Presynaptic imidazoline receptors (Ri-pre) are found in the sympathetic axon terminals of animal and human cardiovascular systems, and they regulate blood pressure by modulating the release of peripheral noradrenaline (NA). The cellular mechanism of Ri-pre-induced inhibition of NA release is unknown. We, therefore, investigated the effect of Ri-pre activation on voltage-dependent Ca2+ channels in rat superior cervical ganglion (SCG) neurons, using the conventional whole-cell patch-clamp method. Cirazoline (30 μm), an Ri-pre agonist as well as an α-adrenoceptor (Rα) agonist, decreased Ca2+ currents (ICa) by about 50% in a voltage-dependent manner with prepulse facilitation. In the presence of low-dose rauwolscine (3 μm), which blocks the α2-adrenoceptor (Rα2), cirazoline still inhibited ICa by about 30%, but prepulse facilitation was significantly attenuated. This inhibitory action of cirazoline was almost completely prevented by high-dose rauwolscine (30 μm), which blocks R i-pre as well as Rα2. In addition, pretreatment with LY320135 (10 μm), another Ri-pre antagonist, in combination with low-dose rauwolscine (3 μm), also blocked the Rα2-resistant effect of cirazoline. Addition of guanosine-5′-O-(2-thiodiphosphate) (2 mm) to the internal solutions significantly attenuated the action of cirazoline. However, pertussis toxin (500 ng ml-1) did not significantly influence the inhibitory effect of cirazoline. Moreover, cirazoline (30 μm) suppressed M current in SCG neurons cultured overnight. Finally, ω-conotoxin (ω-CgTx) GVIA (1 μm) obstructed cirazoline-induced current inhibition, and cirazoline (30 μm) significantly decreased the frequency of action potential firing in a partly reversible manner. This cirazoline-induced inhibition of action potential firing was almost completely occluded in the presence of ω-CgTx. Taken together, our results suggest that activation of Ri-pre in SCG neurons reduced N-type I Ca in a pertussis toxin- and voltage-insensitive pathway, and this inhibition attenuated repetitive action potential firing in SCG neurons.