Cannabinoids depress inhibitory synaptic inputs received by layer 2/3 pyramidal neurons of the neocortex

Terminal type-specific cannabinoid CB1 receptor alterations in patients with schizophrenia: A pilot study

BACKGROUND

Individuals with schizophrenia are at elevated genetic risks for comorbid cannabis use, and often experience exacerbations of cognitive and psychotic symptoms when exposed to cannabis. These findings have led a number of investigators to examine cannabinoid CB1 receptor (CB1R) alterations in schizophrenia, though with conflicting results. We recently demonstrated the presence of CB1R in both excitatory and inhibitory boutons in the human prefrontal cortex, with differential levels of the receptor between bouton types. We hypothesized that the differential enrichment of CB1R between bouton types - a factor previously unaccounted for when examining CB1R changes in schizophrenia - may resolve prior discrepant reports and increase our insight into the effects of CB1R alterations on the pathophysiology of schizophrenia.

METHODS

Using co-labeling immunohistochemistry and fluorescent microscopy, we examined total CB1R levels and CB1R levels within excitatory (vGlut1-positive) and inhibitory (vGAT-positive) boutons of prefrontal cortex samples from ten pairs of individuals (nine male pairs and one female pair) diagnosed with schizophrenia and non-psychiatric comparisons.

RESULTS

Significantly higher total CB1R levels were found within samples from individuals with schizophrenia. Terminal type-specific analyses identified significantly higher CB1R levels within excitatory boutons in samples from individuals with schizophrenia relative to comparisons. In contrast, CB1R levels within the subset of inhibitory boutons that normally express high CB1R levels (presumptive cholecystokinin neuron boutons) were lower in samples from individuals with schizophrenia relative to comparison samples.

CONCLUSION

Given CB1R's role in suppressing neurotransmission upon activation, these results suggest an overall shift in excitatory and inhibitory balance regulation toward a net reduction of excitatory activity in schizophrenia.

Terminal type-specific cannabinoid CB1 receptor alterations in patients with schizophrenia: a pilot study

Background

Individuals with schizophrenia are at elevated genetic risks for comorbid cannabis use, and often experience exacerbations of cognitive and psychotic symptoms when exposed to cannabis. These findings have led a number of investigators to examine cannabinoid CB1 receptor (CB1R) alterations in schizophrenia, though with conflicting results. We recently demonstrated the presence of CB1R in both excitatory and inhibitory boutons in the human prefrontal cortex, with differential levels of the receptor between bouton types. We hypothesized that the differential enrichment of CB1R between bouton types - a factor previously unaccounted for when examining CB1R changes in schizophrenia - may resolve prior discrepant reports and increase our insight into the effects of CB1R alterations on the pathophysiology of schizophrenia.

Methods

Using co-labeling immunohistochemistry and fluorescent microscopy, we examined total CB1R levels and CB1R levels within excitatory (vGlut1-positive) and inhibitory (vGAT-positive) boutons of prefrontal cortex samples from ten pairs of individuals diagnosed with schizophrenia and non-psychiatric comparisons.

Results

Significantly higher total CB1R levels were found within samples from individuals with schizophrenia. Terminal type-specific analyses identified significantly higher CB1R levels within excitatory boutons in samples from individuals with schizophrenia relative to comparisons. In contrast, CB1R levels within the subset of inhibitory boutons that normally express high CB1R levels (presumptive cholecystokinin neuron boutons) were lower in samples from individuals with schizophrenia relative to comparison samples.

Conclusion

Given CB1R's role in suppressing neurotransmission upon activation, these results suggest an overall shift in excitatory and inhibitory balance regulation toward a net reduction of excitatory activity in schizophrenia.

Cell type specific cannabinoid CB1 receptor distribution across the human and non-human primate cortex

Alterations in cannabinoid CB1 receptor (CB1R) are implicated in various psychiatric disorders. CB1R participates in both depolarization induced suppression of inhibition (DSI) and depolarization induced suppression of excitation (DSE), suggesting its involvement in regulating excitatory and inhibitory (E/I) balance. Prior studies examining neuronal cell type specific CB1R distribution have been conducted near exclusively within rodents. Identification of these distribution patterns within the human and non-human primate cortex is essential to increase our insight into its function. Using co-labeling immunohistochemistry and fluorescent microscopy, we examined CB1R protein levels within excitatory and inhibitory boutons of male human and non-human primate prefrontal cortex and auditory cortices, regions involved in the behavioral effects of exogenous cannabinoid exposures. We found that CB1R was present in both bouton populations within all brain regions examined in both species. Significantly higher CB1R levels were found within inhibitory than within excitatory boutons across all regions in both species, although the cell type by brain region interactions differed between the two species. Our results support the importance of conducting more in-depth CB1R examinations to understand how cell type and brain region dependent differences contribute to regional E/I balance regulation, and how aberrations in CB1R distribution may contribute to pathology.

THC-induced behavioral stereotypy in zebrafish as a model of psychosis-like behavior

High doses of the Cannabis constituent Δ9-tetrahydrocannabinol (THC) increase the risk of psychosis in humans. Highly accessible animal models are needed to address underlying mechanisms. Using zebrafish with a conserved endocannabinoid system, this study investigates the acute effects of THC on adult zebrafish behavior and the mechanisms involved. A concentration-dependent THC-induced behavioral stereotypy akin to THC's effect in rats and the psychotropics phencyclidine and ketamine in zebrafish was established. Distinctive circular swimming during THC-exposure was measured using a novel analytical method that we developed, which detected an elevated Repetition Index (RI) compared to vehicle controls. This was reduced upon co-administration of N-methyl-D-aspartate (NMDA) receptor agonist NMDA, suggesting that THC exerts its effects via biochemical or neurobiological mechanisms associated with NMDA receptor antagonism. Co-treatment of γ-aminobutyric acid receptor antagonist pentylenetetrazol also showed signs of reducing the RI. Since THC-induced repetitive behavior remained in co-administrations with cannabinoid receptor 1 inverse agonist AM251, the phenotype may be cannabinoid receptor 1-independent. Conversely, the inverse cannabinoid receptor 2 agonist AM630 significantly reduced THC-induced behavioral stereotypy, indicating cannabinoid receptor 2 as a possible mediator. A significant reduction of the THC-RI was also observed by the antipsychotic sulpiride. Together, these findings highlight this model's potential for elucidating the mechanistic relationship between Cannabis and psychosis.

Cannabinoid Signaling in Auditory Function and Development

Plants of the genus Cannabis have been used by humans for millennia for a variety of purposes. Perhaps most notable is the use of certain Cannabis strains for their psychoactive effects. More recently, several biologically active molecules within the plants of these Cannabis strains, called phytocannabinoids or simply cannabinoids, have been identified. Furthermore, within human cells, endogenous cannabinoids, or endocannabinoids, as well as the receptors and secondary messengers that give rise to their neuromodulatory effects, have also been characterized. This endocannabinoid system (ECS) is composed of two primary ligands-anandamide and 2-arachidonyl glycerol; two primary receptors-cannabinoid receptors 1 and 2; and several enzymes involved in biosynthesis and degradation of endocannabinoid ligands including diacylglycerol lipase (DAGL) and monoacylglycerol lipase (MAGL). Here we briefly summarize cannabinoid signaling and review what has been discerned to date with regard to cannabinoid signaling in the auditory system and its roles in normal physiological function as well as pathological conditions. While much has been uncovered regarding cannabinoid signaling in the central nervous system, less attention has been paid to the auditory system specifically. Still, evidence is emerging to suggest that cannabinoid signaling is critical for the development, maturation, function, and survival of cochlear hair cells (HCs) and spiral ganglion neurons (SGNs). Furthermore, cannabinoid signaling can have profound effects on synaptic connectivity in CNS structures related to auditory processing. While clinical cases demonstrate that endogenous and exogenous cannabinoids impact auditory function, this review highlights several areas, such as SGN development, where more research is warranted.

Cannabinoids, Inner Ear, Hearing, and Tinnitus: A Neuroimmunological Perspective

Cannabis has been used for centuries for recreational and therapeutic purposes. Whereas, the recreative uses are based on the psychotropic effect of some of its compounds, its therapeutic effects range over a wide spectrum of actions, most of which target the brain or the immune system. Several studies have found cannabinoid receptors in the auditory system, both at peripheral and central levels, thus raising the interest in cannabinoid signaling in hearing, and especially in tinnitus, which is affected also by anxiety, memory, and attention circuits where cannabinoid effects are well described. Available studies on animal models of tinnitus suggest that cannabinoids are not likely to be helpful in tinnitus treatment and could even be harmful. However, the pharmacology of cannabinoids is very complex, and most studies focused on neural CB1R-based responses. Cannabinoid effects on the immune system (where CB2Rs predominate) are increasingly recognized as essential in understanding nervous system pathological responses, and data on immune cannabinoid targets have emerged in the auditory system as well. In addition, nonclassical cannabinoid targets (such as TRP channels) appear to play an important role in the auditory system as well. This review will focus on neuroimmunological mechanisms for cannabinoid effects and their possible use as protective and therapeutic agents in the ear and auditory system, especially in tinnitus.

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.

The Role of the Endocannabinoid System and Genetic Variation in Adolescent Brain Development

During adolescence, both rodent and human studies have revealed dynamic changes in the developmental trajectories of corticolimbic structures, which are known to contribute to the regulation of fear and anxiety-related behaviors. The endocannabinoid (eCB) system critically regulates stress responsivity and anxiety throughout the life span. Emerging evidence suggests that during adolescence, changes in eCB signaling contribute to the maturation of local and corticolimbic circuit populations of neurons, such as mediating the balance between excitatory and inhibitory neurotransmission within the prefrontal cortex. This function of the eCB system facilitates efficient communication within and between brain regions and serves a central role in establishing complex and adaptive cognitive and behavioral processing. Although these peri-adolescent changes in eCB signaling promote brain development and plasticity, they also render this period a particularly sensitive one for environmental perturbations to these normative fluctuations in eCB signaling, such as stress, potentially leading to altered developmental trajectories of neural circuits governing emotional behaviors. In this review, we focus on the role of eCB signaling on the regulation of stress and anxiety-related behaviors both during and after adolescence. Moreover, we discuss the functional implications of human genetic variation in the eCB system for the risk for anxiety and consequences of stress across development and into adulthood.

Effects of the cannabinoid CB1 agonist ACEA on salicylate ototoxicity, hyperacusis and tinnitus in guinea pigs

Cannabinoids have been suggested as a therapeutic target for a variety of brain disorders. Despite the presence of their receptors throughout the auditory system, little is known about how cannabinoids affect auditory function. We sought to determine whether administration of arachidonyl-2′-chloroethylamide (ACEA), a highly-selective CB₁ agonist, could attenuate a variety of auditory effects caused by prior administration of salicylate, and potentially treat tinnitus. We recorded cortical resting-state activity, auditory-evoked cortical activity and auditory brainstem responses (ABRs), from chronically-implanted awake guinea pigs, before and after salicylate + ACEA. Salicylate-induced reductions in click-evoked ABR amplitudes were smaller in the presence of ACEA, suggesting that the ototoxic effects of salicylate were less severe. ACEA also abolished salicylate-induced changes in cortical alpha band (6–10 Hz) oscillatory activity. However, salicylate-induced increases in cortical evoked activity (suggestive of the presence of hyperacusis) were still present with salicylate + ACEA. ACEA administered alone did not induce significant changes in either ABR amplitudes or oscillatory activity, but did increase cortical evoked potentials. Furthermore, in two separate groups of non-implanted animals, we found no evidence that ACEA could reverse behavioural identification of salicylate- or noise-induced tinnitus. Together, these data suggest that while ACEA may be potentially otoprotective, selective CB₁ agonists are not effective in diminishing the presence of tinnitus or hyperacusis.

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CB₁ agonist (ACEA) effects were assessed in awake guinea pigs following salicylate.

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Salicylate-induced decreases in brainstem response amplitudes were tempered by ACEA.

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Decreases in alpha band oscillations were not evident following salicylate + ACEA.

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ACEA did not eliminate salicylate-induced increases in cortical evoked potentials.

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ACEA failed to prevent or reverse salicylate- or noise-induced tinnitus behaviour.

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.

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.

A role of CB1R in inducing θ-rhythm coordination between the gustatory and gastrointestinal insula

Anandamide (AEA) and N-oleoylethanolamine (OEA) are produced in the intestine and brain during fasting and satiety, respectively. Subsequently, AEA facilitates food intake via activation of cannabinoid type-1 receptors (CB1Rs) while OEA decreases food intake via activation of peroxisome proliferator-activated receptor-α (PPARα) and/or G-protein-coupled receptor 119 (GPR119). Neuronal activity in the gastrointestinal region of the autonomic insula (GI-Au-I) that rostrally adjoins the gustatory insula (Gu-I) increases during fasting, enhancing appetite while umami and sweet taste sensations in Gu-I enhances appetite in GI-Au-I, strongly suggesting the presence of a neural interaction between the Gu-I and GI-Au-I which changes depending on the concentrations of AEA and OEA. However, this possibility has never been investigated. In rat slice preparations, we demonstrate with voltage-sensitive dye imaging that activation of CB1Rs by AEA induces θ-rhythm oscillatory synchronization in the Gu-I which propagates into the GI-Au-I but stops at its caudal end, displaying an oscillatory coordination. The AEA-induced oscillation was abolished by a CB1R antagonist or OEA through activation of GPR119. Our results demonstrate that the neural coordination between the Gu-I and GI-Au-I is generated or suppressed by the opposing activities between CB1R and GPR119. This mechanism may be involved in the feeding behavior based on taste recognition.

Synthetic cannabinoid JWH-018 and its halogenated derivatives JWH-018-Cl and JWH-018-Br impair Novel Object Recognition in mice: Behavioral, electrophysiological and neurochemical evidence

It is well known that an impairment of learning and memory function is one of the major physiological effects caused by natural or synthetic cannabinoid consumption in rodents, nonhuman primates and in humans. JWH-018 and its halogenated derivatives (JWH-018-Cl and JWH-018-Br) are synthetic CB1/CB2 cannabinoid agonists, illegally marketed as "Spice" and "herbal blend" for their Cannabis-like psychoactive effects. In the present study the effects of acute exposure to JWH-018, JWH-018-Cl, JWH-018-Br (JWH-018-R compounds) and Δ(9)-THC (for comparison) on Novel Object Recognition test (NOR) has been investigated in mice. Moreover, to better characterize the effects of JWH-018-R compounds on memory function, in vitro electrophysiological and neurochemical studies in hippocampal preparations have been performed. JWH-018, JWH-018-Cl and JWH-018-Br dose-dependently impaired both short- and long-memory retention in mice (respectively 2 and 24 h after training session). Their effects resulted more potent respect to that evoked by Δ(9)-THC. Moreover, in vitro studies showed as JWH-018-R compounds negatively affected electrically evoked synaptic transmission, LTP and aminoacid (glutamate and GABA) release in hippocampal slices. Behavioral, electrophysiological and neurochemical effects were fully prevented by CB1 receptor antagonist AM251 pretreatment, suggesting a CB1 receptor involvement. These data support the hypothesis that synthetic JWH-018-R compounds, as Δ(9)-THC, impair cognitive function in mice by interfering with hippocampal synaptic transmission and memory mechanisms. This data outline the danger that the use and/or abuse of these synthetic cannabinoids may represent for the cognitive process in human consumer.

Cannabinoids Occlude the HIV-1 Tat-Induced Decrease in GABAergic Neurotransmission in Prefrontal Cortex Slices

In the era of combined antiretroviral therapy (cART), human immunodeficiency virus type 1 (HIV-1) is now considered a chronic disease that specifically targets the brain and causes HIV-1-associated neurocognitive disorders (HAND). Endocannabinoids exhibit neuroprotective and anti-inflammatory properties in several central nervous system (CNS) disease models, but their effects in HAND are poorly understood. To address this issue, whole-cell recordings were performed on young (14-24 day old) C57BL/6J mice. We investigated the actions of the synthetic cannabinoid WIN55,212-2 (1 μM) and the endocannabinoid N-arachidonoyl ethanolamine (anandamide; AEA, 1 μM) in the presence of HIV-1 Tat on GABAergic neurotransmission in mouse prefrontal cortex (PFC) slices. We found a Tat concentration-dependent (5-50 nM) decrease in the frequency and amplitude of miniature inhibitory postsynaptic currents (mIPSCs). The cannabinoid 1 receptor (CB1R) antagonist rimonabant (1 μM) and zero extracellular calcium prevented the significant Tat-induced decrease in mIPSCs. Further, bath-applied WIN55,212-2 or AEA by itself, significantly decreased the frequency, but not amplitude of mIPSCs and/or spontaneous IPSCs (sIPSCs), and occluded a further downregulation of IPSCs by Tat. Pretreatment with rimonabant but not the CB2R antagonist AM630 (1 μM) prevented the WIN55,212-2- and AEA-induced decrease in IPSCs frequency without any further Tat effect. Results indicated a Tat-induced decrease in GABAergic neurotransmission, which was occluded by cannabinoids via a CB1R-related mechanism. Understanding the relationship between Tat toxicity and endocannabinoid signaling has the potential to identify novel therapeutic interventions to benefit individuals suffering from HAND and other cognitive impairments.

Ultrastructural evidence for synaptic contacts between cortical noradrenergic afferents and endocannabinoid-synthesizing post-synaptic neurons

Endocannabinoids (eCBs) are involved in a myriad of physiological processes that are mediated through the activation of cannabinoid receptors, which are ubiquitously distributed within the nervous system. One neurochemical target at which cannabinoids interact to have global effects on behavior is brain noradrenergic circuitry. We, and others, have previously shown that CB type 1 receptors (CB1r) are positioned to pre-synaptically modulate norepinephrine (NE) release in the rat frontal cortex (FC). Diacylglycerol lipase (DGL) is a key enzyme in the biosynthesis of the endocannabinoid 2-arachidonoylglycerol (2-AG). While DGL-α is expressed in the FC in the rat brain, it is not known whether noradrenergic afferents target neurons expressing synthesizing enzymes for the endocannabinoid, 2-AG. In the present study, we employed high-resolution neuroanatomical approaches to better define cellular sites for interactions between noradrenergic afferents and FC neurons expressing DGL-α. Immunofluorescence microscopy showed close appositions between processes containing the norepinephrine transporter (NET) or dopamine-β-hydroxylase (DβH) and cortical neurons expressing DGL-α-immunoreactivity. Ultrastructural analysis using immunogold-silver labeling for DGL-α and immunoperoxidase labeling for NET or DβH confirmed that NET-labeled axon terminals were directly apposed to FC somata and dendritic processes that exhibited DGL-α-immunoreactivity. Finally, tissue sections were processed for immunohistochemical detection of DGL-α, CB1r and DβH. Triple label immunofluorescence revealed that CB1r and DβH were co-localized in common cellular profiles and these were in close association with DGL-α. Taken together, these data provide anatomical evidence for direct synaptic associations between noradrenergic afferents and cortical neurons exhibiting endocannabinoid synthesizing machinery.

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.

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

The endogenous cannabinoid (endocannabinoid) system is an important regulator of synaptic function. Endocannabinoids acutely modulate inhibitory and excitatory transmission, and also mediate long-term depression at GABAergic and glutamatergic synapses. Typically, endocannabinoid synthesis and release is stimulated by depolarization-induced calcium influx and/or activation of phospholipase-C (PLC) signaling triggered by mGluR activation. Recently it has been shown that brain-derived neurotrophic factor (BDNF) can also induce endocannabinoid release. Although there is growing evidence for cross-talk between BDNF and endocannabinoid signaling, little is known about the functional relevance of these interactions. In the present studies, we examined BDNF - endocannabinoid interactions in regulating activity-dependent long-term depression at inhibitory synapses (iLTD). We found that theta burst stimulation (TBS) in layer 2/3 of mouse somatosensory cortical slices can induce a form of endocannabinoid-mediated iLTD that is independent of metabotropic glutamate receptor (mGluR) activation. This endocannabinoid-dependent iLTD, however, requires endogenous BDNF-trkB signaling, as it is blocked by a trk tyrosine kinase inhibitor and by a trkB receptor antagonist, and also requires activation of diacylglycerol lipase (DAG-lipase, DGL). In addition, endocannabinoid-mediated iLTD can be induced by combining a subthreshold concentration of exogenous BDNF with weak TBS stimulation that by itself is insufficient to induce iLTD. Taken together, our results suggest that TBS can induce the release of endogenous BDNF, which triggers DGL-dependent endocannabinoid mobilization and cannabinoid receptor-dependent iLTD at layer 2/3 cortical synapses.

Involvement of the cannabinoid CB1 receptor in modulation of dopamine output in the prefrontal cortex associated with food restriction in rats

Increase in dopamine output on corticolimbic structures, such as medial prefrontal cortex (mPFC) and nucleus accumbens, has been related to reward effects associated with palatable food or food presentation after a fasting period. The endocannabinoid system regulates feeding behavior through a modulatory action on different neurotransmitter systems, including the dopaminergic system. To elucidate the involvement of type 1 cannabinoid receptors in the regulation of dopamine output in the mPFC associated with feeding in hungry rats, we restricted the food availability to a 2-h period daily for 3 weeks. In food-restricted rats the extracellular dopamine concentration in the mPFC increased starting 80 min before food presentation and returned to baseline after food removal. These changes were attenuated in animals treated with the CB1 receptor antagonist SR141716. To better understand how food restriction can change the response of mesocortical dopaminergic neurons, we studied several components of the neuronal circuit that regulates dopamine output in the mPFC. Patch-clamp experiments revealed that the inhibitory effect of the CB1 receptor agonist WIN 55,212-2 on GABAergic sIPSC frequency was diminished in mPFC neurons of FR compared to fed ad libitum rats. The basal sIPSC frequency resulted reduced in mPFC neurons of food-restricted rats, suggestive of an altered regulation of presynaptic GABA release; these changes were accompanied by an enhanced excitability of mPFC and ventral tegmental area neurons. Finally, type 1 cannabinoid receptor expression in the mPFC was reduced in food-restricted rats. Together, our data support an involvement of the endocannabinoid system in regulation of dopamine release in the mPFC through changes in GABA inhibitory synapses and suggest that the emphasized feeding-associated increase in dopamine output in the mPFC of food-restricted rats might be correlated with an altered expression and function of type 1 cannabinoid receptor in this brain region.

Endocannabinoid modulation of cortical up-states and NREM sleep

Up-/down-state transitions are a form of network activity observed when sensory input into the cortex is diminished such as during non-REM sleep. Up-states emerge from coordinated signaling between glutamatergic and GABAergic synapses and are modulated by systems that affect the balance between inhibition and excitation. We hypothesized that the endocannabinoid (EC) system, a neuromodulatory system intrinsic to the cortical microcircuitry, is an important regulator of up-states and sleep. To test this hypothesis, up-states were recorded from layer V/VI pyramidal neurons in organotypic cultures of wild-type or CB1R knockout (KO) mouse prefrontal cortex. Activation of the cannabinoid 1 receptor (CB1) with exogenous agonists or by blocking metabolism of endocannabinoids, anandamide or 2-arachidonoyl glycerol, increased up-state amplitude and facilitated action potential discharge during up-states. The CB1 agonist also produced a layer II/III-selective reduction in synaptic GABAergic signaling that may underlie its effects on up-state amplitude and spiking. Application of CB1 antagonists revealed that an endogenous EC tone regulates up-state duration. Paradoxically, the duration of up-states in CB1 KO cultures was increased suggesting that chronic absence of EC signaling alters cortical activity. Consistent with increased cortical excitability, CB1 KO mice exhibited increased wakefulness as a result of reduced NREM sleep and NREM bout duration. Under baseline conditions, NREM delta (0.5–4 Hz) power was not different in CB1 KO mice, but during recovery from forced sleep deprivation, KO mice had reduced NREM delta power and increased sleep fragmentation. Overall, these findings demonstrate that the EC system actively regulates cortical up-states and important features of NREM sleep such as its duration and low frequency cortical oscillations.

Pathological plasticity in fragile X syndrome

Deficits in neuronal plasticity are common hallmarks of many neurodevelopmental disorders. In the case of fragile-X syndrome (FXS), disruption in the function of a single gene, FMR1, results in a variety of neurological consequences directly related to problems with the development, maintenance, and capacity of plastic neuronal networks. In this paper, we discuss current research illustrating the mechanisms underlying plasticity deficits in FXS. These processes include synaptic, cell intrinsic, and homeostatic mechanisms both dependent on and independent of abnormal metabotropic glutamate receptor transmission. We place particular emphasis on how identified deficits may play a role in developmental critical periods to produce neuronal networks with permanently decreased capacity to dynamically respond to changes in activity central to learning, memory, and cognition in patients with FXS. Characterizing early developmental deficits in plasticity is fundamental to develop therapies that not only treat symptoms but also minimize the developmental pathology of the disease.

Association of Cannabis Use during Adolescence, Prefrontal CB1 Receptor Signaling, and Schizophrenia

The cannabinoid receptor 1 (CB1R) is the G-protein coupled receptor responsible for the majority of the endocannabinoid signaling in the human brain. It is widely distributed in the limbic system, basal ganglia, and cerebellum, which are areas responsible for cognition, memory, and motor control. Because of this widespread distribution, it is not surprising that drugs that activate CB1R have expected behavioral outcomes consistent with dysregulated signaling from these areas (e.g., memory loss, cognitive deficits, etc). In the context of this review, we present evidence for the role of CB1R signaling in the prefrontal cortex (PFC), an area involved in executive functions, with emphasis on the developmental regulation of CB1R signaling in the acquisition of mature PFC function. We further hypothesize how alterations in CB1R signaling specifically during adolescent maturation might confer liability to psychiatric disorders.

Exogenous and endogenous cannabinoids suppress inhibitory neurotransmission in the human neocortex

Activation of CB(1) receptors on axon terminals by exogenous cannabinoids (eg, Δ(9)-tetrahydrocannabinol) and by endogenous cannabinoids (endocannabinoids) released by postsynaptic neurons leads to presynaptic inhibition of neurotransmission. The aim of this study was to characterize the effect of cannabinoids on GABAergic synaptic transmission in the human neocortex. Brain slices were prepared from neocortical tissues surgically removed to eliminate epileptogenic foci. Spontaneous GABAergic inhibitory postsynaptic currents (sIPSCs) were recorded in putative pyramidal neurons using patch-clamp techniques. To enhance the activity of cannabinoid-sensitive presynaptic axons, muscarinic receptors were continuously stimulated by carbachol. The synthetic cannabinoid receptor agonist WIN55212-2 decreased the cumulative amplitude of sIPSCs. The CB(1) antagonist rimonabant prevented this effect, verifying the involvement of CB(1) receptors. WIN55212-2 decreased the frequency of miniature IPSCs (mIPSCs) recorded in the presence of tetrodotoxin, but did not change their amplitude, indicating that the neurotransmission was inhibited presynaptically. Depolarization of postsynaptic pyramidal neurons induced a suppression of sIPSCs. As rimonabant prevented this suppression, it is very likely that it was due to endocannabinods acting on CB(1) receptors. This is the first demonstration that an exogenous cannabinoid inhibits synaptic transmission in the human neocortex and that endocannabinoids released by postsynaptic neurons suppress synaptic transmission in the human brain. Interferences of cannabinoid agonists and antagonists with synaptic transmission in the cortex may explain the cognitive and memory deficits elicited by these drugs.

Cannabinoid receptor expression at the MNTB-LSO synapse in developing rats

The organization of developing auditory circuits depends on the elimination of aberrant connections and strengthening of appropriate ones. Endocannabinoid mediated plasticity is a proposed mechanism for this refinement. Here we investigated for the anatomical presence of cannabinoid receptors (CB1R) in the lateral superior olive (LSO) and medial nucleus of the trapezoid body (MNTB) of developing rats. We found that CB1R is present within the LSO and that it colocalized with vesicular glutamate transporter (VGLUT3), a presynaptic marker for MTNB terminals. Both before (P5) and around hearing onset (P12), the expression levels of CB1R were higher in the lateral limb of the LSO than in the medial limb. These results suggest that endocannabinoid signaling can modulate the strength of the developing MNTB-LSO synapse.

Endocannabinoid signaling in neural plasticity

Plasticity refers to a physiologically measured change that may last for short or long periods of time. Endocannabinoids (ECBs) are prevalent throughout most of the brain, and modulate synaptic transmission in many ways. This chapter will focus on the roles of ECBs in neural plasticity in the mammalian brain. The topics covered can be divided loosely into two themes: how ECBs regulate synaptic plasticity, and how ECBs' actions themselves are regulated by neuronal activity. Because ECBs regulate synaptic plasticity, the modifiability of ECB mobilization constitutes a form of "metaplasticity" (as reported by Abraham and Bear (Trends Neurosci 19:126-130, 1996)), i.e., an upstream process that determines the nature and extent of synaptic plasticity. Many of their basic functions are still being discovered, and while there is consensus on large issues, many points of divergence exist as well. This chapter concentrates on developments in the roles of ECBs in synaptic plasticity that have come to light since the major review by Chevaleyre et al. (Annu Rev Neurosci 29:37-76, 2006).

Maternal exposure to the CB1 cannabinoid agonist WIN 55212-2 produces robust changes in motor function and intrinsic electrophysiological properties of cerebellar Purkinje neurons in rat offspring

The cerebellum, which controls coordinated and rapid movements, is a potential target for the deleterious effects of drugs of abuse including cannabis (i.e. marijuana, cannabinoids). Prenatal exposure to cannabinoids has been documented to cause abnormalities in motor and cognitive development, but the exact mechanism of this effect at the cellular level has not been fully elucidated. Previous studies indicate that cannabinoids are capable of modulating synaptic neurotransmission. In addition to altering synaptic activity, cannabinoid exposure may also change intrinsic neuronal properties. In the present study several different approaches including behavioral assays, extracellular field potential recordings and whole-cell patch clamp recordings, were used to address whether maternal exposure to the CB1 cannabinoid receptor agonist WIN 55-212-2 (WIN) affects the intrinsic electrophysiological properties of Purkinje neurons. WIN treatment of pregnant rats produced a significant decrease in the rearing frequency, total distance moved and mobility of the offspring, but significantly increased the time of the righting reflex, the grooming frequency and immobility. Neuromotor function, as assessed in the grip test and balance beam test, was also significantly impaired in prenatally WIN-treated group. Prenatal exposure to WIN increased the amplitude of population spikes (PS) recorded from the cerebellar Purkinje cell layer of offspring following synaptic blockage. WIN treatment of pregnant rats also profoundly affected the intrinsic properties of Purkinje neurons in offspring. This treatment increased the firing regularity, firing frequency, amplitude of afterhyperpolarization (AHP), the peak amplitude of action potential and the first spike latency, but decreased significantly the time to peak and duration of action potentials, the instantaneous firing frequency, the rate of rebound action potential and the voltage "sag" ratio. These results raise the possibility that maternal exposure to cannabinoids may profoundly affect the intrinsic membrane properties of cerebellar Purkinje neurons of offspring by altering the membrane excitability through modulation of intrinsic ion channels.

Endocannabinoid modulation of hyperaemia evoked by physiologically relevant stimuli in the rat primary somatosensory cortex

BACKGROUND AND PURPOSE

In vitro studies demonstrate that cannabinoid CB(1) receptors subserve activity-dependent suppression of inhibition in the neocortex. To examine this mechanism in vivo, we assessed the effects of local changes in CB(1) receptor activity on somatosensory cortex neuronal activation by whisker movement in rats.

EXPERIMENTAL APPROACH

Laser Doppler flowmetry and c-Fos immunohistochemistry were used to measure changes in local blood flow and neuronal activation, respectively. All drugs were applied directly to the cranium above the whisker barrel fields of the primary somatosensory cortex.

KEY RESULTS

The CB(1) receptor agonist WIN55212-2 potentiated the hyperaemia induced by whisker movement and this potentiation was occluded by bicuculline. The CB(1) receptor antagonists, rimonabant and AM251, inhibited hyperaemic responses to whisker movement; indicating that activation of endogenous CB(1) receptors increased during whisker movement. Whisker movement-induced expression of c-Fos protein in neurons of the whisker barrel cortex was inhibited by rimonabant. Movement of the whiskers increased the 2-arachidonoylglycerol content in the contralateral, compared to the ipsilateral, sensory cortex.

CONCLUSIONS AND IMPLICATIONS

These results support the hypothesis that endocannabinoid signalling is recruited during physiologically relevant activation of the sensory cortex. These data support the hypothesis that the primary effect of CB(1) receptor activation within the activated whisker barrel cortex is to inhibit GABA release, resulting in disinhibition of neuronal activation. These studies provide physiological data involving endocannabinoid signalling in activity-dependent regulation of neuronal activation and provide a mechanistic basis for the effects of cannabis use on sensory processing in humans.

BDNF evokes release of endogenous cannabinoids at layer 2/3 inhibitory synapses in the neocortex

The neurotrophin brain-derived neurotrophic factor (BDNF) is a potent regulator of inhibitory synaptic transmission, although the locus of this effect and the underlying mechanisms are controversial. We explored a potential interaction between BDNF and endogenous cannabinoid (endocannabinoid) signaling because activation of type 1 cannabinoid (CB1) receptors potently regulates γ-aminobutyric acid (GABA) release and both trkB tyrosine kinase receptors and CB1 receptors are highly expressed at synapses in neocortical layer 2/3. Here, we found that the effects of BDNF at inhibitory cortical synapses are mediated by the release of endocannabinoids acting retrogradely at presynaptic CB1 receptors. Specifically, acute application of BDNF rapidly reduced the amplitude of inhibitory postsynaptic currents (IPSCs) via postsynaptic trkB receptor activation because intracellular delivery of the tyrosine kinase inhibitor K252a completely blocked the BDNF effect. Although triggered by postsynaptic trkB activation, BDNF exposure decreased presynaptic release probability, as evidenced by increases in the paired-pulse ratio and coefficient of variation of evoked responses. In addition, BDNF decreased the frequency but not the amplitude of action potential-independent miniature IPSCs and BDNF did not alter the postsynaptic response to locally applied GABA. These results suggest that BDNF induces the release of a retrograde messenger from the postsynaptic cell that regulates presynaptic neurotransmitter release. Consistent with a role for endocannabinoids as the retrograde signal, the effect of BDNF on IPSCs was blocked by CB1 receptor antagonists and was occluded by a cannabinoid receptor agonist. Furthermore, inhibiting endocannabinoid synthesis or transport also disrupted the BDNF effect, implicating postsynaptic endocannabinoid release triggered by BDNF.

Cannabinoids for the treatment of schizophrenia? A balanced neurochemical framework for both adverse and therapeutic effects of cannabis use

Recent studies have found that cannabinoids may improve neuropsychological performance, ameliorate negative symptoms, and have antipsychotic properties for a subgroup of the schizophrenia population. These findings are in contrast to the longstanding history of adverse consequences of cannabis use, predominantly on the positive symptoms, and a balanced neurochemical basis for these opposing views is lacking. This paper details a review of the neurobiological substrates of schizophrenia and the neurochemical effects of cannabis use in the normal population, in both cortical (in particular prefrontal) and subcortical brain regions. The aim of this paper is to provide a holistic neurochemical framework in which to understand how cannabinoids may impair, or indeed, serve to ameliorate the positive and negative symptoms as well as cognitive impairment. Directions in which future research can proceed to resolve the discrepancies are briefly discussed.

Dopaminergic modulation of endocannabinoid-mediated plasticity at GABAergic synapses in the prefrontal cortex

Similar to dopamine (DA), cannabinoids strongly influence prefrontal cortical functions, such as working memory, emotional learning, and sensory perception. Although endogenous cannabinoid receptors (CB(1)Rs) are abundantly expressed in the prefrontal cortex (PFC), very little is known about endocannabinoid (eCB) signaling in this brain region. Recent behavioral and electrophysiological evidence has suggested a functional interplay between the dopamine and cannabinoid receptor systems, although the cellular mechanisms underlying this interaction remain to be elucidated. We examined this issue by combining neuroanatomical and electrophysiological techniques in PFC of rats and mice (both genders). Using immunoelectron microscopy, we show that CB(1)Rs and dopamine type 2 receptors (D(2)Rs) colocalize at terminals of symmetrical, presumably GABAergic, synapses in the PFC. Indeed, activation of either receptor can suppress GABA release onto layer 5 pyramidal cells. Furthermore, coactivation of both receptors via repetitive afferent stimulation triggers eCB-mediated long-term depression of inhibitory transmission (I-LTD). This I-LTD is heterosynaptic in nature, requiring glutamate release to activate group I metabotropic glutamate receptors. D(2)Rs most likely facilitate eCB signaling at the presynaptic site as disrupting postsynaptic D(2)R signaling does not diminish I-LTD. Facilitation of eCB-LTD may be one mechanism by which DA modulates neuronal activity in the PFC and regulates PFC-mediated behavior in vivo.

Modulation of NMDA and AMPA-mediated synaptic transmission by CB1 receptors in frontal cortical pyramidal cells

Although the endogenous cannabinoid system modulates a variety of physiological and pharmacological processes, the specific role of cannabinoid CB1 receptors in the modulation of glutamatergic neurotransmission and neural plasticity is not well understood. Using whole-cell patch clamp recording techniques, evoked or spontaneous excitatory postsynaptic currents (eEPSCs or sEPSCs) were recorded from visualized, layer II/III pyramidal cells in frontal cortical slices from rat brain. Bath application of the CB1 receptor agonist, WIN 55212-2 (WIN), reduced the amplitude of NMDA receptor-mediated EPSCs in a concentration-dependent manner. When co-applied with the specific CB1 antagonists, AM251 or AM281, WIN did not suppress NMDA receptor-mediated EPSCs. WIN also reduced the amplitude of evoked AMPA receptor-mediated EPSCs, an effect that was also reversed by AM251. Both the frequency and amplitude of spontaneous AMPA receptor-mediated EPSCs were significantly reduced by WIN. In contrast, WIN reduced the frequency, but not the amplitude of miniature EPSCs, suggesting that the suppression of glutamatergic activity by CB1 receptors in the frontal neocortex is mediated by a presynaptic mechanism. Taken together, these data indicate a critical role for endocannabinoid signaling in the regulation of excitatory synaptic transmission in frontal neocortex, and suggest a possible neuronal mechanism whereby THC regulates cortical function.

Endocannabinoid signaling is required for development and critical period plasticity of the whisker map in somatosensory cortex

Type 1 cannabinoid (CB1) receptors mediate widespread synaptic plasticity, but how this contributes to systems-level plasticity and development in vivo is unclear. We tested whether CB1 signaling is required for development and plasticity of the whisker map in rat somatosensory cortex. Treatment with the CB1 antagonist AM251 during an early critical period for layer (L) 2/3 development (beginning postnatal day [P] 12-16) disrupted whisker map development, leading to inappropriate whisker tuning in L2/3 column edges and a blurred map. Early AM251 treatment also prevented experience-dependent plasticity in L2/3, including deprivation-induced synapse weakening and weakening of deprived whisker responses. CB1 blockade after P25 did not disrupt map development or plasticity. AM251 had no acute effect on sensory-evoked spiking and only modestly affected field potentials, suggesting that plasticity effects were not secondary to gross activity changes. These findings implicate CB1-dependent plasticity in systems-level development and early postnatal plasticity of the whisker map.

Endocannabinoid-dependent homeostatic regulation of inhibitory synapses by miniature excitatory synaptic activities

Homeostatic regulation of synaptic strength in response to persistent changes of neuronal activity plays an important role in maintaining the overall level of circuit activity within a normal range. Absence of miniature EPSCs (mEPSCs) for a few hours is known to cause upregulation of excitatory synaptic strength, suggesting that mEPSCs contribute to the maintenance of excitatory synaptic functions. In the present study, we found that the absence of mEPSCs for 1-3 h also resulted in homeostatic suppression of presynaptic functions of inhibitory synapses in acute cortical slices from juvenile rats, as suggested by the reduced frequency (but not amplitude) of miniature IPSCs (mIPSCs) as well as the reduced amplitude of IPSCs. This homeostatic regulation depended on endocannabinoid (eCB) signaling, because blockade of either the activation of cannabinoid type-1 receptors (CB1Rs) or the synthesis of its endogenous ligand 2-arachidonoylglycerol (2-AG) abolished the suppression of inhibitory synapses caused by the absence of mEPSCs. Blockade of group I metabotropic glutamate receptors (mGluR-I) also abolished the suppression of inhibitory synapses, consistent with the mGluR-I requirement for eCB synthesis and release in cortical synapses. Furthermore, this homeostatic regulation also required eukaryotic elongation factor-2 (eEF2)-dependent protein synthesis, but not gene transcription. Activation of eEF2 alone was sufficient to suppress the mIPSC frequency, an effect abolished by inhibiting CB1Rs. Thus, mEPSCs contribute to the maintenance of inhibitory synaptic function and the absence of mEPSCs results in presynaptic suppression of inhibitory synapses via protein synthesis-dependent elevation of eCB signaling.

The endocannabinoid system and the regulation of neural development: potential implications in psychiatric disorders

During brain development, functional neurogenesis is achieved by the concerted action of various steps that include the expansion of progenitor cells, neuronal specification, and establishment of appropriate synapses. Brain patterning and regionalization is regulated by a variety of extracellular signals and morphogens that, together with neuronal activity, orchestrate and regulate progenitor proliferation, differentiation, and neuronal maturation. In the adult brain, CB(1) cannabinoid receptors are expressed at very high levels in selective areas and are engaged by endocannabinoids, which act as retrograde messengers controlling neuronal function and preventing excessive synaptic activity. In addition, the endocannabinoid system is present at early developmental stages of nervous system formation. Recent studies have provided novel information on the role of this endogenous neuromodulatory system in the control of neuronal specification and maturation. Thus, cannabinoid receptors and locally produced endocannabinoids regulate neural progenitor proliferation and pyramidal specification of projecting neurons. CB(1) receptors also control axonal navigation, migration, and positioning of interneurons and excitatory neurons. Loss of function studies by genetic ablation or pharmacological blockade of CB(1) receptors interferes with long-range subcortical projections and, likewise, prenatal cannabinoid exposure induces different functional alterations in the adult brain. Potential implications of these new findings, such as the participation of the endocannabinoid system in the pathogenesis of neurodevelopmental disorders (e.g., schizophrenia) and the regulation of neurogenesis in brain depression, are discussed herein.

Presynaptic and postsynaptic NMDA receptors mediate distinct effects of brain-derived neurotrophic factor on synaptic transmission

In addition to its effects on neuronal survival and differentiation, brain-derived neurotrophic factor (BDNF) plays an important role in modulating synaptic transmission and plasticity in many brain areas, most notably the neocortex and hippocampus. These effects may underlie a role for BDNF in learning and memory as well as developmental plasticity. Consistent with localization of the tropomyosin-related kinase B receptor to both sides of the synapse, BDNF appears to have pre- and postsynaptic effects, but the underlying cellular mechanisms are unclear and it is not known whether pre- and postsynaptic modulations by BDNF occur simultaneously. To address these issues, we recorded dual-component (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [AMPA] and N-methyl-D-aspartate [NMDA]) miniature excitatory postsynaptic currents (mEPSCs) from cortical and hippocampal pyramidal neurons and dentate gyrus granule cells from acute brain slices. BDNF had no effect on the fast component of mEPSC decay or on the peak amplitude, suggesting that BDNF did not modulate postsynaptic AMPA receptors, although BDNF rapidly modulated NMDA receptors, as seen by an enhancement of the slow component of mEPSC decay that was prevented by blocking postsynaptic NMDA receptors. At the same time, BDNF acted presynaptically to enhance mEPSC frequency. Surprisingly, the effect on frequency was also NMDA receptor dependent, but required activation of presynaptic, not postsynaptic, NMDA receptors. BDNF also enhanced action potential-dependent glutamate release via presynaptic NMDA receptors, an effect that was unmasked when voltage-gated calcium channels were partially inhibited. Our results indicate that BDNF acutely modulates presynaptic release and postsynaptic responsiveness through simultaneous effects on pre- and postsynaptic NMDA receptors.

Immunohistochemical localization of CB1 cannabinoid receptors in frontal cortex and related limbic areas in obese Zucker rats: effects of chronic fluoxetine treatment

In the present study, we report on the application of two specific polyclonal antibodies to different intracellular domains of the CB1 cannabinoid receptor to define the expression of the neural CB1 cannabinoid receptor at the histochemical level in frontal cortex and related limbic areas of the obese Zucker rats. Higher levels of CB1 receptor expression in frontal, cingulated and piriform cortex, without differences in temporal, parietal and occipital cortex, were observed in obese Zucker rats, with respect to their lean littermates. CB1 phosphorylated receptor (CB1-P) levels were also higher in frontal, temporal, parietal and occipital cortex in obese rats with respect to lean controls. Potential involvement of brain cortical CB1 cannabinoid receptors in the long-term effects of fluoxetine was studied. Experimental animals were administered with fluoxetine (10 mg/kg, i.p.) daily for 3 weeks, whereas the control group was given 0.9% NaCl solution. In obese Zucker rats, a significant decrease in CB1 receptor levels, measured by western blot, was observed in brain cortex after fluoxetine treatment. Immunostaining for CB1 receptor expression was also carried out, showing a significant decrease in the density of neural cells positive for CB1 receptor in frontal, cingulate and piriform cortex, without changes in parietal, temporal and occipital regions. Regional prosencephalic immunostaining for CB1-P receptor level showed a significant decrease in the density of stained neural cells in frontal, temporal and parietal cortex, without changes in cingulated, piriform and occipital cortex. These results suggest the involvement of endocannabinoid system in the chronic effects of fluoxetine, especially in the frontal cortex.

Dendritic excitability and synaptic plasticity

Most synaptic inputs are made onto the dendritic tree. Recent work has shown that dendrites play an active role in transforming synaptic input into neuronal output and in defining the relationships between active synapses. In this review, we discuss how these dendritic properties influence the rules governing the induction of synaptic plasticity. We argue that the location of synapses in the dendritic tree, and the type of dendritic excitability associated with each synapse, play decisive roles in determining the plastic properties of that synapse. Furthermore, since the electrical properties of the dendritic tree are not static, but can be altered by neuromodulators and by synaptic activity itself, we discuss how learning rules may be dynamically shaped by tuning dendritic function. We conclude by describing how this reciprocal relationship between plasticity of dendritic excitability and synaptic plasticity has changed our view of information processing and memory storage in neuronal networks.

Cannabinoid sensitivity and synaptic properties of 2 GABAergic networks in the neocortex

Distinct networks of gamma-aminobutyric acidergic interneurons connected by electrical synapses can promote different patterns of activity in the neocortex. Cannabinoids affect memory and cognition by powerfully modulating a subset of inhibitory synapses; however, very little is known about the synaptic properties of the cannabinoid receptor-expressing neurons (CB(1)-positive irregular spiking [CB(1)-IS]) in the neocortex. Using paired recordings in neocortical slices, we 1st report here that synapses of CB(1)-IS cells, but not synapses of fast-spiking (FS) cells, are suppressed by release of endocannabinoids from pyramidal neurons. CB(1)-IS synapses were characterized by a very high failure rate that contrasted with the high reliability of FS synapses. Furthermore, CB(1)-IS cells received excitatory inputs less frequently compared with FS cells and made significantly less frequent inhibitory contacts onto local pyramids. These distinct synaptic properties together with their characteristic irregular firing suggest that CB(1)-IS cells play different role in neocortical function than that of FS cells. Thus, whereas the synaptic properties of FS cells can allow them to impose high-frequency rhythmic oscillatory activity, those of CB(1)-IS cells may lead to disruption of fast rhythmic oscillations. Our results suggest that activity-dependent release of cannabinoids, by blocking CB(1)-IS synapses, may alter the role of inhibition in neocortical circuits.

Cannabinoids elicit antidepressant-like behavior and activate serotonergic neurons through the medial prefrontal cortex

Preclinical and clinical studies show that cannabis modulates mood and possesses antidepressant-like properties, mediated by the agonistic activity of cannabinoids on central CB1 receptors (CB1Rs). The action of CB1R agonists on the serotonin (5-HT) system, the major transmitter system involved in mood control and implicated in the mechanism of action of antidepressants, remains however poorly understood. In this study, we demonstrated that, at low doses, the CB1R agonist WIN55,212-2 [R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)]pyrrolo[1,2,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl) methanone mesylate] exerts potent antidepressant-like properties in the rat forced-swim test (FST). This effect is CB1R dependent because it was blocked by the CB1R antagonist rimonabant and is 5-HT mediated because it was abolished by pretreatment with the 5-HT-depleting agent parachlorophenylalanine. Then, using in vivo electrophysiology, we showed that low doses of WIN55,212-2 dose dependently enhanced dorsal raphe nucleus 5-HT neuronal activity through a CB1R-dependent mechanism. Conversely, high doses of WIN55,212-2 were ineffective in the FST and decreased 5-HT neuronal activity through a CB1R-independent mechanism. The CB1R agonist-induced enhancement of 5-HT neuronal activity was abolished by total or medial prefrontocortical, but not by lateral prefrontocortical, transection. Furthermore, 5-HT neuronal activity was enhanced by the local microinjection of WIN55,212-2 into the ventromedial prefrontal cortex (mPFCv) but not by the local microinjection of WIN55,212-2 into the lateral prefrontal cortex. Similarly, the microinjection of WIN55,212-2 into the mPFCv produced a CB1R-dependent antidepressant-like effect in the FST. These results demonstrate that CB1R agonists possess antidepressant-like properties and modulate 5-HT neuronal activity via the mPFCv.

Lack of depolarization-induced suppression of inhibition (DSI) in layer 2/3 interneurons that receive cannabinoid-sensitive inhibitory inputs

In layer 2/3 of neocortex, brief trains of action potentials in pyramidal neurons (PNs) induce the mobilization of endogenous cannabinoids (eCBs), resulting in a depression of GABA release from the terminals of inhibitory interneurons (INs). This depolarization-induced suppression of inhibition (DSI) is mediated by activation of the type 1 cannabinoid receptor (CB1) on presynaptic terminals of a subset of INs. However, it is not clear whether CB1 receptors are also expressed at synapses between INs, and whether INs can release eCBs in response to depolarization. In the present studies, brain slices containing somatosensory cortex were prepared from 14- to 21-day-old CD-1 mice. Whole cell recordings were obtained from layer 2/3 PNs and from INs classified as regular spiking nonpyramidal, irregular spiking, or fast spiking. For all three classes of INs, the cannabinoid agonist WIN55,212-2 suppressed inhibitory synaptic activity, similar to the effect seen in PNs. In addition, trains of action potentials in PNs resulted in significant DSI. In INs, however, DSI was not seen in any cell type, even with prolonged high-frequency spike trains that produced calcium increases comparable to that seen with DSI induction in PNs. In addition, blocking eCB reuptake with AM404, which enhanced DSI in PNs, failed to unmask any DSI in INs. Thus the lack of DSI in INs does not appear to be due to an insufficient increase in intracellular calcium or enhanced reuptake. These results suggest that layer 2/3 INs receive CB1-expressing inhibitory inputs, but that eCBs are not released by these INs.

Supraspinal modulation of pain by cannabinoids: the role of GABA and glutamate

Recent physiological, pharmacological and anatomical studies provide evidence that one of the main roles of the endocannabinoid system in the brain is the regulation of gamma-aminobutyric acid (GABA) and glutamate release. This article aims to review this evidence in the context of its implications for pain. We first provide a brief overview of supraspinal regulation of nociception, followed by a review of the evidence that the brain's endocannabinoid system modulates nociception. We look in detail at regulation of supraspinal GABAergic and glutamatergic neurons by the endocannabinoid system and by exogenously administered cannabinoids. Finally, we review the evidence that cannabinoid-mediated modulation of pain involves modulation of GABAergic and glutamatergic neurotransmission in key brain regions.

Functional CB1 Receptors Are Broadly Expressed in Neocortical GABAergic and Glutamatergic Neurons

The cannabinoid receptor CB1 is found in abundance in brain neurons, whereas CB2 is essentially expressed outside the brain. In the neocortex, CB1 is observed predominantly on large cholecystokinin (CCK)-expressing interneurons. However, physiological evidence suggests that functional CB1 are present on other neocortical neuronal types. We investigated the expression of CB1 and CB2 in identified neurons of rat neocortical slices using single-cell RT-PCR. We found that 63% of somatostatin (SST)-expressing and 69% of vasoactive intestinal polypeptide (VIP)-expressing interneurons co-expressed CB1. As much as 49% of pyramidal neurons expressed CB1. In contrast, CB2 was observed in a small proportion of neocortical neurons. We performed whole cell recordings of pyramidal neurons to corroborate our molecular findings. Inhibitory postsynaptic currents (IPSCs) induced by a mixed muscarinic/nicotinic cholinergic agonist showed depolarization-induced suppression of inhibition and were decreased by the CB1 agonist WIN-55212-2 (WIN-2), suggesting that interneurons excited by cholinergic agonists (mainly SST and VIP neurons) possess CB1. IPSCs elicited by a nicotinic receptor agonist were also reduced in the presence of WIN-2, suggesting that neurons excited by nicotinic agonists (mainly VIP neurons) indeed possess CB1. WIN-2 largely decreased excitatory postsynaptic currents evoked by intracortical electrical stimulation, pointing at the presence of CB1 on glutamatergic pyramidal neurons. All WIN-2 effects were strongly reduced by the CB1 antagonist AM 251. We conclude that CB1 is expressed in various neocortical neuronal populations, including glutamatergic neurons. Our combined molecular and physiological data suggest that CB1 widely mediates endocannabinoid effects on glutamatergic and GABAergic transmission to modulate cortical networks.

Reduced presynaptic efficiency of excitatory synaptic transmission impairs LTP in the visual cortex of BDNF-heterozygous mice

The neurotrophin brain-derived neurotrophic factor (BDNF) plays an important role in neuronal survival, axonal and dendritic growth and synapse formation. BDNF has also been reported to mediate visual cortex plasticity. Here we studied the cellular mechanisms of BDNF-mediated changes in synaptic plasticity, excitatory synaptic transmission and long-term potentiation (LTP) in the visual cortex of heterozygous BDNF-knockout mice (BDNF(+/-)). Patch-clamp recordings in slices showed an approximately 50% reduction in the frequency of miniature excitatory postsynaptic currents (mEPSCs) compared to wild-type animals, in the absence of changes in mEPSC amplitudes. A presynaptic impairment of excitatory synapses from BDNF(+/-) mice was further indicated by decreased paired-pulse ratio and faster synaptic fatigue upon prolonged repetitive stimulation at 40 Hz. In accordance, presynaptic theta-burst stimulation (TBS) failed to induce LTP at layer IV to layers II-III synapses during extracellular field-potential recordings in BDNF(+/-) animals. Changes in postsynaptic function could not be detected, as no changes were observed in either the amplitudes of evoked EPSCs, the ratios of AMPA : NMDA currents or the kinetics of evoked AMPA and NMDA EPSCs. In line with this observation, an LTP pairing paradigm that relies on direct postsynaptic depolarization under patch-clamp conditions could be induced successfully in BDNF(+/-) animals. These data suggest that a chronic reduction in the expression of BDNF to nearly 50% attenuates the efficiency of presynaptic glutamate release in response to repetitive stimulation, thereby impairing presynaptically evoked LTP in the visual cortex.

Depolarization-induced suppression of inhibition mediated by endocannabinoids at synapses from fast-spiking interneurons to medium spiny neurons in the striatum

Endogenous cannabinoids (endocannabinoids) act as retrograde inhibitory messengers in various regions of the brain. We have recently reported that endocannabinoids mediate short-term retrograde suppression of excitatory synaptic transmission from the neocortex to medium spiny (MS) neurons, the major projection neurons from the striatum. However, it remains unclear whether endocannabinoids modulate inhibitory transmission in the striatum. Here we show that depolarization of MS neurons induces transient suppression of inhibition that is mediated by retrograde endocannabinoid signalling. By paired recording from a fast-spiking (FS) interneuron and an MS neuron, we demonstrated that FS-MS inhibitory synapses undergo endocannabinoid-mediated retrograde suppression. We verified that GABAergic inhibitory terminals immunopositive for parvalbumin (PV), a marker for FS interneurons, expressed CB1 receptors. These PV-CB1 double-positive terminals surrounded dopamine D1 receptor-positive and D2 receptor-positive MS neurons; these constitute direct and indirect pathways, respectively. These results suggest that endocannabinoid-mediated retrograde suppression of inhibition influences information flow along both direct and indirect pathways, depending on the activity of MS neurons.

Inhibition of potassium- and ischemia-evoked [3H] D-aspartate release from isolated bovine retina by cannabinoids

We investigated the effect of cannabinoids on potassium chloride (K+)- and ischemia-induced [3H]D-aspartate release from isolated bovine retinae. The superfusion method was employed for studies of [3H]-neurotransmitter release. Cannabinoid receptor CB1 agonists, but not the CB2 agonist JWH 015, inhibited K+ -induced [3H]D-aspartate release from bovine retinae with the following rank order of activity: anandamide > ACEA > methanandamide > WIN 55,212-2. In the ischemic model, the rank order of activity was as follows: methanandamide > ACEA > WIN 55,212-2. The CB1 receptor antagonist AM 251 blocked inhibitory responses produced by cannabinoids in both experimental conditions. In conclusion, cannabinoids inhibit evoked [3H]D-aspartate release from isolated bovine retinae via an effect on CB1 receptors.

Miniature synaptic events elicited by presynaptic Ca2+ rise are selectively suppressed by cannabinoid receptor activation in cerebellar Purkinje cells

Activation of cannabinoid receptors suppresses neurotransmitter release in various brain regions. In cerebellar Purkinje cells (PCs), cannabinoid agonists suppress both EPSC and IPSC evoked by stimulating the corresponding inputs. However, cannabinoid agonists suppress miniature IPSC (mIPSC) but not miniature EPSC (mEPSC) at normal external Ca2+ concentration ([Ca2+]o). Therefore, cannabinoid agonists are thought to suppress release machinery for IPSCs but not that for EPSCs. Here we investigated the possible cause of this difference and found that cannabinoid agonists selectively suppressed Ca2+-enhanced miniature events. A cannabinoid agonist, WIN55,212-2 (5 microM), did not affect mEPSC frequency with 2 mM extracellular Ca2+ (Ca2+(o)). However, WIN55,212-2 became effective when mEPSC frequency was enhanced by elevation of presynaptic Ca2+ level by perfusion with 5 mM Ca2+(o) or bath application of A23187, a Ca2+ ionophore. In contrast, WIN55,212-2 suppressed mIPSC frequency with 2 mM Ca2+(o), but it became ineffective when the presynaptic Ca2+ level was lowered by perfusion with a Ca2+-free solution containing BAPTA-AM. Experiments with systematic [Ca2+]o changes revealed that mIPSC but not mEPSC regularly involved events elicited by presynaptic Ca2+ rise with 2 mM Ca2+(o). Importantly, Ca2+-enhancement of mEPSC and mIPSC was not attributable to activation of voltage-dependent Ca2+ channels. Activation of GABAB receptor or group III metabotropic glutamate receptor, which couple to G(i/o)-protein, also preferentially suppressed Ca2+-enhanced miniature events in PCs. These results suggest that the occurrence of Ca2+-enhanced miniature events at normal [Ca2+]o determines the sensitivity to the presynaptic depression mediated by cannabinoid receptors and other G(i/o)-coupled receptors in PCs.

Endocannabinoid signaling in rat somatosensory cortex: laminar differences and involvement of specific interneuron types

Endocannabinoid-mediated retrograde signaling exerts powerful control over synaptic transmission in many brain areas. However, in the neocortex, the precise laminar, cellular, and subcellular localization of the type 1 cannabinoid receptor (CB1) as well as its function has been elusive. Here we combined multiple immunolabeling with whole-cell recordings to investigate the morpho-functional characteristics of cannabinoid signaling in rat somatosensory cortex. Immunostaining for CB1 revealed axonal and somatic labeling with striking layer specificity: a high density of CB1-positive fibers was seen in layers II-III, in layer VI, and in upper layer V, whereas other layers had sparse (layer IV) or hardly any (layer I) staining. Membrane staining for CB1 was only found in axon terminals, all of which contained GABA and formed symmetric synapses. Double immunostaining also revealed that CB1-positive cells formed two neurochemically distinct subpopulations: two-thirds were cholecystokinin positive and one-third expressed calbindin, each subserving specific inhibitory functions in cortical networks. In addition, cannabinoid sensitivity of GABAergic input showed striking layer specificity, as revealed by both electrophysiological and anatomical experiments. We found a unique population of large pyramidal neurons in layer VB that received much less perisomatic innervation from CB1-expressing GABAergic axon terminals and, accordingly, showed no depolarization-induced suppression of inhibition, unlike pyramidal cells in layer II, and a population of small pyramidal cells in layer V. This suggests that inhibitory control of pyramidal cells involved in intracortical or corticostriatal processing is fine-tuned by activity-dependent endocannabinoid signaling, whereas inhibition of pyramidal cells relaying cortical information to lower subcortical effector centers often lacks this plasticity.

Behaviorally relevant endocannabinoid action in hippocampus: dependence on temporal summation of multiple inputs

Endocannabinoids have been shown to mediate depolarization-induced suppression of GABAergic inhibition (DSI), possibly via release and retrograde diffusion following moderate to severe depolarization of hippocampal pyramidal neurons. However, it is not clear how hippocampal neurons, which have relatively low firing rates in vivo, achieve the degree of depolarization required to release endocannabinoids. Here it is demonstrated that DSI is not dependent on the occurrence of action potentials in the postsynaptic neuron, but is mediated by depolarization-induced calcium entry via voltage-controlled calcium channels (VCCs). The optimal level of calcium entry, and subsequent DSI, are directly related to the frequency of depolarizing pulses, which differs between immature and adult hippocampus. However, it is shown via modeled spike train inputs that the frequency dependence of DSI is overcome if two or more convergent spike trains from different neurons with normal in vivo firing rates converge and overlap in time. In these modeled circumstances, endocannabinoid-mediated DSI occurs most often when converging synaptic inputs from multiple neurons fire in synchrony to allow temporal summation of local membrane events in postsynaptic cells to exceed threshold for calcium entry. It is therefore possible that such suppression of inhibition would only occur during the time that recipient hippocampal neurons receive multiple coincident excitatory synaptic inputs.

Cannabinoids and prefrontal cortical function: Insights from preclinical studies

Marijuana use has been associated with disordered cognition across several domains influenced by the prefrontal cortex (PFC). Here, we review the contribution of preclinical research to understanding the effects of cannabinoids on cognitive ability, and the mechanisms by which cannabinoids may affect the neurochemical processes in the PFC that are associated with these impairments. In rodents, acute administration of cannabinoid agonists produces deficits in working memory, attentional function and reversal learning. These effects appear to be largely dependent on CB1 cannabinoid receptor activation. Preclinical studies also indicate that the endogenous cannabinoid system may tonically regulate some mnemonic processes. Effects of cannabinoids on cognition may be mediated via interaction with neurochemical processes in the PFC and hippocampus. In the PFC, cannabinoids may alter dopaminergic, cholinergic and serotonergic transmission. These mechanisms may underlie cognitive impairments observed following marijuana intake in humans, and may also be relevant to other disorders of cognition. Preclinical research will further enhance our understanding of the interactions between the cannabinoid system and cognitive functioning.

Effects of cannabinoids on neurotransmission

The CB1 cannabinoid receptor is widely distributed in the central and peripheral nervous system. Within the neuron, the CB1 receptor is often localised in axon terminals, and its activation leads to inhibition of transmitter release. The consequence is inhibition of neurotransmission via a presynaptic mechanism. Inhibition of glutamatergic, GABAergic, glycinergic, cholinergic, noradrenergic and serotonergic neurotransmission has been observed in many regions of the central nervous system. In the peripheral nervous system, CB1 receptor-mediated inhibition of adrenergic, cholinergic and sensory neuroeffector transmission has been frequently observed. It is characteristic for the ubiquitous operation of CB1 receptor-mediated presynaptic inhibition that antagonistic components of functional systems (for example, the excitatory and inhibitory inputs of the same neuron) are simultaneously inhibited by cannabinoids. Inhibition of voltage-dependent calcium channels, activation of potassium channels and direct interference with the synaptic vesicle release mechanism are all implicated in the cannabinoid-evoked inhibition of transmitter release. Many presynaptic CB1 receptors are subject to an endogenous tone, i.e. they are constitutively active and/or are continuously activated by endocannabinoids. Compared with the abundant data on presynaptic inhibition by cannabinoids, there are only a few examples for cannabinoid action on the somadendritic parts of neurons in situ.

Dendritic release of retrograde messengers controls synaptic transmission in local neocortical networks

The contribution of retrograde signaling to information processing in the brain has been contemplated for a long time, especially with respect to central nervous system development and long-term synaptic plasticity. During the past few years, however, the concept of retrograde signaling has been expanding to include short-term modifications of synaptic efficacy. The classic point of view on synaptic transmission represents it as a unidirectional transfer of information from presynaptic to postsynaptic sites. This paradigm has, however, been questioned in several experimental studies of neurons in different brain regions. These results suggest that a fast retrograde signal, which provides feedback, exists in active synaptic contacts. In particular, it was found that the dendritic release of retrograde messengers controls the efficacy of synaptic transmission in both excitatory and inhibitory connections between neocortical pyramidal cells and interneurons. The present review discusses these findings and the mechanisms underlying synaptic retrograde signaling.