Modulation of N-type calcium currents by presynaptic imidazoline receptor activation in rat superior cervical ganglion neurons

Presynaptic imidazoline receptors (R_i-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 R_i-pre-induced inhibition of NA release is unknown. We, therefore, investigated the effect of R_i-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 R_i-pre agonist as well as an α-adrenoceptor (R_α) agonist, decreased Ca²⁺ currents (I_Ca) 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 (R_α2), cirazoline still inhibited I_Ca 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 R_α2. In addition, pretreatment with LY320135 (10 μm), another R_i-pre antagonist, in combination with low-dose rauwolscine (3 μm), also blocked the R_α2-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 R_i-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.