Systemic administration of lidocaine suppresses the excitability of rat cervical dorsal horn neurons and tooth-pulp-evoked jaw-opening reflex.

Although systemic lidocaine has been demonstrated to have analgesic actions in neuropathic pain conditions, the effect of intravenous lidocaine on trigeminal pain has not been elucidated. The aim of the present study is to investigate the effect of intravenous lidocaine administration on the excitability of the upper cervical dorsal horn (C1) neuron having convergent inputs from both tooth-pulp (TP) and facial skin as well as nociceptive jaw-opening reflex (JOR). After electrical stimulation of TP, extracellular single-unit recordings from 19 C1 neurons and the digastric muscle electromyogram (dEMG) were made in pentobarbital-anesthetized rats. These neurons also responded to non-noxious and noxious mechanical stimulation (touch and pinch) of facial skin, and every neuron was considered to be a wide dynamic range (WDR) neuron. The TP-evoked C1 neuronal and dEMG activities were dose-dependently inhibited by systematic administration of lidocaine (1-2 mg/kg, i.v.). After intravenous injection of lidocaine, the unit discharges induced by both touch and pinch stimuli were inhibited, and the size of the receptive field for pinch was also significantly decreased. The mean spontaneous discharge frequencies were significantly inhibited by the application of lidocaine. These changes were reversed within -20 min. These results suggest that in the absence of neuropathic pain intravenous lidocaine injection suppresses the trigeminal nociceptive reflex as well as the excitability of C1 neurons having convergent inputs from TP and somatic afferents. Systemic lidocaine administration, therefore, may contribute to the alleviation of trigeminal-referred pain associated with tooth pain.