Astaxanthin Alleviates Inflammatory Mechanical Hyperalgesia by Reducing Hyperexcitability of Trigeminal Nociceptive Secondary Neurons: Potential as an NSAID Alternative

This study investigated the potential of astaxanthin (AST), a natural carotenoid, to mitigate inflammation-induced hyperexcitability in the spinal trigeminal nucleus caudalis (SpVc) and the associated hyperalgesia. The efficacy of systemic AST application was compared to that of celecoxib (CEL). Inflammation was induced by injecting Complete Freund's adjuvant into the whisker pads of rats. The mechanical escape threshold was then assessed by delivering mechanical stimuli to the orofacial region. Although inflamed rats exhibited a significantly lower mechanical threshold compared to naïve rats, this threshold was restored to normal levels two days after treatment with AST, CEL, and the 1/2 CEL + 1/2 AST combination. The activity of SpVc wide-dynamic range (WDR) neurons was measured using extracellular single-unit recordings in response to mechanical stimulation of the orofacial area under anesthesia. In inflamed rats, AST, CEL, and 1/2 CEL + 1/2 AST administration significantly reduced the average firing rate of these neurons elicited by both non-noxious and noxious mechanical stimuli. In addition, all three treatments significantly decreased the heightened average spontaneous activity of SpVc neurons and normalized the increased average receptive field size in inflamed rats. This study provides evidence that systemic AST administration attenuates inflammatory mechanical hyperalgesia. This action is associated with the suppression of hyperexcitability in nociceptive SpVc WDR neurons, likely through the inhibition of the cyclooxygenase-2 signaling pathway. These findings support the potential of AST as a therapeutic agent for complementary and alternative medicine. It may provide a valuable alternative to non-steroidal anti-inflammatory drugs for the prevention of trigeminal inflammatory mechanical hyperalgesia.