Inhibition of transmitter release from rat sympathetic neurons via presynaptic M(1) muscarinic acetylcholine receptors

Nucleic acids based integrated macromolecular complexes for SiRNA delivery: Recent advancements

The therapeutic potential of small interfering RNA (siRNA) is monumental, offering a pathway to silence disease-causing genes with precision. However, the delivery of siRNA to target cells in-vivo remains a formidable challenge, owing to degradation by nucleases, poor cellular uptake and immunogenicity. This overview examines recent advancements in the design and application of nucleic acid-based integrated macromolecular complexes for the efficient delivery of siRNA. We dissect the innovative delivery vectors developed in recent years, including lipid-based nanoparticles, polymeric carriers, dendrimer complexes and hybrid systems that incorporate stimuli-responsive elements for targeted and controlled release. Advancements in bioconjugation techniques, active targeting strategies and nanotechnology-enabled delivery platforms are evaluated for their contribution to enhancing siRNA delivery. It also addresses the complex interplay between delivery system design and biological barriers, highlighting the dynamic progress and remaining hurdles in translating siRNA therapies from bench to bedside. By offering a comprehensive overview of current strategies and emerging technologies, we underscore the future directions and potential impact of siRNA delivery systems in personalized medicine.

Impairment of exocytotic transmitter release by decynium-22 through an inhibition of ion channels

Introduction: In addition to members of the family of Na+/Cl- dependent monoamine transporters, organic cation transporters (OCTs), in particular OCT3, as well as the plasma membrane monoamine transporter (PMAT) may contribute to neuronal reuptake of according neurotransmitters. As opposed to the numerous blockers of monoamine transporters, only a very limited number of specific blockers of OCT3 and PMAT are available. In fact, decynium-22 is the only blocking agent with micromolar affinities for both transport proteins, and this molecule is frequently used to establish roles of OCT3 and/or PMAT as targets for antidepressant drugs and psychostimulants, respectively. Methods/Results: To test for a function of these transporters in the sympathetic nervous system, uptake and release of [3H]1-methyl-4-phenylpyridinium (MPP+) was investigated in primary cultures of rat superior cervical ganglia. Uptake was reduced by cocaine or desipramine, blockers of the noradrenaline transporter, by about 70% and by corticosterone or β-estradiol, blockers of OCT3, by about 30%; decynium-22 achieved complete inhibition of uptake with half maximal effects at 3 μM. Depolarization dependent release was enhanced by corticosterone or β-estradiol, but reduced by decynium-22. As the latter effect is unlikely to be related to actions at OCT3 and/or PMAT, electrophysiological recordings were performed to reveal that decynium-22 inhibits action potential firing and currents through voltage activated calcium channels in superior cervical ganglion neurons. Discussion: These results demonstrate that decynium-22 can impair exocytotic neurotransmitter release by interfering with several types of ion channels. Such transporter-independent effects of decynium-22 that my interfere with basic neuronal functions need to be considered when interpreting results obtained with decynium-22 as prototypic inhibitor of transmitter reuptake via OCT3 and/or PMAT.

Differential regulation of medium spiny and cholinergic neurons in the nucleus accumbens core by the insular and medial prefrontal cortices in the rat

The nucleus accumbens (NAc) receives cortical projections principally from the insular cortex (IC) and medial prefrontal cortex (mPFC). Among NAc neurons, cholinergic interneurons (ChNs) regulate the activities of medium spiny neurons (MSNs), which make up ~ 95% of NAc neurons, by modulating their firing and synaptic properties. However, little is known about the synaptic mechanisms, including their cell-type-dependent corticoaccumbal projection properties and cholinergic effects on the NAc core. Here, we performed whole-cell patch-clamp recordings from NAc MSNs and ChNs in acute brain slice preparations obtained from rats that received an AAV5-hSyn-ChR2(H134R)-mCherry injection into the IC or mPFC. Light stimulation of IC or mPFC axons induced comparable phase-locked excitatory postsynaptic currents (EPSCs) in MSNs. On the other hand, ChNs showed consistent EPSCs evoked by light stimulation of mPFC axons, whereas light stimulation of IC axons evoked much smaller EPSCs, which often showed failure in ChNs. Light-evoked EPSCs were abolished by tetrodotoxin and were recovered by 4-aminopyridine, suggesting that corticoaccumbal projections monosynaptically induce EPSCs in MSNs and ChNs. Carbachol effectively suppressed the amplitude of EPSCs in MSNs and ChNs evoked by light stimulation of IC or mPFC axons and in ChNs evoked by stimulating mPFC axons. The carbachol-induced suppression was recovered by atropine or pirenzepine, while preapplication of gallamine, J104129, PD102807, or AF-DX384 did not block the carbachol-induced EPSC suppression. These results suggest that NAc MSNs and ChNs are differentially regulated by excitatory projections from the IC and mPFC and that these corticoaccumbal excitatory inputs are modulated by M₁ receptor activation.

M1 muscarinic acetylcholine receptor-mediated inhibition of GABA release from striatal medium spiny neurons onto cholinergic interneurons

Acetylcholine (ACh) modulates neurotransmitter release in the central nervous system. Although GABAergic transmission onto the striatal cholinergic interneurons (ChIN) is modulated by dopamine receptors, cholinergic modulation of the same synapse is still unknown. In the present study, modulatory roles of ACh in the GABAergic transmission from striatal medium spiny neurons (MSNs) onto ChIN were investigated using optogenetics and whole-cell patch-clamp technique in juvenile and young-adult mice brain slices. GABA_A receptor-mediated inhibitory postsynaptic currents (IPSCs) were evoked by focal electrical- or blue-light stimulation. Bath application of carbachol, a muscarinic ACh receptor agonist, suppressed the amplitude of IPSCs in a concentration-dependent manner in both age groups. A choline esterase inhibitor, physostigmine, also suppressed the amplitude of IPSCs. In the presence of a membrane permeable M1 muscarine receptor antagonist, pirenzepine, carbachol-induced suppression of IPSCs was antagonized, whereas a M2 muscarine receptor antagonist, a M4 receptor antagonist, or a membrane impermeable M1 receptor antagonist did not antagonize carbachol-induced suppression of IPSCs. Retrograde cannabinoid cascade via cannabinoid receptor 1 was not involved in carbachol-induced inhibition. Furthermore, carbachol did not affect amplitude of inward currents induced by puff application of GABA, whereas coefficient of variation of IPSCs was significantly increased by carbachol. These results suggest that activation of presynaptic M1 muscarine receptors located on the GABAergic terminals including intracellular organelle of MSNs inhibits GABA release onto ChIN.

The alpha- and beta-noradrenergic receptors blockade in the dorsal raphe nucleus impairs the panic-like response elaborated by medial hypothalamus neurons

Dorsal raphe nucleus (DRN) neurons are reciprocally connected to the locus coeruleus (LC) and send neural pathways to the medial hypothalamus (MH). The aim of this work was to investigate whether the blockade of α₁-, α₂- or β-noradrenergic receptors in the DRN or the inactivation of noradrenergic neurons in the LC modify defensive behaviours organised by MH neurons. For this purpose, Wistar male rats received microinjections of WB4101, RX821002, propranolol (α₁-_, α₂- and β-noradrenergic receptor antagonists, respectively) or physiological saline in the DRN, followed 10 min later by MH GABA_A receptor blockade. Other groups of animals received DSP-4 (a noradrenergic neurotoxin), physiological saline or only a needle insertion (sham group) into the LC, and 5 days later, bicuculline or physiological saline was administered in the MH. In all these cases, after MH treatment, the frequency and duration of defensive responses were recorded over 15 min. An anterograde neural tract tracer was also deposited in the DRN. DRN neurons send pathways to lateral and dorsomedial hypothalamus. Blockade of α₁- and β-noradrenergic receptors in the DRN decreased escape reactions elicited by bicuculline microinjections in the MH. In addition, a significant increase in anxiety-like behaviours was observed after the blockade of α₂-noradrenergic receptors in the DRN. LC pretreatment with DSP-4 decreased both anxiety- and panic attack-like behaviours evoked by GABA_A receptor blockade in the MH. In summary, the present findings suggest that the norepinephrine-mediated system modulates defensive reactions organised by MH neurons at least in part via noradrenergic receptors recruitment on DRN neurons.

Presynaptic Calcium Channels

Presynaptic Ca²⁺ entry occurs through voltage-gated Ca²⁺ (Ca_V) channels which are activated by membrane depolarization. Depolarization accompanies neuronal firing and elevation of Ca²⁺ triggers neurotransmitter release from synaptic vesicles. For synchronization of efficient neurotransmitter release, synaptic vesicles are targeted by presynaptic Ca²⁺ channels forming a large signaling complex in the active zone. The presynaptic Ca_V2 channel gene family (comprising Ca_V2.1, Ca_V2.2, and Ca_V2.3 isoforms) encode the pore-forming α1 subunit. The cytoplasmic regions are responsible for channel modulation by interacting with regulatory proteins. This article overviews modulation of the activity of Ca_V2.1 and Ca_V2.2 channels in the control of synaptic strength and presynaptic plasticity.

Opening of ATP-sensitive K(+) (KATP) channels enhance hydroxyl radical generation induced by MPP(+) in rat striatum

The present study examined whether opening of adenosine triphosphate (ATP) sensitive K(+) (KATP) channels can enhance 1-methyl-4-phenylpyridinium (MPP(+))-induced hydroxyl radical (OH) generation in rat striatum. Rats were anesthetized, and sodium salicylate in Ringer's solution (0.5nmol/ml per min) was infused through a microdialysis probe to detect the generation of OH as reflected by the non-enzymatic formation of 2.3-dihydroxybenzoic acid (DHBA) in the striatum. MPP(+) (5mM) enhanced generation of OH with concomitant increased efflux of dopamine (DA). Cromakalim (100μM), a KATP channel opener, through the microdialysis probe significantly increased both DA efflux and OH formation induced by MPP(+). Another KATP channel opener, nicorandil (1mM), also increased the level DA or DHBA, but these changes were not significant. However, in the presence of glibenclamide (10μM), a KATP channel antagonist, and the increase of MPP(+)-induced DA or DHBA were not observed. Cromakalim (10, 50 and 100μM) increased MPP(+)-induced DHBA formation in a concentration-dependent manner. However, the effects of cromakalim in the presence of glibenclamide were abolished. These results suggest that opening of KATP channels may cause OH generation by MPP(+).

Deletion of muscarinic type 1 acetylcholine receptors alters splenic lymphocyte functions and splenic noradrenaline concentration

The existence of interactions between the immune and the sympathetic nervous systems is well established. Noradrenaline can promote or inhibit the immune response, and conversely, the immune response itself can affect noradrenaline concentration in lymphoid organs, such as the spleen. It is also well known that acetylcholine released by pre-ganglionic neurons can modulate noradrenaline release by the postsynaptic neuron. The spleen does not receive cholinergic innervation, but it has been reported that lymphocytes themselves can produce acetylcholine, and express acetylcholine receptors and acetylcholinesterase. We found that the spleen of not overtly immunized mice in which muscarinic type 1 acetylcholine receptors have been knocked out (M1KO) has higher noradrenaline concentrations than that of the wildtype mice, without comparable alterations in the heart, in parallel to a decreased number of IgG-producing B cells. Splenic lymphocytes from M1KO mice displayed increased in vitro-induced cytotoxicity, and this was observed only when CD4(+) T cells were present. In contrast, heterozygous acetylcholinesterase (AChE+/-) mice, had no alterations in splenic noradrenaline concentration, but the in vitro proliferation of AChE+/- CD4(+) T cells was increased. It is theoretically conceivable that reciprocal effects between neuronally and non-neuronally derived acetylcholine and noradrenaline might contribute to the results reported. Our results emphasize the need to consider the balance between the effects of these mediators for the final immunoregulatory outcome.

Voltage-dependent regulation of CaV2.2 channels by Gq-coupled receptor is facilitated by membrane-localized β subunit

The pathway through which preferentially G_qPCRs inhibit Ca_V2.2 channels depends on which β subunits are present.

G protein–coupled receptors (GPCRs) signal through molecular messengers, such as Gβγ, Ca²⁺, and phosphatidylinositol 4,5-bisphosphate (PIP₂), to modulate N-type voltage-gated Ca²⁺ (Ca_V2.2) channels, playing a crucial role in regulating synaptic transmission. However, the cellular pathways through which G_qPCRs inhibit Ca_V2.2 channel current are not completely understood. Here, we report that the location of Ca_V β subunits is key to determining the voltage dependence of Ca_V2.2 channel modulation by G_qPCRs. Application of the muscarinic agonist oxotremorine-M to tsA-201 cells expressing M₁ receptors, together with Ca_V N-type α1B, α2δ1, and membrane-localized β2a subunits, shifted the current-voltage relationship for Ca_V2.2 activation 5 mV to the right and slowed current activation. Muscarinic suppression of Ca_V2.2 activity was relieved by strong depolarizing prepulses. Moreover, when the C terminus of β-adrenergic receptor kinase (which binds Gβγ) was coexpressed with N-type channels, inhibition of Ca_V2.2 current after M₁ receptor activation was markedly reduced and delayed, whereas the delay between PIP₂ hydrolysis and inhibition of Ca_V2.2 current was decreased. When the Gβγ-insensitive Ca_V2.2 α1C-1B chimera was expressed, voltage-dependent inhibition of calcium current was virtually abolished, suggesting that M₁ receptors act through Gβγ to inhibit Ca_V2.2 channels bearing membrane-localized Ca_V β2a subunits. Expression of cytosolic β subunits such as β2b and β3, as well as the palmitoylation-negative mutant β2a(C3,4S), reduced the voltage dependence of M₁ muscarinic inhibition of Ca_V2.2 channels, whereas it increased inhibition mediated by PIP₂ depletion. Together, our results indicate that, with membrane-localized Ca_V β subunits, Ca_V2.2 channels are subject to Gβγ-mediated voltage-dependent inhibition, whereas cytosol-localized β subunits confer more effective PIP₂-mediated voltage-independent regulation. Thus, the voltage dependence of G_qPCR regulation of calcium channels can be determined by the location of isotype-specific Ca_V β subunits.

Excitation of rat sympathetic neurons via M1 muscarinic receptors independently of Kv7 channels

The slow cholinergic transmission in autonomic ganglia is known to be mediated by an inhibition of K_v7 channels via M₁ muscarinic acetylcholine receptors. However, in the present experiments using primary cultures of rat superior cervical ganglion neurons, the extent of depolarisation caused by the M₁ receptor agonist oxotremorine M did not correlate with the extent of K_v7 channel inhibition in the very same neuron. This observation triggered a search for additional mechanisms. As the activation of M₁ receptors leads to a boost in protein kinase C (PKC) activity in sympathetic neurons, various PKC enzymes were inhibited by different means. Interference with classical PKC isoforms led to reductions in depolarisations and in noradrenaline release elicited by oxotremorine M, but left the K_v7 channel inhibition by the muscarinic agonist unchanged. M₁ receptor-induced depolarisations were also altered when extra- or intracellular Cl⁻ concentrations were changed, as were depolarising responses to γ-aminobutyric acid. Depolarisations and noradrenaline release triggered by oxotremorine M were reduced by the non-selective Cl⁻ channel blockers 4-acetamido-4′-isothiocyanato-stilbene-2,2′-disulfonic acid and niflumic acid. Oxotremorine M induced slowly rising inward currents at negative membrane potentials that were blocked by inhibitors of Ca²⁺-activated Cl⁻ and TMEM16A channels and attenuated by PKC inhibitors. These channel blockers also reduced oxotremorine M-evoked noradrenaline release. Together, these results reveal that slow cholinergic excitation of sympathetic neurons involves the activation of classical PKCs and of Ca²⁺-activated Cl⁻ channels in addition to the well-known inhibition of K_v7 channels.

Facilitation of transmitter release from rat sympathetic neurons via presynaptic P2Y(1) receptors

BACKGROUND AND PURPOSE

P2Y(1) , P2Y(2) , P2Y(4) , P2Y(12) and P2Y(13) receptors for nucleotides have been reported to mediate presynaptic inhibition, but unequivocal evidence for facilitatory presynaptic P2Y receptors is not available. The search for such receptors was the purpose of this study.

EXPERIMENTAL APPROACH

In primary cultures of rat superior cervical ganglion neurons and in PC12 cell cultures, currents were recorded via the perforated patch clamp technique, and the release of [(3) H]-noradrenaline was determined.

KEY RESULTS

ADP, 2-methylthio-ATP and ATP enhanced stimulation-evoked (3) H overflow from superior cervical ganglion neurons, treated with pertussis toxin to prevent the signalling of inhibitory G proteins. This effect was abolished by P2Y(1) antagonists and by inhibition of phospholipase C, but not by inhibition of protein kinase C or depletion of intracellular Ca(2+) stores. ADP and a specific P2Y(1) agonist caused inhibition of Kv7 channels, and this was prevented by a respective antagonist. In neurons not treated with pertussis toxin, (3) H overflow was also enhanced by a specific P2Y(1) agonist and by ADP, but only when the P2Y(12) receptors were blocked. ADP also enhanced K(+) -evoked (3) H overflow from PC12 cells treated with pertussis toxin, but only in a clone expressing recombinant P2Y(1) receptors.

CONCLUSIONS AND IMPLICATIONS

These results demonstrate that presynaptic P2Y(1) receptors mediate facilitation of transmitter release from sympathetic neurons most likely through inhibition of Kv7 channels.

Muscarinic pain pharmacology: realizing the promise of novel analgesics by overcoming old challenges

The antinociceptive and analgesic effects of muscarinic receptor ligands in human and nonhuman species have been evident for more than half a century. In this review, we describe the current understanding of the roles of different muscarinic subtypes in pain modulation and their mechanism of action along the pain signaling pathway, including peripheral nociception, spinal cord pain processing, and supraspinal analgesia. Extensive preclinical and clinical validation of these mechanisms points to the development of selective muscarinic agonists as one of the most exciting and promising avenues toward novel pain medications.

Phosphoinositide sensitivity of ion channels, a functional perspective

Phosphoinositides, especially phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] are required for the activity of many different ion channels. This chapter will highlight various aspects of this paradigm, by discussing current knowledge on four different ion channel families: inwardly rectifying K(+) (Kir) channels, KCNQ voltage gated K(+) channels, voltage gated Ca(2+) (VGCC) channels and Transient Receptor Potential (TRP) channels. Our main focus is to discuss functional aspects of this regulation, i.e. how changes in the concentration of PtdIns(4,5)P(2) in the plasma membrane upon phospholipase C activation may modulate the activity of ion channels, and what are the major determinants of this regulation. We also discuss how channels act as coincidence detectors sensing phosphoinositide levels and other signalling molecules. We also briefly discuss the available methods to study phosphoinositide regulation of ion channels, and structural aspects of interaction of ion channel proteins with these phospholipids. Finally, in several cases the effect of PtdIns(4,5)P(2) is more complex than a simple dependence of ion channel activity on the lipid, and we will discuss some these complexities.

Presynaptic muscarinic M(2) receptors modulate glutamatergic transmission in the bed nucleus of the stria terminalis

The anterolateral cell group of the bed nucleus of the stria terminalis (BNST(ALG)) serves as an important relay station in stress circuitry. Limbic inputs to the BNST(ALG) are primarily glutamatergic and activity-dependent changes in this input have been implicated in abnormal behaviors associated with chronic stress and addiction. Significantly, local infusion of acetylcholine (ACh) receptor agonists into the BNST trigger stress-like cardiovascular responses, however, little is known about the effects of these agents on glutamatergic transmission in the BNST(ALG). Here, we show that glutamate- and ACh-containing fibers are found in close association in the BNST(ALG). Moreover, in the presence of the acetylcholinesterase inhibitor, eserine, endogenous ACh release evoked a long-lasting reduction of the amplitude of stimulus-evoked EPSCs. This effect was mimicked by exogenous application of the ACh analog, carbachol, which caused a reversible, dose-dependent, reduction of the evoked EPSC amplitude, and an increase in both the paired-pulse ratio and coefficient of variation, suggesting a presynaptic site of action. Uncoupling of postsynaptic G-proteins with intracellular GDP-β-S, or application of the nicotinic receptor antagonist, tubocurarine, failed to block the carbachol effect. In contrast, the carbachol effect was blocked by prior application of atropine or M(2) receptor-preferring antagonists, and was absent in M(2)/M(4) receptor knockout mice, suggesting that presynaptic M(2) receptors mediate the effect of ACh. Immunoelectron microscopy studies further revealed the presence of M(2) receptors on axon terminals that formed asymmetric synapses with BNST neurons. Our findings suggest that presynaptic M(2) receptors might be an important modulator of the stress circuit and hence a novel target for drug development.

Functional role of M-type (KCNQ) K⁺ channels in adrenergic control of cardiomyocyte contraction rate by sympathetic neurons

M-type (KCNQ) K⁺ channels are known to regulate excitability and firing properties of sympathetic neurons (SNs), but their role in regulating neurotransmitter release is unclear, requiring further study. We sought to use a physiological preparation in which SNs innervate primary cardiomyocytes to evaluate the direct role of M-channels in the release of noradrenaline (NA) from SNs. Co-cultures of rat SNs and mouse cardiomyocytes were prepared, and the contraction rate (CR) of the cardiomyocyte syncytium monitored by video microscopy. We excited the SNs with nicotine, acting on nicotinic acetylcholine receptors, and monitored the increase in CR in the presence or absence of the specific M-channel opener retigabine, or agonists of bradykinin B2 or purinergic P2Y receptors on the SNs. The maximal adrenergic effect on the CR was determined by application of isoproterenol (isoprenaline). To isolate the actions of B2 or P2Y receptor stimulation to the neurons, we prepared cardiomyocytes from B2 receptor or P2Y2 receptor knock-out mice, respectively. We found that co-application of retigabine strongly decreased the nicotine-induced increase in CR. Conversely, co-application of bradykinin or the P2Y-receptor agonist UTP augmented the nicotine-induced increase in CR to about half of the level produced by isoproterenol. All effects on the CR were wholly blocked by propranolol. Our data support the role of M-type K⁺ channels in the control of NA release by SNs at functional adrenergic synapses on cardiomyocytes.We conclude that physiological receptor agonists control the heart rate via the regulation of M-current in SNs.