M1 muscarinic receptors inhibit L-type Ca2+ current and M-current by divergent signal transduction cascades

Modality specific roles for metabotropic GABAergic signaling and calcium induced calcium release mechanisms in regulating cold nociception

Calcium (Ca2+) plays a pivotal role in modulating neuronal-mediated responses to modality-specific sensory stimuli. Recent studies in Drosophila reveal class III (CIII) multidendritic (md) sensory neurons function as multimodal sensors regulating distinct behavioral responses to innocuous mechanical and nociceptive thermal stimuli. Functional analyses revealed CIII-mediated multimodal behavioral output is dependent upon activation levels with stimulus-evoked Ca2+ displaying relatively low vs. high intracellular levels in response to gentle touch vs. noxious cold, respectively. However, the mechanistic bases underlying modality-specific differential Ca2+ responses in CIII neurons remain incompletely understood. We hypothesized that noxious cold-evoked high intracellular Ca2+ responses in CIII neurons may rely upon Ca2+ induced Ca2+ release (CICR) mechanisms involving transient receptor potential (TRP) channels and/or metabotropic G protein coupled receptor (GPCR) activation to promote cold nociceptive behaviors. Mutant and/or CIII-specific knockdown of GPCR and CICR signaling molecules [GABA B -R2, Gαq, phospholipase C, ryanodine receptor (RyR) and Inositol trisphosphate receptor (IP3R)] led to impaired cold-evoked nociceptive behavior. GPCR mediated signaling, through GABA B -R2 and IP3R, is not required in CIII neurons for innocuous touch evoked behaviors. However, CICR via RyR is required for innocuous touch-evoked behaviors. Disruptions in GABA B -R2, IP3R, and RyR in CIII neurons leads to significantly lower levels of cold-evoked Ca2+ responses indicating GPCR and CICR signaling mechanisms function in regulating Ca2+ release. CIII neurons exhibit bipartite cold-evoked firing patterns, where CIII neurons burst during rapid temperature change and tonically fire during steady state cold temperatures. GABA B -R2 knockdown in CIII neurons resulted in disorganized firing patterns during cold exposure. We further demonstrate that application of GABA or the GABA B specific agonist baclofen potentiates cold-evoked CIII neuron activity. Upon ryanodine application, CIII neurons exhibit increased bursting activity and with CIII-specific RyR knockdown, there is an increase in cold-evoked tonic firing and decrease in bursting. Lastly, our previous studies implicated the TRPP channel Pkd2 in cold nociception, and here, we show that Pkd2 and IP3R genetically interact to specifically regulate cold-evoked behavior, but not innocuous mechanosensation. Collectively, these analyses support novel, modality-specific roles for metabotropic GABAergic signaling and CICR mechanisms in regulating intracellular Ca2+ levels and cold-evoked behavioral output from multimodal CIII neurons.

Control of Neuronal Excitability by Cell Surface Receptor Density and Phosphoinositide Metabolism

Phosphoinositides are members of a family of minor phospholipids that make up about 1% of all lipids in most cell types. Despite their low abundance they have been found to be essential regulators of neuronal activities such as action potential firing, release and re-uptake of neurotransmitters, and interaction of cytoskeletal proteins with the plasma membrane. Activation of several different neurotransmitter receptors can deplete phosphoinositide levels by more than 90% in seconds, thereby profoundly altering neuronal behavior; however, despite the physiological importance of this mechanism we still lack a profound quantitative understanding of the connection between phosphoinositide metabolism and neuronal activity. Here, we present a model that describes phosphoinositide metabolism and phosphoinositide-dependent action potential firing in sympathetic neurons. The model allows for a simulation of activation of muscarinic acetylcholine receptors and its effects on phosphoinositide levels and their regulation of action potential firing in these neurons. In this paper, we describe the characteristics of the model, its calibration to experimental data, and use the model to analyze how alterations of surface density of muscarinic acetylcholine receptors or altered activity levels of a key enzyme of phosphoinositide metabolism influence action potential firing of sympathetic neurons. In conclusion, the model provides a comprehensive framework describing the connection between muscarinic acetylcholine signaling, phosphoinositide metabolism, and action potential firing in sympathetic neurons which can be used to study the role of these signaling systems in health and disease.

Muscarinic modulation of M and h currents in gerbil spherical bushy cells

Descending cholinergic fibers innervate the cochlear nucleus. Spherical bushy cells, principal neurons of the anterior part of the ventral cochlear nucleus, are depolarized by cholinergic agonists on two different time scales. A fast and transient response is mediated by alpha-7 homomeric nicotinic receptors while a slow and long-lasting response is mediated by muscarinic receptors. Spherical bushy cells were shown to express M3 receptors, but the receptor subtypes involved in the slow muscarinic response were not physiologically identified yet. Whole-cell patch clamp recordings combined with pharmacology and immunohistochemistry were performed to identify the muscarinic receptor subtypes and the effector currents involved. Spherical bushy cells also expressed both M1 and M2 receptors. The M1 signal was stronger and mainly somatic while the M2 signal was localized in the neuropil and on the soma of bushy cells. Physiologically, the M-current was observed for the gerbil spherical bushy cells and was inhibited by oxotremorine-M application. Surprisingly, long application of carbachol showed only a transient depolarization. Even though no muscarinic depolarization could be detected, the input resistance increased suggesting a decrease in the cell conductance that matched with the closure of M-channels. The hyperpolarization-activated currents were also affected by muscarinic activation and counteracted the effect of the inactivation of M-current on the membrane potential. We hypothesize that this double muscarinic action might allow adaptation of effects during long durations of cholinergic activation.

cPLA2α-/- sympathetic neurons exhibit increased membrane excitability and loss of N-Type Ca2+ current inhibition by M1 muscarinic receptor signaling

Group IVa cytosolic phospholipase A₂ (cPLA₂α) mediates GPCR-stimulated arachidonic acid (AA) release from phosphatidylinositol 4,5-bisphosphate (PIP₂) located in plasma membranes. We previously found in superior cervical ganglion (SCG) neurons that PLA₂ activity is required for voltage-independent N-type Ca²⁺ (N-) current inhibition by M₁ muscarinic receptors (M₁Rs). These findings are at odds with an alternative model, previously observed for M-current inhibition, where PIP₂ dissociation from channels and subsequent metabolism by phospholipase C suffices for current inhibition. To resolve cPLA₂α’s importance, we have investigated its role in mediating voltage-independent N-current inhibition (~40%) that follows application of the muscarinic agonist oxotremorine-M (Oxo-M). Preincubation with different cPLA₂α antagonists or dialyzing cPLA₂α antibodies into cells minimized N-current inhibition by Oxo-M, whereas antibodies to Ca²⁺-independent PLA₂ had no effect. Taking a genetic approach, we found that SCG neurons from cPLA₂α^-/- mice exhibited little N-current inhibition by Oxo-M, confirming a role for cPLA₂α. In contrast, cPLA₂α antibodies or the absence of cPLA₂α had no effect on voltage-dependent N-current inhibition by M₂/M₄Rs or on M-current inhibition by M₁Rs. These findings document divergent M₁R signaling mediating M-current and voltage-independent N-current inhibition. Moreover, these differences suggest that cPLA₂α acts locally to metabolize PIP₂ intimately associated with N- but not M-channels. To determine cPLA₂α’s functional importance more globally, we examined action potential firing of cPLA₂α^+/+ and cPLA₂α^-/- SCG neurons, and found decreased latency to first firing and interspike interval resulting in a doubling of firing frequency in cPLA₂α^-/- neurons. These unanticipated findings identify cPLA₂α as a tonic regulator of neuronal membrane excitability.

Modulation of CaV1.3b L-type calcium channels by M1 muscarinic receptors varies with CaVβ subunit expression

OBJECTIVES

We examined whether two G protein-coupled receptors (GPCRs), muscarinic M1 receptors (M1Rs) and dopaminergic D2 receptors (D2Rs), utilize endogenously released fatty acid to inhibit L-type Ca2+ channels, CaV1.3. HEK-293 cells, stably transfected with M1Rs, were used to transiently transfect D2Rs and CaV1.3b with different CaVβ-subunits, allowing for whole-cell current measurement from a pure channel population.

RESULTS

M1R activation with Oxotremorine-M inhibited currents from CaV1.3b coexpressed with α2δ-1 and a β1b, β2a, β3, or β4-subunit. Surprisingly, the magnitude of inhibition was less with β2a than with other CaVβ-subunits. Normalizing currents revealed kinetic changes after modulation with β1b, β3, or β4, but not β2a-containing channels. We then examined if D2Rs modulate CaV1.3b when expressed with different CaVβ-subunits. Stimulation with quinpirole produced little inhibition or kinetic changes for CaV1.3b coexpressed with β2a or β3. However, quinpirole inhibited N-type Ca2+ currents in a concentration-dependent manner, indicating functional expression of D2Rs. N-current inhibition by quinpirole was voltage-dependent and independent of phospholipase A2 (PLA2), whereas a PLA2 antagonist abolished M1R-mediated N-current inhibition. These findings highlight the specific regulation of Ca2+ channels by different GPCRs. Moreover, tissue-specific and/or cellular localization of CaV1.3b with different CaVβ-subunits could fine tune the response of Ca2+ influx following GPCR activation.

Protein phosphorylation maintains the normal function of cloned human Cav2.3 channels

Ca_v2.3 Ca²⁺ channels are subject to cytosolic regulation, which has been difficult to characterize in native cells. Neumaier et al. demonstrate the role of phosphorylation in the function of these channels and suggest a close relationship between voltage dependence and the phosphorylation state.

R-type currents mediated by native and recombinant Ca_v2.3 voltage-gated Ca²⁺ channels (VGCCs) exhibit facilitation (run-up) and subsequent decline (run-down) in whole-cell patch-clamp recordings. A better understanding of the two processes could provide insight into constitutive modulation of the channels in intact cells, but low expression levels and the need for pharmacological isolation have prevented investigations in native systems. Here, to circumvent these limitations, we use conventional and perforated-patch-clamp recordings in a recombinant expression system, which allows us to study the effects of cell dialysis in a reproducible manner. We show that the decline of currents carried by human Ca_v2.3+β₃ channel subunits during run-down is related to adenosine triphosphate (ATP) depletion, which reduces the number of functional channels and leads to a progressive shift of voltage-dependent gating to more negative potentials. Both effects can be counteracted by hydrolysable ATP, whose protective action is almost completely prevented by inhibition of serine/threonine but not tyrosine or lipid kinases. Protein kinase inhibition also mimics the effects of run-down in intact cells, reduces the peak current density, and hyperpolarizes the voltage dependence of gating. Together, our findings indicate that ATP promotes phosphorylation of either the channel or an associated protein, whereas dephosphorylation during cell dialysis results in run-down. These data also distinguish the effects of ATP on Ca_v2.3 channels from those on other VGCCs because neither direct nucleotide binding nor PIP₂ synthesis is required for protection from run-down. We conclude that protein phosphorylation is required for Ca_v2.3 channel function and could directly influence the normal features of current carried by these channels. Curiously, some of our findings also point to a role for leupeptin-sensitive proteases in run-up and possibly ATP protection from run-down. As such, the present study provides a reliable baseline for further studies on Ca_v2.3 channel regulation by protein kinases, phosphatases, and possibly proteases.

Oxotremorine-M potentiates NMDA receptors by muscarinic receptor dependent and independent mechanisms

Muscarinic acetylcholine M1 receptors play an important role in synaptic plasticity in the hippocampus and cortex. Potentiation of NMDA receptors as a consequence of muscarinic acetylcholine M1 receptor activation is a crucial event mediating the cholinergic modulation of synaptic plasticity, which is a cellular mechanism for learning and memory. In Alzheimer's disease, the cholinergic input to the hippocampus and cortex is severely degenerated, and agonists or positive allosteric modulators of M1 receptors are therefore thought to be of potential use to treat the deficits in cognitive functions in Alzheimer's disease. In this study we developed a simple system in which muscarinic modulation of NMDA receptors can be studied in vitro. Human M1 receptors and NR1/2B NMDA receptors were co-expressed in Xenopus oocytes and various muscarinic agonists were assessed for their modulatory effects on NMDA receptor-mediated responses. As expected, NMDA receptor-mediated responses were potentiated by oxotremorine-M, oxotremorine or xanomeline when the drugs were applied between subsequent NMDA responses, an effect which was fully blocked by the muscarinic receptor antagonist atropine. However, in oocytes expressing NR1/2B NMDA receptors but not muscarinic M1 receptors, oxotremorine-M co-applied with NMDA also resulted in a potentiation of NMDA currents and this effect was not blocked by atropine, demonstrating that oxotremorine-M is able to directly potentiate NMDA receptors. Oxotremorine, which is a close analogue of oxotremorine-M, and xanomeline, a chemically distinct muscarinic agonist, did not potentiate NMDA receptors by this direct mechanism. Comparing the chemical structures of the three different muscarinic agonists used in this study suggests that the tri-methyl ammonium moiety present in oxotremorine-M is important for the compound's interaction with NMDA receptors.

Gq Protein-Coupled Membrane-Initiated Estrogen Signaling Rapidly Excites Corticotropin-Releasing Hormone Neurons in the Hypothalamic Paraventricular Nucleus in Female Mice

CRH neurons in the hypothalamic paraventricular nucleus (PVN) play a central role in regulating the hypothalamus-pituitary-adrenal (HPA) axis and are directly influenced by 17β-estradiol (E2). Although compelling evidence has suggested the existence of membrane-associated estrogen receptors (mERs) in hypothalamic and other central nervous system neurons, it remains unknown whether E2 impacts CRH neuronal excitability through this mechanism. The purpose of the current study is to examine the existence and function of mER signaling in PVN CRH neurons. Whole-cell recordings were made from CRH neurons identified by Alexa Fluor 594 labeling and post hoc immunostaining in ovariectomized female mice. E2 (100nM) rapidly suppressed the M-current (a voltage-dependent K(+) current) and potentiated glutamatergic excitatory postsynaptic currents. The putative Gq-coupled mER (Gq-mER) characterized in hypothalamic proopiomelanocortin neurons initiates a phospholipase C-protein kinase C-protein kinase A pathway; therefore, we examined the involvement of this pathway using selective inhibitors. Indeed, the ER antagonist ICI 182780 and inhibitors of Gq-phospholipase C-protein kinase C-protein kinase A blocked E2's actions, suggesting dependence on the Gq-mER. Furthermore, STX, a selective ligand for the Gq-mER, mimicked E2's actions. Finally, to examine the in vivo effect of Gq-mER activation, E2 or STX injection increased c-fos expression in CRH neurons in the PVN, suggesting CRH neuronal activation. This corresponded to an increase in plasma corticosterone. We conclude that the Gq-mER plays a critical role in the rapid regulation of CRH neuronal activity and the HPA axis. Our findings provide a potential underlying mechanism for E2's involvement in the pathophysiology of HPA-associated mood disorders.

Characterization of ST14A Cells for Studying Modulation of Voltage-Gated Calcium Channels

In medium spiny neurons (MSNs) of the striatum, dopamine D₂ receptors (D₂Rs) specifically inhibit the Ca_v1.3 subtype of L-type Ca²⁺ channels (LTCs). MSNs are heterogeneous in their expression of dopamine receptors making the study of D₂R pathways difficult in primary neurons. Here, we employed the ST14A cell line, derived from embryonic striatum and characterized to have properties of MSNs, to study Ca_v1.3 current and its modulation by neurotransmitters. Round, undifferentiated ST14A cells exhibited little to no endogenous Ca²⁺ current while differentiated ST14A cells expressed endogenous Ca²⁺ current. Transfection with LTC subunits produced functional Ca_v1.3 current from round cells, providing a homogeneous model system compared to native MSNs for studying D₂R pathways. However, neither endogenous nor recombinant Ca_v1.3 current was modulated by the D₂R agonist quinpirole. We confirmed D₂R expression in ST14A cells and also detected D₁Rs, D₄Rs, D₅Rs, G_q, calcineurin and phospholipase A₂ using RT-PCR and/or Western blot analysis. Phospholipase C β-1 (PLCβ-1) expression was not detected by Western blot analysis which may account for the lack of LTC modulation by D₂Rs. These findings raise caution about the assumption that the presence of G-protein coupled receptors in cell lines indicates the presence of complete signaling cascades. However, exogenous arachidonic acid inhibited recombinant Ca_v1.3 current indicating that channels expressed in ST14A cells are capable of modulation since they respond to a known signaling molecule downstream of D₂Rs. Thus, ST14A cells provide a MSN-like cell line for studying channel modulation and signaling pathways that do not involve activation of PLCβ-1.

Scopolamine administration modulates muscarinic, nicotinic and NMDA receptor systems

Studies on the effect of scopolamine on memory are abundant but so far only regulation of the muscarinic receptor (M1) has been reported. We hypothesized that levels of other cholinergic brain receptors as the nicotinic receptors and the N-methyl-D-aspartate (NMDA) receptor, known to be involved in memory formation, would be modified by scopolamine administration.

C57BL/6J mice were used for the experiments and divided into four groups. Two groups were given scopolamine 1 mg/kg i.p. (the first group was trained and the second group untrained) in the multiple T-maze (MTM), a paradigm for evaluation of spatial memory. Likewise, vehicle-treated mice were trained or untrained thus serving as controls. Hippocampal levels of M1, nicotinic receptor alpha 4 (Nic4) and 7 (Nic7) and subunit NR1containing complexes were determined by immunoblotting on blue native gel electrophoresis.

Vehicle-treated trained mice learned the task and showed memory retrieval on day 8, while scopolamine-treatment led to significant impairment of performance in the MTM. At the day of retrieval, hippocampal levels for M1, Nic7 and NR1 were higher in the scopolamine treated groups than in vehicle-treated groups.

The concerted action, i.e. the pattern of four brain receptor complexes regulated by the anticholinergic compound scopolamine, is shown. Insight into probable action mechanisms of scopolamine at the brain receptor complex level in the hippocampus is provided. Scopolamine treatment is a standard approach to test cognitive enhancers and other psychoactive compounds in pharmacological studies and therefore knowledge on mechanisms is of pivotal interest.

Membrane-localized β-subunits alter the PIP2 regulation of high-voltage activated Ca2+ channels

The β-subunits of voltage-gated Ca(2+) (Ca(V)) channels regulate the functional expression and several biophysical properties of high-voltage-activated Ca(V) channels. We find that Ca(V) β-subunits also determine channel regulation by the membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PIP(2)). When Ca(V)1.3, -2.1, or -2.2 channels are cotransfected with the β3-subunit, a cytosolic protein, they can be inhibited by activating a voltage-sensitive lipid phosphatase to deplete PIP(2). When these channels are coexpressed with a β2a-subunit, a palmitoylated peripheral membrane protein, the inhibition is much smaller. PIP(2) sensitivity could be increased by disabling the two palmitoylation sites in the β2a-subunit. To further test effects of membrane targeting of Ca(V) β-subunits on PIP(2) regulation, the N terminus of Lyn was ligated onto the cytosolic β3-subunit to confer lipidation. This chimera, like the Ca(V) β2a-subunit, displayed plasma membrane localization, slowed the inactivation of Ca(V)2.2 channels, and increased the current density. In addition, the Lyn-β3 subunit significantly decreased Ca(V) channel inhibition by PIP(2) depletion. Evidently lipidation and membrane anchoring of Ca(V) β-subunits compete with the PIP(2) regulation of high-voltage-activated Ca(V) channels. Compared with expression with Ca(V) β3-subunits alone, inhibition of Ca(V)2.2 channels by PIP(2) depletion could be significantly attenuated when β2a was coexpressed with β3. Our data suggest that the Ca(V) currents in neurons would be regulated by membrane PIP(2) to a degree that depends on their endogenous β-subunit combinations.

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.

AKAP79/150 signal complexes in G-protein modulation of neuronal ion channels

Voltage-gated M-type (KCNQ) K+ channels play critical roles in regulation of neuronal excitability. Previous work showed A-kinase-anchoring protein (AKAP)79/150-mediated protein kinase C (PKC) phosphorylation of M channels to be involved in M current (I(M)) suppression by muscarinic M1, but not bradykinin B2, receptors. In this study, we first explored whether purinergic and angiotensin suppression of I(M) in superior cervical ganglion (SCG) sympathetic neurons involves AKAP79/150. Transfection into rat SCG neurons of ΔA-AKAP79, which lacks the A domain necessary for PKC binding, or the absence of AKAP150 in AKAP150(-/-) mice, did not affect I(M) suppression by purinergic agonist or by bradykinin, but reduced I(M) suppression by muscarinic agonist and angiotensin II. Transfection of AKAP79, but not ΔA-AKAP79 or AKAP15, rescued suppression of I(M) by muscarinic receptors in AKAP150(-/-) neurons. We also tested association of AKAP79 with M(1), B(2), P2Y(6), and AT(1) receptors, and KCNQ2 and KCNQ3 channels, via Förster resonance energy transfer (FRET) on Chinese hamster ovary cells under total internal refection fluorescence microscopy, which revealed substantial FRET between AKAP79 and M1 or AT1 receptors, and with the channels, but only weak FRET with P2Y(6) or B2 receptors. The involvement of AKAP79/150 in G(q/11)-coupled muscarinic regulation of N- and L-type Ca2+) channels and by cAMP/protein kinase A was also studied. We found AKAP79/150 to not play a role in the former, but to be necessary for forskolin-induced upregulation of L-current. Thus, AKAP79/150 action correlates with the PIP(2) (phosphatidylinositol 4,5-bisphosphate)-depletion mode of I(M) suppression, but does not generalize to G(q/11)-mediated inhibition of N- or L-type Ca2+ channels.

GVI phospholipase A2 role in the stimulatory effect of sphingosine-1-phosphate on TRPC5 cationic channels

The Transient Receptor Potential Canonical 5 (TRPC5) protein forms calcium-permeable cationic channels that are stimulated by G protein-coupled receptor agonists. The signaling pathways of such agonist effects are poorly understood. Here we investigated the potential for involvement of lysophosphatidylcholine (LPC) and arachidonic acid generated by group 6 (GVI) phospholipase A2 (PLA2) enzymes, focusing on stimulation of TRPC5 by sphingosine-1-phosphate (S1P) which acts via a pertussis toxin-sensitive (Gi/o protein) pathway without Ca²⁺-release. Experiments were on HEK 293 cells containing conditional expression of human TRPC5. Channel activity was recorded using an intracellular calcium indicator or whole-cell patch-clamp and PLA2 activity was detected using ³H-arachidonic acid. S1P stimulated PLA2 and TRPC5 activities. Both effects were suppressed by the GVI PLA2 inhibitor bromoenol lactone. Knock-down of GVI PLA2 by RNA interference suppressed channel activity evoked by S1P whereas activity evoked by the direct channel stimulator LPC was unaffected. Arachidonic acid did not stimulate the channels. Prior exposure of channels to LPC but not arachidonic acid suppressed channel activity evoked by S1P but not gadolinium, a putative direct stimulator of the channels. The data suggest roles of LPC and GVI PLA2 in S1P-evoked TRPC5 activity.

A comparison of scopolamine and biperiden as a rodent model for cholinergic cognitive impairment

RATIONALE

The nonselective muscarinic antagonist scopolamine hydrobromide (SCOP) is employed as the gold standard for inducing memory impairments in healthy humans and animals. However, its use remains controversial due to the wide spectrum of behavioral effects of this drug.

OBJECTIVE

The present study investigated whether biperiden (BIP), a muscarinic m1 receptor antagonist, is to be preferred over SCOP as a pharmacological model for cholinergic memory deficits in rats. This was done by comparing the effects of SCOP and BIP using a battery of operant tasks: fixed ratio (FR5) and progressive ratio (PR10) schedules of reinforcement, an attention paradigm and delayed nonmatching to position task.

RESULTS

SCOP induced diffuse behavioral disruption, which included sensorimotor responding (FR5, 0.3 and 1 mg/kg), food motivation (PR10, 1 mg/kg), attention (0.3 mg/kg, independent of stimulus duration), and short-term memory (delayed nonmatching to position (DNMTP), 0.1 and 0.3 mg/kg, delay-dependent but also impairment at the zero second delay). BIP induced relatively more selective deficits, as it slowed sensorimotor responding (FR5, 10 mg/kg) and disrupted short-term memory (DNMTP, 3 mg/kg, delay-dependent but no impairment at the zero second delay). BIP had no effect on food motivation (PR10) or attention.

CONCLUSION

Muscarinic m1 antagonists should be considered an interesting alternative for SCOP as a pharmacological model for cholinergic mnemonic deficits in animals.

Modulation of high-voltage activated Ca(2+) channels by membrane phosphatidylinositol 4,5-bisphosphate

Modulation of voltage-gated Ca(2+) channels controls activities of excitable cells. We show that high-voltage activated Ca(2+) channels are regulated by membrane phosphatidylinositol 4,5-bisphosphate (PIP(2)) with different sensitivities. Plasma membrane PIP(2) depletion by rapamycin-induced translocation of an inositol lipid 5-phosphatase or by a voltage-sensitive 5-phosphatase (VSP) suppresses Ca(V)1.2 and Ca(V)1.3 channel currents by approximately 35% and Ca(V)2.1 and Ca(V)2.2 currents by 29% and 55%, respectively. Other Ca(V) channels are less sensitive. Inhibition is not relieved by strong depolarizing prepulses. It changes the voltage dependence of channel gating little. Recovery of currents from inhibition needs intracellular hydrolysable ATP, presumably for PIP(2) resynthesis. When PIP(2) is increased by overexpressing PIP 5-kinase, activation and inactivation of Ca(V)2.2 current slow and voltage-dependent gating shifts to slightly higher voltages. Thus, endogenous membrane PIP(2) supports high-voltage activated L-, N-, and P/Q-type Ca(2+) channels, and stimuli that activate phospholipase C deplete PIP(2) and reduce those Ca(2+) channel currents.

Arachidonic acid inhibition of L-type calcium (CaV1.3b) channels varies with accessory CaVbeta subunits

Arachidonic acid (AA) inhibits the activity of several different voltage-gated Ca²⁺ channels by an unknown mechanism at an unknown site. The Ca²⁺ channel pore-forming subunit (Ca_Vα₁) is a candidate for the site of AA inhibition because T-type Ca²⁺ channels, which do not require accessory subunits for expression, are inhibited by AA. Here, we report the unanticipated role of accessory Ca_Vβ subunits on the inhibition of Ca_V1.3b L-type (L-) current by AA. Whole cell Ba²⁺ currents were measured from recombinant channels expressed in human embryonic kidney 293 cells at a test potential of −10 mV from a holding potential of −90 mV. A one-minute exposure to 10 µM AA inhibited currents with β_1b, β₃, or β₄ 58, 51, or 44%, respectively, but with β_2a only 31%. At a more depolarized holding potential of −60 mV, currents were inhibited to a lesser degree. These data are best explained by a simple model where AA stabilizes Ca_V1.3b in a deep closed-channel conformation, resulting in current inhibition. Consistent with this hypothesis, inhibition by AA occurred in the absence of test pulses, indicating that channels do not need to open to become inhibited. AA had no effect on the voltage dependence of holding potential–dependent inactivation or on recovery from inactivation regardless of Ca_Vβ subunit. Unexpectedly, kinetic analysis revealed evidence for two populations of L-channels that exhibit willing and reluctant gating previously described for Ca_V2 channels. AA preferentially inhibited reluctant gating channels, revealing the accelerated kinetics of willing channels. Additionally, we discovered that the palmitoyl groups of β_2a interfere with inhibition by AA. Our novel findings that the Ca_Vβ subunit alters kinetic changes and magnitude of inhibition by AA suggest that Ca_Vβ expression may regulate how AA modulates Ca²⁺-dependent processes that rely on L-channels, such as gene expression, enzyme activation, secretion, and membrane excitability.

Central cholinesterase inhibition enhances glutamatergic synaptic transmission

Central cholinergic overstimulation results in prolonged seizures of status epilepticus in humans and experimental animals. Cellular mechanisms of underlying seizures caused by cholinergic stimulation remain uncertain, but enhanced glutamatergic transmission is a potential mechanism. Paraoxon, an organophosphate cholinesterase inhibitor, enhanced glutamatergic transmission on hippocampal granule cells synapses by increasing the frequency and amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) in a concentration-dependent fashion. The amplitude of mEPSCs was not increased, which suggested the possibility of enhanced action potential-dependent release. Analysis of EPSCs evoked by minimal stimulation revealed reduced failures and increased amplitude of evoked responses. The ratio of amplitudes of EPSCs evoked by paired stimuli was also altered. The effect of paraoxon on glutamatergic transmission was blocked by the muscarinic antagonist atropine and partially mimicked by carbachol. The nicotinic receptor antagonist α -bungarotoxin did not block the effects of paraoxon; however, nicotine enhanced glutamatergic transmission. These studies suggested that cholinergic overstimulation enhances glutamatergic transmission by enhancing neurotransmitter release from presynaptic terminals.

Orientation of palmitoylated CaVbeta2a relative to CaV2.2 is critical for slow pathway modulation of N-type Ca2+ current by tachykinin receptor activation

The G(q)-coupled tachykinin receptor (neurokinin-1 receptor [NK-1R]) modulates N-type Ca(2+) channel (Ca(V)2.2 or N channel) activity at two distinct sites by a pathway involving a lipid metabolite, most likely arachidonic acid (AA). In another study published in this issue (Heneghan et al. 2009. J. Gen Physiol. doi:10.1085/jgp.200910203), we found that the form of modulation observed depends on which Ca(V)beta is coexpressed with Ca(V)2.2. When palmitoylated Ca(V)beta2a is coexpressed, activation of NK-1Rs by substance P (SP) enhances N current. In contrast, when Ca(V)beta3 is coexpressed, SP inhibits N current. However, exogenously applied palmitic acid minimizes this inhibition. These findings suggested that the palmitoyl groups of Ca(V)beta2a may occupy an inhibitory site on Ca(V)2.2 or prevent AA from interacting with that site, thereby minimizing inhibition. If so, changing the orientation of Ca(V)beta2a relative to Ca(V)2.2 may displace the palmitoyl groups and prevent them from antagonizing AA's actions, thereby allowing inhibition even in the presence of Ca(V)beta2a. In this study, we tested this hypothesis by deleting one (Bdel1) or two (Bdel2) amino acids proximal to the alpha interacting domain (AID) of Ca(V)2.2's I-II linker. Ca(V)betas bind tightly to the AID, whereas the rigid region proximal to the AID is thought to couple Ca(V)beta's movements to Ca(V)2.2 gating. Although Bdel1/beta2a currents exhibited more variable enhancement by SP, Bdel2/beta2a current enhancement was lost at all voltages. Instead, inhibition was observed that matched the profile of N-current inhibition from Ca(V)2.2 coexpressed with Ca(V)beta3. Moreover, adding back exogenous palmitic acid minimized inhibition of Bdel2/beta2a currents, suggesting that when palmitoylated Ca(V)beta2a is sufficiently displaced, endogenously released AA can bind to the inhibitory site. These findings support our previous hypothesis that Ca(V)beta2a's palmitoyl groups directly interact with an inhibitory site on Ca(V)2.2 to block N-current inhibition by SP.

The Ca2+ channel beta subunit determines whether stimulation of Gq-coupled receptors enhances or inhibits N current

In superior cervical ganglion (SCG) neurons, stimulation of M₁ receptors (M₁Rs) produces a distinct pattern of modulation of N-type calcium (N-) channel activity, enhancing currents elicited with negative test potentials and inhibiting currents elicited with positive test potentials. Exogenously applied arachidonic acid (AA) reproduces this profile of modulation, suggesting AA functions as a downstream messenger of M₁Rs. In addition, techniques that diminish AA's concentration during M₁R stimulation minimize N-current modulation. However, other studies suggest depletion of phosphatidylinositol-4,5-bisphosphate during M₁R stimulation suffices to elicit modulation. In this study, we used an expression system to examine the physiological mechanisms regulating modulation. We found the β subunit (Ca_Vβ) acts as a molecular switch regulating whether modulation results in enhancement or inhibition. In human embryonic kidney 293 cells, stimulation of M₁Rs or neurokinin-1 receptors (NK-1Rs) inhibited activity of N channels formed by Ca_V2.2 and coexpressed with Ca_Vβ1b, Ca_Vβ3, or Ca_Vβ4 but enhanced activity of N channels containing Ca_Vβ2a. Exogenously applied AA produced the same pattern of modulation. Coexpression of Ca_Vβ2a, Ca_Vβ3, and Ca_Vβ4 recapitulated the modulatory response previously seen in SCG neurons, implying heterogeneous association of Ca_Vβ with Ca_V2.2. Further experiments with mutated, chimeric Ca_Vβ subunits and free palmitic acid revealed that palmitoylation of Ca_Vβ2a is essential for loss of inhibition. The data presented here fit a model in which Ca_Vβ2a blocks inhibition, thus unmasking enhancement. Our discovery that the presence or absence of palmitoylated Ca_Vβ2a toggles M₁R- or NK-1R–mediated modulation of N current between enhancement and inhibition identifies a novel role for palmitoylation. Moreover, these findings predict that at synapses, modulation of N-channel activity by M₁Rs or NK-1Rs will fluctuate between enhancement and inhibition based on the presence of palmitoylated Ca_Vβ2a.

Selective activation of the M1 muscarinic acetylcholine receptor achieved by allosteric potentiation

The forebrain cholinergic system promotes higher brain function in part by signaling through the M(1) muscarinic acetylcholine receptor (mAChR). During Alzheimer's disease (AD), these cholinergic neurons degenerate, therefore selectively activating M(1) receptors could improve cognitive function in these patients while avoiding unwanted peripheral responses associated with non-selective muscarinic agonists. We describe here benzyl quinolone carboxylic acid (BQCA), a highly selective allosteric potentiator of the M(1) mAChR. BQCA reduces the concentration of ACh required to activate M(1) up to 129-fold with an inflection point value of 845 nM. No potentiation, agonism, or antagonism activity on other mAChRs is observed up to 100 microM. Furthermore studies in M(1)(-/-) mice demonstrates that BQCA requires M(1) to promote inositol phosphate turnover in primary neurons and to increase c-fos and arc RNA expression and ERK phosphorylation in the brain. Radioligand-binding assays, molecular modeling, and site-directed mutagenesis experiments indicate that BQCA acts at an allosteric site involving residues Y179 and W400. BQCA reverses scopolamine-induced memory deficits in contextual fear conditioning, increases blood flow to the cerebral cortex, and increases wakefulness while reducing delta sleep. In contrast to M(1) allosteric agonists, which do not improve memory in scopolamine-challenged mice in contextual fear conditioning, BQCA induces beta-arrestin recruitment to M(1), suggesting a role for this signal transduction mechanism in the cholinergic modulation of memory. In summary, BQCA exploits an allosteric potentiation mechanism to provide selectivity for the M(1) receptor and represents a promising therapeutic strategy for cognitive disorders.

Regulation of voltage-gated Ca2+ channels by lipids

Great skepticism has surrounded the question of whether modulation of voltage-gated Ca(2+) channels (VGCCs) by the polyunsaturated free fatty acid arachidonic acid (AA) has any physiological basis. Here we synthesize findings from studies of both native and recombinant channels where micromolar concentrations of AA consistently inhibit both native and recombinant activity by stabilizing VGCCs in one or more closed states. Structural requirements for these inhibitory actions include a chain length of at least 18 carbons and multiple double bonds located near the fatty acid's carboxy terminus. Acting at a second site, AA increases the rate of VGCC activation kinetics, and in Ca(V)2.2 channels, increases current amplitude. We present evidence that phosphatidylinositol 4,5-bisphosphate (PIP(2)), a palmitoylated accessory subunit (beta(2a)) of VGCCs and AA appear to have overlapping sites of action giving rise to complex channel behavior. Their actions converge in a physiologically relevant manner during muscarinic modulation of VGCCs. We speculate that M(1) muscarinic receptors may stimulate multiple lipases to break down the PIP(2) associated with VGCCs and leave PIP(2)'s freed fatty acid tails bound to the channels to confer modulation. This unexpectedly simple scheme gives rise to unanticipated predictions and redirects thinking about lipid regulation of VGCCs.

Cholinergic responses and intrinsic membrane properties of developing thalamic parafascicular neurons

Parafascicular (Pf) neurons receive cholinergic input from the pedunculopontine nucleus (PPN), which is active during waking and REM sleep. There is a developmental decrease in REM sleep in humans between birth and puberty and 10-30 days in rat. Previous studies have established an increase in muscarinic and 5-HT1 serotonergic receptor-mediated inhibition and a transition from excitatory to inhibitory GABA(A) responses in the PPN during the developmental decrease in REM sleep. However, no studies have been conducted on the responses of Pf cells to the cholinergic input from the PPN during development, which is a major target of ascending cholinergic projections and may be an important mechanism for the generation of rhythmic oscillations in the cortex. Whole cell patch-clamp recordings were performed in 9- to 20-day-old rat Pf neurons in parasagittal slices, and responses to the cholinergic agonist carbachol (CAR) were determined. Three types of responses were identified: inhibitory (55.3%), excitatory (31.1%), and biphasic (fast inhibitory followed by slow excitatory, 6.8%), whereas 6.8% of cells showed no response. The proportion of CAR-inhibited Pf neurons increased with development. Experiments using cholinergic antagonists showed that M2 receptors mediated the inhibitory response, whereas excitatory modulation involved M1, nicotinic, and probably M3 or M5 receptors, and the biphasic response was caused by the activation of multiple types of muscarinic receptors. Compared with CAR-inhibited cells, CAR-excited Pf cells showed 1) a decreased membrane time constant, 2) higher density of hyperpolarization-activated channels (I(h)), 3) lower input resistance (R(in)), 4) lower action potential threshold, and 5) shorter half-width duration of action potentials. Some Pf cells exhibited spikelets, and all were excited by CAR. During development, we observed decreases in I(h) density, R(in), time constant, and action potential half-width. These results suggest that cholinergic modulation of Pf differentially affects separate populations, perhaps including electrically coupled cells. Pf cells tend to show decreased excitability and cholinergic activation during the developmental decrease in REM sleep.

Arachidonic acid-containing phosphatidylcholine species are increased in selected brain regions of a depressive animal model: implications for pathophysiology

The Flinders Sensitive Line (FSL) rat is a genetic animal model of depression. Following recent findings that the brain fatty acid composition of FSL is characterised by increased arachidonic acid (AA), we used electrospray tandem mass spectrometry and (1)H-NMR to examine lipid species in different brain areas. Cholesterol and sphingolipids were increased in the hypothalamus of the FSL rats. Furthermore, arachidonic acid-containing phosphatidylcholine (AA-PC) species were elevated with PC16:0/20:4, PC18:1/20:4 and PC18:0/20:4 (p<0.003) increased in the hypothalamus and striatum. In contrast, there was a decrease in some docosahexaenoic acid (DHA)-containing species, specifically PC18:1/22:6 (p<0.003) in the striatum and PE18:1/22:6 (p<0.004) in the prefrontal cortex. Since no significant differences were observed in the erythrocyte fatty acid concentrations, dietary or environmental causes for these observations are unlikely. The increase in AA-PC species which in this animal model may be associated with altered neuropathy target esterase activity, an enzyme involved in membrane PC homeostasis, may contribute to the depressive phenotype of the FSL rats.

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

BACKGROUND AND PURPOSE

M(2), M(3) and/or M(4) muscarinic acetylcholine receptors have been reported to mediate presynaptic inhibition in sympathetic neurons. M(1) receptors mediate an inhibition of K(v)7, Ca(V)1 and Ca(V)2.2 channels. These effects cause increases and decreases in transmitter release, respectively, but presynaptic M(1) receptors are generally considered facilitatory. Here, we searched for inhibitory presynaptic M(1) receptors.

EXPERIMENTAL APPROACH

In primary cultures of rat superior cervical ganglion neurons, Ca(2+) currents were recorded via the perforated patch-clamp technique, and the release of [(3)H]-noradrenaline was determined.

KEY RESULTS

The muscarinic agonist oxotremorine M (OxoM) transiently enhanced (3)H outflow and reduced electrically evoked release, once the stimulant effect had faded. The stimulant effect was enhanced by pertussis toxin (PTX) and was abolished by blocking M(1) receptors, by opening K(v)7 channels and by preventing action potential propagation. The inhibitory effect was not altered by preventing action potentials or by opening K(v)7 channels, but was reduced by PTX and omega-conotoxin GVIA. The inhibition remaining after PTX treatment was abolished by blockage of M(1) receptors or inhibition of phospholipase C. When [(3)H]-noradrenaline release was triggered independently of voltage-activated Ca(2+) channels (VACCs), OxoM failed to cause any inhibition. The inhibition of Ca(2+) currents by OxoM was also reduced by omega-conotoxin and PTX and was abolished by M(1) antagonism in PTX-treated neurons.

CONCLUSIONS AND IMPLICATIONS

These results demonstrate that M(1), in addition to M(2), M(3) and M(4), receptors mediate presynaptic inhibition in sympathetic neurons using phospholipase C to close VACCs.

Arachidonic acid and ion channels: an update

Arachidonic acid (AA), a polyunsaturated fatty acid with four double bonds, has multiple actions on living cells. Many of these effects are mediated by an action of AA or its metabolites on ion channels. During the last 10 years, new types of ion channels, transient receptor potential (TRP) channels, store-operated calcium entry (SOCE) channels and non-SOCE channels have been studied. This review summarizes our current knowledge about the effects of AA on TRP and non-SOCE channels as well as classical ion channels. It aims to distinguish between effects of AA itself and effects of AA metabolites. Lipid mediators are of clinical interest because some of them (for example, leukotrienes) play a role in various diseases, others (such as prostaglandins) are targets for pharmacological therapeutic intervention.

L- and N-current but not M-current inhibition by M1 muscarinic receptors requires DAG lipase activity

Stimulation of postsynaptic M(1) muscarinic receptors (M(1)Rs) increases firing rates of both sympathetic and central neurons that underlie increases in vasomotor tone, heart rate, and cognitive memory functioning. At the cellular level, M(1)R stimulation modulates currents through various voltage-gated ion channels, including KCNQ K+ channels (M-current) and both L- and N-type Ca2+ channels (L- and N-current) by a pertussis toxin-insensitive, slow signaling pathway. Depletion of phosphatidylinositol-4,5-bisphosphate (PIP2) during M(1)R stimulation suffices to inhibit M-current. We found previously that following PIP2 hydrolysis by phospholipase C, activation of phospholipase A2 and liberation of a lipid metabolite, most likely arachidonic acid (AA) are necessary for L- and N-current modulation. Here we examined the involvement of a third lipase, diacylglycerol lipase (DAGL), in the slow pathway. We documented the presence of DAGL in superior cervical ganglion neurons, and then tested the highly selective DAGL inhibitor, RHC-80267, for its capacity to antagonize M(1)R-mediated modulation of whole-cell Ca2+ currents. RHC-80267 significantly reduced L- and N-current inhibition by the muscarinic agonist oxotremorine-M (Oxo-M) but did not affect their inhibition by exogenous AA. Moreover, voltage-dependent inhibition of N-current by Oxo-M remained in the presence of RHC-80267, indicating selective action on the slow pathway. RHC also blocked inhibition of recombinant N-current. In contrast, RHC-80267 had no effect on native M-current inhibition. These data are consistent with a role for DAGL in mediating L- and N-current inhibition. These results extend our previous findings that the signaling pathway mediating L- and N-current inhibition diverges from the pathway initiating M-current inhibition.

Presynaptic lonotropic receptors

The release of transmitters through vesicle exocytosis from nerve terminals is not constant but is subject to modulation by various mechanisms, including prior activity at the synapse and the presence of neurotransmitters or neuromodulators in the synapse. Instantaneous responses of postsynaptic cells to released transmitters are mediated by ionotropic receptors. In contrast to metabotropic receptors, ionotropic receptors mediate the actions of agonists in a transient manner within milliseconds to seconds. Nevertheless, transmitters can control vesicle exocytosis not only via slowly acting metabotropic, but also via fast acting ionotropic receptors located at the presynaptic nerve terminals. In fact, members of the following subfamilies of ionotropic receptors have been found to control transmitter release: ATP P2X, nicotinic acetylcholine, GABA(A), ionotropic glutamate, glycine, 5-HT(3), andvanilloid receptors. As these receptors display greatly diverging structural and functional features, a variety of different mechanisms are involved in the regulation of transmitter release via presynaptic ionotropic receptors. This text gives an overview of presynaptic ionotropic receptors and briefly summarizes the events involved in transmitter release to finally delineate the most important signaling mechanisms that mediate the effects of presynaptic ionotropic receptor activation. Finally, a few examples are presented to exemplify the physiological and pharmacological relevance of presynaptic ionotropic receptors.

Ciliary neurotrophic factor-treated astrocyte conditioned medium regulates the L-type calcium channel activity in rat cortical neurons

Astrocytes are activated by ciliary neurotrophic factor (CNTF) in vivo and in vitro, however, the consequences on the L-type calcium channel (LCC) of neurons are still poorly understood. Therefore, in the present study, whole-cell patch clamp, western-blot and RT-PCR assay were performed to evaluate the effects of CNTF-treated astrocyte conditioned medium (CNTF-ACM) on LCC current (I(Ca)-L) and the expression of Cav1.2 and Cav1.3 in Sprague-Dawley rat cortical neurons. The results revealed that CNTF-ACM enhanced the amplitude of Ica-L and the expression of Cav1.3 significantly, but had no effects on Cav1.2 expression. We also found an increase in the concentration of fibroblast growth factor-2 (FGF-2) in CNTF-ACM by ELISA assay. Taken together, these findings indicate that CNTF induces the release of factors, including FGF-2, from astrocytes, thereby potentiating the activity of LCC in cortical neurons.

The lipid connection-regulation of voltage-gated Ca(2+) channels by phosphoinositides

Recent findings have revealed a pivotal role for phospholipids phosphatidylinositol -4,5-biphosphate (PIP(2)) and phosphatidylinositol -3,4,5-trisphosphate (PIP(3)) in the regulation of high voltage-activated (HVA) Ca(2+) channels. PIP(2) exerts two opposing actions on HVA Ca(2+) channels: It stabilizes their activity but also produces a voltage-dependent inhibition that can be antagonized by protein kinase A (PKA) phosphorylation. PIP(2) depletion and arachidonic acid together mediate the slow, voltage-independent inhibition of HVA Ca(2+) channels by G( q/11 )-coupled receptors in neurons. A sufficient level of plasma membrane PIP(2) also appears to be necessary for G( betagamma )-mediated inhibition. On the other hand, increased production of PIP(3) by PI-3 kinases promotes trafficking of HVA Ca(2+) channels to the plasma membrane. This review discusses these findings and their implications.