Subcutaneous l-tyrosine elicits cutaneous analgesia in response to local skin pinprick in rats

The purpose of the study was to estimate the ability of l-tyrosine to induce cutaneous analgesia and to investigate the interaction between l-tyrosine and the local anesthetic lidocaine. After subcutaneously injecting the rats with l-tyrosine and lidocaine in a dose-dependent manner, cutaneous analgesia (by blocking the cutaneous trunci muscle reflex-CTMR) was evaluated in response to the local pinprick. The drug-drug interaction was analyzed by using an isobolographic method. We showed that both l-tyrosine and lidocaine produced dose-dependent cutaneous analgesia. On the 50% effective dose (ED₅₀) basis, the rank of drug potency was lidocaine (5.09 [4.88-5.38] μmol)>l-tyrosine (39.1 [36.5-41.8] μmol) (P<0.05). At the equipotent doses (ED₂₅, ED₅₀, and ED₇₅), the duration of cutaneous analgesia caused by l-tyrosine lasted longer than that caused by lidocaine (P<0.01). Lidocaine co-administered with l-tyrosine exhibited an additive effect on infiltrative cutaneous analgesia. Our pre-clinical study demonstrated that l-tyrosine elicits the local/cutaneous analgesia, and the interaction between l-tyrosine and lidocaine is additive. l-tyrosine has a lower potency but much greater duration of cutaneous analgesia than lidocaine. Adding l-tyrosine to lidocaine preparations showed greater duration of cutaneous analgesia compared with lidocaine alone.