Taick-Sang Nam

Modulation of N-type Ca2+ currents by moxonidine via imidazoline I1 receptor activation in rat superior cervical ganglion neurons

Moxonidine, an imidazoline deriviatives, suppress the vasopressor sympathetic outflow to produce hypotension. This effect has been known to be mediated in part by suppressing sympathetic outflow via acting imidazoline I(1) receptors (IR(1)) at postganglionic sympathetic neurons. But, the cellular mechanism of IR(1)-induced inhibition of noradrenaline (NA) release is still unknown. We therefore, investigated the effect of IR(1) activation on voltage-dependent Ca(2+) channels which is known to play an pivotal role in regulating NA in rat superior cervical ganglion (SCG) neurons, using the conventional whole-cell patch-clamp method. In the presence of rauwolscine (3 μΜ), which blocks α(2)-adrenoceptor (R(α2)), moxonidine inhibited voltage-dependent Ca(2+) current (I(Ca)) by about 30%. This moxonidine-induced inhibition was almost completely prevented by efaroxan (10 μΜ) which blocks IR(1) as well as R(α2). In addition, ω-conotoxin (CgTx) GVIA (1 μΜ) occluded moxonidine-induced inhibition of I(Ca), but, moxonidine-induced I(Ca) inhibition was not affected by pertussis toxin (PTX) nor shows any characteristics of voltage-dependent inhibition. These data suggest that moxonidine inhibit voltage-dependent N-type Ca(2+) current (I(Ca-N)) via activating IR(1). Finally, moxonidine significantly decreased the frequency of AP firing in a partially reversible manner. This inhibition of AP firing was almost completely occluded in the presence of ω-CgTx. Taken together, our results suggest that activation of IR(1) in SCG neurons reduced I(Ca-N) in a PTX-and voltage-insensitive pathway, and this inhibition attenuated repetitive AP firing in SCG neurons.

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

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 Ri‐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 ICa in a pertussis toxin‐ and voltage‐insensitive pathway, and this inhibition attenuated repetitive action potential firing in SCG neurons.

Experimental Physiology –Research Paper: Modulation of N‐type calcium currents by presynaptic imidazoline receptor activation in rat superior cervical ganglion neurons

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 Ri‐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 ICa in a pertussis toxin‐ and voltage‐insensitive pathway, and this inhibition attenuated repetitive action potential firing in SCG neurons.

Intracellular acidification evoked by moderate extracellular acidosis attenuates transient receptor potential V1 (TRPV1) channel activity in rat dorsal root ganglion neurons

Transient receptor potential V1 (TRPV1) has been suggested to play an important role in detecting decreases in extracellular pH (pHo). Results from recentu2002in vivou2002studies, however, have suggested that TRPV1 channels play less of a role in sensing a moderately acidic pHo (6.0 < pH < 7.0) than predicted from theu2002in vitrou2002experiments. A clear explanation for this discrepancy between theu2002in vitrou2002andu2002in vivou2002data has not yet been provided. We report here that intracellular acidification induced by a moderately low pHo (6.4) almost completely inhibited the effect of extracellular acidosis on TRPV1 activity. In our experiments, sodium acetate (20 mm), which was used to induce intracellular acidosis, attenuated the capsaicin‐evoked TRPV1 current (ICAP) in a reversible manner in whole‐cell patch‐clamp mode and shifted the concentration–response curve to the right. Likewise, the concentration–response curve was significantly shifted to the right by lowering the pH of the pipette solution from 7.2 to 6.5. In addition, application of an acidic bath solution (pH 6.4) to the intracellular side also significantly suppressedu2002ICAP in inside‐out patch mode. In cell‐attached patch mode, the single‐channel activity ofu2002iCAP was significantly attenuated by intracellular acidosis that was induced by a decrease in pHo (6.4). These results suggested that intracellular acidification induced by a low pHo inhibited TRPV1 activity. When studied in perforated patch mode or by acidifying the intracellular pipette solution, potentiation or activation of TRPV1 by extracellular acidosis (pH 6.4) at 37°C was almost completely inhibited. Likewise, enhancement of neuronal excitability by a moderately acidic pHo (6.4) at a physiological temperature (37°C) was attenuated by lowering the pH of the pipette solution to 6.5 or using perforated patch mode. Taken together, these results suggest that extracellular acidosis of moderate intensity may not significantly modulate TRPV1 activity in physiological conditions at which intracellular pH can be readily affected by pHo, and this phenomenon is due to attenuation of TRPV1 channel activity by low‐pHo‐induced intracellular acidification.

Inhibition of carbachol-evoked oscillatory currents by the NO donor sodium nitroprusside in guinea-pig ileal myocytes

The effect of sodium nitroprusside (SNP) on carbachol (CCh)‐evoked inward cationic current (Icat) oscillations in guinea‐pig ileal longitudinal myocytes was investigated using the whole‐cell patch‐clamp technique and permeabilized longitudinal muscle strips. SNP (10 μm) completely inhibited Icat oscillations evoked by 1 μm CCh. 1H‐(1,2,4) Oxadiazole [4,3‐a] quinoxaline‐1‐one (ODQ; 1 μm) almost completely prevented the inhibitory effect of SNP on Icat oscillations. 8‐Bromo‐guanosine 3′,5′‐cyclic monophosphate (8‐Br‐cGMP; 30 μm) in the pipette solution completely abolished Icat oscillations. However, a pipette solution containing Rp‐8‐Br‐cGMP (30 μm) almost completely abolished the inhibitory effect of SNP on Icat oscillations. When the intracellular calcium concentration ([Ca2+]i) was held at a resting level using BAPTA (10 mm) and Ca2+ (4.6 μm) in the pipette solution, CCh (1 μm) evoked only the sustained component of Icat without any oscillations and SNP did not affect the current. A high concentration of inositol 1,4,5‐trisphosphate (IP3; 30 μm) in the patch pipette solutions significantly reduced the inhibitory effect of SNP (10 μm) on Icat oscillations. SNP significantly inhibited the Ca2+ release evoked by either CCh or IP3 but not by caffeine in permeabilized preparations of longitudinal muscle strips. These results suggest that the inhibitory effects of SNP on Icat oscillations are mediated, in part, by functional modulation of the IP3 receptor, and not by the inhibition of cationic channels themselves or by muscarinic receptors in the plasma membrane. This inhibition seems to be mediated by an increased cGMP concentration in a protein kinase G‐dependent manner.

Experimental Physiology -Research Paper: Modulation of N-type calcium currents by presynaptic imidazoline receptor activation in rat superior cervical ganglion neurons: Cirazoline inhibits N-type Ca2+channels

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 Ri‐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 ICa in a pertussis toxin‐ and voltage‐insensitive pathway, and this inhibition attenuated repetitive action potential firing in SCG neurons.