Calcium channel currents in acutely dissociated intracardiac neurons from adult rats

Ion channel expression in intrinsic cardiac neurons: new players in cardiac channelopathies?

The autonomic nervous system is an important modulator of electrical disorders observed in cardiac pathologies through changes in the balance between sympathetic and parasympathetic tone. The final common pathway for cardiac neuronal autonomic control resides in the intrinsic cardiac nervous system (ICNS), composed of intracardiac neurons (ICN), and which allows sympathetic-parasympathetic efferent neuronal interactions at intracardiac sites. The ICNS is a complex system that plays a crucial role in the regulation of cardiac physiological parameters and has been shown to contribute to cardiac diseases, in particular cardiac arrhythmias. It is therefore crucial to understand the molecular determinants, such as ion channels, that control the excitability of the ICNS and their potential modulation in pathological conditions. This review discusses several ion channels expressed by ICN, including potassium channels (e.g., inward rectifier, calcium-dependent, voltage-activated, muscarinic-sensitive), voltage-gated sodium channels (VGSC), voltage-gated calcium channels (VGCC), hyperpolarization-activated cyclic nucleotide-gated (HCN) channels and Transient Receptor Potential (TRP) Channels, and their potential involvement in cardiac pathologies. We highlight the need for further research on ICN ion channels, particularly under pathological conditions, to develop therapies for cardiac arrhythmias.

Cellular and Molecular Mechanisms Underlying Altered Excitability of Cardiac Efferent Neurons in Cirrhotic Rats

Patients with cirrhosis often exhibit cardiac autonomic dysfunction (CAD), characterized by enhanced cardiac sympathetic activity and diminished cardiac vagal tone, leading to increased morbidity and mortality. This study delineates the cellular and molecular mechanisms associated with altered neuronal activities causing cirrhosis-induced CAD. Biliary and nonbiliary cirrhotic rats were produced by common bile duct ligation (CBDL) and intraperitoneal injections of thioacetamide (TAA), respectively. Three weeks after CBDL or TAA injection, the assessment of heart rate variability revealed autonomic imbalance in cirrhotic rats. We observed increased excitability in stellate ganglion (SG) neurons and decreased excitability in intracardiac ganglion (ICG) neurons in cirrhotic rats compared to sham-operated controls. Additionally, threshold, rheobase, and action potential duration exhibited opposite alterations in SG and ICG neurons, along with changes in afterhyperpolarization duration. A- and M-type K⁺ channels were significantly downregulated in SG neurons, while M-type K⁺ channels were upregulated, with downregulation of the N- and L-type Ca2⁺ channels in the ICG neurons of cirrhotic rats, both in transcript expression and functional activity. Collectively, these findings suggest that cirrhosis induces an imbalance between cardiac sympathetic and parasympathetic neuronal activities via the differential regulation of K+ and Ca2+ channels. Thus, cirrhosis-induced CAD may be associated with impaired autonomic efferent functions within the homeostatic reflex arc that regulates cardiac functions.

Recording plasticity in neuronal activity in the rodent intrinsic cardiac nervous system using calcium imaging techniques

The intrinsic cardiac nervous system (ICNS) is composed of interconnected clusters of neurons called ganglionated plexi (GP) which play a major role in controlling heart rate and rhythm. The function of these neurons is particularly important due to their involvement in cardiac arrhythmias such as atrial fibrillation (AF), and previous work has shown that plasticity in GP neural networks could underpin aberrant activity patterns that drive AF. As research in this field increases, developing new techniques to visualize the complex interactions and plasticity in this GP network is essential. In this study we have developed a calcium imaging method enabling the simultaneous recording of plasticity in neuronal activity from multiple neurons in intact atrial GP networks. Calcium imaging was performed with Cal-520 AM labeling in aged spontaneously hypertensive rats (SHRs), which display both spontaneous and induced AF, and age-matched Wistar Kyoto (WKY) controls to determine the relationship between chronic hypertension, arrhythmia and GP calcium dynamics. Our data show that SHR GPs have significantly larger calcium responses to cholinergic stimulation compared to WKY controls, as determined by both higher amplitude and longer duration calcium responses. Responses were significantly but not fully blocked by hexamethonium, indicating multiple cholinergic receptor subtypes are involved in the calcium response. Given that SHRs are susceptible to cardiac arrhythmias, our data provide evidence for a potential link between arrhythmia and plasticity in calcium dynamics that occur not only in cardiomyocytes but also in the GP neurons of the heart.

Hydrogen Peroxide Scavenging Restores N-Type Calcium Channels in Cardiac Vagal Postganglionic Neurons and Mitigates Myocardial Infarction-Evoked Ventricular Arrhythmias in Type 2 Diabetes Mellitus

Objective

Withdrawal of cardiac vagal activity is associated with ventricular arrhythmia-related high mortality in patients with type 2 diabetes mellitus (T2DM). Our recent study found that reduced cell excitability of cardiac vagal postganglionic (CVP) neurons is involved in cardiac vagal dysfunction and further exacerbates myocardial infarction (MI)-evoked ventricular arrhythmias and mortality in T2DM. However, the mechanisms responsible for T2DM-impaired cell excitability of CVP neurons remain unclear. This study tested if and how elevation of hydrogen peroxide (H2O2) inactivates CVP neurons and contributes to cardiac vagal dysfunction and ventricular arrhythmogenesis in T2DM.

Methods and Results

Rat T2DM was induced by a high-fat diet plus streptozotocin injection. Local in vivo transfection of adenoviral catalase gene (Ad.CAT) successfully induced overexpression of catalase and subsequently reduced cytosolic H2O2 levels in CVP neurons in T2DM rats. Ad.CAT restored protein expression and ion currents of N-type Ca2+ channels and increased cell excitability of CVP neurons in T2DM. Ad.CAT normalized T2DM-impaired cardiac vagal activation, vagal control of ventricular function, and heterogeneity of ventricular electrical activity. Additionally, Ad.CAT not only reduced the susceptibility to ventricular arrhythmias, but also suppressed MI-evoked lethal ventricular arrhythmias such as VT/VF in T2DM.

Conclusions

We concluded that endogenous H2O2 elevation inhibited protein expression and activation of N-type Ca2+ channels and reduced cell excitability of CVP neurons, which further contributed to the withdrawal of cardiac vagal activity and ventricular arrhythmogenesis in T2DM. Our current study suggests that the H2O2-N-type Ca2+ channel signaling axis might be an effective therapeutic target to suppress ventricular arrhythmias in T2DM patients with MI.

Macrophage depletion in stellate ganglia alleviates cardiac sympathetic overactivation and ventricular arrhythmogenesis by attenuating neuroinflammation in heart failure

Cardiac sympathetic overactivation is involved in arrhythmogenesis in patients with chronic heart failure (CHF). Inflammatory infiltration in the stellate ganglion (SG) is a critical factor for cardiac sympathoexcitation in patients with ventricular arrhythmias. This study aims to investigate if macrophage depletion in SGs decreases cardiac sympathetic overactivation and ventricular arrhythmogenesis in CHF. Surgical ligation of the coronary artery was used for induction of CHF. Clodronate liposomes were microinjected into bilateral SGs of CHF rats for macrophage depletion. Using cytokine array, immunofluorescence staining, and Western blot analysis, we found that macrophage expansion and expression of TNFα and IL-1β in SGs were markedly increased in CHF rats. Flow cytometry data confirmed that the percentage of macrophages in SGs was higher in CHF rats than that in sham rats. Clodronate liposomes significantly reduced CHF-elevated proinflammatory cytokine levels and macrophage expansion in SGs. Clodronate liposomes also reduced CHF-increased N-type Ca2+ currents and excitability of cardiac sympathetic postganglionic neurons and inhibited CHF-enhanced cardiac sympathetic nerve activity. ECG data from 24-h, continuous telemetry recording in conscious rats demonstrated that clodronate liposomes not only restored CHF-induced heterogeneity of ventricular electrical activities, but also decreased the incidence and duration of ventricular tachycardia/fibrillation in CHF. Macrophage depletion with clodronate liposomes attenuated CHF-induced cardiac sympathetic overactivation and ventricular arrhythmias through reduction of macrophage expansion and neuroinflammation in SGs.

The role of the tripartite synapse in the heart: how glial cells may contribute to the physiology and pathophysiology of the intracardiac nervous system

One of the major roles of the intracardiac nervous system (ICNS) is to act as the final site of signal integration for efferent information destined for the myocardium to enable local control of heart rate and rhythm. Multiple subtypes of neurons exist in the ICNS where they are organized into clusters termed ganglionated plexi (GP). The majority of cells in the ICNS are actually glial cells; however, despite this, ICNS glial cells have received little attention to date. In the central nervous system, where glial cell function has been widely studied, glia are no longer viewed simply as supportive cells but rather have been shown to play an active role in modulating neuronal excitability and synaptic plasticity. Pioneering studies have demonstrated that in addition to glia within the brain stem, glial cells within multiple autonomic ganglia in the peripheral nervous system, including the ICNS, can also act to modulate cardiovascular function. Clinically, patients with atrial fibrillation (AF) undergoing catheter ablation show high plasma levels of S100B, a protein produced by cardiac glial cells, correlated with decreased AF recurrence. Interestingly, S100B also alters GP neuron excitability and neurite outgrowth in the ICNS. These studies highlight the importance of understanding how glial cells can affect the heart by modulating GP neuron activity or synaptic inputs. Here, we review studies investigating glia both in the central and peripheral nervous systems to discuss the potential role of glia in controlling cardiac function in health and disease, paying particular attention to the glial cells of the ICNS.

Drosophila Voltage-Gated Calcium Channel α1-Subunits Regulate Cardiac Function in the Aging Heart

Ion channels maintain numerous physiological functions and regulate signaling pathways. They are the key targets for cellular reactive oxygen species (ROS), acting as signaling switches between ROS and ionic homeostasis. We have carried out a paraquat (PQ) screen in Drosophila to identify ion channels regulating the ROS handling and survival in Drosophila melanogaster. Our screen has revealed that α1-subunits (D-type, T-type, and cacophony) of voltage-gated calcium channels (VGCCs) handle PQ-mediated ROS stress differentially in a gender-based manner. Since ROS are also involved in determining the lifespan, we discovered that the absence of T-type and cacophony decreased the lifespan while the absence of D-type maintained a similar lifespan to that of the wild-type strain. VGCCs are also responsible for electrical signaling in cardiac cells. The cardiac function of each mutant was evaluated through optical coherence tomography (OCT), which revealed that α1-subunits of VGCCs are essential in maintaining cardiac rhythmicity and cardiac function in an age-dependent manner. Our results establish specific roles of α1-subunits of VGCCs in the functioning of the aging heart.

Reduced N-Type Ca2+ Channels in Atrioventricular Ganglion Neurons Are Involved in Ventricular Arrhythmogenesis

Background

Attenuated cardiac vagal activity is associated with ventricular arrhythmogenesis and related mortality in patients with chronic heart failure. Our recent study has shown that expression of N‐type Ca²⁺ channel α‐subunits (Ca_v2.2‐α) and N‐type Ca²⁺ currents are reduced in intracardiac ganglion neurons from rats with chronic heart failure. Rat intracardiac ganglia are divided into the atrioventricular ganglion (AVG) and sinoatrial ganglion. Ventricular myocardium receives projection of neuronal terminals only from the AVG. In this study we tested whether a decrease in N‐type Ca²⁺ channels in AVG neurons contributes to ventricular arrhythmogenesis.

Methods and Results

Lentiviral Ca_v2.2‐α shRNA (2 μL, 2×10⁷ pfu/mL) or scrambled shRNA was in vivo transfected into rat AVG neurons. Nontransfected sham rats served as controls. Using real‐time single‐cell polymerase chain reaction and reverse‐phase protein array, we found that in vivo transfection of Ca_v2.2‐α shRNA decreased expression of Ca_v2.2‐α mRNA and protein in rat AVG neurons. Whole‐cell patch‐clamp data showed that Ca_v2.2‐α shRNA reduced N‐type Ca²⁺ currents and cell excitability in AVG neurons. The data from telemetry electrocardiographic recording demonstrated that 83% (5 out of 6) of conscious rats with Ca_v2.2‐α shRNA transfection had premature ventricular contractions (P<0.05 versus 0% of nontransfected sham rats or scrambled shRNA‐transfected rats). Additionally, an index of susceptibility to ventricular arrhythmias, inducibility of ventricular arrhythmias evoked by programmed electrical stimulation, was higher in rats with Ca_v2.2‐α shRNA transfection compared with nontransfected sham rats and scrambled shRNA‐transfected rats.

Conclusions

A decrease in N‐type Ca²⁺ channels in AVG neurons attenuates vagal control of ventricular myocardium, thereby initiating ventricular arrhythmias.

Correlation of Ventricular Arrhythmogenesis with Neuronal Remodeling of Cardiac Postganglionic Parasympathetic Neurons in the Late Stage of Heart Failure after Myocardial Infarction

Introduction: Ventricular arrhythmia is a major cause of sudden cardiac death in patients with chronic heart failure (CHF). Our recent study demonstrates that N-type Ca²⁺ currents in intracardiac ganglionic neurons are reduced in the late stage of CHF rats. Rat intracardiac ganglia are divided into the atrioventricular ganglion (AVG) and sinoatrial ganglion. Only AVG nerve terminals innervate the ventricular myocardium. In this study, we tested the correlation of electrical remodeling in AVG neurons with ventricular arrhythmogenesis in CHF rats. Methods and Results: CHF was induced in male Sprague-Dawley rats by surgical ligation of the left coronary artery. The data from 24-h continuous radiotelemetry ECG recording in conscious rats showed that ventricular tachycardia/fibrillation (VT/VF) occurred in 3 and 14-week CHF rats but not 8-week CHF rats. Additionally, as an index for vagal control of ventricular function, changes of left ventricular systolic pressure (LVSP) and the maximum rate of left ventricular pressure rise (LV dP/dt_max) in response to vagal efferent nerve stimulation were blunted in 14-week CHF rats but not 3 or 8-week CHF rats. Results from whole-cell patch clamp recording demonstrated that N-type Ca²⁺ currents in AVG neurons began to decrease in 8-week CHF rats, and that there was also a significant decrease in 14-week CHF rats. Correlation analysis revealed that N-type Ca²⁺ currents in AVG neurons negatively correlated with the cumulative duration of VT/VF in 14-week CHF rats, whereas there was no correlation between N-type Ca²⁺ currents in AVG neurons and the cumulative duration of VT/VF in 3-week CHF. Conclusion: Malignant ventricular arrhythmias mainly occur in the early and late stages of CHF. Electrical remodeling of AVG neurons highly correlates with the occurrence of ventricular arrhythmias in the late stage of CHF.

Multiple nickel-sensitive targets elicit cardiac arrhythmia in isolated mouse hearts after pituitary adenylate cyclase-activating polypeptide-mediated chronotropy

The pituitary adenylate cyclase-activating polypeptide (PACAP)-27 modulates various biological processes, from the cellular level to function specification. However, the cardiac actions of this neuropeptide are still under intense studies. Using control (+|+) and mice lacking (-|-) either R-type (Ca_v2.3) or T-type (Ca_v3.2) Ca²⁺ channels, we investigated the effects of PACAP-27 on cardiac activity of spontaneously beating isolated perfused hearts. Superfusion of PACAP-27 (20nM) caused a significant increase of baseline heart frequency in Ca_v2.3(+|+) (156.9±10.8 to 239.4±23.4 bpm; p<0.01) and Ca_v2.3(-|-) (190.3±26.4 to 270.5±25.8 bpm; p<0.05) hearts. For Ca_v3.2, the heart rate was significantly increased in Ca_v3.2(-|-) (133.1±8.5 bpm to 204.6±27.9 bpm; p<0.05) compared to Ca_v3.2(+|+) hearts (185.7±11.2 bpm to 209.3±22.7 bpm). While the P wave duration and QTc interval were significantly increased in Ca_v2.3(+|+) and Ca_v2.3(-|-) hearts following PACAP-27 superfusion, there was no effect in Ca_v3.2(+|+) and Ca_v3.2(-|-) hearts. The positive chronotropic effects observed in the four study groups, as well as the effect on P wave duration and QTc interval were abolished in the presence of Ni²⁺ (50μM) and PACAP-27 (20nM) in hearts from Ca_v2.3(+|+) and Ca_v2.3(-|-) mice. In addition to suppressing PACAP's response, Ni²⁺ also induced conduction disturbances in investigated hearts. In conclusion, the most Ni²⁺-sensitive Ca²⁺ channels (R- and T-type) may modulate the PACAP signaling cascade during cardiac excitation in isolated mouse hearts, albeit to a lesser extent than other Ni²⁺-sensitive targets.

Nickel suppresses the PACAP-induced increase in guinea pig cardiac neuron excitability

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a potent intercellular signaling molecule involved in multiple homeostatic functions. PACAP/PAC1 receptor signaling increases excitability of neurons within the guinea pig cardiac ganglia, making them a unique system to establish mechanisms underlying PACAP modulation of neuronal function. Calcium influx is required for the PACAP-increased cardiac neuron excitability, although the pathway is unknown. This study tested whether PACAP enhancement of calcium influx through either T-type or R-type channels contributed to the modulation of excitability. Real-time quantitative polymerase chain reaction analyses indicated transcripts for Cav3.1, Cav3.2, and Cav3.3 T-type isoforms and R-type Cav2.3 in cardiac neurons. These neurons often exhibit a hyperpolarization-induced rebound depolarization that remains when cesium is present to block hyperpolarization-activated nonselective cationic currents (Ih). The T-type calcium channel inhibitors, nickel (Ni(2+)) or mibefradil, suppressed the rebound depolarization, and treatment with both drugs hyperpolarized cardiac neurons by 2-4 mV. Together, these results are consistent with the presence of functional T-type channels, potentially along with R-type channels, in these cardiac neurons. Fifty micromolar Ni(2+), a concentration that suppresses currents in both T-type and R-type channels, blunted the PACAP-initiated increase in excitability. Ni(2+) also blunted PACAP enhancement of the hyperpolarization-induced rebound depolarization and reversed the PACAP-mediated increase in excitability, after being initiated, in a subset of cells. Lastly, low voltage-activated currents, measured under perforated patch whole cell recording conditions and potentially flowing through T-type or R-type channels, were enhanced by PACAP. Together, our results suggest that a PACAP-enhanced, Ni(2+)-sensitive current contributes to PACAP-induced modulation of neuronal excitability.

Heart failure-induced changes of voltage-gated Ca2+ channels and cell excitability in rat cardiac postganglionic neurons

Chronic heart failure (CHF) is characterized by decreased cardiac parasympathetic and increased cardiac sympathetic nerve activity. This autonomic imbalance increases the risk of arrhythmias and sudden death in patients with CHF. We hypothesized that the molecular and cellular alterations of cardiac postganglionic parasympathetic (CPP) neurons located in the intracardiac ganglia and sympathetic (CPS) neurons located in the stellate ganglia (SG) possibly link to the cardiac autonomic imbalance in CHF. Rat CHF was induced by left coronary artery ligation. Single-cell real-time PCR and immunofluorescent data showed that L (Ca(v)1.2 and Ca(v)1.3), P/Q (Ca(v)2.1), N (Ca(v)2.2), and R (Ca(v)2.3) types of Ca2+ channels were expressed in CPP and CPS neurons, but CHF decreased the mRNA and protein expression of only the N-type Ca2+ channels in CPP neurons, and it did not affect mRNA and protein expression of all Ca2+ channel subtypes in the CPS neurons. Patch-clamp recording confirmed that CHF reduced N-type Ca2+ currents and cell excitability in the CPP neurons and enhanced N-type Ca2+ currents and cell excitability in the CPS neurons. N-type Ca2+ channel blocker (1 μM ω-conotoxin GVIA) lowered Ca2+ currents and cell excitability in the CPP and CPS neurons from sham-operated and CHF rats. These results suggest that CHF reduces the N-type Ca2+ channel currents and cell excitability in the CPP neurons and enhances the N-type Ca2+ currents and cell excitability in the CPS neurons, which may contribute to the cardiac autonomic imbalance in CHF.

Pharmacoresistant Cav 2·3 (E-type/R-type) voltage-gated calcium channels influence heart rate dynamics and may contribute to cardiac impulse conduction

Voltage-gated Ca(2+) channels regulate cardiac automaticity, rhythmicity and excitation-contraction coupling. Whereas L-type (Cav 1·2, Cav 1·3) and T-type (Cav 3·1, Cav 3·2) channels are widely accepted for their functional relevance in the heart, the role of Cav 2·3 Ca(2+) channels expressing R-type currents remains to be elucidated. We have investigated heart rate dynamics in control and Cav 2·3-deficient mice using implantable electrocardiogram radiotelemetry and pharmacological injection experiments. Autonomic block revealed that the intrinsic heart rate does not differ between both genotypes. Systemic administration of isoproterenol resulted in a significant reduction in interbeat interval in both genotypes. It remained unaffected after administering propranolol in Cav 2·3(-|-) mice. Heart rate from isolated hearts as well as atrioventricular conduction for both genotypes differed significantly. Additionally, we identified and analysed the developmental expression of two splice variants, i.e. Cav 2·3c and Cav 2·3e. Using patch clamp technology, R-type currents could be detected in isolated prenatal cardiomyocytes and be related to R-type Ca(2+) channels. Our results indicate that on the systemic level, the pharmacologically inducible heart rate range and heart rate reserve are impaired in Cav 2·3 (-|-) mice. In addition, experiments on Langendorff perfused hearts elucidate differences in basic properties between both genotypes. Thus, Cav 2·3 does not only contribute to the cardiac autonomous nervous system but also to intrinsic rhythm propagation.

Changes of calcium channel mRNA, protein and current in NG108-15 cells after cell differentiation

Based on the characteristics of differentiated NG108-15 cells (cell membrane excitability, acetylcholine release, and activities of choline acetyltransferase and acetylcholinesterase), NG108-15 cells are extensively used to explore neuronal functions as a cholinergic cell line. In the present study, differentiation-induced alterations of voltage-gated Ca(2+) channel mRNA, protein, and current were investigated in the NG108-15 cells. Real-time PCR, Western blot, and whole-cell patch-clamp data showed that differentiation caused mRNA, protein, and ion current changes of all Ca(2+) channel subunits. However, the changes of mRNA, protein, and ion current are inconsistent in all Ca(2+) channel subunits. Especially, P/Q- and R-type Ca(2+) channel proteins do not form the functional P/Q- and R-type Ca(2+) channels even if the mRNA and protein of P/Q- and R-type Ca(2+) channels can be detected in NG108-15 cells. These results indicate that differentiation can modulate gene transcription, protein translation, and post-translation of the Ca(2+) channels to induce the alteration of the Ca(2+) ion currents in NG108-15 cells. From these data, we understand that combining real-time PCR, Western blot, and patch-clamp techniques can comprehensively unveil the modulation of the Ca(2+) channels.

Alterations of calcium channels and cell excitability in intracardiac ganglion neurons from type 2 diabetic rats

Clinical study has demonstrated that patients with type 2 diabetes with attenuated arterial baroreflex have higher mortality rate compared with those without arterial baroreflex dysfunction. As a final pathway for the neural control of the cardiac function, functional changes of intracardiac ganglion (ICG) neurons might be involved in the attenuated arterial baroreflex in the type 2 diabetes mellitus (T2DM). Therefore, we measured the ICG neuron excitability and Ca(2+) channels in the sham and T2DM rats. T2DM was induced by a combination of both high-fat diet and low-dose streptozotocin (STZ, 30 mg/kg ip) injection. After 12-14 wk of the above treatment, the T2DM rats presented hyperglycemia, hyperlipidemia, and insulin resistance but no hyperinsulinemia, which closely mimicked the clinical features of the patients with T2DM. Data from immunofluorescence staining showed that L, N, P/Q, and R types of Ca(2+) channels were expressed in the ICG neurons, but only protein expression of N-type Ca(2+) channels was decreased in the ICG neurons from T2DM rats. Using whole cell patch-clamp technique, we found that T2DM significantly reduced the Ca(2+) currents and cell excitability in the ICG neurons. ω-Conotoxin GVIA (a specific N-type Ca(2+) channel blocker, 1 μM) lowered the Ca(2+) currents and cell excitability toward the same level in sham and T2DM rats. These results indicate that the decreased N-type Ca(2+) channels contribute to the suppressed ICG neuron excitability in T2DM rats. From this study, we think high-fat diet/STZ injection-induced T2DM might be an appropriate animal model to test the cellular and molecular mechanisms of cardiovascular autonomic dysfunction.

Expression profiles of high voltage-activated calcium channels in sympathetic and parasympathetic pelvic ganglion neurons innervating the urogenital system

Among the autonomic ganglia, major pelvic ganglia (MPG) innervating the urogenital system are unique because both sympathetic and parasympathetic neurons are colocalized within one ganglion capsule. Sympathetic MPG neurons are discriminated from parasympathetic ones by expression of low voltage-activated Ca2+ channels that primarily arise from T-type alpha1H isoform and contribute to the generation of low-threshold spikes. Until now, however, expression profiles of high voltage-activated (HVA) Ca2+ channels in these two populations of MPG neurons remain unknown. Thus, in the present study, we dissected out HVA Ca2+ channels using pharmacological and molecular biological tools. Reverse transcription-polymerase chain reaction analysis showed that MPG neurons contained transcripts encoding all of the known HVA Ca2+ channel isoforms (alpha1B, alpha1C, alpha1D and alpha1E), with the exception of alpha1A. Western blot analysis and pharmacology with omega-agatoxin IVA (1 microM) confirmed that MPG neurons lack the alpha1A Ca2+ channels. Unexpectedly, the expression profile of HVA Ca2+ channel isoforms was identical in the sympathetic and parasympathetic neurons of the MPG. Of the total Ca2+ currents, omega-conotoxin GVIA-sensitive N-type (alpha1B) currents constituted 57 +/- 5% (n = 9) and 60 +/- 3% (n = 6), respectively; nimodipine-sensitive L-type (alpha1C and alpha1D) currents made up 17 +/- 4% and 14 +/- 2%, respectively; and nimodipine-resistant and omega-conotoxin GVIA-resistant R-type currents were 25 +/- 3% and 22 +/- 2%, respectively. The R-type Ca2+ currents were sensitive to NiCl2 (IC50 = 22 +/- 0.1 microM) but not to SNX-482, which was able to potently (IC50 = 76 +/- 0.4 nM) block the recombinant alpha1E/beta2a/alpha2delta Ca2+ currents expressed in human embryonic kidney 293 cells. Taken together, our data suggest that sympathetic and parasympathetic MPG neurons share a similar but unique profile of HVA Ca2+ channel isoforms.

Ca2+ Influx, But Not Ca2+ Release From Internal Stores, Is Required for the PACAP-Induced Increase in Excitability in Guinea Pig Intracardiac Neurons

Mechanisms modulating the pituitary adenylate cyclase activating polypeptide (PACAP)-induced increase in excitability have been studied using dissociated guinea pig intrinsic cardiac neurons and intact ganglion preparations. Measurements of intracellular calcium (Ca2+) with the fluorescent Ca2+ indicator dye fluo-3 indicated that neither PACAP nor vasoactive intestinal polypeptide (VIP) at either 100 nM or 1 μM produced a discernible elevation of intracellular Ca2+ in dissociated intracardiac neurons. For neurons in ganglion whole mount preparations kept in control bath solution, local application of PACAP significantly increased excitability, as indicated by the number of action potentials generated by long depolarizing current pulses. However, in a Ca2+-deficient solution in which external Ca2+ was replaced by Mg2+ or when cells were bathed in control solution containing 200 μM Cd2+, PACAP did not enhance action potential firing. In contrast, in a Ca2+-deficient solution with Ca2+ replaced by strontium (Sr2+), PACAP increased excitability. PACAP increased excitability in cells treated with a combination of 20 μM ryanodine and 10 mM caffeine to interrupt release of Ca2+ from internal stores. Experiments using fluo-3 showed that ryanodine/caffeine pretreatment eliminated subsequent caffeine-induced Ca2+ release from intracellular stores, whereas exposure to the Ca2+-deficient solution did not. In dissociated intracardiac neurons voltage clamped with the perforated patch recording technique, 100 nM PACAP decreased the voltage-dependent barium current ( IBa). These results show that, in the guinea pig intracardiac neurons, the PACAP-induced increase in excitability apparently requires Ca2+ influx through Cd2+-sensitive calcium permeable channels other than voltage-dependent Ca2+ channels, but not Ca2+ release from internal stores.

Ablation of Cav2.3 / E?type voltage?gated calcium channel results in cardiac arrhythmia and altered autonomic control within the murine cardiovascular system

Voltage-gated calcium channels are key components in cardiac electrophysiology. We demonstrate that Ca(v)2.3 is expressed in mouse and human heart and that mice lacking the Ca(v)2.3 voltage-gated calcium channel exhibit severe alterations in cardiac function. Amplified cDNA fragments from murine heart and single cardiomyocytes reveal the expression of three different Ca(v)2.3 splice variants. The ablation of Ca(v)2.3 was found to be accompanied by a compensatory upregulation of the Ca(v)3.1 T-type calcium channel, while other voltage-gated calcium channels remained unaffected. Telemetric ECG recordings from Ca(v)2.3 deficient mice displayed subsidiary escape rhythm, altered atrial activation patterns, atrioventricular conduction disturbances and alteration in QRS-morphology. Furthermore, time domain analysis of heart rate variability (HRV) in Ca(v)2.3(-/-) mice exhibited a significant increase in heart rate as well as in the coefficient of variance (CV) compared to control mice. Administration of atropin/propranolol revealed that increased heart rate was due to enhanced sympathetic tonus and that partial decrease of CV in Ca(v)2.3(-/-) mice after autonomic block was in accordance with a complete abolishment of 2(nd) degree atrioventricular block. However, escape rhythms, atrial activation disturbances and QRS-dysmorphology remained unaffected, indicating that these are intrinsic cardiac features in Ca(v)2.3(-/-) mice. We conclude that the expression of Ca(v)2.3 is essential for normal impulse generation and conduction in murine heart.

Pituitary adenylate cyclase activating polypeptide (PACAP) decreases neuronal somatostatin immunoreactivity in cultured guinea-pig parasympathetic cardiac ganglia

Postganglionic parasympathetic neurons in guinea-pig cardiac ganglia exhibit choline acetyltransferase (ChAT)-immunoreactivity, and a large fraction (60%) of the ChAT-positive cardiac neurons co-express somatostatin-immunoreactivity. This co-expression remained when the cardiac ganglia explants were maintained in culture for 72 h (40% somatostatin-immunoreactive). The guinea-pig cardiac ganglia neurons express the high affinity pituitary adenylate cyclase activating polypeptide (PACAP)-selective PAC1 receptor, and treatment of the ganglia explants with 20 nM PACAP27 for 72 h to evaluate PACAP regulation of somatostatin expression revealed a dramatic 85% decrease in the number of somatostatin-IR neurons (6% somatostatin-IR neurons) compared with untreated control explant preparations. The decrease in percentage of somatostatin-IR neurons by PACAP27 was time- and concentration-dependent, and selective for PACAP27; PACAP38 and vasoactive intestinal polypeptide were less effective. PACAP6-38, a PACAP antagonist, eliminated the PACAP27-induced change in somatostatin positive neurons. The PACAP-mediated decrease in somatostatin-IR neurons was eliminated in calcium-deficient solutions and by the addition of nifedipine, indicating a requirement for calcium influx through L-type calcium channels. The addition of either the calmodulin inhibitor N-(4-aminobutyl)-1-naphthalenesulfonamide or the MEK inhibitor PD98059, also eliminated the PACAP27-induced decrease in somatostatin-IR cells. The PACAP27-mediated effect on somatostatin expression was not affected by inhibitors of protein kinase A or phospholipase C, but was reduced by the adenylyl cyclase inhibitor SQ22356, suggesting cAMP involvement. Semiquantitative and quantitative reverse transcription PCR prosomatostatin transcript measurements showed that cardiac ganglia prosomatostatin mRNA levels were not diminished by chronic PACAP27 exposure despite the dramatic decrement in somatostatin-expressing neurons. Neuronal peptide-IR content represents a balance between production and secretion. These results suggested that one of the primary effects of PACAP exposure may be enhanced levels of neuropeptide release that exceeded production levels, resulting in somatostatin depletion and a decrement in the number of identifiable somatostatin-expressing cardiac neurons.

The calcium channel ligand FPL 64176 enhances L-type but inhibits N-type neuronal calcium currents

One strategy for isolating neuronal L-type calcium (Ca(2+)) currents, which typically comprise a minority of the whole cell current in neurons, has been to use pharmacological agents that increase channel activity. This study examines the effects of the benzoyl pyrrole FPL 64176 (FPL) on L-type Ca(2+) currents and compares them to those of the dihydropyridine (+)-202-791. At micromolar concentrations, both agonists increased whole cell current amplitude in PC12 cells. However, FPL also significantly slowed the rate of activation and elicited a longer-lasting slow component of the tail current compared to (+)-202-791. In single channel cell-attached patch recordings, FPL increased open probability, first latency, mean closed time and mean open time more than (+)-202-791, with no difference in unitary conductance. These gating differences suggest that, compared to (+)-202-791, FPL decreases transition rates between open and closed conformations. Where examined, the actions of FPL and (+)-202-791 on whole cell L-type currents in sympathetic neurons appeared similar to those in PC12 cells. In contrast to its effects on L-type current, 10 microM FPL inhibited the majority of the whole cell current in HEK cells expressing a recombinant N-type Ca(2+) channel, raising caution concerning the use of FPL as a selective L-type Ca(2+) channel agonist in neurons.

Sigma receptors inhibit high-voltage-activated calcium channels in rat sympathetic and parasympathetic neurons

Studies on the expression and cellular function of sigma receptors in autonomic neurons were conducted in neonatal rat intracardiac and superior cervical (SCG) ganglia. Individual neurons from SCG and intracardiac ganglia were shown to express transcripts encoding the sigma-1 receptor using single-cell RT-PCR techniques. The relationship between sigma receptors and calcium channels was studied in isolated neurons of these ganglia under voltage-clamp mode using the perforated-patch configuration of the whole cell patch-clamp recording technique. Bath application of sigma receptor agonists was shown to rapidly depress peak calcium channel currents in a reversible manner in both SCG and intracardiac ganglion neurons. The inhibition of barium (I(Ba)) currents was dose-dependent, and half-maximal inhibitory concentration (IC50) values for haloperidol, ibogaine, (+)-pentazocine, and 1,3-Di-O-tolylguanidin (DTG) were 6, 31, 61, and 133 microM, respectively. The rank order potency of haloperidol > ibogaine > (+)-pentazocine > DTG is consistent with the effects on calcium channels being mediated by a sigma-2 receptor. Preincubation of neurons with the irreversible sigma receptor antagonist, metaphit, blocked DTG-mediated inhibition of Ca2+ channel currents. Maximum inhibition of calcium channel currents was > or =95%, suggesting that sigma receptors block all calcium channel subtypes found on the cell body of these neurons, which includes N-, L-, P/Q-, and R-type calcium channels. In addition to depressing peak Ca2+ channel current, sigma receptors altered the biophysical properties of these channels. Following sigma receptor activation, Ca2+ channel inactivation rate was accelerated, and the voltage dependence of both steady-state inactivation and activation shifted toward more negative potentials. Experiments on the signal transduction cascade coupling sigma receptors and Ca2+ channels demonstrated that neither cell dialysis nor intracellular application of 100 microM guanosine 5'-O-(2-thiodiphosphate) trilithium salt (GDP-beta-S) abolished the modulation of I(Ba) by sigma receptor agonists. These data suggest that neither a diffusible cytosolic second messenger nor a G protein is involved in this pathway. Activation of sigma receptors on sympathetic and parasympathetic neurons is likely to modulate cell-to-cell signaling in autonomic ganglia and thus the regulation of cardiac function by the peripheral nervous system.

Inhibition of human alpha1E subunit-mediated ca2+ channels by the antipsychotic agent chlorpromazine

Chlorpromazine is a neuroleptic antipsychotic agent with a long history of clinical use. Its primary mode of action is thought to be through modulation of monoaminergic inter-neuronal communication; however, its side-effect profile indicates substantial activities in other systems. Recent work has begun to uncover actions of this compound on ion channels. In this light we have investigated the actions of chlorpromazine on the recombinant alpha1E subunit-encoded voltage-sensitive Ca2+ channel (VSCC) that is believed to encode drug-resistant R-type currents found in neurones and other cells. Chlorpromazine produced a dose-dependent antagonism of these channels that was reversed on drug removal. The mean IC50 was close to 10 microM. At this concentration, the level of antagonism observed was dependent on the membrane potential, with greater inhibition being observed at more negative test potentials. Furthermore, chlorpromazine induced substantial changes in the steady-state inactivation properties of alpha1Ebeta3-mediated currents, although it was not seen to elicit a corresponding change in inactivation kinetics. These results are discussed with regard to the possible clinical mechanisms of chlorpromazine actions.

Analysis and neuronal modeling of the nonlinear characteristics of a local cardiac reflex in the rat

Previous experimental results have suggested the existence of a local cardiac reflex in the rat. In this study, the putative role of such a local reflex in cardiovascular regulation is quantitatively analyzed. A model for the local reflex is developed from anatomical experimental results and physiological data in the literature. Using this model, a systems-level analysis is conducted. Simulation results indicate that the neuromodulatory mechanism of the local reflex attenuates the nonlinearity of the relationship between cardiac vagal drive and arterial pressure. This behavior is characterized through coherence analysis. Furthermore, the modulation of phase-related characteristics of the cardiovascular system is suggested as a plausible mechanism for the nonlinear attenuation. Based on these results, it is plausible that the functional role of the local reflex is highly robust nonlinear compensation at the heart, which results in less complex dynamics in other parts of the reflex.

Developmental changes in hyperpolarization-activated currents I(h) and I(K(IR)) in isolated rat intracardiac neurons

The hyperpolarization-activated nonselective cation current, I(h), was investigated in neonatal and adult rat intracardiac neurons. I(h) was observed in all neurons studied and displayed slow time-dependent rectification. I(h) was isolated by blockade with external Cs(+) (2 mM) and was inhibited irreversibly by the bradycardic agent, ZD 7288. Current density of I(h) was approximately twofold greater in neurons from neonatal (-4.1 pA/pF at -130 mV) as compared with adult (-2.3 pA/pF) rats; however, the reversal potential and activation parameters were unchanged. The reversal potential and amplitude of I(h) was sensitive to changes in external Na(+) and K(+) concentrations. An inwardly rectifying K(+) current, I(K(IR)), was also present in intracardiac neurons from adult but not neonatal rats and was blocked by extracellular Ba(2+). I(K(IR)) was present in approximately one-third of the adult intracardiac neurons studied, with a current density of -0.6 pA/pF at -130 mV. I(K(IR)) displayed rapid activation kinetics and no time-dependent rectification consistent with the rapidly activating, inward K(+) rectifier described in other mammalian autonomic neurons. I(K(IR)) was sensitive to changes in external K(+), whereby raising the external K(+) concentration from 3 to 15 mM shifted the reversal potential by approximately +36 mV. Substitution of external Na(+) had no effect on the reversal potential or amplitude of I(K(IR)). I(K(IR)) density increases as a function of postnatal development in a population of rat intracardiac neurons, which together with a concomitant decrease in I(h) may contribute to changes in the modulation of neuronal excitability in adult versus neonatal rat intracardiac ganglia.

Functional expression of L-, N-, P/Q-, and R-type calcium channels in the human NT2-N cell line

The biophysical and pharmacological properties of voltage-gated calcium channel currents in the human teratocarcinoma cell line NT2-N were studied using the whole cell patch-clamp technique. When held at -80 mV, barium currents (I(Ba)s) were evoked by voltage commands to above -35 mV that peaked at +5 mV. When holding potentials were reduced to -20 mV or 5 mM barium was substituted for 5 mM calcium, there was a reduction in peak currents and a right shift in the current-voltage curve. A steady-state inactivation curve for I(Ba) was fit with a Boltzmann curve (V(1/2) = -43.3 mV; slope = -17.7 mV). Maximal current amplitude increased from 1-wk (232 pA) to 9-wk (1025 pA) postdifferentiation. Whole cell I(Ba)s were partially blocked by specific channel blockers to a similar extent in 1- to 3-wk and 7- to 9-wk postdifferentiation NT2-N cells: 10 microM nifedipine (19 vs. 25%), 10 microM conotoxin GVIA (27 vs. 25%), 10 microM conotoxin MVIIC (15 vs. 16%), and 1.75 microM SNX-482 (31 vs. 33%). Currents were completely blocked by 300 microM cadmium. In the presence of nifedipine, GVIA, and MVIIC, approximately 35% of current remained, which was reduced further by SNX-482 (7-14% of current remained), consistent with functional expression of L-, N-, and P/Q-calcium channel types and one or more R-type channel. The presence of multiple calcium currents in this human neuronal-type cell line provides a potentially useful model for study of the regulation, expression and cellular function of human derived calcium channel currents; in particular the R-type current(s).

T-type calcium currents in rat cardiomyocytes during postnatal development: contribution to hormone secretion

T-type Ca(2+) channels have been suggested to play a role in cardiac automaticity, cell growth, and cardiovascular remodeling. Although three genes encoding for a T-type Ca(2+) channel have been identified, the nature of the isoform(s) supporting the cardiac T-type Ca(2+) current (I(Ca,T)) has not yet been determined. We describe the postnatal evolution of I(Ca,T) density in freshly dissociated rat atrial and ventricular myocytes and its functional properties at peak current density in young atrial myocytes. I(Ca,T) displays a classical low activation threshold, rapid inactivation kinetics, negative steady-state inactivation, slow deactivation, and the presence of a window current. Interestingly, I(Ca,T) is poorly sensitive to Ni(2+) and insensitive to R-type current toxin SNX-482. RT-PCR experiments and comparison of functional properties with recombinant Ca(2+) channel subtypes suggest that neonatal I(Ca,T) is related to the alpha(1G)-subunit. Atrial natriuretic factor (ANF) secretion was measured using peptide radioimmunoassays in atrial tissue. Pharmacological dissection of ANF secretion indicates an important contribution of I(Ca,T) to Ca(2+) signaling during the neonatal period.

VIP and PACAP potentiation of nicotinic ACh-evoked currents in rat parasympathetic neurons is mediated by G-protein activation

The effects of vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP27 and PACAP38) on isolated parasympathetic neurons of rat intracardiac and submandibular ganglia were examined under voltage clamp using whole-cell patch-clamp recording techniques. VIP and PACAP (</= 10 nM) selectively and reversibly increased the affinity of nicotinic acetylcholine receptor channels (nAChRs) for their agonists resulting in a potentiation of acetylcholine (ACh)-evoked whole-cell currents at low agonist concentrations. VIP-induced potentiation was observed with either ACh or nicotine as the cholinergic agonist. The VIP- but not the PACAP-induced potentiation of ACh-evoked currents was inhibited by [Ac-Tyr1, D-Phe2]-GRF 1-29, amide (100 nM), a selective antagonist of VPAC1 and VPAC2 receptors; whereas the PACAP38- but not the VIP-induced potentiation was inhibited by 100 nM PACAP6-38, a PAC1 and VPAC2 receptor antagonist. The signal transduction pathway mediating VIP- and PACAP-induced potentiation of nicotinic ACh-evoked currents involves a pertussis toxin (PTX)-sensitive G-protein. Intracellular application of 200 microM GTPgammaS or GDPbetaS inhibited VIP-induced potentiation of ACh-evoked whole-cell currents. GTPgammaS alone potentiated ACh- and nicotine-evoked currents and the magnitude of these currents was not further increased by VIP or PACAP. The G-protein subtype modulating the neuronal nAChRs was examined by intracellular dialysis with antibodies directed against alphao, alphai-1,2, alphai-3 or beta G-protein subunits. Only the anti-Galphao and anti-Gbeta antibodies significantly inhibited the effect of VIP and PACAP on ACh-evoked currents. The potentiation of ACh-evoked currents by VIP and PACAP may be mediated by a membrane-delimited signal transduction cascade involving the PTX-sensitive Go protein.

Characterization of calcium currents in functionally mature mouse spinal motoneurons

Motoneurons integrate synaptic input and produce output in the form of trains of action potentials such that appropriate muscle contraction occurs. Motoneuronal calcium currents play an important role in the production of this repetitive firing. Because these currents change in the postnatal period, it is necessary to study them in animals in which the motor system is 'functionally mature', that is, animals that are able to weight-bear and walk. In this study, calcium currents were recorded using whole-cell patch-clamp techniques from large (> 20 microm) ventral horn cells in lumbar spinal cord slices prepared from mature mice. Ninety percent (nine out of 10) of the recorded cells processed for choline acetyltransferase were found to be cholinergic, confirming their identity as motoneurons. A small number of motoneurons were found to have currents with low-voltage-activated (T-type) characteristics. Pharmacological dissection of the high-voltage-activated current demonstrated omega-agatoxin-TK- (P/Q-type), omega-conotoxin GVIA- (N-type), and dihydropyridine- and FPL-64176-sensitive (L-type) components. A cadmium-sensitive component of the current that was insensitive to these chemicals (R-type) was also seen in these cells. These results indicate that the calcium current in lumbar spinal motoneurons from functionally mature mice is mediated by a number of different channel subtypes. The characterization of these calcium channels in mature mammalian motoneurons will allow for the future study of their modulation and their roles during behaviours such as locomotion.

Determinants of voltage-dependent inactivation affect Mibefradil block of calcium channels

The voltage gated calcium channel family is a major target for a range of therapeutic drugs. Mibefradil (Ro 40-5967) belongs to a new chemical class of these molecules which differs from other Ca2+ antagonists by its ability to potently block T-type Ca2+ channels. However, this molecule has also been shown to inhibit other Ca2+ channel subtypes. To further analyze the mechanism governing the Ca2+ channel-Mibefradil interaction, we examined the effect of Mibefradil on various recombinant Ca2+ channels expressed in mammalian cells from their cloned cDNAs, using Ca2+ as the permeant ion at physiological concentration. Expression of alpha1A, alpha1C, and alpha1E in tsA 201 cells resulted in Ca2+ currents with functional characteristics closely related to those of their native counterparts. Mibefradil blocked alpha1A and alpha1E with a Kd comparable to that reported for T-type channels, but had a lower affinity (approximately 30-fold) for alpha1C. For each channel, inhibition by Mibefradil was consistent with high-affinity binding to the inactivated state. Modulation of the voltage-dependent inactivation properties by the nature of the coexpressed beta subunit or the alpha1 splice variant altered block at the Mibefradil receptor site. Therefore, we conclude that the tissue and sub-cellular localization of calcium channel subunits as well as their specific associations are essential parameters to understand the in vivo effects of Mibefradil.

L-Type calcium channels in enterochromaffin cells from guinea pig and human duodenal crypts: an in situ study

BACKGROUND & AIMS

This study has investigated stimulus-secretion coupling of enterochromaffin cells by studying the cellular location and function of voltage-gated Ca(2+) channels within small intestinal crypts.

METHODS

Digital fluorescence imaging and electrochemical detection were used to measure intracellular Ca(2+) responses and serotonin (5-hydroxytryptamine [5-HT]) secretion in intact crypts isolated from guinea pig and human duodenum.

RESULTS

In fluo-3-loaded crypts, electrical depolarization with high K(+) solution increased cytosolic free [Ca(2+)] only in single cells subsequently identified by immunocytochemistry as enterochromaffin cells. In guinea pig enterochromaffin cells, the L-type Ca(2+) channel agonist FPL 64176 (3 micromol/L) did not change resting intracellular [Ca(2+)] but potentiated the depolarization-evoked increase in [Ca(2+)] (298 +/- 72 nmol/L) by 19 +/- 3-fold. In the majority of human enterochromaffin cells, FPL 64176 alone increased resting [Ca(2+)] by 423 +/- 171 nmol/L. Secretion studies in guinea pig crypts showed that high K(+) and FPL 64176 caused a 12-fold increase in 5-HT release. Noradrenaline caused increases in both enterochromaffin cell [Ca(2+)] and 5-HT release.

CONCLUSIONS

Using this approach, we have found that in duodenal crypts, enterochromaffin cells, but not other epithelial cells, contain L-type voltage-gated Ca(2+) channels involved in regulating 5-HT secretion. These data have implications for the pharmacological control of intestinal disorders involving enterochromaffin cell dysfunction.

Activation of various G-protein coupled receptors modulates Ca2+ channel currents via PTX-sensitive and voltage-dependent pathways in rat intracardiac neurons

In the present study, we examined the ability of several putative neurotransmitters and neuromodulators to modulate voltage-dependent Ca2+ channel currents in adult rat intracardiac neurons. Of 17 compounds tested, acetylcholine (Ach), neuropeptide Y (NPY), norepinephrine (NE), and met-enkephalin (met-Enk) were effective modulators of the Ca2+ currents. The neurotransmitter-induced current inhibition was associated with slow activation kinetics and relief by a strong depolarizing prepulse. Overnight pretreatment of neurons with pertussis toxin (PTX, 500 ng/ml) significantly attenuated the neurotransmitter-induced current inhibition. Heterologous expression of transducin, a known chelator of G-protein betagamma subunits, almost completely abolished the neurotransmitter-induced current inhibition. Taken together, our data suggest that four different neurotransmitters inhibit the Ca2+ channel currents in adult rat intracardiac neurons via a pathway that is voltage-dependent, membrane-delimited, and utilizes betagamma subunits released from PTX-sensitive G-proteins. The Ca2+ channel inhibition by non-cholinergic neurotransmitters may play important roles in regulation of neuronal excitability and Ach release at synapses in intracardiac ganglia, thereby contributing to cholinergic control of cardiac functions.

Calcium currents of rhythmic neurons recorded in the isolated respiratory network of neonatal mice

To obtain a quantitative characterization of voltage-activated calcium currents in respiratory neurons, we performed voltage-clamp recordings in the transverse brainstem slice of mice from neurons located within the ventral respiratory group. It is assumed that this medullary region contains the neuronal network responsible for generating the respiratory rhythm. This study represents one of the first attempts to analyze quantitatively the currents in respiratory neurons. The inward calcium currents of VRG neurons consisted of two components: a high voltage-activated (HVA) and a low voltage-activated (LVA) calcium current. The activation threshold of the HVA current was at -40 mV. It was fully activated (peak voltage) between -10 and 0 mV. The half-maximal activation (V50) was at -27. 29 mV +/- 1.15 (n = 24). The HVA current was inactivated completely at a holding potential of -35 mV and fully deinactivated at a holding potential of -65 mV (V50, -52.26 mV +/- 0.27; n = 18). The threshold for the activation of the LVA current was at -65 mV. This current had its peak voltage between -50 and -40 mV (mean, V50 = -59. 15 mV +/- 0.21; n = 15). The LVA current was inactivated completely at a holding potential of -65 mV and deinactivated fully at a holding potential of -95 mV (mean, V50 = -82.40 mV +/- 0.32; n = 38). These properties are consistent with other studies suggesting that the LVA current is a T-type current. The properties of these inward currents are discussed with respect to their role in generating Ca2+ potentials that may contribute to the generation of the mammalian respiratory rhythm.

Pituitary adenylate cyclase-activating polypeptide expression and modulation of neuronal excitability in guinea pig cardiac ganglia

Cardiac output is regulated by the coordinate interactions of stimulatory sympathetic and inhibitory parasympathetic signals. Intracardiac parasympathetic ganglia are integrative centers of cardiac regulation, and modulation of the parasympathetic drive on the heart is accomplished by altering intrinsic cardiac ganglion neuron excitability. The pituitary adenylate cyclase-activating polypeptide (PACAP)/vasoactive intestinal peptide (VIP) family of peptides modulates cardiac function, and in guinea pig heart, PACAP appears to act directly on intrinsic parasympathetic cardiac ganglia neurons through PACAP-selective receptors. A multidisciplinary project tested whether cardiac PACAP peptides act through PACAP-selective receptors as excitatory neuromodulators amplifying the parasympathetic inhibition from guinea pig cardiac ganglia. The in vivo sources of regulatory PACAP peptides were localized immunocytochemically to neuronal fibers and a subpopulation of intrinsic postganglionic cardiac neurons. RT-PCR confirmed that cardiac ganglia expressed proPACAP transcripts and have PACAP peptide biosynthetic capabilities. Messenger RNA encoding PACAP-selective PAC1 receptor isoforms were also present in cardiac ganglia. Alternative splicing of PAC1 receptor transcripts produced predominant expression of the very short variant with neither HIP nor HOP cassettes; lower levels of the PAC1HOP2 receptor mRNA were present. Almost all of the parasympathetic neurons expressed membrane-associated PAC1 receptor proteins, localized immunocytochemically, which correlated with the population of cells that responded physiologically to PACAP peptides. PACAP depolarized cardiac ganglia neurons and increased neuronal membrane excitability. The rank order of peptide potency on membrane excitability in response to depolarizing currents was PACAP27>PACAP38>VIP. The PACAP-induced increase in excitability was not a function of membrane depolarization nor was it caused by alterations in action potential configuration. These results support roles for PACAP peptides as integrative modulators amplifying, through PACAP-selective receptors, the parasympathetic cardiac ganglia inhibition of cardiac output.

Stellate neurones innervating the rat heart express N, L and P/Q calcium channels

The aim of the study was to investigate the kinetic properties and identify the subtypes of Ca2+ currents in the cardiac postganglionic sympathetic neurones of rats. Neurones were labelled with a fluorescent tracer--Fast-Blue, injected into the pericardial cavity. Voltage-dependent Ca2+ currents were recorded from dispersed stellate ganglion cells that showed Fast Blue labelling. Only high threshold voltage-dependent Ca2+ currents were found in the somata of cardiac sympathetic neurones. Their maximum amplitude, mean cell capacitance and current density were respectively: 0.67 nA, 19.3 pF and 36.4 pA/pF (n = 21). The maximum Ca2+ conductance was 51.3 nS (n = 14). Half activation voltage equalled +11.0 mV and the slope factor for conductance 11.1 (n = 14). As tested with a 10 s pre-pulse, the Ca2+ current began to inactivate at -80 mV. Half inactivation voltage and slope factor for steady-state inactivation were -36.6 mV and 14.1 (n = 9), respectively. Saturating concentration of L channel blocker (nifedipine), N channel blocker (omega-conotoxin-GVIA), P/Q channel blocker (omega-Agatoxin-IVA) and N/P/Q channel blocker (omega-conotoxin-MVIIC) reduced the total Ca2+ current by 26.8% (n = 7), 57.1% (n = 12), 25.9% (n = 6) and 69.4% (n = 6), respectively. These results show that the somata of cardiac postganglionic cardiac sympathetic neurones contain significant populations of N, L and P/Q high threshold Ca2+ channels.

Ca2+ and K+ currents regulate accommodation and firing frequency in guinea pig bronchial ganglion neurons

Intracellular microelectrode recordings were obtained from neurons located in adult guinea pig bronchial parasympathetic ganglia in situ to determine the calcium and potassium currents regulating repetitive action potential activity and firing rates by these neurons. Neurons in these ganglia respond to prolonged suprathreshold depolarizing current steps with either a burst of action potentials at the onset of the stimulus (accommodating or phasic neurons) or repetitive action potentials throughout the stimulus (nonaccommodating or tonic neurons). Instantaneous and adapted firing rates during prolonged threshold and suprathreshold stimuli were lower in tonic than in phasic neurons, indicating a longer interspike interval between repetitive action potentials in tonic neurons. In tonic neurons, blockade of A-type current with 4-aminopyridine increased accommodation; 4-aminopyridine or apamin decreased the interspike interval in tonic neurons. Calcium-free buffer, cadmium ions, or omega-conotoxin GVIA also increased accommodation in tonic neurons but did not affect the interspike interval; nifedipine or verapamil did not affect the tonic firing pattern. Accommodation in phasic neurons could be decreased by a conditioning hyperpolarization step of the resting potential, which could be subsequently blocked by 4-aminopyridine or calcium-free buffer. Accommodation in phasic neurons could also be decreased by apamin or barium ions: the repetitive action potentials observed during these treatments could be reversed by cadmium ions or calcium-free buffer. These results indicate that tonic and phasic neurons in guinea pig bronchial parasympathetic ganglia have similar types of calcium currents, but potassium channels may ultimately regulate the accommodation pattern, the firing rate, and, consequently, the output from these neurons.

Opioid receptor-mediated inhibition of omega-conotoxin GVIA-sensitive calcium channel currents in rat intracardiac neurons

Modulation of depolarization-activated ionic conductances by opioid receptor agonists was investigated in isolated parasympathetic neurons from neonatal rat intracardiac ganglia by using the whole cell perforated patch clamp technique. Met-enkephalin (10 muM) altered the action potential waveform, reducing the maximum amplitude and slowing the rate of rise and repolarization but the afterhyperpolarization was not appreciably altered. Under voltage clamp, 10 muM Met-enkephalin selectively and reversibly inhibited the peak amplitude of high-voltage-activated Ca2+ channel currents elicited at 0 mV by approximately 52% and increased three- to fourfold the time to peak. Met-enkephalin had no effect on the voltage dependence of steady-state inactivation but shifted the voltage dependence of activation to more positive membrane potentials whereby stronger depolarization was required to open Ca2+ channels. Half-maximal inhibition of Ba2+ current (IBa) amplitude was obtained with 270 nM Met-enkephalin or Leu-enkephalin. The opioid receptor subtype selective agonists, DAMGO and DADLE, but not DPDPE, inhibited IBa and were antagonized by the opioid receptor antagonists, naloxone and naltrindole with IC50s of 84 nM and 1 muM, respectively. The kappa-opioid receptor agonists, bremazocine and dynorphin A, did not affect Ca2+ channel current amplitude or kinetics. Taken together, these data suggest that enkephalin-induced inhibition of Ca2+ channels in rat intracardiac neurons is mediated primarily by the mu-opioid receptor type. Addition of Met-enkephalin after exposure to 300 nM omega-conotoxin GVIA, which blocked approximately 75% of the total Ca2+ channel current, failed to cause a further decrease of the residual current. Met-enkephalin inhibited the omega-conotoxin GVIA-sensitive but not the omega-conotoxin-insensitive IBa in rat intracardiac neurons. Dialysis of the cell with a GTP-free intracellular solution or preincubation of the neurons in Pertussis toxin (PTX) abolished the attenuation of IBa by Met-enkephalin, suggesting the involvement of a PTX-sensitive Gprotein in the signal transduction pathway. The activation of mu-opioid receptors and subsequent inhibition of N-type Ca2+ channels in the soma and terminals of postganglionic intracardiac neurons is likely to inhibit the release of ACh and thereby regulate vagal transmission to the mammalian heart.

Muscarinic receptor activation modulates Ca2+ channels in rat intracardiac neurons via a PTX- and voltage-sensitive pathway

With use of the whole cell patch-clamp technique, effects of the potent muscarinic agonist oxotremorine methiodide (oxo-M) on voltage-activated Ca2+ channel currents were investigated in acutely dissociated adult rat intracardiac neurons. In all tested neurons oxo-M reversibly inhibited the peak Ba2+ current. Inhibition of the peak Ba2+ current by oxo-M was associated with slowing of activation kinetics and was concentration dependent. The concentration of oxo-M necessary to produce a half-maximal inhibition of current and the maximal inhibition were 40.8 nM and 75.9%, respectively. Inhibitory effect of oxo-M was completely abolished by atropine. Among different muscarinic receptor antagonists, methoctramine (100 and 300 nM) significantly antagonized the current inhibition by oxo-M, with a negative logarithm of dissociation constant of 8.3 in adult rat intracardiac neurons. Internal dialysis of neurons with guanosine 5'-(thio)triphosphate (GTPgammaS, 0.5 mM) could mimic the muscarinic inhibition of the peak Ba2+ current and significantly occlude inhibitory effects of oxo-M. In addition, the internal dialysis of guanosine-5'-O-(2-thiodiphosphate) (GDPbetaS, 2 mM) also significantly reduced the muscarinic inhibition of the peak Ba2+ current by oxo-M. Inhibitory effects of oxo-M were significantly abolished by pertussis toxin (PTX, 200 and 400 ng/ml) but not by cholera toxin (400 ng/ml). Furthermore, the bath application of N-ethylmaleimide (50 microM) significantly reduced the inhibition of the peak Ba2+ current by oxo-M. The oxo-M shifted the activation curve derived from measurments of tail currents toward more positive potentials. A strong conditioning prepulse to +100 mV significantly relieved the muscarinic inhibition of peak Ba2+ currents by oxo-M and the GTPgammaS-induced current inhibition. In a series of experiments, changes in intracellular concentration of bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid and protein kinase activities failed to mimic or occlude the current inhibition by oxo-M. The dihydropyridine antagonist nifedipine (10 microM) was not able to occlude any of the inhibitory effects of oxo-M, and oxo-M (3 microM) failed to reduce the slow tail currents induced by the L-type agonist methyl 2,5-dimethyl-4-[2-(phenylmethyl)benzoyl]-1H-pyrrole-3-carboxylate (FPL 64176; 2 microM). However, omega-conotoxin (omega-CgTX) GVIA (1 microM) significantly occluded the muscarinic inhibition of the Ba2+ currents. In the presence of omega-CgTX GVIA (1 microM) and nifedipine (10 microM), oxo-M could further inhibit approximately 20% of the total Ca2+ current. After complete removal of N-, Q-, and L-type currents with use of omega-CgTX GVIA, omega-agatoxin IVA, and nifedipine, 70% of the R-type current (approximately 6-7% of the total current) was inhibited by oxo-M (3 microM). In conclusion, the M2 muscarinic receptor activation selectively inhibits N-, Q-, and R-type Ca2+ channel currents, sparing L-type Ca2+ channel currents mainly via a PTX- and voltage-sensitive pathway in adult rat intracardiac neurons.