Maternal treatment with a selective delta-opioid receptor agonist during gestation has a sex-specific pro-cognitive action in offspring: mechanisms involved

Background: There is growing evidence that the treatment of several mental disorders can potentially benefit from activation of delta-opioid receptors. In the future, delta-agonists with a safe pharmacological profile can be used for the treatment of mood disorders in pregnant women. However, the data on prenatal exposure to delta-opioid agonists are missing. The present study is aimed to test the hypothesis that the activation of delta-opioid receptors during gravidity has positive effects on the behaviour accompanied by changes in glutamate and monoamine neurotransmission. Methods: Gestating Wistar rats were chronically treated with a selective delta-agonist SNC80 or vehicle. Adult male and female offspring underwent novel object recognition (for the assessment of cognition) and open field (for the assessment of anxiety and habituation) tests, followed by in vivo electrophysiological examination of the activity of hippocampal glutamate and midbrain serotonin (5-HT) and dopamine neurons. Results: We found that the maternal treatment with SNC80 did not affect the offspring's anxiety, habituation, and 5-HT neuronal firing activity. Female offspring of SNC80-treated dams exhibited improved novelty recognition associated with decreased firing rate and burst activity of glutamate and dopamine neurons. Conclusion: Maternal treatment with delta-opioid agonists during gestation may have a pro-cognitive effect on offspring without any negative effects on anxiety and habituation. The putative pro-cognitive effect might be mediated via mechanism(s) involving the firing activity of hippocampal glutamate and mesolimbic dopamine neurons.

Effects of the acute administration of delta-opioid receptor ligands on the excitability of rat hippocampal glutamate and brainstem monoamine neurons in vivo

It was previously reported that the delta opioid receptor (DOR) agonist SNC80 and antagonist naltrindole modulate the excitability of hippocampal glutamate neurons in primary cultures. The present study aimed to investigate the acute effects of these ligands on the firing activity of hippocampal cornu ammonis 1/3 (CA1/3) glutamate, dorsal raphe nucleus (DRN) serotonin (5-HT), locus coeruleus (LC) noradrenaline, and ventral tegmental area (VTA) dopamine neurons in in vivo conditions. Adult Wistar male rats were used. SNC80 and naltrindole were administered intravenously. Neuronal firing activity was assessed using extracellular single-unit electrophysiology. SNC80, administered first at 1-3 mg/kg, dose-dependently inhibited CA1/3 glutamate, DRN 5-HT, and VTA dopamine neurons. Naltrindole, administered at 1-3 mg/kg after SNC80, did not have any additional effect. Naltrindole, administered first at 1-3 mg/kg, stimulated DRN 5-HT neurons in a dose-dependent manner; this stimulation was dose-dependently reversed by 1-3 mg/kg of SNC80. SNC80 and naltrindole inhibited LC noradrenaline neurons when only they were co-administered at 3 mg/kg, and only when SNC80 was administered first. In conclusion, DOR ligands alter the firing activity of hippocampal glutamate and brainstem monoamine neurons in in vivo conditions. The psychoactive effects of DOR ligands, reported in previous studies, might be explained, at least in part, by their ability to modulate the firing activity of hippocampal glutamate and brainstem monoamine neurons.

Effects of chronic delta-opioid receptor agonist on the excitability of hippocampal glutamate and brainstem monoamine neurons, anxiety, locomotion, and habituation in rats

BACKGROUND

Short-term treatment with non-peptide agonists of delta-opioid receptors, such as agonist SNC80, induced behavioral effects in rodents, which could be modulated via changes in central neurotransmission. The present experiments aimed at testing the hypothesis that chronic treatment with SNC80 induces anxiolytic effects associated with changes in hippocampal glutamate and brainstem monoamine pathways.

METHODS

Adult male Wistar rats were used in experiments. Rats were treated with SNC80 (3 mg/kg/day) for fourteen days. Neuronal excitability was assessed using extracellular in vivo single-unit electrophysiology. The behavioral parameters were examined using the elevated plus maze and open field tests.

RESULTS

Chronic SNC80 treatment increased the excitability of hippocampal glutamate and ventral tegmental area dopamine neurons and had no effect on the firing activity of dorsal raphe nucleus serotonin cells. Chronic SNC80 treatment induced anxiolytic effects, which were, however, confounded by increased locomotor activity clearly confirmed in an open field test. The ability to cope with stressful situations and habituation processes in a novel environment was not influenced by chronic treatment with SNC80.

CONCLUSION

Our study suggests that the psychoactive effects of SNC80 might be explained by its ability to stimulate hippocampal glutamate and mesolimbic dopamine transmission.

Stress, depression, and hippocampus: from biochemistry to electrophysiology

Major depressive disorder is a very common serious mental illness with increasing prevalence in the population. Its pathology includes biochemical, morphological, and electrophysiological changes in various brain areas. In spite of decades of extensive research pathophysiology of depression is still not sufficiently understood. When depression occurs just before or during pregnancy, it may have a detrimental effect on perinatal and/or postnatal brain development, affecting the offspring's behavior. An important role in the pathology of depression is the hippocampus as a center for cognition and memory. Here we review changes in morphology, biochemical, and electrical signaling caused by depression in first and second generation identified in various animal models.

Clozapine Reverses Dysfunction of Glutamatergic Neurons Derived From Clozapine-Responsive Schizophrenia Patients

The cellular pathology of schizophrenia and the potential of antipsychotics to target underlying neuronal dysfunctions are still largely unknown. We employed glutamatergic neurons derived from induced pluripotent stem cells (iPSC) obtained from schizophrenia patients with known histories of response to clozapine and healthy controls to decipher the mechanisms of action of clozapine, spanning from molecular (transcriptomic profiling) and cellular (electrophysiology) levels to observed clinical effects in living patients. Glutamatergic neurons derived from schizophrenia patients exhibited deficits in intrinsic electrophysiological properties, synaptic function and network activity. Deficits in K⁺ and Na⁺ currents, network behavior, and glutamatergic synaptic signaling were restored by clozapine treatment, but only in neurons from clozapine-responsive patients. Moreover, neurons from clozapine-responsive patients exhibited a reciprocal dysregulation of gene expression, particularly related to glutamatergic and downstream signaling, which was reversed by clozapine treatment. Only neurons from clozapine responders showed return to normal function and transcriptomic profile. Our results underscore the importance of K⁺ and Na⁺ channels and glutamatergic synaptic signaling in the pathogenesis of schizophrenia and demonstrate that clozapine might act by normalizing perturbances in this signaling pathway. To our knowledge this is the first study to demonstrate that schizophrenia iPSC-derived neurons exhibit a response phenotype correlated with clinical response to an antipsychotic. This opens a new avenue in the search for an effective treatment agent tailored to the needs of individual patients.

Secretory carrier-associated membrane protein 2 (SCAMP2) regulates cell surface expression of T-type calcium channels

Low-voltage-activated T-type Ca²⁺ channels are key regulators of neuronal excitability both in the central and peripheral nervous systems. Therefore, their recruitment at the plasma membrane is critical in determining firing activity patterns of nerve cells. In this study, we report the importance of secretory carrier-associated membrane proteins (SCAMPs) in the trafficking regulation of T-type channels. We identified SCAMP2 as a novel Ca_v3.2-interacting protein. In addition, we show that co-expression of SCAMP2 in mammalian cells expressing recombinant Ca_v3.2 channels caused an almost complete drop of the whole cell T-type current, an effect partly reversed by single amino acid mutations within the conserved cytoplasmic E peptide of SCAMP2. SCAMP2-induced downregulation of T-type currents was also observed in cells expressing Ca_v3.1 and Ca_v3.3 channel isoforms. Finally, we show that SCAMP2-mediated knockdown of the T-type conductance is caused by the lack of Ca_v3.2 expression at the cell surface as evidenced by the concomitant loss of intramembrane charge movement without decrease of total Ca_v3.2 protein level. Taken together, our results indicate that SCAMP2 plays an important role in the trafficking of Ca_v3.2 channels at the plasma membrane.

De novo SCN8A and inherited rare CACNA1H variants associated with severe developmental and epileptic encephalopathy

Developmental and epileptic encephalopathies (DEEs) are a group of severe epilepsies that are characterized by seizures and developmental delay. DEEs are primarily attributed to genetic causes and an increasing number of cases have been correlated with variants in ion channel genes. In this study, we report a child with an early severe DEE. Whole exome sequencing showed a de novo heterozygous variant (c.4873–4881 duplication) in the SCN8A gene and an inherited heterozygous variant (c.952G > A) in the CACNA1H gene encoding for Na_v1.6 voltage-gated sodium and Ca_v3.2 voltage-gated calcium channels, respectively. In vitro functional analysis of human Na_v1.6 and Ca_v3.2 channel variants revealed mild but significant alterations of their gating properties that were in general consistent with a gain- and loss-of-channel function, respectively. Although additional studies will be required to confirm the actual pathogenic involvement of SCN8A and CACNA1H, these findings add to the notion that rare ion channel variants may contribute to the etiology of DEEs.

Di-4-ANEPPS Modulates Electrical Activity and Progress of Myocardial Ischemia in Rabbit Isolated Heart

Aims

Although voltage-sensitive dye di-4-ANEPPS is a common tool for mapping cardiac electrical activity, reported effects on electrophysiological parameters are rather. The main goals of the study were to reveal effects of the dye on rabbit isolated heart and to verify, whether rabbit isolated heart stained with di-4-ANEPPS is a suitable tool for myocardial ischemia investigation.

Methods and Results

Study involved experiments on stained (n = 9) and non-stained (n = 11) Langendorff perfused rabbit isolated hearts. Electrophysiological effects of the dye were evaluated by analysis of various electrogram (EG) parameters using common paired and unpaired statistical tests. It was shown that staining the hearts with di-4-ANEPPS leads to only short-term sporadic prolongation of impulse conduction through atria and atrioventricular node. On the other hand, significant irreversible slowing of heart rate and ventricular conduction were found in stained hearts as compared to controls. In patch clamp experiments, significant inhibition of sodium current density was observed in differentiated NG108-15 cells stained by the dye. Although no significant differences in mean number of ventricular premature beats were found between the stained and the non-stained hearts in ischemia as well as in reperfusion, all abovementioned results indicate increased arrhythmogenicity. In isolated hearts during ischemia, prominent ischemic patterns appeared in the stained hearts with 3–4 min delay as compared to the non-stained ones. Moreover, the ischemic changes did not achieve the same magnitude as in controls even after 10 min of ischemia. It resulted in poor performance of ischemia detection by proposed EG parameters, as was quantified by receiver operating characteristics analysis.

Conclusion

Our results demonstrate significant direct irreversible effect of di-4-ANEPPS on spontaneous heart rate and ventricular impulse conduction in rabbit isolated heart model. Particularly, this should be considered when di-4-ANEPPS is used in ischemia studies in rabbit. Delayed attenuated response of such hearts to ischemia might lead to misinterpretation of obtained results.

Maternal immune activation in rats attenuates the excitability of monoamine-secreting neurons in adult offspring in a sex-specific way

Higher risk of depression and schizophrenia in descendants of mothers experienced acute infection during the pregnancy has been reported. Since monoamines are fundamental in mentioned psychopathologies, it is possible that maternal immune activation leads to impaired functioning of serotonin (5-HT), noradrenaline, and dopamine neurons in offspring. To test this hypothesis, we examined the effect of maternal immune activation by lipopolysaccharide (LPS) in rats on the excitability of monoamine-secreting neurons in the offspring. LPS was administered during days 15-19 of the gestation in the rising doses of 20-80 µg/kg; control dams received vehicle. During days 53-63 postpartum, rats were anesthetized and electrodes were inserted into the dorsal raphe nucleus, locus coeruleus, and ventral tegmental area for in vivo excitability assessment of 5-HT, noradrenaline, and dopamine neurons. Maternal immune activation suppressed the firing rate of 5-HT neurons in both sexes and stimulated the firing rate of dopamine neurons in males. Decrease in the firing rate of 5-HT neurons was accompanied with an increase, and increase in the firing rate of dopamine neurons with a decrease, in the density of spontaneously active cells. Maternal immune activation also decreased the variability of interspike intervals in 5-HT and dopamine neurons. It is possible that the alteration of excitability of 5-HT and dopamine neurons by maternal immune activation is involved in the psychopathologies induced by infectious disease during the pregnancy. Stimulation of dopamine excitability in males might be a compensatory mechanism secondary to the maternal immune challenge-induced suppression of 5-HT neurons.

Modulation of voltage-gated CaV2.2 Ca2+ channels by newly identified interaction partners

Voltage-gated Ca²⁺ channels are typically integrated in a complex network of protein-protein-interactions, also referred to as Ca²⁺ channel nanodomains. Amongst the neuronal Ca_V2 channel family, Ca_V2.2 is of particular importance due to its general role for signal transmission from the periphery to the central nervous system, but also due to its significance for pain perception. Thus, Ca_V2.2 is an ideal target candidate to search for pharmacological inhibitors but also for novel modulatory interactors. In this review we summarize the last years findings of our intense screenings and characterization of the six Ca_V2.2 interaction partners, tetraspanin-13 (TSPAN-13), reticulon 1 (RTN1), member 1 of solute carrier family 38 (SLC38), prostaglandin D2 synthase (PTGDS), transmembrane protein 223 (TMEM223), and transmembrane BAX inhibitor motif 3 (Grina/TMBIM3) containing protein. Each protein shows a unique way of channel modulation as shown by extensive electrophysiological studies. Amongst the newly identified interactors, Grina/TMBIM3 is most striking due to its modulatory effect which is rather comparable to G-protein regulation.

A Study among the Genotype, Functional Alternations, and Phenotype of 9 SCN1A Mutations in Epilepsy Patients

Mutations in the voltage-gated sodium channel Na_v1.1 (SCN1A) are linked to various epileptic phenotypes with different severities, however, the consequences of newly identified SCN1A variants on patient phenotype is uncertain so far. The functional impact of nine SCN1A variants, including five novel variants identified in this study, was studied using whole-cell patch-clamp recordings measurement of mutant Na_v1.1 channels expressed in HEK293T mammalian cells. E78X, W384X, E1587K, and R1596C channels failed to produce measurable sodium currents, indicating complete loss of channel function. E788K and M909K variants resulted in partial loss of function by exhibiting reduced current density, depolarizing shifts of the activation and hyperpolarizing shifts of the inactivation curves, and slower recovery from inactivation. Hyperpolarizing shifts of the activation and inactivation curves were observed in D249E channels along with slower recovery from inactivation. Slower recovery from inactivation was observed in E78D and T1934I with reduced current density in T1934I channels. Various functional effects were observed with the lack of sodium current being mainly associated with severe phenotypes and milder symptoms with less damaging channel alteration. In vitro functional analysis is thus fundamental for elucidation of the molecular mechanisms of epilepsy, to guide patients' treatment, and finally indicate misdiagnosis of SCN1A related epilepsies.

A rare CACNA1H variant associated with amyotrophic lateral sclerosis causes complete loss of Cav3.2 T-type channel activity

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder characterized by the progressive loss of cortical, brain stem and spinal motor neurons that leads to muscle weakness and death. A previous study implicated CACNA1H encoding for Cav3.2 calcium channels as a susceptibility gene in ALS. In the present study, two heterozygous CACNA1H variants were identified by whole genome sequencing in a small cohort of ALS patients. These variants were functionally characterized using patch clamp electrophysiology, biochemistry assays, and molecular modeling. A previously unreported c.454GTAC > G variant produced an inframe deletion of a highly conserved isoleucine residue in Cav3.2 (p.ΔI153) and caused a complete loss-of-function of the channel, with an additional dominant-negative effect on the wild-type channel when expressed in trans. In contrast, the c.3629C > T variant caused a missense substitution of a proline with a leucine (p.P1210L) and produced a comparatively mild alteration of Cav3.2 channel activity. The newly identified ΔI153 variant is the first to be reported to cause a complete loss of Cav3.2 channel function. These findings add to the notion that loss-of-function of Cav3.2 channels associated with rare CACNA1H variants may be risk factors in the complex etiology of ALS.

Role of glucocorticoid- and monoamine-metabolizing enzymes in stress-related psychopathological processes

Glucocorticoid signaling is fundamental in healthy stress coping and in the pathophysiology of stress-related diseases, such as post-traumatic stress disorder (PTSD). Glucocorticoids are metabolized by cytochrome P450 (CYP) as well as 11-β-hydroxysteroid dehydrogenase type 1 (11βHSD1) and 2 (11βHSD2). Acute stress-induced increase in glucocorticoid concentrations stimulates the expression of several CYP sub-types. CYP is primarily responsible for glucocorticoid metabolism and its increased activity can result in decreased circulating glucocorticoids in response to repeated stress stimuli. In addition, repeated stress-induced glucocorticoid release can promote 11βHSD1 activation and 11βHSD2 inhibition, and the 11βHSD2 suppression can lead to apparent mineralocorticoid excess. The activation of CYP and 11βHSD1 and the suppression of 11βHSD2 may at least partly contribute to development of the blunted glucocorticoid response to stressors characteristic in high trait anxiety, PTSD, and other stress-related disorders. Glucocorticoids and glucocorticoid-metabolizing enzymes interact closely with other biomolecules such as inflammatory cytokines, monoamines, and some monoamine-metabolizing enzymes, namely the monoamine oxidase type A (MAO-A) and B (MAO-B). Glucocorticoids boost MAO activity and this decreases monoamine levels and induces oxidative tissue damage which then activates inflammatory cytokines. The inflammatory cytokines suppress CYP expression and activity. This dynamic cross-talk between glucocorticoids, monoamines, and their metabolizing enzymes could be a critical factor in the pathophysiology of stress-related disorders.Lay summaryGlucocorticoids, which are produced and released under the control by brain regulatory centers, are fundamental in the stress response. This review emphasizes the importance of glucocorticoid metabolism and particularly the interaction between the brain and the liver as the major metabolic organ in the body. The activity of enzymes involved in glucocorticoid metabolism is proposed to play not only an important role in positive, healthy glucocorticoid effects, but also to contribute to the development and course of stress-related diseases.

Role of individual S4 segments in gating of Cav3.1 T-type calcium channel by voltage

Contributions of voltage sensing S4 segments in domains I – IV of Ca_V3.1 channel to channel activation were analyzed. Neutralization of the uppermost charge in individual S4 segments by exchange of arginine for cysteine was employed. Mutant channels with single exchange in domains I – IV, in two adjacent domains, and in all four domains were constructed and expressed in HEK 293 cells. Changes in maximal gating charge Q_max and the relation between Q_max and maximal conductance G_max were evaluated. Q_max was the most affected by single mutation in domain I and by double mutations in domains I + II and I + IV. The ratio G_max/Q_max proportional to opening probability of the channel was significantly decreased by the mutation in domain III and increased by mutations in domains I and II. In channels containing double mutations G_max/Q_max ratio increased significantly when the mutation in domain I was included. Mutations in domains II and III zeroed each other. Mutation in domain IV prevented the decrease caused by the mutation in domain III. Neither ion current nor gating current was observed when channels with quadruple mutations were expressed. Immunocytochemistry analysis did not reveal the presence of channel protein in the cell membrane. Likely, quadruple mutation results in a structural change that affects the channel’s trafficking mechanism. Altogether, S4 segments in domains I-IV of the Ca_V3.1 channel unequally contribute to channel gating by voltage. We suggest the most important role of the voltage sensor in the domain I and lesser roles of voltage sensors in domains II and III.

Identification of a molecular gating determinant within the carboxy terminal region of Cav3.3 T-type channels

The physiological functions controlled by T-type channels are intrinsically dependent on their gating properties, and alteration of T-type channel activity is linked to several human disorders. Therefore, it is essential to develop a clear understanding of the structural determinants responsible for the unique gating features of T-type channels. Here, we have investigated the specific role of the carboxy terminal region by creating a series a deletion constructs expressed in tsA-201 cells and analyzing them by patch clamp electrophysiology. Our data reveal that the proximal region of the carboxy terminus contains a structural determinant essential for shaping several gating aspects of Cav3.3 channels, including voltage-dependence of activation and inactivation, inactivation kinetics, and coupling between the voltage sensing and the pore opening of the channel. Altogether, our data are consistent with a model in which the carboxy terminus stabilizes the channel in a closed state.

Structure, function and regulation of CaV 2.2 N-type calcium channels

N-type or Ca_V2.2 high-voltage activated calcium channels are distinguished by exclusively neuronal tissue distribution, sensitivity to ω-conotoxins, prominent inhibition by G-proteins, and a unique role in nociception. Most investigated modulatory pathway regulating the Ca_V2.2 channels is G-protein-coupled receptor-activated pathway leading to current inhibition by Gβγ subunit of G-protein. Binding of Gβγ dimer to α₁ subunit of the Ca_V2.2 channel transfers the channel form "willing" to "reluctant" gating state. Channel phosphorylation by protein kinase C potentiates N-type calcium current. Ca_V2.2 channels could be functionally regulated also by a number of protein-protein interactions. Ca _V2.2 null mice are hyposensitive to inflammatory and neuropathic pain, otherwise they have a mild phenotype. Consistent with the mild phenotype of the Ca_V2.2-/- mice, reports on mutations linked to a disease phenotype are scarce. Only one mutation related to human heritable diseases was identified until now. Pharmaceutical inhibition of Ca_V2.2 channels either by direct inhibition of the channel, by an activation of G-protein coupled receptors, or by inhibition of membrane targeting of the channel protein are promising strategies for treatment of severe chronic and/or neuropathic pain.

Correction to: Four novel interaction partners demonstrate diverse modulatory effects on voltage-gated CaV2.2 Ca2+ channels

The article was originally published with one author missing. The name of the co-author Roman Moravcik was inadvertently omitted. His name and affiliation have now been added to the author list. The original article has been corrected.

Four novel interaction partners demonstrate diverse modulatory effects on voltage-gated CaV2.2 Ca2+ channels

Voltage-gated Ca²⁺ channels are embedded in a network of protein interactions that are fundamental for channel function and modulation. Different strategies such as high-resolution quantitative MS analyses and yeast-two hybrid screens have been used to uncover these Ca²⁺ channel nanodomains. We applied the yeast split-ubiquitin system with its specific advantages to search for interaction partners of the Ca_V2.2 Ca²⁺ channel and identified four proteins: reticulon 1 (RTN1), member 1 of solute carrier family 38 (SLC38), prostaglandin D2 synthase (PTGDS) and transmembrane protein 223 (TMEM223). Interactions were verified using the yeast split-ubiquitin system and narrowed down to Ca_V2.2 domain IV. Colocalization studies using fluorescent constructs demonstrated defined regions of subcellular localization. Detailed electrophysiological studies revealed that coexpression of RTN1 modulated Ca_V2.2 channels only to a minor extent. SLC38 accelerated the cumulative current inactivation during a high-frequency train of brief depolarizing pulses. As neurons expressing Ca_V2.2 channels were exposed to high-frequency bursts under physiological conditions, observed regulation may have a negative modulatory effect on transmitter release. Coexpression of PTGDS significantly lowered the average current density and slowed the kinetics of cumulative current inactivation. Since the latter effect was not significant, it may only partly compensate the first one under physiological conditions. Expression of TMEM223 lowered the average current density, accelerated the kinetics of cumulative current inactivation and slowed the kinetics of recovery from inactivation. Therefore, TMEM223 and, to a lesser extent, PTGDS, may negatively modulate Ca²⁺ entry required for transmitter release and/or for dendritic plasticity under physiological conditions.

Cellular and molecular mechanisms underlying the treatment of depression: focusing on hippocampal G-protein-coupled receptors and voltage-dependent calcium channels

Depression is a brain disorder characterized by severe emotional, cognitive, neuroendocrine and somatic dysfunction. Although the latest generation of antidepressant drugs has improved clinical efficacy and safety, the onset of their clinical effect is significantly delayed after treatment commencement, and a large number of patients exhibit inadequate response to these drugs and/or depression relapse even following initially successful treatment. It is therefore essential to develop new antidepressant drugs and/or adjuncts to existing ones. Recent studies suggest that the beneficial effect of antidepressant drugs is mediated, at least in part, via stimulation of adult hippocampal neurogenesis and subsequent increase in hippocampal plasticity. Since the stimulatory effect of antidepressant drugs on hippocampal neurogenesis involves G-protein coupled receptors (GPCR) and voltage-dependent calcium channels (VDCC), greater efficacy may be available if future antidepressant drugs directly target these specific GPCR and VDCC. The potential advantages and limitations of these treatment strategies are discussed in the article.

Modulation of Cav3.2 T-type calcium channel permeability by asparagine-linked glycosylation

Low-voltage-gated T-type calcium channels are expressed throughout the nervous system where they play an essential role in shaping neuronal excitability. Defects in T-type channel expression have been linked to various neuronal disorders including neuropathic pain and epilepsy. Currently, little is known about the cellular mechanisms controlling the expression and function of T-type channels. Asparagine-linked glycosylation has recently emerged as an essential signaling pathway by which the cellular environment can control expression of T-type channels. However, the role of N-glycans in the conducting function of T-type channels remains elusive. In the present study, we used human Cav3.2 glycosylation-deficient channels to assess the role of N-glycosylation on the gating of the channel. Patch-clamp recordings of gating currents revealed that N-glycans attached to hCav3.2 channels have a minimal effect on the functioning of the channel voltage-sensor. In contrast, N-glycosylation on specific asparagine residues may have an essential role in the conducting function of the channel by enhancing the channel permeability and / or the pore opening of the channel. Our data suggest that modulation of N-linked glycosylation of hCav3.2 channels may play an important physiological role, and could also support the alteration of T-type currents observed in disease states.

Effects of AP39, a novel triphenylphosphonium derivatised anethole dithiolethione hydrogen sulfide donor, on rat haemodynamic parameters and chloride and calcium Cav3 and RyR2 channels

H2S donor molecules have the potential to be viable therapeutic agents. The aim of this current study was (i) to investigate the effects of a novel triphenylphosphonium derivatised dithiolethione (AP39), in the presence and absence of reduced nitric oxide bioavailability and (ii) to determine the effects of AP39 on myocardial membrane channels; CaV3, RyR2 and Cl(-). Normotensive, L-NAME- or phenylephrine-treated rats were administered Na2S, AP39 or control compounds (AP219 and ADT-OH) (0.25-1 µmol kg(-1)i.v.) and haemodynamic parameters measured. The involvement of membrane channels T-type Ca(2+) channels CaV3.1, CaV3.2 and CaV3.3 as well as Ca(2+) ryanodine (RyR2) and Cl(-) single channels derived from rat heart sarcoplasmic reticulum were also investigated. In anaesthetised Wistar rats, AP39 (0.25-1 µmol kg(-1) i.v) transiently decreased blood pressure, heart rate and pulse wave velocity, whereas AP219 and ADT-OH and Na2S had no significant effect. In L-NAME treated rats, AP39 significantly lowered systolic blood pressure for a prolonged period, decreased heart rate and arterial stiffness. In electrophysiological studies, AP39 significantly inhibited Ca(2+) current through all three CaV3 channels. AP39 decreased RyR2 channels activity and increased conductance and mean open time of Cl(-) channels. This study suggests that AP39 may offer a novel therapeutic opportunity in conditions whereby (•)NO and H2S bioavailability are deficient such as hypertension, and that CaV3, RyR2 and Cl(-) cardiac membrane channels might be involved in its biological actions.

A Ca(v)3.2/syntaxin-1A signaling complex controls T-type channel activity and low-threshold exocytosis

T-type calcium channels represent a key pathway for Ca(2+) entry near the resting membrane potential. Increasing evidence supports a unique role of these channels in fast and low-threshold exocytosis in an action potential-independent manner, but the underlying molecular mechanisms have remained unknown. Here, we report the existence of a syntaxin-1A/Ca(v)3.2 T-type calcium channel signaling complex that relies on molecular determinants that are distinct from the synaptic protein interaction site (synprint) found in synaptic high voltage-activated calcium channels. This interaction potently modulated Ca(v)3.2 channel activity, by reducing channel availability. Other members of the T-type calcium channel family were also regulated by syntaxin-1A, but to a smaller extent. Overexpression of Ca(v)3.2 channels in MPC 9/3L-AH chromaffin cells induced low-threshold secretion that could be prevented by uncoupling the channels from syntaxin-1A. Altogether, our findings provide compelling evidence for the existence of a syntaxin-1A/T-type Ca(2+) channel signaling complex and provide new insights into the molecular mechanism by which these channels control low-threshold exocytosis.

Type 1 and 2 IP3 receptors respond differently to catecholamines and stress

Type 1 and 2 inositol 1,4,5-trisphosphate (IP3) receptors have been found in cardiac tissue, although they are localized in different types of cells. While the type 1 predominates in neuronal cells and cardiac ganglia, type 2 IP3 receptor is localized mainly in cardiomyocytes. In the heart, gene expression of the type 1 IP3 receptor is modulated by catecholamines, while type 2 is not affected. In this study we compared the effects of two stressors--cold exposure and hypoxia--on gene expression and protein levels of type 1 and 2 IP3 receptors. We found that both types of IP3 receptors were increased by these stressors, although the response of the type 2 IP3 receptor was delayed. We propose that regulation of these types of IP3 receptors in the heart might differ in control conditions and during stress.

Hypoxia differently modulates gene expression of inositol 1,4,5-trisphosphate receptors in mouse kidney and HEK 293 cell line

Hypoxia is a state of insufficient oxygen supply of the tissue or cell. Kidney tissue is highly sensitive to oxygen deprivation and easily develops renal ischemic injury. Calcium transporters very sensitively react to oxygen deficiency. We investigated whether hypoxia affects the gene expression of intracellular calcium transporters in the intact kidney, and we compared the response to that of HEK 293 cells. Our results showed that, while in mouse kidney tissue hypoxia elevates mRNA for inositol 1,4,5-trisphosphate receptors (IP3R) type 1 (IP3R1) and ryanodine receptors (RyR) type 2 (RyR2), in culture of HEK 293 cells the gene expression of all IP3Rs decreased without affecting viability of the cells. RyR2 mRNA in HEK 293 cells was not significantly changed, but RyR1 gene expression was significantly increased by hypoxia. The different response of kidney tissue and HEK 293 cells to hypoxia could be due to unequal differentiation state of the cells in intact tissue and cultured embryonic cell line. The physiological relevance of this observation remains to be determined.

Effects of phenylalkylamines and benzothiazepines on Cav1.3-mediated Ca2+ currents in neonatal mouse inner hair cells

Calcium currents (I(Ca)) in inner hair cells (IHCs) are carried by the Ca(v)1.3 subtype of L-type calcium channels. They play an important role in synaptic transmission of sound-evoked mechanical stimuli. L-type calcium channels are targets of the organic blocker classes dihydropyridines, phenylalkylamines and benzothiazepines. Previously a low sensitivity of the Ca(v)1.3 subtype towards dihydropyridines has been demonstrated. Therefore, this study evaluates the effect of two phenylalkylamines (verapamil and gallopamil) and the benzothiazepine diltiazem on I(Ca) through Ca(v)1.3 channels in mouse IHCs. Whole-cell I(Ca) was measured using the patch-clamp technique in mouse IHCs aged postnatal day 3-7 with 5 mM calcium as a charge carrier. The phenylalkylamines verapamil and gallopamil and the benzothiazepine diltiazem inhibited I(Ca) in IHCs in a concentration-dependent manner. This block was largely reversible. Dose-response curves revealed IC(50) values of 199+/-19 microM for verapamil, 466+/-151 microM for gallopamil and 326+/-67 microM for diltiazem. The inhibition of peak I(Ca) by phenylalkylamines and benzothiazepines was voltage-independent. Verapamil (300 microM) enhanced current inactivation from -20 to +20 mV while diltiazem (300 microM) did so only at very depolarised potentials (+20 mV). In conclusion, the concentrations of phenylalkylamines and benzothiazepine necessary to inhibit 50% of I(Ca) in IHCs were one order larger compared to concentrations which inhibited I(Ca) through Ca(v)1.2 channels in native cells or expression systems. However, inhibitory concentrations were in the same range as those required for block of I(Ca) in turtle hair cells.

Cav1.2 calcium channels modulate the spiking pattern of hippocampal pyramidal cells

Ca(v)1.2 L-type calcium channels support hippocampal synaptic plasticity, likely by facilitating dendritic Ca2+ influx evoked by action potentials (AP) back-propagated from the soma. Ca2+ influx into hippocampal neurons during somatic APs is sufficient to activate signalling pathways associated with late phase LTP. Thus, mechanisms controlling AP firing of hippocampal neurons are of major functional relevance. We examined the excitability of CA1 pyramidal cells using somatic current-clamp recordings in brain slices from control type mice and mice with the Ca(v)1.2 gene inactivated in principal hippocampal neurons. Lack of the Ca(v)1.2 protein did not affect either affect basic characteristics, such as resting membrane potential and input resistance, or parameters of single action potentials (AP) induced by 5 ms depolarising current pulses. However, CA1 hippocampal neurons from control and mutant mice differed in their patterns of AP firing during 500 ms depolarising current pulses: threshold voltage for repetitive firing was shifted significantly by about 5 mV to more depolarised potentials in the mutant mice (p<0.01), and the latency until firing of the first AP was prolonged (73.2+/-6.6 ms versus 48.1+/- 7.8 ms in control; p<0.05). CA1 pyramidal cells from the mutant mice also showed a lowered initial spiking frequency within an AP train. In control cells, isradipine had matching effects, while BayK 8644 facilitated spiking. Our data demonstrate that Ca(v)1.2 channels are involved in regulating the intrinsic excitability of CA1 pyramidal neurons. This cellular mechanism may contribute to the known function of Ca(v)1.2 channels in supporting synaptic plasticity and memory.

Modulation of expression of the sigma receptors in the heart of rat and mouse in normal and pathological conditions

The aim of the present work was to study the effect of various stressors (hypoxia, cold, immobilization) on the gene expression of sigma receptors in the left ventricles of rat heart. We have clearly shown that gene expression of sigma receptors is upregulated by strong stress stimuli, such as immobilization and/or hypoxia. Nevertheless, cold as a milder stressor has no effect on sigma receptor's mRNA levels. Signalling cascade of sigma receptors is dependent on IP(3) receptors, since silencing of both, type 1 and 2 IP(3) receptors resulted in decreased mRNA levels of sigma receptors. Physiological relevance of sigma receptors in the heart is not clear yet. Nevertheless, based on the already published data we can assume that sigma receptors might participate in contractile responses in cardiomyocytes.

Modulation of expression of Na+/Ca2+ exchanger in heart of rat and mouse under stress

AIM

The Na(+)/Ca(2+) exchanger (NCX) is a major Ca(2+) extrusion system in the plasma membrane of cardiomyocytes and an important component participating on the excitation-contraction coupling process in muscle cells. NCX1 isoform is the most abundant in the heart and is known to be changed after development of ischaemia or myocardial infarction. Objective of this study was to investigate the effect of stress factors (immobilization, cold and short-term hypoxia) on the expression of NCX1, in vivo, in the heart of rat and mouse.

METHODS

We compared gene expression and protein levels of control and stressed animals. The activity of NCX was measured by the whole cell configuration using the patch clamp. We also measured physiological parameters of the heart in physiological conditions and under ischaemia-reperfusion to compare response of control and stressed hearts.

RESULTS

We have found that only strong stress stimulus (hypoxia, immobilization) applied repeatedly for several days elevated the NCX1 mRNA level. Cold, which is a weaker stressor that activates mainly sympathoneural, and only marginally adrenomedullary system did not affect the gene expression of NCX1. Thus, from these results it appears that hormones produced by the adrenal medulla (mainly adrenaline) might be involved in this process. To study possible mechanism of the NCX1 regulation by stress, we focused on the possible role of the hypothalamo-pituitary-adrenocortical pathway in the activation of catecholamine synthesis in the adrenal medulla. We have already published that cortisol affects activity, but not the gene expression of NCX1. In this work, we used corticotropin-releasing hormone (CRH) knockout mice, where secretion of corticosterone and subsequently adrenaline is significantly suppressed. As no increase in NCX1 mRNA was observed in CRH knockout mice due to immobilization stress, we proposed that adrenaline (probably regulated via corticosterone) is involved in the regulation of NCX1 gene expression during stress.

CONCLUSIONS

The gene expression and protein levels of the NCX1 are increased by the strong stress stimuli, e.g. hypoxia, or immobilization stress. The activity of NCX1 is decreased. Based on these results, we assume that the gene expression of NCX is increased as a consequence of suppressed activity of this transport system.

Modulation by 6-hydroxydopamine of expression of the phenylethanolamine N-methyltransferase (PNMT) gene in the rat heart during immobilization stress

Phenylethanolamine N-methyltransferase (PNMT) is the final enzyme in the catecholamine synthesizing cascade that converts noradrenaline (NA) to adrenaline (Adr). Both of these catecholamines are physiologically important hormones and neurotransmitters in mammals with profound influence on the activity of the cardiovascular system. Although PNMT activity and gene expression have been reported in the neonatal and also adult rat heart, little is known about the identity of the cells expressing PNMT mRNA. In this study, we have shown that besides PNMT in neuronal and intrinsic cardiac cells, this enzyme is expressed also in rat cardiomyocytes, as shown by immunofluorescence in isolated cardiomyocytes. To determine which cells in the heart more sensitively show stress-induced changes in PNMT mRNA expression, we performed chemical sympathectomy by administration of 6-hydroxydopamine (6-OHDA), which destroys catecholaminergic terminals. We determined PNMT mRNA levels in the left atria and ventricles of control and stressed rats. In the rats treated with 6-OHDA, PNMT mRNA levels were not changed under normal, physiological conditions compared to vehicle treated rats. Similar results were observed on isolated cardiomyocytes from control and 6-OHDA treated rats. However, 6-OHDA treatment prevented immobilization-induced increase in PNMT mRNA expression. The results allow us to propose that in the heart, the immobilization-induced increase in PNMT gene expression is probably not in cardiomyocytes, but in neuronal cells.

IP3 type 1 receptors in the heart: Their predominance in atrial walls with ganglion cells

Previously we have shown that inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs) are abundantly expressed in the atria of rat hearts. Since arrangement of atria is very heterogeneous, in this work we focused on the precise localization of IP3 receptors in the left atrium, where the gene expression of the type 1 IP3R was the highest. The mRNA levels of the IP3 type 1 receptors in the left atrium, left ventricle and myocytes were determined using real-time polymerase chain reaction and Taqman probe. For precise localization, immunohistochemistry with the antibody against type 1 IP3Rs was performed. The mRNA of type 1 IP3 receptor was more than three times higher in the left atrium than in the left ventricle, as determined by real-time PCR. Expression of the type 1 IP3 receptor mRNA was higher in the atria, especially in parts containing cardiac ganglion cells. The atrial auricles, which are particularly free of ganglion cells, and the ventricles (wall of the right and left ventricle and ventricular septum) contained four to five times less IP3 receptors than atrial samples with ganglia. IP3R type 1 immunoreactivity detected by a confocal microscope attributed the most condensed signal on ganglionic cells, although light immunoreactivity was also seen in cardiomyocytes. These results show that type 1IP3 receptors predominate in intrinsic neuronal ganglia of cardiac atria.

Role of hippocampal Cav1.2 Ca2+ channels in NMDA receptor-independent synaptic plasticity and spatial memory

Current knowledge about the molecular mechanisms of NMDA receptor (NMDAR)-independent long-term potentiation (LTP) in the hippocampus and its function for memory formation in the behaving animal is limited. NMDAR-independent LTP in the CA1 region is thought to require activity of postsynaptic L-type voltage-dependent Ca2+ channels (Cav1.x), but the underlying channel isoform remains unknown. We evaluated the function of the Cav1.2 L-type Ca2+ channel for spatial learning, synaptic plasticity, and triggering of learning-associated biochemical processes using a mouse line with an inactivation of the CACNA1C (Cav1.2) gene in the hippocampus and neocortex (Cav1.2(HCKO)). This model shows (1) a selective loss of protein synthesis-dependent NMDAR-independent Schaffer collateral/CA1 late-phase LTP (L-LTP), (2) a severe impairment of hippocampus-dependent spatial memory, and (3) decreased activation of the mitogen-activated protein kinase (MAPK) pathway and reduced cAMP response element (CRE)-dependent transcription in CA1 pyramidal neurons. Our results provide strong evidence for a role of L-type Ca2+ channel-dependent, NMDAR-independent hippocampal L-LTP in the formation of spatial memory in the behaving animal and for a function of the MAPK/CREB (CRE-binding protein) signaling cascade in linking Cav1.2 channel-mediated Ca2+ influx to either process.

The amino side of the C-terminus determines fast inactivation of the T-type calcium channel alpha1G

We analysed the kinetic properties of the fast inactivating T-type calcium channel alpha1G in HEK 293 cells transfected with different alpha1G chimeras, containing the N-terminus, III-IV linker or various C-terminal regions of the slowly inactivating L-type alpha1C. A highly negatively charged region of 23 amino acids at the amino side of the intracellular carboxy terminus of alpha1G was found to be critical for fast inactivation. The N-terminus of alpha1G does not seem to be necessary for inactivation of the T-type calcium channel because replacement of the a1G N-terminus with the alpha1C N-terminus did not influence channel kinetics at all. Replacing the III-IV linker of alpha1G with that of a1C decreased the rate of inactivation at -20 mV from 15.8 +/- 1.8 to 8.5 +/- 1.1 ms, and shifted the potential for half-maximal inactivation from -69.6 +/- 0.8 to -54.0 +/- 1.7 mV. However, these parameters were not significantly different at other potentials. We suggest a putative 'ball-and-chain'-like mechanism for inactivation in which the negative charges function as an acceptor domain for a ball, hypothetically located at a different intracellular part of the channel. In addition, transferring the IQ motif and EF hand of alpha1C to alpha1G does not confer Ca2+-dependent inactivation on alpha1G, suggesting that other sequences besides the C-terminus are needed for Ca2+-dependent inactivation of alpha1C.

Neuronal distribution and functional characterization of the calcium channel alpha2delta-2 subunit

The auxiliary calcium channel alpha2delta subunit comprises a family of three genes, alpha2delta-1 to 3, which are expressed in a tissue-specific manner. alpha2delta-2 mRNA is found in the heart, skeletal muscle, brain, kidney, liver and pancreas. We report here for the first time the identification and functional characterization of alpha2delta-2 splice variants and their mRNA distribution in the mouse brain. The splice variants differ in the alpha2 and delta protein by eight and three amino acid residues, respectively, and are differentially expressed in cardiac tissue and human medullary thyroid carcinoma (hMTC) cells. In situ hybridization of mouse brain sections revealed the highest expression of alpha2delta-2 mRNA in the Purkinje cell layer of the cerebellum, habenulae and septal nuclei, and a lower expression in the cerebral cortex, olfactory bulb, thalamic and hypothalamic nuclei, as well as the inferior and superior colliculus. As the in situ data did not suggest a specific colocalization with any alpha1 subunit, coexpression studies of alpha2delta-2 were carried out either with the high-voltage-gated calcium channels, alpha1C, alpha1E or alpha1A, or with the low-voltage-gated calcium channel, alpha1G. Coexpression of alpha2delta-2 increased the current density, shifted the voltage dependence of channel activation and inactivation of alpha1C, alpha1E and alpha1A subunits in a hyperpolarizing direction, and accelerated the decay and shifted the steady-state inactivation of the alpha1G current.

Structure and functional expression of a new member of the tetrodotoxin-sensitive voltage-activated sodium channel family from human neuroendocrine cells

A member of a new subclass of the voltage-activated sodium channel genes has been cloned from the human medullary thyroid carcinoma (hMTC) cell line. The cDNA of hNE-Na (human neuroendocrine sodium channel) encodes a 1977 amino acid protein which phylogenetically represents a link between sodium channels isolated from skeletal muscle and brain. The hNE-Na alpha subunit was transiently expressed in human embryonic kidney cells either alone or in combination with the human sodium channel beta 1 subunit. The channel exhibited rapid activation and inactivation kinetics, and was blocked by tetrodotoxin and cadmium with IC50 values of 24.5 nM and 1.1 mM, respectively. Action potentials were generated in cells expressing high levels of hNE-Na. Northern blot and reverse transcription-polymerase chain reaction (RT-PCR) analyses demonstrated its expression in hMTC cells, in a C-cell carcinoma, and in thyroid and adrenal gland. Transcripts were not identified in pituitary gland, brain, heart, liver or kidney, indicating that the hNE-Na is a sodium channel solely expressed in neuroendocrine cells.

Expression of the L-type calcium channel with two different beta subunits and its modulation by Ro 40-5967

The smooth muscle alpha 1Cb subunit of the L-type calcium channel was expressed alone (CHO alpha 1 cell) or together with the skeletal beta 1 (CHO alpha 1 beta 1 cell) subunit or smooth muscle beta 3 (CHO alpha 1 beta 3 cell) subunit in Chinese hamster ovary (CHO) cells. The interaction of the expressed calcium channel with the non-dihydropyridine calcium channel blocker Ro 40-5967 was studied. Ro 40-5967 decreased isradipine binding by an apparent allosteric interaction and blocked the barium inward currents (IBa) in a voltage- and use-dependent manner in all cells. The steady-state inactivation curves were shifted to hyperpolarizing potentials in the presence of Ro 40-5967. The rate of channel inactivation was increased in CHO alpha 1 and CHO alpha 1 beta 3 cells. The shift in the steady-state inactivation curve and the increase in channel inactivation were less pronounced in CHO alpha 1 beta 1 cells than in the other cell lines. Low concentrations of Ro 40-5967 increased IBa by up to 198% in 33% of the CHO alpha 1 beta 1 cells. In addition, higher concentrations of Ro 40-5967 were required to inhibit IBa in 60% of the CHO alpha 1 beta 3 cells. These results suggest that the beta subunits modify the interaction of the non-dihydropyridine Ro 40-5967 with the expressed calcium channel alpha 1 subunit.

Ca2+ channel modulating effects of heparin in mammalian cardiac myocytes

1. The effect of heparin on L-type Ca2+ channels in rabbit, rat and guinea-pig cardiac myocytes was studied using the whole-cell patch clamp method. 2. Sodium salts of heparin uniformly suppressed the Ca2+ current, ICa, independent of their molecular weight, in the rat and guinea-pig ventricular and rabbit atrial myocytes. The suppression of ICa by heparin was dose dependent and reached its maximum, about 30%, around 10 microM. Heparin did not alter the voltage-dependence or the steady-state inactivation properties of ICa. These effects were specific to heparin as another polysaccharide, dextran, failed to have any effect on ICa. 3. The suppressive effect of heparin was not diminished when [Ca2+]o was increased to 10 mM, or when Ba2+ was the charge carrier through the Ca2+ channel. 4. Spectrophotometric assays showed that heparin-induced changes in [Ca2+]o generally were too small to alter ICa significantly. 5. In myocytes buffered with 0.1 mM EGTA, the suppressive effect of heparin was more prominent on the inactivating than on the maintained component of ICa. 6. When extracellular Na+ was replaced by Cs+, the heparin suppressive effect was accompanied by a 10 mV shift of both the voltage dependence of activation and the steady-state inactivation parameters toward more negative potentials. 7. When both Mg2+ and Na+ were omitted from the bathing solutions, the suppressive effect of heparin was significantly enhanced such that almost 80% of the current was blocked. 8. In Cs(+)-based solutions 10 mM [Mg2+]o suppressed ICa by about 70% and heparin partially relieved this block. Heparin, however, did not counteract the Mg(2+)-induced suppression of ICa in Na(+)-based solution. 9. Extracellularly applied heparin did not alter the isoprenaline-induced enhancement of ICa or interfere with the blocking effect of phorbol esters on ICa. 10. Heparin thus appears to interfere with the permeation of Ca2+ through the channel by a mechanism regulating the Ca(2+)-induced inactivation of the Ca2+ channel. Na+ and Mg2+ appear to alter the kinetics and the magnitude of the suppressive effect of heparin on the Ca2+ channel, suggesting an interaction of these cations with either the Ca2+ or the heparin-binding sites of the channel.