Endocannabinoid-mediated short-term synaptic plasticity: depolarization-induced suppression of inhibition (DSI) and depolarization-induced suppression of excitation (DSE)

Cannabinoids Modulate Light Signaling in ON-Sustained Retinal Ganglion Cells of the Mouse

The sole output of the retina to the brain is a signal that results from the integration of excitatory and inhibitory synaptic inputs at the level of retinal ganglion cells (RGCs). Endogenous cannabinoids (eCBs) are found throughout the central nervous system where they modulate synaptic excitability. Cannabinoid receptors and their ligands have been localized to most retinal neurons in mammals, yet their impact on retinal processing is not well known. Here, we set out to investigate the role of the cannabinoid system in retinal signaling using electrophysiological recordings from ON-sustained (ON-S) RGCs that displayed morphological and physiological signatures of ON alpha RGCs in dark adapted mouse retina. We studied the effect of the cannabinoid agonist WIN55212-2 and the inverse agonist AM251 on the spatial tuning of ON-S RGCs. WIN55212-2 significantly reduced their spontaneous spiking activity and responses to optimal spot size as well as altered their spatial tuning by reducing light driven excitatory and inhibitory inputs to RGCs. AM251 produced the opposite effect, increasing spontaneous spiking activity and peak response as well as increasing inhibitory and excitatory inputs. In addition, AM251 sharpened the spatial tuning of ON-S RGCs by increasing the inhibitory effect of the surround. These results demonstrate the presence of a functional cannabinergic system in the retina as well as sensitivity of ON-RGCs to cannabinoids. These results reveal a neuromodulatory system that can regulate the sensitivity and excitability of retinal synapses in a dynamic, activity dependent manner and that endocannabinoids may play a significant role in retinal processing.

Prenatal cannabinoid exposure and altered neurotransmission

Marijuana is one of the most commonly used illicit drugs worldwide. In addition, use of synthetic cannabinoids is increasing, especially among adolescents and young adults. Although human studies have shown that the use of marijuana during pregnancy leads to adverse behavioral effects, such as deficiencies in attention and executive function in affected offspring, the rate of marijuana use among pregnant women is steadily increasing. Various aspects of human behavior including emotion, learning, and memory are dependent on complex interactions between multiple neurotransmitter systems that are especially vulnerable to alterations during the developmental period. Thus, exploration of neurotransmitter changes in response to prenatal cannabinoid exposure is crucial to develop an understanding of how homeostatic imbalance and various long-term neurobehavioral deficits manifest following the abuse of marijuana or other synthetic cannabinoids during pregnancy. Current literature confirms that vast alterations to neurotransmitter systems are present following prenatal cannabinoid exposure, and many of these alterations within the brain are region specific, time-dependent, and sexually dimorphic. In this review, we aim to provide a summary of observed changes to various neurotransmitter systems following cannabinoid exposure during pregnancy and to draw possible correlations to reported behavioral alterations in affected offspring.

ABHD6: Its Place in Endocannabinoid Signaling and Beyond

The endocannabinoid (eCB) signaling system modulates neurotransmission and inflammation, among other physiological functions. Its newest member, α/β-hydrolase domain-containing 6 (ABHD6), has emerged as a promising therapeutic target to treat several devastating diseases, including epilepsy. Here, we review the molecular mechanisms that mediate and control eCB signaling and, within it, the specific role of ABHD6. We also discuss how ABHD6 controls the abundance of additional lipids and the trafficking of ionotropic receptors to plasma membranes. We finish with several unexplored questions regarding this novel enzyme. Our current understanding of the molecular mechanism and biological function of ABHD6 provides a strong foundation for the development of small-molecule therapeutics to treat devastating diseases.

Cortical GABAergic Dysfunction in Stress and Depression: New Insights for Therapeutic Interventions

Major depressive disorder (MDD) is a debilitating illness characterized by neuroanatomical and functional alterations in limbic structures, notably the prefrontal cortex (PFC), that can be precipitated by exposure to chronic stress. For decades, the monoaminergic deficit hypothesis of depression provided the conceptual framework to understand the pathophysiology of MDD. However, accumulating evidence suggests that MDD and chronic stress are associated with an imbalance of excitation-inhibition (E:I) within the PFC, generated by a deficit of inhibitory synaptic transmission onto principal glutamatergic neurons. MDD patients and chronically stressed animals show a reduction in GABA and GAD67 levels in the brain, decreased expression of GABAergic interneuron markers, and alterations in GABA_A and GABA_B receptor levels. Moreover, genetically modified animals with deletion of specific GABA receptors subunits or interneuron function show depressive-like behaviors. Here, we provide further evidence supporting the role of cortical GABAergic interneurons, mainly somatostatin- and parvalbumin-expressing cells, required for the optimal E:I balance in the PFC and discuss how the malfunction of these cells can result in depression-related behaviors. Finally, considering the relatively low efficacy of current available medications, we review new fast-acting pharmacological approaches that target the GABAergic system to treat MDD. We conclude that deficits in cortical inhibitory neurotransmission and interneuron function resulting from chronic stress exposure can compromise the integrity of neurocircuits and result in the development of MDD and other stress-related disorders. Drugs that can establish a new E:I balance in the PFC by targeting the glutamatergic and GABAergic systems show promising as fast-acting antidepressants and represent breakthrough strategies for the treatment of depression.

Cortical GABAergic Dysfunction in Stress and Depression: New Insights for Therapeutic Interventions

Major depressive disorder (MDD) is a debilitating illness characterized by neuroanatomical and functional alterations in limbic structures, notably the prefrontal cortex (PFC), that can be precipitated by exposure to chronic stress. For decades, the monoaminergic deficit hypothesis of depression provided the conceptual framework to understand the pathophysiology of MDD. However, accumulating evidence suggests that MDD and chronic stress are associated with an imbalance of excitation-inhibition (E:I) within the PFC, generated by a deficit of inhibitory synaptic transmission onto principal glutamatergic neurons. MDD patients and chronically stressed animals show a reduction in GABA and GAD67 levels in the brain, decreased expression of GABAergic interneuron markers, and alterations in GABA_A and GABA_B receptor levels. Moreover, genetically modified animals with deletion of specific GABA receptors subunits or interneuron function show depressive-like behaviors. Here, we provide further evidence supporting the role of cortical GABAergic interneurons, mainly somatostatin- and parvalbumin-expressing cells, required for the optimal E:I balance in the PFC and discuss how the malfunction of these cells can result in depression-related behaviors. Finally, considering the relatively low efficacy of current available medications, we review new fast-acting pharmacological approaches that target the GABAergic system to treat MDD. We conclude that deficits in cortical inhibitory neurotransmission and interneuron function resulting from chronic stress exposure can compromise the integrity of neurocircuits and result in the development of MDD and other stress-related disorders. Drugs that can establish a new E:I balance in the PFC by targeting the glutamatergic and GABAergic systems show promising as fast-acting antidepressants and represent breakthrough strategies for the treatment of depression.

Cannabinoid signalling in embryonic and adult neurogenesis: possible implications for psychiatric and neurological disorders

Cannabinoid signalling modulates several aspects of brain function, including the generation and survival of neurons during embryonic and adult periods. The present review intended to summarise evidence supporting a role for the endocannabinoid system on the control of neurogenesis and neurogenesis-dependent functions. Studies reporting participation of cannabinoids on the regulation of any step of neurogenesis and the effects of cannabinoid compounds on animal models possessing neurogenesis-dependent features were selected from Medline. Qualitative evaluation of the selected studies indicated that activation of cannabinoid receptors may change neurogenesis in embryonic or adult nervous systems alongside rescue of phenotypes in animal models of different psychiatric and neurological disorders. The text offers an overview on the effects of cannabinoids on central nervous system development and the possible links with psychiatric and neurological disorders such as anxiety, depression, schizophrenia, brain ischaemia/stroke and Alzheimer's disease. An understanding of the mechanisms by which cannabinoid signalling influences developmental and adult neurogenesis will help foster the development of new therapeutic strategies for neurodevelopmental, psychiatric and neurological disorders.

Psychopharmacology of chronic pain

Chronic pain is a frequent condition that affects an estimated 20% of people worldwide, accounting for 15%-20% of doctors' appointments (Treede et al., 2015). It lacks the acute warning function of physiologic nociception, and instead involves the activation of multiple neurophysiologic mechanisms in the somatosensory system, a complex neuronal network under the control of powerful autoregulatory loops and able to undergo rapid neuroplastic alteration (Verdu et al., 2008). There is a growing body of research suggesting that some such pathways are shared by major psychologic disorders such as depression and anxiety, opening new avenues in co-treatment strategies. In particular, besides anticonvulsants, which are today used as analgesics, other psychopharmaceuticals, such as the tricyclic antidepressants, are displaying efficacy in the treatment of neuropathic and nociceptive chronic pain. The state of the art regarding the mechanisms of nociception and the pharmacology of both the neurotransmitters involved and the wide range of psychoactive compounds that may be useful in the treatment of chronic pain are discussed.

Inhibition of fatty acid amide hydrolase in the central amygdala alleviates co-morbid expression of innate anxiety and excessive alcohol intake

Fatty acid amide hydrolase (FAAH) is an enzyme that prominently degrades the major endocannabinoid N-arachidonoylethanolamine (anandamide). Inhibition of this enzyme leads to increased anandamide levels in brain regions that modulate stress and anxiety. Recently, we found that genetically selected Marchigian Sardinian alcohol-preferring (msP) rats display hyperactive FAAH in amygdalar regions that was associated with increased stress sensitivity and a hyper-anxious phenotype. Our previous work has also demonstrated that msPs display an innate preference for and excessive consumption of alcohol, potentially reflecting a form of self-medication to gain relief from hyper-anxious states. Here, we expand on our previous work by microinjecting the selective FAAH inhibitor URB597 (vehicle, 0.03, 0.1 and 1.0 μg per rat) into the central amygdala (CeA) and basolateral amygdala in msP versus non-selected Wistar rats to evaluate the effects of localized FAAH inhibition on operant alcohol self-administration and restraint-induced anxiety using the elevated plus maze. Intra-CeA URB597 significantly reduced alcohol self-administration in msP but not in Wistar rats. Intra-basolateral amygdala URB597 also attenuated alcohol drinking in msPs, although the effect was less pronounced relative to CeA treatment. In contrast, control experiments administering URB597 into the ventral tegmental area produced no genotypic differences in drinking. We also found that URB597 treatment in the CeA significantly reduced the anxiogenic effects of restraint stress in msPs, although no effects were detected in Wistars. Dysregulation of FAAH regulated systems in the major output region of the amygdala may drive the propensity for co-morbid expression of anxiety and excessive alcohol use.

Functional Relevance of Endocannabinoid-Dependent Synaptic Plasticity in the Central Nervous System

The endocannabinoid (eCB) signaling system plays a key role in short-term and long-term synaptic plasticity in brain regions involved in various neural functions ranging from action selection to appetite control. This review will explore the role of eCBs in shaping neural circuit function to regulate behaviors. In particular, we will discuss the behavioral consequences of eCB mediated long-term synaptic plasticity in different brain regions. This review brings together evidence from in vitro and ex vivo studies and points out the need for more in vivo studies.

Cannabigerol Action at Cannabinoid CB1 and CB2 Receptors and at CB1-CB2 Heteroreceptor Complexes

Cannabigerol (CBG) is one of the major phytocannabinoids present in Cannabis sativa L. that is attracting pharmacological interest because it is non-psychotropic and is abundant in some industrial hemp varieties. The aim of this work was to investigate in parallel the binding properties of CBG to cannabinoid CB₁ (CB₁R) and CB₂ (CB₂R) receptors and the effects of the compound on agonist activation of those receptors and of CB₁–CB₂ heteroreceptor complexes. Using [³H]-CP-55940, CBG competed with low micromolar K_i values the binding to CB₁R and CB₂R. Homogeneous binding in living cells, which is only technically possible for the CB₂R, provided a 152 nM K_i value. Also interesting, CBG competed the binding of [³H]-WIN-55,212-2 to CB₂R but not to CB₁R (K_i: 2.7 versus >30 μM). The phytocannabinoid modulated signaling mediated by receptors and receptor heteromers even at low concentrations of 0.1–1 μM. cAMP, pERK, β-arrestin recruitment and label-free assays in HEK-293T cells expressing the receptors and treated with endocannabinoids or selective agonists proved that CBG is a partial agonist of CB₂R. The action on cells expressing heteromers was similar to that obtained in cells expressing the CB₂R. The effect of CBG on CB₁R was measurable but the underlying molecular mechanisms remain uncertain. The results indicate that CBG is indeed effective as regulator of endocannabinoid signaling.

Endocannabinoid CB1 receptors are involved in antiepileptogenic effect of low frequency electrical stimulation during perforant path kindling in rats

INTRODUCTION

Administration of low-frequency electrical stimulation (LFS) at the kindling site has an antiepileptogenic effect. In the present study, we investigated the role of cannabinoid receptors type 1 (CB1) in mediating the inhibitory effects of LFS on the development of perforant path kindled seizures.

METHODS

For seizure generation, rats were kindled by electrical stimulation of perforant path in semi-rapid kindling manner (12 stimulations per day at 10 min intervals at afterdischarge threshold intensity).To determine the effect of LFS (0.1 ms pulse duration at 1 Hz, 800 pulses) on seizure generation, LFS was applied to the perforant path 5 min after the last kindling stimulation daily. AM281, a CB1 receptor antagonist, was microinjected into the lateral ventricle immediately after the last kindling stimulation (before LFS application) at the doses of 0.5 and 2 μg/μl during kindling procedure. The expression of cannabinoid receptors in the dentate gyrus was also investigated using immunohistochemistry.

RESULTS

Application of LFS had inhibitory effect on development of kindled seizures (kindling rate). Microinjection of AM281 (0.5 μg/μl) immediately after the last kindling stimulation (before LFS application) reduced the inhibitory effect of LFS on the kindling rate and suppressed the effects of LFS on potentiation (increasing the magnitude) of both population spike amplitude and population excitatory postsynaptic potential slope during kindling acquisition. AM281 pretreatment also prevented the effects of LFS on kindling-induced increase in early and late paired pulse depression. The higher dose of AM281 (2 μg/μl) failed to exert the effects observed with its lower dose (0.5 μg/μl). In addition, there was a decreased CB1 receptors immunostaining in kindled animals compared to control. However, application of LFS following kindling stimulations led to overexpression of CB1 receptors in the dentate gyrus.

CONCLUSION

Obtained results showed that activation of overexpressed cannabinoid CB1 receptors by endogenous cannabinoids may have a role in mediating the inhibitory effect of LFS on perforant path kindled seizures.

Species-specific susceptibility to cannabis-induced convulsions

Background and Purpose

Numerous claims are made for cannabis' therapeutic utility upon human seizures, but concerns persist about risks. A potential confounder is the presence of both Δ⁹‐tetrahydrocannabinol (THC), variously reported to be pro‐ and anticonvulsant, and cannabidiol (CBD), widely confirmed as anticonvulsant. Therefore, we investigated effects of prolonged exposure to different THC/CBD cannabis extracts on seizure activity and associated measures of endocannabinoid (eCB) system signalling.

Experimental Approach

Cannabis extract effects on in vivo neurological and behavioural responses, and on bioanalyte levels, were measured in rats and dogs. Extract effects on seizure activity were measured using electroencephalography telemetry in rats. eCB signalling was also investigated using radioligand binding in cannabis extract‐treated rats and treatment‐naïve rat, mouse, chicken, dog and human tissue.

Key Results

Prolonged exposure to cannabis extracts caused spontaneous, generalized seizures, subserved by epileptiform discharges in rats, but not dogs, and produced higher THC, but lower 11‐hydroxy‐THC (11‐OH‐THC) and CBD, plasma concentrations in rats versus dogs. In the same rats, prolonged exposure to cannabis also impaired cannabinoid type 1 receptor (CB₁ receptor)‐mediated signalling. Profiling CB₁ receptor expression, basal activity, extent of activation and sensitivity to THC suggested interspecies differences in eCB signalling, being more pronounced in a species that exhibited cannabis extract‐induced seizures (rat) than one that did not (dog).

Conclusions and Implications

Sustained cannabis extract treatment caused differential seizure, behavioural and bioanalyte levels between rats and dogs. Supporting radioligand binding data suggest species differences in eCB signalling. Interspecies variations may have important implications for predicting cannabis‐induced convulsions from animal models.

Linked Articles

This article is part of a themed section on 8^th European Workshop on Cannabinoid Research. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.10/issuetoc

Constitutive Increases in Amygdalar Corticotropin-Releasing Factor and Fatty Acid Amide Hydrolase Drive an Anxious Phenotype

BACKGROUND

Corticotropin-releasing factor (CRF) mediates anxiogenic responses by activating CRF type 1 (CRF1) receptors in limbic brain regions. Anxiety is further modulated by the endogenous cannabinoid (eCB) system that attenuates the synaptic effects of stress. In the amygdala, acute stress activates the enzymatic clearance of the eCB N-arachidonoylethanolamine via fatty acid amide hydrolase (FAAH), although it is unclear whether chronic dysregulation of CRF systems induces maladaptive changes in amygdalar eCB signaling. Here, we used genetically selected Marchigian Sardinian P (msP) rats carrying an innate overexpression of CRF1 receptors to study the role of constitutive upregulation in CRF systems on amygdalar eCB function and persistent anxiety-like effects.

METHODS

We applied behavioral, pharmacological, and biochemical methods to broadly characterize anxiety-like behaviors and amygdalar eCB clearance enzymes in msP versus nonselected Wistar rats. Subsequent studies examined the influence of dysregulated CRF and FAAH systems in altering excitatory transmission in the central amygdala (CeA).

RESULTS

msPs display an anxious phenotype accompanied by elevations in amygdalar FAAH activity and reduced dialysate N-arachidonoylethanolamine levels in the CeA. Elevations in CRF-CRF1 signaling dysregulate FAAH activity, and this genotypic difference is normalized with pharmacological blockade of CRF1 receptors. msPs also exhibit elevated baseline glutamatergic transmission in the CeA, and dysregulated CRF-FAAH facilitates stress-induced increases in glutamatergic activity. Treatment with an FAAH inhibitor relieves sensitized glutamatergic responses in msPs and attenuates the anxiety-like phenotype.

CONCLUSIONS

Pathological anxiety and stress hypersensitivity are driven by constitutive increases in CRF1 signaling that dysregulate N-arachidonoylethanolamine signaling mechanisms and reduce neuronal inhibitory control of CeA glutamatergic synapses.

Molecular Targets of the Phytocannabinoids: A Complex Picture

For centuries, hashish and marihuana, both derived from the Indian hemp Cannabis sativa L., have been used for their medicinal, as well as, their psychotropic effects. These effects are associated with the phytocannabinoids which are oxygen containing C₂₁ aromatic hydrocarbons found in Cannabis sativa L. To date, over 120 phytocannabinoids have been isolated from Cannabis. For many years, it was assumed that the beneficial effects of the phytocannabinoids were mediated by the cannabinoid receptors, CB₁ and CB₂. However, today we know that the picture is much more complex, with the same phytocannabinoid acting at multiple targets. This contribution focuses on the molecular pharmacology of the phytocannabinoids, including Δ⁹-THC and CBD, from the prospective of the targets at which these important compounds act.

Enhanced ability of TRPV1 channels in regulating glutamatergic transmission after repeated morphine exposure in the nucleus accumbens of rat

Glutamatergic projections to nucleus accumbens (NAc) drive drug-seeking behaviors during opioids withdrawal. Modulating glutamatergic neurotransmission provides a novel pharmacotherapeutic avenue for treatment of opioids dependence. Great deals of researches have verified that transient receptor potential vanilloid 1 (TRPV1) channels alters synaptic transmitter release and regulate neural plasticity. In the present study, whole-cell patch clamp recordings were adopted to examine the activity of TRPV1 Channels in regulating glutamate-mediated excitatory postsynaptic currents (EPSCs) in NAc of rat during morphine withdrawal for 3days and 3weeks. The data showed that the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) and the amplitudes of evoked excitatory postsynaptic currents (eEPSCs) were increased during morphine withdrawal after applied with capsaicin (TRPV1 agonist). Capsaicin decreased the paired pulse ratio (PPR) and increased sEPSCs frequency but not their amplitudes suggesting a presynaptic locus of action during morphine withdrawal. All these effects were fully blocked by the TRPV1 antagonist Capsazepine. Additionally, In the presence of AM251 (CB1 receptor antagonist), depolarization-induced release of endogenous cannabinoids activated TRPV1 channels to enhance glutamatergic neurotransmission during morphine withdrawal. The functional enhancement of TRPV1 Channels in facilitating glutamatergic transmission was not recorded in dorsal striatum. Our findings demonstrate the ability of TRPV1 in regulating excitatory glutamatergic transmission is enhanced during morphine withdrawal in NAc, which would deepen our understanding of glutamatergic modulation during opioids withdrawal.

Does cannabidiol have a role in the treatment of schizophrenia?

Schizophrenia is a debilitating psychiatric disorder which places a significant emotional and economic strain on the individual and society-at-large. Unfortunately, currently available therapeutic strategies do not provide adequate relief and some patients are treatment-resistant. In this regard, cannabidiol (CBD), a non-psychoactive constituent of Cannabis sativa, has shown significant promise as a potential antipsychotic for the treatment of schizophrenia. However, there is still considerable uncertainty about the mechanism of action of CBD as well as the brain regions which are thought to mediate its putative antipsychotic effects. We argue that further research on CBD is required to fast-track its progress to the clinic and in doing so, we may generate novel insights into the neurobiology of schizophrenia.

Cannabinoid-induced depression of synaptic transmission is switched to stimulation when dopaminergic tone is increased in the globus pallidus of the rodent

Because activation of D2 receptors reverses the neurochemical effects of cannabinoids, we examined whether increasing dopaminergic tone in the globus pallidus (GPe) switches cannabinoid induced depression of synaptic transmission. GABAergic synaptic currents evoked in pallidal neurons by stimulation of striatal projections (IPSCs) were depressed by perfusion with the CB1R agonist ACEA. Coactivation of D2Rs with quinpirole converted the depression into stimulation. Pretreatment with pertussis toxin (PTX) to limit Gi/o protein coupling also switched the CB1R-induced depression of IPSCs. The stimulation of IPSCs was blocked by the selective PKA blocker H89. Changes in the paired pulse ratio during both inhibitory and stimulatory responses indicate that the effects are due to changes in transmitter release. Postsynaptic depolarization induces endocannabinoid release that inhibits transmitter release (DSI). When D2Rs were activated with quinpirole, depolarization increased transmission instead of depressing it. This increase was blocked by AM251. We also examined the effects of CB1R/D2R coactivation on cAMP accumulation in the GPe to further verify that the AC/PKA cascade is involved. CB1R/D2R coactivation converted the inhibition of cAMP seen when each receptor is stimulated alone into a stimulation. We also determined the effects on turning behavior of unilateral injection of ACEA into the GPe of awake animals and its modification by dopamine antagonists. Blockade of D2 family receptors with sulpiride antagonized the motor effects of ACEA. We show, for the first time, that cannabinoid-inhibition of synaptic transmission in the GPe becomes a stimulation after D2Rs or PTX treatment and that the switch is probably relevant for the control of motor behavior.

Inhibition of Endocannabinoid Degradation Improves Outcomes from Mild Traumatic Brain Injury: A Mechanistic Role for Synaptic Hyperexcitability

Traumatic brain injury (TBI) is an increasingly prevalent condition affecting soldiers, athletes, and motor vehicle accident victims. Unfortunately, it currently lacks effective therapeutic interventions. TBI is defined as a primary mechanical insult followed by a secondary cascade involving inflammation, apoptosis, release of reactive oxygen species, and excitotoxicity, all of which can cause synaptic changes, altered neuronal signaling, and, ultimately, behavioral changes. Previously we showed that preventing degradation of the endocannabinoid (EC) 2-acylglycerol (2-AG) with JZL184 after mild TBI attenuated neuroinflammation and improved recovery of neurobehavioral function during the early 24 h post-TBI period. The aim of this study was to extend the timeline of observations to 2 weeks post-injury and to investigate JZL184's impact on synaptic transmission, which we view as a potential mechanism for TBI-induced cellular and behavioral pathology. Adult male rats underwent mild TBI (mTBI) followed by a single intraperitoneal injection of JZL184 or vehicle 30 min post-injury. JZL184 administered-TBI animals showed improved neurobehavioral recovery compared with vehicle-injected TBI animals beginning 24 h post-injury and persisting for 2 weeks. JZL184-treated animals had significantly diminished gray and white matter astrocyte activation when compared with vehicle-treated animals at day 7 post-TBI. JZL184 administration significantly attenuated the increased pGluR1^S845/GluR1 and pERK 1/2/ERK and the increases in miniature excitatory postsynaptic potential (mEPSC) frequency and amplitude observed in layer 5 pyramidal neurons at 10 days post-TBI. These results suggest a neuroprotective role for ECs in ameliorating the TBI-induced neurobehavioral, neuroinflammatory, and glutamate dyshomeostasis from mTBI. Further studies elucidating the cellular mechanisms involved are warranted.

The neuronal identity bias behind neocortical GABAergic plasticity

In the neocortex, different types of excitatory and inhibitory neurons connect to one another following a detailed blueprint, defining functionally-distinct subnetworks, whose activity and modulation underlie complex cognitive functions. We review the cell-autonomous plasticity of perisomatic inhibition onto principal excitatory neurons. We propose that the tendency of different cortical layers to exhibit depression or potentiation of perisomatic inhibition is dictated by the specific identities of principal neurons (PNs). These are mainly defined by their projection targets and by their preference to be innervated by specific perisomatic-targeting basket cell types. Therefore, principal neurons responsible for relaying information to subcortical nuclei are differentially inhibited and show specific forms of plasticity compared to other PNs that are specialized in more associative functions.

Emerging Trends in Retrograde Signaling

Retrograde signaling is defined as the signaling events leading from the plastids to the nucleus in plants and across the chemical synapse, from the postsynaptic neuron to the presynaptic neuron in animals. The discovery of various retrograde messengers has opened many avenues and clouds of thoughts as to the role of retrograde signaling. They have been implicated particularly in long-term potentiation (LTP) and synaptic plasticity. But the basic assumptions about retrograde signaling have not been studied upon for many years. This review focuses on established facts and hypothesis put forward in retrograde signaling.

Endocannabinoids in synaptic plasticity and neuroprotection

Endocannabinoids (eCBs) are endogenous lipid mediators involved in a variety of physiological, pharmacological, and pathological processes. While activation of the eCB system primarily induces inhibitory effects on both GABAergic and glutamatergic synaptic transmission and plasticity through acting on presynaptically expressed CB1 receptors in the brain, accumulated information suggests that eCB signaling is also capable of facilitating or potentiating excitatory synaptic transmission in the hippocampus. Recent studies show that a long-lasting potentiation of excitatory synaptic transmission at Schaffer collateral (SC)-CA1 synapses is induced by spatiotemporally primed inputs, accompanying with a long-term depression of inhibitory synaptic transmission (I-LTD) in hippocampal CA1 pyramidal neurons. This input timing-dependent long-lasting synaptic potentiation at SC-CA1 synapses is mediated by 2-arachidonoylglycerol (2-AG) signaling triggered by activation of postsynaptic N-methyl-D-aspartate receptors, group I metabotropic glutamate receptors (mGluRs), and a concurrent rise in intracellular Ca(2+). Emerging evidence now also indicates that 2-AG is an important signaling mediator keeping brain homeostasis by exerting its anti-inflammatory and neuroprotective effects in response to harmful insults through CB1/2 receptor-dependent and/or -independent mechanisms. Activation of the nuclear receptor protein peroxisome proliferator-activated receptor-γ apparently is one of the important mechanisms in resolving neuroinflammation and protecting neurons produced by 2-AG signaling. Thus, the information summarized in this review suggests that the role of eCB signaling in maintaining integrity of brain function is greater than what we thought previously.

Phasic and Tonic Inhibition are Maintained Respectively by CaMKII and PKA in the Rat Visual Cortex

Phasic and tonic γ-aminobutyric acid_A (GABA_A) receptor-mediated inhibition critically regulate neuronal information processing. As these two inhibitory modalities have distinctive features in their receptor composition, subcellular localization of receptors, and the timing of receptor activation, it has been thought that they might exert distinct roles, if not completely separable, in the regulation of neuronal function. Inhibition should be maintained and regulated depending on changes in network activity, since maintenance of excitation-inhibition balance is essential for proper functioning of the nervous system. In the present study, we investigated how phasic and tonic inhibition are maintained and regulated by different signaling cascades. Inhibitory postsynaptic currents were measured as either electrically evoked events or spontaneous events to investigate regulation of phasic inhibition in layer 2/3 pyramidal neurons of the rat visual cortex. Tonic inhibition was assessed as changes in holding currents by the application of the GABA_A receptor blocker bicuculline. Basal tone of phasic inhibition was maintained by intracellular Ca²⁺ and Ca²⁺/calmodulin-dependent protein kinase II (CaMKII). However, maintenance of tonic inhibition relied on protein kinase A activity. Depolarization of membrane potential (5 min of 0 mV holding) potentiated phasic inhibition via Ca²⁺ and CaMKII but tonic inhibition was not affected. Thus, phasic and tonic inhibition seem to be independently maintained and regulated by different signaling cascades in the same cell. These results suggest that neuromodulatory signals might differentially regulate phasic and tonic inhibition in response to changes in brain states.

Enhanced endocannabinoid-mediated modulation of rostromedial tegmental nucleus drive onto dopamine neurons in Sardinian alcohol-preferring rats

The progressive predominance of rewarding effects of addictive drugs over their aversive properties likely contributes to the transition from drug use to drug dependence. By inhibiting the activity of DA neurons in the VTA, GABA projections from the rostromedial tegmental nucleus (RMTg) are well suited to shift the balance between drug-induced reward and aversion. Since cannabinoids suppress RMTg inputs to DA cells and CB1 receptors affect alcohol intake in rodents, we hypothesized that the endocannabinoid system, by modulating this pathway, might contribute to alcohol preference. Here we found that RMTg afferents onto VTA DA neurons express CB1 receptors and display a 2-arachidonoylglycerol (2-AG)-dependent form of short-term plasticity, that is, depolarization-induced suppression of inhibition (DSI). Next, we compared rodents with innate opposite alcohol preference, the Sardinian alcohol-preferring (sP) and alcohol-nonpreferring (sNP) rats. We found that DA cells from alcohol-naive sP rats displayed a decreased probability of GABA release and a larger DSI. This difference was due to the rate of 2-AG degradation. In vivo, we found a reduced RMTg-induced inhibition of putative DA neurons in sP rats that negatively correlated with an increased firing. Finally, alcohol failed to enhance RMTg spontaneous activity and to prolong RMTg-induced silencing of putative DA neurons in sP rats. Our results indicate functional modifications of RMTg projections to DA neurons that might impact the reward/aversion balance of alcohol attributes, which may contribute to the innate preference observed in sP rats and to their elevated alcohol intake.

Multifractal analysis of information processing in hippocampal neural ensembles during working memory under Δ⁹-tetrahydrocannabinol administration

BACKGROUND

Multifractal analysis quantifies the time-scale-invariant properties in data by describing the structure of variability over time. By applying this analysis to hippocampal interspike interval sequences recorded during performance of a working memory task, a measure of long-range temporal correlations and multifractal dynamics can reveal single neuron correlates of information processing.

NEW METHOD

Wavelet leaders-based multifractal analysis (WLMA) was applied to hippocampal interspike intervals recorded during a working memory task. WLMA can be used to identify neurons likely to exhibit information processing relevant to operation of brain-computer interfaces and nonlinear neuronal models.

RESULTS

Neurons involved in memory processing ("Functional Cell Types" or FCTs) showed a greater degree of multifractal firing properties than neurons without task-relevant firing characteristics. In addition, previously unidentified FCTs were revealed because multifractal analysis suggested further functional classification. The cannabinoid type-1 receptor (CB1R) partial agonist, tetrahydrocannabinol (THC), selectively reduced multifractal dynamics in FCT neurons compared to non-FCT neurons.

COMPARISON WITH EXISTING METHODS

WLMA is an objective tool for quantifying the memory-correlated complexity represented by FCTs that reveals additional information compared to classification of FCTs using traditional z-scores to identify neuronal correlates of behavioral events.

CONCLUSION

z-Score-based FCT classification provides limited information about the dynamical range of neuronal activity characterized by WLMA. Increased complexity, as measured with multifractal analysis, may be a marker of functional involvement in memory processing. The level of multifractal attributes can be used to differentially emphasize neural signals to improve computational models and algorithms underlying brain-computer interfaces.

Developmental but not adult cannabinoid treatments persistently alter axonal and dendritic morphology within brain regions important for zebra finch vocal learning

Prior work shows developmental cannabinoid exposure alters zebra finch vocal development in a manner associated with altered CNS physiology, including changes in patterns of CB1 receptor immunoreactivity, endocannabinoid concentrations and dendritic spine densities. These results raise questions about the selectivity of developmental cannabinoid effects: are they a consequence of a generalized developmental disruption, or are effects produced through more selective and distinct interactions with biochemical pathways that control receptor, endogenous ligand and dendritic spine dynamics? To begin to address this question we have examined effects of developmental cannabinoid exposure on the pattern and density of expression of proteins critical to dendritic (MAP2) and axonal (Nf-200) structure to determine the extent to which dendritic vs. axonal neuronal morphology may be altered. Results demonstrate developmental, but not adult cannabinoid treatments produce generalized changes in expression of both dendritic and axonal cytoskeletal proteins within brain regions and cells known to express CB1 cannabinoid receptors. Results clearly demonstrate that cannabinoid exposure during a period of sensorimotor development, but not adulthood, produce profound effects upon both dendritic and axonal morphology that persist through at least early adulthood. These findings suggest an ability of exogenous cannabinoids to alter general processes responsible for normal brain development. Results also further implicate the importance of endocannabinoid signaling to peri-pubertal periods of adolescence, and underscore potential consequences of cannabinoid abuse during periods of late-postnatal CNS development.

Sex-specific tonic 2-arachidonoylglycerol signaling at inhibitory inputs onto dopamine neurons of Lister Hooded rats

Addiction as a psychiatric disorder involves interaction of inherited predispositions and environmental factors. Similarly to humans, laboratory animals self-administer addictive drugs, whose appetitive properties result from activation and suppression of brain reward and aversive pathways, respectively. The ventral tegmental area (VTA) where dopamine (DA) cells are located is a key component of brain reward circuitry, whereas the rostromedial tegmental nucleus (RMTg) critically regulates aversive behaviors. Reduced responses to either aversive intrinsic components of addictive drugs or to negative consequences of compulsive drug taking might contribute to vulnerability to addiction. In this regard, female Lister Hooded (LH) rats are more vulnerable than male counterparts to cannabinoid self-administration. We, therefore, took advantage of sex differences displayed by LH rats, and studied VTA DA neuronal properties to unveil functional differences. Electrophysiological properties of DA cells were examined performing either single cell extracellular recordings in anesthetized rats or whole-cell patch-clamp recordings in slices. In vivo, DA cell spontaneous activity was similar, though sex differences were observed in RMTg-induced inhibition of DA neurons. In vitro, DA cells showed similar intrinsic and synaptic properties. However, females displayed larger depolarization-induced suppression of inhibition (DSI) than male LH rats. DSI, an endocannabinoid-mediated form of short term plasticity, was mediated by 2-arachidonoylglycerol (2-AG) activating type 1-cannabinoid (CB1) receptors. We found that sex-dependent differences in DSI magnitude were not ascribed to CB1 number and/or function, but rather to a tonic 2-AG signaling. We suggest that sex specific tonic 2-AG signaling might contribute to regulate responses to aversive intrinsic properties to cannabinoids, thus resulting in faster acquisition/initiation of cannabinoid taking and, eventually, in progression to addiction.

Mechanisms of heterosynaptic metaplasticity

Synaptic plasticity is fundamental to the neural processes underlying learning and memory. Interestingly, synaptic plasticity itself can be dynamically regulated by prior activity, in a process termed 'metaplasticity', which can be expressed both homosynaptically and heterosynaptically. Here, we focus on heterosynaptic metaplasticity, particularly long-range interactions between synapses spread across dendritic compartments, and review evidence for intracellular versus intercellular signalling pathways leading to this effect. Of particular interest is our previously reported finding that priming stimulation in stratum oriens of area CA1 in the hippocampal slice heterosynaptically inhibits subsequent long-term potentiation and facilitates long-term depression in stratum radiatum. As we have excluded the most likely intracellular signalling pathways that might mediate this long-range heterosynaptic effect, we consider the hypothesis that intercellular communication may be critically involved. This hypothesis is supported by the finding that extracellular ATP hydrolysis, and activation of adenosine A2 receptors are required to induce the metaplastic state. Moreover, delivery of the priming stimulation in stratum oriens elicited astrocytic calcium responses in stratum radiatum. Both the astrocytic responses and the metaplasticity were blocked by gap junction inhibitors. Taken together, these findings support a novel intercellular communication system, possibly involving astrocytes, being required for this type of heterosynaptic metaplasticity.

Cannabinoids, Neurogenesis and Antidepressant Drugs: Is there a Link?

Similar to clinically used antidepressants, cannabinoids can also regulate anxiety and depressive symptoms. Although the mechanisms of these effects are not completely understood, recent evidence suggests that changes in endocannabinoid system could be involved in some actions of antidepressants. Chronic antidepressant treatment modifies the expression of CB1 receptors and endocannabinoid (EC) content in brain regions related to mood and anxiety control. Moreover, both antidepressant and cannabinoids activate mitogen-activated protein (MAP) kinase and phosphoinositide 3-kinase(PI3-K)/Akt or PKB signaling, intracellular pathways that regulate cell proliferation and neural cell survival. Facilitation of hippocampal neurogenesis is proposed as a common effect of chronic antidepressant treatment. Genetic or pharmacological manipulations of cannabinoid receptors (CB1 and CB2) or enzymes responsible for endocannabinoid-metabolism have also been shown to control proliferation and neurogenesis in the hippocampus. In the present paper we reviewed the studies that have investigated the potential contribution of cannabinoids and neurogenesisto antidepressant effects. Considering the widespread brain distribution of the EC system, a better understanding of this possible interaction could contribute to the development of therapeutic alternatives to mood and anxiety disorders.

Developmental pattern of diacylglycerol lipase-α (DAGLα) immunoreactivity in brain regions important for song learning and control in the zebra finch (Taeniopygia guttata)

Zebra finch song is a learned behavior dependent upon successful progress through a sensitive period of late-postnatal development. This learning is associated with maturation of distinct brain nuclei and the fiber tract interconnections between them. We have previously found remarkably distinct and dense CB1 cannabinoid receptor expression within many of these song control brain regions, implying a normal role for endocannabinoid signaling in vocal learning. Activation of CB1 receptors via daily treatments with exogenous agonist during sensorimotor stages of song learning (but not in adulthood) results in persistent alteration of song patterns. Now we are working to understand physiological changes responsible for this cannabinoid-altered vocal learning. We have found that song-altering developmental treatments are associated with changes in expression of endocannabinoid signaling elements, including CB1 receptors and the principal CNS endogenous agonist, 2-AG. Within CNS, 2-AG is produced largely through activity of the α isoform of the enzyme diacylglycerol lipase (DAGLα). To better appreciate the role of 2-AG production in normal vocal development we have determined the spatial distribution of DAGLα expression within zebra finch CNS during vocal development. Early during vocal development at 25 days, DAGLα staining is typically light and of fibroid processes. Staining peaks late in the sensorimotor stage of song learning at 75 days and is characterized by fiber, neuropil and some staining of both small and large cell somata. Results provide insight to the normal role for endocannabinoid signaling in the maturation of brain regions responsible for song learning and vocal-motor output, and suggest mechanisms by which exogenous cannabinoid exposure alters acquisition of this form of vocal communication.

PUFA-derived endocannabinoids: an overview

Following on from the discovery of cannabinoid receptors, of their endogenous agonists (endocannabinoids) and of the biosynthetic and metabolic enzymes of the endocannabinoids, significant progress has been made towards the understanding of the role of the endocannabinoid system in both physiological and pathological conditions. Endocannabinoids are mainly n-6 long-chain PUFA (LCPUFA) derivatives that are synthesised by neuronal cells and inactivated via a two-step process that begins with their transport from the extracellular to the intracellular space and culminates in their intracellular degradation by hydrolysis or oxidation. Although the enzymes responsible for the biosynthesis and metabolism of endocannabinoids have been well characterised, the processes involved in their cellular uptake are still a subject of debate. Moreover, little is yet known about the roles of endocannabinoids derived from n-3 LCPUFA such as EPA and DHA. Here, I provide an overview of what is currently known about the mechanisms for the biosynthesis and inactivation of endocannabinoids, together with a brief analysis of the involvement of the endocannabinoids in both food intake and obesity. Owing to limited space, recent reviews will be often cited instead of original papers.

CB1 agonists, locally applied to the cortico-thalamic circuit of rats with genetic absence epilepsy, reduce epileptic manifestations

Drugs that modulate the endocannabinoid system and endocannabinoids typically play an anticonvulsant role although some proconvulsant effects have been reported both in humans and animal models. Moreover, no evidence for a role of the cannabinoid system in human absence epilepsy has been found although limited evidence of efficacy in relevant experimental animal models has been documented. This study aims to characterize the role of cannabinoids in specific areas of the cortico-thalamic network involved in oscillations that underlie seizures in a genetic animal model of absence epilepsy, the WAG/Rij rat. We assessed the effects of focal injection of the endogenous cannabinoid, anandamide (AEA), a non-selective CB receptor agonist (WIN55,212) and a selective CB1 receptor antagonist/inverse agonist (SR141716A) into thalamic nuclei and primary somatosensory cortex (S1po) of the cortico-thalamic network. AEA and WIN both reduced absence seizures independently from the brain focal site of infusion while, conversely, rimonabant increased absence seizures but only when focally administered to the ventroposteromedial thalamic nucleus (VPM). These results, together with previous reports, support therapeutic potential for endocannabinoid system modulators in absence epilepsy and highlight that attenuated endocannabinergic function may contribute to the generation and maintenance of seizures. Furthermore, the entire cortico-thalamic network responds to cannabinoid treatment, indicating that in all areas considered, CB receptor activation inhibits the pathological synchronization that subserves absence seizures. In conclusion, our result might be useful for the identification of future drug therapies in absence epilepsy.

Additive effect of rimonabant and citalopram on extracellular serotonin levels monitored with in vivo microdialysis in rat brain

Current pharmacological therapies for depression, including selective serotonin reuptake inhibitors (SSRI), are far from ideal. The cannabinoid system has been implicated in control of mood and neural processing of emotional information, and the modulation of serotonin (5-HT) release in the synaptic clefts. The aim of the present study was to evaluate whether the combination of a selective SSRI (citalopram) with a selective cannabinoid CB1 receptor antagonist (rimonabant) represents a more effective strategy than the antidepressant alone to enhance serotonergic transmission. For this purpose extracellular 5-HT levels were monitored with microdialysis in forebrain (prefrontal cortex, PFC) and mesencephalic (locus coeruleus, LC) serotonergic terminal areas in freely awake rats. Rimonabant at 10 mg/kg, i.p., but not at 3mg/kg i.p. increased 5-HT in both areas. Citalopram at 3, 5 and 10 mg/kg i.p. increased 5-HT both in PFC and LC in a dose-dependent manner. The effect of citalopram (5mg/kg, i.p.) on 5-HT levels was significantly enhanced by rimonabant at 10 mg/kg, i.p. but not at 3 mg/kg i.p. in both areas. The present results demonstrate that the cannabinoid CB1 receptor antagonist rimonabant is able to enhance in an additive manner the citalopram-induced increase of 5-HT concentrations in serotonergic terminal areas. The combination of a cannabinoid antagonist and a SSRI may provide a novel strategy to increase 5-HT availability, reducing the dose of SSRIs, and potentially decreasing the time lag for the clinical onset of the antidepressant effect.

A biophysical model of endocannabinoid-mediated short term depression in hippocampal inhibition

Memories are believed to be represented in the synaptic pathways of vastly interconnected networks of neurons. The plasticity of synapses, that is, their strengthening and weakening depending on neuronal activity, is believed to be the basis of learning and establishing memories. An increasing number of studies indicate that endocannabinoids have a widespread action on brain function through modulation of synap–tic transmission and plasticity. Recent experimental studies have characterised the role of endocannabinoids in mediating both short- and long-term synaptic plasticity in various brain regions including the hippocampus, a brain region strongly associated with cognitive functions, such as learning and memory. Here, we present a biophysically plausible model of cannabinoid retrograde signalling at the synaptic level and investigate how this signalling mediates depolarisation induced suppression of inhibition (DSI), a prominent form of short-term synaptic depression in inhibitory transmission in hippocampus. The model successfully captures many of the key characteristics of DSI in the hippocampus, as observed experimentally, with a minimal yet sufficient mathematical description of the major signalling molecules and cascades involved. More specifically, this model serves as a framework to test hypotheses on the factors determining the variability of DSI and investigate under which conditions it can be evoked. The model reveals the frequency and duration bands in which the post-synaptic cell can be sufficiently stimulated to elicit DSI. Moreover, the model provides key insights on how the state of the inhibitory cell modulates DSI according to its firing rate and relative timing to the post-synaptic activation. Thus, it provides concrete suggestions to further investigate experimentally how DSI modulates and is modulated by neuronal activity in the brain. Importantly, this model serves as a stepping stone for future deciphering of the role of endocannabinoids in synaptic transmission as a feedback mechanism both at synaptic and network level.

Recent advances in pharmacotherapy of chemotherapy-induced nausea and vomiting

Nausea and vomiting remain among the most feared side effects of chemotherapy for cancer patients. Significant progress has been made in the last 15 years in developing more effective and better-tolerated measures to minimize chemotherapy-induced nausea and vomiting (CINV). During the 1990s, the selective 5-hydroxytryptamine receptor antagonists were first introduced for the treatment of CINV, and resulted in more effective and better tolerated treatment of CINV. Despite recent progress, however, a significant number of patients still develop CINV, particularly during the 2-5-day period (delayed emesis) following chemotherapy. There is evidence that this may be an underappreciated problem on the part of some caregivers. Recently, two new antiemetics, aprepitant, the first member of the neurokinin-1 antagonists, and palonosetron, a second-generation 5-hydroxytryptamine receptor antagonist, received regulatory approval in the U.S. Both represent useful additions to the therapeutic armamentarium for the management of CINV.

Cytosolic phospholipase A2 alpha/arachidonic acid signaling mediates depolarization-induced suppression of excitation in the cerebellum

Background

Depolarization-induced suppression of excitation (DSE) at parallel fiber-Purkinje cell synapse is an endocannabinoid-mediated short-term retrograde plasticity. Intracellular Ca²⁺ elevation is critical for the endocannabinoid production and DSE. Nevertheless, how elevated Ca²⁺ leads to DSE is unclear.

Methodology/Principal Findings

We utilized cytosolic phospholipase A₂ alpha (cPLA₂α) knock-out mice and whole-cell patch clamp in cerebellar slices to observed the action of cPLA₂α/arachidonic acid signaling on DSE at parallel fiber-Purkinje cell synapse. Our data showed that DSE was significantly inhibited in cPLA₂α knock-out mice, which was rescued by arachidonic acid. The degradation enzyme of 2-arachidonoylglycerol (2-AG), monoacylglycerol lipase (MAGL), blocked DSE, while another catabolism enzyme for N-arachidonoylethanolamine (AEA), fatty acid amide hydrolase (FAAH), did not affect DSE. These results suggested that 2-AG is responsible for DSE in Purkinje cells. Co-application of paxilline reversed the blockade of DSE by internal K⁺, indicating that large conductance Ca²⁺-activated potassium channel (BK) is sufficient to inhibit cPLA₂α/arachidonic acid-mediated DSE. In addition, we showed that the release of 2-AG was independent of soluble NSF attachment protein receptor (SNARE), protein kinase C and protein kinase A.

Conclusions/Significance

Our data first showed that cPLA₂α/arachidonic acid/2-AG signaling pathway mediates DSE at parallel fiber-Purkinje cell synapse.

Putative depolarisation-induced retrograde signalling accelerates the repeated hypoxic depression of excitatory synaptic transmission in area CA1 of rat hippocampus via group I metabotropic glutamate receptors

Excitatory synaptic transmission in area CA1 of the mammalian hippocampus is rapidly depressed during hypoxia. The depression is largely attributable to an increase in extracellular adenosine and activation of inhibitory adenosine A(1) receptors on presynaptic glutamatergic terminals. However, sequential exposure to hypoxia results in a slower subsequent hypoxic depression of excitatory synaptic transmission, a phenomenon we have previously ascribed to a reduction in the release of extracellular adenosine. In the present study we show that this delayed depression of excitatory postsynaptic currents (EPSCs) to repeated hypoxia can be reversed by a period of postsynaptic depolarisation delivered to an individual CA1 neuron, under whole-cell voltage clamp, between two periods of hypoxia. The depolarisation-induced acceleration of the hypoxic depression of the EPSC is dependent upon postsynaptic Ca(2+) influx, the activation of PKC and is blocked by intracellular application of GDP-β-S and N-ethylmaleimide (NEM), inhibitors of membrane fusion events. In addition, the acceleration of the hypoxic depression of the EPSC was prevented by the GI mGluR antagonist AIDA, but not by the CB1 cannabinoid receptor antagonist AM251. Our results suggest a process initiated in the postsynaptic cell that can influence glutamate release during subsequent metabolic stress. This may reflect a novel neuroprotective strategy potentially involving retrograde release of adenosine and/or glutamate.

Adenosine A(2A)-cannabinoid CB(1) receptor interaction: an integrative mechanism in striatal glutamatergic neurotransmission

The striatum is a subcortical area involved in sensorimotor, cognitive and emotional processes. Adenosine A(2A) receptors (A(2A)Rs) are highly expressed in the striatum, and their ability to establish functional and molecular interactions with many other receptors attributes to a pivotal role in the modulation and integration of striatal neurotransmission. This review will focus on the interaction between A(2A)Rs and cannabinoid CB(1) receptors (CB(1)Rs), taking it as a paradigmatic example of synaptic integration. Indeed, A(2A)Rs can exert an opposite (permissive vs. inhibitory) influence on CB1-dependent synaptic effect. These apparently irreconcilable functions could depend on a different role of pre- vs. postsynaptic A(2A)Rs, on their interaction with other receptors (namely adenosine A(1), metabotropic glutamate 5 and dopamine D2 receptors), and on whether A(2A)Rs form or not heteromers with CB(1)Rs. Besides providing a good example of the intricate pattern of events taking place in striatal synapses, the A(2A)/CB(1)R interaction proves very informative to understand the physiology of the basal ganglia and the mechanisms of related diseases. This article is part of a Special Issue entitled: Brain Integration.

The role of metaplasticity mechanisms in regulating memory destabilization and reconsolidation

Memory allows organisms to predict future events based on prior experiences. This requires encoded information to persist once important predictors are extracted, while also being modifiable in response to changes within the environment. Memory reconsolidation may allow stored information to be modified in response to related experience. However, there are many boundary conditions beyond which reconsolidation may not occur. One interpretation of these findings is that the event triggering memory retrieval must contain new information about a familiar stimulus in order to induce reconsolidation. Presently, the mechanisms that affect the likelihood of reconsolidation occurring under these conditions are not well understood. Here we speculate on a number of systems that may play a role in protecting memory from being destabilized during retrieval. We conclude that few memories may enter a state in which they cannot be modified. Rather, metaplasticity mechanisms may serve to alter the specific reactivation cues necessary to destabilize a memory. This might imply that destabilization mechanisms can differ depending on learning conditions.

Long-lasting increase in [³H]CP55,940 binding to CB1 receptors following cocaine self-administration and its withdrawal in rats

The present work has aimed on the neuroadaptive changes in CB1 receptor density that are evoked by self-administered cocaine use and subsequent withdrawal in rats. We employed a quantitative autoradiographic analysis using labeled [³H]CP55,940, a CB1 receptor agonist. To distinguish the passive pharmacological effects of cocaine from those related to motivation and the cognitive processes evoked by active cocaine self-administration, the "yoked" procedure was used. Our results demonstrate that repeated cocaine administration over 14 days induced up-regulation of CB1 receptors in the cortical and subcortical brain areas of animals who received cocaine, whether the cocaine was actively self-administered or received passively (the "yoked" control group) and that the neuroadaptation of CB1 receptors persisted after the 10-day extinction phase. On the other hand, we found that only self-administering rats showed CB1 receptor up-regulation in numerous brain areas, which suggests that these structures may be directly linked to CB1 receptor control over motivational and cognitive processes. Moreover, the observed increase in [³H]CP55,940 binding in these brain areas likely indicates long-lasting neurobiological adaptations resulting from chronic cocaine self-administration. In conclusion, we demonstrated that chronic cocaine self-administration leads to increased CB1 receptor levels in numerous brain areas and that this neuroadaptation is maintained over a long-lasting extinction period.

The biology that underpins the therapeutic potential of cannabis-based medicines for the control of spasticity in multiple sclerosis

Cannabis-based medicines have recently been approved for the treatment of pain and spasticity in multiple sclerosis (MS). This supports the original perceptions of people with MS, who were using illegal street cannabis for symptom control and pre-clinical testing in animal models of MS. This activity is supported both by the biology of the disease and the biology of the cannabis plant and the endocannabinoid system. MS results from disease that impairs neurotransmission and this is controlled by cannabinoid receptors and endogenous cannabinoid ligands. This can limit spasticity and may also influence the processes that drive the accumulation of progressive disability.

Endocannabinoid tone versus constitutive activity of cannabinoid receptors

This review evaluates the cellular mechanisms of constitutive activity of the cannabinoid (CB) receptors, its reversal by inverse agonists, and discusses the pitfalls and problems in the interpretation of the research data. The notion is presented that endogenously produced anandamide (AEA) and 2-arachidonoylglycerol (2-AG) serve as autocrine or paracrine stimulators of the CB receptors, giving the appearance of constitutive activity. It is proposed that one cannot interpret inverse agonist studies without inference to the receptors' environment vis-à-vis the endocannabinoid agonists which themselves are highly lipophilic compounds with a preference for membranes. The endocannabinoid tone is governed by a combination of synthetic pathways and inactivation involving transport and degradation. The synthesis and degradation of 2-AG is well characterized, and 2-AG has been strongly implicated in retrograde signalling in neurons. Data implicating endocannabinoids in paracrine regulation have been described. Endocannabinoid ligands can traverse the cell's interior and potentially be stored on fatty acid-binding proteins (FABPs). Molecular modelling predicts that the endocannabinoids derived from membrane phospholipids can laterally diffuse to enter the CB receptor from the lipid bilayer. Considering that endocannabinoid signalling to CB receptors is a much more likely scenario than is receptor activation in the absence of agonist ligands, researchers are advised to refrain from assuming constitutive activity except for experimental models known to be devoid of endocannabinoid ligands.

Action potential-triggered somatic exocytosis in mesencephalic trigeminal nucleus neurons in rat brain slices

The neurons in the mesencephalic trigeminal nucleus (MeV) play essential roles in proprioceptive sensation of the face and oral cavity. The somata of MeV neurons are generally assumed to carry out neuronal functions but not to play a direct role in synaptic transmission. Using whole-cell recording and membrane capacitance (C(m)) measurements, we found that the somata of MeV neurons underwent robust exocytosis (C(m) jumps) upon depolarization and with the normal firing of action potentials in brain slices. Both removing [Ca(2+)](o) and buffering [Ca(2+)](i) with BAPTA blocked this exocytosis, indicating that it was completely Ca(2+) dependent. In addition, an electron microscopic study showed synaptic-like vesicles approximated to the plasma membrane in somata. There was a single Ca(2+)-dependent releasable vesicle pool with a peak release rate of 1912 fF s(-1). Importantly, following depolarization-induced somatic exocytosis, GABA-mediated postsynaptic currents were transiently reduced by 31%, suggesting that the somatic vesicular release had a retrograde effect on afferent GABAergic transmission. These results provide strong evidence that the somata of MeV neurons undergo robust somatic secretion and may play a crucial role in bidirectional communication between somata and their synaptic inputs in the central nervous system.

Endocannabinoids and prostaglandins both contribute to GnRH neuron-GABAergic afferent local feedback circuits

Gonadotropin-releasing hormone (GnRH) neurons form the final common pathway for central control of fertility. Regulation of GnRH neurons by long-loop gonadal steroid feedback through steroid receptor-expressing afferents such as GABAergic neurons is well studied. Recently, local central feedback circuits regulating GnRH neurons were identified. GnRH neuronal depolarization induces short-term inhibition of their GABAergic afferents via a mechanism dependent on metabotropic glutamate receptor (mGluR) activation. GnRH neurons are enveloped in astrocytes, which express mGluRs. GnRH neurons also produce endocannabinoids, which can be induced by mGluR activation. We hypothesized the local GnRH-GABA circuit utilizes glia-derived and/or cannabinoid mechanisms and is altered by steroid milieu. Whole cell voltage-clamp was used to record GABAergic postsynaptic currents (PSCs) from GnRH neurons before and after action potential-like depolarizations were mimicked. In GnRH neurons from ovariectomized (OVX) mice, this depolarization reduced PSC frequency. This suppression was blocked by inhibition of prostaglandin synthesis with indomethacin, by a prostaglandin receptor antagonist, or by a specific glial metabolic poison, together suggesting the postulate that prostaglandins, potentially glia-derived, play a role in this circuit. This circuit was also inhibited by a CB1 receptor antagonist or by blockade of endocannabinoid synthesis in GnRH neurons, suggesting an endocannabinoid element, as well. In females, local circuit inhibition persisted in androgen-treated mice but not in estradiol-treated mice or young ovary-intact mice. In contrast, local circuit inhibition was present in gonad-intact males. These data suggest GnRH neurons interact with their afferent neurons using multiple mechanisms and that these local circuits can be modified by both sex and steroid feedback.