Acute Intravenous Astaxanthin Administration Modulates Hyperexcitability in Rat Nociceptive Secondary Sensory Neurons Induced by Inflammation

Previous in vivo studies have clearly demonstrated that the intravenous administration of the carotenoid astaxanthin (AST) suppresses the excitability of rat trigeminal spinal nucleus caudalis (SpVc) neurons. This action is hypothesized to be mediated through the inhibition of both voltage-gated Ca²⁺ (Cav) channels and excitatory glutamate receptor transmission. The objective of this study was to determine whether acute intravenous administration of AST alleviates the hyperexcitability of SpVc wide dynamic range (WDR) neurons in a rat model of inflammation. Neuronal responses to both nociceptive and non-nociceptive mechanical stimulation were evaluated using an in vivo electrophysiological model. One day following inflammation induced by Complete Freund's Adjuvant (CFA), the mechanical escape threshold was significantly reduced compared to pre-injection baseline values. Subsequently, extracellular single-unit recordings were performed on SpVc WDR neurons in anesthetized, inflamed rats. The neuronal responses to both non-noxious and noxious orofacial mechanical stimuli were then analyzed. Acute intravenous administration of AST at 1 and 5 mM elicited a dose-dependent reduction in the mean firing frequency of SpVc WDR neurons in response to noxious mechanical stimuli. This inhibition peaked within 10 min and was fully reversed after approximately 25 min. Importantly, AST preferentially inhibited the discharge frequency of SpVc WDR neurons in response to noxious stimulation, exhibiting a significantly greater effect than on the response evoked by non-noxious stimulation (41.5 ± 3.0% vs. 20.7 ± 4.2%, <i>p</i> < 0.05). Collectively, these findings demonstrate that acute intravenous administration of AST effectively suppresses noxious synaptic transmission within the SpVc during inflammation. We propose that this suppressive effect is mediated by the inhibition of upregulated Cav channels and glutamate receptors. Consequently, AST is implicated as a promising therapeutic candidate for the management of trigeminal inflammatory pain, given its potential for a favorable safety profile compared to conventional treatments.