Antinociceptive effect of intrathecal administration of hypotaurine in rat models of inflammatory and neuropathic pain

Hypotaurine is an intermediate in taurine biosynthesis from cysteine in astrocytes. Although hypotaurine functions as an antioxidant and organic osmolyte, its physiological role in the central nervous system remains unclear. This study used behavioral assessments to determine whether hypotaurine influenced nociceptive transmission in acute, inflammatory, and neuropathic pain. The tail flick, paw pressure, and formalin tests were performed in male Sprague-Dawley rats to examine the effects of the intrathecal administration of hypotaurine (100, 200, 400, 600 μg) on thermal, mechanical, and chemical nociception. Chronic constriction injury (CCI) to the sciatic nerve was induced in the rats, and the electronic von Frey test and plantar test were performed to assess the effects on neuropathic pain. To determine which neurotransmitter pathway(s) was involved in the action of hypotaurine, in this study, we examined how the antagonists of spinal pain processing receptors altered the effect of 600 μg hypotaurine. To explore whether hypotaurine affected motor performance, the Rotarod test was conducted. Hypotaurine had antinociceptive effects on thermal, mechanical, and chemical nociception in the spinal cord. In CCI rats, hypotaurine alleviated mechanical allodynia and thermal hyperalgesia. These effects were reversed completely by pretreatment with an intrathecal injection of strychnine, a glycine receptor antagonist. Conversely, hypotaurine did not affect motor performance. This study demonstrated that intrathecal hypotaurine suppressed acute, inflammatory, and neuropathic pain. Hypotaurine may regulate nociceptive transmission physiologically by activating glycinergic neurons in the spinal cord, and it is a promising candidate for treating various pain states.