Determinants of PKC-dependent modulation of a family of neuronal calcium channels

Number and relative abundance of synaptic vesicles in functionally distinct priming states determine synaptic strength and short-term plasticity

Heterogeneity in synaptic strength and short-term plasticity (STP) was characterized in post-hearing rat calyx of Held synapses at near-physiological external [Ca2+] under control conditions and after experimentally induced synaptic potentiation. Kinetic modelling was combined with non-negative tensor factorization (NTF) to separate changes in synaptic vesicle (SV) priming kinetics from those in SV fusion probability (pfu sion). Heterogeneous synaptic strength and STP under control conditions can be fully accounted for by assuming a uniform pfusion among calyx synapses yet profound synapse-to-synapse variation in the resting equilibrium of SVs in functionally distinct priming states. Although synaptic potentiation induced by either elevated resting [Ca2+]i, elevated external [Ca2+] or stimulation of the diacylglycerol (DAG) signalling pathway leads to seemingly similar changes, that is, stronger synapses with less facilitation and more pronounced depression, the underlying mechanisms are different. Specifically, synaptic potentiation induced by the DAG mimetic and Munc13/PKC activator phorbol 12,13-dibutyrate (PDBu) only moderately enhances pfusion but strongly increases the abundance of fusion-competent maturely primed SVs, demonstrating that the dynamic equilibrium of differentially primed SVs critically determines synaptic strength and STP. Activation of the DAG pathway not only stimulates priming at resting [Ca2+]i but further promotes SV pool replenishment at elevated [Ca2+]i following pool-depleting stimulus trains. A two-step priming and fusion scheme which recapitulates the sequential build-up of the molecular SV fusion machinery is capable of reproducing experimentally induced changes in synaptic strength and STP in numerical simulations with a small number of plausible model parameter changes. KEY POINTS: A relatively simple two-step synaptic vesicle (SV) priming and fusion scheme is capable of reproducing experimentally induced changes in synaptic strength and short-term plasticity with a small number of plausible parameter changes. The combination of non-negative tensor factorization (NTF)-decomposition analysis and state modelling allows one to separate experimentally induced changes in SV priming kinetics from those in SV fusion probability. A relatively low sensitivity of the SV priming equilibrium to changes in resting [Ca2+]i suggests that the amplitude of the 'effective' action potential (AP)-induced Ca2+ transient is quite large, likely representing contributions of global and local Ca2+ signals. Enhanced synaptic strength and stronger depression after stimulation of the diacylglycerol (DAG) signalling pathway is primarily caused by enhanced SV priming, leading to increased abundance of maturely primed SVs at rest with comparably small changes in SV fusion probability. Application of DAG mimetics enhances the Ca2+-dependent acceleration of SV priming causing a faster recovery of synaptic strength after pool-depleting stimuli.

PKC regulation of ion channels: The involvement of PIP2

Ion channels are integral membrane proteins whose gating has been increasingly shown to depend on the presence of the low-abundance membrane phospholipid, phosphatidylinositol (4,5) bisphosphate. The expression and function of ion channels is tightly regulated via protein phosphorylation by specific kinases, including various PKC isoforms. Several channels have further been shown to be regulated by PKC through altered surface expression, probability of channel opening, shifts in voltage dependence of their activation, or changes in inactivation or desensitization. In this review, we survey the impact of phosphorylation of various ion channels by PKC isoforms and examine the dependence of phosphorylated ion channels on phosphatidylinositol (4,5) bisphosphate as a mechanistic endpoint to control channel gating.

Distribution of functional opioid receptors in human dorsal root ganglion neurons

Preclinical evidence has highlighted the importance of the μ-opioid peptide (MOP) receptor on primary afferents for both the analgesic actions of MOP receptor agonists, as well as the development of tolerance, if not opioid-induced hyperalgesia. There is also growing interest in targeting other opioid peptide receptor subtypes (δ-opioid peptide [DOP], κ-opioid peptide [KOP], and nociceptin/orphanin-FQ opioid peptide [NOP]) on primary afferents, as alternatives to MOP receptors, which may not be associated with as many deleterious side effects. Nevertheless, results from several recent studies of human sensory neurons indicate that although there are many similarities between rodent and human sensory neurons, there may also be important differences. Thus, the purpose of this study was to assess the distribution of opioid receptor subtypes among human sensory neurons. A combination of pharmacology, patch-clamp electrophysiology, Ca imaging, and single-cell semiquantitative polymerase chain reaction was used. Our results suggest that functional MOP-like receptors are present in approximately 50% of human dorsal root ganglion neurons. δ-opioid peptide-like receptors were detected in a subpopulation largely overlapping that with MOP-like receptors. Furthermore, KOP-like and NOP-like receptors are detected in a large proportion (44% and 40%, respectively) of human dorsal root ganglion neurons with KOP receptors also overlapping with MOP receptors at a high rate (83%). Our data confirm that all 4 opioid receptor subtypes are present and functional in human sensory neurons, where the overlap of DOP, KOP, and NOP receptors with MOP receptors suggests that activation of these other opioid receptor subtypes may also have analgesic efficacy.

Lysophosphatidic Acid-Activated Calcium Signaling Is Elevated in Red Cells from Sickle Cell Disease Patients

(1) Background: It is known that sickle cells contain a higher amount of Ca2+ compared to healthy red blood cells (RBCs). The increased Ca2+ is associated with the most severe symptom of sickle cell disease (SCD), the vaso-occlusive crisis (VOC). The Ca2+ entry pathway received the name of Psickle but its molecular identity remains only partly resolved. We aimed to map the involved Ca2+ signaling to provide putative pharmacological targets for treatment. (2) Methods: The main technique applied was Ca2+ imaging of RBCs from healthy donors, SCD patients and a number of transgenic mouse models in comparison to wild-type mice. Life-cell Ca2+ imaging was applied to monitor responses to pharmacological targeting of the elements of signaling cascades. Infection as a trigger of VOC was imitated by stimulation of RBCs with lysophosphatidic acid (LPA). These measurements were complemented with biochemical assays. (3) Results: Ca2+ entry into SCD RBCs in response to LPA stimulation exceeded that of healthy donors. LPA receptor 4 levels were increased in SCD RBCs. Their activation was followed by the activation of Gi protein, which in turn triggered opening of TRPC6 and CaV2.1 channels via a protein kinase Cα and a MAP kinase pathway, respectively. (4) Conclusions: We found a new Ca2+ signaling cascade that is increased in SCD patients and identified new pharmacological targets that might be promising in addressing the most severe symptom of SCD, the VOC.

Gabapentin and Duloxetine Prevent Oxaliplatin- and Paclitaxel-Induced Peripheral Neuropathy by Inhibiting Extracellular Signal-Regulated Kinase 1/2 (ERK1/2) Phosphorylation in Spinal Cords of Mice

Chemotherapy-induced peripheral neuropathy is a common factor in limiting therapy which can result in therapy cessation or dose reduction. Gabapentin, a calcium channel inhibitor, and duloxetine, a serotonin noradrenaline reuptake inhibitor, are used to treat a variety of pain conditions such as chronic low back pain, postherpetic neuralgia, and diabetic neuropathy. It has been reported that administration of gabapentin suppressed oxaliplatin- and paclitaxel-induced mechanical hyperalgesia in rats. Moreover, duloxetine has been shown to suppress oxaliplatin-induced cold allodynia in rats. However, the mechanisms by which these drugs prevent oxaliplatin- and paclitaxel-induced neuropathy remain unknown. Behavioral assays were performed using cold plate and the von Frey test. The expression levels of proteins were examined using western blot analysis. In this study, we investigated the mechanisms by which gabapentin and duloxetine prevent oxaliplatin- and paclitaxel-induced neuropathy in mice. We found that gabapentin and duloxetine prevented the development of oxaliplatin- and paclitaxel-induced cold and mechanical allodynia. In addition, our results revealed that gabapentin and duloxetine suppressed extracellular signal-regulated protein kinase 1/2 (ERK1/2) phosphorylation in the spinal cord of mice. Moreover, PD0325901 prevented the development of oxaliplatin- and paclitaxel-induced neuropathic-like pain behavior by inhibiting ERK1/2 activation in the spinal cord of mice. In summary, our findings suggest that gabapentin, duloxetine, and PD0325901 prevent the development of oxaliplatin- and paclitaxel-induced neuropathic-like pain behavior by inhibiting ERK1/2 phosphorylation in mice. Therefore, inhibiting ERK1/2 phosphorylation could be an effective preventive strategy against oxaliplatin- and paclitaxel-induced neuropathy.

Dopamine receptors in emesis: Molecular mechanisms and potential therapeutic function

Dopamine is a member of the catecholamine family and is associated with multiple physiological functions. Together with its five receptor subtypes, dopamine is closely linked to neurological disorders such as schizophrenia, Parkinson's disease, depression, attention deficit-hyperactivity, and restless leg syndrome. Unfortunately, several dopamine receptor-based agonists used to treat some of these diseases cause nausea and vomiting as impending side-effects. The high degree of cross interactions of dopamine receptor ligands with many other targets including G-protein coupled receptors, transporters, enzymes, and ion-channels, add to the complexity of discovering new targets for the treatment of nausea and vomiting. Using activation status of signaling cascades as mechanism-based biomarkers to foresee drug sensitivity combined with the development of dopamine receptor-based biased agonists may hold great promise and seems as the next step in drug development for the treatment of such multifactorial diseases. In this review, we update the present knowledge on dopamine and dopamine receptors and their potential roles in nausea and vomiting. The pre- and clinical evidence provided in this review supports the implication of both dopamine and dopamine receptor agonists in the incidence of emesis. Besides the conventional dopaminergic antiemetic drugs, potential novel antiemetic targeting emetic protein signaling cascades may offer superior selectivity profile and potency.

Focus on the pedunculopontine nucleus. Consensus review from the May 2018 brainstem society meeting in Washington, DC, USA

The pedunculopontine nucleus (PPN) is located in the mesopontine tegmentum and is best delimited by a group of large cholinergic neurons adjacent to the decussation of the superior cerebellar peduncle. This part of the brain, populated by many other neuronal groups, is a crossroads for many important functions. Good evidence relates the PPN to control of reflex reactions, sleep-wake cycles, posture and gait. However, the precise role of the PPN in all these functions has been controversial and there still are uncertainties in the functional anatomy and physiology of the nucleus. It is difficult to grasp the extent of the influence of the PPN, not only because of its varied functions and projections, but also because of the controversies arising from them. One controversy is its relationship to the mesencephalic locomotor region (MLR). In this regard, the PPN has become a new target for deep brain stimulation (DBS) for the treatment of parkinsonian gait disorders, including freezing of gait. This review is intended to indicate what is currently known, shed some light on the controversies that have arisen, and to provide a framework for future research.

Bottom-up gamma and bipolar disorder, clinical and neuroepigenetic implications

OBJECTIVES

This limited review examines the role of the reticular activating system (RAS), especially the pedunculopontine nucleus (PPN), one site of origin of bottom-up gamma, in the symptoms of bipolar disorder (BD).

METHODS

The expression of neuronal calcium sensor protein 1 (NCS-1) in the brains of BD patients is increased. It has recently been found that all PPN neurons manifest intrinsic membrane beta/gamma frequency oscillations mediated by high threshold calcium channels, suggesting that it is one source of bottom-up gamma. This review specifically addresses the involvement of these channels in the manifestation of BD.

RESULTS

Excess NCS-1 was found to dampen gamma band oscillations in PPN neurons. Lithium, a first line treatment for BD, was found to decrease the effects of NCS-1 on gamma band oscillations in PPN neurons. Moreover, gamma band oscillations appear to epigenetically modulate gene transcription in PPN neurons, providing a new direction for research in BD.

CONCLUSIONS

This is an area needing much additional research, especially since the dysregulation of calcium channels may help explain many of the disorders of arousal in, elicit unwanted neuroepigenetic modulation in, and point to novel therapeutic avenues for, BD.

Voltage-Sensitive Calcium Channels in the Brain: Relevance to Alcohol Intoxication and Withdrawal

Voltage-sensitive Ca²⁺ (Ca_V) channels are the primary route of depolarization-induced Ca²⁺ entry in neurons and other excitable cells, leading to an increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i). The resulting increase in [Ca²⁺]_i activates a wide range of Ca²⁺-dependent processes in neurons, including neurotransmitter release, gene transcription, activation of Ca²⁺-dependent enzymes, and activation of certain K⁺ channels and chloride channels. In addition to their key roles under physiological conditions, Ca_V channels are also an important target of alcohol, and alcohol-induced changes in Ca²⁺ signaling can disturb neuronal homeostasis, Ca²⁺-mediated gene transcription, and the function of neuronal circuits, leading to various neurological and/or neuropsychiatric symptoms and disorders, including alcohol withdrawal induced-seizures and alcoholism.

In vitro and in vivo phosphorylation of the Cav2.3 voltage-gated R-type calcium channel

During the recording of whole cell currents from stably transfected HEK-293 cells, the decline of currents carried by the recombinant human Cav2.3+β3 channel subunits is related to adenosine triphosphate (ATP) depletion after rupture of the cells. It reduces the number of functional channels and leads to a progressive shift of voltage-dependent gating to more negative potentials (Neumaier F., et al., 2018). 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. These findings indicate that ATP promotes phosphorylation of either the channel or an associated protein, whereas dephosphorylation during cell dialysis results in run-down. Protein phosphorylation is required for Cav2.3 channel function and could directly influence the normal features of current carried by these channels. Therefore, results from in vitro and in vivo phosphorylation of Cav2.3 are summarized to come closer to a functional analysis of structural variations in Cav2.3 splice variants.

Arousal and drug abuse

The reticular activating system (RAS) is not an amorphous region but distinct nuclei with specific membrane properties that dictate their firing during waking and sleep. The locus coeruleus and raphe nucleus fire during waking and slow wave sleep, with the pedunculopontine nucleus (PPN) firing during both waking and REM sleep, the states manifesting arousal-related EEG activity. Two important discoveries in the PPN in the last 10 years are, 1) that some PPN cells are electrically coupled, and 2) every PPN cell manifests high threshold calcium channels that allow them to oscillate at beta/gamma band frequencies. The role of arousal in drug abuse is considered here in terms of the effects of drugs of abuse on these two mechanisms. Drug abuse and the perception of withdrawal/relapse are mediated by neurobiological processes that occur only when we are awake, not when we are asleep. These relationships focus on the potential role of arousal, more specifically of RAS electrical coupling and gamma band activity, in the addictive process as well as the relapse to drug use.

Bottom-up gamma and stages of waking

Gamma activity has been proposed to promote the feed forward or "bottom-up" flow of information from lower to higher regions of the brain during perception. The pedunculopontine nucleus (PPN) modulates waking and REM sleep, and is part of the reticular activating system (RAS). The properties of PPN cells are unique in that all PPN neurons fire maximally at gamma band frequency regardless of electrophysiological or transmitter type, thus proposed as one origin of "bottom-up" gamma. This property is based on the presence of intrinsic membrane oscillations subserved by high threshold, voltage-dependent calcium channels. Moreover, some PPN cells are electrically coupled. Assuming that the population of PPN neurons has the capacity to fire at ∼40Hz coherently, then the population as a whole can be expected to generate a stable gamma band signal. But what if not all the neurons are firing at the peaks of the oscillations? That means that some cells may fire only at the peaks of every second oscillation. Therefore, the population as a whole can be expected to be firing at a net ∼20Hz. If some cells are firing at the peaks of every fourth oscillation, then the PPN as a whole would be firing at ∼10Hz. Firing at rates below 10Hz would imply that the system is seldom firing at the peaks of any oscillation, basically asleep, in slow wave sleep, thus the activation of the RAS is insufficient to promote waking. This hypothesis carries certain implications, one of which is that we awaken in stages as more and more cells are recruited to fire at the peaks of more and more oscillations. For this system, it would imply that, as we awaken, we step from ∼10Hz to ∼20Hz to ∼30Hz to ∼40Hz, that is, in stages and presumably at different levels of awareness. A similar process can be expected to take place as we fall asleep. Awakening can then be considered to be stepwise, not linear. That is, the implication is that the process of waking is a stepwise event, not a gradual increase, suggesting that the brain can spend time at each of these different stages of arousal.

Stimulatory and inhibitory effects of PKC isozymes are mediated by serine/threonine PKC sites of the Cav2.3α1 subunits

Protein kinase C (PKC) isozymes modulate voltage-gated calcium (Ca_v) currents through Ca_v2.2 and Ca_v2.3 channels by targeting serine/threonine (Ser/Thr) phosphorylation sites of Ca_vα₁ subunits. Stimulatory (Thr-422, Ser-2108 and Ser-2132) and inhibitory (Ser-425) sites were identified in the Ca_v2.2α₁ subunits to PKCs βII and ε. In the current study, we investigated if the homologous sites of Ca_v2.3α₁ subunits (stimulatory: Thr-365, Ser-1995 and Ser-2011; inhibitory: Ser-369) behaved in similar manner. Several Ala and Asp mutants were constructed in Ca_v2.3α₁ subunits in such a way that the Ser/Thr sites can be examined in isolation. These mutants or WT Ca_v2.3α₁ along with auxiliary β_1b and α₂/δ subunits were expressed in Xenopus oocytes and the effects of PKCs βII and ε studied on the barium current (I_Ba). Among these sites, stimulatory Thr-365 and Ser-1995 and inhibitory Ser-369 behaved similar to their homologs in Ca_v2.2α₁ subunits. Furthermore PKCs produced neither stimulation nor inhibition when stimulatory Thr-365 or Ser-1995 and inhibitory Ser-369 were present together. However, the PKCs potentiated the I_Ba when two stimulatory sites, Thr-365 and Ser-1995 were present together, thus overcoming the inhibitory effect of Ser-369. Taken together net PKC effect may be the difference between the responses of the stimulatory and inhibitory sites.

Bilirubin augments Ca2+ load of developing bushy neurons by targeting specific subtype of voltage-gated calcium channels

Neonatal brain is particularly vulnerable to pathological levels of bilirubin which elevates and overloads intracellular Ca²⁺, leading to neurotoxicity. However, how voltage-gated calcium channels (VGCCs) are functionally involved in excess calcium influx remains unknown. By performing voltage-clamp recordings from bushy cells in the ventral cochlear nucleus (VCN) in postnatal rat pups (P4-17), we found the total calcium current density was more than doubled over P4-17, but the relative weight of VGCC subtypes changed dramatically, being relatively equal among T, L, N, P/Q and R-type at P4-6 to predominantly L, N, R over T and P/Q at P15-17. Surprisingly, acute administration of bilirubin augmented the VGCC currents specifically mediated by high voltage-activated (HVA) P/Q-type calcium currents. This augment was attenuated by intracellular loading of Ca²⁺ buffer EGTA or calmodulin inhibitory peptide. Our findings indicate that acute exposure to bilirubin increases VGCC currents, primarily by targeting P/Q-type calcium channels via Ca²⁺ and calmodulin dependent mechanisms to overwhelm neurons with excessive Ca²⁺. Since P/Q-subtype calcium channels are more prominent in neonatal neurons (e.g. P4-6) than later stages, we suggest this subtype-specific enhancement of P/Q-type Ca²⁺ currents likely contributes to the early neuronal vulnerability to hyperbilirubinemia in auditory and other brain regions.

Inhibition of oxytocin and vasopressin neuron activity in rat hypothalamic paraventricular nucleus by relaxin-3-RXFP3 signalling

KEY POINTS

Relaxin-3 is a stress-responsive neuropeptide that acts at its cognate receptor, RXFP3, to alter behaviours including feeding. In this study, we have demonstrated a direct, RXFP3-dependent, inhibitory action of relaxin-3 on oxytocin and vasopressin paraventricular nucleus (PVN) neuron electrical activity, a putative cellular mechanism of orexigenic actions of relaxin-3. We observed a Gα_i/o -protein-dependent inhibitory influence of selective RXFP3 activation on PVN neuronal activity in vitro and demonstrated a direct action of RXFP3 activation on oxytocin and vasopressin PVN neurons, confirmed by their abundant expression of RXFP3 mRNA. Moreover, we demonstrated that RXFP3 activation induces a cadmium-sensitive outward current, which indicates the involvement of a characteristic magnocellular neuron outward potassium current. Furthermore, we identified an abundance of relaxin-3-immunoreactive axons/fibres originating from the nucleus incertus in close proximity to the PVN, but associated with sparse relaxin-3-containing fibres/terminals within the PVN.

ABSTRACT

The paraventricular nucleus of the hypothalamus (PVN) plays an essential role in the control of food intake and energy expenditure by integrating multiple neural and humoral inputs. Recent studies have demonstrated that intracerebroventricular and intra-PVN injections of the neuropeptide relaxin-3 or selective relaxin-3 receptor (RXFP3) agonists produce robust feeding in satiated rats, but the cellular and molecular mechanisms of action associated with these orexigenic effects have not been identified. In the present studies, using rat brain slices, we demonstrated that relaxin-3, acting through its cognate G-protein-coupled receptor, RXFP3, hyperpolarized a majority of putative magnocellular PVN neurons (88%, 22/25), including cells producing the anorexigenic neuropeptides, oxytocin and vasopressin. Importantly, the action of relaxin-3 persisted in the presence of tetrodotoxin and glutamate/GABA receptor antagonists, indicating its direct action on PVN neurons. Similar inhibitory effects on PVN oxytocin and vasopressin neurons were produced by the RXFP3 agonist, RXFP3-A2 (82%, 80/98 cells). In situ hybridization histochemistry revealed a strong colocalization of RXFP3 mRNA with oxytocin and vasopressin immunoreactivity in rat PVN neurons. A smaller percentage of putative parvocellular PVN neurons was sensitive to RXFP3-A2 (40%, 16/40 cells). These data, along with a demonstration of abundant peri-PVN and sparse intra-PVN relaxin-3-immunoreactive nerve fibres, originating from the nucleus incertus, the major source of relaxin-3 neurons, identify a strong inhibitory influence of relaxin-3-RXFP3 signalling on the electrical activity of PVN oxytocin and vasopressin neurons, consistent with the orexigenic effect of RXFP3 activation observed in vivo.

Role of calcium channels in bipolar disorder

Bipolar disorder is characterized by a host of sleep-wake abnormalities that suggests that the reticular activating system (RAS) is involved in these symptoms. One of the signs of the disease is a decrease in high frequency gamma band activity, which accounts for a number of additional deficits. Bipolar disorder has also been found to overexpress neuronal calcium sensor protein 1 (NCS-1). Recent studies showed that elements in the RAS generate gamma band activity that is mediated by high threshold calcium (Ca2+) channels. This mini-review provides a description of recent findings on the role of Ca2+ and Ca2+ channels in bipolar disorder, emphasizing the involvement of arousal-related systems in the manifestation of many of the disease symptoms. This will hopefully bring attention to a much-needed area of research and provide novel avenues for therapeutic development.

Bottom-up Gamma: the Pedunculopontine Nucleus and Reticular Activating System

Gamma rhythms have been proposed to promote the feed forward or "bottom-up" flow of information from lower to higher regions in the brain during perception. On the other hand, beta rhythms have been proposed to represent feed back or "top-down" influence from higher regions to lower. The pedunculopontine nucleus (PPN) has been implicated in sleep-wake control and arousal, and is part of the reticular activating system (RAS). This review describes the properties of the cells in this nucleus. These properties are unique, and perhaps it is the particular characteristics of these cells that allow the PPN to be involved in a host of functions and disorders. The fact that all PPN neurons fire maximally at gamma band frequency regardless of electrophysiological or transmitter type, make this an unusual cell group. In other regions, for example in the cortex, cells with such a property represent only a sub-population. More importantly, the fact that this cell group's functions are related to the capacity to generate coherent activity at a preferred natural frequency, gamma band, speaks volumes about how the PPN functions. We propose that "bottom-up" gamma band influence arises in the RAS and contributes to the build-up of the background of activity necessary for preconscious awareness and gamma activity at cortical levels.

Review: Cav2.3 R-type Voltage-Gated Ca2+ Channels - Functional Implications in Convulsive and Non-convulsive Seizure Activity

Background:

Researchers have gained substantial insight into mechanisms of synaptic transmission, hyperexcitability, excitotoxicity and neurodegeneration within the last decades. Voltage-gated Ca²⁺ channels are of central relevance in these processes. In particular, they are key elements in the etiopathogenesis of numerous seizure types and epilepsies. Earlier studies predominantly targeted on Ca_v2.1 P/Q-type and Ca_v3.2 T-type Ca²⁺ channels relevant for absence epileptogenesis. Recent findings bring other channels entities more into focus such as the Ca_v2.3 R-type Ca²⁺ channel which exhibits an intriguing role in ictogenesis and seizure propagation. Ca_v2.3 R-type voltage gated Ca²⁺ channels (VGCC) emerged to be important factors in the pathogenesis of absence epilepsy, human juvenile myoclonic epilepsy (JME), and cellular epileptiform activity, e.g. in CA1 neurons. They also serve as potential target for various antiepileptic drugs, such as lamotrigine and topiramate.

Objective:

This review provides a summary of structure, function and pharmacology of VGCCs and their fundamental role in cellular Ca²⁺ homeostasis. We elaborate the unique modulatory properties of Ca_v2.3 R-type Ca²⁺ channels and point to recent findings in the proictogenic and proneuroapoptotic role of Ca_v2.3 R-type VGCCs in generalized convulsive tonic–clonic and complex-partial hippocampal seizures and its role in non-convulsive absence like seizure activity.

Conclusion:

Development of novel Ca_v2.3 specific modulators can be effective in the pharmacological treatment of epilepsies and other neurological disorders.

Implications of gamma band activity in the pedunculopontine nucleus

The fact that the pedunculopontine nucleus (PPN) is part of the reticular activating system places it in a unique position to modulate sensory input and fight-or-flight responses. Arousing stimuli simultaneously activate ascending projections of the PPN to the intralaminar thalamus to trigger cortical high-frequency activity and arousal, as well as descending projections to reticulospinal systems to alter posture and locomotion. As such, the PPN has become a target for deep brain stimulation for the treatment of Parkinson's disease, modulating gait, posture, and higher functions. This article describes the latest discoveries on PPN physiology and the role of the PPN in a number of disorders. It has now been determined that high-frequency activity during waking and REM sleep is controlled by two different intracellular pathways and two calcium channels in PPN cells. Moreover, there are three different PPN cell types that have one or both calcium channels and may be active during waking only, REM sleep only, or both. Based on the new discoveries, novel mechanisms are proposed for insomnia as a waking disorder. In addition, neuronal calcium sensor protein-1 (NCS-1), which is over expressed in schizophrenia and bipolar disorder, may be responsible for the dysregulation in gamma band activity in at least some patients with these diseases. Recent results suggest that NCS-1 modulates PPN gamma band activity and that lithium acts to reduce the effects of over expressed NCS-1, accounting for its effectiveness in bipolar disorder.

Synergistic analgesic effect of propofol-alfentanil combination through detecting the inhibition of cAMP signal pathway

OBJECTIVE

The study aims to investigate the possible mechanism of the synergistic analgesic effect of propofol-alfentanil combination.

METHODS

The synergistic analgesic effects of propofol-alfentanil combination in Sprague-Dawley (SD) rats were analysed through the von Frey test. Then, we examined the activity of phospholipase C (PLC) and the intracellular levels of Ca(2+) and adenosine 3', 5'cyclic monophosphate (cAMP) in primary neuronal cells of fetal SD rats. We detected the intracellular Ca(2+) concentration by fluorescence and flow cytometry. The PLC activity of the primary neuronal cells was assayed using the EnzChek(®) Direct Phospholipase C Assay Kit. The cAMP content of the cells was assayed using the cAMP Direct Immunoassay Kit (Fluorometric).

KEY FINDINGS

Both propofol and alfentanil treatments depressed cAMP levels and PLC activity, but propofol-alfentanil combination decreased these parameters to a greater extent than alfentanil treatment alone. Propofol and alfentanil both inhibited Ca(2+) channel, but propofol-alfentanil combination suppressed this channel to a greater extent than alfentanil treatment alone. Fluorescent image analysis revealed that both propofol and alfentanil reduced the intracellular levels of Ca(2+) , and propofol-alfentanil combination showed weaker signals than alfentanil alone. Propofol-alfentanil combination significantly reduced intracellular Ca(2+) level, cAMP level and PLC activity.

CONCLUSION

Propofol and alfentanil exert synergistic analgesic effects through the adenylyl cyclase pathway.

Dual Regulation of R-Type CaV2.3 Channels by M1 Muscarinic Receptors

Voltage-gated Ca(2+) (CaV) channels are dynamically modulated by G protein-coupled receptors (GPCR). The M1 muscarinic receptor stimulation is known to enhance CaV2.3 channel gating through the activation of protein kinase C (PKC). Here, we found that M1 receptors also inhibit CaV2.3 currents when the channels are fully activated by PKC. In whole-cell configuration, the application of phorbol 12-myristate 13-acetate (PMA), a PKC activator, potentiated CaV2.3 currents by ∼two-fold. After the PMA-induced potentiation, stimulation of M1 receptors decreased the CaV2.3 currents by 52 ± 8%. We examined whether the depletion of phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) is responsible for the muscarinic suppression of CaV2.3 currents by using two methods: the Danio rerio voltage-sensing phosphatase (Dr-VSP) system and the rapamycin-induced translocatable pseudojanin (PJ) system. First, dephosphorylation of PI(4,5)P2 to phosphatidylinositol 4-phosphate (PI(4)P) by Dr-VSP significantly suppressed CaV2.3 currents, by 53 ± 3%. Next, dephosphorylation of both PI(4)P and PI(4,5)P2 to PI by PJ translocation further decreased the current by up to 66 ± 3%. The results suggest that CaV2.3 currents are modulated by the M1 receptor in a dual mode-that is, potentiation through the activation of PKC and suppression by the depletion of membrane PI(4,5)P2. Our results also suggest that there is rapid turnover between PI(4)P and PI(4,5)P2 in the plasma membrane.

The 10 Hz Frequency: A Fulcrum For Transitional Brain States

A 10 Hz rhythm is present in the occipital cortex when the eyes are closed (alpha waves), in the precentral cortex at rest (mu rhythm), in the superior and middle temporal lobe (tau rhythm), in the inferior olive (projection to cerebellar cortex), and in physiological tremor (underlying all voluntary movement). These are all considered resting rhythms in the waking brain which are "replaced" by higher frequency activity with sensorimotor stimulation. That is, the 10 Hz frequency fulcrum is replaced on the one hand by lower frequencies during sleep, or on the other hand by higher frequencies during volition and cognition. The 10 Hz frequency fulcrum is proposed as the natural frequency of the brain during quiet waking, but is replaced by higher frequencies capable of permitting more complex functions, or by lower frequencies during sleep and inactivity. At the center of the transition shifts to and from the resting rhythm is the reticular activating system, a phylogenetically preserved area of the brain essential for preconscious awareness.

Pedunculopontine Gamma Band Activity and Development

This review highlights the most important discovery in the reticular activating system in the last 10 years, the manifestation of gamma band activity in cells of the reticular activating system (RAS), especially in the pedunculopontine nucleus, which is in charge of waking and rapid eye movement (REM) sleep. The identification of different cell groups manifesting P/Q-type Ca(2+) channels that control waking vs. those that manifest N-type channels that control REM sleep provides novel avenues for the differential control of waking vs. REM sleep. Recent discoveries on the development of this system can help explain the developmental decrease in REM sleep and the basic rest-activity cycle.

The role of auxiliary subunits for the functional diversity of voltage-gated calcium channels

Voltage-gated calcium channels (VGCCs) represent the sole mechanism to convert membrane depolarization into cellular functions like secretion, contraction, or gene regulation. VGCCs consist of a pore-forming α(1) subunit and several auxiliary channel subunits. These subunits come in multiple isoforms and splice-variants giving rise to a stunning molecular diversity of possible subunit combinations. It is generally believed that specific auxiliary subunits differentially regulate the channels and thereby contribute to the great functional diversity of VGCCs. If auxiliary subunits can associate and dissociate from pre-existing channel complexes, this would allow dynamic regulation of channel properties. However, most auxiliary subunits modulate current properties very similarly, and proof that any cellular calcium channel function is indeed modulated by the physiological exchange of auxiliary subunits is still lacking. In this review we summarize available information supporting a differential modulation of calcium channel functions by exchange of auxiliary subunits, as well as experimental evidence in support of alternative functions of the auxiliary subunits. At the heart of the discussion is the concept that, in their native environment, VGCCs function in the context of macromolecular signaling complexes and that the auxiliary subunits help to orchestrate the diverse protein-protein interactions found in these calcium channel signalosomes. Thus, in addition to a putative differential modulation of current properties, differential subcellular targeting properties and differential protein-protein interactions of the auxiliary subunits may explain the need for their vast molecular diversity.

Pedunculopontine arousal system physiology - Implications for insomnia

We consider insomnia a disorder of waking rather than a disorder of sleep. This review examines the role of the reticular activating system, especially the pedunculopontine nucleus, in the symptoms of insomnia, mainly representing an overactive waking drive. We determined that high frequency activity during waking and REM sleep is controlled by two different intracellular pathways and channel types in PPN cells. We found three different PPN cell types that have one or both channels and may be active during waking only, REM sleep only, or both. These discoveries point to a specific mechanism and novel therapeutic avenues for insomnia.

Potentiation of high voltage-activated calcium channels by 4-aminopyridine depends on subunit composition

4-Aminopyridine (4-AP, fampridine) is used clinically to improve neuromuscular function in patients with multiple sclerosis, spinal cord injury, and myasthenia gravis. 4-AP can increase neuromuscular and synaptic transmission by directly stimulating high voltage-activated (HVA) Ca(2+) channels independent of its blocking effect on voltage-activated K(+) channels. Here we provide new evidence that the potentiating effect of 4-AP on HVA Ca(2+) channels depends on the specific combination of voltage-activated calcium channel α1 (Cavα1) and voltage-activated calcium channel β (Cavβ) subunits. Among the four Cavβ subunits examined, Cavβ3 was the most significant subunit involved in the 4-AP-induced potentiation of both L-type and N-type currents. Of particular note, 4-AP at micromolar concentrations selectively potentiated L-type currents reconstituted with Cav1.2, α2δ1, and Cavβ3. In contrast, 4-AP potentiated N-type currents only at much higher concentrations and had little effect on P/Q-type currents. In a phrenic nerve-diaphragm preparation, blocking L-type Ca(2+) channels eliminated the potentiating effect of low concentrations of 4-AP on end-plate potentials. Furthermore, 4-AP enhanced the physical interaction of Cav1.2 and Cav2.2 subunits to Cavβ3 and also increased their trafficking to the plasma membrane. Site-directed mutagenesis identified specific regions in the guanylate kinase, HOOK, and C-terminus domains of the Cavβ3 subunit crucial to the ability of 4-AP to potentiate L-type and N-type currents. Our findings indicate that 4-AP potentiates HVA Ca(2+) channels by enhancing reciprocal Cav1.2-Cavβ3 and Cav2.2-Cavβ3 interactions. The therapeutic effect of 4-AP on neuromuscular function is probably mediated by its actions on Cavβ3-containing L-type Ca(2+) channels.

Contribution of protein kinase Cα in the stimulation of insulin by the down-regulation of Cavβ subunits

Voltage-gated calcium (Cav) channels and protein kinase C (PKC) isozymes are involved in insulin secretion. In addition, Cavβ, one of the auxiliary subunits of Cav channels, also regulates the secretion of insulin as knockout of Cavβ3 (β3(-/-)) subunits in mice led to efficient glucose homeostasis and increased insulin levels. We examined whether other types of Cavβ subunits also have similar properties. In this regard, we used small interfering RNA (siRNA) of these subunits (20 μg each) to down-regulate them and examined blood glucose, serum insulin and PKC translocation in isolated pancreatic β cells of mice. While the down-regulation of Cavβ2 and β3 subunits increased serum insulin levels and caused efficient glucose homeostasis, the down-regulation of Cavβ1 and β4 subunits failed to affect both these parameters. Examination of PKC isozymes in the pancreatic β-cells of Cavβ2- or β3 siRNA-injected mice showed that three PKC isozymes, viz., PKC α, βII and θ, translocated to the membrane. This suggests that when present, Cavβ2 and β3 subunits inhibited PKC activation. Among these three isozymes, only PKCα siRNA inhibited insulin and increased glucose concentrations. It is possible that the activation of PKCs βII and θ is not sufficient for the release of insulin and PKCα is the mediator of insulin secretion under the control of Cavβ subunits. Since Cavβ subunits are present intracellularly, it is possible that they (1) inhibited the translocation of PKC isozymes to the membrane and (2) decreased the interaction between Cav channels and PKC isozymes and thus the secretion of insulin.

Exogenous α-synuclein decreases raft partitioning of Cav2.2 channels inducing dopamine release

α-Synuclein is thought to regulate neurotransmitter release through multiple interactions with presynaptic proteins, cytoskeletal elements, ion channels, and synaptic vesicles membrane. α-Synuclein is abundant in the presynaptic compartment, and its release from neurons and glia has been described as responsible for spreading of α-synuclein-derived pathology. α-Synuclein-dependent dysregulation of neurotransmitter release might occur via its action on surface-exposed calcium channels. Here, we provide electrophysiological and biochemical evidence to show that α-synuclein, applied to rat neurons in culture or striatal slices, selectively activates Cav2.2 channels, and said activation correlates with increased neurotransmitter release. Furthermore, in vivo perfusion of α-synuclein into the striatum also leads to acute dopamine release. We further demonstrate that α-synuclein reduces the amount of plasma membrane cholesterol and alters the partitioning of Cav2.2 channels, which move from raft to cholesterol-poor areas of the plasma membrane. We provide evidence for a novel mechanism through which α-synuclein acts from the extracellular milieu to modulate neurotransmitter release and propose a unifying hypothesis for the mechanism of α-synuclein action on multiple targets: the reorganization of plasma membrane microdomains.

Effects of calcium (Ca(2+)) extrusion mechanisms on electrophysiological properties in a hypoglossal motoneuron: insight from a mathematical model

Spike-frequency dynamics and spike shape can provide insight into the types of ion channels present in any given neuron and give a sense for the precise response any neuron may have to a given input stimulus. Motoneuron firing frequency over time is especially important due to its direct effect on motor output. Of particular interest is intracellular Ca(2+), which exerts a powerful influence on both firing properties over time and spike shape. In order to better understand the cellular mechanisms for the regulation of intracellular Ca(2+) and their effect on spiking behavior, we have modified a computational model of an HM to include a variety of Ca(2+) handling processes. For the current study, a series of HM models that include Ca(2+) pumps, Na(+)/Ca(2+) exchangers, and a generic exponential decay of excess Ca(2+) were generated. Simulations from these models indicate that although each extrusion mechanism exerts a similar effect on voltage, the firing properties change distinctly with the inclusion of additional Ca(2+)-related mechanisms: BK channels, Ca(2+) buffering, and diffusion of [Ca(2+)]i modeled via a linear diffusion partial differential equation. While an exponential decay of Ca(2+) seems to adequately capture short-term changes in firing frequency seen in biological data, internal diffusion of Ca(2+) appears to be necessary for capturing longer term frequency changes.

Phorbol ester modulation of Ca2+ channels mediates nociceptive transmission in dorsal horn neurones

Phorbol esters are analogues of diacylglycerol which activate C1 domain proteins, such as protein kinase C (PKC). Phorbol ester/PKC pathways have been proposed as potential therapeutic targets for chronic pain states, potentially by phosphorylating proteins involved in nociception, such as voltage-dependent Ca²⁺ channels (VDCCs). In this brief report, we investigate the potential involvement of Ca_V2 VDCC subtypes in functional effects of the phorbol ester, phorbol 12-myristate 13-acetate (PMA) on nociceptive transmission in the spinal cord. Effects of PMA and of selective pharmacological blockers of Ca_V2 VDCC subtypes on nociceptive transmission at laminae II dorsal horn neurones were examined in mouse spinal cord slices. Experiments were extended to Ca_V2.3(−/−) mice to complement pharmacological studies. PMA increased the mean frequency of spontaneous postsynaptic currents (sPSCs) in dorsal horn neurones, without an effect on event amplitude or half-width. sPSC frequency was reduced by selective VDCC blockers, ω-agatoxin-IVA (AgTX; Ca_V2.1), ω-conotoxin-GVIA (CTX; Ca_V2.2) or SNX-482 (Ca_V2.3). PMA effects were attenuated in the presence of each VDCC blocker and, also, in Ca_V2.3(−/−) mice. These initial data demonstrate that PMA increases nociceptive transmission at dorsal horn neurones via actions on different Ca_V2 subtypes suggesting potential anti-nociceptive targets in this system.

Calcium and potassium channels in experimental subarachnoid hemorrhage and transient global ischemia

Healthy cerebrovascular myocytes express members of several different ion channel families which regulate resting membrane potential, vascular diameter, and vascular tone and are involved in cerebral autoregulation. In animal models, in response to subarachnoid blood, a dynamic transition of ion channel expression and function is initiated, with acute and long-term effects differing from each other. Initial hypoperfusion after exposure of cerebral vessels to oxyhemoglobin correlates with a suppression of voltage-gated potassium channel activity, whereas delayed cerebral vasospasm involves changes in other potassium channel and voltage-gated calcium channels expression and function. Furthermore, expression patterns and function of ion channels appear to differ between main and small peripheral vessels, which may be key in understanding mechanisms behind subarachnoid hemorrhage-induced vasospasm. Here, changes in calcium and potassium channel expression and function in animal models of subarachnoid hemorrhage and transient global ischemia are systematically reviewed and their clinical significance discussed.

Molecular and biophysical basis of glutamate and trace metal modulation of voltage-gated Ca(v)2.3 calcium channels

Here, we describe a new mechanism by which glutamate (Glu) and trace metals reciprocally modulate activity of the Ca_v2.3 channel by profoundly shifting its voltage-dependent gating. We show that zinc and copper, at physiologically relevant concentrations, occupy an extracellular binding site on the surface of Ca_v2.3 and hold the threshold for activation of these channels in a depolarized voltage range. Abolishing this binding by chelation or the substitution of key amino acid residues in IS1–IS2 (H111) and IS2–IS3 (H179 and H183) loops potentiates Ca_v2.3 by shifting the voltage dependence of activation toward more negative membrane potentials. We demonstrate that copper regulates the voltage dependence of Ca_v2.3 by affecting gating charge movements. Thus, in the presence of copper, gating charges transition into the “ON” position slower, delaying activation and reducing the voltage sensitivity of the channel. Overall, our results suggest a new mechanism by which Glu and trace metals transiently modulate voltage-dependent gating of Ca_v2.3, potentially affecting synaptic transmission and plasticity in the brain.

Functional coupling of the metabotropic glutamate receptor, InsP3 receptor and L-type Ca2+ channel in mouse CA1 pyramidal cells

Activity-dependent regulation of calcium dynamics in neuronal cells can play significant roles in the modulation of many cellular processes such as intracellular signalling, neuronal activity and synaptic plasticity. Among many calcium influx pathways into neurons, the voltage-dependent calcium channel (VDCC) is the major source of calcium influx, but its modulation by synaptic activity has still been under debate. While the metabotropic glutamate receptor (mGluR) is supposed to modulate L-type VDCCs (L-VDCCs), its reported actions include both facilitation and suppression, probably reflecting the uncertainty of both the molecular targets of the mGluR agonists and the source of the recorded calcium signal in previous reports. In this study, using subtype-specific knockout mice, we have shown that mGluR5 induces facilitation of the depolarization-evoked calcium current. This facilitation was not accompanied by the change in single-channel properties of the VDCC itself; instead, it required the activation of calcium-induced calcium release (CICR) that was triggered by VDCC opening, suggesting that the opening of CICR-coupled cation channels was essential for the facilitation. This facilitation was blocked or reduced by the inhibitors of both L-VDCCs and InsP3 receptors (InsP3Rs). Furthermore, L-VDCCs and mGluR5 were shown to form a complex by coimmunoprecipitation, suggesting that the specific functional coupling between mGluR5, InsP3Rs and L-VDCCs played a pivotal role in the calcium-current facilitation. Finally, we showed that mGluR5 enhanced VDCC-dependent long-term potentiation (LTP) of synaptic transmission. Our study has identified a novel mechanism of the interaction between the mGluR and calcium signalling, and suggested a contribution of mGluR5 to synaptic plasticity.

Activation of alpha1-adrenoceptors enhances glutamate release onto ventral tegmental area dopamine cells

The ventral tegmental area (VTA) plays an important role in reward and motivational processes that facilitate the development of drug addiction. Glutamatergic inputs into the VTA contribute to dopamine (DA) neuronal activation related to reward and response-initiating effects in drug abuse. Previous investigations indicate that alpha1-adrenoreceptors (α1-ARs) are primarily localized at presynaptic elements in the ventral midbrain. Studies from several brain regions have shown that presynaptic α1-AR activation enhances glutamate release. Therefore, we hypothesized that glutamate released onto VTA-DA neurons is modulated by pre-synaptic α1-AR. Recordings were obtained from putative VTA-DA cells of male Sprague-Dawley rats (28-50 days postnatal) using voltage clamp techniques. Phenylephrine (10 μM) and methoxamine (80μM), both α1-AR agonists, increased AMPA receptor-mediated excitatory postsynaptic currents' (EPSCs) amplitude evoked by electrical stimulation of afferent fibers (p<0.05). This effect was blocked by the α1-AR antagonist prazosin (1 μM). Phenylephrine decreased the paired-pulse ratio (PPR) and increased spontaneous EPSCs' frequencies but not their amplitudes suggesting a presynaptic locus of action. No changes in miniature EPSCs (0.5μM, tetrodotoxin [TTX]) were observed after phenylephrine's application which suggests that α1-AR effect was action potential dependent. Normal extra- and intracellular Ca(2+) concentration seems necessary for the α1-AR effect since phenylephrine in low Ca(2+) artificial cerebrospinal fluid (ACSF) and depletion of intracellular Ca(2+) stores with thapsigargin (10 μM) failed to increase the AMPA EPSCs' amplitude. Chelerythrine (1μM, protein kinase C (PKC) inhibitor) but not Rp-cAMPS (11 μM, PKA inhibitor) blocked the α1-AR activation effect on AMPA EPSCs, indicating that a PKC intracellular pathway is required. These results demonstrated that presynaptic α1-AR activation modulates glutamatergic inputs that affect VTA-DA neuronal excitability. α1-AR action might be heterosynaptically localized at glutamatergic fibers terminating onto VTA-DA neurons. It is suggested that drug-induced changes in α1-AR could be part of the neuroadaptations occurring in the mesocorticolimbic circuitry during the addiction process.

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.

ERK, synaptic plasticity and acid-induced muscle pain

Chronic pain is characterized by post-injury pain hypersensitivity. Current evidence suggests that it might result from altered neuronal excitability and/or synaptic functions in pain-related pathways and brain areas, an effect known as central sensitization. Increased activity of extracellular signal-regulated kinase (ERK) has been well-demonstrated in the dorsal horn of the spinal cord in chronic pain animal models. Recently, increased ERK activity has also been identified in two supraspinal areas, the central amygdala and the paraventricular thalamic nucleus anterior. Our recent work on the capsular central amygdala has shown that this increased ERK activity can enhance synaptic transmission, which might account for central sensitization and behavior hypersensitivity in animals receiving noxious stimuli.

Role of extracellular signal-regulated kinase in synaptic transmission and plasticity of a nociceptive input on capsular central amygdaloid neurons in normal and acid-induced muscle pain mice

Application of phorbol 12,13-diacetate (PDA) caused marked enhancement of synaptic transmission of nociceptive parabrachio-amygdaloid (PBA) input onto neurons of the capsular central amygdaloid (CeAC) nucleus. The potentiation of PBA-CeAC EPSCs by PDA involved a presynaptic protein kinase C (PKC)-dependent component and a postsynaptic PKC-extracellular-regulated kinase (ERK)-dependent component. NMDA glutamatergic receptor (NMDAR)-dependent long-term potentiation (LTP) of PBA-CeAC EPSCs, which was also dependent on the PKC-ERK signaling pathway, was induced by tetanus stimulation at 100 Hz. In slices from mice subjected to acid-induced muscle pain (AIMP), phosphorylated ERK levels in the CeAC increased, and PBA-CeAC synaptic transmission was postsynaptically enhanced. The enhanced PBA-CeAC synaptic transmission in AIMP mice shared common mechanisms with the postsynaptic potentiation effect of PDA and induction of NMDAR-dependent LTP by high-frequency stimulation in normal slices, both of which required ERK activation. Since the CeAC plays an important role in the emotionality of pain, enhanced synaptic function of nociceptive (PBA) inputs onto CeAC neurons might partially account for the supraspinal mechanisms underlying central sensitization.

A post-burst after depolarization is mediated by group i metabotropic glutamate receptor-dependent upregulation of Ca(v)2.3 R-type calcium channels in CA1 pyramidal neurons

The excitability of hippocampal pyramidal neurons is regulated by activation of metabotropic glutamate receptors, an effect that is mediated by modulation of R-type calcium channels.

Activation of group I metabotropic glutamate receptors (subtypes mGluR1 and mGluR5) regulates neural activity in a variety of ways. In CA1 pyramidal neurons, activation of group I mGluRs eliminates the post-burst afterhyperpolarization (AHP) and produces an afterdepolarization (ADP) in its place. Here we show that upregulation of Ca_v2.3 R-type calcium channels is responsible for a component of the ADP lasting several hundred milliseconds. This medium-duration ADP is rapidly and reversibly induced by activation of mGluR5 and requires activation of phospholipase C (PLC) and release of calcium from internal stores. Effects of mGluR activation on subthreshold membrane potential changes are negligible but are large following action potential firing. Furthermore, the medium ADP exhibits a biphasic activity dependence consisting of short-term facilitation and longer-term inhibition. These findings suggest that mGluRs may dramatically alter the firing of CA1 pyramidal neurons via a complex, activity-dependent modulation of Ca_v2.3 R-type channels that are activated during spiking at physiologically relevant rates and patterns.

The hippocampus is an essential structure in the brain for the formation of new declarative memories. Understanding the cellular basis of memory formation, storage, and recall in the hippocampus requires a knowledge of the properties of the relevant neurons and how they are modulated by activity in the neural circuit. For many years, we have known that various chemical neurotransmitters can modulate the electrical excitability of neurons in the hippocampus. Here, we report new experiments to reveal how the chemical neurotransmitter glutamate increases neuronal excitability. The effect we study is the conversion of the afterhyperpolarization (a cellular consequence of firing an action potential) to an afterdepolarization. We identified the metabotropic glutamate receptors involved in this conversion (receptors called mGluR1 and mGluR5) as well as the final target of modulation (R-type calcium channels composed of Ca_v2.3 subunits), which cause the neurons to exhibit altered excitability in the presence of glutamate. We also determined some of the intermediate steps between activation of the glutamate receptors and modulation of the calcium channels responsible for the change in excitability, offering further mechanistic insight into how synaptic transmission can regulate cellular and network activity.

The ß subunit of voltage-gated Ca2+ channels

Calcium regulates a wide spectrum of physiological processes such as heartbeat, muscle contraction, neuronal communication, hormone release, cell division, and gene transcription. Major entryways for Ca(2+) in excitable cells are high-voltage activated (HVA) Ca(2+) channels. These are plasma membrane proteins composed of several subunits, including α(1), α(2)δ, β, and γ. Although the principal α(1) subunit (Ca(v)α(1)) contains the channel pore, gating machinery and most drug binding sites, the cytosolic auxiliary β subunit (Ca(v)β) plays an essential role in regulating the surface expression and gating properties of HVA Ca(2+) channels. Ca(v)β is also crucial for the modulation of HVA Ca(2+) channels by G proteins, kinases, and the Ras-related RGK GTPases. New proteins have emerged in recent years that modulate HVA Ca(2+) channels by binding to Ca(v)β. There are also indications that Ca(v)β may carry out Ca(2+) channel-independent functions, including directly regulating gene transcription. All four subtypes of Ca(v)β, encoded by different genes, have a modular organization, consisting of three variable regions, a conserved guanylate kinase (GK) domain, and a conserved Src-homology 3 (SH3) domain, placing them into the membrane-associated guanylate kinase (MAGUK) protein family. Crystal structures of Ca(v)βs reveal how they interact with Ca(v)α(1), open new research avenues, and prompt new inquiries. In this article, we review the structure and various biological functions of Ca(v)β, with both a historical perspective as well as an emphasis on recent advances.

Down-modulation of Ca2+ channels by endogenously released ATP and opioids: from the isolated chromaffin cell to the slice of adrenal medullae

Modifications in Ca(2+) influx may lead to profound changes in the cell activity associated with Ca(2+)-dependent processes, from muscle contraction and neurotransmitter release to calcium-mediated cell death. Therefore, calcium entry into the cell requires fine regulation. In this context, understanding of the modulation of voltage-dependent Ca(2+) channels seems to be critical. The modulatory process results in the enhancement or decrement of calcium influx that may regulate the local and global cytosolic Ca(2+) concentrations. Here, we summarize the well-established data on this matter described in isolated chromaffin cells by our laboratory and others, and the new results we have obtained in a more physiological preparation: freshly isolated slices of mouse adrenal medullae.

FMRFamide-like peptides (FLPs) enhance voltage-gated calcium currents to elicit muscle contraction in the human parasite Schistosoma mansoni

Schistosomes are amongst the most important and neglected pathogens in the world, and schistosomiasis control relies almost exclusively on a single drug. The neuromuscular system of schistosomes is fertile ground for therapeutic intervention, yet the details of physiological events involved in neuromuscular function remain largely unknown. Short amidated neuropeptides, FMRFamide-like peptides (FLPs), are distributed abundantly throughout the nervous system of every flatworm examined and they produce potent myoexcitation. Our goal here was to determine the mechanism by which FLPs elicit contractions of schistosome muscle fibers. Contraction studies showed that the FLP Tyr-Ile-Arg-Phe-amide (YIRFamide) contracts the muscle fibers through a mechanism that requires Ca²⁺ influx through sarcolemmal voltage operated Ca²⁺ channels (VOCCs), as the contractions are inhibited by classical VOCC blockers nicardipine, verapamil and methoxyverapamil. Whole-cell patch-clamp experiments revealed that inward currents through VOCCs are significantly and reversibly enhanced by the application of 1 µM YIRFamide; the sustained inward currents were increased to 190% of controls and the peak currents were increased to 180%. In order to examine the biochemical link between the FLP receptor and the VOCCs, PKC inhibitors calphostin C, RO 31–8220 and chelerythrine were tested and all produced concentration dependent block of the contractions elicited by 1 µM YIRFamide. Taken together, the data show that FLPs elicit contractions by enhancing Ca²⁺ influx through VOCC currents using a PKC-dependent pathway.

Schistosomiasis (bilharzia) is caused by infection with trematodes of the genus Schistosoma. The disease afflicts over 200 million people, with the bulk of the disease burden focused in some of the world's poorest countries. Schistosomiasis control rests largely on chemotherapy with a single drug, praziquantel, a precarious situation calling for the discovery and development of new antischistosomal agents. One hindrance to the discovery of new drugs is a deficiency of knowledge regarding some basic biological processes of these parasitic worms. Here, we take significant steps toward the elucidation of signaling and pathways involved in schistosome neuromuscular control, a central biological function with proven vulnerability to chemotherapeutic intervention. Neuropeptides are known to be important in flatworm muscle control and here we find that FMRFamide-like peptides act to contract schistosome muscle by enhancing calcium influx through voltage-operated calcium channels. We also found that the receptor for the myoexcitatory neuropeptides uses a protein kinase C pathway to stimulate the voltage-operated calcium channels. Understanding the molecules involved in the neuromuscular physiology of these worms helps to identify potentially useful targets for a new generation of antischistosomal drugs.

Multiple kinase pathways regulate voltage-dependent Ca2+ influx and migration in oligodendrocyte precursor cells

It is becoming increasingly clear that voltage-operated Ca(2+) channels (VOCCs) play a fundamental role in the development of oligodendrocyte progenitor cells (OPCs). Because direct phosphorylation by different kinases is one of the most important mechanisms involved in VOCC modulation, the aim of this study was to evaluate the participation of serine-threonine kinases and tyrosine kinases (TKs) on Ca(2+) influx mediated by VOCCs in OPCs. Calcium imaging revealed that OPCs exhibited Ca(2+) influx after plasma membrane depolarization via L-type VOCCs. Furthermore, VOCC-mediated Ca(2+) influx declined with OPC differentiation, indicating that VOCCs are developmentally regulated in OPCs. PKC activation significantly increased VOCC activity in OPCs, whereas PKA activation produced the opposite effect. The results also indicated that OPC morphological changes induced by PKC activation were partially mediated by VOCCs. Our data clearly suggest that TKs exert an activating influence on VOCC function in OPCs. Furthermore, using the PDGF response as a model to probe the role of TK receptors (TKr) on OPC Ca(2+) uptake, we found that TKr activation potentiated Ca(2+) influx after membrane depolarization. Interestingly, this TKr modulation of VOCCs appeared to be essential for the PDGF enhancement of OPC migration rate, because cell motility was completely blocked by TKr antagonists, as well as VOCC inhibitors, in migration assays. The present study strongly demonstrates that PKC and TKrs enhance Ca(2+) influx induced by depolarization in OPCs, whereas PKA has an inhibitory effect. These kinases modulate voltage-operated Ca(2+) uptake in OPCs and participate in the modulation of process extension and migration.