Exogenous and endogenous cannabinoids control synaptic transmission in mice nucleus accumbens

Endocannabinoids and addiction memory: Relevance to methamphetamine/morphine abuse

AIM

This review aims to summarise the role of endocannabinoid system (ECS), incluing cannabinoid receptors and their endogenous lipid ligands in the modulation of methamphetamine (METH)/morphine-induced memory impairments.

METHODS

Here, we utilized the results from researches which have investigated regulatory role of ECS (including cannabinoid receptor agonists and antagonists) on METH/morphine-induced memory impairments.

RESULTS

Among the neurotransmitters, glutamate and dopamine seem to play a critical role in association with the ECS to heal the drug-induced memory damages. Also, the amygdala, hippocampus, and prefrontal cortex are three important brain regions that participate in both drug addiction and memory task processes, and endocannabinoid neurotransmission have been investigated.

CONCLUSION

ECS can be regarded as a treatment for the side effects of METH and morphine, and their memory-impairing effects.

Mechanisms of endocannabinoid control of synaptic plasticity

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

CB1R activation in nucleus accumbens core promotes stress-induced reinstatement of cocaine seeking by elevating extracellular glutamate in a drug-paired context

Preclinical models of stress-induced relapse to drug use have shown that the dysregulation of glutamatergic transmission within the nucleus accumbens (NA) contributes notably to the reinstatement of cocaine-seeking behavior in rodents. In this sense, there has been increasing interest in the cannabinoid type-1 receptor (CB1R), due to its crucial role in modulating glutamatergic neurotransmission within brain areas involved in drug-related behaviors. This study explored the involvement of CB1R within the NA subregions in the restraint stress-induced reinstatement of cocaine-conditioned place preference (CPP), as well as in the regulation of glutamatergic transmission, by using a pharmacological approach and the in vivo microdialysis sampling technique in freely moving rats. CB1R blockade by the antagonist/inverse agonist AM251 (5 nmol/0.5 μl/side) or CB1R activation by the agonist ACEA (0.01 fmol/0.5 μl/side), prevented or potentiated restraint stress-induced reinstatement of cocaine-CPP, respectively, after local administration into NAcore, but not NAshell. In addition, microdialysis experiments demonstrated that restraint stress elicited a significant increase in extracellular glutamate in NAcore under reinstatement conditions, with the local administration of AM251 or ACEA inhibiting or potentiating this, respectively. Interestingly, this rise specifically corresponded to the cocaine-associated CPP compartment. We also showed that this context-dependent change in glutamate paralleled the expression of cocaine-CPP, and disappeared after the extinction of this response. Taken together, these findings demonstrated the key role played by CB1R in mediating reinstatement of cocaine-CPP after restraint stress, through modulation of the context-specific glutamate release within NAcore. Additionally, CB1R regulation of basal extracellular glutamate was demonstrated and proposed as the underlying mechanism.

Heterosynaptic GABAB Receptor Function within Feedforward Microcircuits Gates Glutamatergic Transmission in the Nucleus Accumbens Core

Complex circuit interactions within the nucleus accumbens (NAc) facilitate goal-directed behavior. Medium spiny neurons (MSNs) mediate NAc output by projecting to functionally divergent brain regions, a property conferred, in part, by the differential projection patterns of D1- and D2 dopamine receptor-expressing MSNs. Glutamatergic afferents to the NAc direct MSN output by recruiting feedforward inhibitory microcircuits comprised of parvalbumin (PV)-expressing interneurons (INs). Furthermore, the GABA_B heteroreceptor (GABA_BR), a G_i/o-coupled G-protein-coupled receptor, is expressed at glutamatergic synapses throughout the mesolimbic network, yet its physiological context and synaptic mechanism within the NAc remains unknown. Here, we explored GABA_BR function at glutamatergic synapses within PV-IN-embedded microcircuits in the NAc core of male mice. We found that GABA_BR is expressed presynaptically and recruits a noncanonical signaling mechanism to reduce glutamatergic synaptic efficacy at D1(+) and D1(-) (putative D2) MSN subtypes. Furthermore, PV-INs, a robust source of neuronal GABA in the NAc, heterosynaptically target GABA_BR to selectively modulate glutamatergic transmission onto D1(+) MSNs. These findings elucidate a new mechanism of feedforward inhibition and refine mechanisms by which GABA_B heteroreceptors modulate mesolimbic circuit function.SIGNIFICANCE STATEMENT Glutamatergic transmission in the nucleus accumbens (NAc) critically contributes to goal-directed behaviors. However, intrinsic microcircuit mechanisms governing the integration of these synapses remain largely unknown. Here, we show that parvalbumin-expressing interneurons within feedforward microcircuits heterosynaptically target GABA_B heteroreceptors (GABA_BR) on glutamate terminals. Activation of presynaptically-expressed GABA_BR decreases glutamatergic synaptic strength by engaging a non-canonical signaling pathway that interferes with vesicular exocytotic release machinery. These findings offer mechanistic insight into the role of GABA_B heteroreceptors within reward circuitry, elucidate a novel arm to feedforward inhibitory networks, and inform the growing use of GABA_BR-selective pharmacotherapy for various motivational disorders, including addiction, major depressive disorder, and autism (Cousins et al., 2002; Kahn et al., 2009; Jacobson et al., 2018; Stoppel et al., 2018; Pisansky et al., 2019).

Synaptic Plasticity in the Nucleus Accumbens: Lessons Learned from Experience

Synaptic plasticity contributes to behavioral adaptations. As a key node in the reward pathway, the nucleus accumbens (NAc) is important for determining motivation-to-action outcomes. Across animal models of motivation including addiction, depression, anxiety, and hedonic feeding, selective recruitment of neuromodulatory signals and plasticity mechanisms have been a focus of physiologists and behaviorists alike. Experience-dependent plasticity mechanisms within the NAc vary depending on the distinct afferents and cell-types over time. A greater understanding of molecular mechanisms determining how these changes in synaptic strength track with behavioral adaptations will provide insight into the process of learning and memory along with identifying maladaptations underlying pathological behavior. Here, we summarize recent findings detailing how changes in NAc synaptic strength and mechanisms of plasticity manifest in various models of motivational disorders.

Endogenous dopamine and endocannabinoid signaling mediate cocaine-induced reversal of AMPAR synaptic potentiation in the nucleus accumbens shell

Repeated exposure to drugs of abuse alters the structure and function of neural circuits mediating reward, generating maladaptive plasticity in circuits critical for motivated behavior. Within meso-corticolimbic dopamine circuitry, repeated exposure to cocaine induces progressive alterations in AMPAR-mediated glutamatergic synaptic transmission. During a 10-14 day period of abstinence from cocaine, AMPAR signaling is potentiated at synapses on nucleus accumbens (NAc) medium spiny neurons (MSNs), promoting a state of heightened synaptic excitability. Re-exposure to cocaine during abstinence, however, rapidly reverses and depotentiates enhanced AMPAR signaling. To understand how re-exposure to cocaine alters AMPAR synaptic transmission, we investigated the roles of dopamine and endocannabinoid (eCB) signaling in modifying synaptic strength in the NAc shell. Using patch-clamp recordings from NAc slices prepared after 10-14 days of abstinence from repeated cocaine, we found that AMPAR-mediated depotentiation is rapidly induced in the NAc shell within 20 min of cocaine re-exposure ex vivo, and persists for up to five days before synapses return to levels of potentiation observed during abstinence. In cocaine-treated animals, global dopamine receptor activation was both necessary and sufficient for the cocaine-evoked depotentiation of AMPAR synaptic function. Additionally, we identified that CB1 receptors are engaged by endogenous endocannabinoids (eCBs) during re-exposure to cocaine ex vivo. Overall, these results indicate the central role that dopamine and eCB signaling mechanisms play in modulating cocaine-induced AMPAR plasticity in the NAc shell.

mGluR5 antagonism inhibits cocaine reinforcement and relapse by elevation of extracellular glutamate in the nucleus accumbens via a CB1 receptor mechanism

Metabotropic glutamate receptor 5 (mGluR5) antagonism inhibits cocaine self-administration and reinstatement of drug-seeking behavior. However, the cellular and molecular mechanisms underlying this action are poorly understood. Here we report a presynaptic glutamate/cannabinoid mechanism that may underlie this action. Systemic or intra-nucleus accumbens (NAc) administration of the mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) dose-dependently reduced cocaine (and sucrose) self-administration and cocaine-induced reinstatement of drug-seeking behavior. The reduction in cocaine-taking and cocaine-seeking was associated with a reduction in cocaine-enhanced extracellular glutamate, but not cocaine-enhanced extracellular dopamine (DA) in the NAc. MPEP alone, when administered systemically or locally into the NAc, elevated extracellular glutamate, but not DA. Similarly, the cannabinoid CB1 receptor antagonist, rimonabant, elevated NAc glutamate, not DA. mGluR5s were found mainly in striatal medium-spiny neurons, not in astrocytes, and MPEP-enhanced extracellular glutamate was blocked by a NAc CB1 receptor antagonist or N-type Ca⁺⁺ channel blocker, suggesting that a retrograde endocannabinoid-signaling mechanism underlies MPEP-induced glutamate release. This interpretation was further supported by our findings that genetic deletion of CB1 receptors in CB1-knockout mice blocked both MPEP-enhanced extracellular glutamate and MPEP-induced reductions in cocaine self-administration. Together, these results indicate that the therapeutic anti-cocaine effects of mGluR5 antagonists are mediated by elevation of extracellular glutamate in the NAc via an endocannabinoid-CB1 receptor disinhibition mechanism.

The Nucleus Accumbens: Mechanisms of Addiction across Drug Classes Reflect the Importance of Glutamate Homeostasis

The nucleus accumbens is a major input structure of the basal ganglia and integrates information from cortical and limbic structures to mediate goal-directed behaviors. Chronic exposure to several classes of drugs of abuse disrupts plasticity in this region, allowing drug-associated cues to engender a pathologic motivation for drug seeking. A number of alterations in glutamatergic transmission occur within the nucleus accumbens after withdrawal from chronic drug exposure. These drug-induced neuroadaptations serve as the molecular basis for relapse vulnerability. In this review, we focus on the role that glutamate signal transduction in the nucleus accumbens plays in addiction-related behaviors. First, we explore the nucleus accumbens, including the cell types and neuronal populations present as well as afferent and efferent connections. Next we discuss rodent models of addiction and assess the viability of these models for testing candidate pharmacotherapies for the prevention of relapse. Then we provide a review of the literature describing how synaptic plasticity in the accumbens is altered after exposure to drugs of abuse and withdrawal and also how pharmacological manipulation of glutamate systems in the accumbens can inhibit drug seeking in the laboratory setting. Finally, we examine results from clinical trials in which pharmacotherapies designed to manipulate glutamate systems have been effective in treating relapse in human patients. Further elucidation of how drugs of abuse alter glutamatergic plasticity within the accumbens will be necessary for the development of new therapeutics for the treatment of addiction across all classes of addictive substances.

The Sodium Channel β4 Auxiliary Subunit Selectively Controls Long-Term Depression in Core Nucleus Accumbens Medium Spiny Neurons

Voltage-gated sodium channels are essential for generating the initial rapid depolarization of neuronal membrane potential during action potentials (APs) that enable cell-to-cell communication, the propagation of signals throughout the brain, and the induction of synaptic plasticity. Although all brain neurons express one or several variants coding for the core pore-forming sodium channel α subunit, the expression of the β (β1–4) auxiliary subunits varies greatly. Of particular interest is the β4 subunit, encoded by the Scn4b gene, that is highly expressed in dorsal and ventral (i.e., nucleus accumbens – NAc) striata compared to other brain regions, and that endows sodium channels with unique gating properties. However, its role on neuronal activity, synaptic plasticity, and behaviors related to drugs of abuse remains poorly understood. Combining whole-cell patch-clamp recordings with two-photon calcium imaging in Scn4b knockout (KO) and knockdown mice, we found that Scn4b altered the properties of APs in core accumbens medium spiny neurons (MSNs). These alterations are associated with a reduction of the probability of MSNs to evoke spike-timing-dependent long-term depression (tLTD) and a reduced ability of backpropagating APs to evoke dendritic calcium transients. In contrast, long-term potentiation (tLTP) remained unaffected. Interestingly, we also showed that amphetamine-induced locomotor activity was significantly reduced in male Scn4b KO mice compared to wild-type controls. Taken together, these data indicate that the Scn4b subunit selectively controls tLTD by modulating dendritic calcium transients evoked by backpropagating APs.

Interacting Cannabinoid and Opioid Receptors in the Nucleus Accumbens Core Control Adolescent Social Play

Social play behavior is a highly rewarding, developmentally important form of social interaction in young mammals. However, its neurobiological underpinnings remain incompletely understood. Previous work has suggested that opioid and endocannabinoid neurotransmission interact in the modulation of social play. Therefore, we combined behavioral, pharmacological, electrophysiological, and genetic approaches to elucidate the role of the endocannabinoid 2-arachidonoylglycerol (2-AG) in social play, and how cannabinoid and opioid neurotransmission interact to control social behavior in adolescent rodents. Systemic administration of the 2-AG hydrolysis inhibitor JZL184 or the opioid receptor agonist morphine increased social play behavior in adolescent rats. These effects were blocked by systemic pretreatment with either CB1 cannabinoid receptor (CB1R) or mu-opioid receptor (MOR) antagonists. The social play-enhancing effects of systemic morphine or JZL184 treatment were also prevented by direct infusion of the CB1R antagonist SR141716 and the MOR antagonist naloxone into the nucleus accumbens core (NAcC). Searching for synaptic correlates of these effects in adolescent NAcC excitatory synapses, we observed that CB1R antagonism blocked the effect of the MOR agonist DAMGO and, conversely, that naloxone reduced the effect of a cannabinoid agonist. These results were recapitulated in mice, and completely abolished in CB1R and MOR knockout mice, suggesting that the functional interaction between CB1R and MOR in the NAcC in the modulation of social behavior is widespread in rodents. The data shed new light on the mechanism by which endocannabinoid lipids and opioid peptides interact to orchestrate rodent socioemotional behaviors.

The GABAA receptor agonist muscimol induces an age- and region-dependent form of long-term depression in the mouse striatum

Several forms of long-term depression (LTD) of glutamatergic synaptic transmission have been identified in the dorsal striatum and in the nucleus accumbens (NAc). Such experience-dependent synaptic plasticity might play important roles in reward-related learning. The GABAA receptor agonist muscimol was recently found to trigger a long-lasting depression of glutamatergic synaptic transmission in the NAc of adolescent mice, but the mechanisms that underlie this novel form of LTD were not studied. Here we examined the effect of muscimol applied in the perfusion solution on the amplitude of field excitatory postsynaptic potentials/population spikes (fEPSP/PSs) in mouse brain slices. We found that muscimol depressed the fEPSP/PS in the NAc of adolescent mice but not adult mice, through both postsynaptic and presynaptic mechanisms. Indeed, muscimol altered the fEPSP/PS paired-pulse ratio, depolarized the membrane of projection neurons, and decreased the frequency, but not amplitude, of spontaneous excitatory postsynaptic currents in the NAc of adolescent mice. The LTD induced by muscimol likely involved endocannabinoids, metabotropic glutamate receptors (mGluRs), but not TRPV1 receptors. Muscimol-LTD was occluded by prior induction of LTD through low-frequency stimulation (LFS) of the slice, demonstrating a common pathway in the induction of LFS-LTD and muscimol-LTD. We also found that muscimol induced a form of LTD in the dorsolateral striatum of adult but not adolescent mice. This LTD was mediated by endocannabinoids but did not involve mGluRs or TRPV1 receptors. These results identify a novel form of synaptic plasticity, and its mechanisms of induction, which is age and region dependent. These findings may contribute to a better understanding of the increased susceptibility of the adolescent brain to long-term synaptic changes in regions associated with reward mechanisms.

Hierarchical glucocorticoid-endocannabinoid interplay regulates the activation of the nucleus accumbens by insulin

Here we asked if insulin activation of the nucleus accumbens in vitro is reflected by an increase in (3)H-deoxyglucose ([(3)H]DG) uptake, thus subserving a new model to study molecular mechanisms of central insulin actions. Additionally, we investigated the dependence of this insulin effect on endocannabinoids and corticosteroids, two major culprits in insulin resistance. We found that in acute accumbal slices, insulin (3 and 300nM but not at 0.3nM) produced an increase in [(3)H]DG uptake. The synthetic cannabinoid agonist, WIN55212-2 (500nM) and the glucocorticoid dexamethasone (10μM), impaired insulin (300nM) action on [(3)H]DG uptake. The glucocorticoid receptor (GcR) antagonist, mifepristone (10μM) prevented dexamethasone from inhibiting insulin's action. Strikingly, this anti-insulin action of dexamethasone was also blocked by two CB1 cannabinoid receptor (CB1R) antagonists, O-2050 (500nM) and SR141716A (500nM), as well as by tetrahydrolipstatin (10μM), an inhibitor of diacylglycerol lipases-the enzymes responsible for the synthesis of the endocannabinoid, 2-arachidonoyl-glycerol (2-AG). On the other hand, the blockade of the post-synaptic 2-AG metabolizing enzymes, α,β-serine hydrolase domain 6/12 by WWL70 (1μM) also prevented the action of insulin, probably via increasing endogenous 2-AG tone. Additionally, an anti-insulin receptor (InsR) antibody immunoprecipitated CB1Rs from accumbal homogenates, indicating a physical complexing of CB1Rs with InsRs that supports their functional interaction. Altogether, insulin stimulates glucose uptake in the nucleus accumbens. Accumbal GcR activation triggers the synthesis of 2-AG that in turn binds to the known CB1R-InsR heteromer, thus impeding insulin signaling.

Induction of cannabinoid- and N-methyl-D-aspartate receptor-mediated long-term depression in the nucleus accumbens and dorsolateral striatum is region and age dependent

BACKGROUND

The adolescent brain is sensitive to experience-dependent plasticity and might be more vulnerable than the adult brain to the effects of some drugs of abuse. The factors that contribute to these differences are not fully identified. We have examined the ability of cannabinoids to induce a form of synaptic plasticity, long-term depression, in the nucleus accumbens and dorsolateral striatum of adolescent and adult mice.

METHODS

We measured field excitatory postsynaptic potentials/population spikes in brain slices.

RESULTS

We found that the cannabinoid receptor agonist WIN 55,212-2 (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate) induced long-term depression in the nucleus accumbens of adolescent but not adult mice and failed to induce long-term depression in the dorsolateral striatum of adolescent or adult mice. Similar results were obtained with the group I metabotropic glutamate receptor agonist (S)-3,5- dihydroxyphenylglycine, which has previously been shown to promote the release of endocannabinoids. These age-related differences were associated with reduced protein levels of the cannabinoid type 1 receptor and metabotropic glutamate receptor 1 in adult nucleus accumbens and dorsolateral striatum and with an increased tone of endocannabinoids in the dorsolateral striatum of adult mice. We also found that N-methyl-D-aspartate receptor-dependent long-term depression, which was induced in the nucleus accumbens of adolescent mice, was blunted in adult mice, possibly because of decreased levels of GluN1, the obligatory subunit of N-methyl-D-aspartate receptors.

CONCLUSIONS

This study identifies region- and age-specific differences in the ability of endogenous and exogenous cannabinoids, and of N-methyl-D-aspartate receptors, to induce long-term depression in the striatal complex. These observations might contribute to a better understanding of the increased sensitivity of the adolescent brain to drug induced-plasticity.

Endocannabinoid CB1 receptor-mediated rises in Ca(2+) and depolarization-induced suppression of inhibition within the laterodorsal tegmental nucleus

Cannabinoid type 1 receptors (CB1Rs) are functionally active within the laterodorsal tegmental nucleus (LDT), which is critically involved in control of rapid eye movement sleep, cortical arousal, and motivated states. To further characterize the cellular consequences of activation of CB1Rs in this nucleus, we examined whether CB1R activation led to rises in intracellular Ca(2+) ([Ca(2+)]i) and whether processes shown in other regions to involve endocannabinoid (eCB) transmission were present in the LDT. Using a combination of Ca(2+) imaging in multiple cells loaded with Ca(2+) imaging dye via 'bulk-loading' or in single cells loaded with dye via a patch-clamp electrode, we found that WIN 55212-2 (WIN-2), a potent CB1R agonist, induced increases in [Ca(2+)]i which were sensitive to AM251, a CB1R antagonist. A proportion of rises persisted in TTX and/or low-extracellular Ca(2+) conditions. Attenuation of these increases by a reversible inhibitor of sarcoplasmic reticulum Ca(2+)-ATPases, suggests these rises occurred following release of Ca(2+) from intracellular stores. Under voltage clamp conditions, brief, direct depolarization of LDT neurons resulted in a decrease in the frequency and amplitude of AM251-sensitive, inhibitory postsynaptic currents (IPSCs), which was an action sensitive to presence of a Ca(2+) chelator. Finally, actions of DHPG, a mGlu1R agonist, on IPSC activity were examined and found to result in an AM251- and BAPTA-sensitive inhibition of both the frequency and amplitude of sIPSCs. Taken together, our data further characterize CB1R and eCB actions in the LDT and indicate that eCB transmission could play a role in the processes governed by this nucleus.

Interplay between synaptic endocannabinoid signaling and metaplasticity in neuronal circuit function and dysfunction

Synaptic neuromodulation acts across different functional domains to regulate cognitive processing and behavior. Recent challenges are related to elucidating the molecular and cellular mechanisms through which neuromodulatory pathways act on multiple time scales to signal state-dependent contingencies at the synaptic level or to stabilise synaptic connections during behavior. Here, we present a framework with the synaptic neuromodulators endocannabinoids (eCBs) as key players in dynamic synaptic changes. Modulation of various molecular components of the eCB pathway yields interconnected functional activation states of eCB signaling (prior, tonic, and persistent), which may contribute to metaplastic control of synaptic and behavioral functions in health and disease. The emerging picture supports aberrant metaplasticity as a contributor to cognitive dysfunction associated with several pathological states in which eCB signaling, or other neuromodulatory pathways, are deregulated.

Achieving cannabis cessation -- evaluating N-acetylcysteine treatment (ACCENT): design and implementation of a multi-site, randomized controlled study in the National Institute on Drug Abuse Clinical Trials Network

Despite recent advances in behavioral interventions for cannabis use disorders, effect sizes remain modest, and few individuals achieve long-term abstinence. One strategy to enhance outcomes is the addition of pharmacotherapy to complement behavioral treatment, but to date no efficacious medications targeting cannabis use disorders in adults through large, randomized controlled trials have been identified. The National Institute on Drug Abuse Clinical Trials Network (NIDA CTN) is currently conducting a study to test the efficacy of N-acetylcysteine (NAC) versus placebo (PBO), added to contingency management, for cannabis cessation in adults (ages 18-50). This study was designed to replicate positive findings from a study in cannabis-dependent adolescents that found greater odds of abstinence with NAC compared to PBO. This paper describes the design and implementation of an ongoing 12-week, intent-to-treat, double-blind, randomized, placebo-controlled study with one follow-up visit four weeks post-treatment. Approximately 300 treatment-seeking cannabis-dependent adults will be randomized to NAC or PBO across six study sites in the United States. The primary objective of this 12-week study is to evaluate the efficacy of twice-daily orally-administered NAC (1200 mg) versus matched PBO, added to contingency management, on cannabis abstinence. NAC is among the first medications to demonstrate increased odds of abstinence in a randomized controlled study among cannabis users in any age group. The current study will assess the cannabis cessation efficacy of NAC combined with a behavioral intervention in adults, providing a novel and timely contribution to the evidence base for the treatment of cannabis use disorders.

New rules governing synaptic plasticity in core nucleus accumbens medium spiny neurons

The nucleus accumbens is a forebrain region responsible for drug reward and goal-directed behaviors. It has long been believed that drugs of abuse exert their addictive properties on behavior by altering the strength of synaptic communication over long periods of time. To date, attempts at understanding the relationship between drugs of abuse and synaptic plasticity have relied on the high-frequency long-term potentiation model of T.V. Bliss & T. Lømo [(1973) Journal of Physiology, 232, 331-356]. We examined synaptic plasticity using spike-timing-dependent plasticity, a stimulation paradigm that reflects more closely the in vivo firing patterns of mouse core nucleus accumbens medium spiny neurons and their afferents. In contrast to other brain regions, the same stimulation paradigm evoked bidirectional long-term plasticity. The magnitude of spike-timing-dependent long-term potentiation (tLTP) changed with the delay between action potentials and excitatory post-synaptic potentials, and frequency, whereas that of spike-timing-dependent long-term depression (tLTD) remained unchanged. We showed that tLTP depended on N-methyl-d-aspartate receptors, whereas tLTD relied on action potentials. Importantly, the intracellular calcium signaling pathways mobilised during tLTP and tLTD were different. Thus, calcium-induced calcium release underlies tLTD but not tLTP. Finally, we found that the firing pattern of a subset of medium spiny neurons was strongly inhibited by dopamine receptor agonists. Surprisingly, these neurons were exclusively associated with tLTP but not with tLTD. Taken together, these data point to the existence of two subgroups of medium spiny neurons with distinct properties, each displaying unique abilities to undergo synaptic plasticity.

Phospholipase A₂: the key to reversing long-term memory impairment in a gastropod model of aging

Memory failure associated with changes in neuronal circuit functions rather than cell death is a common feature of normal aging in diverse animal species. The (neuro)biological foundations of this phenomenon are not well understood although oxidative stress, particularly in the guise of lipid peroxidation, is suspected to play a key role. Using an invertebrate model system of age-associated memory impairment that supports direct correlation between behavioral deficits and changes in the underlying neural substrate, we show that inhibition of phospholipase A(2) (PLA(2)) abolishes both long-term memory (LTM) and neural defects observed in senescent subjects and subjects exposed to experimental oxidative stress. Using a combination of behavioral assessments and electrophysiological techniques, we provide evidence for a close link between lipid peroxidation, provocation of phospholipase A(2)-dependent free fatty acid release, decline of neuronal excitability, and age-related long-term memory impairments. This supports the view that these processes suspend rather than irreversibly extinguish the aging nervous system's intrinsic capacity for plasticity.

Withdrawal from THC during adolescence: sex differences in locomotor activity and anxiety

Research suggests that the use and abuse of marijuana can be especially harmful if it occurs during adolescence, a period of vast developmental changes throughout the brain. Due to the localization of cannabinoid receptors within the limbic system and the established effects of cannabinoids on emotional states and anxiety levels of rats and humans, we studied the sex- and dose-related effects of Δ⁹-tetrahydrocannabinol (THC, the main psychoactive component in marijuana) on behavior and anxiety during spontaneous withdrawal. Male and female Sprague Dawley rats were administered 2, 7.5 or 15 mg/kg THC or vehicle from postnatal day 35-41 (approximating mid-adolescence in humans). Locomotor activity and anxiety-related behaviors were measured during drug administration and abstinence. THC caused significant dose-dependent locomotor depression during drug administration. Locomotor depression initially abated upon drug cessation, but re-emerged by the end of the abstinence period and was greater in female than male rats. We found sensitization to the locomotor-depressing effects of THC in middle- and high-dose rats and the subsequent development of tolerance in high-dose rats. The high dose of THC increased anxiety-like behaviors while the low dose decreased anxiety-like behaviors during drug administration, with females more sensitive to the anxiogenic effects of THC than males. During abstinence, females were again especially sensitive to the anxiogenic effects of THC. This study demonstrates sexually-dimorphic effects of THC on anxiety-related behaviors and locomotor activity during and after THC administration during adolescence. This information may be useful in the development of therapeutic approaches for the treatment of marijuana withdrawal in adolescents.

Neurobiologia da Cannabis: do sistema endocanabinoide aos transtornos por uso de Cannabis

OBJETIVOS: Diante das lacunas na efetividade das terapêuticas para transtornos por uso de Cannabis, a droga ilícita mais consumida no mundo, este trabalho propõe-se a rever os conhecimentos sobre o substrato neuroanatômico, biomolecular e celular do sistema endocanabinoide, descrever os mecanismos de neuroplasticidade dependente dos canabinoides e relacioná-los com a neurobiologia dos transtornos por uso de Cannabis (abuso e dependência). MÉTODOS: Recorreu-se às bases de dados Medline, Scopus e ISI Web of Knowledge; as palavras-chave pesquisadas foram "Cannabis", "neurobiology", "endocannabinoid system", "endocannabinoids", "receptors, cannabinoid", "neuronal plasticity", "long-term synaptic depression", "long-term potentiation", "marijuana abuse" e "tetrahydrocannabinol". Foram incluídos 80 trabalhos nesta revisão. DISCUSSÃO: A distribuição neuroanatômica, celular e biomolecular do sistema endocanabinoide adequa-se perfeitamente às suas funções de neuromodulação (via neuroplasticidade e metaplasticidade), nomeadamente em vias relacionadas aos transtornos por uso de substâncias. Os canabinoides exógenos perturbam essas funções. CONCLUSÃO: O sistema endocanabinoide contribui para a definição de setpoints em diversas vias neuronais, incluindo vias cruciais na instalação de transtornos por uso de substâncias; com o uso de Cannabis, esses setpoints tornar-se-ão mais permissivos, facilitando os transtornos por uso de Cannabis. Os avanços no entendimento da neurobiologia da Cannabis abrem uma janela de oportunidades para novas estratégias terapêuticas nos transtornos por uso de Cannabis.

Endocannabinoid signaling in midbrain dopamine neurons: more than physiology?

Different classes of neurons in the CNS utilize endogenous cannabinoids as retrograde messengers to shape afferent activity in a short- and long-lasting fashion. Transient suppression of excitation and inhibition as well as long-term depression or potentiation in many brain regions require endocannabinoids to be released by the postsynaptic neurons and activate presynaptic CB1 receptors. Memory consolidation and/or extinction and habit forming have been suggested as the potential behavioral consequences of endocannabinoid-mediated synaptic modulation. HOWEVER, ENDOCANNABINOIDS HAVE A DUAL ROLE: beyond a physiological modulation of synaptic functions, they have been demonstrated to participate in the mechanisms of neuronal protection under circumstances involving excessive excitatory drive, glutamate excitotoxicity, hypoxia-ischemia, which are key features of several neurodegenerative disorders. In this framework, the recent discovery that the endocannabinoid 2-arachidonoyl-glycerol is released by midbrain dopaminergic neurons, under both physiological synaptic activity to modulate afferent inputs and pathological conditions such as ischemia, is particularly interesting for the possible implication of these molecules in brain functions and dysfunctions. Since dopamine dysfunctions underlie diverse neuropsychiatric disorders including schizophrenia, psychoses, and drug addiction, the importance of better understanding the correlation between an unbalanced endocannabinoid signal and the dopamine system is even greater. Additionally, we will review the evidence of the involvement of the endocannabinoid system in the pathogenesis of Parkinson's disease, where neuroprotective actions of cannabinoid-acting compounds may prove beneficial.The modulation of the endocannabinoid system by pharmacological agents is a valuable target in protection of dopamine neurons against functional abnormalities as well as against their neurodegeneration.

Converging action of alcohol consumption and cannabinoid receptor activation on adult hippocampal neurogenesis

Alcoholism is characterized by successive periods of abstinence and relapse, resulting from long-lasting changes in various circuits of the central nervous system. Accumulating evidence points to the endocannabinoid system as one of the most relevant biochemical systems mediating alcohol addiction. The endocannabinoid system regulates adult neurogenesis, a form of long-lasting adult plasticity that occurs in a few areas of the brain, including the dentate gyrus. Because exposure to psychotropic drugs regulates adult neurogenesis, it is possible that neurogenesis might be implicated in the pathophysiology, and hence treatment, of neurobiological illnesses related to drugs of abuse. Here, we investigated the sensitivity of adult hippocampal neurogenesis to alcohol and the cannabinoid receptor agonist WIN 55,212-2 (WIN). Specifically, we analysed the potential link between alcohol relapse, cannabinoid receptor activation, and adult neurogenesis. Adult rats were exposed to subchronic alcohol binge intoxication and received the cannabinoid receptor agonist WIN. Another group of rats were subjected to an alcohol operant self-administration task. Half of these latter animals had continuous access to alcohol, while the other half were subjected to alcohol deprivation, with or without WIN administration. WIN treatment, when administered during alcohol deprivation, resulted in the greatest increase in alcohol consumption during relapse. Together, forced alcohol binge intoxication and WIN administration dramatically reduced hippocampal neurogenesis. Furthermore, adult neurogenesis inversely correlated with voluntary consumption of alcohol. These findings suggest that adult hippocampal neurogenesis is a key factor involved in drug abuse and that it may provide a new strategy for the treatment of alcohol addiction and dependence.

Palatable high-energy diet decreases the expression of cannabinoid type 1 receptor messenger RNA in specific brain regions in the rat

In laboratory rodents, a palatable high-energy diet (PHED) is usually consumed in a higher quantity than a standard laboratory diet, leading to the development of an obese phenotype. The central effects of PHED are not fully understood. Nonetheless, the long-term consumption of PHED can decrease cannabinoid type 1 receptor (CB1R) protein density in particular brain regions. However, little is known about the diet-dependent regulation of the brain expression of CB1R mRNA. The present study aimed to investigate the effects of the long-term consumption of PHED and short-term (12 h) food deprivation on the brain expression of CB1R mRNA. For 13 weeks, rats were fed a standard laboratory chow or PHED presented as a free choice of chow, shortcake biscuits and pork spread. In total, the food intake of PHED rats was higher than that of chow-fed animals. Expectedly, PHED rats demonstrated higher body weight than chow-fed animals. The difference in body weight between PHED- and chow-fed rats was as result of the fat but not the lean mass. PHED-fed rats had significantly higher plasma levels of leptin and insulin and significantly higher levels of expression of suppressor of cytokine signalling 3 (SOCS-3) in the arcuate hypothalamic nucleus. The long-term consumption of PHED significantly decreased the levels of CB1R mRNA expression in the cingulate (Cg) cortex, ventromedial hypothalamic nucleus and the descending/autonomic divisions of the parvocellular hypothalamic nucleus (PVH), the ventrolateral parvocellular PVH and, to a lesser extent, the dorsomedial parvocellular PVH. Acute food deprivation decreased the levels of CB1R transcript in the Cg and ventrolateral parvocellular PVH. Altogether, the present results demonstrate that long-term PHED leads to an increase in the hypothalamic expression of SOCS-3 mRNA and a decrease in expression of CB1R mRNA in the Cg cortex and specific hypothalamic regions.

Are CB(1) Receptor Antagonists Nootropic or Cognitive Impairing Agents?

For more than a decade, a considerable amount of research has examined the effects of rimonabant (SR 141716) and other CB(1) receptor antagonists in both in vivo and in vitro models of learning and memory. In addition to its utility in determining whether the effects of drugs are mediated though a CB(1) receptor mechanism of action, these antagonists are useful in providing insight into the physiological function of the endogenous cannabinoid system. Several groups have reported that CB(1) receptor antagonists enhance memory duration in a variety of spatial and operant paradigms, but not in all paradigms. Conversely, disruption of CB(1) receptor signaling also impairs extinction learning in which the animal actively suppresses a learned response when reinforcement has been withheld. These extinction deficits occur in aversively motivated tasks, such as in fear conditioning or escape behavior in the Morris water maze task, but not in appetitively motivated tasks. Similarly, in electrophysiological models, CB(1) receptor antagonists elicit a variety of effects, including enhancement of long-term potentiation (LTP), while disrupting long-term depression (LTD) and interfering with transient forms of plasticity, including depolarization-induced suppression of inhibition (DSI) and depolarization-induced suppression of excitation (DSE). The collective results of the in vivo and in vitro studies employing CB(1) receptor antagonists, demonstrate that these receptors play integral roles in different components of cognitive processing. Functionally, pharmacological blockade of CB(1) receptors may strengthen memory duration, but interferes with extinction of learned behaviors that are associated with traumatic or aversive memories.

Role of cannabis and endocannabinoids in the genesis of schizophrenia

RATIONALE

Cannabis abuse and endocannabinoids are associated to schizophrenia.

OBJECTIVES

It is important to discern the association between schizophrenia and exogenous Cannabis sativa, on one hand, and the endogenous cannabinoid system, on the other hand.

RESULTS

On one hand, there is substantial evidence that cannabis abuse is a risk factor for psychosis in genetically predisposed people, may lead to a worse outcome of the disease, or it can affect normal brain development during adolescence, increasing the risk for schizophrenia in adulthood. Regarding genetic predisposition, alterations affecting the cannabinoid CNR1 gene could be related to schizophrenia. On the other hand, the endogenous cannabinoid system is altered in schizophrenia (i.e., increased density of cannabinoid CB1 receptor binding in corticolimbic regions, enhanced cerebrospinal fluid anandamide levels), and dysregulation of this system can interact with neurotransmitter systems in such a way that a "cannabinoid hypothesis" can be integrated in the neurobiological hypotheses of schizophrenia. Finally, there is also evidence that some genetic alterations of the CNR1 gene can act as a protectant factor against schizophrenia or can induce a better pharmacological response to atypical antipsychotics.

CONCLUSIONS

Cannabis abuse is a risk factor for psychosis in predisposed people, it can affect neurodevelopment during adolescence leading to schizophrenia, and a dysregulation of the endocannabinoid system can participate in schizophrenia. It is also worth noting that some specific cannabinoid alterations can act as neuroprotectant for schizophrenia or can be a psychopharmacogenetic rather than a vulnerability factor.

Interoceptive effects of alcohol require mGlu5 receptor activity in the nucleus accumbens

The interoceptive effects of alcohol are major determinants of addiction liability. Metabotropic glutamate (mGlu) receptors are widely expressed in striatal circuits known to modulate drug-seeking. Given that the interoceptive effects of drugs can be important determinants of abuse liability, we hypothesized that striatal mGlu receptors modulate the interoceptive effects of alcohol. Using drug discrimination learning, rats were trained to discriminate alcohol (1 g/kg, i.g.) versus water. We found that systemic antagonism of metabotropic glutamate subtype 5 (mGlu5) receptors [10 mg/kg 2-methyl-6-(phenylethynyl)pyridine (MPEP) and 3 mg/kg 3-((2-methyl-1,3-thiazol-4-yl)ethynyl)pyridine], but not mGlu1 receptors ([0.3-3 mg/kg JNJ16259685) (3,4-dihydro-2H-pyrano[2,3]beta-quinolin-7-yl)(cis-4-methoxycyclohexyl) methanone)], inhibited the discriminative stimulus effects of alcohol. Furthermore, mGlu5 receptor antagonism (10 mg/kg MPEP) significantly inhibited neuronal activity in the nucleus accumbens core as levels of the transcription factor c-Fos were significantly reduced. Accordingly, targeted inhibition of mGlu5 receptors (20 microg of MPEP) in the nucleus accumbens core blunted the discriminative stimulus effects of alcohol (1 g/kg). Anatomical specificity was confirmed by the lack of effect of inhibition of mGlu5 receptors (10-30 microg of MPEP) in the dorsomedial caudate-putamen and the similar cytological expression patterns and relative density of mGlu5 receptors between the brain regions. Functional involvement of intra-accumbens mGlu5 receptors was confirmed as activation of mGlu5 receptors [10 microg of (RS)-2-amino-2-(2-chloro-5-hydroxyphenyl)acetic acid sodium salt] enhanced the discriminative stimulus effects of a low alcohol dose (0.5 g/kg), and mGlu5 receptor inhibition (20 microg of MPEP) prevented the agonist-induced enhancement. These results show that mGlu5 receptor activity in the nucleus accumbens is required for the expression of the interoceptive effects of alcohol.

Influence of intracerebroventricular or intraperitoneal administration of cannabinoid receptor agonist (WIN 55,212-2) and inverse agonist (AM 251) on the regulation of food intake and hypothalamic serotonin levels

The effect of intracerebroventricular or intraperitoneal administration of cannabinoid receptor agonist WIN 55,212-2 or inverse agonist AM 251 on food intake and extracellular levels of serotonin and acetic acid 5-hydroxy-indol from presatiated rats was studied. Compared to the vehicle-injected control, the intracerebroventricular administration of WIN 55,212-2 was associated with a significant increase in food intake, whereas the administration of AM 251 caused a significant reduction in this respect. These results were accompanied by considerable reductions or increases in serotonin and acetic acid 5-hydroxy-indol levels compared to the vehicle-injected control and the baseline values for the different experimental groups studied. Intraperitoneal administration of WIN 55,212-2 at doses of 1 and 2 mg/kg promoted hyperphagia up to 6 h after injection, whereas administration of a higher dose (5 mg/kg) significantly inhibited food intake and motor behaviour in partially satiated rats. Administration of any of the AM 251 doses studied (0.5, 1, 2, 5 mg/kg) led to a significant decrease in the amount of food ingested from 2 h after the injection, compared to the vehicle-injected control group, with the most striking effect being observed when the 5 mg/kg dose was injected.

Neurochemical evidence that stimulation of CB1 cannabinoid receptors on GABAergic nerve terminals activates the dopaminergic reward system by increasing dopamine release in the rat nucleus accumbens

We examined the effect of cannabinoid receptor activation on basal and electrical field simulation-evoked (25 V, 2 Hz, 240 shocks) [(3)H]dopamine efflux in the isolated rat nucleus accumbens in a preparation, in which any effect on the dendrites or somata of ventral tegmental projection neurons was excluded. The cannabinoid agonist (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate (WIN55,212-2, 100 nM) significantly enhanced stimulation-evoked [(3)H]dopamine release in the presence of the selective dopamine transporter inhibitor 1-[2-[bis-(4-fluorophenyl)methoxy]ethyl]-4-(3-phenylpropyl)piperazine dihydrochloride (GBR12909, 100 nM). GBR12909 (100 nM-1 microM), when added alone, increased the evoked [(3)H]dopamine efflux in a concentration-dependent manner. The stimulatory effect of WIN55,212-2 on the evoked tritium efflux was inhibited by the selective CB1 cannabinoid receptor antagonist N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251, 100 nM) and by the GABA(A) receptor antagonist bicuculline (10 microM). Repeated application of N-methyl-d aspartate (1 mM) under Mg(2+)-free conditions, which directly acts on dopaminergic terminals, reversibly increased the tritium efflux, but WIN55,212-2 did not affect N-methyl-d aspartate-evoked [(3)H]dopamine efflux, indicating that WIN55,212-2 has no direct action on dopaminergic nerve terminals. AM251 (100 nM) alone also did not have an effect on electrical stimulation-evoked [(3)H]dopamine efflux. Likewise, the selective CB2 receptor antagonist 6-iodo-2-methyl-1-[2-(4-morpholinyl)ethyl]-1H-indol-3-yl](4-methoxyphenyl)methanone (AM630, 0.3 microM) and the anandamide transport inhibitor (5Z,8Z,11Z,14Z)-N-(4-hydroxy-2-methylphenyl)-5,8,11,14-eicosatetraenamide (VDM11, 10 microM) had no significant effect on electrically evoked [(3)H]dopamine release. This is the first neurochemical evidence that the activation of CB1 cannabinoid receptors leads to the augmentation of [(3)H]dopamine efflux via a local GABA(A) receptor-mediated disinhibitory mechanism in the rat nucleus accumbens.

Serotonergic mechanisms in addiction-related memories

Drug-associated memories are a hallmark of addiction and a contributing factor in the continued use and relapse to drugs of abuse. Repeated association of drugs of abuse with conditioned stimuli leads to long-lasting behavioral responses that reflect reward-controlled learning and participate in the establishment of addiction. A greater understanding of the mechanisms underlying the formation and retrieval of drug-associated memories may shed light on potential therapeutic approaches to effectively intervene with drug use-associated memory. There is evidence to support the involvement of serotonin (5-HT) neurotransmission in learning and memory formation through the families of the 5-HT(1) receptor (5-HT(1)R) and 5-HT(2)R which have also been shown to play a modulatory role in the behavioral effects induced by many psychostimulants. While there is a paucity of studies examining the effects of selective 5-HT(1A)R ligands, the available dataset suggests that 5-HT(1B)R agonists may inhibit retrieval of cocaine-associated memories. The 5-HT(2A)R and 5-HT(2C)R appear to be integral in the strong conditioned associations made between cocaine and environmental cues with 5-HT(2A)R antagonists and 5-HT(2C)R agonists possessing potency in blocking retrieval of cocaine-associated memories following cocaine self-administration procedures. The complex anatomical connectivity between 5-HT neurons and other neuronal phenotypes in limbic-corticostriatal brain structures, the heterogeneity of 5-HT receptors (5-HT(X)R) and the conflicting results of behavioral experiments which employ non-specific 5-HT(X)R ligands contribute to the complexity of interpreting the involvement of 5-HT systems in addictive-related memory processes. This review briefly traces the history of 5-HT involvement in retrieval of drug-cue associations and future targets of serotonergic manipulation that may reduce the impact that drug cues have on addictive behavior and relapse.

Methamphetamine-seeking behavior is due to inhibition of nicotinic cholinergic transmission by activation of cannabinoid CB1 receptors

We previously reported the involvement of cannabinoid CB1 receptors (CB1Rs) and nicotinic acetylcholine receptors (nAChRs) in the reinstatement of methamphetamine (MAP)-seeking behavior (lever-pressing response for MAP reinforcement under saline infusion). The present study examined whether the reinstatement involves interactions between these receptors. Rats were trained to self-administer MAP with a light and tone (MAP-associated cues). Then, extinction sessions under saline infusion without cues were conducted. After that, a reinstatement tests were conducted by either presenting the cues or a MAP-priming injection. Systemic and intracranial administration of HU210, a cannabinoid CB1R agonist, into the nucleus accumbens core (NAC) and prelimbic cortex (PrC) reinstated MAP-seeking behavior. The reinstatement caused by the systemic HU210 treatment was attenuated by intracranial administration of AM251, a cannabinoid CB1R antagonist, into each region mentioned above. Meanwhile, reinstatement induced by the MAP-associated cues and MAP-priming injection was also attenuated by intracranial administration of AM251 in each region. In these regions, the attenuating effects of AM251 on the reinstatement induced by each stimulus were blocked by the intracranial administration of mecamylamine, a non-selective nAChR antagonist, but not by scopolamine, a muscarinic ACh receptor (mAChR) antagonist. Furthermore, the intracranial administration of DHbetaE, an alpha4beta2 nAChR antagonist, but not MLA, an alpha7 nAChR antagonist, into each region blocked the AM251-induced attenuation of the reinstatement. These findings suggest that relapses to MAP-seeking behavior may be due to two steps, first inhibition of ACh transmission by the activation of cannabinoid CB1Rs and then the inactivation of alpha4beta2 nAChRs.

The pharmacology of the endocannabinoid system: functional and structural interactions with other neurotransmitter systems and their repercussions in behavioral addiction

Addiction is a chronic, recurring and complex disorder. It is characterized by anomalous behaviors that are linked to permanent or long-lasting neurobiological alterations. Furthermore, the endocannabinoid system has a crucial role in mediating neurotransmitter release as one of the main neuromodulators of the mammalian central nervous system. The purpose of the present review is to instruct readers about the functional and structural interactions between the endocannabinoid system and the main neurotransmitter systems of the central nervous system in the context of drug addiction. With this aim, we have systematically reviewed the main findings of most of the existing literature that explores cross-talk in the five brain areas that are most traditionally implicated in addiction: amygdala, prefrontal cortex, nucleus accumbens, hippocampus and ventral tegmental area (VTA). The neurotransmission systems influenced by the pharmacology of the endocannabinoid system in these brain areas, which are reviewed here, are gamma-aminobutyric acid (GABA), glutamate, the main biogenic amines (dopamine, noradrenaline and serotonin), acetylcholine and opioids. We show that all of these neurotransmitter systems can be modulated differentially in each brain area by the activation or deactivation of cannabinoid CB1 brain receptors. Specifically, most of the studies relate to the hippocampus and nucleus accumbens. Moreover, the neurotransmitter with the fewest number of related studies is acetylcholine (excepting in the hippocampus), whereas there is a large number that evaluates GABA, glutamate and dopamine. Finally, we propose a possible interpretation of the role of the endocannabinoid system in the phenomenon of addiction.

Cannabis use and brain structural alterations in first episode schizophrenia--a region of interest, voxel based morphometric study

Structural alterations of the brain in schizophrenia have been associated with genetic and environmental factors. Among the environmental factors, cannabis use has been associated with increased risk for patients with schizophrenia, but the effect of cannabis on their brain structure is unclear. We examined gray matter alterations in first episode schizophrenia patients (FES) with cannabis use (FES+C; n=15) compared to FES without cannabis use (FES-C; n=24) and 42 healthy controls who did not use cannabis. We conducted a voxel based morphometric analysis of a priori determined regions of interest consisting of the CB1 receptor rich brain regions. We observed a decrease in gray matter density in the right posterior cingulate cortex (PCC) in FES+C when compared with FES-C. The results suggest that cannabis use may be associated with altered brain structure, in particular regions rich in CB1 receptors. These findings need to be confirmed by larger, prospective studies.

Glutamatergic substrates of drug addiction and alcoholism

The past two decades have witnessed a dramatic accumulation of evidence indicating that the excitatory amino acid glutamate plays an important role in drug addiction and alcoholism. The purpose of this review is to summarize findings on glutamatergic substrates of addiction, surveying data from both human and animal studies. The effects of various drugs of abuse on glutamatergic neurotransmission are discussed, as are the effects of pharmacological or genetic manipulation of various components of glutamate transmission on drug reinforcement, conditioned reward, extinction, and relapse-like behavior. In addition, glutamatergic agents that are currently in use or are undergoing testing in clinical trials for the treatment of addiction are discussed, including acamprosate, N-acetylcysteine, modafinil, topiramate, lamotrigine, gabapentin and memantine. All drugs of abuse appear to modulate glutamatergic transmission, albeit by different mechanisms, and this modulation of glutamate transmission is believed to result in long-lasting neuroplastic changes in the brain that may contribute to the perseveration of drug-seeking behavior and drug-associated memories. In general, attenuation of glutamatergic transmission reduces drug reward, reinforcement, and relapse-like behavior. On the other hand, potentiation of glutamatergic transmission appears to facilitate the extinction of drug-seeking behavior. However, attempts at identifying genetic polymorphisms in components of glutamate transmission in humans have yielded only a limited number of candidate genes that may serve as risk factors for the development of addiction. Nonetheless, manipulation of glutamatergic neurotransmission appears to be a promising avenue of research in developing improved therapeutic agents for the treatment of drug addiction and alcoholism.

Modulation of the endocannabinoid system: therapeutic potential against cocaine dependence

Dependence on cocaine is still a main unresolved medical and social concern, and in spite of research efforts, no pharmacological therapy against cocaine dependence is yet available. Recent studies have shown that the endocannabinoid system participates in specific stages and aspects of drug dependence in general, and some of this evidence suggests an involvement of the cannabinoid system in cocaine effects. For example, cocaine administration has been shown to alter brain endocannabinoid levels, and the endocannabinoid system has been involved in long-term modifications of brain processes that might play a role in neuro/behavioral effects of psychostimulant drugs like cocaine. Human studies show that marijuana dependence is frequently associated with cocaine dependence, and that the cannabinoid receptor CNR1 gene polymorphism might be related to cocaine addiction. This article will review the main papers in the field showing how a modulation of different components of the cannabinoid system might interact with some of the neurobiological/behavioral effects of cocaine related to its reinforcing effects, evaluated in preclinical models or in clinical settings. The goal of this review will be to provide insights into the complex picture of cocaine abuse and addiction, and to extrapolate from such endocannabinoid-cocaine interactions useful information to test the therapeutic potential of cannabinoid ligands and endocannabinoid-level enhancers against cocaine dependence for future preclinical/clinical trials.

Inhibition of fatty-acid amide hydrolase accelerates acquisition and extinction rates in a spatial memory task

Recent reports have demonstrated that disruption of CB(1) receptor signaling impairs extinction of learned responses in conditioned fear and Morris water maze paradigms. Here, we test the hypothesis that elevating brain levels of the endogenous cannabinoid anandamide through either genetic deletion or pharmacological inhibition of its primary catabolic enzyme fatty-acid amide hydrolase (FAAH) will potentiate extinction in a fixed platform water maze task. FAAH (-/-) mice and mice treated with the FAAH inhibitor OL-135, did not display any memory impairment or motor disruption, but did exhibit a significant increase in the rate of extinction. Unexpectedly, FAAH-compromised mice also exhibited a significant increase in acquisition rate. The CB(1) receptor antagonist SR141716 (rimonabant) when given alone had no effects on acquisition, but disrupted extinction. Additionally, SR141716 blocked the effects of OL-135 on both acquisition and extinction. Collectively, these results indicate that endogenous anandamide plays a facilitatory role in extinction through a CB(1) receptor mechanism of action. In contrast, the primary psychoactive constituent of marijuana, Delta(9)-tetrahydrocannabinol, failed to affect extinction rates, suggesting that FAAH is a more effective target than a direct acting CB(1) receptor agonist in facilitating extinction. More generally, these findings suggest that FAAH inhibition represents a promising pharmacological approach to treat psychopathologies hallmarked by an inability to extinguish maladaptive behaviors, such as post-traumatic stress syndrome and obsessive-compulsive disorder.

mGluR5 stimulates gliotransmission in the nucleus accumbens

Although metabotropic glutamate receptor 5 (mGluR5) is essential for cocaine self-administration and drug-seeking behavior, there is limited knowledge of the cellular actions of this receptor in the nucleus accumbens (NAc). Although mGluR5 has the potential to regulate neurons directly, recent studies have shown the importance of mGluR5 in regulating Ca(2+) signaling in astrocytes and, as a consequence, the Ca(2+)-dependent release of excitatory transmitters from these glia. In this study, we demonstrate that activation of mGluR5 induces Ca(2+) oscillations in NAc astrocytes with the correlated appearance of NMDA receptor-dependent slow inward currents detected in medium spiny neurons (MSNs). Photolysis of caged Ca(2+) loaded specifically into astrocytes evoked slow inward currents demonstrating that Ca(2+) elevations in astrocytes are responsible for these excitatory events. Pharmacological evaluation of these glial-evoked NMDA currents shows that they are mediated by NR2B-containing NMDA receptors, whereas synaptic NMDA receptors rely on NR2A-containing receptors. Stimulation of glutamatergic afferents activates mGluR5-dependent astrocytic Ca(2+) oscillations and gliotransmission that is sustained for minutes beyond the initial stimulus. Because gliotransmission is mediated by NMDA receptors, depolarized membrane potentials exhibited during up-states augment excitation provided by gliotransmission, which drives bursts of MSN action potentials. Because the predominant mGluR5-dependent action of glutamatergic afferents is to cause the sustained activation of astrocytes, which in turn excite MSNs through extrasynaptic NMDA receptors, our results raise the potential for gliotransmission being involved in prolonged mGluR5-dependent adaptation in the NAc.

AM 251 produces sustained reductions in food intake and body weight that are resistant to tolerance and conditioned taste aversion

The cannabinoid 1 (CB(1)) receptor has been implicated in the regulation of food intake. Here, we examine the effect of the CB(1) receptor antagonist AM 251 on food intake and body weight over a prolonged period. Further, we examine whether AM 251 produces conditioned taste aversion (CTA) and if sustained antagonism at central receptors contributes to its anorectic effect. The effect of AM 251 of food intake and body weight was examined in daily (1 mg kg(-1)) and 5-day (5 mg kg(-1)) dosing schedules. Matching reductions in food intake and body weight were observed in both paradigms. A single administration of AM 251 (5 mg kg(-1)) significantly reduced food intake for 4 days. Tolerance to the anorectic effects of AM 251 did not develop in either dosing strategy. Active avoidance of AM 251 (3; 5 mg kg(-1), i.p.) was examined using a CTA assay. Rats showed no evidence of CTA associated with AM 251. We investigated the sustained effect of AM 251 (5 mg kg(-1), i.p.) on CB(1) receptors in the hypothalamus using Delta(9)-tetrahydrocannabinol (8 mg kg(-1), i.p.) induced hypothermia. AM 251 initially blocked hypothermia, but this effect was not seen 2 or 4 days later. The results demonstrate that smaller, or infrequent, administrations of AM 251 can produce sustained reductions in food intake and body weight in rat. Reductions in food intake were sustained longer than AM 251 antagonized the effects of a CB(1) receptor agonist in the hypothalamus, and occurred independently of CTA.

Microinjection of the L-type calcium channel antagonist diltiazem into the ventral nucleus accumbens shell facilitates cocaine-induced conditioned place preferences

BACKGROUND

Calcium (Ca2+) influx within the nucleus accumbens shell (NASh) can influence brain reward processes. We found previously that rats self-administer NMDA receptor antagonists (which block Ca2+ influx through NMDA receptors) into the NASh. We also found that manipulations which increase expression of Ca2+-permeable AMPA receptors within this region make cocaine aversive. Here we examined if Ca2+ influx via L-type Ca2+ channels within the NASh would influence cocaine reward.

METHODS

Rats received bilateral microinjections of the L-type Ca2+ channel antagonist diltiazem into the ventral NASh prior to place conditioning with systemic cocaine.

RESULTS

Microinjections of diltiazem (10 nmol/hemisphere) into the ventral NASh facilitated the ability of a sub-threshold dose of cocaine (5.0 mg/kg) to establish place preferences, but did not affect place conditioning on their own (5.0-40 nmol/hemisphere). Microinjections into more dorsal regions had no effects.

CONCLUSIONS

Blockade of Ca2+ influx through L-type channels Ca2+ within the ventral NASh increases cocaine reward.

Extracellular-signal regulated kinase 1-dependent metabotropic glutamate receptor 5-induced long-term depression in the bed nucleus of the stria terminalis is disrupted by cocaine administration

The bed nucleus of the stria terminalis (BNST) is a key component of the CNS stress and reward circuit. Synaptic plasticity in this region could in part underlie the persistent behavioral alterations in generalized anxiety and addiction. Group I metabotropic glutamate receptors (mGluRs) have been implicated in stress, addiction, and synaptic plasticity, but their roles in the BNST are unknown. We find that activation of group I mGluRs in the dorsal BNST induces depression of excitatory synaptic transmission through two distinct mechanisms. First, a combined activation of group I mGluRs (mGluR1 and mGluR5) induces a transient depression that is cannabinoid 1 receptor dependent. Second, as with endocannabinoid-independent group I mGluR long-term depression (LTD) in the adult hippocampus, we find that activation of mGluR5 induces an extracellular signal-regulated kinase (ERK)-dependent LTD. Surprisingly, our data demonstrate that this LTD requires the ERK1 rather than ERK2 isoform, establishing a key role for this isoform in the CNS. Finally, we find that this LTD is dramatically reduced after multiple exposures but not a single exposure to cocaine, suggesting a role for this form of plasticity in the actions of psychostimulants on anxiety and reward circuitries and their emergent control of animal behavior.

Cannabinoids, electrophysiology, and retrograde messengers: challenges for the next 5 years

Most of the behavioral effects of cannabis and its active ingredients, the cannabinoids (delta9THC being the most abundant of these), appear to be mediated by cannabinoid receptors. Endogenous cannabinoids (endocannabinoids) are lipid mediators that activate these same cannabinoid receptors. Elegant work from several laboratories over the past 5 years has established that endocannabinoids, possibly acting as retrograde messengers, mediate several forms of neuronal plasticity. Endocannabinoid-mediated neuronal plasticity is common, apparently occurring at all neurons that express cannabinoid receptors. Thus, it is likely that delta9THC produces its effects by interacting with endocannabinoid-mediated neuronal plasticity, though whether it does so cooperatively or antagonistically remains an open question. In this review we will briefly discuss the work establishing endocannabinoids as mediators of neuronal plasticity and then present evidence that a major effect of delta9THC may be to antagonize the actions of endocannabinoids.

Cannabinoid modulation of opiate reinforcement through the ventral striatopallidal pathway

Recent evidence indicates that cannabinoid-1 (CB1) receptors play a role in the mediation of opiate reward, though the neural mechanisms for this process have not been characterized. The present experiments investigated the influence of CB1 receptors in the ventral striatopallidal system on opiate-induced neurochemical events and opiate self-administration behavior in rats. Acute morphine administration (3 mg/kg) significantly reduced ventral pallidal GABA efflux in a manner similar to that produced by heroin self-administration. This neurochemical effect was reversed by doses of the selective CB1 antagonist SR 141716A (Rimonabant; 1 and 3 mg/kg) that also significantly reduce opiate reward. Morphine-induced increases in nucleus accumbens dopamine levels were unaltered by SR 141716A. Intravenous heroin self-administration (0.02 mg/infusion) was significantly reduced by intra-accumbens, but not intraventral pallidal SR 141716A infusions (1 and 3 microg/side), implicating nucleus accumbens CB1 receptors in the modulation of opiate reinforcement. In contrast, SR14716A did not alter cocaine self-administration (0.125 mg/inf), cocaine-induced (10 mg/kg) decrements in ventral pallidal GABA efflux or cocaine-induced increases in accumbens dopamine. This is consistent with evidence that selective inactivation of CB1 receptors reduces opiate-, but not psychostimulant-maintained self-administration. The CB1 receptor agonist WIN 55,212-2 (5 mg/kg) reduced pallidal GABA efflux in a manner similar to morphine, and this effect was reversed by the opiate receptor antagonist naloxone. Collectively these findings suggest that CB1 receptors modulate opiate reward through the ventral striatopallidal projection and that the modulation of this projection system may be involved in the reciprocal behavioral effects between cannabinoids, and opioids.

Differential regulation of synaptic inputs by constitutively released endocannabinoids and exogenous cannabinoids

Endocannabinoid release from a single neuron has been shown to cause presynaptic inhibition of transmitter release at many different sites. Here, we demonstrate that hypothalamic proopiomelanocortin (POMC) neurons release endocannabinoids continuously under basal conditions, unlike other release sites at which endocannabinoid production must be stimulated. The basal endocannabinoid release selectively inhibited GABA release onto POMC neurons, although exogenous administration of cannabinoid agonists also inhibited glutamate release. The CB1 cannabinoid receptor antagonist AM 251 [N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide] blocked endocannabinoid-mediated inhibition of GABA release without affecting excitatory synaptic currents, whereas the CB1 receptor agonist WIN 55,212-2 [R-(+)-(2,3-dihydro-5-methyl-3-[(4-morpholinyl)methyl]pyrol [1,2,3-de]-1,4-benzoxazin-6-yl)(1-naphthalenyl) methanone monomethanesulfonate] inhibited both inhibitory and excitatory synaptic currents in POMC neurons. These data demonstrate that endogenously released cannabinoids and exogenously applied CB1 receptor agonists can have markedly different effects on synaptic inputs. Furthermore, the data suggest a novel form of endocannabinoid-mediated retrograde inhibition, whereby the regulation of a subset of inputs requires either the removal of tonic presynaptic inhibition caused by endocannabinoids or the engagement of a mechanism that actively inhibits endocannabinoid production.

Chronic exposure to Delta9-tetrahydrocannabinol downregulates oxytocin and oxytocin-associated neurophysin in specific brain areas

Cannabinoids are widely abused drugs. Our goal was to identify genes modulated by Delta9-tetrahydrocannabinol (Delta9-THC) treatment. We found that chronic administration of Delta9-THC (1.5 mg/kg/day, i.p.; 7 days) to rats, downregulates the expression of oxytocin-neurophysin (OT-NP) mRNA and of OT and oxytocin-associated NP (NPOT) immunoreactivity in nucleus accumbens (NAc) and ventral tegmental area (VTA), brain areas involved in reward and addiction. Real-time PCR revealed a 60% and 53% reduction of OT-NP mRNA in NAc and VTA, respectively, under chronic treatment, while no changes were observed in NAc after 24 h. Immunohistochemistry showed a large decrease in number of OT and NPOT-stained fibers in NAc (by 59% and 52%, respectively) and VTA (by 50% and 56%, respectively). No changes in cell staining were observed in the paraventricular nucleus and supraoptic nucleus. As OT is known to inhibit development of drug tolerance and attenuate withdrawal symptoms, we suggest that OT downregulation could play a role during the establishment of the chronic effects of Delta9-THC.

Molecular mapping of striatal subdivisions in juvenile Macaca Mulata

The striatum of the primate brain can be subdivided into three distinct anatomical subregions: caudate (CAU), putamen (PUT), and ventral striatum (VS). Although these subregions share several anatomical connections, cell morphological, and histochemical features, they differ considerably in their vulnerability to different neurological and psychiatric diseases, and these brain regions have significantly different functions in health and disease. In order to better understand the molecular underpinnings of the different disease and functional vulnerabilities, transcriptional profiles were generated from the CAU, PUT, and VS of five juvenile rhesus macaques (Macaca mulatta) using human cDNA neuromicroarrays containing triplicate spots of 1227 cDNAs. Differences in microarray gene expression were assessed using z score analysis and 1.5-fold change between paired subregions. Clustering of genes based on dissimilarity of expression patterns between regions revealed subregion specific expression profiles encoding G-protein-coupled receptor signaling transcripts, transcription factors, kinases and phosphatases, and cell signaling and signal transduction transcripts. Twelve transcripts were examined using quantitative real-time PCR (qPCR), and 81% demonstrated alterations similar to those seen with microarray analysis, some of which were statistically significant. Subregion specific transcription profiles support the anatomical differentiation and potential disease vulnerabilities of the respective subregions.

Lipids and drugs of abuse

Drug abuse continues to take an enormous economic and social toll on the world. Among the costs are reduced productivity, increased need for medical services and stress on families. Treatments that allow affected individuals to reduce compulsive drug use are lacking and novel approaches to their development will likely come from increased understanding of the consequences of chronic exposure to reinforcing drugs. The purpose of this review is to explore the role of lipids in drug abuse and to present a rationale for an increased focus on the interactions between drugs of abuse and lipids in the brain. Small molecular weight lipids function as neuromodulators in the brain and, as such, play a role in the synaptic plasticity that occurs following exposure to drugs of abuse. In addition, the membrane lipid bilayer consists of lipid subdomains and emerging evidence suggests that protein function can be altered by transient associations with these subdomains. Finally, lipidomics is a very new field devoted to the exploration of changes in cellular lipid constituents during phenotypic alterations. Enhanced research in all of these areas will likely provide useful insights into and, perhaps, therapeutic targets for the treatment of drug abuse.

Calcium and endocannabinoids in the modulation of inhibitory synaptic transmission

Synapses in the central nervous system can be highly plastic devices, being able to modify their efficacy in relaying information in response to several factors. Calcium ions are often fundamental in triggering synaptic plasticity. Here, we will shortly review the effects induced by postsynaptic increases of calcium concentration at GABAergic and glycinergic synapses. Both postsynaptic and presynaptic mechanisms mediating changes in synaptic strength will be examined. Particular attention will be devoted to phenomena of retrograde signaling and, specifically, to the recently discovered role, played by the endocannabinoid system in retrograde synaptic modulation.

Inhibition of restraint stress-induced neural and behavioural activation by endogenous cannabinoid signalling

The role of endocannabinoid (eCB) signalling in restraint stress-induced neuronal activation was studied. Male mice exposed to 30 min of restraint exhibit increased Fos protein within prefrontal cortex (PFC), lateral septum (LS), nucleus accumbens (Acb) and medial amygdala. SR141716 (2 mg/kg) itself had no effect on Fos but pretreatment with SR141716 significantly potentiated restraint-induced Fos expression in cingulate, LS and Acb. SR141716 also significantly increased the time spent in active escape behaviours during the restraint. In restraint-habituated mice (mice exposed to four previous restraint episodes), the fifth restraint exposure resulted in decreased expression of active escape behaviours compared to the first exposure and only induced Fos protein in the central and medial amygdala. Administration of SR141716 prior to the fifth restraint episode resulted in greater potentiation of restraint-induced Fos induction than the first; significant increases occurred within all regions of PFC examined, LS and Acb. Brain regional eCB content was measured immediately after restraint. N-arachidonylethanolamine content within the amygdala was significantly decreased after both restraint episodes. 2-Arachidonylglycerol content was significantly increased in both the limbic forebrain and amygdala after the fifth restraint but not the first. Restraint had no effect on cerebellar eCB content. These data suggest that eCB activation of CB(1) receptors opposes the behavioural and neuronal responses to aversive stimuli. Because repeated homotypic stress increased both limbic 2-AG and resulted in a greater effect of SR141716 on limbic Fos expression, we hypothesize that increased CB(1) receptor activity contributes to the expression of habituation to homotypic stress.

In vivo up-regulation of brain-derived neurotrophic factor in specific brain areas by chronic exposure to Delta9-tetrahydrocannabinol

Cannabinoids are widely abused drugs. Here we show that chronic administration of Delta(9)-tetrahydrocannabinol (Delta(9)-THC), the active psychotropic agent in marijuana and hashish, at 1.5 mg per kg per day intraperitoneally for 7 days, increases the expression, at both mRNA and protein levels, of brain-derived neurotrophic factor (BDNF), in specific rat brain areas, notably in those involved in reward and addiction. Real-time PCR revealed a 10-fold up-regulation of BDNF mRNA in the nucleus accumbens (NAc) upon chronic Delta(9)-THC treatment, but there was no change at 3 or 24 h after a single injection. Smaller increases in mRNA levels were found in the ventral tegmental area (VTA), medial prefrontal cortex and paraventricular nucleus (PVN). Immunohistochemistry showed large increases in BDNF-stained cells in the NAc (5.5-fold), posterior VTA (4-fold) and PVN (1.7-fold), but no change was observed in the anterior VTA, hippocampus or dorsal striatum. Altogether, our study indicates that chronic exposure to Delta(9)-THC up-regulates BDNF in specific brain areas involved with reward, and provides evidence for different BDNF expression in the anterior and posterior VTA. Moreover, BDNF is known to modulate synaptic plasticity and adaptive processes underlying learning and memory, leading to long-term functional and structural modification of synaptic connections. We suggest that Delta(9)-THC up-regulation of BDNF expression has an important role in inducing the neuroadaptive processes taking place upon exposure to cannabinoids.