Role for Endogenous BDNF in Endocannabinoid-Mediated Long-Term Depression at Neocortical Inhibitory Synapses

VPS13A knockdown impairs corticostriatal synaptic plasticity and locomotor behavior in a new mouse model of chorea-acanthocytosis

Chorea-acanthocytosis (ChAc) is an inherited neurodegenerative movement disorder caused by VPS13A gene mutations leading to the absence of protein expression. The striatum is the most affected brain region in ChAc patients. However, the study of the VPS13A function in the brain has been poorly addressed. Here we generated a VPS13A knockdown (KD) model and aimed to elucidate the contribution of VPS13A to synaptic plasticity and neuronal communication in the corticostriatal circuit. First, we infected primary cortical neurons with miR30-shRNA against VPS13A and analyzed its effects on neuronal plasticity. VPS13A-KD neurons showed a higher degree of branching than controls, accompanied by decreased BDNF and PSD-95 levels, indicative of synaptic alterations. We then injected AAV-KD bilaterally in the frontal cortex and two different regions of the striatum of mice and analyzed the effects of VPS13A-KD on animal behavior and synaptic plasticity. VPS13A-KD mice showed modification of the locomotor behavior pattern, with increased exploratory behavior and hyperlocomotion. Corticostriatal dysfunction in VPS13A-KD mice was evidenced by impaired striatal long-term depression (LTD) after stimulation of cortical afferents, which was partially recovered by BDNF administration. VPS13A-KD did not lead to neuronal loss in the cortex or the striatum but induced a decrease in the neuronal release of CX3CL1 and triggered a microglial reaction, especially in the striatum. Notably, CX3CL1 administration partially restored the impaired corticostriatal LTD in VPS13A-KD mice. Our results unveil the involvement of VPS13A in neuronal connectivity modifying BDNF and CX3CL1 release. Moreover, the involvement of VPS13A in synaptic plasticity and motor behavior provides key information to further understand not only ChAc pathophysiology but also other neurological disorders.

Endocannabinoid signaling in synaptic function

In the last decades, astrocytes have emerged as important regulatory cells actively involved in brain function by exchanging signaling with neurons. The endocannabinoid (eCB) signaling is widely present in many brain areas, being crucially involved in multiple brain functions and animal behaviors. The present review presents and discusses current evidence demonstrating that astrocytes sense eCBs released during neuronal activity and subsequently release gliotransmitters that regulate synaptic transmission and plasticity. The eCB signaling to astrocytes and the synaptic regulation mediated by astrocytes activated by eCBs are complex phenomena that exhibit exquisite spatial and temporal properties, a wide variety of downstream signaling mechanisms, and a large diversity of functional synaptic outcomes. Studies investigating this topic have revealed novel regulatory processes of synaptic function, like the lateral regulation of synaptic transmission and the active involvement of astrocytes in the spike-timing dependent plasticity, originally thought to be exclusively mediated by the coincident activity of pre- and postsynaptic neurons, following Hebbian rules for associative learning. Finally, the critical influence of astrocyte-mediated eCB signaling on animal behavior is also discussed.

Visual Deprivation Selectively Reduces Thalamic Reticular Nucleus-Mediated Inhibition of the Auditory Thalamus in Adults

Sensory loss leads to widespread cross-modal plasticity across brain areas to allow the remaining senses to guide behavior. While multimodal sensory interactions are often attributed to higher-order sensory areas, cross-modal plasticity has been observed at the level of synaptic changes even across primary sensory cortices. In particular, vision loss leads to widespread circuit adaptation in the primary auditory cortex (A1) even in adults. Here we report using mice of both sexes in which cross-modal plasticity occurs even earlier in the sensory-processing pathway at the level of the thalamus in a modality-selective manner. A week of visual deprivation reduced inhibitory synaptic transmission from the thalamic reticular nucleus (TRN) to the primary auditory thalamus (MGBv) without changes to the primary visual thalamus (dLGN). The plasticity of TRN inhibition to MGBv was observed as a reduction in postsynaptic gain and short-term depression. There was no observable plasticity of the cortical feedback excitatory synaptic transmission from the primary visual cortex to dLGN or TRN and A1 to MGBv, which suggests that the visual deprivation-induced plasticity occurs predominantly at the level of thalamic inhibition. We provide evidence that visual deprivation-induced change in the short-term depression of TRN inhibition to MGBv involves endocannabinoid CB1 receptors. TRN inhibition is considered critical for sensory gating, selective attention, and multimodal performances; hence, its plasticity has implications for sensory processing. Our results suggest that selective disinhibition and altered short-term dynamics of TRN inhibition in the spared thalamic nucleus support cross-modal plasticity in the adult brain.SIGNIFICANCE STATEMENT Losing vision triggers adaptation of the brain to enhance the processing of the remaining senses, which can be observed as better auditory performance in blind subjects. We previously found that depriving vision of adult rodents produces widespread circuit reorganization in the primary auditory cortex and enhances auditory processing at a neural level. Here we report that visual deprivation-induced plasticity in adults occurs much earlier in the auditory pathway, at the level of thalamic inhibition. Sensory processing is largely gated at the level of the thalamus via strong cortical feedback inhibition mediated through the thalamic reticular nucleus (TRN). We found that TRN inhibition of the auditory thalamus is selectively reduced by visual deprivation, thus playing a role in adult cross-modal plasticity.

2-AG and anandamide enhance hippocampal long-term potentiation via suppression of inhibition

It is widely accepted that exogenous cannabinoids can impair short-term memory and cognition in humans and other animals. This is likely related to the inhibition of long-term potentiation (LTP), a form of synaptic plasticity, by the global and sustained activation of CB1 cannabinoid receptors in the presence of exogenous agonists. Conversely, the temporally and spatially restricted release of endogenous cannabinoid (eCB) ligands may enhance synaptic plasticity in a synapse-specific manner. We examined the role of eCB signaling in LTP by recording field excitatory postsynaptic potentials (fEPSPs) in the CA1 stratum radiatum in hippocampal slices from juvenile mice. LTP was induced either electrically, by theta burst stimulation (TBS), or pharmacologically, by treatment for 15 min with a solution designed to increase intracellular cAMP (chem-LTP). A stable and long-lasting potentiation in fEPSP slope following TBS was significantly reduced by blocking cannabinoid receptor activation with CB1 receptor antagonists. Chem-LTP caused a sustained 2-fold increase in fEPSP slope and was also blocked by CB1 receptor antagonists. TBS-LTP was partially reduced by inhibiting the synthesis of the endogenous ligands 2-arachidonylglycerol (2-AG) and anandamide. A similar effect was observed with chem-LTP. Blocking inhibitory synapses completely prevented the effect of CB1 receptor antagonists or inhibition of eCB synthesis on TBS-LTP and chem-LTP. These results indicate that simultaneous activation of CB1 receptors by 2-AG and anandamide enhances TBS-induced and pharmacologically-induced LTP, and this effect is mediated by the suppression of inhibition at GABAergic synapses.

Prenatal exposure to Cannabis smoke induces early and lasting damage to the brain

Cannabis is the most widely used illegal drug during pregnancy, however, the effects of gestational exposure to Cannabis smoke (CS) on the central nervous system development remain uncharacterised. This study investigates the effects of maternal CS inhalation on brain function in the offspring. Pregnant mice were exposed daily to 5 min of CS during gestational days (GD) 5.5-17.5. On GD 18.5 half of the dams were euthanized for foetus removal. The offspring from the remaining dams were euthanized on postnatal days (PND) 20 and 60 for evaluation. Brain volume, cortex cell number, SOX2, histone-H3, parvalbumin, NeuN, and BDNF immunoreactivity were assessed in all groups. In addition, levels of NeuN, CB1 receptor, and BDNF expression were assessed and cortical primary neurons from rats were treated with Cannabis smoke extract (CSE) for assessment of cell viability. We found that male foetuses from the CS exposed group had decreased brain volume, whereas mice at PND 60 from the exposed group presented with increased brain volume. Olfactory bulb and diencephalon volume were found lower in foetuses exposed to CS. Mice at PND 60 from the exposed group had a smaller volume in the thalamus and hypothalamus while the cerebellum presented with a greater volume. Also, there was an increase in cortical BDNF immunoreactivity in CS exposed mice at PND 60. Protein expression analysis showed an increase in pro-BDNF in foetus brains exposed to CS. Mice at PND 60 presented an increase in mature BDNF in the prefrontal cortex (PFC) in the exposed group and a higher CB1 receptor expression in the PFC. Moreover, hippocampal NeuN expression was higher in adult animals from the exposed group. Lastly, treatment of cortical primary neurons with doses of CSE resulted in decreased cell viability. These findings highlight the potential negative neurodevelopmental outcomes induced by gestational CS exposure.

Rapid treatments for depression: Endocannabinoid system as a therapeutic target

Current first-line treatments for major depressive disorder (MDD), i.e., antidepressant drugs and psychotherapy, show delayed onset of therapeutic effect as late as 2-3 weeks or more. In the clinic, the speed of beginning of the actions of antidepressant drugs or other interventions is vital for many reasons. Late-onset means that depression, its related disability, and the potential danger of suicide remain a threat for some patients. There are some rapid-acting antidepressant interventions, such as sleep deprivation, ketamine, acute exercise, which induce a significant response, ranging from a few hours to maximally one week, and most of them share a common characteristic that is the activation of the endocannabinoid (eCB) system. Activation of this system, i.e., augmentation of eCB signaling, appears to have anti-depressant-like actions. This article puts the idea forward that the activation of eCB signaling represents a critical mechanism of rapid-acting therapeutic interventions in MDD, and this system might contribute to the development of novel rapid-acting treatments for MDD.

Obesity-induced astrocyte dysfunction impairs heterosynaptic plasticity in the orbitofrontal cortex

Overconsumption of highly palatable, energy-dense food is considered a key driver of the obesity pandemic. The orbitofrontal cortex (OFC) is critical for reward valuation of gustatory signals, yet how the OFC adapts to obesogenic diets is poorly understood. Here, we show that extended access to a cafeteria diet impairs astrocyte glutamate clearance, which leads to a heterosynaptic depression of GABA transmission onto pyramidal neurons of the OFC. This decrease in GABA tone is due to an increase in extrasynaptic glutamate, which acts via metabotropic glutamate receptors to liberate endocannabinoids. This impairs the induction of endocannabinoid-mediated long-term plasticity. The nutritional supplement, N-acetylcysteine rescues this cascade of synaptic impairments by restoring astrocytic glutamate transport. Together, our findings indicate that obesity targets astrocytes to disrupt the delicate balance between excitatory and inhibitory transmission in the OFC.

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).

SNP-SNP interactions in the BDNF, COMT, CBR1 and CCK genes, associated with post-traumatic stress disorder in urban residents of Itagüí, Colombia

Introducción. Los polimorfismos de un solo nucleótido (SNP, por su sigla en inglés) en los genes BDNF, COMT, CBR1 y CCK han sido asociados con el proceso de extinción del miedo en humanos. Dado que la extinción del miedo es clave para la recuperación del trauma psicológico, es posible que estos genes modulen el riesgo de desarrollar trastorno de estrés postraumático (TEPT).Objetivo. Evaluar las asociaciones unilocus y multilocus entre los SNP en los genes BDNF, COMT, CBR1 y CCK y el riesgo de desarrollar TEPT.Materiales y métodos. 129 habitantes del municipio de Itagüí, Colombia, que habían experimentado trauma psicológico al menos una vez, fueron genotipificados para estos polimorfismos (38 casos de TEPT y 91 controles). Se realizaron pruebas de asociación unilocus y multilocus por regresión logística para SNP únicos y las combinaciones genotípicas SNP-SNP existentes.Resultados. No se encontraron asociaciones unilocus, pero se observaron interacciones entre BDNF y CBR1, y CCK y COMT. De estas interacciones, las combinaciones genotípicas que se comportaron como factores de riesgo fueron AG-AA (OR=13.52, p<0.05) de BDNF-CBR1 y TC-AA (OR=13.70, p<0.05) de CCK-COMT.Conclusiones: Los dos pares de polimorfismos en interacción encontrados en el presente estudio podrían actuar de forma aditiva y generar un mayor riesgo de desarrollar TEPT después de sufrir trauma psicológico. Quienes portan un solo alelo tienen un menor riesgo de desarrollar el trastorno que quienes portan dos alelos en genes que interactúan entre sí.

The physiology of regulated BDNF release

The neurotrophic factor BDNF is an important regulator for the development of brain circuits, for synaptic and neuronal network plasticity, as well as for neuroregeneration and neuroprotection. Up- and downregulations of BDNF levels in human blood and tissue are associated with, e.g., neurodegenerative, neurological, or even cardiovascular diseases. The changes in BDNF concentration are caused by altered dynamics in BDNF expression and release. To understand the relevance of major variations of BDNF levels, detailed knowledge regarding physiological and pathophysiological stimuli affecting intra- and extracellular BDNF concentration is important. Most work addressing the molecular and cellular regulation of BDNF expression and release have been performed in neuronal preparations. Therefore, this review will summarize the stimuli inducing release of BDNF, as well as molecular mechanisms regulating the efficacy of BDNF release, with a focus on cells originating from the brain. Further, we will discuss the current knowledge about the distinct stimuli eliciting regulated release of BDNF under physiological conditions.

Activation of tyrosine phosphatase PTP1B in pyramidal neurons impairs endocannabinoid signaling by tyrosine receptor kinase trkB and causes schizophrenia-like behaviors in mice

Schizophrenia is a debilitating disorder affecting young adults displaying symptoms of cognitive impairment, anxiety, and early social isolation prior to episodes of auditory hallucinations. Cannabis use has been tied to schizophrenia-like symptoms, indicating that dysregulated endogenous cannabinoid signaling may be causally linked to schizophrenia. Previously, we reported that glutamatergic neuron-selective ablation of Lmo4, an endogenous inhibitor of the tyrosine phosphatase PTP1B, impairs endocannabinoid (eCB) production from the metabotropic glutamate receptor mGluR5. These Lmo4-deficient mice display anxiety-like behaviors that are alleviated by local shRNA knockdown or pharmacological inhibition of PTP1B that restores mGluR5-dependent eCB production in the amygdala. Here, we report that these Lmo4-deficient mice also display schizophrenia-like behaviors: impaired working memory assessed in the Y maze and defective sensory gating by prepulse inhibition of the acoustic startle response. Modulation of inhibitory inputs onto layer 2/3 pyramidal neurons of the prefrontal cortex relies on eCB signaling from the brain-derived neurotrophic factor receptor trkB, rather than mGluR5, and this mechanism was defective in Lmo4-deficient mice. Genetic ablation of PTP1B in the glutamatergic neurons lacking Lmo4 restored tyrosine phosphorylation of trkB, trkB-mediated eCB signaling, and ameliorated schizophrenia-like behaviors. Pharmacological inhibition of PTP1B with trodusquemine also restored trkB phosphorylation and improved schizophrenia-like behaviors by restoring eCB signaling, since the CB1 receptor antagonist 1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-1-piperidinyl-1H-pyrazole-3-carboxamide blocked this effect. Thus, activation of PTP1B in pyramidal neurons contributes to schizophrenia-like behaviors in Lmo4-deficient mice and genetic or pharmacological intervention targeting PTP1B ameliorates schizophrenia-related deficits.

Lights on Endocannabinoid-Mediated Synaptic Potentiation

The endocannabinoid (eCB) system is a lipid-based neurotransmitter complex that plays crucial roles in the neural control of learning and memory. The current model of eCB-mediated retrograde signaling is that eCBs released from postsynaptic elements travel retrogradely to presynaptic axon terminals, where they activate cannabinoid type-1 receptors (CB₁Rs) and ultimately decrease neurotransmitter release on a short- or long-term scale. An increasing body of evidence has enlarged this view and shows that eCBs, besides depressing synaptic transmission, are also able to increase neurotransmitter release at multiple synapses of the brain. This indicates that eCBs act as bidirectional regulators of synaptic transmission and plasticity. Recently, studies unveiled links between the expression of eCB-mediated long-term potentiation (eCB-LTP) and learning, and between its dysregulation and several pathologies. In this review article, we first distinguish the various forms of eCB-LTP based on their mechanisms, resulting from homosynaptically or heterosynaptically-mediated processes. Next, we consider the neuromodulation of eCB-LTP, its behavioral impact on learning and memory, and finally, eCB-LTP disruptions in various pathologies and its potential as a therapeutic target in disorders such as stress coping, addiction, Alzheimer’s and Parkinson’s disease, and pain. Cannabis is gaining popularity as a recreational substance as well as a medicine, and multiple eCB-based drugs are under development. In this context, it is critical to understand eCB-mediated signaling in its multi-faceted complexity. Indeed, the bidirectional nature of eCB-based neuromodulation may offer an important key to interpret the functions of the eCB system and how it is impacted by cannabis and other drugs.

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.

Resveratrol Regulates BDNF, trkB, PSA-NCAM, and Arc Expression in the Rat Cerebral Cortex after Bilateral Common Carotid Artery Occlusion and Reperfusion

The polyphenol resveratrol (RVT) may drive protective mechanisms of cerebral homeostasis during the hypoperfusion/reperfusion triggered by the transient bilateral common carotid artery occlusion followed by reperfusion (BCCAO/R). This immunochemical study investigates if a single dose of RVT modulates the plasticity-related markers brain-derived neurotrophic factor (BDNF), the tyrosine kinase trkB receptor, Polysialylated-Neural Cell Adhesion Molecule (PSA-NCAM), and Activity-regulated cytoskeleton-associated (Arc) protein in the brain cortex after BCCAO/R. Frontal and temporal-occipital cortical regions were examined in male Wistar rats randomly subdivided in two groups, sham-operated and submitted to BCCAO/R. Six hours prior to surgery, half the rats were gavage fed a dose of RVT (180 mg·kg⁻¹ in 300 µL of sunflower oil as the vehicle), while the second half was given the vehicle alone. In the frontal cortex of BCCAO/R vehicle-treated rats, BDNF and PSA-NCAM decreased, while trkB increased. RVT pre-treatment elicited an increment of all examined markers in both sham- and BCCAO/R rats. No variations occurred in the temporal-occipital cortex. The results highlight a role for RVT in modulating neuronal plasticity through the BDNF-trkB system and upregulation of PSA-NCAM and Arc, which may provide both trophic and structural local support in the dynamic changes occurring during the BCCAO/R, and further suggest that dietary supplements such as RVT are effective in preserving the tissue potential to engage plasticity-related events and control the functional response to the hypoperfusion/reperfusion challenge.

BDNF Controls Bidirectional Endocannabinoid Plasticity at Corticostriatal Synapses

The dorsal striatum exhibits bidirectional corticostriatal synaptic plasticity, NMDAR and endocannabinoids (eCB) mediated, necessary for the encoding of procedural learning. Therefore, characterizing factors controlling corticostriatal plasticity is of crucial importance. Brain-derived neurotrophic factor (BDNF) and its receptor, the tropomyosine receptor kinase-B (TrkB), shape striatal functions, and their dysfunction deeply affects basal ganglia. BDNF/TrkB signaling controls NMDAR plasticity in various brain structures including the striatum. However, despite cross-talk between BDNF and eCBs, the role of BDNF in eCB plasticity remains unknown. Here, we show that BDNF/TrkB signaling promotes eCB-plasticity (LTD and LTP) induced by rate-based (low-frequency stimulation) or spike-timing–based (spike-timing–dependent plasticity, STDP) paradigm in striatum. We show that TrkB activation is required for the expression and the scaling of both eCB-LTD and eCB-LTP. Using 2-photon imaging of dendritic spines combined with patch-clamp recordings, we show that TrkB activation prolongs intracellular calcium transients, thus increasing eCB synthesis and release. We provide a mathematical model for the dynamics of the signaling pathways involved in corticostriatal plasticity. Finally, we show that TrkB activation enlarges the domain of expression of eCB-STDP. Our results reveal a novel role for BDNF/TrkB signaling in governing eCB-plasticity expression in striatum and thus the engram of procedural learning.

Dual mechanism of TRKB activation by anandamide through CB1 and TRPV1 receptors

BACKGROUND

Administration of anandamide (AEA) or 2-arachidonoylglycerol (2AG) induces CB1 coupling and activation of TRKB receptors, regulating the neuronal migration and maturation in the developing cortex. However, at higher concentrations AEA also engages vanilloid receptor TRPV1, usually with opposed consequences on behavior.

METHODS AND RESULTS

Using primary cell cultures from the cortex of rat embryos (E18) we determined the effects of AEA on phosphorylated TRKB (pTRK). We observed that AEA (at 100 and 200 nM) induced a significant increase in pTRK levels. Such effect of AEA at 100 nM was blocked by pretreatment with the CB1 antagonist AM251 (200 nM) and, at the higher concentration of 200 nM by the TRPV1 antagonist capsazepine (200 nM), but mildly attenuated by AM251. Interestingly, the effect of AEA or capsaicin (a TRPV1 agonist, also at 200 nM) on pTRK was blocked by TRKB.Fc (a soluble form of TRKB able to bind BDNF) or capsazepine, suggesting a mechanism dependent on BDNF release. Using the marble-burying test (MBT) in mice, we observed that the local administration of ACEA (a CB1 agonist) into the prelimbic region of prefrontal cortex (PL-PFC) was sufficient to reduce the burying behavior, while capsaicin or BDNF exerted the opposite effect, increasing the number of buried marbles. In addition, both ACEA and capsaicin effects were blocked by previous administration of k252a (an antagonist of TRK receptors) into PL-PFC. The effect of systemically injected CB1 agonist WIN55,212-2 was blocked by previous administration of k252a. We also observed a partial colocalization of CB1/TRPV1/TRKB in the PL-PFC, and the localization of TRPV1 in CaMK2+ cells.

CONCLUSION

Taken together, our data indicate that anandamide engages a coordinated activation of TRKB, via CB1 and TRPV1. Thus, acting upon CB1 and TRPV1, AEA could regulate the TRKB-dependent plasticity in both pre- and postsynaptic compartments.

LTD at mossy fiber synapses onto stratum lucidum interneurons requires TrkB and retrograde endocannabinoid signaling

Hippocampal mossy fiber axons simultaneously activate CA3 pyramidal cells and stratum lucidum interneurons (SLINs), the latter providing feedforward inhibition to control CA3 pyramidal cell excitability. Filopodial extensions of giant boutons of mossy fibers provide excitatory synaptic input to the SLIN. These filopodia undergo extraordinary structural plasticity causally linked to execution of memory tasks, leading us to seek the mechanisms by which activity regulates these synapses. High-frequency stimulation of the mossy fibers induces long-term depression (LTD) of their calcium-permeable AMPA receptor synapses with SLINs; previous work localized the site of induction to be postsynaptic and the site of expression to be presynaptic. Yet, the underlying signaling events and the identity of the retrograde signal are incompletely understood. We used whole cell recordings of SLINs in hippocampal slices from wild-type and mutant mice to explore the mechanisms. Genetic and pharmacologic perturbations revealed a requirement for both the receptor tyrosine kinase TrkB and its agonist, brain-derived neurotrophic factor (BDNF), for induction of LTD. Inclusion of inhibitors of Trk receptor kinase and PLC in the patch pipette prevented LTD. Endocannabinoid receptor antagonists and genetic deletion of the CB1 receptor prevented LTD. We propose a model whereby release of BDNF from mossy fiber filopodia activates TrkB and PLCγ1 signaling postsynaptically within SLINs, triggering synthesis and release of an endocannabinoid that serves as a retrograde signal, culminating in reduced glutamate release. Insights into the signaling pathways by which activity modifies function of these synapses will facilitate an understanding of their contribution to the local circuit and behavioral consequences of hippocampal granule cell activity. NEW & NOTEWORTHY We investigated signaling mechanisms underlying plasticity of the hippocampal mossy fiber filopodial synapse with interneurons in stratum lucidum. High-frequency stimulation of the mossy fibers induces long-term depression of this synapse. Our findings are consistent with a model in which brain-derived neurotrophic factor released from filopodia activates TrkB of a stratum lucidum interneuron; the ensuing activation of PLCγ1 induces synthesis of an endocannabinoid, which provides a retrograde signal leading to reduced release of glutamate presynaptically.

Brain-Derived Neurotrophic Factor (BDNF) Role in Cannabinoid-Mediated Neurogenesis

The adult mammalian brain can produce new neurons in a process called adult neurogenesis, which occurs mainly in the subventricular zone (SVZ) and in the hippocampal dentate gyrus (DG). Brain-derived neurotrophic factor (BDNF) signaling and cannabinoid type 1 and 2 receptors (CB1R and CB2R) have been shown to independently modulate neurogenesis, but how they may interact is unknown. We now used SVZ and DG neurosphere cultures from early (P1-3) postnatal rats to study the CB1R and CB2R crosstalk with BDNF in modulating neurogenesis. BDNF promoted an increase in SVZ and DG stemness and cell proliferation, an effect blocked by a CB2R selective antagonist. CB2R selective activation promoted an increase in DG multipotency, which was inhibited by the presence of a BDNF scavenger. CB1R activation induced an increase in SVZ and DG cell proliferation, being both effects dependent on BDNF. Furthermore, SVZ and DG neuronal differentiation was facilitated by CB1R and/or CB2R activation and this effect was blocked by sequestering endogenous BDNF. Conversely, BDNF promoted neuronal differentiation, an effect abrogated in SVZ cells by CB1R or CB2R blockade while in DG cells was inhibited by CB2R blockade. We conclude that endogenous BDNF is crucial for the cannabinoid-mediated effects on SVZ and DG neurogenesis. On the other hand, cannabinoid receptor signaling is also determinant for BDNF actions upon neurogenesis. These findings provide support for an interaction between BDNF and endocannabinoid signaling to control neurogenesis at distinct levels, further contributing to highlight novel mechanisms in the emerging field of brain repair.

Endogenous cannabinoids mediate the effect of BDNF at CA1 inhibitory synapses in the hippocampus

Brain-derived neurotrophic factor (BDNF), traditionally known for promoting neuronal growth and development, is also a modulator of synaptic transmission. In addition to the well-characterized effects at excitatory synapses, BDNF has been shown to acutely suppress inhibitory neurotransmission; however, the underlying mechanisms are unclear. We have previously shown that at inhibitory synapses in layer 2/3 of the somatosensory cortex, BDNF induces the mobilization of endogenous cannabinoids (eCBs) that act retrogradely to suppress GABA release. Here, we hypothesized that in the hippocampus, BDNF acts similarly via eCB signaling to suppress GABAergic transmission. We found that the acute application of BDNF reduced the spontaneous inhibitory postsynaptic currents (sIPSCs) via postsynaptic TrkB receptor activation. The suppressive effects of BDNF required eCB signaling, as this effect on sIPSCs was prevented by a CB1 receptor antagonist. Further, blocking the postsynaptic eCB release prevented the effect of BDNF, whereas eCB reuptake inhibition enhanced the effect of BDNF. These results suggest that BDNF triggers the postsynaptic release of eCBs. To identify the specific eCB release by BDNF, we tested the effects of disrupting the synthesis or degradation of 2-arachidonoylcglycerol (2-AG). Blocking 2-AG synthesis prevented the effect of BDNF and blocking 2-AG degradation enhanced the effect of BDNF. However, there was no change in the effect of BDNF when anandamide degradation was blocked. Collectively, these results suggest that in the hippocampus, BDNF-TrkB signaling induces the postsynaptic release of the endogenous cannabinoid 2-AG, which acts retrogradely on the presynaptic CB1 receptors to suppress GABA release.

2-Arachidonoylglycerol: A signaling lipid with manifold actions in the brain

2-Arachidonoylglycerol (2-AG) is a signaling lipid in the central nervous system that is a key regulator of neurotransmitter release. 2-AG is an endocannabinoid that activates the cannabinoid CB₁ receptor. It is involved in a wide array of (patho)physiological functions, such as emotion, cognition, energy balance, pain sensation and neuroinflammation. In this review, we describe the biosynthetic and metabolic pathways of 2-AG and how chemical and genetic perturbation of these pathways has led to insight in the biological role of this signaling lipid. Finally, we discuss the potential therapeutic benefits of modulating 2-AG levels in the brain.

Molecular mechanisms of experience-dependent maturation in cortical GABAergic inhibition

Critical periods (CP) in early post-natal life are periods of plasticity during which the neuronal circuitry is most receptive to environmental stimuli. These early experiences translate to a more permanent and sophisticated neuronal connection in the adult brain systems. Multiple studies have pointed to the development of inhibitory circuitry as one of the central factors for the onset of critical periods. We discuss several molecular mechanisms regulating inhibitory circuit maturation and CP, from gene transcription level to protein signaling level. Also, beyond the level of gene sequences, we briefly consider recent information on dynamic epigenetic regulation of gene expression through histone methylation and acetylation and their implication on timed development of the inhibitory circuitry for the onset of CP.

Perspectives on the Role of Endocannabinoids in Autism Spectrum Disorders

Autism spectrum disorders (ASDs) are diagnosed on the basis of three behavioral features, namely, (1) deficits in social communication, (2) absence or delay in language and (3) stereotypy. The consensus regarding the neurological pathogenesis of ASDs is aberrant synaptogenesis and synapse function. Further, it is now widely accepted that ASD is neurodevelopmental in nature, placing emphasis on derangements occurring at the level of intra- and intercellular signaling during corticogenesis. At present, there is an ever-growing list of mutations in putative susceptibility genes in affected individuals, preventing effective transformation of knowledge gathered from basic science research to the clinic. In response, the focus of ASD biology has shifted toward the identification of cellular signaling pathways that are common to various ASD-related mutations in hopes that these shared pathways may serve as more promising treatment targets than targeting individual genes or proteins. To this end, the endogenous cannabinoid (endocannabinoid, eCB) system has recently emerged as a promising therapeutic target in the field of ASD research. The eCB system is altered in several neurological disorders, but the role of these bioactive lipids in ASD etiology remains poorly understood. In this perspective, we review current evidence linking eCB signaling to ASDs and put forth the notion that continued focus on eCBs in autism research may provide valuable insight into pathophysiology and treatment strategies. In addition to its role in modulating transmitter release at mature synapses, the eCB signaling system plays important roles in many aspects of cortical development, and disruption of these effects of eCBs may also be related to ASD pathophysiology.

Neurotrophin receptors in the pathogenesis, diagnosis and therapy of neurodegenerative diseases

In the last few years, exciting properties have emerged regarding the activation, signaling, mechanisms of action, and therapeutic targeting of the two types of neurotrophin receptors: the p75^NTR with its intracellular and extracellular peptides, the Trks, their precursors and their complexes. This review summarizes these new developments, with particular focus on neurodegenerative diseases. Based on the evolving knowledge, innovative concepts have been formulated regarding the pathogenesis of these diseases, especially the Alzheimer's and two other, the Parkinson's and Huntington's diseases. The medical progresses include original procedures of diagnosis, started from studies in mice and now investigated for human application, based on innovative classes of receptor agonists and blockers. In parallel, comprehensive studies have been and are being carried out for the development of drugs. The relevance of these studies is based on the limitations of the therapies employed until recently, especially for the treatment of Alzheimer's patients. Starting from well known drugs, previously employed for non-neurodegenerative diseases, the ongoing progress has lead to the development of small molecules that cross rapidly the blood-brain barrier. Among these molecules the most promising are specific blockers of the p75^NTR receptor. Additional drugs, that activate Trk receptors, were shown effective against synaptic loss and memory deficits. In the near future such approaches, coordinated with treatments with monoclonal antibodies and with developments in the microRNA field, are expected to improve the therapy of neurodegenerative diseases, and may be relevant also for other human disease conditions.

BDNF-induced endocannabinoid release modulates neocortical glutamatergic neurotransmission

Endocannabinoids (eCBs) and neurotrophins, particularly brain-derived neurotrophic factor (BDNF), are potent neuromodulators found throughout the mammalian neocortex. Both eCBs and BDNF play critical roles in many behavioral and neurophysiological processes and are targets for the development of novel therapeutics. The effects of eCBs and BDNF are primarily mediated by the type 1 cannabinoid (CB1) receptor and the trkB tyrosine kinase receptor, respectively. Our laboratory and others have previously established that BDNF potentiates excitatory transmission by enhancing presynaptic glutamate release and modulating NMDA receptors. In contrast, we have shown that BDNF attenuates inhibitory transmission by inducing postsynaptic release of eCBs that act retrogradely to suppress GABA release in layer 2/3 of somatosensory cortex. Here, we hypothesized that BDNF also induces release of eCBs at excitatory synapses, which could have a mitigating or opposing effect on the direct presynaptic effects of BDNF. We found the highest levels of expression of CB1 and trkB and receptors in layers 2/3 and 5. Surprisingly, BDNF did not increase the frequency of spontaneous miniature excitatory postsynaptic currents (mEPSCs) onto layer 5 pyramidal neurons in somatosensory cortex, in contrast to its effects in the hippocampus and visual cortex. However, the effect of BDNF on mEPSC frequency in somatosensory cortex was unmasked by blocking CB1 receptors or disrupting eCB release. Thus, BDNF-trKB signaling regulates glutamate release in the somatosensory cortex via opposing effects, a direct presynaptic enhancement of release probability, and simultaneous postsynaptically-induced eCB release that decreases release probability via presynaptic CB1 receptors.

Regulation of fear extinction by long-term depression: The roles of endocannabinoids and brain derived neurotrophic factor

The extinction of a conditioned fear response is of great interest in the search for a means of ameliorating adverse neurobiological changes resulting from stress. The discovery that endocannibinoid (EC) levels are inversely related to the extent of such stress, and that the amygdala is a primary site mediating stress, suggests that ECs in this brain region might play a major role in extinction. Supporting this are the observations that the basolateral complex of the amygdala shows an increase in ECs only during extinction and that early clinical trials indicate that cannabinoid-like agents, when taken orally by patients suffering from post traumatic stress disorder (PTSD), reduce insomnia and nightmares. In order to optimize the potential of these agents to ameliorate symptoms of PTSD four important questions need to be answered: first, what is the identity of the cells that release ECs in the amygdala during extinction; second, what are their sites of action; third, what roles do the ECs play in the alleviation of long- depression (LTD), a process central to extinction; and finally, to what extent does brain derived neurotrophic factor (BDNF) facilitate the release of ECs? A review of the relevant literature is presented in an attempt to answer these questions. It is suggested that the principal cell involved in EC synthesis and release during extinction is the so-called excitatory extinction neuron in the basal nucleus of the amygdala. Furthermore that the main site of action of the ECs is the adjacent calcitonin gene-related peptide inhibitory interneurons, whose normal role of blocking the excitatory neurons is greatly diminished. The molecular pathways leading (during extinction trials) to the synthesis and release of ECs from synaptic spines of extinction neurons, that is potentiated by BDNF, are also delineated in this review. Finally, consideration is given to how the autocrine action of BDNF, linked to the release of ECs, can lead to the sustained release of these, so maintaining extinction over long times.