Hippocampal endocannabinoids play an important role in induction of long-term potentiation and regulation of contextual fear memory formation

Interplay between endocannabinoid and endovanilloid mechanisms in fear conditioning

OBJECTIVE

The transient receptor potential cation channel, subfamily V (vanilloid), member 1 (TRPV1) mediates pain perception to thermal and chemical stimuli in peripheral neurons. The cannabinoid receptor type 1 (CB1), on the other hand, promotes analgesia in both the periphery and the brain. TRPV1 and CB1 have also been implicated in learned fear, which involves the association of a previously neutral stimulus with an aversive event. In this review, we elaborate on the interplay between CB1 receptors and TRPV1 channels in learned fear processing.

METHODS

We conducted a PubMed search for a narrative review on endocannabinoid and endovanilloid mechanisms on fear conditioning.

RESULTS

TRPV1 and CB1 receptors are activated by a common endogenous agonist, arachidonoyl ethanolamide (anandamide), Moreover, they are expressed in common neuroanatomical structures and recruit converging cellular pathways, acting in concert to modulate fear learning. However, evidence suggests that TRPV1 exerts a facilitatory role, whereas CB1 restrains fear responses.

CONCLUSION

TRPV1 and CB1 seem to mediate protective and aversive roles of anandamide, respectively. However, more research is needed to achieve a better understanding of how these receptors interact to modulate fear learning.

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

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

Lights on Endocannabinoid-Mediated Synaptic Potentiation

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

Endocannabinoid interactions in the regulation of acquisition of contextual conditioned fear

Endocannabinoids (eCBs) anandamide (AEA) and 2-arachidonoylglycerol (2-AG) were shown to be involved in the basis of trauma-induced behavioral changes, particularly contextual conditioned fear, however, their ligand-specific effects and possible interactions are poorly understood. Here we assessed specific eCB effects and interactions on acquisition of contextual conditioned fear employing electric footshocks in a rat model. We selectively increased eCB levels by pharmacological blockade of the degrading enzymes of AEA by URB597 and 2-AG by JZL184 before traumatization either systemically or locally in relevant brain areas, the prelimbic cortex (PrL), ventral hippocampus (vHC) and basolateral amygdala (BLA). Following traumatization, a series of contextual reminders were conducted during which conditioned fear was assessed. While systemic URB597-treatment during traumatization only slightly enhanced the acquisition of contextual conditioned fear, administration of the compound in the PrL and vHC led to the acquisition of stable, lasting conditioned fear, resistant to extinction. These effects of URB597 were blocked by simultaneous administration of JZL184. Similar treatment effects did not occur in the BLA. Treatment effects were not secondary to alterations in locomotor activity or nociception. Our findings suggest that AEA and 2-AG functionally interact in the regulation of acquisition of contextual conditioned fear. AEA signaling in the PrL and vHC is a crucial promoter of fear acquisition while 2-AG potentially modulates this effect. The lack of eCB effects in the BLA suggests functional specificity of eCBs at distinct brain sites.

Dopamine-endocannabinoid interactions mediate spike-timing-dependent potentiation in the striatum

Dopamine modulates striatal synaptic plasticity, a key substrate for action selection and procedural learning. Thus, characterizing the repertoire of activity-dependent plasticity in striatum and its dependence on dopamine is of crucial importance. We recently unraveled a striatal spike-timing-dependent long-term potentiation (tLTP) mediated by endocannabinoids (eCBs) and induced with few spikes (~5–15). Whether this eCB-tLTP interacts with the dopaminergic system remains to be investigated. Here, we report that eCB-tLTP is impaired in a rodent model of Parkinson’s disease and rescued by L-DOPA. Dopamine controls eCB-tLTP via dopamine type-2 receptors (D₂R) located presynaptically in cortical terminals. Dopamine–endocannabinoid interactions via D₂R are required for the emergence of tLTP in response to few coincident pre- and post-synaptic spikes and control eCB-plasticity by modulating the long-term potentiation (LTP)/depression (LTD) thresholds. While usually considered as a depressing synaptic function, our results show that eCBs in the presence of dopamine constitute a versatile system underlying bidirectional plasticity implicated in basal ganglia pathophysiology.

Dopamine tightly regulates plasticity at corticostriatal synapses. Here, the authors report that endocannabinoid dependent LTP induced with few spikes in the striatum is impaired in a rodent model of Parkinson’s disease, requires dopamine through presynaptic D2 receptors located on corticostriatal inputs.

Endocannabinoid-LTP Mediated by CB1 and TRPV1 Receptors Encodes for Limited Occurrences of Coincident Activity in Neocortex

Synaptic efficacy changes, long-term potentiation (LTP) and depression (LTD), underlie various forms of learning and memory. Synaptic plasticity is generally assessed under prolonged activation, whereas learning can emerge from few or even a single trial. Here, we investigated the existence of rapid responsiveness of synaptic plasticity in response to a few number of spikes, in neocortex in a synaptic Hebbian learning rule, the spike-timing-dependent plasticity (STDP). We investigated the effect of lowering the number of pairings from 100 to 50, and 10 on STDP expression, using whole-cell recordings from pyramidal cells in rodent somatosensory cortical brain slices. We found that a low number of paired stimulations induces LTP at neocortical layer 4–2/3 synapses. Besides the asymmetric Hebbian STDP reported in the neocortex induced by 100 pairings, we observed a symmetric anti-Hebbian LTD for 50 pairings and unveiled a unidirectional Hebbian spike-timing-dependent LTP (tLTP) induced by 10–15 pairings. This tLTP was not mediated by NMDA receptor activation but requires CB₁ receptors and transient receptor potential vanilloid type-1 (TRPV1) activated by endocannabinoids (eCBs). eCBs have been widely described as mediating short- and long-term synaptic depression. Here, the eCB-tLTP reported at neocortical synapses could constitute a substrate operating in the online learning of new associative memories or during the initial stages of learning. In addition, these findings should provide useful insight into the mechanisms underlying eCB-plasticity occurring during marijuana intoxication.

Robustness of STDP to spike timing jitter

In Hebbian plasticity, neural circuits adjust their synaptic weights depending on patterned firing. Spike-timing-dependent plasticity (STDP), a synaptic Hebbian learning rule, relies on the order and timing of the paired activities in pre- and postsynaptic neurons. Classically, in ex vivo experiments, STDP is assessed with deterministic (constant) spike timings and time intervals between successive pairings, thus exhibiting a regularity that differs from biological variability. Hence, STDP emergence from noisy inputs as occurring in in vivo-like firing remains unresolved. Here, we used noisy STDP pairings where the spike timing and/or interval between pairings were jittered. We explored with electrophysiology and mathematical modeling, the impact of jitter on three forms of STDP at corticostriatal synapses: NMDAR-LTP, endocannabinoid-LTD and endocannabinoid-LTP. We found that NMDAR-LTP was highly fragile to jitter, whereas endocannabinoid-plasticity appeared more resistant. When the frequency or number of pairings was increased, NMDAR-LTP became more robust and could be expressed despite strong jittering. Our results identify endocannabinoid-plasticity as a robust form of STDP, whereas the sensitivity to jitter of NMDAR-LTP varies with activity frequency. This provides new insights into the mechanisms at play during the different phases of learning and memory and the emergence of Hebbian plasticity in in vivo-like activity.

Cannabinoid Modulation of the Stressed Hippocampus

Exposure to stressful situations is one of the risk factors for the precipitation of several psychiatric disorders, including Major Depressive Disorder, Posttraumatic Stress Disorder and Schizophrenia. The hippocampal formation is a forebrain structure highly associated with emotional, learning and memory processes; being particularly vulnerable to stress. Exposure to stressful stimuli leads to neuroplastic changes and imbalance between inhibitory/excitatory networks. These changes have been associated with an impaired hippocampal function. Endocannabinoids (eCB) are one of the main systems controlling both excitatory and inhibitory neurotransmission, as well as neuroplasticity within the hippocampus. Cannabinoids receptors are highly expressed in the hippocampus, and several lines of evidence suggest that facilitation of cannabinoid signaling within this brain region prevents stress-induced behavioral changes. Also, chronic stress modulates hippocampal CB₁ receptors expression and endocannabinoid levels. Moreover, cannabinoids participate in mechanisms related to synaptic plasticity and adult neurogenesis. Here, we discussed the main findings supporting the involvement of hippocampal cannabinoid neurotransmission in stress-induced behavioral and neuroplastic changes.

Nicotinamide adenine dinucleotide suppresses epileptogenesis at an early stage

The pathophysiologic mechanisms of epileptogenesis are poorly understood, and no effective therapy exists for suppressing epileptogenesis. Numerous reports have shown that nicotinamide adenine dinucleotide (NAD⁺) has neuroprotective effects, suggesting its potential use for treating epileptogenesis. Here we evaluated the effects of NAD⁺ on epileptogenesis and the mechanisms underlying these effects. In pilocarpine-induced status epilepticus (SE) model mice, NAD⁺ was injected three times within 24.5 h after SE. NAD⁺ intervention significantly reduced the incidence of spontaneous recurrent seizure (SRS) and abnormal electroencephalogram (EEG) activity, rescued contextual fear memory formation, reduced neuronal loss in the CA1 region of the hippocampus at SRS stage. Furthermore, exogenous supply of NAD⁺ distinctly reversed the seizure-induced depletion of endogenous NAD⁺, reduced neuronal apoptosis in the CA1 region of the hippocampus, and reversed the augmented Acp53/p53 ratio at the early stage of epileptogenesis. Our findings demonstrated that early-stage intervention with NAD⁺ prevents epileptogenesis in pilocarpine-induced SE mice by suppressing neuronal apoptosis.

The interactive role of CB1 receptors and L-type calcium channels in hippocampal long-term potentiation in rats

Long-term potentiation (LTP) of synaptic responses is a widely researched model of synaptic plasticity that occurs during learning and memory. The cannabinoid system is an endogenous system that modulate this kind of synaptic plasticity. In addition, voltage dependent calcium channels is essential for induction of LTP at some synapses in the hippocampus. However, there is currently debate over the interaction between L-type calcium channels and cannabinoid system on the synaptic plasticity. In this study, we examined the effects of an acute administration of the cannabinoid antagonist AM251 following a chronic administration of the Ca²⁺ channel blocker verapamil on LTP induction in the hippocampal dentate gyrus(DG) of rats. Male Wistar rats were administered verapamil(10,25,50mg/kg) or saline intraperitoneally(IP) daily for 13days(n=10/group). After this treatment period, animals were anesthetized with an IP injection of urethane; the recording and stimulating electrodes were positioned in the DG and the perforant pathway. After obtaining a steady state baseline response, a single IP injection of saline or AM251(1 or 5mg/kg) was administered. LTP was induced by high-frequency stimulation(HFS). The population spike(PS) amplitude and the slope of excitatory postsynaptic potentials(EPSP) were compared between the experimental groups. The acute administration of the CB1 antagonist AM251 increased LTP induction. The EPSP slopes and PS amplitude in the verapamil and AM251 groups differed after HFS, such that AM251 increased LTP, whereas verapamil decreased LTP induction. These findings suggest that there are functional interactions between the L-type calcium channels and cannabinoid system in this model of synaptic plasticity in the hippocampus.

Interaction between NMDA and CB2 function in the dorsal hippocampus on memory consolidation impairment: an isobologram analysis

RATIONALE

Convincing evidence has supported the pivotal role of N-methyl-D-aspartate receptors (NMDARs) and CB2Rs in the regulation of learning and memory.

OBJECTIVE

In this study, the role of hippocampal (CA1 region) CB2 receptors on aversive memory consolidation deficit induced by D-AP5, a NMDA receptor antagonist, was evaluated.

METHODS

Adult male Wistar rats received cannula implants that bilaterally targeted the CA1 region. Long-term memory was examined using the step-through type of passive avoidance task.

RESULTS

Post-training, intra-CA1 microinjection of D-AP5 (0.5 and 0.75 μg/rat), GP1a (CB2 receptor agonist at dose of 150 ng/rat) and AM630 (CB2 receptor antagonist at doses 75 and 100 ng/rat) impaired memory consolidation processes. Intra-CA1 microinjection of a lower dose of GP1a or AM630 restored memory impairment induced by D-AP5 at the two higher doses, while AM630 decreased D-AP5 memory response at the lower dose. The isobologram analysis showed that there is a synergistic effect between D-AP5 and AM630 on memory consolidation deficit.

CONCLUSIONS

These results suggest that CA1 CB2 receptors modulate memory consolidation impairment induced by D-AP5.

Precondition of right frontal region with anodal tDCS can restore the fear memory impairment induced by ACPA in male mice

Fear memory and learning cause behavioural patterns such as fight or flight responses, which increase survival probability, but unfit processing of fear memory and learning can lead to maladaptive behaviours and maladies such as phobias, Post-Traumatic Stress Disorder (PTSD) and anxiety disorders. The growing prevalence of these maladies shows the need to quest novel methods for their treatment. We used anodal transcranial direct current stimulation (tDCS) on the right frontal region as a precondition neuromodulator and arachidonylcyclopropylamide (ACPA), a selective CB1 cannabinoid receptor agonist, as a fear memory impairing agent to assess their effects on contextual and auditory fear conditioning (reliable model for fear studies). Right frontal anodal tDCS (0.2 mA for. 20 minutes) 24 hours before the train did not alter contextual and auditory learning and memory in short-term (24 hrs after the training phase). Moreover, intraperitoneal pre-train injection of ACPA (0.1 mg/kg) alone, decreased both contextual and auditory learning and memory in short- but not long-term. Right frontal anodal tDCS improved short-term contextual fear memory in subthreshold doses of ACPA. On the other hand, right frontal anodal tDCS in long-term improved (lower doses of ACPA) and restored (higher doses of ACPA) both fear memories. These findings showed that, aforementioned approach could cause durable learning and memory improvements. Also this combined modality could be useful for fear extinction training and maladies which inflict amnesia.

Endocannabinoid System: the Direct and Indirect Involvement in the Memory and Learning Processes-a Short Review

The endocannabinoid system via cannabinoid (CB: CB1 and CB2) receptors and their endogenous ligands is directly and indirectly involved in many physiological functions, especially in memory and learning processes. Extensive studies reported that this system strictly modulates cognition-related processes evaluated in various animal models. However, the effects of cannabinoids on the cognition have been contradictory. The cannabinoid compounds were able to both impair or improve different phases of memory processes through direct (receptor related) or indirect (non-receptor related) mechanism. The memory-related effects induced by the cannabinoids can be depended on the kind of cannabinoid compound used, dosage, and route of administration as well as on the memory task chosen. Therefore, the objectives of this paper are to review and summarize the results describing the role of endocannabinoid system in cognition, including various stages of memory.

A Primary Cortical Input to Hippocampus Expresses a Pathway-Specific and Endocannabinoid-Dependent Form of Long-Term Potentiation

The endocannabinoid 2-arachidonoyl-sn-glycerol (2-AG), a key modulator of synaptic transmission in mammalian brain, is produced in dendritic spines and then crosses the synaptic junction to depress neurotransmitter release. Here we report that 2-AG-dependent retrograde signaling also mediates an enduring enhancement of glutamate release, as assessed with independent tests, in the lateral perforant path (LPP), one of two cortical inputs to the granule cells of the dentate gyrus. Induction of this form of long-term potentiation (LTP) involved two types of glutamate receptors, changes in postsynaptic calcium, and the postsynaptic enzyme that synthesizes 2-AG. Stochastic optical reconstruction microscopy confirmed that CB₁ cannabinoid receptors are localized presynaptically to LPP terminals, while the inhibition or knockout of the receptors eliminated LPP-LTP. Suppressing the enzyme that degrades 2-AG dramatically enhanced LPP potentiation, while overexpressing it produced the opposite effect. Priming with a CB₁ agonist markedly reduced the threshold for LTP. Latrunculin A, which prevents actin polymerization, blocked LPP-LTP when applied extracellularly but had no effect when infused postsynaptically into granule cells, indicating that critical actin remodeling resides in the presynaptic compartment. Importantly, there was no evidence for the LPP form of potentiation in the Schaffer-commissural innervation of field CA1 or in the medial perforant path. Peripheral injections of compounds that block or enhance LPP-LTP had corresponding effects on the formation of long-term memory for cues conveyed to the dentate gyrus by the LPP. Together, these results indicate that the encoding of information carried by a principal hippocampal afferent involves an unusual, regionally differentiated form of plasticity.

Endocannabinoid dynamics gate spike-timing dependent depression and potentiation

Synaptic plasticity is a cardinal cellular mechanism for learning and memory. The endocannabinoid (eCB) system has emerged as a pivotal pathway for synaptic plasticity because of its widely characterized ability to depress synaptic transmission on short- and long-term scales. Recent reports indicate that eCBs also mediate potentiation of the synapse. However, it is not known how eCB signaling may support bidirectionality. Here, we combined electrophysiology experiments with mathematical modeling to question the mechanisms of eCB bidirectionality in spike-timing dependent plasticity (STDP) at corticostriatal synapses. We demonstrate that STDP outcome is controlled by eCB levels and dynamics: prolonged and moderate levels of eCB lead to eCB-mediated long-term depression (eCB-tLTD) while short and large eCB transients produce eCB-mediated long-term potentiation (eCB-tLTP). Moreover, we show that eCB-tLTD requires active calcineurin whereas eCB-tLTP necessitates the activity of presynaptic PKA. Therefore, just like glutamate or GABA, eCB form a bidirectional system to encode learning and memory.

Cannabinoid receptors contribute to astroglial Ca²⁺-signalling and control of synaptic plasticity in the neocortex

Communication between neuronal and glial cells is thought to be very important for many brain functions. Acting via release of gliotransmitters, astrocytes can modulate synaptic strength. The mechanisms underlying ATP release from astrocytes remain uncertain with exocytosis being the most intriguing and debated pathway. We have demonstrated that ATP and d-serine can be released from cortical astrocytes in situ by a SNARE-complex-dependent mechanism. Exocytosis of ATP from astrocytes can activate post-synaptic P2X receptors in the adjacent neurons, causing a downregulation of synaptic and extrasynaptic GABA receptors in cortical pyramidal neurons. We showed that release of gliotransmitters is important for the NMDA receptor-dependent synaptic plasticity in the neocortex. Firstly, induction of long-term potentiation (LTP) by five episodes of theta-burst stimulation (TBS) was impaired in the neocortex of dominant-negative (dn)-SNARE mice. The LTP was rescued in the dn-SNARE mice by application of exogenous non-hydrolysable ATP analogues. Secondly, we observed that weak sub-threshold stimulation (two TBS episodes) became able to induce LTP when astrocytes were additionally activated via CB-1 receptors. This facilitation was dependent on activity of ATP receptors and was abolished in the dn-SNARE mice. Our results strongly support the physiological relevance of glial exocytosis for glia-neuron communications and brain function.

Endocannabinoids mediate bidirectional striatal spike-timing-dependent plasticity

KEY POINTS

Although learning can arise from few or even a single trial, synaptic plasticity is commonly assessed under prolonged activation. Here, we explored the existence of rapid responsiveness of synaptic plasticity at corticostriatal synapses in a major synaptic learning rule, spike-timing-dependent plasticity (STDP). We found that spike-timing-dependent depression (tLTD) progressively disappears when the number of paired stimulations (below 50 pairings) is decreased whereas spike-timing-dependent potentiation (tLTP) displays a biphasic profile: tLTP is observed for 75-100 pairings, is absent for 25-50 pairings and re-emerges for 5-10 pairings. This tLTP induced by low numbers of pairings (5-10) depends on activation of the endocannabinoid system, type-1 cannabinoid receptor and the transient receptor potential vanilloid type-1. Endocannabinoid-tLTP may represent a physiological mechanism operating during the rapid learning of new associative memories and behavioural rules characterizing the flexible behaviour of mammals or during the initial stages of habit learning.

ABSTRACT

Synaptic plasticity, a main substrate for learning and memory, is commonly assessed with prolonged stimulations. Since learning can arise from few or even a single trial, synaptic strength is expected to adapt rapidly. However, whether synaptic plasticity occurs in response to limited event occurrences remains elusive. To answer this question, we investigated whether a low number of paired stimulations can induce plasticity in a major synaptic learning rule, spike-timing-dependent plasticity (STDP). It is known that 100 pairings induce bidirectional STDP, i.e. spike-timing-dependent potentiation (tLTP) and depression (tLTD) at most central synapses. In rodent striatum, we found that tLTD progressively disappears when the number of paired stimulations is decreased (below 50 pairings) whereas tLTP displays a biphasic profile: tLTP is observed for 75-100 pairings, absent for 25-50 pairings and re-emerges for 5-10 pairings. This tLTP, induced by very few pairings (∼5-10) depends on the endocannabinoid (eCB) system. This eCB-dependent tLTP (eCB-tLTP) involves postsynaptic endocannabinoid synthesis, requires paired activity (post- and presynaptic) and the activation of type-1 cannabinoid receptor (CB1R) and transient receptor potential vanilloid type-1 (TRPV1). eCB-tLTP occurs in both striatopallidal and striatonigral medium-sized spiny neurons (MSNs) and is dopamine dependent. Lastly, we show that eCB-LTP and eCB-LTD can be induced sequentially in the same neuron, depending on the cellular conditioning protocol. Thus, while endocannabinoids are usually thought simply to depress synaptic function, they also constitute a versatile system underlying bidirectional plasticity. Our results reveal a novel form of synaptic plasticity, eCB-tLTP, which may underlie rapid learning capabilities characterizing behavioural flexibility.

The interactive role of cannabinoid and vanilloid systems in hippocampal synaptic plasticity in rats

Long-term potentiation (LTP) has been most thoroughly studied in the hippocampus, which has a key role in learning and memory. Endocannabinoids are one of the endogenous systems that modulate this kind of synaptic plasticity. The activation of the vanillioid system has also been shown to mediate synaptic plasticity in the hippocampus. In addition, immunohistochemical studies have shown that cannabinoid receptor type 1 (CB1) and vanilloid receptor 1 (TRPV1) are closely located in the hippocampus. In this study, we examined the hippocampal effects of co-administrating WIN55-212-2 and capsaicin, which are CB1 and TRPV1 agonists, respectively, on the induction of LTP in the dentate gyrus (DG) of rats. LTP in the hippocampal area was induced by high-frequency stimulation (HFS). Our results indicated that the cannabinoid agonist reduced both field excitatory post-synaptic potential (fEPSP) slope and population spike (PS) amplitude after HFS with respect to the control group, whereas the vanilloid agonist increased these parameters along with the increased induction of LTP as compared to the control group. We also showed that the co-administration of cannabinoid and vanilloid agonists had different effects on fEPSP slope and PS amplitude. It seems that agonists of the vanilloid system modulate cannabinoid outputs that cause an increase in synaptic plastisity, while in contemporary consumption of two agonist, TRPV1 agonist can change production of endocannabinoid, which in turn result to enhancement of LTP induction. These findings suggest that the two systems may interact or share certain common signaling pathways in the hippocampus.

Effects of cannabinoid and glutamate receptor antagonists and their interactions on learning and memory in male rats

INTRODUCTION

Despite previous findings on the effects of cannabinoid and glutamatergic systems on learning and memory, the effects of the combined stimulation or the simultaneous inactivation of these two systems on learning and memory have not been studied. In addition, it is not clear whether the effects of the cannabinoid system on learning and memory occur through the modulation of glutamatergic synaptic transmission. Hence, in this study, we examined the effects of the simultaneous inactivation of the cannabinoid and glutamatergic systems on learning and memory using a passive avoidance (PA) test in rats.

MATERIALS AND METHODS

On the test day, AM251, which is a CB1 cannabinoid receptor antagonist; MK-801, which is a glutamate receptor antagonist; or both substances were injected intraperitoneally into male Wistar rats 30min before placing the animal in a shuttle box. A learning test (acquisition) was then performed, and a retrieval test was performed the following day.

RESULTS

Learning and memory in the PA test were significantly different among the groups. The CB1 receptor antagonist improved the scores on the PA acquisition and retention tests. However, the glutamatergic receptor antagonist decreased the acquisition and retrieval scores on the PA task. The CB1 receptor antagonist partly decreased the glutamatergic receptor antagonist effects on PA learning and memory.

CONCLUSIONS

These results indicated that the acute administration of a CB1 antagonist improved cognitive performance on a PA task in normal rats and that a glutamate-related mechanism may underlie the antagonism of cannabinoid by AM251 in learning and memory.

Interaction between the cholecystokinin and endogenous cannabinoid systems in cued fear expression and extinction retention

Post-traumatic stress disorder (PTSD) is thought to develop, in part, from improper inhibition of fear. Accordingly, one of the most effective treatment strategies for PTSD is exposure-based psychotherapy. Ideally, neuroscience would inform adjunct therapies that target the neurotransmitter systems involved in extinction processes. Separate studies have implicated the cholecystokinin (CCK) and endocannabinoid systems in fear; however, there is a high degree of anatomical colocalization between the cannabinoid 1 receptor (Cnr1) and CCK in the basolateral amygdala (BLA), a brain region critical for emotion regulation. Although most research has focused on GABA and GABAergic plasticity as the mechanism by which Cnr1 mediates fear inhibition, we hypothesize that a functional interaction between Cnr1 and CCKB receptor (CCKBR) is critical for fear extinction processes. In this study, systemic pharmacological manipulation of the cannabinoid system modulated cued fear expression in C57BL/6J mice after consolidation of auditory fear conditioning. Knockout of the CCKBR, however, had no effect on fear- or anxiety-like behaviors. Nonetheless, administration of a Cnr1 antagonist increased freezing behavior during a cued fear expression test in wild-type subjects, but had no effect on freezing behavior in CCKBR knockout littermates. In addition, we found that Cnr1-positive fibers form perisomatic clusters around CCKBR-positive cell bodies in the BLA. These CCKBR-positive cells comprise a molecularly heterogenous population of excitatory and inhibitory neurons. These findings provide novel evidence that Cnr1 contributes to cued fear expression via an interaction with the CCK system. Dysfunctional Cnr1-CCKBR interactions might contribute to the etiology of, or result from, fear-related psychiatric disease.

Endocannabinoids and Mental Disorders

Preclinical and clinical data fully support the involvement of the endocannabinoid system in the etiopathogenesis of several mental diseases. In this review we will briefly summarize the most common alterations in the endocannabinoid system, in terms of cannabinoid receptors and endocannabinoid levels, present in mood disorders (anxiety, posttraumatic stress disorder, depression, bipolar disorder, and suicidality) as well as psychosis (schizophrenia) and autism. The arising picture for each pathology is not always straightforward; however, both animal and human studies seem to suggest that pharmacological modulation of this system might represent a novel approach for treatment.

Toward a translational approach to targeting the endocannabinoid system in posttraumatic stress disorder: a critical review of preclinical research

Despite the lack of clinical research, marijuana and synthetic cannabinoids have been approved to treat posttraumatic stress disorder (PTSD) in several states in the United States. This review critically examines preclinical research on the endocannabinoid system (ECS) in order to evaluate three key questions that are relevant to PTSD: (1) Does ECS dysfunction impact fear extinction? (2) Can stress-related symptoms be prevented by ECS modulation? (3) Is the ECS a potential target for enhancing PTSD treatment? Disruption of the ECS impaired fear extinction in rodents, and ECS abnormalities have been observed in PTSD. Targeting fear memories via the ECS had mixed results in rodents, whereas augmented cannabinoid receptor activation typically facilitated extinction. However, the translational value of these findings is limited by the paucity and inconsistency of human research. Further investigation is necessary to determine whether incorporating cannabinoids in treatment would benefit individuals with PTSD, with cautious attention to risks.

The endocannabinoid system: an emotional buffer in the modulation of memory function

Extensive evidence indicates that endocannabinoids modulate cognitive processes in animal models and human subjects. However, the results of endocannabinoid system manipulations on cognition have been contradictory. As for anxiety behavior, a duality has indeed emerged with regard to cannabinoid effects on memory for emotional experiences. Here we summarize findings describing cannabinoid effects on memory acquisition, consolidation, retrieval and extinction. Additionally, we review findings showing how the endocannabinoid system modulates memory function differentially, depending on the level of stress and arousal associated with the experimental context. Based on the evidence reviewed here, we propose that the endocannabinoid system is an emotional buffer that moderates the effects of environmental context and stress on cognitive processes.

A role for the endocannabinoid system in exercise-induced spatial memory enhancement in mice

It is well known that physical exercise has positive effects on cognitive functions and hippocampal plasticity. However, the underlying mechanisms have remained to be further investigated. Here we investigated the hypothesis that the memory-enhancement promoted by physical exercise relies on facilitation of the endocannabinoid system. We observed that the spatial memory tested in the object location paradigm did not persist in sedentary mice, but could be improved by 1 week of treadmill running. In addition, exercise up-regulated CB1 receptor and BDNF expression in the hippocampus. To verify if these changes required CB1 activation, we treated the mice with the selective antagonist, AM251, before each period of physical activity. In line with our hypothesis, this drug prevented the exercise-induced memory enhancement and BDNF expression. Furthermore, AM251 reduced CB1 expression. To test if facilitating the endocannabinoid system signaling would mimic the alterations observed after exercise, we treated sedentary animals during 1 week with the anandamide-hydrolysis inhibitor, URB597. Mice treated with this drug recognized the object in a new location and have increased levels of CB1 and BDNF expression in the hippocampus, showing that potentiating the endocanabinoid system equally benefits memory. In conclusion, the favorable effects of exercise upon spatial memory and BDNF expression depend on facilitation of CB1 receptor signaling, which can be mimic by inhibition of anandamide hydrolysis in sedentary animals. Our results suggest that, at least in part, the promnesic effect of the exercise is dependent of CB1 receptor activation and is mediated by BDNF.

The endocannabinoid 2-arachidonoylglycerol negatively regulates habituation by suppressing excitatory recurrent network activity and reducing long-term potentiation in the dentate gyrus

Endocannabinoids are known to mediate retrograde suppression of synaptic transmission, modulate synaptic plasticity, and influence learning and memory. The 2-arachidonoylglycerol (2-AG) produced by diacylglycerol lipase α (DGLα) is regarded as the major endocannabinoid that causes retrograde synaptic suppression. To determine how 2-AG signaling influences learning and memory, we subjected DGLα knock-out mice to two learning tasks. We tested the mice using habituation and odor-guided transverse patterning tasks that are known to involve the dentate gyrus and the CA1, respectively, of the hippocampus. We found that DGLα knock-out mice showed significantly faster habituation to an odor and a new environment than wild-type littermates with normal performance in the transverse patterning task. In freely moving animals, long-term potentiation (LTP) induced by theta burst stimulation was significantly larger at perforant path-granule cell synapses in the dentate gyrus of DGLα knock-out mice. Importantly, prior induction of synaptic potentiation at this synapse caused a significant retardation of habituation in DGLα knock-out but not in wild-type littermates. The excitability of granule cells became higher in DGLα knock-out mice after they generated action potentials. Since no differences were found in intrinsic membrane properties and responses to odor stimuli in granule cells, the elevated excitability is considered to result from enhanced activity of an excitatory recurrent network composed of granule cells and mossy cells. These results suggest that retrograde 2-AG signaling negatively regulates habituation by suppressing excitatory recurrent network activity and reducing LTP in the dentate gyrus.

Activation of extrasynaptic NMDA receptors induces LTD in rat hippocampal CA1 neurons

In the adult rat hippocampus, activation of N-methyl-d-aspartate receptors (NMDARs) is required for the induction of certain forms of synaptic plasticity, including long-term potentiation (LTP) and long-term depression (LTD). Several studies have indicated the opposing role of synaptic NMDARS (S-NMDARs) versus extrasynaptic NMDARs (ES-NMDARs) in CREB-dependent gene regulation and neuronal survival/death. The contribution of ES-NMDARs in synaptic plasticity, however, remains unclear. Here we investigated the contribution of ES-NMDARs on LTD induction in CA1 neurons of rat hippocampal slices. ES-NMDARs were selectively activated by theta burst stimulation (TBS) after selective blockade of S-NMDARs with pairing of 5 Hz stimulation and MK-801, an irreversible use-dependent antagonist of NMDARs. Application of TBS in naïve slices evoked a transient potentiation. In contrast, the activation of ES-NMDARs evoked a robust LTD. These results suggest the involvement of ES-NMDARs in LTD induction.

The decrease of dopamine D₂/D₃ receptor densities in the putamen and nucleus caudatus goes parallel with maintained levels of CB₁ cannabinoid receptors in Parkinson's disease: a preliminary autoradiographic study with the selective dopamine D₂/D₃ antagonist [³H]raclopride and the novel CB₁ inverse agonist [¹²⁵I]SD7015

Cannabinoid type-1 receptors (CB₁Rs) modulate synaptic neurotransmission by participating in retrograde signaling in the adult brain. Increasing evidence suggests that cannabinoids through CB₁Rs play an important role in the regulation of motor activities in the striatum. In the present study, we used human brain samples to examine the relationship between CB₁R and dopamine receptor density in case of Parkinson's disease (PD). Post mortem putamen, nucleus caudatus and medial frontal gyrus samples obtained from PD patients were used for CB₁R and dopamine D₂/D₃ receptor autoradiography. [¹²⁵I]SD7015, a novel selective CB₁R inverse agonist, developed by a number of the present co-authors, and [³H]raclopride, a dopamine D₂/D₃ antagonist, were used as radioligands. Our results demonstrate unchanged CB₁R density in the putamen and nucleus caudatus of deceased PD patients, treated with levodopa (L-DOPA). At the same time dopamine D₂/D₃ receptors displayed significantly decreased density levels in case of PD putamen (control: 47.97 ± 10.00 fmol/g, PD: 3.73 ± 0.07 fmol/g (mean ± SEM), p<0.05) and nucleus caudatus (control: 30.26 ± 2.48 fmol/g, PD: 12.84 ± 5.49 fmol/g, p<0.0005) samples. In contrast to the putamen and the nucleus caudatus, in the medial frontal gyrus neither receptor densities were affected. Our data suggest the presence of an unaltered CB₁R population even in late stages of levodopa treated PD. This further supports the presence of an intact CB₁R population which, in line with the conclusion of earlier publications, may be utilized as a pharmacological target in the treatment of PD. Furthermore we found discrepancy between a maintained CB₁R population and a decreased dopamine D₂/D₃ receptor population in PD striatum. The precise explanation of this conundrum requires further studies with simultaneous examination of the central cannabinoid and dopaminergic systems in PD using higher sample size.

Changes in inhibitory CA1 network in dual pathology model of epilepsy

The combination of two precipitating factors appears to be more and more recognized in patients with temporal lobe epilepsy. Using a two-hit rat model, with a neonatal freeze lesion mimicking a focal cortical malformation combined with hyperthermia-induced seizures mimicking febrile seizures, we have previously reported an increase of inhibition in CA1 pyramidal cells at P20. Here, we investigated the changes affecting excitatory and inhibitory drive onto CA1 interneurons to better define the changes in CA1 inhibitory networks and their paradoxical role in epileptogenesis, using electrophysiological recordings in CA1 hippocampus from rat pups (16-20 d old). We investigated interneurons in CA1 hippocampal area located in stratum oriens (Or) and at the border of strata lacunosum and moleculare (L-M). Our results revealed an increase of the excitatory drive to both types of interneurons with no change in the inhibitory drive. The mechanisms underlying the increase of excitatory synaptic currents (EPSCs) in both types of interneurons are different. In Or interneurons, the amplitude of spontaneous and miniature EPSCs increased, while their frequency was not affected suggesting changes at the post-synaptic level. In L-M interneurons, the frequency of spontaneous EPSCs increases, but the amplitude is not affected. Analyses of miniature EPSCs showed no changes in both their frequency and amplitude. We concluded that L-M interneurons increase in excitatory drive is due to a change in Shaffer collateral axon excitability. The changes described here in CA1 inhibitory network may actually contribute to the epileptogenicity observed in this dual pathology model by increasing pyramidal cell synchronization.