Evidence for differential modulation of conditioned aversion and fear-conditioned analgesia by CB1 receptors

Effects of Intra-BLA Administration of PPAR Antagonists on Formalin-Evoked Nociceptive Behaviour, Fear-Conditioned Analgesia, and Conditioned Fear in the Presence or Absence of Nociceptive Tone in Rats

There is evidence for the involvement of peroxisome proliferator-activated receptors (PPARs) in pain, cognition, and anxiety. However, their role in pain–fear interactions is unknown. The amygdala plays a key role in pain, conditioned fear, and fear-conditioned analgesia (FCA). We investigated the effects of intra-basolateral amygdala (BLA) administration of PPARα, PPARβ/δ, and PPARγ antagonists on nociceptive behaviour, FCA, and conditioned fear in the presence or absence of nociceptive tone. Male Sprague-Dawley (SD) rats received footshock (FC) or no footshock (NFC) in a conditioning arena. Twenty-three and a half hours later, rats received an intraplantar injection of formalin or saline and, 15 min later, intra-BLA microinjections of vehicle, PPARα (GW6471) PPARβ/δ (GSK0660), or PPARγ (GW9662) antagonists before arena re-exposure. Pain and fear-related behaviour were assessed, and neurotransmitters/endocannabinoids measured post-mortem. Intra-BLA administration of PPARα or PPARγ antagonists potentiated freezing in the presence of nociceptive tone. Blockade of all PPAR subtypes in the BLA increased freezing and BLA dopamine levels in NFC rats in the absence of nociceptive tone. Administration of intra-BLA PPARα and PPARγ antagonists increased levels of dopamine in the BLA compared with the vehicle-treated counterparts. In conclusion, PPARα and PPARγ in the BLA play a role in the expression or extinction of conditioned fear in the presence or absence of nociceptive tone.

Sexually Dimorphic Expression of Fear-conditioned Analgesia in Rats and Associated Alterations in the Endocannabinoid System in the Periaqueductal Grey

The endocannabinoid system within the periaqueductal grey (PAG) has been implicated in fear-conditioned analgesia (FCA), the profound suppression of pain upon re-exposure to a context previously paired with an aversive stimulus. Since the endocannabinoid and nociceptive systems exhibit sexual dimorphism, the aim of the present study was to assess possible sex differences in the expression of FCA, fear in the presence of nociceptive tone, and associated sex-dependent alterations in the endocannabinoid system within the PAG. Male and female Sprague-Dawley rats received footshock (10 × 1s; 0.4 mA; every 60 s) or no-footshock in a conditioning arena and 23.5 h later received intraplantar injection of formalin (2.5%) under brief isoflourane anaesthetic into the right hind paw. Nociceptive and fear-related behaviours were assessed 30 min later. Levels of endocannabinoids, N-acylethanolamines and neurotransmitters in the PAG were assessed by LC-MS/MS and expression of endocannabinoid system-related proteins by Western immunoblotting. Male, but not female, rats exhibited robust FCA and greater expression of fear-related behaviours than females. Fear-conditioned formalin-treated males, but not females, had higher levels of N-oleoylethanolamine (OEA) and γ-aminobutyric acid (GABA) in the PAG, compared with non-fear-conditioned controls. There was no effect of fear conditioning on the levels of FAAH or CB₁ receptor expression (CB1R) in the PAG of male or female formalin-treated rats. Non-fear-conditioned females had higher levels of CB1R and PPARγ expression than non-fear-conditioned male counterparts. In summary, our results provide evidence of sexual dimorphism in the expression of FCA and fear-related behaviours, and associated alterations in components of the endocannabinoid system and GABA within the PAG.

Cannabinoids, the endocannabinoid system, and pain: a review of preclinical studies

ABSTRACT

This narrative review represents an output from the International Association for the Study of Pain's global task force on the use of cannabis, cannabinoids, and cannabis-based medicines for pain management, informed by our companion systematic review and meta-analysis of preclinical studies in this area. Our aims in this review are (1) to describe the value of studying cannabinoids and endogenous cannabinoid (endocannabinoid) system modulators in preclinical/animal models of pain; (2) to discuss both pain-related efficacy and additional pain-relevant effects (adverse and beneficial) of cannabinoids and endocannabinoid system modulators as they pertain to animal models of pathological or injury-related persistent pain; and (3) to identify important directions for future research. In service of these goals, this review (1) provides an overview of the endocannabinoid system and the pharmacology of cannabinoids and endocannabinoid system modulators, with specific relevance to animal models of pathological or injury-related persistent pain; (2) describes pharmacokinetics of cannabinoids in rodents and humans; and (3) highlights differences and discrepancies between preclinical and clinical studies in this area. Preclinical (rodent) models have advanced our understanding of the underlying sites and mechanisms of action of cannabinoids and the endocannabinoid system in suppressing nociceptive signaling and behaviors. We conclude that substantial evidence from animal models supports the contention that cannabinoids and endocannabinoid system modulators hold considerable promise for analgesic drug development, although the challenge of translating this knowledge into clinically useful medicines is not to be underestimated.

Reduced behavioral withdrawal responses during fear retrieval in adult mice and rats

Pain triggers emotional changes in humans and animals, including fear and anxiety. Conversely, fear and anxiety may enhance suffering of patients with pain. However, in animal models of acute pain, it has been reported that fear may inhibit pain by activating endogenous inhibitory systems. In this study, we wanted to examine if behavioral withdrawal responses may be affected during fear retrieval, a condition where fear-associated tone is applied. We found that thermal pain thresholds were significantly increased during fear retrieval. Our results indicate that animals are suffering fear like-events, while their behavioral responses are inhibited. These results indicate that it will be important to evaluate both emotional and behavioral withdrawal responses for future development of new pain medicine.

Attenuation of fear-conditioned analgesia in rats by monoacylglycerol lipase inhibition in the anterior cingulate cortex: Potential role for CB2 receptors

BACKGROUND AND PURPOSE

Improved understanding of brain mechanisms regulating endogenous analgesia is important from a fundamental physiological perspective and for identification of novel therapeutic strategies for pain. The endocannabinoid system plays a key role in stress-induced analgesia, including fear-conditioned analgesia (FCA), a potent form of endogenous analgesia. Here, we studied the role of the endocannabinoid 2-arachidonoyl glycerol (2-AG) within the anterior cingulate cortex (ACC; a brain region implicated in the affective component of pain) in FCA in rats.

EXPERIMENTAL APPROACH

FCA was modelled in male Lister-hooded rats by assessing formalin-evoked nociceptive behaviour in an arena previously paired with footshock. The effects of intra-ACC administration of MJN110 (inhibitor of monoacylglycerol lipase [MGL], the primary enzyme catabolizing 2-AG), AM630 (CB2 receptor antagonist), AM251 (CB1 receptor antagonist) or MJN110 + AM630 on FCA were assessed.

KEY RESULTS

MJN110 attenuated FCA when microinjected into the ACC, an effect associated with increased levels of 2-AG in the ACC. This effect of MJN110 on FCA was unaltered by co-administration of AM251 but was blocked by AM630, which alone reduced nociceptive behaviour in non-fear-conditioned rats. RT-qPCR confirmed that mRNA encoding CB1 and CB2 receptors was detectable in the ACC of formalin-injected rats and unchanged in those expressing FCA.

CONCLUSION AND IMPLICATIONS

These results suggest that an MGL substrate in the ACC, likely 2-AG, modulates FCA and that within the ACC, 2-AG-CB2 receptor signalling may suppress this form of endogenous analgesia. These results may facilitate increased understanding and improved treatment of pain- and fear-related disorders and their co-morbidity.

Impact of anesthesia exposure in early development on learning and sensory functions

Each year, millions of children undergo anesthesia, and both human and animal studies have indicated that exposure to anesthesia at an early age can lead to neuronal damage and learning deficiency. However, disorders of sensory functions were not reported in children or animals exposed to anesthesia during infancy, which is surprising, given the significant amount of damage to brain tissue reported in many animal studies. In this review, we discuss the relationship between the systems in the brain that mediate sensory input, spatial learning, and classical conditioning, and how these systems could be affected during anesthesia exposure. Based on previous reports, we conclude that anesthesia can induce structural, functional, and compensatory changes in both sensory and learning systems. Changes in myelination following anesthesia exposure were observed as well as the neurodegeneration in the gray matter across variety of brain regions. Disproportionate cell death between excitatory and inhibitory cells induced by anesthesia exposure can lead to a long-term shift in the excitatory/inhibitory balance, which affects both learning-specific networks and sensory systems. Anesthesia may directly affect synaptic plasticity which is especially critical to learning acquisition. However, sensory systems appear to have better ability to compensate for damage than learning-specific networks.

European Pain Federation (EFIC) position paper on appropriate use of cannabis-based medicines and medical cannabis for chronic pain management

Cannabis-based medicines are being approved for pain management in an increasing number of European countries. There are uncertainties and controversies on the role and appropriate use of cannabis-based medicines for the management of chronic pain. EFIC convened a European group of experts, drawn from a diverse range of basic science and relevant clinical disciplines, to prepare a position paper to empower and inform specialist and nonspecialist prescribers on appropriate use of cannabis-based medicines for chronic pain. The expert panel reviewed the available literature and harnessed the clinical experience to produce these series of recommendations. Therapy with cannabis-based medicines should only be considered by experienced clinicians as part of a multidisciplinary treatment and preferably as adjunctive medication if guideline-recommended first- and second-line therapies have not provided sufficient efficacy or tolerability. The quantity and quality of evidence are such that cannabis-based medicines may be reasonably considered for chronic neuropathic pain. For all other chronic pain conditions (cancer, non-neuropathic noncancer pain), the use of cannabis-based medicines should be regarded as an individual therapeutic trial. Realistic goals of therapy have to be defined. All patients must be kept under close clinical surveillance. As with any other medical therapy, if the treatment fails to reach the predefined goals and/or the patient is additionally burdened by an unacceptable level of adverse effects and/or there are signs of abuse and misuse of the drug by the patient, therapy with cannabis-based medicines should be terminated.

SIGNIFICANCE

This position paper provides expert recommendations for nonspecialist and specialist healthcare professionals in Europe, on the importance and the appropriate use of cannabis-based medicines as part of a multidisciplinary approach to pain management, in properly selected and supervised patients.

The prefrontal cortical endocannabinoid system modulates fear-pain interactions in a subregion-specific manner

BACKGROUND AND PURPOSE

The emotional processing and coordination of top-down responses to noxious and conditioned aversive stimuli involves the medial prefrontal cortex (mPFC). Evidence suggests that subregions of the mPFC [infralimbic (IfL), prelimbic (PrL) and anterior cingulate (ACC) cortices] differentially alter the expression of contextually induced fear and nociceptive behaviour. We investigated the role of the endocannabinoid system in the IfL, PrL and ACC in formalin-evoked nociceptive behaviour, fear-conditioned analgesia (FCA) and conditioned fear in the presence of nociceptive tone.

EXPERIMENTAL APPROACH

FCA was modelled in male Lister-hooded rats by assessing formalin-evoked nociceptive behaviour in an arena previously paired with footshock. The effects of intra-mPFC administration of AM251 [cannabinoid type 1 (CB₁ ) receptor antagonist/inverse agonist], URB597 [fatty acid amide hydrolase (FAAH) inhibitor] or URB597 + AM251 on FCA and freezing behaviour were assessed.

KEY RESULTS

AM251 attenuated FCA when injected into the IfL or PrL and reduced contextually induced freezing behaviour when injected intra-IfL but not intra-PrL or intra-ACC. Intra-ACC administration of AM251 alone or in combination with URB597 had no effect on FCA or freezing. URB597 attenuated FCA and freezing behaviour when injected intra-IfL, prolonged the expression of FCA when injected intra-PrL and had no effect on these behaviours when injected intra-ACC.

CONCLUSIONS AND IMPLICATIONS

These results suggest important and differing roles for FAAH substrates or CB₁ receptors in the PrL, IfL and ACC in the expression of FCA and conditioned fear in the presence of nociceptive tone.

LINKED ARTICLES

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

Mechanisms of placebo analgesia: A dual-process model informed by insights from cross-species comparisons

Placebo treatments are pharmacologically inert, but are known to alleviate symptoms across a variety of clinical conditions. Associative learning and cognitive expectations both play important roles in placebo responses, however we are just beginning to understand how interactions between these processes lead to powerful effects. Here, we review the psychological principles underlying placebo effects and our current understanding of their brain bases, focusing on studies demonstrating both the importance of cognitive expectations and those that demonstrate expectancy-independent associative learning. To account for both forms of placebo analgesia, we propose a dual-process model in which flexible, contextually driven cognitive schemas and attributions guide associative learning processes that produce stable, long-term placebo effects. According to this model, the placebo-induction paradigms with the most powerful effects are those that combine reinforcement (e.g., the experience of reduced pain after placebo treatment) with suggestions and context cues that disambiguate learning by attributing perceived benefit to the placebo. Using this model as a conceptual scaffold, we review and compare neurobiological systems identified in both human studies of placebo analgesia and behavioral pain modulation in rodents. We identify substantial overlap between the circuits involved in human placebo analgesia and those that mediate multiple forms of context-based modulation of pain behavior in rodents, including forebrain-brainstem pathways and opioid and cannabinoid systems in particular. This overlap suggests that placebo effects are part of a set of adaptive mechanisms for shaping nociceptive signaling based on its information value and anticipated optimal response in a given behavioral context.

The cannabinoid system and pain

Chronic pain states are highly prevalent and yet poorly controlled by currently available analgesics, representing an enormous clinical, societal, and economic burden. Existing pain medications have significant limitations and adverse effects including tolerance, dependence, gastrointestinal dysfunction, cognitive impairment, and a narrow therapeutic window, making the search for novel analgesics ever more important. In this article, we review the role of an important endogenous pain control system, the endocannabinoid (EC) system, in the sensory, emotional, and cognitive aspects of pain. Herein, we briefly cover the discovery of the EC system and its role in pain processing pathways, before concentrating on three areas of current major interest in EC pain research; 1. Pharmacological enhancement of endocannabinoid activity (via blockade of EC metabolism or allosteric modulation of CB1receptors); 2. The EC System and stress-induced modulation of pain; and 3. The EC system & medial prefrontal cortex (mPFC) dysfunction in pain states. Whilst we focus predominantly on the preclinical data, we also include extensive discussion of recent clinical failures of endocannabinoid-related therapies, the future potential of these approaches, and important directions for future research on the EC system and pain. This article is part of the Special Issue entitled "A New Dawn in Cannabinoid Neurobiology".

Impact of stress, fear and anxiety on the nociceptive responses of larval zebrafish

Both adult and larval zebrafish have been demonstrated to show behavioural responses to noxious stimulation but also to potentially stress- and fear or anxiety- eliciting situations. The pain or nociceptive response can be altered and modulated by these situations in adult fish through a mechanism called stress-induced analgesia. However, this phenomenon has not been described in larval fish yet. Therefore, this study explores the behavioural changes in larval zebrafish after noxious stimulation and exposure to challenges that can trigger a stress, fear or anxiety reaction. Five-day post fertilization zebrafish were exposed to either a stressor (air emersion), a predatory fear cue (alarm substance) or an anxiogenic (caffeine) alone or prior to immersion in acetic acid 0.1%. Pre- and post-stimulation behaviour (swimming velocity and time spent active) was recorded using a novel tracking software in 25 fish at once. Results show that larvae reduced both velocity and activity after exposure to the air emersion and alarm substance challenges and that these changes were attenuated using etomidate and diazepam, respectively. Exposure to acetic acid decreased velocity and activity as well, whereas air emersion and alarm substance inhibited these responses, showing no differences between pre- and post-stimulation. Therefore, we hypothesize that an antinociceptive mechanism, activated by stress and/or fear, occur in 5dpf zebrafish, which could have prevented the larvae to display the characteristic responses to pain.

Dorsolateral periaqueductal gray matter CB1 and TRPV1 receptors exert opposite modulation on expression of contextual fear conditioning

Cannabinoid type 1 (CB1) and Transient Potential Vanilloid type 1 (TRPV1) receptors in the dorsolateral periaqueductal gray (dlPAG) matter are involved in the modulation of conditioned response. Both CB1 and TRPV1 receptors are related to glutamate release and nitric oxide (NO) synthesis. It was previously demonstrated that both NMDA glutamate receptors and NO are involved in the conditioned emotional response. Therefore, one aim of this work was to verify whether dlPAG CB1 and TRPV1 receptors modulate the expression of contextual conditioned emotional response. Moreover, we also investigated the involvement of NMDA receptors and the NO pathway in this response. Male Wistar rats with local dlPAG guide cannula were submitted to contextual fear conditioning. Following 24 h, a polyethylene catheter was implanted in the femoral artery for cardiovascular recordings. After an additional 24 h, drugs were administered in the dlPAG and freezing behavior and autonomic responses were recorded during chamber re-exposure. Both a CB1 antagonist (AM251) and a TRPV1 agonist (Capsaicin; CPS) increased the expression of a conditioned emotional response. This response was prevented by an NMDA antagonist, a preferential neuronal NO synthase inhibitor, an NO scavenger and a soluble guanylate cyclase inhibitor (sGC). Furthermore, pretreatment with a TRPV1 antagonist also prevented the increased conditioned emotional response induced by AM251. Considering that GABA can counterbalance glutamate effects, we also investigated whether GABAA receptors were involved in the effect of a higher dose of AM251. Pretreatment with a GABAA receptor antagonist caused an increased conditioned emotional response by AM251. Our results support the possibility that dlPAG CB1 and TRPV1 receptors are involved in the expression of conditioned emotional response through the NMDA/NO/sGC pathway. Moreover, the opposite effects exerted by GABA and glutamate could produce different outcomes of drugs modulating eCBs.

High Times for Painful Blues: The Endocannabinoid System in Pain-Depression Comorbidity

Depression and pain are two of the most debilitating disorders worldwide and have an estimated cooccurrence of up to 80%. Comorbidity of these disorders is more difficult to treat, associated with significant disability and impaired health-related quality of life than either condition alone, resulting in enormous social and economic cost. Several neural substrates have been identified as potential mediators in the association between depression and pain, including neuroanatomical reorganization, monoamine and neurotrophin depletion, dysregulation of the hypothalamo-pituitary-adrenal axis, and neuroinflammation. However, the past decade has seen mounting evidence supporting a role for the endogenous cannabinoid (endocannabinoid) system in affective and nociceptive processing, and thus, alterations in this system may play a key role in reciprocal interactions between depression and pain. This review will provide an overview of the preclinical evidence supporting an interaction between depression and pain and the evidence supporting a role for the endocannabinoid system in this interaction.

Involvement of the endocannabinoid system in attentional modulation of nociceptive behaviour in rats

BACKGROUND

Distraction is used clinically to relieve and manage pain. It is hypothesized that pain demands attention and that exposure to another attention-demanding stimulus causes withdrawal of attention away from painful stimuli, thereby reducing perceived pain. We have recently developed a rat model that provides an opportunity to investigate the neurobiological mechanisms mediating distraction-induced analgesia, as these mechanisms are, at present, poorly understood. Given the well-described role of the endogenous cannabinoid (endocannabinoid; EC) system in the modulation of pain and attentional processing, the present study investigated its role in distraction-induced antinociception in rats.

METHODS

Animals received the CB1 receptor antagonist/inverse agonist, rimonabant or vehicle intraperitoneally, 30 min prior to behavioural evaluation. Formalin-evoked nociceptive behaviour was measured in the presence or absence of a novel-object distractor. Liquid chromatography-tandem mass spectrometry was used to determine the levels of the endogenous cannabinoids anandamide and 2-arachidonoylglycerol (2-AG) in the ventral hippocampus (vHip).

RESULTS

Exposure to a novel object distractor significantly reduced formalin-evoked nociceptive behaviour. The novel object-induced reduction in nociceptive behaviour was attenuated by rimonabant. Novel object exposure was also associated with increased tissue levels of anandamide and 2-AG in the vHip.

CONCLUSIONS

These data suggest that the reduction in formalin-evoked nociceptive behaviour that occurs as a result of exposure to a novel object may be mediated by engagement of the EC system, in particular in the vHip. The results provide evidence that the EC system may be an important neural substrate subserving attentional modulation of pain.

Microinjection of 2-arachidonoyl glycerol into the rat ventral hippocampus differentially modulates contextually induced fear, depending on a persistent pain state

The endogenous cannabinoid (endocannabinoid) system plays a key role in the modulation of aversive and nociceptive behaviour. The components of the endocannabinoid system are expressed throughout the hippocampus, a brain region implicated in both conditioned fear and pain. In light of evidence that pain can impact on the expression of fear-related behaviour, and vice versa, we hypothesised that exogenous administration of the endocannabinoid 2-arachidonoyl glycerol (2-AG) into the ventral hippocampus (vHip) would differentially regulate fear responding in the absence vs. the presence of formalin-evoked nociceptive tone. Fear-conditioned rats showed significantly increased freezing and a reduction in formalin-evoked nociceptive behaviour upon re-exposure to a context previously paired with footshock. Bilateral microinjection of 2-AG into the vHip significantly reduced contextually induced freezing in non-formalin-treated rats, and reduced formalin-evoked nociceptive behaviour in non-fear-conditioned rats. In contrast, 2-AG microinjection had no effect on fear responding in formalin-treated rats, and no effect on nociceptive behaviour in fear-conditioned rats. The inhibitory effect of 2-AG on fear-related behaviour, but not pain-related behaviour, was blocked by co-administration of the cannabinoid receptor 1 (CB1) antagonist/inverse agonist rimonabant. Tissue levels of the endocannabinoids N-arachidonoylethanolamide (anandamide, AEA) and 2-AG were similar in the vHip of fear-conditioned rats receiving formalin injection and the vHip of fear-conditioned rats receiving saline injection. However, the levels of AEA and 2-AG were significantly lower in the contralateral ventrolateral periaqueductal grey of formalin-treated fear-conditioned rats than in that of their saline-treated counterparts. These data suggest that 2-AG-CB1 receptor signalling in the vHip has an anti-aversive effect, and that this effect is abolished in the presence of a persistent pain state.

No more pain upon Gq-protein-coupled receptor activation: role of endocannabinoids

Marijuana has been used to relieve pain for centuries. The analgesic mechanism of its constituents, the cannabinoids, was only revealed after the discovery of cannabinoid receptors (CB1 and CB2) two decades ago. The subsequent identification of the endocannabinoids, anandamide and 2-arachidonoylglycerol (2-AG), and their biosynthetic and degradation enzymes discloses the therapeutic potential of compounds targeting the endocannabinoid system for pain control. Inhibitors of the anandamide and 2-AG degradation enzymes, fatty acid amide hydrolase and monoacylglycerol lipase, respectively, may be superior to direct cannabinoid receptor ligands as endocannabinoids are synthesized on demand and rapidly degraded, focusing action at generating sites. Recently, a promising strategy for pain relief was revealed in the periaqueductal gray (PAG). It is initiated by Gq-protein-coupled receptor (Gq PCR) activation of the phospholipase C-diacylglycerol lipase enzymatic cascade, generating 2-AG that produces inhibition of GABAergic transmission (disinhibition) in the PAG, thereby leading to analgesia. Here, we introduce the antinociceptive properties of exogenous cannabinoids and endocannabinoids, involving their biosynthesis and degradation processes, particularly in the PAG. We also review recent studies disclosing the Gq PCR-phospholipase C-diacylglycerol lipase-2-AG retrograde disinhibition mechanism in the PAG, induced by activating several Gq PCRs, including metabotropic glutamatergic (type 5 metabotropic glutamate receptor), muscarinic acetylcholine (M1/M3), and orexin 1 receptors. Disinhibition mediated by type 5 metabotropic glutamate receptor can be initiated by glutamate transporter inhibitors or indirectly by substance P, neurotensin, cholecystokinin and capsaicin. Finally, the putative role of 2-AG generated after activating the above neurotransmitter receptors in stress-induced analgesia is discussed.

Amygdala FAAH and anandamide: mediating protection and recovery from stress

A long-standing literature linking endocannabinoids (ECBs) to stress, fear, and anxiety has led to growing interest in developing novel anxiolytics targeting the ECB system. Following rapid on-demand biosynthesis and degradation upon neuronal activation, the ECB N-arachidonoylethanolamide (anandamide, AEA) is actively degraded by the serine hydrolase enzyme, fatty acid amide hydrolase (FAAH). Exposure to stress rapidly mobilizes FAAH to deplete the signaling pool of AEA and increase neuronal excitability in a key anxiety-mediating region--the basolateral amygdala (BLA). Gene deletion or pharmacological inhibition of FAAH prevents stress-induced reductions in AEA and associated increases in BLA dendritic hypertrophy and anxiety-like behavior. Additionally, inhibition of FAAH facilitates long-term fear extinction and rescues deficient fear extinction in rodent models by enhancing AEA-CB1 (cannabinoid type 1) receptor signaling and synaptic plasticity in the BLA. These preclinical findings propose restoring deficient BLA AEA levels by pharmacologically inhibiting FAAH as a mechanism to therapeutically mitigate the effects of traumatic stress.

Stress-induced antinociception in fish reversed by naloxone

Pain perception in non-mammalian vertebrates such as fish is a controversial issue. We demonstrate that, in the fish Leporinus macrocephalus, an imposed restraint can modulate the behavioral response to a noxious stimulus, specifically the subcutaneous injection of 3% formaldehyde. In the first experiment, formaldehyde was applied immediately after 3 or 5 min of the restraint. Inhibition of the increase in locomotor activity in response to formaldehyde was observed, which suggests a possible restraint-induced antinociception. In the second experiment, the noxious stimulus was applied 0, 5, 10 and 15 min after the restraint, and both 3 and 5 min of restraint promoted short-term antinociception of approximately 5 min. In experiments 3 and 4, an intraperitoneal injection of naloxone (30 mg.kg(-1)) was administered 30 min prior to the restraint. The 3- minute restraint-induced antinociception was blocked by pretreatment with naloxone, but the corresponding 5-minute response was not. One possible explanation for this result is that an opioid and a non-preferential μ-opioid and/or non-opioid mechanism participate in this response modulation. Furthermore, we observed that both the 3- and 5- minutes restraint were severely stressful events for the organism, promoting marked increases in serum cortisol levels. These data indicate that the response to a noxious stimulus can be modulated by an environmental stressor in fish, as is the case in mammals. To our knowledge, this study is the first evidence for the existence of an endogenous antinociceptive system that is activated by an acute standardized stress in fish. Additionally, it characterizes the antinociceptive response induced by stress in terms of its time course and the opioid mediation, providing information for understanding the evolution of nociception modulation.

Behavioural, neurochemical and neuroendocrine effects of the endogenous β-carboline harmane in fear-conditioned rats

The putative endogenous imidazoline binding site ligand harmane enhances neuronal activation in response to psychological stress and alters behaviour in animal models of anxiety and antidepressant efficacy. However, the neurobiological mechanisms underlying harmane's psychotropic effects are poorly understood. We investigated the effects of intraperitoneal injection of harmane (2.5 and 10 mg/kg) on fear-conditioned behaviour, hypothalamo-pituitary-adrenal axis activity, and monoaminergic activity within specific fear-associated areas of the rat brain. Harmane had no significant effect on the duration of contextually induced freezing or 22 kHz ultrasonic vocalisations and did not alter the contextually induced suppression of motor activity, including rearing. Harmane reduced the duration of rearing and tended to increase freezing in non-fear-conditioned controls, suggesting potential sedative effects. Harmane increased plasma ACTH and corticosterone concentrations, and serotonin (in hypothalamus, amygdaloid cortex, prefrontal cortex and hippocampus) and noradrenaline (prefrontal cortex) content, irrespective of fear-conditioning. Furthermore, harmane reduced dopamine and serotonin turnover in the PFC and hypothalamus, and serotonin turnover in the amygdaloid cortex in both fear-conditioned and non-fear-conditioned rats. In contrast, harmane increased dopamine and noradrenaline content and reduced dopamine turnover in the amygdala of fear-conditioned rats only, suggesting differential effects on catecholaminergic transmission in the presence and absence of fear. The precise mechanism(s) mediating these effects of harmane remain to be determined but may involve its inhibitory action on monoamine oxidases. These findings support a role for harmane as a neuromodulator, altering behaviour, brain neurochemistry and neuroendocrine function.

The endocannabinoid system in the rat dorsolateral periaqueductal grey mediates fear-conditioned analgesia and controls fear expression in the presence of nociceptive tone

BACKGROUND AND PURPOSE

Endocannabinoids in the midbrain periaqueductal grey (PAG) modulate nociception and unconditioned stress-induced analgesia; however, their role in fear-conditioned analgesia (FCA) has not been examined. The present study examined the role of the endocannabinoid system in the dorsolateral (dl) PAG in formalin-evoked nociceptive behaviour, conditioned fear and FCA in rats.

EXPERIMENTAL APPROACH

Rats received intra-dlPAG administration of the CB(1) receptor antagonist/inverse agonist rimonabant, or vehicle, before re-exposure to a context paired 24 h previously with foot shock. Formalin-evoked nociceptive behaviour and fear-related behaviours (freezing and 22 kHz ultrasonic vocalization) were assessed. In a separate cohort, levels of endocannabinoids [2-arachidonoyl glycerol (2-AG) and N-arachidonoyl ethanolamide (anandamide; AEA)] and the related N-acylethanolamines (NAEs) [N-palmitoyl ethanolamide (PEA) and N-oleoyl ethanolamide (OEA)] were measured in dlPAG tissue following re-exposure to conditioned context in the presence or absence of formalin-evoked nociceptive tone.

KEY RESULTS

Re-exposure of rats to the context previously associated with foot shock resulted in FCA. Intra-dlPAG administration of rimonabant significantly attenuated FCA and fear-related behaviours expressed in the presence of nociceptive tone. Conditioned fear without formalin-evoked nociceptive tone was associated with increased levels of 2-AG, AEA, PEA and OEA in the dlPAG. FCA was specifically associated with an increase in AEA levels in the dlPAG.

CONCLUSIONS AND IMPLICATIONS

Conditioned fear to context mobilises endocannabinoids and NAEs in the dlPAG. These data support a role for endocannabinoids in the dlPAG in mediating the potent suppression of pain responding which occurs during exposure to conditioned aversive contexts.

LINKED ARTICLES

This article is part of a themed section on Cannabinoids in Biology and Medicine. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-8. To view Part I of Cannabinoids in Biology and Medicine visit http://dx.doi.org/10.1111/bph.2011.163.issue-7.

Anxiogenic-like effects of chronic cannabidiol administration in rats

RATIONALE

Several pre-clinical and human-based studies have shown that acutely administered cannabidiol (CBD) can produce anxiolytic-like effects

OBJECTIVES

The present study investigated the effects of chronic administration of CBD on rat behaviour and on the expression of brain proteins.

METHODS

Male Lister-hooded rats (150-200 g, n = 8 per group) received daily injections of CBD (10 mg/kg, i.p.) for 14 days. The rats were subjected to two behavioural tests: locomotor activity and conditioned emotional response (CER). The expression of brain-derived neurotrophic factor (BDNF), its receptor tyrosine kinase B (Trk B), extracellular signal-regulated kinases (ERK1/2) and phospho-ERK1/2 and the transcription factor cyclic AMP response element binding protein activation (CREB) and phospho-CREB were determined in brain regions such as the frontal cortex and hippocampus using Western immunoblotting.

RESULTS

CBD significantly increased the time spent freezing in the CER test with no effect on locomotor activity. CBD significantly reduced BDNF expression in the hippocampus and frontal cortex with no change in the striatum. In addition, CBD significantly reduced TrkB expression in the hippocampus with a strong trend towards reduction in the striatum but had no effect in the frontal cortex. In the hippocampus, CBD had no effect on ERK1/2 or phospho-ERK2, but in the frontal cortex, CBD significantly reduced phospho-ERK1/2 expression without affecting total ERK.

CONCLUSION

Chronic administration of CBD produced an anxiogenic-like effect in clear opposition to the acute anxiolytic profile previously reported. In addition, CBD decreased the expression of proteins that have been shown to be enhanced by chronic treatment with antidepressant/anxiolytic drugs.

Fear-induced suppression of nociceptive behaviour and activation of Akt signalling in the rat periaqueductal grey: role of fatty acid amide hydrolase

The endocannabinoid system regulates nociception and aversion and mediates fear-conditioned analgesia (FCA). We investigated the effects of the fatty acid amide hydrolase (FAAH) inhibitor URB597, which inhibits the catabolism of the endocannabinoid anandamide and related N-acylethanolamines, on expression of FCA and fear and pain related behaviour per se in rats. We also examined associated alterations in the expression of the signal transduction molecule phospho-Akt in the periaqueductal grey (PAG) by immunoblotting. FCA was modelled by assessing formalin-evoked nociceptive behaviour in an arena previously paired with footshock. URB597 (0.3 mg/kg, i.p.) enhanced FCA and increased fear-related behaviour in formalin-treated rats. Conditioned fear per se in non-formalin-treated rats was associated with increased expression of phospho-Akt in the PAG. URB597 reduced the expression of fear-related behaviour in the early part of the trial, an effect that was accompanied by attenuation of the fear-induced increase in phospho-Akt expression in the PAG. Intra-plantar injection of formalin also reduced the fear-induced increase in phospho-Akt expression. These data provide evidence for a role of FAAH in FCA, fear responding in the presence or absence of nociceptive tone, and fear-evoked increases in PAG phospho-Akt expression. In addition, the results suggest that fear-evoked activation of Akt signalling in the PAG is abolished in the presence of nociceptive tone.

Neuroanatomical substrates involved in cannabinoid modulation of defensive responses

Administration of Cannabis sativa derivatives causes anxiolytic or anxiogenic effects in humans and laboratory animals, depending on the specific compound and dosage used. In agreement with these findings, several studies in the last decade have indicated that the endocannabinoid system modulates neuronal activity in areas involved in defensive responses. The mechanisms of these effects, however, are still not clear. The present review summarizes recent data suggesting that they involve modulation of glutamate and GABA-mediated neurotransmission in brain sites such as the medial prefrontal cortex, amygdaloid complex, bed nucleus of the stria terminalis, hippocampus and dorsal periaqueductal gray. Moreover, we also discuss results indicating that, in these regions, the endocannabinoid system could be particularly engaged by highly stressful situations.

Fear relief-toward a new conceptual frame work and what endocannabinoids gotta do with it

The endocannabinoid system seems to play very specific roles in fear extinction, which can only be described within a well-defined model of the various fear relief processes. We, therefore, seek to clarify the current conceptual framework of fear relief within classical and operant fear conditioning paradigms as well as propose new clarifications within this framework where necessary. Based on these revisions as well as previous research involving the endocannabinoid system and fear relief, we are able to pinpoint the processes in which endocannabinoids seem to play a significant role. Following auditory-cued fear conditioning, this applies in particular to habituation and its involvement in acute and long-lasting fear relief. Following contextual conditioning, in contrast, endocannabinoids seem to affect relearning processes as well. Furthermore, we describe how the involvement of the endocannabinoid system develops over the course of the fear relief process and what this may imply for the clinical use of pharmacotherapies targeting the endocannabinoid system in treating fear and anxiety disorders.

A role for the ventral hippocampal endocannabinoid system in fear-conditioned analgesia and fear responding in the presence of nociceptive tone in rats

The endogenous cannabinoid (endocannabinoid) system plays an important role in fear-conditioned analgesia (FCA) and expression and extinction of conditioned fear. The hippocampus has an established role in both pain and conditioned fear and is a substrate for endocannabinoid activity. This study aimed to investigate the role of the endocannabinoid system in the ventral hippocampus (vHip) in FCA and in fear responding in the presence of nociceptive tone. Fear-conditioned rats displayed significantly increased freezing and 22-kHz ultrasonic vocalisation and a reduction in formalin-evoked nociceptive behaviour (ie, FCA) upon re-exposure to a context previously paired with footshock. Tissue levels of the endocannabinoids, anandamide, and 2-arachidonoylglycerol, as well as the fatty acid amide, palmitoylethanolamide, were significantly higher in the vHip of fear-conditioned rats compared with non-fear-conditioned controls. URB597 (inhibitor of fatty acid amide hydrolase [FAAH]), administered bilaterally into the vHip, significantly enhanced FCA during the entire trial and increased fear responding in formalin-treated rats early in the trial. The URB597-induced enhancement of FCA was blocked by intra-vHip administration of the cannabinoid(1) (CB(1)) receptor antagonist/inverse agonist rimonabant. Intra-vHip rimonabant alone had no effect on the expression of FCA, and URB597 did not significantly alter formalin-evoked nociceptive behaviour in non-fear-conditioned rats. These data suggest an important role for the endocannabinoid system in the vHip in FCA, whereby levels of 2-arachidonoylglycerol and the FAAH substrates palmitoylethanolamide and anandamide are increased in rats expressing FCA, and pharmacological inhibition of FAAH in the vHip enhances this form of endogenous analgesia via a CB(1) receptor-dependent mechanism.

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

Recent studies show contradictory results regarding the contribution of endocannabinoids in fear memory formation and long-term synaptic plasticity. In this study, we investigated the effects of both cannabinoid receptor type 1 (CB1 receptor) antagonist AM281 and anandamide reuptake inhibitor AM404 on the formation of contextual fear memory in adult mice. Both i.p. and intra-hippocampal injections of AM281 promoted contextual fear memory while a high dose of AM404 inhibited it. These findings demonstrate that CB1 receptor-mediated signaling negatively contributes to contextual fear memory formation. We further investigated the induction of long-term potentiation (LTP) in CA1 pyramidal neurons of hippocampal slices and found that AM281 impaired the induction of LTP. Additionally, the blockade of LTP by AM281 was completely prevented by bath application of picrotoxin, a selective antagonist of GABA(A) receptor. Taken together, these results indicate that activation of CB1 receptor contributes to induction of LTP via a GABA(A) receptor-mediated mechanism.

Endocannabinoids and stress

Endogenous cannabinoids play an important role in the physiology and behavioral expression of stress responses. Activation of the hypothalamic-pituitary-adrenal (HPA) axis, including the release of glucocorticoids, is the fundamental hormonal response to stress. Endocannabinoid (eCB) signaling serves to maintain HPA-axis homeostasis, by buffering basal activity as well as by mediating glucocorticoid fast feedback mechanisms. Following chronic stressor exposure, eCBs are also involved in physiological and behavioral habituation processes. Behavioral consequences of stress include fear and stress-induced anxiety as well as memory formation in the context of stress, involving contextual fear conditioning and inhibitory avoidance learning. Chronic stress can also lead to depression-like symptoms. Prominent in these behavioral stress responses is the interaction between eCBs and the HPA-axis. Future directions may differentiate among eCB signaling within various brain structures/neuronal subpopulations as well as between the distinct roles of the endogenous cannabinoid ligands. Investigation into the role of the eCB system in allostatic states and recovery processes may give insight into possible therapeutic manipulations of the system in treating chronic stress-related conditions in humans.

The role of cannabinoids in modulating emotional and non-emotional memory processes in the hippocampus

Cannabinoid agonists generally have a disruptive effect on memory, learning, and operant behavior that is considered to be hippocampus-dependent. Nevertheless, under certain conditions, cannabinoid receptor activation may facilitate neuronal learning processes. For example, CB₁ receptors are essential for the extinction of conditioned fear associations, indicating an important role for this receptor in neuronal emotional learning and memory. This review examines the diverse effects of cannabinoids on hippocampal memory and plasticity. It shows how the effects of cannabinoid receptor activation may vary depending on the route of administration, the nature of the task (aversive or not), and whether it involves emotional memory formation (e.g., conditioned fear and extinction learning) or non-emotional memory formation (e.g., spatial learning). It also examines the memory stage under investigation (acquisition, consolidation, retrieval, extinction), and the brain areas involved. Differences between the effects of exogenous and endogenous agonists are also discussed. The apparently biphasic effects of cannabinoids on anxiety is noted as this implies that the effects of cannabinoid receptor agonists on hippocampal learning and memory may be attributable to a general modulation of anxiety or stress levels and not to memory per se. The review concludes that cannabinoids have diverse effects on hippocampal memory and plasticity that cannot be categorized simply into an impairing or an enhancing effect. A better understanding of the involvement of cannabinoids in memory processes will help determine whether the benefits of the clinical use of cannabinoids outweigh the risks of possible memory impairments.

Modulation of conditioned fear, fear-conditioned analgesia, and brain regional c-Fos expression following administration of muscimol into the rat basolateral amygdala

Evidence suggests that gamma-aminobutyric acid (GABA) signalling in the basolateral amygdala (BLA) is involved in pain, fear, and fear-conditioned analgesia (FCA). In this study, we investigated the effects of intra-BLA administration of the GABA(A) receptor agonist muscimol on the expression of conditioned-fear, formalin-evoked nociception, and fear-conditioned analgesia in rats, and the associated alterations in brain regional expression of the immediate early gene product and marker of neuronal activity, c-Fos. Formalin-evoked nociceptive behavior, conditioned-fear and fear-conditioned analgesia were apparent in animals receiving intra-BLA saline. Intra-BLA muscimol suppressed fear behavior and prevented fear-conditioned analgesia, but had no significant effect on the expression of formalin-evoked nociception. The suppression of fear behavior by intra-BLA muscimol was associated with increased c-Fos expression in the central nucleus of the amygdala (CeA) and throughout the periaqueductal grey (PAG). These intra-BLA muscimol-induced increases in c-Fos expression were abolished in rats receiving intraplantar formalin injection. These data suggest that alterations in neuronal activity in the CeA and PAG as a result of altered GABAergic signalling in the BLA may be involved in the behavioral expression of fear and associated analgesia. Furthermore, these alterations in neuronal activity are susceptible to modulation by formalin-evoked nociceptive input in a state-dependent manner.

PERSPECTIVE

The expression of learned fear and associated analgesia are under the control of GABA(A) receptors in the basolateral amygdala, through a mechanism which may involve altered neuronal activity in key components of the descending inhibitory pain pathway. The results enhance our understanding of the neural mechanisms subserving fear-pain interactions.

The CB1 inverse agonist AM251, but not the CB1 antagonist AM4113, enhances retention of contextual fear conditioning in rats

The effects of CB1 antagonist/inverse agonists on the acquisition and consolidation of conditioned fear remain uncertain. Recent studies suggest that the CB1 antagonist/inverse agonist AM251 affects acquisition or consolidation of both contextual and discretely cued fear memories. AM251 is frequently referred to as a CB1 antagonist; however in vitro signal transduction assays indicate that this drug also elicits inverse agonist activity at CB1 receptors. The present studies were undertaken to compare the effects of AM251 on conditioned fear with those produced by AM4113, a novel CB1 antagonist with minimal inverse agonist activity. All drugs were administered prior to conditioning. In retention tests conducted two weeks after conditioning, both AM251 (4.0 mg/kg) and AM4113 (6.0 mg/kg)-treated animals exhibited reduced freezing during a conditioned tone cue played within a novel context. In contextual fear retention tests, animals previously treated with 4.0 or 8.0 mg/kg AM251 exhibited enhanced freezing. By contrast, no dose of AM4113 had any significant effect on contextual fear memory, which is consistent with the lower signal transduction activity of AM4113 at CB1 receptors compared to AM251. These results suggest that CB1 neutral antagonists may be less likely than CB1 inverse agonists to facilitate the acquisition or consolidation of contextual fear that may contribute to some clinical disorders.

Integrated cannabinoid CB1 receptor transmission within the amygdala-prefrontal cortical pathway modulates neuronal plasticity and emotional memory encoding

The cannabinoid CB1 receptor system is functionally involved in the processing and encoding of emotionally salient sensory information, learning and memory. The CB1 receptor is found in high concentrations in brain structures that are critical for emotional processing, including the basolateral amygdala (BLA) and the medial prefrontal cortex (mPFC). In addition, synaptic plasticity in the form of long-term potentiation (LTP) within the BLA > mPFC pathway is an established correlate of exposure to emotionally salient events. We performed a series of in vivo LTP studies by applying tetanic stimulation to the BLA combined with recordings of local field potentials within prelimbic cortical (PLC) region of the rat mPFC. Systemic pretreatment with AM-251 dose dependently blocked LTP along the BLA-PLC pathway and also the behavioral acquisition of conditioned fear memories. We next performed a series of microinfusion experiments wherein CB1 receptor transmission within the BLA > PLC circuit was pharmacologically blocked. Asymmetrical, interhemispheric blockade of CB1 receptor transmission along the BLA > PLC pathway prevented the acquisition of emotionally salient associative memory. Our results indicate that coordinated CB1 receptor transmission within the BLA > PLC pathway is critically involved in the encoding of emotional fear memories and modulates neural plasticity related to the encoding of emotionally salient associative learning.

Endocannabinoid system and fear conditioning

The endocannabinoid system has been proposed to modulate neuronal functions involved in distinct types of defensive reactions, possibly counteracting the harmful consequences of stressful stimuli. However, the precise brain sites for this action remain to be further explored. This chapter summarizes the data about the role of the endocannabinoid system in the processing of conditioned fear as well as the potential neural subtract for its actions.

Alterations in extracellular levels of gamma-aminobutyric acid in the rat basolateral amygdala and periaqueductal gray during conditioned fear, persistent pain and fear-conditioned analgesia

Evidence suggests an important role for supraspinal gamma-aminobutyric acid (GABA) in conditioned fear and pain. Using dual probe microdialysis coupled to HPLC, we investigated alterations in extracellular levels of GABA simultaneously in the rat basolateral amygdala and dorsal periaqueductal gray during expression of conditioned fear, formalin-evoked nociception, and fear-conditioned analgesia. Re-exposure to a context previously paired with footshock significantly increased the duration of freezing and 22-kilohertz ultrasonic vocalization, and reduced formalin-evoked nociceptive behavior. Upon re-exposure to the context, GABA levels in the basolateral amygdala were significantly lower in fear-conditioned animals compared with non-fear-conditioned controls, irrespective of intraplantar formalin/saline injection. GABA levels in the dorsal periaqueductal gray were lower in rats receiving intraplantar injection of formalin, compared with saline-treated controls. GABA levels sampled were sensitive to nipecotic acid and calcium infusion. No specific fear-conditioned analgesia-related alterations in GABA efflux were observed in these regions despite the ability of rats undergoing dual probe microdialysis to express this important survival response. In conclusion, expression of contextually induced fear- and pain-related behavior are accompanied by suppression of GABA release in the basolateral amygdala and dorsal periaqueductal gray, respectively, compared with non-fear, non-pain controls.

PERSPECTIVE

This study investigates alterations in levels of the neurotransmitter GABA simultaneously in the rat amygdala and periaqueductal grey during expression of pain- and fear-related behavior and fear-induced analgesia. The results enhance our understanding of the role of this neurotransmitter in pain, memory of pain and control of pain during fear.

Stress-induced analgesia

For over 30 years, scientists have been investigating the phenomenon of pain suppression upon exposure to unconditioned or conditioned stressful stimuli, commonly known as stress-induced analgesia. These studies have revealed that individual sensitivity to stress-induced analgesia can vary greatly and that this sensitivity is coupled to many different phenotypes including the degree of opioid sensitivity and startle response. Furthermore, stress-induced analgesia is influenced by age, gender, and prior experience to stressful, painful, or other environmental stimuli. Stress-induced analgesia is mediated by activation of the descending inhibitory pain pathway. Pharmacological and neurochemical studies have demonstrated involvement of a large number of neurotransmitters and neuropeptides. In particular, there are key roles for the endogenous opioid, monoamine, cannabinoid, gamma-aminobutyric acid and glutamate systems. The study of stress-induced analgesia has enhanced our understanding of the fundamental physiology of pain and stress and can be a useful approach for uncovering new therapeutic targets for the treatment of pain and stress-related disorders.

Activation of CB1 cannabinoid receptors impairs memory consolidation and hippocampal polysialylated neural cell adhesion molecule expression in contextual fear conditioning

We investigated the role of CB1 receptors in hippocampal-dependent memory consolidation mediated by polysialylated neural cell adhesion molecule (PSA-NCAM) during contextual fear conditioning (CFC). The CB1 receptor agonist 3-(1,1-dimethylheptyl)-(-)-11-hydroxy-Delta(8)-tetrahydrocannabinol (HU-210) (0.1 mg/kg) was given immediately after training during the memory consolidation phase, and freezing behavior was measured 24 h after conditioning. Administration of HU-210 attenuated freezing behavior measured in CFC. Western blot analysis showed that CFC induced a decrease in the expression of NCAM-180, but did not change the level of NCAM-140 and increased PSA-NCAM expression measured 24 h after training in the rat hippocampus. HU-210 (0.1 mg/kg) injection did not affect the reduction in NCAM-180 levels induced by CFC, but it blocked the increase in PSA-NCAM expression. Since the dentate gyrus (DG) of the hippocampus is known to be involved in memory consolidation and expresses a high level of PSA-NCAM protein, we measured the effects of CFC and HU-210 administration on PSA-NCAM-immunoreactive (IR) cells in the DG. CFC caused an increase in the number of PSA-NCAM-IR cells in the DG, but not K(i)-67- or doublecortin (DCX)-IR cells. This increase in PSA-NCAM-IR cells was abolished by HU-210 injection. Administration of the CB1 receptor antagonist N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM-251) (3 mg/kg immediately before HU-210) inhibited the effects of HU-210 on freezing behavior and PSA-NCAM expression in the DG. These results indicate that activation of CB1 receptors disturbs consolidation of fear memory in CFC, likely by affecting PSA-NCAM expression in the DG, which plays an important role in synaptic rearrangement during the formation of memory traces.

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

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