptors represent peripheral targets for analgesia but selective GABA(B) agonists like baclofen cross the blood-brain barrier. Recently, we found that the CNS-impermeant amino acid, isovaline, produces analgesia without apparent CNS effects. On observing that isovaline has GABA(B) activity in brain slices, we examined the hypothesis that isovaline produces peripheral analgesia mediated by GABA(B) receptors. We compared the peripheral analgesic and CNS effect profiles of isovaline, baclofen, and GABA (a CNS-impermeant, unselective GABA(B) agonist). All three amino acids attenuated allodynia induced by prostaglandin E2 injection into the mouse hindpaw and tested with von Frey filaments. The antiallodynic actions of isovaline, baclofen, and GABA were blocked by the GABA(B) antagonist, CGP52432, and potentiated by the GABA(B) modulator, CGP7930. We measured Behavioural Hyperactivity Scores and temperature change as indicators of GABAergic action in the CNS. ED(95) doses of isovaline and GABA produced no CNS effects while baclofen produced substantial sedation and hypothermia. In a mouse model of osteoarthritis, isovaline restored performance during forced exercise to baseline values. Immunohistochemical staining of cutaneous layers of the analgesic test site demonstrated co-localization of GABA(B1) and GABA(B2) receptor subunits on fine nerve endings and keratinocytes. Isovaline represents a new class of peripherally restricted analgesics without CNS effects, mediated by cutaneous GABA(B) receptors.