CB1 receptors diminish both Ca(2+) influx and glutamate release through two different mechanisms active in distinct populations of cerebrocortical nerve terminals

Munc13- and SNAP25-dependent molecular bridges play a key role in synaptic vesicle priming

Synaptic vesicle tethering, priming, and neurotransmitter release require a coordinated action of multiple protein complexes. While physiological experiments, interaction data, and structural studies of purified systems were essential for our understanding of the function of the individual complexes involved, they cannot resolve how the actions of individual complexes integrate. We used cryo-electron tomography to simultaneously image multiple presynaptic protein complexes and lipids at molecular resolution in their native composition, conformation, and environment. Our detailed morphological characterization suggests that sequential synaptic vesicle states precede neurotransmitter release, where Munc13-comprising bridges localize vesicles <10 nanometers and soluble N-ethylmaleimide-sensitive factor attachment protein 25-comprising bridges <5 nanometers from the plasma membrane, the latter constituting a molecularly primed state. Munc13 activation supports the transition to the primed state via vesicle bridges to plasma membrane (tethers), while protein kinase C promotes the same transition by reducing vesicle interlinking. These findings exemplify a cellular function performed by an extended assembly comprising multiple molecularly diverse complexes.

The activation of mGluR4 rescues parallel fiber synaptic transmission and LTP, motor learning and social behavior in a mouse model of Fragile X Syndrome

BACKGROUND

Fragile X syndrome (FXS), the most common inherited intellectual disability, is caused by the loss of expression of the Fragile X Messenger Ribonucleoprotein (FMRP). FMRP is an RNA-binding protein that negatively regulates the expression of many postsynaptic as well as presynaptic proteins involved in action potential properties, calcium homeostasis and neurotransmitter release. FXS patients and mice lacking FMRP suffer from multiple behavioral alterations, including deficits in motor learning for which there is currently no specific treatment.

METHODS

We performed electron microscopy, whole-cell patch-clamp electrophysiology and behavioral experiments to characterise the synaptic mechanisms underlying the motor learning deficits observed in Fmr1KO mice and the therapeutic potential of positive allosteric modulator of mGluR4.

RESULTS

We found that enhanced synaptic vesicle docking of cerebellar parallel fiber to Purkinje cell Fmr1KO synapses was associated with enhanced asynchronous release, which not only prevents further potentiation, but it also compromises presynaptic parallel fiber long-term potentiation (PF-LTP) mediated by β adrenergic receptors. A reduction in extracellular Ca2+ concentration restored the readily releasable pool (RRP) size, basal synaptic transmission, β adrenergic receptor-mediated potentiation, and PF-LTP. Interestingly, VU 0155041, a selective positive allosteric modulator of mGluR4, also restored both the RRP size and PF-LTP in mice of either sex. Moreover, when injected into Fmr1KO male mice, VU 0155041 improved motor learning in skilled reaching, classical eyeblink conditioning and vestibuloocular reflex (VOR) tests, as well as the social behavior alterations of these mice.

LIMITATIONS

We cannot rule out that the activation of mGluR4s via systemic administration of VU0155041 can also affect other brain regions. Further studies are needed to stablish the effect of a specific activation of mGluR4 in cerebellar granule cells.

CONCLUSIONS

Our study shows that an increase in synaptic vesicles, SV, docking may cause the loss of PF-LTP and motor learning and social deficits of Fmr1KO mice and that the reversal of these changes by pharmacological activation of mGluR4 may offer therapeutic relief for motor learning and social deficits in FXS.

Trans-synaptic assemblies link synaptic vesicles and neuroreceptors

Synaptic transmission is characterized by fast, tightly coupled processes and complex signaling pathways that require a precise protein organization, such as the previously reported nanodomain colocalization of pre- and postsynaptic proteins. Here, we used cryo-electron tomography to visualize synaptic complexes together with their native environment comprising interacting proteins and lipids on a 2- to 4-nm scale. Using template-free detection and classification, we showed that tripartite trans-synaptic assemblies (subcolumns) link synaptic vesicles to postsynaptic receptors and established that a particular displacement between directly interacting complexes characterizes subcolumns. Furthermore, we obtained de novo average structures of ionotropic glutamate receptors in their physiological composition, embedded in plasma membrane. These data support the hypothesis that synaptic function is carried by precisely organized trans-synaptic units. It provides a framework for further exploration of synaptic and other large molecular assemblies that link different cells or cellular regions and may require weak or transient interactions to exert their function.

Development and Validation of CRISPR Activator Systems for Overexpression of CB1 Receptors in Neurons

Gene therapy approaches using viral vectors for the overexpression of target genes have been for several years the focus of gene therapy research against neurological disorders. These approaches deliver robust expression of therapeutic genes, but are typically limited to the delivery of single genes and often do not manipulate the expression of the endogenous locus. In the last years, the advent of CRISPR-Cas9 technologies have revolutionized many areas of scientific research by providing novel tools that allow simple and efficient manipulation of endogenous genes. One of the applications of CRISPR-Cas9, termed CRISPRa, based on the use of a nuclease-null Cas9 protein (dCas9) fused to transcriptional activators, enables quick and efficient increase in target endogenous gene expression. CRISPRa approaches are varied, and different alternatives exist with regards to the type of Cas9 protein and transcriptional activator used. Several of these approaches have been successfully used in neurons in vitro and in vivo, but have not been so far extensively applied for the overexpression of genes involved in synaptic transmission. Here we describe the development and application of two different CRISPRa systems, based on single or dual Lentiviral and Adeno-Associated viral vectors and VP64 or VPR transcriptional activators, and demonstrate their efficiency in increasing mRNA and protein expression of the Cnr1 gene, coding for neuronal CB1 receptors. Both approaches were similarly efficient in primary neuronal cultures, and achieved a 2–5-fold increase in Cnr1 expression, but the AAV-based approach was more efficient in vivo. Our dual AAV-based VPR system in particular, based on Staphylococcus aureus dCas9, when injected in the hippocampus, displayed almost complete simultaneous expression of both vectors, high levels of dCas9 expression, and good efficiency in increasing Cnr1 mRNA as measured by in situ hybridization. In addition, we also show significant upregulation of CB1 receptor protein in vivo, which is reflected by an increased ability in reducing neurotransmitter release, as measured by electrophysiology. Our results show that CRISPRa techniques could be successfully used in neurons to target overexpression of genes involved in synaptic transmission, and can potentially represent a next-generation gene therapy approach against neurological disorders.

Template-free detection and classification of membrane-bound complexes in cryo-electron tomograms

With faithful sample preservation and direct imaging of fully hydrated biological material, cryo-electron tomography provides an accurate representation of molecular architecture of cells. However, detection and precise localization of macromolecular complexes within cellular environments is aggravated by the presence of many molecular species and molecular crowding. We developed a template-free image processing procedure for accurate tracing of complex networks of densities in cryo-electron tomograms, a comprehensive and automated detection of heterogeneous membrane-bound complexes and an unsupervised classification (PySeg). Applications to intact cells and isolated endoplasmic reticulum (ER) allowed us to detect and classify small protein complexes. This classification provided sufficiently homogeneous particle sets and initial references to allow subsequent de novo subtomogram averaging. Spatial distribution analysis showed that ER complexes have different localization patterns forming nanodomains. Therefore, this procedure allows a comprehensive detection and structural analysis of complexes in situ.

Cryo-Electron Tomography of the Mammalian Synapse

Characterizing the detailed structure of the mammalian synapse is of crucial importance to understand its mechanisms of function. Here I describe a protocol to study synaptic architecture by cryo-electron tomography (cryo-ET), a powerful electron microscopy technique that enables 3D visualization of unstained, fully hydrated cellular structures at molecular resolution. The protocol focuses on purified synaptic terminals ("synaptosomes"), currently the most suitable preparation to analyze mammalian synaptic architecture by cryo-ET.

Synucleins Have Multiple Effects on Presynaptic Architecture

Synucleins (α, β, γ-synuclein) are a family of abundant presynaptic proteins. α-Synuclein is causally linked to the pathogenesis of Parkinson's disease (PD). In an effort to define their physiological and pathological function or functions, we investigated the effects of deleting synucleins and overexpressing α-synuclein PD mutations, in mice, on synapse architecture using electron microscopy (EM) and cryoelectron tomography (cryo-ET). We show that synucleins are regulators of presynapse size and synaptic vesicle (SV) pool organization. Using cryo-ET, we observed that deletion of synucleins increases SV tethering to the active zone but decreases the inter-linking of SVs by short connectors. These ultrastructural changes were correlated with discrete protein phosphorylation changes in αβγ-synuclein-/- neurons. We also determined that α-synuclein PD mutants (PARK1/hA30P and PARK4/hα-syn) primarily affected presynaptic cytomatrix proximal to the active zone, congruent with previous findings that these PD mutations decrease neurotransmission. Collectively, our results suggest that synucleins are important orchestrators of presynaptic terminal topography.

Hierarchical detection and analysis of macromolecular complexes in cryo-electron tomograms using Pyto software

Molecular complexes, arguably the basic units carrying cellular function, can be visualized directly in their native environment by cryo-electron tomography. Here we describe a procedure for the detection of small, pleomorphic membrane-bound molecular complexes in cryo-tomograms by a hierarchical connectivity segmentation. Validation on phantom and real data showed above 90% true positive rates. This segmentation procedure is implemented in the Pyto software package, together with methods for quantitative characterization and classification of complexes detected by our segmentation procedure and for statistical analysis between experimental conditions. Therefore, the methods presented provide a means for the detection and quantitative interpretation of structures captured in cryo-electron tomograms, as well as for the elucidation of their cellular function.

Removing Contamination-Induced Reconstruction Artifacts from Cryo-electron Tomograms

Imaging of fully hydrated, vitrified biological samples by electron tomography yields structural information about cellular protein complexes in situ. Here we present a computational procedure that removes artifacts of three-dimensional reconstruction caused by contamination present in samples during imaging by electron microscopy. Applying the procedure to phantom data and electron tomograms of cellular samples significantly improved the resolution and the interpretability of tomograms. Artifacts caused by surface contamination associated with thinning by focused ion beam, as well as those arising from gold fiducial markers and from common, lower contrast contamination, could be removed. Our procedure is widely applicable and is especially suited for applications that strive to reach a higher resolution and involve the use of recently developed, state-of-the-art instrumentation.

Lack of presynaptic interaction between glucocorticoid and CB1 cannabinoid receptors in GABA- and glutamatergic terminals in the frontal cortex of laboratory rodents

Corticosteroid and endocannabinoid actions converge on prefrontocortical circuits associated with neuropsychiatric illnesses. Corticosteroids can also modulate forebrain synapses by using endocannabinoid effector systems. Here, we determined whether corticosteroids can modulate transmitter release directly in the frontal cortex and, in doing so, whether they affect presynaptic CB1 cannabinoid receptor- (CB1R) mediated neuromodulation. By Western blotting of purified subcellular fractions of the rat frontal cortex, we found glucocorticoid receptors (GcRs) and CB1Rs enriched in isolated frontocortical nerve terminals (synaptosomes). CB1Rs were predominantly presynaptically located while GcRs showed preference for the post-synaptic fraction. Additional confocal microscopy analysis of cortical and hippocampal regions revealed vesicular GABA transporter-positive and vesicular glutamate transporter 1-positive nerve terminals endowed with CB1R immunoreactivity, apposing GcR-positive post-synaptic compartments. In functional transmitter release assay, corticosteroids, corticosterone (0.1-10 microM) and dexamethasone (0.1-10 microM) did not significantly affect the evoked release of [(3)H]GABA and [(14)C]glutamate in superfused synaptosomes, isolated from both rats and mice. In contrast, the synthetic cannabinoid, WIN55212-2 (1 microM) diminished the release of both [(3)H]GABA and [(14)C]glutamate, evoked with various depolarization paradigms. This effect of WIN55212-2 was abolished by the CB1R neutral antagonist, O-2050 (1 microM), and was absent in the CB1R KO mice. CB2R-selective agonists did not affect the release of either neurotransmitter. The lack of robust presynaptic neuromodulation by corticosteroids was unchanged upon either CB1R activation or genetic inactivation. Altogether, corticosteroids are unlikely to exert direct non-genomic presynaptic neuromodulation in the frontal cortex, but they may do so indirectly, via the stimulation of trans-synaptic endocannabinoid signaling.

Palmitoylethanolamide inhibits glutamate release in rat cerebrocortical nerve terminals

The effect of palmitoylethanolamide (PEA), an endogenous fatty acid amide displaying neuroprotective actions, on glutamate release from rat cerebrocortical nerve terminals (synaptosomes) was investigated. PEA inhibited the Ca²⁺-dependent release of glutamate, which was triggered by exposing synaptosomes to the potassium channel blocker 4-aminopyridine. This release inhibition was concentration dependent, associated with a reduction in cytosolic Ca²⁺ concentration, and not due to a change in synaptosomal membrane potential. The glutamate release-inhibiting effect of PEA was prevented by the Cav2.1 (P/Q-type) channel blocker ω-agatoxin IVA or the protein kinase A inhibitor H89, not affected by the intracellular Ca²⁺ release inhibitors dantrolene and CGP37157, and partially antagonized by the cannabinoid CB1 receptor antagonist AM281. Based on these results, we suggest that PEA exerts its presynaptic inhibition, likely through a reduction in the Ca²⁺ influx mediated by Cav2.1 (P/Q-type) channels, thereby inhibiting the release of glutamate from rat cortical nerve terminals. This release inhibition might be linked to the activation of presynaptic cannabinoid CB1 receptors and the suppression of the protein kinase A pathway.

Benzophenanthridine alkaloid, piperonyl butoxide and (S)-methoprene action at the cannabinoid-1 receptor (CB1-receptor) pathway of mouse brain: Interference with [(3)H]CP55940 and [(3)H]SR141716A binding and modification of WIN55212-2-dependent inhibition of synaptosomal l-glutamate release

Benzophenanthridine alkaloids (chelerythrine and sanguinarine) inhibited binding of [(3)H]SR141716A to mouse brain membranes (IC50s: <1µM). Piperonyl butoxide and (S)-methoprene were less potent (IC50s: 21 and 63µM respectively). Benzophenanthridines and piperonyl butoxide were more selective towards brain CB1 receptors versus spleen CB2 receptors. All compounds reduced Bmax of [(3)H]SR141716A binding to CB1 receptors, but only methoprene and piperonyl butoxide increased Kd (3-5-fold). Benzophenanthridines increased the Kd of [(3)H]CP55940 binding (6-fold), but did not alter Bmax. (S)-methoprene increased the Kd of [(3)H]CP55940 binding (by almost 4-fold) and reduced Bmax by 60%. Piperonyl butoxide lowered the Bmax of [(3)H]CP55940 binding by 50%, but did not influence Kd. All compounds reduced [(3)H]SR141716A and [(3)H]CP55940 association with CB1 receptors. Combined with a saturating concentration of SR141716A, only piperonyl butoxide and (S)-methoprene increased dissociation of [(3)H]SR141716A above that of SR141716A alone. Only piperonyl butoxide increased dissociation of [(3)H]CP55940 to a level greater than CP55940 alone. Binding results indicate predominantly allosteric components to the study compounds action. 4-Aminopyridine-(4-AP-) evoked release of l-glutamate from synaptosomes was partially inhibited by WIN55212-2, an effect completely neutralized by AM251, (S)-methoprene and piperonyl butoxide. With WIN55212-2 present, benzophenanthridines enhanced 4-AP-evoked l-glutamate release above 4-AP alone. Modulatory patterns of l-glutamate release (with WIN-55212-2 present) align with previous antagonist/inverse agonist profiling based on [(35)S]GTPγS binding. Although these compounds exhibit lower potencies compared to many classical CB1 receptor inhibitors, they may have potential to modify CB1-receptor-dependent behavioral/physiological outcomes in the whole animal.

Cryo-electron tomography reveals a critical role of RIM1α in synaptic vesicle tethering

Synaptic vesicles are embedded in a complex filamentous network at the presynaptic terminal. Before fusion, vesicles are linked to the active zone (AZ) by short filaments (tethers). The identity of the molecules that form and regulate tethers remains unknown, but Rab3-interacting molecule (RIM) is a prominent candidate, given its central role in AZ organization. In this paper, we analyzed presynaptic architecture of RIM1α knockout (KO) mice by cryo-electron tomography. In stark contrast to previous work on dehydrated, chemically fixed samples, our data show significant alterations in vesicle distribution and AZ tethering that could provide a structural basis for the functional deficits of RIM1α KO synapses. Proteasome inhibition reversed these structural defects, suggesting a functional recovery confirmed by electrophysiological recordings. Altogether, our results not only point to the ubiquitin-proteasome system as an important regulator of presynaptic architecture and function but also show that the tethering machinery plays a critical role in exocytosis, converging into a structural model of synaptic vesicle priming by RIM1α.

Glutamatergic antipsychotic drugs: a new dawn in the treatment of schizophrenia?

Growing evidence for glutamate abnormalities in schizophrenia support the development of novel antipsychotic agents targeting this system. Early studies investigating modulation of the glutamate system using glycine, D-serine and sarcosine in patients with schizophrenia have demonstrated significant effects, particularly on negative symptoms, conventionally thought to be refractory to antipsychotic drug treatment. Drugs targeting the glutamate system also have a completely different side-effect profile to dopamine D2 antagonists, with no propensity to extrapyramidal side effects, prolactinaemia or weight gain. It has been hypothesized that glutamatergic drugs may be of benefit to the 20-30% of individuals with schizophrenia who fail to show any response to dopaminergic agents, and may be particularly useful in the early stages of the illness, where they may be disease-modifying. A number of glutamatergic compounds have been reported as having promising results in phase II drug trials. If these reach the clinic, they will represent the first truly novel approach to pharmacotherapy in schizophrenia for more than 50 years.

The interplay of cannabinoid and NMDA glutamate receptor systems in humans: preliminary evidence of interactive effects of cannabidiol and ketamine in healthy human subjects

BACKGROUND

Interactions between glutamatergic and endocannabinoid systems may contribute to schizophrenia, dissociative states, and other psychiatric conditions. Cannabidiol (CBD), a cannabinoid-1/2 (CB1/2) receptor weak partial agonist or antagonist, may play a role in the treatment of schizophrenia.

OBJECTIVE

This study tested the hypothesis that CBD would attenuate the behavioral effects of the NMDA receptor antagonist, ketamine, in healthy human subjects.

METHODS

Ten male healthy volunteers were evaluated twice in a randomized order. In both sessions they received ketamine (bolus of 0.26 mg/kg/1 min followed by IV infusion of 0.25mg/kg over 30 min) preceded by either CBD (600 mg) or placebo. Psychopathology was assessed using the Brief Psychiatric Rating Scale (BPRS) and the CADSS (Clinician Administered Dissociative States Scale) at regular intervals from 30 min before to 90 min after ketamine administration.

RESULTS

CBD significantly augmented the activating effects of ketamine, as measured by the activation subscales of the BPRS. However, CBD also showed a non-significant trend to reduce ketamine-induced depersonalization, as measured by the CADSS.

CONCLUSION

These data describe a complex pattern of psychopharmacologic interactions between CBD and ketamine at the doses of each agent studied in this experiment.

Quantitative analysis of the native presynaptic cytomatrix by cryoelectron tomography

The filamentous structures that tether exocytic vesicles to the plasma membrane in the active zone are rearranged in response to synaptic stimulation.

The presynaptic terminal contains a complex network of filaments whose precise organization and functions are not yet understood. The cryoelectron tomography experiments reported in this study indicate that these structures play a prominent role in synaptic vesicle release. Docked synaptic vesicles did not make membrane to membrane contact with the active zone but were instead linked to it by tethers of different length. Our observations are consistent with an exocytosis model in which vesicles are first anchored by long (>5 nm) tethers that give way to multiple short tethers once vesicles enter the readily releasable pool. The formation of short tethers was inhibited by tetanus toxin, indicating that it depends on soluble N-ethyl-maleimide sensitive fusion protein attachment protein receptor complex assembly. Vesicles were extensively interlinked via a set of connectors that underwent profound rearrangements upon synaptic stimulation and okadaic acid treatment, suggesting a role of these connectors in synaptic vesicle mobilization and neurotransmitter release.

Attenuation of morphine antinociceptive tolerance by a CB(1) receptor agonist and an NMDA receptor antagonist: Interactive effects

CB(1) cannabinoid (CB(1)) receptor agonists and N-Methyl-d-Aspartate (NMDA) receptor antagonists attenuate the development of morphine antinociceptive tolerance. The present study used dose-addition analysis to evaluate CB(1)/NMDA receptor interactions on this endpoint. Chronic morphine administration (5 days, 100 mg/kg, twice daily) resulted in a 2.8-fold rightward shift in the morphine dose-effect curve. Co-administration of either the CB(1) receptor agonist CP-55940 (5-(1,1-Dimethylheptyl)-2-[5-hydroxy-2-(3-hydroxypropyl)cyclohexyl]phenol; 0.32-1.0 mg/kg) or the NMDA receptor antagonist (-)-6-phosphonomethyl-deca-hydroisoquinoline-3-carboxylic acid (LY235959; 1.0-3.2 mg/kg) with morphine dose-dependently attenuated morphine tolerance. The relative potency of each drug alone was quantified using a defined level of effect (one-quarter log shift in the morphine dose-effect curve), resulting in equieffective doses of 0.42 mg/kg and 1.1 mg/kg for CP-55940 and LY235959, respectively. Subsequent experiments assessed CP-55940/LY235959 interactions using a fixed-proportion design. Co-administration of CP-55940/LY235959 mixtures (1:1, 1:3.2, or 1:10 CP-55940/LY235959) with morphine dose-dependently attenuated morphine tolerance. Isobolographic and dose-addition analysis were used to statistically compare the experimentally determined potency for each mixture (z(mix)) with predicted additive potency (z(add)). Mixtures of 1:1 and 1:3.2 CP-55940/LY235959 produced additive effects (z(add) = z(mix)), while the mixture of 1:10 CP-55940/LY235959 produced a supra-additive effect (z(add) > z(mix)). These results suggest that CP-55940 and LY235959 produce additive or supra-additive attenuation of morphine antinociceptive tolerance after repeated morphine administration, depending on their relative concentrations.

The inhibition of release by mGlu7 receptors is independent of the Ca2+ channel type but associated to GABAB and adenosine A1 receptors

Neurotransmitter release is inhibited by G-protein coupled receptors (GPCRs) through signalling pathways that are negatively coupled to Ca2+ channels and adenylyl cyclase. Through Ca2+ imaging and immunocytochemistry, we have recently shown that adenosine A1, GABAB and the metabotropic glutamate type 7 receptors coexist in a subset of cerebrocortical nerve terminals. As these receptors inhibit glutamate release through common intracellular signalling pathways, their co-activation occluded each other responses. Here we have addressed whether the occlusion of receptor responses is restricted to the glutamate release mediated by N-type Ca2+ channels by analysing this process in nerve terminals from mice lacking the alpha1B subunit (Cav 2.2) of these channels. We found that glutamate release from cerebrocortical nerve terminals without these channels, in which release relies exclusively on P/Q type Ca2+ channels, is not modulated by mGlu7 receptors. Furthermore, there is no occlusion of the release inhibition by GABAB and adenosine A1. Hence, in the cerebrocortical preparation, these three receptors only appear to coexist in N-type channel containing nerve terminals. In contrast, in hippocampal nerve terminals lacking this subunit, where mGlu7 receptors modulate glutamate release via P/Q type channels, the occlusion of inhibitory responses by co-stimulation of adenosine A1, GABAB and mGlu7 receptors was observed. Thus, occlusion of the responses by the three GPCRs is independent of the Ca2+ channel type but rather, it is associated to functional mGlu7 receptors.

Feedback mechanisms in the regulation of intracellular calcium ([Ca2+]i) in the peripheral nociceptive system: role of TRPV-1 and pain related receptors

Multimodal stimuli like heat, cold, bacterial or mechanical events are able to elicit pain, which is necessary to guarantee survival. However, the control of pain is of major clinical importance. The perception and transduction of pain is differentially modulated in the peripheral and central nervous system (CNS): while peripheral structures modulate these signals, the perception of pain occurs in the CNS. In recent years major advances have been made in the understanding of the processes which are involved in pain sensation. For the peripheral pain reception, the importance of specific pain receptors of the transition receptor pore (TRP)-family (e.g. the TRPV-1 receptor) has been analyzed. These receptors/channels are localized at the cell membrane of nociceptive neurones as well as in membranes of intracellular calcium stores like the endoplasmic reticulum. While the associated channel conducts different ions, a major proportion is calcium. Therefore, this review focuses on (1) the modulations of intracellular calcium ([Ca2+]i) initiated by the activation of pain receptors and (2) the consequences of [Ca2+]i changes for the processing of pain signals at the peripheral side. The possible interference of TRPV-1 induced [Ca2+]i modulations to the function of other membrane receptors and channels, like voltage gated calcium, sodium or potassium channels, or co-expressed CB1-receptors will be discussed. The latter interactions are of specific interest since the analgetic properties of endo- and exo-cannabinoids are mediated by CB1 receptors and their activation significantly modulates the calcium induced release of pain related transmitters. Furthermore, multiple cross links between different pain modulating intracellular pathways and their dependence on [Ca2+]i modulations will be illuminated. Overall, this review will summarize new insights resulting in the understanding of the prominent influence of [Ca2+]i for processes which are involved in pain sensation.