| Summary: | There is a great need for new analgesics. The current problem in treatment of severe pain is that side effects limit the effectiveness of therapy. Glycine receptors are important in modulation of nociception, suggesting a novel class of analgesics. Previous studies in rats show that intrathecal administration of glycine agonists and amino acids structurally similar to glycine have
antinociceptive effects. The effects of isovaline, a unique, non-proteogenic glycine-like aminoacid, have not been studied. Isovaline is absorbed from the gut and transported across the blood-brain-barrier.
We examined the hypothesis that isovaline produces antinociception in mice. Administration of strychnine, an antagonist at glycine receptors, into the cisterna magna or lumbar intrathecal space resulted in allodynia, localized to the somatotopic distribution of the trigeminal and lumbar nerves. These findings provided a basis for models of lumbar and trigeminal neuralgia.
Racemic isovaline blocked strychnine induced allodynia in both models without apparent side effects. We next investigated the antinociceptive effects of glycine-like amino acids in formalin foot assay, a conventional rodent model of acute and chronic pain. Antinociceptive effects were demonstrated on intrathecal administration of glycine, beta-alanine, and isovaline. Intravenous isovaline produced significant antinociceptive effects in the formalin foot model.
The toxicity of isovaline and related amino acids were determined. Exploratory behavior, gait, and responses to stimuli were used to assess sedation. The rotarod test was used to examine central nervous system (CNS) and neuromuscular toxicities of intravenous isovaline. Lumbar administration of glycine and beta-alanine caused scratching and/or lower body weakness. Isovaline at 7-times intrathecal ED50 produced lower body weakness in some animals. None of the amino acids produced sedation comparable to morphine. At 6-times ED50, beta-alanine produced weakness. Both glycine (ED50) and beta-alanine (3x ED50) but not isovaline produced local nerve irritation. Intracisternal injection of glycine did not reverse allodynia and resulted in death. Neither R nor S enantiomers of isovaline impaired performance on the rotarod test.
Isovaline has significant antinociceptive properties. Given the absence of apparent CNS or motor toxicity, isovaline has potential as a clinical analgesic.
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