The present in vivo study investigated whether systemic administration of
theanine attenuates the
inflammation-induced hyperexcitability of trigeminal spinal nucleus caudalis (SpVc) neurons associated with
hyperalgesia. Complete
Freund's adjuvant (CFA) was injected into the whisker pads of 24 rats to induce
inflammation, and then mechanical stimulation was applied to the orofacial area to assess the threshold of escape. The mechanical threshold was statistically significantly lower in CFA-inflamed rats compared to uninjected naïve rats, and this lowered threshold returned to control levels after 2 days of
theanine administration. The mean discharge frequency of SpVc wide-dynamic range (WDR) neurons to mechanical stimuli in anesthetized CFA-inflamed rats was statistically significantly lower after two days of
theanine administration. In addition, the increased mean spontaneous discharge of SpVc WDR neurons in CFA-inflamed rats statistically significantly decreased after
theanine administration. Similarly,
theanine restored the expanded mean receptive field size in CFA-inflamed rats to control levels. Taken together, these results suggest that administration of
theanine attenuates inflammatory
hyperalgesia associated with hyperexcitability of nociceptive SpVc WDR neurons. These findings support the potential of
theanine as a therapeutic agent in complementary
alternative medicine strategies to prevent inflammatory
hyperalgesia.