Efffects of pain inducing agents on voltage dependent calcium currents in the trigeminal ganglion

Abstract

Trigeminal ganglion (TG) neurons are primary sensory neurons involved in the transmission of orofacial sensation, including pain. It is known that intracellular free calcium concentration plays a critical role in neurotransmitter release. In the present study, we examined the effects of pain-inducing agents, bradykinin and histamine, on voltage dependent Ca^2＋ current (I_Ca) and compared these with the effects of the analgesic agent, eugenol. I_Ca was measured using a whole-cell patch clamp method. Bradykinin (10μM) inhibited I_Ca in TG neurons. Maximum inward I_Ca was reduced by 26% by bradykinin compared with the controls (n＝13, p＜0.05). Decreased I_Ca induced by bradykinin did not recover to the prestimulus level after washing out bradykinin. In contrast, histamine (100μM) increased I_Ca by 25% compared with controls (n＝7,p＜0.05) in an irreversible manner. Eugenol (250μM), commonly used to produce analgesia in dental clinics, decreased I_Ca to 22.6% (n＝9,p＜0.05) and this effect on the I_Ca was reversible. although the effect of eugenol on the I_Ca was consistent, the effects of the two pain-inducing agents, bradykinin and histamine, were different. Our results suggest that the modulation of I_Ca in TG neurons is not a decisive factor underlying the pain mechanism evoked by pain-inducing agents.