Attenuation of anxiety-related behaviour after the antagonism of transient receptor potential vanilloid type 1 channels in the rat ventral hippocampus

TRPV1 channel in the pathophysiology of epilepsy and its potential as a molecular target for the development of new antiseizure drug candidates

Identification of transient receptor potential cation channel, subfamily V member 1 (TRPV1), also known as capsaicin receptor, in 1997 was a milestone achievement in the research on temperature sensation and pain signalling. Very soon after it became evident that TRPV1 is implicated in a wide array of physiological processes in different peripheral tissues, as well as in the central nervous system, and thereby could be involved in the pathophysiology of numerous diseases. Increasing evidence suggests that modulation of TRPV1 may also affect seizure susceptibility and epilepsy. This channel is localized in brain regions associated with seizures and epilepsy, and its overexpression was found both in animal models of seizures and in brain samples from epileptic patients. Moreover, modulation of TRPV1 on non-neuronal cells (microglia, astrocytes, and/or peripheral immune cells) may have an impact on the neuroinflammatory processes that play a role in epilepsy and epileptogenesis. In this paper, we provide a comprehensive and critical overview of currently available data on TRPV1 as a possible molecular target for epilepsy management, trying to identify research gaps and future directions. Overall, several converging lines of evidence implicate TRPV1 channel as a potentially attractive target in epilepsy research but more studies are needed to exploit the possible role of TRPV1 in seizures/epilepsy and to evaluate the value of TRPV1 ligands as candidates for new antiseizure drugs.

Therapeutic potential of transient receptor potential (TRP) channels in psychiatric disorders

Psychiatric disorders such as Bipolar disorder, Anxiety, Major depressive disorder, Schizophrenia, Attention-deficit/hyperactivity disorder, as well as neurological disorders such as Migraine, are linked by the evidence of altered calcium homeostasis. The disturbance of intra-cellular calcium homeostasis disrupts the activity of numerous ion channels including transient receptor potential (TRP) channels. TRP channel families comprise non-selective calcium-permeable channels that have been implicated in variety of physiological processes in the brain, as well as in the pathogenesis of psychiatric disorders. Through a comprehensive review of current research and experimentation, this investigation elucidates the role of TRP channels in psychiatric disorders. Furthermore, this review discusses about the exploration of epigenetics and TRP channels in psychiatric disorders.

Interrelated involvement of the endocannabinoid/endovanilloid (TRPV1) systems and epigenetic processes in anxiety- and working memory impairment-related behavioural effects of nicotine as a stressor

The addictive use of nicotine contained in tobacco is associated with stressor-like emotional and cognitive effects such as anxiety and working memory impairment, and the involvement of epigenetic mechanisms such as histone acetylation has recently been reported. Although the precise nature of behavioural plasticity remains unclear, both anxiogenic- and working memory impairment-like effects were observed in the present experimental model of mice treated with repeated subcutaneous nicotine and/or immobilization stress, and these effects were commonly attenuated by the histone deacetylase (HDAC) inhibitors that induce histone acetylation. Such HDAC inhibitor-induced resilience was mimicked by ligands for the endocannabinoid (ECB) system, a neurotransmitter system that is closely associated with nicotine-induced addiction-related behaviours: the anxiogenic-like effects were mitigated by the cannabinoid type 1 (CB1) agonist arachidonylcyclopropylamide (ACPA), whereas the working memory impairment-like effects were mitigated by the CB1 antagonist SR 141716A. Moreover, the effects of the HDAC inhibitors were also mimicked by ligands for the endovanilloid (transient receptor potential vanilloid 1 [TRPV1]) system, a system that shares common characteristics with the ECB system: the anxiogenic-like effects were mitigated by the TRPV1 antagonist capsazepine, whereas the working memory impairment-like effects were mitigated by the TRPV1 agonist olvanil. Notably, the HDAC inhibitor-induced anxiolytic-like effects were attenuated by SR 141716A, which were further counteracted by capsazepine, whereas the working memory improvement-like effects were attenuated by capsazepine, which were further counteracted by SR 141716A. These results suggest the contribution of interrelated control of the ECB/TRPV1 systems and epigenetic processes such as histone acetylation to novel therapeutic approaches.

Mitochondrial dysfunction and neurological disorders: A narrative review and treatment overview

Mitochondria play a vital role in the nervous system, as they are responsible for generating energy in the form of ATP and regulating cellular processes such as calcium (Ca2+) signaling and apoptosis. However, mitochondrial dysfunction can lead to oxidative stress (OS), inflammation, and cell death, which have been implicated in the pathogenesis of various neurological disorders. In this article, we review the main functions of mitochondria in the nervous system and explore the mechanisms related to mitochondrial dysfunction. We discuss the role of mitochondrial dysfunction in the development and progression of some neurological disorders including Parkinson's disease (PD), multiple sclerosis (MS), Alzheimer's disease (AD), depression, and epilepsy. Finally, we provide an overview of various current treatment strategies that target mitochondrial dysfunction, including pharmacological treatments, phototherapy, gene therapy, and mitotherapy. This review emphasizes the importance of understanding the role of mitochondria in the nervous system and highlights the potential for mitochondrial-targeted therapies in the treatment of neurological disorders. Furthermore, it highlights some limitations and challenges encountered by the current therapeutic strategies and puts them in future perspective.

Multiple Chemical Sensitivity: It's time to catch up to the science

Multiple chemical sensitivity (MCS) is a complex medical condition associated with low dose chemical exposures. MCS is characterized by diverse features and common comorbidities, including fibromyalgia, cough hypersensitivity, asthma, and migraine, and stress/anxiety, with which the syndrome shares numerous neurobiological processes and altered functioning within diverse brain regions. Predictive factors linked to MCS comprise genetic influences, gene-environment interactions, oxidative stress, systemic inflammation, cell dysfunction, and psychosocial influences. The development of MCS may be attributed to the sensitization of transient receptor potential (TRP) receptors, notably TRPV1 and TRPA1. Capsaicin inhalation challenge studies demonstrated that TRPV1 sensitization is manifested in MCS, and functional brain imaging studies revealed that TRPV1 and TRPA1 agonists promote brain-region specific neuronal variations. Unfortunately, MCS has often been inappropriately viewed as stemming exclusively from psychological disturbances, which has fostered patients being stigmatized and ostracized, and often being denied accommodation for their disability. Evidence-based education is essential to provide appropriate support and advocacy. Greater recognition of receptor-mediated biological mechanisms should be incorporated in laws, and regulation of environmental exposures.

Transient Receptor Potential (TRP) Channels in Pain, Neuropsychiatric Disorders, and Epilepsy

Pharmacomodulation of membrane channels is an essential topic in the study of physiological conditions and disease status. Transient receptor potential (TRP) channels are one such family of nonselective cation channels that have an important influence. In mammals, TRP channels consist of seven subfamilies with a total of twenty-eight members. Evidence shows that TRP channels mediate cation transduction in neuronal signaling, but the full implication and potential therapeutic applications of this are not entirely clear. In this review, we aim to highlight several TRP channels which have been shown to mediate pain sensation, neuropsychiatric disorders, and epilepsy. Recent findings suggest that TRPM (melastatin), TRPV (vanilloid), and TRPC (canonical) are of particular relevance to these phenomena. The research reviewed in this paper validates these TRP channels as potential targets of future clinical treatment and offers patients hope for more effective care.

TRPV1 modulation of contextual fear memory depends on stimulus intensity and endocannabinoid signalling in the dorsal hippocampus

The transient receptor potential vanilloid type-1 (TRPV1) channels have been implicated in the modulation of aversive responses. The endocannabinoid anandamide acts as an endogenous TRPV1 agonist, exerting opposite functions at TRPV1 and type-1 cannabinoid receptors (CB₁R). Here we tested the hypothesis that hippocampal TRPV1 modulates contextual fear memory retrieval and investigated the influence of the aversive stimulus intensity as well as the role of endocannabinoid signaling. Male C57BL/6J mice were tested for contextual fear memory after low-, moderate-, or high-intensity shock protocols. The selective TRPV1 blockers SB366791 (1-10 nmol) and 6-I-NC (2 nmol) were infused via intra-dorsal hippocampus before the retrieval test session. The local levels of endocannabinoids and Arc and Zif268 mRNAs, involved in synaptic plasticity and memory, were quantified. First, both TRPV1 blockers reduced memory retrieval in animals exposed to moderate or high (but not low) intensity training protocols. In the second series of results, the magnitude of the freezing responses positively correlated with the hippocampal anandamide levels; TRPV1 and CB₁R were found co-localized in this brain region; and the CB₁R antagonist, AM251, prevented the effects of SB366791. Thus, endocannabinoid signaling possibly mediates the effects of TRPV1 blockers. Finally, inhibition of memory retrieval by TRPV1 blockers increased Arc and Zif268 mRNAs and impaired fear memory reinstatement. In conclusion, the modulation of fear memories by dorsal hippocampal TRPV1 channels may depend on the aversive stimulus intensity and occur via anandamide/CB₁ signaling. Moreover, TRPV1 blockers promote Arc and Zif268 transcription, with subsequent attenuation of aversive memory reinstatement.

Increased TRPV1 Channels and FosB Protein Expression Are Associated with Chronic Epileptic Seizures and Anxiogenic-like Behaviors in a Preclinical Model of Temporal Lobe Epilepsy

Epilepsies are neurological disorders characterized by chronic seizures and their related neuropsychiatric comorbidities, such as anxiety. The Transient Receptor Potential Vanilloid type-1 (TRPV1) channel has been implicated in the modulation of seizures and anxiety-like behaviors in preclinical models. Here, we investigated the impact of chronic epileptic seizures in anxiety-like behavior and TRPV1 channels expression in a genetic model of epilepsy, the Wistar Audiogenic Rat (WAR) strain. WARs were submitted to audiogenic kindling (AK), a preclinical model of temporal lobe epilepsy (TLE) and behavioral tests were performed in the open-field (OF), and light-dark box (LDB) tests 24 h after AK. WARs displayed increased anxiety-like behavior and TRPV1R expression in the hippocampal CA1 area and basolateral amygdala nucleus (BLA) when compared to control Wistar rats. Chronic seizures increased anxiety-like behaviors and TRPV1 and FosB expression in limbic and brainstem structures involved with epilepsy and anxiety comorbidity, such as the hippocampus, superior colliculus, and periaqueductal gray matter. Therefore, these results highlight previously unrecognized alterations in TRPV1 expression in brain structures involved with TLE and anxiogenic-like behaviors in a genetic model of epilepsy, the WAR strain, supporting an important role of TRPV1 in the modulation of neurological disorders and associated neuropsychiatric comorbidities.

TRPV1 blockers as potential new treatments for psychiatric disorders

The transient receptor potential vanilloid-1 channel (TRPV1) is responsible for decoding physical and chemical stimuli. TRPV1 is activated by capsaicin (a compound from chili peppers), heat (above 43°C) and acid environment, playing a major role in pain, inflammation and body temperature. Molecular and histological studies have suggested TRPV1 expression in specific brain regions, where it can be activated primarily by the endocannabinoid anandamide, fostering studies on its potential role in psychiatric disorders. TRPV1 blockers are effective in various animal models predictive of anxiolytic and antipanic activities, in addition to reducing conditioned fear. In models of antidepressant activity, these compounds reduce behavioral despair and promote active stress-coping behavior. TRPV1 blockers also reduce the effects of certain drugs of abuse and revert behavioral changes in animal models of neurodevelopmental disorders. The main limiting factor in developing TRPV1 blockers as therapeutic agents concerns their effects on body temperature, particularly hyperthermia. New compounds, which block specific states of the channel, could represent an alternative. Moreover, compounds blocking both TRPV1 and the anandamide-hydrolyzing enzyme, fatty acid amide hydrolase (FAAH), termed dual TRPV1/FAAH blockers, have been investigated with promising results. Overall, preclinical studies yield favorable results with TRPV1 blockers in animal models of psychiatric disorders.

Therapeutic potential of pharmacological agents targeting TRP channels in CNS disorders

Central nervous system (CNS) disorders like Alzheimer's disease (AD), Parkinson disease (PD), stroke, epilepsy, depression, and bipolar disorder have a high impact on both medical and social problems due to the surge in their prevalence. All of these neuronal disorders share some common etiologies including disruption of Ca²⁺ homeostasis and accumulation of misfolded proteins. These misfolded proteins further disrupt the intracellular Ca²⁺ homeostasis by disrupting the activity of several ion channels including transient receptor potential (TRP) channels. TRP channel families include non-selective Ca²⁺ permeable channels, which act as cellular sensors activated by various physio-chemical stimuli, exogenous, and endogenous ligands responsible for maintaining the intracellular Ca²⁺ homeostasis. TRP channels are abundantly expressed in the neuronal cells and disturbance in their activity leads to various neuronal diseases. Under the pathological conditions when the activity of TRP channels is perturbed, there is a disruption of the neuronal homeostasis through increased inflammatory response, generation of reactive oxygen species, and mitochondrial dysfunction. Therefore, there is a potential of pharmacological interventions targeting TRP channels in CNS disorders. This review focuses on the role of TRP channels in neurological diseases; also, we have highlighted the current insights into the pharmacological modulators targeting TRP channels.

Pleiotropic Pharmacological Actions of Capsazepine, a Synthetic Analogue of Capsaicin, against Various Cancers and Inflammatory Diseases

Capsazepine is a synthetic analogue of capsaicin that can function as an antagonist of TRPV1. Capsazepine can exhibit diverse effects on cancer (prostate cancer, breast cancer, colorectal cancer, oral cancer, and osteosarcoma) growth and survival, and can be therapeutically used against other major disorders such as colitis, pancreatitis, malaria, and epilepsy. Capsazepine has been reported to exhibit pleiotropic anti-cancer effects against numerous tumor cell lines. Capsazepine can modulate Janus activated kinase (JAK)/signal transducer and activator of the transcription (STAT) pathway, intracellular Ca²⁺ concentration, and reactive oxygen species (ROS)-JNK-CCAAT/enhancer-binding protein homologous protein (CHOP) pathways. It can inhibit cell proliferation, metastasis, and induce apoptosis. Moreover, capsazepine can exert anti-inflammatory effects through the downregulation of lipopolysaccharide (LPS)-induced nuclear transcription factor-kappa B (NF-κB), as well as the blockage of activation of both transient receptor potential cation channel subfamily V member 1 (TRPV1) and transient receptor potential cation channel, subfamily A, and member 1 (TRPA1). This review briefly summarizes the diverse pharmacological actions of capsazepine against various cancers and inflammatory conditions.

Capsaicin upregulates HDAC2 via TRPV1 and impairs neuronal maturation in mice

Transient receptor potential vanilloid 1 (TRPV1) affects mood and neuroplasticity in the brain, where its role is poorly understood. In the present study we investigated whether capsaicin (8-methyl-N-vanillyl-trans-6-nonenamide), an agonist of TRPV1, induced chromatin remodeling and thereby altered gene expression related to synaptic plasticity. We found that capsaicin treatment resulted in upregulation of histone deacetylase 2 (HDAC2) in the mouse hippocampus and HDAC2 was enriched at Psd95, synaptophysin, GLUR1, GLUR2 promoters. Viral-mediated hippocampal knockdown of HDAC2 induced expression of Synapsin I and prevented the detrimental effects of capsaicin on Synapsin I expression in mice, supporting the role of HDAC2 in regulation of capsaicin-induced Synapsin I expression. Taken together, our findings implicate HDAC2 in capsaicin-induced transcriptional regulation of synaptic molecules and support the view that HDAC2 is a molecular link between TRPV1 activity and synaptic plasticity.

N-arachidonoyl-serotonin, a dual FAAH and TRPV1 blocker, inhibits the retrieval of contextual fear memory: Role of the cannabinoid CB1 receptor in the dorsal hippocampus

Anandamide, an endocannabinoid, inhibits aversive responses by activating the CB₁ cannabinoid receptor. At high concentrations, however, anandamide may exert pro-aversive activities mediated by the transient receptor potential vanilloid type-1 channel (TRPV1). Accordingly, N-arachidonoyl-serotonin (AA-5-HT), a dual blocker of the anandamide-hydrolysing enzyme fatty acid amide hydrolase (FAAH) and the TRPV1 channel, induces anxiolytic-like effects. Here we tested the hypothesis that AA-5-HT inhibits the expression of contextual fear conditioning by facilitating CB₁ receptor signalling in the dorsal hippocampus of mice. Intraperitoneal injection of AA-5-HT (0.1, 0.3, 1 mg/kg) inhibited the retrieval of contextual fear memory (freezing response). The effect of AA-5-HT (0.3 mg/kg) was prevented by systemic injection of the CB₁ receptor antagonist, AM251 (1.0 mg/kg), and mimicked by simultaneous FAAH inhibition (URB597, 0.3 mg/kg) and TRPV1 blockage (SB366791, 1 mg/kg). Injection of AA-5-HT (0.125, 0.25, 0.5 nmol) into the dorsal hippocampus also reduced freezing. Finally, the effect of systemic AA-5-HT (0.3 mg/kg) was prevented by intra-hippocampal injection of AM251 (1 nmol). In conclusion, dual FAAH and TRPV1 blockage inhibits contextual fear memory by facilitating anandamide-induced CB₁ receptor activation in the dorsal hippocampus. This approach may lead to new pharmacological treatments for traumatic memories and related psychiatric disorders.

Psychiatric Disorders and TRP Channels: Focus on Psychotropic Drugs

Psychiatric and neurological disorders are mostly associated with the changes in neural calcium ion signaling pathways required for activity-triggered cellular events. One calcium channel family is the TRP cation channel family, which contains seven subfamilies. Results of recent papers have discovered that calcium ion influx through TRP channels is important. We discuss the latest advances in calcium ion influx through TRP channels in the etiology of psychiatric disorders. Activation of TRPC4, TRPC5, and TRPV1 cation channels in the etiology of psychiatric disorders such as anxiety, fear-associated responses, and depression modulate calcium ion influx. Evidence substantiates that anandamide and its analog (methanandamide) induce an anxiolytic-like effect via CB1 receptors and TRPV1 channels. Intracellular calcium influx induced by oxidative stress has an significant role in the etiology of bipolar disorders (BDs), and studies recently reported the important role of TRP channels such as TRPC3, TRPM2, and TRPV1 in converting oxidant or nitrogen radical signaling to cytosolic calcium ion homeostasis in BDs. The TRPV1 channel also plays a function in morphine tolerance and hyperalgesia. Among psychotropic drugs, amitriptyline and capsazepine seem to have protective effects on psychiatric disorders via the TRP channels. Some drugs such as cocaine and methamphetamine also seem to have an important role in alcohol addiction and substance abuse via activation of the TRPV1 channel. Thus, we explore the relationships between the etiology of psychiatric disorders and TRP channel-regulated mechanisms. Investigation of the TRP channels in psychiatric disorders holds the promise of the development of new drug treatments.

Beyond the CB1 Receptor: Is Cannabidiol the Answer for Disorders of Motivation?

The Cannabis sativa plant has been used to treat various physiological and psychiatric conditions for millennia. Current research is focused on isolating potentially therapeutic chemical constituents from the plant for use in the treatment of many central nervous system disorders. Of particular interest is the primary nonpsychoactive constituent cannabidiol (CBD). Unlike Δ(9)-tetrahydrocannabinol (THC), CBD does not act through the cannabinoid type 1 (CB1) receptor but has many other receptor targets that may play a role in psychiatric disorders. Here we review preclinical and clinical data outlining the therapeutic efficacy of CBD for the treatment of motivational disorders such as drug addiction, anxiety, and depression. Across studies, findings suggest promising treatment effects and potentially overlapping mechanisms of action for CBD in these disorders and indicate the need for further systematic investigation of the viability of CBD as a psychiatric pharmacotherapy.

Involvement of TRPV1 channels in the periaqueductal grey on the modulation of innate fear responses

OBJECTIVES

The transient receptor potential vanilloid type-1 channel (TRPV1) is expressed in the midbrain periaqueductal grey (PAG), a region of the brain related to aversive responses. TRPV1 antagonism in the dorsolateral PAG (dlPAG) induces anxiolytic-like effects in models based on conflict situations. No study, however, has investigated whether these receptors could contribute to fear responses to proximal threat. Thus, we tested the hypothesis that TRPV1 in the PAG could mediate fear response in rats exposed to a predator.

METHODS

We verified whether exposure to a live cat (a natural predator) would activate TRPV1-expressing neurons in the PAG. Double-staining immunohistochemistry was used as a technique to detect c-Fos, a marker of neuronal activation, and TRPV1 expression. We also investigated whether intra-dlPAG injections of the TRPV1 antagonist, capsazepine (CPZ), would attenuate the behavioural consequences of predator exposure.

RESULTS

Exposure to a cat increased c-Fos expression in TRPV1-positive neurons, mainly in the dorsal columns of the PAG, suggesting that TRPV1-expressing neurons are activated by threatening stimuli. Accordingly, local injection of CPZ inhibited the fear responses.

CONCLUSION

These data support the hypothesis that TRPV1 channels mediate fear reactions in the dlPAG. This may have an implication for the development of TRPV1-antagonists as potential drugs for the treatment of certain psychiatric disorders.

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.

The blockade of transient receptor potential ankirin 1 (TRPA1) signalling mediates antidepressant- and anxiolytic-like actions in mice

BACKGROUND AND PURPOSE

Transient receptor potential vanilloid 1 (TRPV1) and TRP ankyrin 1 (TRPA1) are involved in many biological processes, including nociception and hyperalgesia. Whereas the involvement of TRPV1 in psychiatric disorders such as anxiety and depression has been reported, little is known regarding the role of TRPA1 in these conditions.

EXPERIMENTAL APPROACH

We investigated the role of TRPA1 in mice models of depression [forced swimming test (FST)] and anxiety [elevated plus maze (EPM) test].

KEY RESULTS

Administration of the TRPA1 antagonist (HC030031, 30 nmol in 2 μL, i.c.v.) reduced immobility time in the FST. Similar results were obtained after oral administration of HC030031 (30-300 mg·kg(-1) ). The reduction in immobility time in FST induced by HC030031 (100 mg·kg(-1) ) was completely prevented by pretreatment with TRPA1 agonist, cinnamaldehyde (50 mg·kg(-1) , p.o.), which per se was inactive. In the EPM test, pretreatment with cinnamaldehyde (50 mg·kg(-1) , p.o.), which per se did not affect behaviour response, prevented the anxiolytic-like effect (increased open arm exploration) evoked by TRPA1 blockade (HC030031, 100 mg·kg(-1) , p.o.). Treatment with either cinnamaldehyde or HC030031 did not affect spontaneous ambulation. Furthermore, TRPA1-deficient mice showed anxiolytic- and antidepressant-like phenotypes in the FST and EPM test respectively.

CONCLUSION AND IMPLICATIONS

The present findings indicate that genetic deletion or pharmacological blockade of TRPA1 produces inhibitory activity in mouse models of anxiety and depression. These results imply that TRPA1 exerts tonic control, promoting anxiety and depression, and that TRPA1 antagonism has potential as an innovative strategy for the treatment of anxiety and mood disorders.

TRPV1 in the central nervous system: synaptic plasticity, function, and pharmacological implications

The function of TRPV1 in the peripheral nervous system is increasingly being investigated for its anti-inflammatory and antinociceptive properties in an effort to find a novel target to fight pain that is nonaddictive. However, in recent years, it was discovered that TRPV1 is also associated with a wide array of functions and behaviors in the central nervous system, such as fear, anxiety, stress, thermoregulation, pain, and, more recently, synaptic plasticity, the cellular mechanism that allows the brain to adapt to its environment. This suggests a new role for brain TRPV1 in areas such as learning and memory, reward and addiction, and development. This wide array of functional aspects of TRPV1 in the central nervous system (CNS) is in part due to its multimodal form of activation and highlights the potential pharmacological implications of TRPV1 in the brain. As humans also express a TRPV1 homologue, it is likely that animal research will be translational to humans and therefore worthy of exploration. This review outlines the basic expression patterns of TRPV1 in the CNS along with what is known regarding its signaling mechanisms and its role in the aforementioned brain functions. As TRPV1 involvement in synaptic plasticity has never been fully reviewed elsewhere, it will be a focus of this review. The chapter concludes with some of the potential pharmaceutical implications of further TRPV1 research.

Effects of glutamate NMDA and TRPV1 receptor antagonists on the biphasic responses to anandamide injected into the dorsolateral periaqueductal grey of Wistar rats

RATIONALE

The endocannabinoid and endovanniloid anandamide (AEA) exerts biphasic effects when injected into the dorsolateral periaqueductal grey (dlPAG) in rats submitted to threatening situations. Whereas lower doses of AEA induce anxiolytic-like effects by activating cannabinoid CB1 receptors, no effects are observed with higher doses, possibly due to the simultaneous activation of transient receptor potential vanilloid type 1 (TRPV1) receptors. This activation would facilitate glutamatergic neurotransmission.

OBJECTIVE

Considering that the blockade of TRPV1 or NMDA receptors in the dlPAG induces anxiolytic-like effects, we tested the hypothesis that facilitation of glutamate transmission through TRPV1 is responsible for the lack of anxiolytic-like effect observed with high AEA doses.

METHODS

Male Wistar rats with a unilateral cannula aimed at the dlPAG received injections of an ineffective dose of AP7 (an NMDA antagonist, 1 nmol) or capsazepine (CPZ, a TRPV1 antagonist, 10 nmol), followed by a high dose of AEA (50 and 200 pmol) and were exposed to the elevated plus maze (EPM) or the Vogel conflict test (VCT).

RESULTS

AP7, CPZ, or AEA did not induce any significant effects when administered alone. However, AP7 or CPZ prior to AEA significantly increased the percentage of entries and time spent in the open arms of EPM and the number of punished licks in the VCT suggesting an anxiolytic-like effect.

CONCLUSIONS

These results suggest that the lack of anxiolytic-like effect of higher AEA doses is due to facilitation of glutamate release in the dlPAG, probably via activation of TRPV1 receptors in this structure.

Calcineurin is required for TRPV1-induced long-term depression of hippocampal interneurons

Transient receptor potential vanilloid 1 (TRPV1) mediates a novel form of presynaptic long-term depression (LTD) in hippocampal interneurons. To date, while TRPV1 is currently being heavily studied in the PNS for its anti-nociceptive and anti-inflammatory properties, much less is known regarding TRPV1 signaling and function in the CNS, including the mechanism mediating hippocampal interneuron LTD. Here we performed whole-cell voltage clamp electrophysiology experiments on CA1 hippocampal interneurons from Sprague-Dawley male rats to identify this signaling mechanism. Because calcineurin is linked to multiple synaptic plasticity types, we investigated whether TRPV1 activates presynaptic calcineurin, which in turn induces LTD. To do so we employed calcineurin inhibitors cyclosporin A or FK-506. To determine the location of the calcineurin involved we either bath applied calcineurin antagonists, blocking calcineurin activity ubiquitously in the slice, presynaptically and postsynaptically, or applied antagonists to the internal solution to block calcineurin postsynaptically. Both calcineurin antagonists applied to the bath blocked TRPV1-dependent LTD, indicating calcineurin involvement in LTD. Because calcineurin antagonist applied to the internal solution did not block TRPV1-LTD, it suggests presynaptic calcineurin is required for LTD. Finally, because high frequency stimulus used to induce LTD could potentially activate receptors besides TRPV1, we confirmed that bath, but not intracellularly applied cyclosporin A, also blocked TRPV1 agonist-induced depression of CA1 interneurons. In conclusion, these data illustrate that presynaptic calcineurin activity is required for both TRPV1-induced LTD and TRPV1 agonist-induced depression. This finding is the first to demonstrate the TRPV1-induced signaling mechanism in CA1 hippocampus.

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.

Opposing roles for cannabinoid receptor type-1 (CB₁) and transient receptor potential vanilloid type-1 channel (TRPV1) on the modulation of panic-like responses in rats

The midbrain dorsal periaqueductal gray (dPAG) has an important role in orchestrating anxiety- and panic-related responses. Given the cellular and behavioral evidence suggesting opposite functions for cannabinoid type 1 receptor (CB₁) and transient receptor potential vanilloid type-1 channel (TRPV1), we hypothesized that they could differentially influence panic-like reactions induced by electrical stimulation of the dPAG. Drugs were injected locally and the expression of CB₁ and TRPV1 in this structure was assessed by immunofluorescence and confocal microscopy. The CB₁-selective agonist, ACEA (0.01, 0.05 and 0.5 pmol) increased the threshold for the induction of panic-like responses solely at the intermediary dose, an effect prevented by the CB₁-selective antagonist, AM251 (75 pmol). Panicolytic-like effects of ACEA at the higher dose were unmasked by pre-treatment with the TRPV1 antagonist capsazepine (0.1 nmol). Similarly to ACEA, capsazepine (1 and 10 nmol) raised the threshold for triggering panic-like reactions, an effect mimicked by another TRPV1 antagonist, SB366791 (1 nmol). Remarkably, the effects of both capsazepine and SB366791 were prevented by AM251 (75 pmol). These pharmacological data suggest that a common endogenous agonist may have opposite functions at a given synapse. Supporting this view, we observed that several neurons in the dPAG co-expressed CB₁ and TRPV1. Thus, the present work provides evidence that an endogenous substance, possibly anandamide, may exert both panicolytic and panicogenic effects via its actions at CB₁ receptors and TRPV1 channels, respectively. This tripartite set-point system might be exploited for the pharmacotherapy of panic attacks and anxiety-related disorders.

Transient receptor potential vanilloid 1 channels modulate the anxiolytic effect of diazepam

The present study investigated the interaction between the vanilloid and GABAergic systems on anxiety. Swiss mice were subjected to social interaction test, an animal model for assessing anxiety-related behavior, after intracerebroventricular administration of capsaicin, (TRPV1 agonist) or capsazepine, (TRPV1 antagonist) either alone or in combination with traditional anxiolytic drug, diazepam. Results showed that capsaicin (1, 10, and 100 μg/mouse) decreased the interaction time exhibiting an anxiogenic-like response, while capsazepine (10, and 100 μg/mouse) produced anxiolytic-like response similar to that of diazepam (0.25-4 mg/kg, i.p). Prior administration of capsaicin at a dose, inactive per se (0.1 μg/mouse) attenuated the anxiolytic effect of diazepam, whereas, co-administration of capsazepine and diazepam both in their sub-effective as well as effective doses exhibited significant anxiolytic-like effect. Interestingly, the combined treatment of diazepam (2mg/kg) and capsazepine (100μg/mouse) produced no sedative or locomotor deficit effects. On the contrary, a higher dose of diazepam (>2mg/kg) alone was found to be a sedative or locomotor depressant, indicating that the anxiolytic effect of diazepam, at least in part involve TRPV1 receptor. Morever, capsazepine pretreatment blocked the anxiogenic effect of capsaicin (1, and 100 μg/mouse). Taken together, these findings suggest that blockade of TRPV1 might be a functional tool to prevent the risks associated with the long-term use of benzodiazepines.

Cannabinoid type 1 receptors and transient receptor potential vanilloid type 1 channels in fear and anxiety-two sides of one coin?

The transient receptor potential vanilloid type 1 channel (TRPV1; originally vanilloid receptor VR1) is activated in peripheral terminals of nociceptive fibers by noxious heat, low pH, and natural products such as capsaicin, the pungent ingredient of red-hot chilli peppers. Evidence has been accumulating that TRPV1 is expressed also in the brain, where it seems to be involved in antinociception, locomotor control, and regulation of affective behaviors. This ion channel might be activated by arachidonoyl ethanolamide (anandamide), the endogenous agonist of the cannabinoid type 1 (CB(1)) receptor. However, while CB(1) activation leads to a decrease in intracellular calcium and attenuation of synaptic transmission, anandamide binding to TRPV1 results in elevated calcium levels and potentiated synaptic transmission. This suggests a tripartite regulatory system with antagonistic effects of CB(1) and TRPV1, which are tied together by the same endogenous ligand. Such a system may have important implication for the modulation of behavioral responses. The present commentary elaborates on this interplay between CB(1) receptors and TRPV1 channels in the context of fear- and anxiety-related behaviors.

Endocannabinoid influence in drug reinforcement, dependence and addiction-related behaviors

The endogenous cannabinoid system is an important regulatory system involved in physiological homeostasis. Endocannabinoid signaling is known to modulate neural development, immune function, metabolism, synaptic plasticity and emotional state. Accumulating evidence also implicates brain endocannabinoid signaling in the etiology of drug addiction which is characterized by compulsive drug seeking, loss of control in limiting drug intake, emergence of a negative emotional state in the absence of drug use and a persistent vulnerability toward relapse to drug use during protracted abstinence. In this review we discuss the effects of drug intake on brain endocannabinoid signaling, evidence implicating the endocannabinoid system in the motivation for drug consumption, and drug-induced alterations in endocannabinoid function that may contribute to various aspects of addiction including dysregulated synaptic plasticity, increased stress responsivity, negative affective states, drug craving and relapse to drug taking. Current knowledge of genetic variants in endocannabinoid signaling associated with addiction is also discussed.

Endocannabinoid-mediated synaptic plasticity and addiction-related behavior

Endogenous cannabinoids (eCBs) are retrograde messengers that provide feedback inhibition of both excitatory and inhibitory transmission in brain through the activation of presynaptic CB₁ receptors. Substantial evidence indicates that eCBs mediate various forms of short- and long-term plasticity in brain regions involved in the etiology of addiction. The present review provides an overview of the mechanisms through which eCBs mediate various forms of synaptic plasticity and discusses evidence that eCB-mediated plasticity is disrupted following exposure to a variety of abused substances that differ substantially in pharmacodynamic mechanism including alcohol, psychostimulants and cannabinoids. The possible involvement of dysregulated eCB signaling in maladaptive behaviors that evolve over long-term drug exposure is also discussed, with a particular focus on altered behavioral responses to drug exposure, deficient extinction of drug-related memories, increased drug craving and relapse, heightened stress sensitivity and persistent affective disruption (anxiety and depression).

Anxiolytic-like effects induced by blockade of transient receptor potential vanilloid type 1 (TRPV1) channels in the medial prefrontal cortex of rats

RATIONALE

The endocannabinoid anandamide, in addition to activating cannabinoid type 1 receptors (CB1), may act as an agonist at transient receptor potential vanilloid type 1 (TRPV1) channels. In the periaqueductal gray, CB1 activation inhibits, whereas TRPV1 increases, anxiety-like behavior. In the medial prefrontal cortex (mPFC), another brain region related to defensive responses, CB1 activation induces anxiolytic-like effects. However, a possible involvement of TRPV1 is still unclear.

OBJECTIVES

In the present study, we tested the hypothesis that TRPV1 channel contributes to the modulation of anxiety-like behavior in the mPFC.

MATERIALS AND METHODS

Male Wistar rats (n = 5-7 per group) received microinjections of the TRPV1 antagonist capsazepine (1-60 nmol) in the ventral portion of the mPFC and were exposed to the elevated plus maze (EPM) or to the Vogel conflict test.

RESULTS

Capsazepine increased exploration of open arms in the EPM as well as the number of punished licks in the Vogel conflict test, suggesting anxiolytic-like effects. No changes in the number of entries into the enclosed arms were observed in the EPM, indicating that there were no changes in motor activity. Moreover, capsazepine did not interfere with water consumption or nociceptive threshold, discarding potential confounding factors for the Vogel conflict test.

CONCLUSIONS

These data suggest that TRPV1 in the ventral mPFC tonically inhibits anxiety-like behavior. TRPV1 could facilitate defensive responses opposing, therefore, the anxiolytic-like effects reported after local activation of CB1 receptors.