Endocannabinoid-mediated long-term plasticity requires cAMP/PKA signaling and RIM1alpha

SGIP1 binding to the α-helical H9 domain of cannabinoid receptor 1 promotes axonal surface expression

Endocannabinoid signalling mediated by cannabinoid receptor 1 (CB1R, also known as CNR1) is critical for homeostatic neuromodulation of both excitatory and inhibitory synapses. This requires highly polarised axonal surface expression of CB1R, but how this is achieved remains unclear. We previously reported that the α-helical H9 domain in the intracellular C terminus of CB1R contributes to axonal surface expression by an unknown mechanism. Here, we show in rat primary neuronal cultures that the H9 domain binds to the endocytic adaptor protein SGIP1 to promote CB1R expression in the axonal membrane. Overexpression of SGIP1 increases CB1R axonal surface localisation but has no effect on CB1R lacking the H9 domain (CB1RΔH9). Conversely, SGIP1 knockdown reduces axonal surface expression of CB1R but does not affect CB1RΔH9. Furthermore, SGIP1 knockdown diminishes CB1R-mediated inhibition of presynaptic Ca2+ influx in response to neuronal activity. Taken together, these data advance mechanistic understanding of endocannabinoid signalling by demonstrating that SGIP1 interaction with the H9 domain underpins axonal CB1R surface expression to regulate presynaptic responsiveness.

Cyclic nucleotide-induced bidirectional long-term synaptic plasticity in Drosophila mushroom body

Activation of the cAMP pathway is one of the common mechanisms underlying long-term potentiation (LTP). In the Drosophila mushroom body, simultaneous activation of odour-coding Kenyon cells (KCs) and reinforcement-coding dopaminergic neurons activates adenylyl cyclase in KC presynaptic terminals, which is believed to trigger synaptic plasticity underlying olfactory associative learning. However, learning induces long-term depression (LTD) at these synapses, contradicting the universal role of cAMP as a facilitator of transmission. Here, we developed a system to electrophysiologically monitor both short-term and long-term synaptic plasticity at KC output synapses and demonstrated that they are indeed an exception in which activation of the cAMP-protein kinase A pathway induces LTD. Contrary to the prevailing model, our cAMP imaging found no evidence for synergistic action of dopamine and KC activity on cAMP synthesis. Furthermore, we found that forskolin-induced cAMP increase alone was insufficient for plasticity induction; it additionally required simultaneous KC activation to replicate the presynaptic LTD induced by pairing with dopamine. On the other hand, activation of the cGMP pathway paired with KC activation induced slowly developing LTP, proving antagonistic actions of the two second-messenger pathways predicted by behavioural study. Finally, KC subtype-specific interrogation of synapses revealed that different KC subtypes exhibit distinct plasticity duration even among synapses on the same postsynaptic neuron. Thus, our work not only revises the role of cAMP in synaptic plasticity by uncovering the unexpected convergence point of the cAMP pathway and neuronal activity, but also establishes the methods to address physiological mechanisms of synaptic plasticity in this important model. KEY POINTS: Although presynaptic cAMP increase generally facilitates synapses, olfactory associative learning in Drosophila, which depends on dopamine and cAMP signalling genes, induces long-term depression (LTD) at the mushroom body output synapses. By combining electrophysiology, pharmacology and optogenetics, we directly demonstrate that these synapses are an exception where activation of the cAMP-protein kinase A pathway leads to presynaptic LTD. Dopamine- or forskolin-induced cAMP increase alone is not sufficient for LTD induction; neuronal activity, which has been believed to trigger cAMP synthesis in synergy with dopamine input, is required in the downstream pathway of cAMP. In contrast to cAMP, activation of the cGMP pathway paired with neuronal activity induces presynaptic long-term potentiation, which explains behaviourally observed opposing actions of transmitters co-released by dopaminergic neurons. Our work not only revises the role of cAMP in synaptic plasticity, but also provides essential methods to address physiological mechanisms of synaptic plasticity in this important model system.

A quantitative pharmacology model for cannabinoid CB1 receptor mediated by Gi/Gs protein competition

BACKGROUND AND PURPOSE

Orthosteric agonism of the CB1 receptor normally associates with Gi signalling resulting in a net inhibition of cAMP production. Empirical evidence shows CB1 causes a net cAMP stimulation through Gs coupling under two conditions: co-stimulation with the D2 receptor and high-level CB1 expression. Two hypotheses have been proposed to account for these paradoxical effects, (1) Gi is consumed by coupling to D2 or extra CB1 and excess CB1 binds to Gs and (2), the formation of dimers CB1 -CB1 or CB1 -D2 switches Gi/Gs preference. This study explored the mechanisms of Gi/Gs preference based on a mathematical model of the CB1 receptor.

EXPERIMENTAL APPROACH

The model was based on Hypothesis 1 and known mechanisms. The model was calibrated to align with multiple types of data (cAMP, Gi dissociation and internalisation). The key step of Hypothesis 1 was examined by simulation from the model. An experiment was proposed to distinguish Hypothesis 1 and 2.

KEY RESULTS

The model successfully descripted multiple types of data under Hypothesis 1. Simulations from the model indicated that precoupling of G protein with receptors is necessary for this hypothesis. The model designed experiments to distinguish Hypothesis 1 and 2 by increasing Gi & Gs in parallel with CB1 overexpression. The two hypotheses result in distinct cAMP responses.

CONCLUSION AND IMPLICATIONS

A mathematical model of CB1 -regulated Gi/Gs pathways was developed. It indicated Hypothesis 1 is feasible and G protein precoupling is a key step causing cAMP signalling switch. The model-designed experiments provided guides for future experimentation.

Cyclic nucleotide-induced bidirectional long-term synaptic plasticity in Drosophila mushroom body

Activation of the cAMP pathway is one of the common mechanisms underlying long-term potentiation (LTP). In the Drosophila mushroom body, simultaneous activation of odor-coding Kenyon cells (KCs) and reinforcement-coding dopaminergic neurons activates adenylyl cyclase in KC presynaptic terminals, which is believed to trigger synaptic plasticity underlying olfactory associative learning. However, learning induces long-term depression (LTD) at these synapses, contradicting the universal role of cAMP as a facilitator of transmission. Here, we develop a system to electrophysiologically monitor both short-term and long-term synaptic plasticity at KC output synapses and demonstrate that they are indeed an exception where activation of the cAMP/protein kinase A pathway induces LTD. Contrary to the prevailing model, our cAMP imaging finds no evidence for synergistic action of dopamine and KC activity on cAMP synthesis. Furthermore, we find that forskolin-induced cAMP increase alone is insufficient for plasticity induction; it additionally requires simultaneous KC activation to replicate the presynaptic LTD induced by pairing with dopamine. On the other hand, activation of the cGMP pathway paired with KC activation induces slowly developing LTP, proving antagonistic actions of the two second-messenger pathways predicted by behavioral study. Finally, KC subtype-specific interrogation of synapses reveals that different KC subtypes exhibit distinct plasticity duration even among synapses on the same postsynaptic neuron. Thus, our work not only revises the role of cAMP in synaptic plasticity by uncovering the unexpected convergence point of the cAMP pathway and neuronal activity, but also establishes the methods to address physiological mechanisms of synaptic plasticity in this important model.

Local and long-range GABAergic circuits in hippocampal area CA1 and their link to Alzheimer's disease

GABAergic inhibitory neurons are the principal source of inhibition in the brain. Traditionally, their role in maintaining the balance of excitation-inhibition has been emphasized. Beyond homeostatic functions, recent circuit mapping and functional manipulation studies have revealed a wide range of specific roles that GABAergic circuits play in dynamically tilting excitation-inhibition coupling across spatio-temporal scales. These span from gating of compartment- and input-specific signaling, gain modulation, shaping input-output functions and synaptic plasticity, to generating signal-to-noise contrast, defining temporal windows for integration and rate codes, as well as organizing neural assemblies, and coordinating inter-regional synchrony. GABAergic circuits are thus instrumental in controlling single-neuron computations and behaviorally-linked network activity. The activity dependent modulation of sensory and mnemonic information processing by GABAergic circuits is pivotal for the formation and maintenance of episodic memories in the hippocampus. Here, we present an overview of the local and long-range GABAergic circuits that modulate the dynamics of excitation-inhibition and disinhibition in the main output area of the hippocampus CA1, which is crucial for episodic memory. Specifically, we link recent findings pertaining to GABAergic neuron molecular markers, electrophysiological properties, and synaptic wiring with their function at the circuit level. Lastly, given that area CA1 is particularly impaired during early stages of Alzheimer's disease, we emphasize how these GABAergic circuits may contribute to and be involved in the pathophysiology.

Ca2+-stimulated ADCY1 and ADCY8 regulate distinct aspects of synaptic and cognitive flexibility

The type 1 and 8 adenylyl cyclase (ADCY1 and ADCY8) exclusively account for Ca2+-stimulated cyclic AMP (cAMP) production and regulate activity-dependent synaptic modification. In this study, we examined distinct forms of synaptic plasticity in the hippocampus of Adcy1-/- and Adcy8-/- mice. We found that, at the Schaffer collateral-CA1 synapses, while the Adcy8-/- mice displayed normal long-term potentiation (LTP) following various induction protocols with high-frequency stimulation (HFS), the Adcy1-/- mice showed protocol-dependent deficits in LTP. We also found that long-term depression (LTD) requires ADCY1 but not ADCY8. Interestingly, both Adcy1-/- and Adcy8-/- mice showed defective synaptic depotentiation (i.e., activity-dependent reversal of LTP); the deficits in Adcy8-/- mice were dependent on the induction protocol. Examination of spatial memory found that ADCY1 is required for the formation of both initial and reversal memory. ADCY8 is only required for reversal memory formation. These data demonstrate that ADCY1 and ADCY8 play distinct roles in regulating synaptic and cognitive flexibility that involves bidirectional modification of synaptic function.

Chronic Δ9-tetrahydrocannabinol treatment has dose-dependent effects on open field exploratory behavior and [3H] SR141716A receptor binding in the rat brain

AIMS

Acute and chronic Δ⁹-THC exposure paradigms affect the body differently. More must be known about the impact of chronic Δ⁹-THC on cannabinoid-1 (CB1R) and mu-opioid (MOR) receptor levels in the brain. The present study examined chronic Δ⁹-THC's effects on CB1R and MOR levels and locomotor activity.

MAIN METHODS

Adolescent Sprague-Dawley rats were given daily intraperitoneal injections of Δ⁹-THC [0.75mg/kg (low dose or LD) or 2.0 mg/kg (high dose or HD)] or vehicle for 24 days, and locomotion in the open field was tested after the first and fourth weeks of chronic Δ⁹-THC exposure. Brains were harvested at the end of treatment. [³H] SR141716A and [³H] DAMGO autoradiography assessed CB1R and MOR levels, respectively.

KEY FINDINGS

Relative to each other, chronic HD rats showed reduced vertical plane (VP) entries and time, while LD rats had increased VP entries and time for locomotion, as assessed by open-field testing; no effects were found relative to the control. Autoradiography analyses showed that HD Δ⁹-THC significantly decreased CB1R binding relative to LD Δ⁹-THC in the cingulate (33%), primary motor (42%), secondary motor (33%) somatosensory (38%), rhinal (38%), and auditory (50%) cortices; LD Δ⁹-THC rats displayed elevated binding in the primary motor (33% increase) and hypothalamic (33 % increase) regions compared with controls. No significant differences were observed in MOR binding for the LD or HD compared to the control.

SIGNIFICANCE

These results demonstrate that chronic Δ⁹-THC dose-dependently altered CB1R levels throughout the brain and locomotor activity in the open field.

Pannexin-1 Modulates Inhibitory Transmission and Hippocampal Synaptic Plasticity

Pannexin-1 (Panx1) hemichannel is a non-selective transmembrane channel that may play important roles in intercellular signaling by allowing the permeation of ions and metabolites, such as ATP. Although recent evidence shows that the Panx1 hemichannel is involved in controlling excitatory synaptic transmission, the role of Panx1 in inhibitory transmission remains unknown. Here, we studied the contribution of Panx1 to the GABAergic synaptic efficacy onto CA1 pyramidal neurons (PyNs) by using patch-clamp recordings and pharmacological approaches in wild-type and Panx1 knock-out (Panx1-KO) mice. We reported that blockage of the Panx1 hemichannel with the mimetic peptide ¹⁰Panx1 increases the synaptic level of endocannabinoids (eCB) and the activation of cannabinoid receptors type 1 (CB1Rs), which results in a decrease in hippocampal GABAergic efficacy, shifting excitation/inhibition (E/I) balance toward excitation and facilitating the induction of long-term potentiation. Our finding provides important insight unveiling that Panx1 can strongly influence the overall neuronal excitability and play a key role in shaping synaptic changes affecting the amplitude and direction of plasticity, as well as learning and memory processes.

2-AG-Mediated Control of GABAergic Signaling Is Impaired in a Model of Epilepsy

Repeated seizures result in a persistent maladaptation of endocannabinoid (eCB) signaling, mediated part by anandamide signaling deficiency in the basolateral amygdala (BLA) that manifests as aberrant synaptic function and altered emotional behavior. Here, we determined the effect of repeated seizures (kindling) on 2-arachidonoylglycerol (2-AG) signaling on GABA transmission by directly measuring tonic and phasic eCB-mediated retrograde signaling in an in vitro BLA slice preparation from male rats. We report that both activity-dependent and muscarinic acetylcholine receptor (mAChR)-mediated depression of GABA synaptic transmission was reduced following repeated seizure activity. These effects were recapitulated in sham rats by preincubating slices with the 2-AG synthesizing enzyme inhibitor DO34. Conversely, preincubating slices with the 2-AG degrading enzyme inhibitor KML29 rescued activity-dependent 2-AG signaling, but not mAChR-mediated synaptic depression, over GABA transmission in kindled rats. These effects were not attributable to a change in cannabinoid type 1 (CB1) receptor sensitivity or altered 2-AG tonic signaling since the application of the highly selective CB1 receptor agonist CP55,940 provoked a similar reduction in GABA synaptic activity in both sham and kindled rats, while no effect of either DO34 or of the CB1 inverse agonist AM251 was observed on frequency and amplitude of spontaneous IPSCs in either sham or kindled rats. Collectively, these data provide evidence that repeated amygdala seizures persistently alter phasic 2-AG-mediated retrograde signaling at BLA GABAergic synapses, probably by impairing stimulus-dependent 2-AG synthesis/release, which contributes to the enduring aberrant synaptic plasticity associated with seizure activity.SIGNIFICANCE STATEMENT The plastic reorganization of endocannabinoid (eCB) signaling after seizures and during epileptogenesis may contribute to the negative neurobiological consequences associated with seizure activity. Therefore, a deeper understanding of the molecular basis underlying the pathologic long-term eCB signaling remodeling following seizure activity will be crucial to the development of novel therapies for epilepsy that not only target seizure activity, but, most importantly, the epileptogenesis and the comorbid conditions associated with epilepsy.

Sex-dependent changes of hippocampal synaptic plasticity and cognitive performance in C57BL/6J mice exposed to neonatal repeated maternal separation

The repeated maternal separation (RMS) is a useful experimental model useful in rodents to study the long-term influence of early-life stress on brain neurophysiology. We here investigated the influence of RMS exposure on hippocampal inhibitory and excitatory synaptic transmission, long-term synaptic plasticity and the related potential alterations in learning and memory performance in adult male and female C57Bl/6J mice. Mice were separated daily from their dam for 360 min, from postnatal day 2 (PND2) to PND17, and experiments were performed at PND 60. Patch-clamp recordings in hippocampal CA1 pyramidal neurons revealed a significant enhancement of GABAergic miniature IPSC (mIPSC) frequency, and a decrease in the amplitude of glutamatergic mEPSCs in male mice exposed to RMS. Only a slight but significant reduction in the amplitude of GABAergic mIPSCs was observed in females exposed to RMS compared to the relative controls. A marked increase in long-term depression (LTD) at CA3-CA1 glutamatergic synapses and in the response to the CB1r agonist win55,212 were detected in RMS male, but not female mice. An impaired spatial memory and a reduced preference for novelty was observed in males exposed to RMS but not in females. A single injection of β-ethynyl estradiol at PND2, prevented the changes observed in RMS male mice, suggesting that estrogens may play a protective role early in life against the exposure to stressful conditions. Our findings strengthen the idea of a sex-dependent influence of RMS on long-lasting modifications in synaptic transmission, effects that may be relevant for cognitive performance.

Electrophysiology of Endocannabinoid Signaling

Electrophysiological technique is an efficient tool for investigating the synaptic regulatory effects mediated by the endocannabinoid system. Stimulation of presynaptic type 1 cannabinoid receptor (CB1) is the principal mode by which endocannabinoids suppress transmitter release in the central nervous system, but a non-retrograde manner of functioning and other receptors have also been described. Endocannabinoids are key modulators of both short- and long-term plasticity. Here, we discuss ex vivo electrophysiological approaches to examine synaptic signaling induced by cannabinoid and endocannabinoid molecules in the mammalian brain.

Accumulation of systematic TPM1 mediates inflammation and neuronal remodeling by phosphorylating PKA and regulating the FABP5/NF-κB signaling pathway in the retina of aged mice

The molecular mechanisms underlying functional decline during normal brain aging are poorly understood. Here, we identified the actin‐associated protein tropomyosin 1 (TPM1) as a new systemic pro‐aging factor associated with function deficits in normal aging retinas. Heterochronic parabiosis and blood plasma treatment confirmed that systemic factors regulated age‐related inflammatory responses and the ectopic dendritic sprouting of rod bipolar (RBC) and horizontal (HC) cells in the aging retina. Proteomic analysis revealed that TPM1 was a potential systemic molecule underlying structural and functional deficits in the aging retina. Recombinant TPM1 protein administration accelerated the activation of glial cells, the dendritic sprouting of RBCs and HCs and functional decline in the retina of young mice, whereas anti‐TPM1 neutralizing antibody treatment ameliorated age‐related structural and function changes in the retina of aged mice. Old mouse plasma (OMP) induced glial cell activation and the dendritic outgrowth of RBCs and HCs in young mice, and yet TMP1‐depleted OMP failed to reproduce the similar effect in young mice. These results confirmed that TPM1 was a systemic pro‐aging factor. Moreover, we demonstrated that systematic TPM1 was an immune‐related molecule, which elicited endogenous TPM1 expression and inflammation by phosphorylating PKA and regulating FABP5/NF‐κB signaling pathway in normal aging retinas. Interestingly, we observed TPM1 upregulation and the ectopic dendritic sprouting of RBCs and HCs in young mouse models of Alzheimer's disease, indicating a potential role of TPM1 in age‐related neurodegenerative diseases. Our data indicate that TPM1 could be targeted for combating the aging process.

Prenatal Opioid Exposure Impairs Endocannabinoid and Glutamate Transmission in the Dorsal Striatum

The opioid crisis has contributed to a growing population of children exposed to opioids during fetal development; however, many of the long-term effects of opioid exposure on development are unknown. We previously demonstrated that opioids have deleterious effects on endocannabinoid plasticity at glutamate synapses in the dorsal striatum of adolescent rodents, but it is unclear whether prenatal opioid exposure produces similar neuroadaptations. Using a mouse model of prenatal methadone exposure (PME), we performed proteomics, phosphoproteomics, and patch-clamp electrophysiology in the dorsolateral striatum (DLS) and dorsomedial striatum (DMS) to examine synaptic functioning in adolescent PME offspring. PME impacted the proteome and phosphoproteome in a region- and sex-dependent manner. Many proteins and phosphorylated proteins associated with glutamate transmission were differentially abundant in PME offspring, which was associated with reduced glutamate release in the DLS and altered the rise time of excitatory events in the DMS. Similarly, the intrinsic excitability properties of DMS neurons were significantly affected by PME. Last, pathway analyses revealed an enrichment in retrograde endocannabinoid signaling in the DLS, but not in the DMS, of males. Electrophysiology studies confirmed that endocannabinoid-mediated synaptic depression was impaired in the DLS, but not DMS, of PME-males. These results indicate that PME induces persistent neuroadaptations in the dorsal striatum and could contribute to the aberrant behavioral development described in offspring with prenatal opioid exposure.

Between Science and Big Business: Tapping Mary Jane's Uncharted Potential

At the intersection of science and medicine, government policy, and pop culture, cannabis has prompted society since the beginning of recorded history. And yet, there is comparatively little replicable data on the plant, its constituents, and their capacity to modify human physiology. Over the past decades, several findings have pointed toward the importance of the endogenous cannabinoid system in maintaining homeostasis, making it an important target for various diseases. Here, we summarize the current state of knowledge on endogenous- and plant-based cannabinoids, address the issues related to cannabinoid-based drug discovery, and incite efforts to utilize their polypharmacological profile toward tackling diseases with a complex underlying pathophysiology. By fusing modern science and technology with the empirical data that has been gathered over centuries, we propose an outlook that could help us overcome the dearth of innovation for new drugs and synchronously redefine the future of drug discovery. Simultaneously, we call attention to the startling disconnect between the scientific, regulatory, and corporate entities that is becoming increasingly evident in this booming industry.

Engineered synaptic tools reveal localized cAMP signaling in synapse assembly

The physiological mechanisms driving synapse formation are elusive. Although numerous signals are known to regulate synapses, it remains unclear which signaling mechanisms organize initial synapse assembly. Here, we describe new tools, referred to as "SynTAMs" for synaptic targeting molecules, that enable localized perturbations of cAMP signaling in developing postsynaptic specializations. We show that locally restricted suppression of postsynaptic cAMP levels or of cAMP-dependent protein-kinase activity severely impairs excitatory synapse formation without affecting neuronal maturation, dendritic arborization, or inhibitory synapse formation. In vivo, suppression of postsynaptic cAMP signaling in CA1 neurons prevented formation of both Schaffer-collateral and entorhinal-CA1/temporoammonic-path synapses, suggesting a general principle. Retrograde trans-synaptic rabies virus tracing revealed that postsynaptic cAMP signaling is required for continuous replacement of synapses throughout life. Given that postsynaptic latrophilin adhesion-GPCRs drive synapse formation and produce cAMP, we suggest that spatially restricted postsynaptic cAMP signals organize assembly of postsynaptic specializations during synapse formation.

Imaging the endocannabinoid signaling system

The endocannabinoid (eCB) system is one of the most widespread neuromodulatory systems in the mammalian brain, with a multifaceted role in functions ranging from development to synaptic plasticity. Endocannabinoids are synthesized on demand from membrane lipid precursors, and act primarily on a single G-protein coupled receptor type, CB1, to carry out diverse functions. Despite the importance of the eCB system both in healthy brain function and in disease, critically important details of eCB signaling remained unknown. How eCBs are released from the membrane, how these lipid molecules are transported between cells, and how the distribution of their receptors is controlled, remained elusive. Recent advances in optical microscopy methods and biosensor engineering may open up new avenues for studying eCB signaling. We summarize applications of superresolution microscopy using single molecule localization to reveal distinct patterns of nanoscale CB1 distribution in neuronal axons and axon terminals. We review single particle tracking studies using quantum dots that allowed visualizing CB1 trajectories. We highlight the recent development of fluorescent eCB biosensors, that revealed spatiotemporally specific eCB release in live cells and live animals. Finally, we discuss future directions where method development may help to advance a precise understanding of eCB signaling.

The Synaptic Interactions of Alcohol and the Endogenous Cannabinoid System

PURPOSE

A growing body of evidence has implicated the endocannabinoid (eCB) system in the acute, chronic, and withdrawal effects of alcohol/ethanol on synaptic function. These eCB-mediated synaptic effects may contribute to the development of alcohol use disorder (AUD). Alcohol exposure causes neurobiological alterations similar to those elicited by chronic cannabinoid (CB) exposure. Like alcohol, cannabinoids alter many central processes, such as cognition, locomotion, synaptic transmission, and neurotransmitter release. There is a strong need to elucidate the effects of ethanol on the eCB system in different brain regions to understand the role of eCB signaling in AUD.

SEARCH METHODS

For the scope of this review, preclinical studies were identified through queries of the PubMed database.

SEARCH RESULTS

This search yielded 459 articles. Clinical studies and papers irrelevant to the topic of this review were excluded.

DISCUSSION AND CONCLUSIONS

The endocannabinoid system includes, but is not limited to, cannabinoid receptors 1 (CB₁), among the most abundantly expressed neuronal receptors in the brain; cannabinoid receptors 2 (CB₂); and endogenously formed CB₁ ligands, including arachidonoylethanolamide (AEA; anandamide), and 2-arachidonoylglycerol (2-AG). The development of specific CB₁ agonists, such as WIN 55,212-2 (WIN), and antagonists, such as SR 141716A (rimonabant), provide powerful pharmacological tools for eCB research. Alcohol exposure has brain region-specific effects on the eCB system, including altering the synthesis of endocannabinoids (e.g., AEA, 2-AG), the synthesis of their precursors, and the density and coupling efficacy of CB₁. These alcohol-induced alterations of the eCB system have subsequent effects on synaptic function including neuronal excitability and postsynaptic conductance. This review will provide a comprehensive evaluation of the current literature on the synaptic interactions of alcohol exposure and eCB signaling systems, with an emphasis on molecular and physiological synaptic effects of alcohol on the eCB system. A limited volume of studies has focused on the underlying interactions of alcohol and the eCB system at the synaptic level in the brain. Thus, the data on synaptic interactions are sparse, and future research addressing these interactions is much needed.

Endocannabinoid-Mediated Control of Neural Circuit Excitability and Epileptic Seizures

Research on endocannabinoid signaling has greatly advanced our understanding of how the excitability of neural circuits is controlled in health and disease. In general, endocannabinoid signaling at excitatory synapses suppresses excitability by inhibiting glutamate release, while that at inhibitory synapses promotes excitability by inhibiting GABA release, although there are some exceptions in genetically epileptic animal models. In the epileptic brain, the physiological distributions of endocannabinoid signaling molecules are disrupted during epileptogenesis, contributing to the occurrence of spontaneous seizures. However, it is still unknown how endocannabinoid signaling changes during seizures and how the redistribution of endocannabinoid signaling molecules proceeds during epileptogenesis. Recent development of cannabinoid sensors has enabled us to investigate endocannabinoid signaling in much greater spatial and temporal details than before. Application of cannabinoid sensors to epilepsy research has elucidated activity-dependent changes in endocannabinoid signaling during seizures. Furthermore, recent endocannabinoid research has paved the way for the clinical use of cannabidiol for the treatment of refractory epilepsy, such as Dravet syndrome, Lennox-Gastaut syndrome and tuberous sclerosis complex. Cannabidiol significantly reduces seizures and is considered to have comparable tolerability to conventional antiepileptic drugs. In this article, we introduce recent advances in research on the roles of endocannabinoid signaling in epileptic seizures and discuss future directions.

Basic mechanisms of plasticity and learning

The last century was characterized by a significant scientific effort aimed at unveiling the neurobiological basis of learning and memory. Thanks to the characterization of the mechanisms regulating the long-term changes of neuronal synaptic connections, it was possible to understand how specific neural networks shape themselves during the acquisition of memory traces or complex motor tasks. In this chapter, we will summarize the mechanisms underlying the main forms of synaptic plasticity taking advantage of the studies performed in the hippocampus and in the nucleus striatum, key brain structures that play a crucial role in cognition. Moreover, we will discuss how the molecular pathways involved in the induction of physiologic synaptic long-term changes could be disrupted during neurodegenerative and neuroinflammatory disorders, highlighting the translational relevance of this intriguing research field.

Axonal CB1 Receptors Mediate Inhibitory Bouton Formation via cAMP Increase and PKA

Experience-dependent formation and removal of inhibitory synapses are essential throughout life. For instance, GABAergic synapses are removed to facilitate learning, and strong excitatory activity is accompanied by the formation of inhibitory synapses to maintain coordination between excitation and inhibition. We recently discovered that active dendrites trigger the growth of inhibitory synapses via CB1 receptor-mediated endocannabinoid signaling, but the underlying mechanism remained unclear. Using two-photon microscopy to monitor the formation of individual inhibitory boutons in hippocampal organotypic slices from mice (both sexes), we found that CB1 receptor activation mediated the formation of inhibitory boutons and promoted their subsequent stabilization. Inhibitory bouton formation did not require neuronal activity and was independent of G_i/o-protein signaling, but was directly induced by elevating cAMP levels using forskolin and by activating G_s-proteins using DREADDs. Blocking PKA activity prevented CB1 receptor-mediated inhibitory bouton formation. Our findings reveal that axonal CB1 receptors signal via unconventional downstream pathways and that inhibitory bouton formation is triggered by an increase in axonal cAMP levels. Our results demonstrate an unexpected role for axonal CB1 receptors in axon-specific, and context-dependent, inhibitory synapse formation.SIGNIFICANCE STATEMENT Coordination between excitation and inhibition is required for proper brain function throughout life. It was previously shown that new inhibitory synapses can be formed in response to strong excitation to maintain this coordination, and this was mediated by endocannabinoid signaling via CB1 receptors. As activation of CB1 receptors generally results in the suppression of synaptic transmission, it remained unclear how CB1 receptors can mediate the formation of inhibitory synapses. Here we show that CB1 receptors on inhibitory axons signal via unconventional intracellular pathways and that inhibitory bouton formation is triggered by an increase in axonal cAMP levels and requires PKA activity. Our findings point to a central role for axonal cAMP signaling in activity-dependent inhibitory synapse formation.

Activating mGlu3 Metabotropic Glutamate Receptors Rescues Schizophrenia-like Cognitive Deficits Through Metaplastic Adaptations Within the Hippocampus

BACKGROUND

Polymorphisms in GRM3, the gene encoding the mGlu₃ metabotropic glutamate receptor, are associated with impaired cognition and neuropsychiatric disorders such as schizophrenia. Limited availability of selective genetic and molecular tools has hindered progress in developing a clear understanding of the mechanisms through which mGlu₃ receptors regulate synaptic plasticity and cognition.

METHODS

We examined associative learning in mice with trace fear conditioning, a hippocampal-dependent learning task disrupted in patients with schizophrenia. Underlying cellular mechanisms were assessed using ex vivo hippocampal slice preparations with selective pharmacological tools and selective genetic deletion of mGlu₃ receptor expression in specific neuronal subpopulations.

RESULTS

mGlu₃ receptor activation enhanced trace fear conditioning and reversed deficits induced by subchronic phencyclidine. Mechanistic studies revealed that mGlu₃ receptor activation induced metaplastic changes, biasing afferent stimulation to induce long-term potentiation through an mGlu₅ receptor-dependent, endocannabinoid-mediated, disinhibitory mechanism. Selective genetic deletion of either mGlu₃ or mGlu₅ from hippocampal pyramidal cells eliminated effects of mGlu₃ activation, revealing a novel mechanism by which mGlu₃ and mGlu₅ interact to enhance cognitive function.

CONCLUSIONS

These data demonstrate that activation of mGlu₃ receptors in hippocampal pyramidal cells enhances hippocampal-dependent cognition in control and impaired mice by inducing a novel form of metaplasticity to regulate circuit function, providing a clear mechanism through which genetic variation in GRM3 can contribute to cognitive deficits. Developing approaches to positively modulate mGlu₃ receptor function represents an encouraging new avenue for treating cognitive disruption in schizophrenia and other psychiatric diseases.

The Safety Factor for Neuromuscular Transmission: Effects of Dimethylsulphoxide, Cannabinoids and Synaptic Homeostasis

BackgroundIn myasthenia gravis, impaired postsynaptic sensitivity to acetylcholine results in failure of neuromuscular transmission and fatiguing muscle weakness.ObjectiveDevelop an ex vivo muscle contraction assay to test cannabinoids and other substances that might act on the myasthenic neuromuscular junction to restore control of the muscle.MethodsTubocurarine was added to an ex vivo, mouse phrenic nerve-hemidiaphragm muscle preparation to reduce acetylcholine sensitivity. This produced a myasthenia-like decrement in twitch force during a train of 10 nerve impulses (3 / sec). Endplate potential (EPP) recordings were used to confirm and extend the findings.ResultsSurprisingly, addition to the bath of dimethylsulphoxide (DMSO), at concentrations as low as 0.1%(v/v), partially reversed the decrement in nerve-evoked force. Intracellular electrophysiology, conducted in the presence of tubocurarine, showed that DMSO increased the amplitudes of both the spontaneous miniature EPP (MEPP) and the (nerve-evoked) EPP. In the absence of tubocurarine (synaptic potentials at physiological levels), an adaptive fall in quantal content negated the DMSO-induced rise in EPP amplitude. The effects of cannabinoid receptor agonists (solubilized with DMSO) in the contraction assay do not support their further exploration as useful therapeutic agents for myasthenia gravis. CP 55,940 (a dual agonist for cannabinoid receptor types 1 and 2) reversed the beneficial effects of DMSO.Conclusions:We demonstrate a powerful effect of DMSO upon quantal amplitude that might mislead pharmacological studies of synaptic function wherever DMSO is used as a drug vehicle. Our results also show that compounds targeting impaired neuromuscular transmission should be tested under myasthenic-like conditions, so as to avoid confounding effects of synaptic homeostasis.

Mechanisms of endocannabinoid control of synaptic plasticity

The endogenous cannabinoid transmitter system regulates synaptic transmission throughout the nervous system. Unlike conventional transmitters, specific stimuli induce synthesis of endocannabinoids (eCBs) in the postsynaptic neuron, and these travel backwards to modulate presynaptic inputs. In doing so, eCBs can induce short-term changes in synaptic strength and longer-term plasticity. While this eCB regulation is near ubiquitous, it displays major regional and synapse specific variations with different synapse specific forms of short-versus long-term plasticity throughout the brain. These differences are due to the plethora of pre- and postsynaptic mechanisms which have been implicated in eCB signalling, the intricacies of which are only just being realised. In this review, we shall describe the current understanding and highlight new advances in this area, with a focus on the retrograde action of eCBs at CB1 receptors (CB₁Rs).

Multiple cannabinoid signaling cascades powerfully suppress recurrent excitation in the hippocampus

Recurrent excitatory neural networks are unstable. In the hippocampus, excitatory mossy cells (MCs) receive strong excitatory inputs from dentate granule cells (GCs) and project back onto the proximal dendrites of GCs. By targeting the ipsi- and contralateral dentate gyrus (DG) along the dorsoventral axis of the hippocampus, MCs form an extensive recurrent excitatory circuit (GC-MC-GC) whose dysregulation can promote epilepsy. We recently reported that a physiologically relevant pattern of MC activity induces a robust form of presynaptic long-term potentiation (LTP) of MC-GC transmission which enhances GC output. Left unchecked, this LTP may interfere with DG-dependent learning, like pattern separation-which relies on sparse GC firing-and may even facilitate epileptic activity. Intriguingly, MC axons display uniquely high expression levels of type-1 cannabinoid receptors (CB1Rs), but their role at MC-GC synapses is poorly understood. Using rodent hippocampal slices, we report that constitutively active CB1Rs, presumably via βγ subunits, selectively inhibited MC inputs onto GCs but not MC inputs onto inhibitory interneurons or CB1R-sensitive inhibitory inputs onto GCs. Tonic CB1R activity also inhibited LTP and GC output. Furthermore, brief endocannabinoid release from GCs dampened MC-GC LTP in two mechanistically distinct ways: during induction via βγ signaling and before induction via α_i/o signaling in a form of presynaptic metaplasticity. Lastly, a single in vivo exposure to exogenous cannabinoids was sufficient to induce this presynaptic metaplasticity. By dampening excitatory transmission and plasticity, tonic and phasic CB1R activity at MC axon terminals may preserve the sparse nature of the DG and protect against runaway excitation.

Cannabis use, abuse, and withdrawal: Cannabinergic mechanisms, clinical, and preclinical findings

Cannabis sativa is the most widely used illicit drug in the world. Its main psychoactive component is delta‐9‐tetrahydrocannabinol (THC), one of over 100 phytocannabinoid compounds produced by the cannabis plant. THC is the primary compound that drives cannabis abuse potential and is also used and prescribed medically for therapeutic qualities. Despite its therapeutic potential, a significant subpopulation of frequent cannabis or THC users will develop a drug use syndrome termed cannabis use disorder. Individuals suffering from cannabis use disorder exhibit many of the hallmarks of classical addictions including cravings, tolerance, and withdrawal symptoms. Currently, there are no efficacious treatments for cannabis use disorder or withdrawal symptoms. This makes both clinical and preclinical research on the neurobiological mechanisms of these syndromes ever more pertinent. Indeed, basic research using animal models has provided valuable evidence of the neural molecular and cellular actions of cannabis that mediate its behavioral effects. One of the main components being central action on the cannabinoid type‐one receptor and downstream intracellular signaling related to the endogenous cannabinoid system. Back‐translational studies have provided insight linking preclinical basic and behavioral biology research to better understand symptoms observed at the clinical level. This narrative review aims to summarize major research elucidating the molecular, cellular, and behavioral manifestations of cannabis/THC use that play a role in cannabis use disorder and withdrawal.

Cnr2 Is Important for Ribbon Synapse Maturation and Function in Hair Cells and Photoreceptors

The role of the cannabinoid receptor 2 (CNR2) is still poorly described in sensory epithelia. We found strong cnr2 expression in hair cells (HCs) of the inner ear and the lateral line (LL), a superficial sensory structure in fish. Next, we demonstrated that sensory synapses in HCs were severely perturbed in larvae lacking cnr2. Appearance and distribution of presynaptic ribbons and calcium channels (Ca_v1.3) were profoundly altered in mutant animals. Clustering of membrane-associated guanylate kinase (MAGUK) in post-synaptic densities (PSDs) was also heavily affected, suggesting a role for cnr2 for maintaining the sensory synapse. Furthermore, vesicular trafficking in HCs was strongly perturbed suggesting a retrograde action of the endocannabinoid system (ECs) via cnr2 that was modulating HC mechanotransduction. We found similar perturbations in retinal ribbon synapses. Finally, we showed that larval swimming behaviors after sound and light stimulations were significantly different in mutant animals. Thus, we propose that cnr2 is critical for the processing of sensory information in the developing larva.

Mechanisms of endocannabinoid transport in the brain

The endocannabinoids 2-arachidonoylglycerol (2-AG) and anandamide are among the best studied lipid messengers in the brain. By activating cannabinoid receptors in the CNS, endocannabinoids tune synaptic function, thereby influencing a variety of physiological and behavioural processes. Extensive research conducted over the last few decades has considerably enhanced our understanding of the molecular mechanisms and physiological functions of the endocannabinoid system. It is now well-established that endocannabinoids are synthesized by postsynaptic neurons and serve as retrograde messengers that suppress neurotransmitter release at central synapses. While the detailed mechanisms by which endocannabinoids gate synaptic function and behavioural processes are relatively well characterized, the mechanisms governing endocannabinoid transport at central synapses remain ill defined. Recently, several studies have begun to unravel the mechanisms governing intracellular and intercellular endocannabinoid transport. In this review, we will focus on new advances in the mechanisms of intracellular and synaptic endocannabinoid transport in the CNS.

Endocannabinoids and Dopamine Balance Basal Ganglia Output

The entopeduncular nucleus is one of the basal ganglia's output nuclei, thereby controlling basal ganglia information processing. Entopeduncular nucleus neurons integrate GABAergic inputs from the Striatum and the globus pallidus, together with glutamatergic inputs from the subthalamic nucleus. We show that endocannabinoids and dopamine interact to modulate the long-term plasticity of all these primary afferents to the entopeduncular nucleus. Our results suggest that the interplay between dopamine and endocannabinoids determines the balance between direct pathway (striatum) and indirect pathway (globus pallidus) in entopeduncular nucleus output. Furthermore, we demonstrate that, despite the lack of axon collaterals, information is transferred between neighboring neurons in the entopeduncular nucleus via endocannabinoid diffusion. These results transform the prevailing view of the entopeduncular nucleus as a feedforward “relay” nucleus to an intricate control unit, which may play a vital role in the process of action selection.

Endocannabinoid-serotonin systems interaction in health and disease

Endocannabinoid (eCB) and serotonin (5-HT) neuromodulatory systems work both independently and together to finely orchestrate neuronal activity throughout the brain to strongly sculpt behavioral functions. Surprising parallelism between the behavioral effects of 5-HT and eCB activity has been widely reported, including the regulation of emotional states, stress homeostasis, cognitive functions, food intake and sleep. The distribution pattern of the 5-HT system and the eCB molecular elements in the brain display a strong overlap and several studies report a functional interplay and even a tight interdependence between eCB/5-HT signaling. In this review, we examine the available evidence of the interaction between the eCB and 5-HT systems. We first introduce the eCB system, then we describe the eCB/5-HT crosstalk at the neuronal and synaptic levels. Finally, we explore the potential eCB/5-HT interaction at the behavioral level with the implication for psychiatric and neurological disorders. The precise elucidation of how this neuromodulatory interaction dynamically regulates biological functions may lead to the development of more targeted therapeutic strategies for the treatment of depressive and anxiety disorders, psychosis and epilepsy.

Resilience to fear: The role of individual factors in amygdala response to stressors

Resilience to stress is an adaptive process that varies individually. Resilience refers to the adaptation, or the ability to maintain or regain mental health, despite being subject to adverse situation. Resilience is a dynamic concept that reflects a combination of internal individual factors, including age and gender interacting with external factors such as social, cultural and environmental factors. In the last decade, we have witnessed an increase in the prevalence of anxiety disorders, including post-traumatic stress disorder. Given that stress in unavoidable, it is of great interest to understand the neurophysiological mechanisms of resilience, the individual factors that may contribute to susceptibility and promote efficacious approaches to improve resilience. Here, we address this complex question, attempting at defining clear and operational definitions that may allow us to improve our analysis of behavior incorporating individuality. We examine how individual perception of the stressor can alter the outcome of an adverse situation using as an example, the fear-conditioning paradigm and discuss how individual differences in the reward system can contribute to resilience. Given the central role of the endocannabinoid system in regulating fear responses and anxiety, we discuss the evidence that polymorphisms in several molecules of this signaling system contribute to different anxiety phenotypes. The endocannabinoid system is highly interconnected with the serotoninergic and dopaminergic modulatory systems, contributing to individual differences in stress perception and coping mechanisms. We review how the individual variability in these modulatory systems can be used towards a multivariable assessment of stress risk. Incorporating individuality in our research will allow us to define biomarkers of anxiety disorders as well as assess prognosis, towards a personalized clinical approach to mental health.

Maternal hypercaloric diet affects factors involved in lipid metabolism and the endogenous cannabinoid systems in the hypothalamus of adult offspring: sex-specific response of astrocytes to palmitic acid and anandamide

Aim: We aimed to investigate whether maternal malnutrition during gestation/lactation induces long-lasting changes on inflammation, lipid metabolism and endocannabinoid signaling in the adult offspring hypothalamus and the role of hypothalamic astrocytes in these changes.Methods: We analyzed the effects of a free-choice hypercaloric palatable diet (P) during (pre)gestation, lactation and/or post-weaning on inflammation, lipid metabolism and endogenous cannabinoid signaling in the adult offspring hypothalamus. We also evaluated the response of primary hypothalamic astrocytes to palmitic acid and anandamide.Results: Postnatal exposure to a P diet induced factors involved in hypothalamic inflammation (Tnfa and Il6) and gliosis (Gfap, vimentin and Iba1) in adult offspring, being more significant in females. In contrast, maternal P diet reduced factors involved in astrogliosis (vimentin), fatty acid oxidation (Cpt1a) and monounsaturated fatty acid synthesis (Scd1). These changes were accompanied by an increase in the expression of the genes for the cannabinoid receptor (Cnr1) and Nape-pld, an enzyme involved in endocannabinoid synthesis, in females and a decrease in the endocannabinoid degradation enzyme Faah in males. These changes suggest that the maternal P diet results in sex-specific alterations in hypothalamic endocannabinoid signaling and lipid metabolism. This hypothesis was tested in hypothalamic astrocyte cultures, where palmitic acid (PA) and the polyunsaturated fatty acid N-arachidonoylethanolamine (anandamide or AEA) were found to induce similar changes in the endocannabinoid system (ECS) and lipid metabolism.Conclusion: These results stress the importance of both maternal diet and sex in long term metabolic programming and suggest a possible role of hypothalamic astrocytes in this process.

BDNF Activates Postsynaptic TrkB Receptors to Induce Endocannabinoid Release and Inhibit Presynaptic Calcium Influx at a Calyx-Type Synapse

Brain-derived neurotropic factor (BDNF) has been shown to play critical roles in neural development, plasticity, and neurodegenerative diseases. The main function of BDNF in the brain is widely accepted to be synaptic regulation. However, how BDNF modulates synaptic transmission, especially the underlying signaling cascades between presynaptic and postsynaptic neurons, remains controversial.

Brain-derived neurotropic factor (BDNF) has been shown to play critical roles in neural development, plasticity, and neurodegenerative diseases. The main function of BDNF in the brain is widely accepted to be synaptic regulation. However, how BDNF modulates synaptic transmission, especially the underlying signaling cascades between presynaptic and postsynaptic neurons, remains controversial. In the present study, we investigated the actions of BDNF at rat calyx-type synapses of either sex by measuring the excitatory postsynaptic current (EPSC) and presynaptic calcium current and capacitance changes. We found that BDNF inhibits the EPSC, presynaptic calcium influx, and exocytosis/endocytosis via activation of the presynaptic cannabinoid Type 1 receptors (CB1Rs). Inhibition of the CB1Rs abolished the BDNF-induced presynaptic inhibition, whereas CB1R agonist mimicked the effect of BDNF. Exploring the underlying signaling cascade, we found that BDNF specifically activates the postsynaptic TrkB receptors, inducing the release of endocannabinoids via the PLCγ/DGL pathway and retrogradely activating presynaptic CB1Rs. We also reported the involvement of AC/PKA in modulating vesicle endocytosis, which may account for the BDNF-induced calcium-dependent and -independent regulation of endocytosis. Thus, our study provides new insights into the BDNF/endocannabinoid-associated modulation of neurotransmission in physiological and pathologic processes.

SIGNIFICANCE STATEMENT BDNF plays critical roles in the modulation of synaptic strength. However, how BDNF regulates synaptic transmission and its underlying signaling cascade(s) remains elusive. By measuring EPSC and the presynaptic calcium current and capacitance changes at rat calyces, we found that BDNF inhibits synaptic transmission via BDNF-TrkB-eCB signaling pathway. Activation of postsynaptic TrkB receptors induces endocannabinoid release via the PLCγ/DGL pathway, retrogradely activating the presynaptic CB1Rs, inhibiting the AC/PKA, and suppressing calcium influx. Our findings provide a comprehensive understanding of BDNF/endocannabinoid-associated modulation of neuronal activities.

CB1-receptor-mediated inhibitory LTD triggers presynaptic remodeling via protein synthesis and ubiquitination

Long-lasting forms of postsynaptic plasticity commonly involve protein synthesis-dependent structural changes of dendritic spines. However, the relationship between protein synthesis and presynaptic structural plasticity remains unclear. Here, we investigated structural changes in cannabinoid-receptor 1 (CB₁)-mediated long-term depression of inhibitory transmission (iLTD), a form of presynaptic plasticity that involves a protein-synthesis-dependent long-lasting reduction in GABA release. We found that CB₁-iLTD in acute rat hippocampal slices was associated with protein synthesis-dependent presynaptic structural changes. Using proteomics, we determined that CB₁ activation in hippocampal neurons resulted in increased ribosomal proteins and initiation factors, but decreased levels of proteins involved in regulation of the actin cytoskeleton, such as ARPC2 and WASF1/WAVE1, and presynaptic release. Moreover, while CB₁-iLTD increased ubiquitin/proteasome activity, ubiquitination but not proteasomal degradation was critical for structural and functional presynaptic CB₁-iLTD. Thus, CB₁-iLTD relies on both protein synthesis and ubiquitination to elicit structural changes that underlie long-term reduction of GABA release.

Cannabinoids, Endocannabinoids and Sleep

Sleep is a vital function of the nervous system that contributes to brain and bodily homeostasis, energy levels, cognitive ability, and other key functions of a variety of organisms. Dysfunctional sleep induces neural problems and is a key part of almost all human psychiatric disorders including substance abuse disorders. The hypnotic effects of cannabis have long been known and there is increasing use of phytocannabinoids and other formulations as sleep aids. Thus, it is crucial to gain a better understanding of the neurobiological basis of cannabis drug effects on sleep, as well as the role of the endogenous cannabinoid system in sleep physiology. In this review article, we summarize the current state of knowledge concerning sleep-related endogenous cannabinoid function derived from research on humans and rodent models. We also review information on acute and chronic cannabinoid drug effects on sleep in these organisms, and molecular mechanisms that may contribute to these effects. We point out the potential benefits of acute cannabinoids for sleep improvement, but also the potential sleep-disruptive effects of withdrawal following chronic cannabinoid drug use. Prescriptions for future research in this burgeoning field are also provided.

The ins and outs of inhibitory synaptic plasticity: Neuron types, molecular mechanisms and functional roles

GABAergic interneurons are highly diverse, and their synaptic outputs express various forms of plasticity. Compelling evidence indicates that activity-dependent changes of inhibitory synaptic transmission play a significant role in regulating neural circuits critically involved in learning and memory and circuit refinement. Here, we provide an updated overview of inhibitory synaptic plasticity with a focus on the hippocampus and neocortex. To illustrate the diversity of inhibitory interneurons, we discuss the case of two highly divergent interneuron types, parvalbumin-expressing basket cells and neurogliaform cells, which support unique roles on circuit dynamics. We also present recent progress on the molecular mechanisms underlying long-term, activity-dependent plasticity of fast inhibitory transmission. Lastly, we discuss the role of inhibitory synaptic plasticity in neuronal circuits' function. The emerging picture is that inhibitory synaptic transmission in the CNS is extremely diverse, undergoes various mechanistically distinct forms of plasticity and contributes to a much more refined computational role than initially thought. Both the remarkable diversity of inhibitory interneurons and the various forms of plasticity expressed by GABAergic synapses provide an amazingly rich inhibitory repertoire that is central to a variety of complex neural circuit functions, including memory.

Mu-Opioids Suppress GABAergic Synaptic Transmission onto Orbitofrontal Cortex Pyramidal Neurons with Subregional Selectivity

The orbitofrontal cortex (OFC) plays a critical role in evaluating outcomes in a changing environment. Administering opioids to the OFC can alter the hedonic reaction to food rewards and increase their consumption in a subregion-specific manner. However, it is unknown how mu-opioid signaling influences synaptic transmission in the OFC. Thus, we investigated the cellular actions of mu-opioids within distinct subregions of the OFC. Using in vitro patch-clamp electrophysiology in brain slices containing the OFC, we found that the mu-opioid agonist DAMGO produced a concentration-dependent inhibition of GABAergic synaptic transmission onto medial OFC (mOFC), but not lateral OFC (lOFC) neurons. This effect was mediated by presynaptic mu-opioid receptor activation of local parvalbumin (PV⁺)-expressing interneurons. The DAMGO-induced suppression of inhibition was long lasting and not reversed on washout of DAMGO or by application of the mu-opioid receptor antagonist CTAP, suggesting an inhibitory long-term depression (LTD) induced by an exogenous mu-opioid. We show that LTD at inhibitory synapses is dependent on downstream cAMP/protein kinase A (PKA) signaling, which differs between the mOFC and lOFC. Finally, we demonstrate that endogenous opioid release triggered via moderate physiological stimulation can induce LTD. Together, these results suggest that presynaptic mu-opioid stimulation of local PV⁺ interneurons induces a long-lasting suppression of GABAergic synaptic transmission, which depends on subregional differences in mu-opioid receptor coupling to the downstream cAMP/PKA intracellular cascade. These findings provide mechanistic insight into the opposing functional effects produced by mu-opioids within the OFC.SIGNIFICANCE STATEMENT Considering that both the orbitofrontal cortex (OFC) and the opioid system regulate reward, motivation, and food intake, understanding the role of opioid signaling within the OFC is fundamental for a mechanistic understanding of the sequelae for several psychiatric disorders. This study makes several novel observations. First, mu-opioids induce a long-lasting suppression of inhibitory synaptic transmission onto OFC pyramidal neurons in a regionally selective manner. Second, mu-opioids recruit parvalbumin inputs to suppress inhibitory synaptic transmission in the mOFC. Third, the regional selectivity of mu-opioid action of endogenous opioids is due to the efficacy of mu-opioid receptor coupling to the downstream cAMP/PKA intracellular cascades. These experiments are the first to reveal a cellular mechanism of opioid action within the OFC.

Protein Interactors and Trafficking Pathways That Regulate the Cannabinoid Type 1 Receptor (CB1R)

The endocannabinoid system (ECS) acts as a negative feedback mechanism to suppress synaptic transmission and plays a major role in a diverse range of brain functions including, for example, the regulation of mood, energy balance, and learning and memory. The function and dysfunction of the ECS are strongly implicated in multiple psychiatric, neurological, and neurodegenerative diseases. Cannabinoid type 1 receptor (CB1R) is the most abundant G protein-coupled receptor (GPCR) expressed in the brain and, as for any synaptic receptor, CB1R needs to be in the right place at the right time to respond appropriately to changing synaptic circumstances. While CB1R is found intracellularly throughout neurons, its surface expression is highly polarized to the axonal membrane, consistent with its functional expression at presynaptic sites. Surprisingly, despite the importance of CB1R, the interacting proteins and molecular mechanisms that regulate the highly polarized distribution and function of CB1R remain relatively poorly understood. Here we set out what is currently known about the trafficking pathways and protein interactions that underpin the surface expression and axonal polarity of CB1R, and highlight key questions that still need to be addressed.

Disrupted inhibitory plasticity and homeostasis in Fragile X syndrome

Fragile X Syndrome (FXS) is a neurodevelopmental disorder instigated by the absence of a key translation regulating protein, Fragile X Mental Retardation Protein (FMRP). The loss of FMRP in the CNS leads to abnormal synaptic development, disruption of critical periods of plasticity, and an overall deficiency in proper sensory circuit coding leading to hyperexcitable sensory networks. However, little is known about how this hyperexcitable environment affects inhibitory synaptic plasticity. Here, we show that in vivo layer 2/3 of the primary somatosensory cortex of the Fmr1 KO mouse exhibits basal hyperexcitability and an increase in neuronal firing rate suppression during whisker activation. This aligns with our in vitro data that indicate an increase in GABAergic spontaneous activity, a faulty mGluR-mediated inhibitory input and impaired inhibitory plasticity processes. Specifically, we find that mGluR activation sensitivity is overall diminished in the Fmr1 KO mouse leading to both a decreased spontaneous inhibitory postsynaptic input to principal cells and a disrupted form of inhibitory long-term depression (I-LTD). These data suggest an adaptive mechanism that acts to homeostatically counterbalance the cortical hyperexcitability observed in FXS.

PUFA and their derivatives in neurotransmission and synapses: a new hallmark of synaptopathies

PUFA of the n-3 and n-6 families are present in high concentration in the brain where they are major components of cell membranes. The main forms found in the brain are DHA (22 :6, n-3) and arachidonic acid (20:4, n-6). In the past century, several studies pinpointed that modifications of n-3 and n-6 PUFA levels in the brain through dietary supply or genetic means are linked to the alterations of synaptic function. Yet, synaptopathies emerge as a common characteristic of neurodevelopmental disorders, neuropsychiatric diseases and some neurodegenerative diseases. Understanding the mechanisms of action underlying the activity of PUFA at the level of synapses is thus of high interest. In this frame, dietary supplementation in PUFA aiming at restoring or promoting the optimal function of synapses appears as a promising strategy to treat synaptopathies. This paper reviews the link between dietary PUFA, synapse formation and the role of PUFA and their metabolites in synaptic functions.

Targeting the endocannabinoid system: a predictive, preventive, and personalized medicine-directed approach to the management of brain pathologies

Cannabis-inspired medical products are garnering increasing attention from the scientific community, general public, and health policy makers. A plethora of scientific literature demonstrates intricate engagement of the endocannabinoid system with human immunology, psychology, developmental processes, neuronal plasticity, signal transduction, and metabolic regulation. Despite the therapeutic potential, the adverse psychoactive effects and historical stigma, cannabinoids have limited widespread clinical application. Therefore, it is plausible to weigh carefully the beneficial effects of cannabinoids against the potential adverse impacts for every individual. This is where the concept of "personalized medicine" as a promising approach for disease prediction and prevention may take into the account. The goal of this review is to provide an outline of the endocannabinoid system, including endocannabinoid metabolizing pathways, and will progress to a more in-depth discussion of the therapeutic interventions by endocannabinoids in various neurological disorders.

Role of cyclic nucleotides and their downstream signaling cascades in memory function: Being at the right time at the right spot

A plethora of studies indicate the important role of cAMP and cGMP cascades in neuronal plasticity and memory function. As a result, altered cyclic nucleotide signaling has been implicated in the pathophysiology of mnemonic dysfunction encountered in several diseases. In the present review we provide a wide overview of studies regarding the involvement of cyclic nucleotides, as well as their upstream and downstream molecules, in physiological and pathological mnemonic processes. Next, we discuss the regulation of the intracellular concentration of cyclic nucleotides via phosphodiesterases, the enzymes that degrade cAMP and/or cGMP, and via A-kinase-anchoring proteins that refine signal compartmentalization of cAMP signaling. We also provide an overview of the available data pointing to the existence of specific time windows in cyclic nucleotide signaling during neuroplasticity and memory formation and the significance to target these specific time phases for improving memory formation. Finally, we highlight the importance of emerging imaging tools like Förster resonance energy transfer imaging and optogenetics in detecting, measuring and manipulating the action of cyclic nucleotide signaling cascades.

Priming of GABAergic Long-term Potentiation by Muscarinic Receptors

Growing evidence indicates that GABAergic interneurons play a pivotal role to generate brain oscillation patterns, which are fundamental for the mnemonic processing of the hippocampus. While acetylcholine (ACh) is a powerful modulator of synaptic plasticity and brain function, few studies have been focused on the role of cholinergic signaling in the regulation of GABAergic inhibitory synaptic plasticity. We have previously shown that co-activation of endocannabinoids (CB1R) and muscarinic receptor (mAChR) in hippocampal interneurons can induce activity-dependent GABAergic long-term depression in CA1 pyramidal neurons. Here, using electrophysiological and pharmacological approaches in acute rat hippocampal slices, we show that activation of cholinergic receptors followed by either high-frequency stimulation of Schaeffer collaterals or exogenous activation of metabotropic glutamate receptor (mGluR) induces a robust long-term potentiation at GABAergic synapses (iLTP). These forms of iLTP are blocked by the M1 type of mAChR (MR1) or by the group I of mGluR (mGluR1/5) antagonists. These results suggest the existence of spatiotemporal cooperativity between cholinergic and glutamatergic pathways where activation of mAChR serves as a metaplastic switch making glutamatergic synapses capable to induce long-term potentiation at inhibitory synapses, that may contribute to the modulation of brain mechanisms of learning and memory.

Preserving the balance: diverse forms of long-term GABAergic synaptic plasticity

Cellular mechanisms that regulate the interplay of synaptic excitation and inhibition are thought to be central to the functional stability of healthy neuronal circuits. A growing body of literature demonstrates the capacity for inhibitory GABAergic synapses to exhibit long-term plasticity in response to changes in neuronal activity. Here, we review this expanding field of research, focusing on the diversity of mechanisms that link glutamatergic signalling, postsynaptic action potentials and inhibitory synaptic strength. Several lines of evidence indicate that multiple, parallel forms of plasticity serve to regulate activity at both the input and output domains of individual neurons. Overall, these varied phenomena serve to promote both stability and flexibility over the life of the organism.

The readily-releasable pool dynamically regulates multivesicular release

The number of neurotransmitter-filled vesicles released into the synaptic cleft with each action potential dictates the reliability of synaptic transmission. Variability of this fundamental property provides diversity of synaptic function across brain regions, but the source of this variability is unclear. The prevailing view is that release of a single (univesicular release, UVR) or multiple vesicles (multivesicular release, MVR) reflects variability in vesicle release probability, a notion that is well-supported by the calcium-dependence of release mode. However, using mouse brain slices, we now demonstrate that the number of vesicles released is regulated by the size of the readily-releasable pool, upstream of vesicle release probability. Our results point to a model wherein protein kinase A and its vesicle-associated target, synapsin, dynamically control release site occupancy to dictate the number of vesicles released without altering release probability. Together these findings define molecular mechanisms that control MVR and functional diversity of synaptic signaling.

Our nervous system allows us to rapidly sense and respond to the world around us via cells called neurons that relay electrical signals around the brain and body. When an electrical impulse travelling along one neuron reaches a junction – called a synapse – with a neighboring neuron, it stimulates small containers known as vesicles from the first cell to release their contents into the synapse. These contents then travel across to the neighboring cell and may generate a new electrical impulse.

The number of vesicles at a synapse that are ready to be released varies from one to ten. The more vesicles the neuron releases, the more likely the second cell will produce an electrical signal of its own. However, not all electrical signals reaching a synapse stimulate vesicles to be released and some signals only release a single vesicle.

What determines how many vesicles are released by a single electrical signal? Some vesicles have a higher likelihood of being released than others, but this “eagerness” does not always predict how many vesicles an individual synapse will actually discharge. Now, Vaden et al. have used brain tissue from mice to test an alternative possibility: the simple idea that the number of vesicles available at the synapse affects how many vesicles are released without altering their eagerness for release.

Vaden et al. found that activating an enzyme called protein kinase A increased the number of vesicles released from synapses without changing how likely individual vesicles were to be released. Inhibiting protein kinase A also did not change individual vesicle’s eagerness to be released, but did decrease the number of vesicles that were discharged. Further experiments found that protein kinase A modifies a molecule on the surface of vesicles, known as synapsin, which controls the number of vesicles that are available for release.

These findings show that the number of vesicles released at a synapse is controlled by two independently regulated parameters: the number of vesicles that are available, as well as how eager individual vesicles are to be released. The ability of neurons to communicate with each other is disrupted in autism spectrum disorders, Alzheimer’s disease and many other diseases. Learning how neurons communicate in healthy brains will help us understand what happens in the neurons of individuals with these conditions.

Activity-Independent Effects of CREB on Neuronal Survival and Differentiation during Mouse Cerebral Cortex Development

Neuronal survival and morphological maturation depends on the action of the transcription factor calcium responsive element binding protein (CREB), which regulates expression of several target genes in an activity-dependent manner. However, it remains largely unknown whether CREB-mediated transcription could play a role at early stages of neuronal differentiation, prior to the establishment of functional synaptic contacts. Here, we show that CREB is phosphorylated at very early stages of neuronal differentiation in vivo and in vitro, even in the absence of depolarizing agents. Using genetic tools, we also show that inhibition of CREB-signaling affects neuronal growth and survival in vitro without affecting cell proliferation and neurogenesis. Expression of A-CREB or M-CREB, 2 dominant-negative inhibitors of CREB, decreases cell survival and the complexity of neuronal arborization. Similar changes are observed in neurons treated with protein kinase A (PKA) and Ca2+/calmodulin-dependent protein kinase II (CaMKII) inhibitors, which also show decreased levels of pCREBSer133. Notably, expression of CREB-FY, a Tyr134Phe CREB mutant with a lower Km for phosphorylation, partly rescues the effects of PKA and CaMKII inhibition. Our data indicate that CREB-mediated signaling play important roles at early stages of cortical neuron differentiation, prior to the establishment of fully functional synaptic contacts.

Active Zone Proteins RIM1αβ Are Required for Normal Corticostriatal Transmission and Action Control

Dynamic regulation of synaptic transmission at cortical inputs to the dorsal striatum is considered critical for flexible and efficient action learning and control. Presynaptic mechanisms governing the properties and plasticity of glutamate release from these inputs are not fully understood, and the corticostriatal synaptic processes that support normal action learning and control remain unclear. Here we show in male and female mice that conditional deletion of presynaptic proteins RIM1αβ (RIM1) from excitatory cortical neurons impairs corticostriatal synaptic transmission in the dorsolateral striatum. Key forms of presynaptic G-protein-coupled receptor-mediated short- and long-term striatal plasticity are spared following RIM1 deletion. Conditional RIM1 KO mice show heightened novelty-induced locomotion and impaired motor learning on the accelerating rotarod. They further show heightened self-paced instrumental responding for food and impaired learning of a habitual instrumental response strategy. Together, these findings reveal a selective role for presynaptic RIM1 in neurotransmitter release at prominent basal ganglia synapses, and provide evidence that RIM1-dependent processes help to promote the refinement of skilled actions, constrain goal-directed behaviors, and support the learning and use of habits.SIGNIFICANCE STATEMENT Our daily functioning hinges on the ability to flexibly and efficiently learn and control our actions. How the brain encodes these capacities is unclear. Here we identified a selective role for presynaptic proteins RIM1αβ in controlling glutamate release from cortical inputs to the dorsolateral striatum, a brain structure critical for action learning and control. Behavioral analysis of mice with restricted genetic deletion of RIM1αβ further revealed roles for RIM1αβ-dependent processes in the learning and refinement of motor skills and the balanced expression of goal-directed and habitual actions.