Activation of TRPV1 in the VTA excites dopaminergic neurons and increases chemical- and noxious-induced dopamine release in the nucleus accumbens

Metabolic pathways, pharmacokinetic, and brain neurochemicals effects of capsaicin: Comprehensively insights from in vivo studies

BACKGROUND

Capsaicin (CAP), a prominent component of chili pepper known for its potent agonistic effects on TRPV1, has attracted significant attention for its diverse physiological effects. Nevertheless, there remains a paucity of data concerning its in vivo distribution, metabolism, pharmacodynamic properties, and influence on the metabolic profile of the brain.

METHODS

Stable isotope tracing, in vitro enzyme incubation, microdialysis coupled with UHPLC-MS/MS techniques were employed to investigate the in vivo metabolic pathways, distribution, and pharmacokinetic properties of CAP, and the potential biases in metabolic pathways was elucidate through molecular docking. Furthermore, the effect of CAP on brain metabolic profiles was assessed using untargeted metabolomics, and spatial visualization analysis was conducted through mass spectrometry imaging.

RESULTS

CAP was distributed predominantly in the kidneys, with lower content in the liver, heart, lungs, brain, and spleen following peripheral administration, and the absorption half-life in the body was about 20 min. CAP primarily underwent alkyl terminal dehydrogenation, hydroxylation, and macrocyclization metabolic pathways under the action of CYP2C9, CYP2C19 and CYP2D6, resulting in at least four metabolites. Among them, the hydroxylation products were main metabolites and the dehydrogenation product 16,17-dihydrocapsaicin could interact with the key binding sites Leu515 and Thr550 of TRPV1 like CAP. CAP quickly diffused to various brain regions and the metabolic characteristics in the striatum were relatively different from that in the blood. The distribution of CAP in the brain primarily triggered the release of neurotransmitters in areas associated with reward, cognition, and memory. Both acute and chronic exposure to CAP elevated amino acid levels in cortical regions, while producing contrasting effects on nucleotide metabolites.

CONCLUSION

This study offers an initial in-depth analysis of the distribution patterns, metabolic pathways and pharmacodynamic properties of CAP in the body and brain. These findings established a basis for further studies on CAP's pharmacology properties and its influence on the central nervous system.

Interplay between endocannabinoid and endovanilloid mechanisms in fear conditioning

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

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.

Interplay between endocannabinoids and dopamine in the basal ganglia: implications for pain in Parkinson's disease

Pain is a complex phenomenon, and basal ganglia circuitry integrates many aspects of pain including motor, emotional, autonomic, and cognitive responses. Perturbations in dopamine (DA) signaling are implicated in the pathogenesis of chronic pain due to its involvement in both pain perception and relief. Several lines of evidence support the role of endocannabinoids (eCBs) in the regulation of many electrical and chemical aspects of DAergic neuron function including excitability, synaptic transmission, integration, and plasticity. However, eCBs play an even more intricate and intimate relationship with DA, as indicated by the adaptive changes in the eCB system following DA depletion. Although the precise mechanisms underlying DA control on pain are not fully understood, given the high correlation of eCB and DAergic system, it is conceivable that eCBs may be part of these mechanisms.In this brief survey, we describe the reciprocal regulation of eCB-DA neurotransmission with a particular emphasis on the actions of eCBs on ionic and synaptic signaling in DAergic neurons mediated by CB receptors or independent on them. Furthermore, we analyze the eCB-DA imbalance which characterizes pain condition and report the implications of reduced DA levels for pain in Parkinson's disease. Lastly, we discuss the potential of the eCB-DA system in the development of future therapeutic strategies for the treatment of pain.

TRPV1: Receptor structure, activation, modulation and role in neuro-immune interactions and pain

In the 1990s, the identification of a non-selective ion channel, especially responsive to capsaicin, revolutionized the studies of somatosensation and pain that were to follow. The TRPV1 channel is expressed mainly in neuronal cells, more specifically, in sensory neurons responsible for the perception of noxious stimuli. However, its presence has also been detected in other non-neuronal cells, such as immune cells, β- pancreatic cells, muscle cells and adipocytes. Activation of the channel occurs in response to a wide range of stimuli, such as noxious heat, low pH, gasses, toxins, endocannabinoids, lipid-derived endovanilloid, and chemical agents, such as capsaicin and resiniferatoxin. This activation results in an influx of cations through the channel pore, especially calcium. Intracellular calcium triggers different responses in sensory neurons. Dephosphorylation of the TRPV1 channel leads to its desensitization, which disrupts its function, while its phosphorylation increases the channel's sensitization and contributes to the channel's rehabilitation after desensitization. Kinases, phosphoinositides, and calmodulin are the main signaling pathways responsible for the channel's regulation. Thus, in this review we provide an overview of TRPV1 discovery, its tissue expression as well as on the mechanisms by which TRPV1 activation (directly or indirectly) induces pain in different disease models.

An Ascending Excitatory Circuit from the Dorsal Raphe for Sensory Modulation of Pain

The dorsal raphe nucleus (DRN) is an important nucleus in pain regulation. However, the underlying neural pathway and the function of specific cell types remain unclear. Here, we report a previously unrecognized ascending facilitation pathway, the DRN to the mesoaccumbal dopamine (DA) circuit, for regulating pain. Chronic pain increased the activity of DRN glutamatergic, but not serotonergic, neurons projecting to the ventral tegmental area (VTA) (DRNGlu-VTA) in male mice. The optogenetic activation of DRNGlu-VTA circuit induced a pain-like response in naive male mice, and its inhibition produced an analgesic effect in male mice with neuropathic pain. Furthermore, we discovered that DRN ascending pathway regulated pain through strengthened excitatory transmission onto the VTA DA neurons projecting to the ventral part of nucleus accumbens medial shell (vNAcMed), thereby activated the mesoaccumbal DA neurons. Correspondingly, optogenetic manipulation of this three-node pathway bilaterally regulated pain behaviors. These findings identified a DRN ascending excitatory pathway that is crucial for pain sensory processing, which can potentially be exploited toward targeting pain disorders.

Therapeutic Molecular Insights into the Active Engagement of Cannabinoids in the Therapy of Parkinson's Disease: A Novel and Futuristic Approach

Parkinson's disease is a neurodegenerative disorder which is characterised mostly by loss of dopaminergic nerve cells throughout the nigral area mainly as a consequence of oxidative stress. Muscle stiffness, disorganised bodily responses, disturbed sleep, weariness, amnesia, and voice impairment are all symptoms of dopaminergic neuron degeneration and existing symptomatic treatments are important to arrest additional neuronal death. Some cannabinoids have recently been demonstrated as robust antioxidants that might protect the nerve cells from degeneration even when cannabinoid receptors are not triggered. Cannabinoids are likely to have property to slow or presumably cease the steady deterioration of the brain's dopaminergic systems, a condition for which there is now no treatment. The use of cannabinoids in combination with currently available drugs has the potential to introduce a radically new paradigm for treatment of Parkinson's disease, making it immensely useful in the treatment of such a debilitating illness.

Medial and dorsal lateral septum involving social disruption stress-primed escalation in acid-induced writhes

Introduction

Stress may cause prospective escalations in abdominal pain magnitude and accumbal TRPV1 expression, while central neural circuits mediating these stress effects remain unclear.

Methods

Using retrograde tracing methods, we first demonstrated the existence of a medial septal-dorsal lateral septal -accumbal circuit very likely involving social disruption stress-primed escalations in acid-induced writhes and accumbal TRPV1 level. An intersectional viral strategy and virus-carrying hM3Dq and hM4Di DREADDs were, then, employed to selectively modulate GABAergic and cholinergic neuronal activity in medial and dorsal lateral septum.

Results

Exciting medial septal GABAergic neuron was found to prevent social disruption stress-primed escalations in acid-induced writhes and accumbal TRPV1 and PKCε expressions. Likewise, inactivating dorsal lateral septal cholinergic neurons was also effective in abolishing these stress-primed escalations. Inactivating GABAergic neuron in non-stressed animals' medial septum was found to reproduce the stress-primed effects in causing heightened acid-induced writhes and accumbal TRPV1 and PKCε levels.

Discussion

These results, taken together, prompt us to conclude that social disruption stress may produce plastic changes in a newly-identified medial septal-dorsal lateral septal-accumbal circuit. Moreover, medial septal GABAergic hypoactivity and dorsal lateral septal cholinergic hyperactivity are, at least, two likely causes reflecting such stress-produced escalations in abdominal pain magnitude and pain transduction-related protein over-expression in nucleus accumbens.

Magnetic nanomaterials for wireless thermal and mechanical neuromodulation

Magnetic fields are very attractive for non-invasive neuromodulation because they easily penetrate trough the skull and tissue. Cell specific neuromodulation requires the magnetic field energy to be converted by an actuator to a biologically relevant signal. Miniaturized actuators available today range from small, isotropic magnetic nanoparticles to larger, submicron anisotropic magnetic nanomaterials. Depending on the parameters of external magnetic fields and the properties of the nanoactuators, they create either a thermal or a mechanical stimulus. Ferromagnetic nanomaterials generate heat in response to high frequency alternating magnetic fields associated with dissipative losses. Anisotropic nanomaterials with large magnetic moments are capable of exerting forces at stationary or slowly varying magnetic fields. These tools allow exploiting thermosensitive or mechanosensitive neurons in circuit or cell specific tetherless neuromodulation schemes. This review will address assortment of available magnetic nanomaterial-based neuromodulation techniques that rely on application of external magnetic fields.

Endocannabinoid signaling in microglia

Microglia, the innate immune cells of the central nervous system (CNS), execute their sentinel, housekeeping and defense functions through a panoply of genes, receptors and released cytokines, chemokines and neurotrophic factors. Moreover, microglia functions are closely linked to the constant communication with other cell types, among them neurons. Depending on the signaling pathway and type of stimuli involved, the outcome of microglia operation can be neuroprotective or neurodegenerative. Accordingly, microglia are increasingly becoming considered cellular targets for therapeutic intervention. Among signals controlling microglia activity, the endocannabinoid (EC) system has been shown to exert a neuroprotective role in many neurological diseases. Like neurons, microglia express functional EC receptors and can produce and degrade ECs. Interestingly, boosting EC signaling leads to an anti-inflammatory and neuroprotective microglia phenotype. Nonetheless, little evidence is available on the microglia-mediated therapeutic effects of EC compounds. This review focuses on the EC signals acting on the CNS microglia in physiological and pathological conditions, namely on the CB1R, CB2R and TRPV1-mediated regulation of microglia properties. It also provides new evidence, which strengthens the understanding of mechanisms underlying the control of microglia functions by ECs. Given the broad expression of the EC system in glial and neuronal cells, the resulting picture is the need for in vivo studies in transgenic mouse models to dissect the contribution of EC microglia signaling in the neuroprotective effects of EC-derived compounds.

Remote neural regulation mediated by nanomaterials

Neural regulation techniques play an essential role in the functional dissection of neural circuits and also the treatment of neurological diseases. Recently, a series of nanomaterials, including upconversion nanoparticles (UCNPs), magnetic nanoparticles (MNPs), and silicon nanomaterials (SNMs) that are responsive to remote optical or magnetic stimulation, have been applied as transducers to facilitate localized control of neural activities. In this review, we summarize the latest advances in nanomaterial-mediated neural regulation, especially in a remote and minimally invasive manner. We first give an overview of existing neural stimulation techniques, including electrical stimulation, transcranial magnetic stimulation, chemogenetics, and optogenetics, with an emphasis on their current limitations. Then we focus on recent developments in nanomaterial-mediated neural regulation, including UCNP-mediated fiberless optogenetics, MNP-mediated magnetic neural regulation, and SNM-mediated non-genetic neural regulation. Finally, we discuss the possibilities and challenges for nanomaterial-mediated neural regulation.

Side Effects of Opioids Are Ameliorated by Regulating TRPV1 Receptors

Humans have used opioids to suppress moderate to severe pain for thousands of years. However, the long-term use of opioids has several adverse effects, such as opioid tolerance, opioid-induced hyperalgesia, and addiction. In addition, the low efficiency of opioids in controlling neuropathic pain limits their clinical applications. Combining nonopioid analgesics with opioids to target multiple sites along the nociceptive pathway may alleviate the side effects of opioids. This study reviews the feasibility of reducing opioid side effects by regulating the transient receptor potential vanilloid 1 (TRPV1) receptors and summarizes the possible underlying mechanisms. Blocking and activating TRPV1 receptors can improve the therapeutic profile of opioids in different manners. TRPV1 and μ-opioid receptors are bidirectionally regulated by β-arrestin2. Thus, drug combinations or developing dual-acting drugs simultaneously targeting μ-opioid and TRPV1 receptors may mitigate opioid tolerance and opioid-induced hyperalgesia. In addition, TRPV1 receptors, especially expressed in the dorsal striatum and nucleus accumbens, participate in mediating opioid reward, and its regulation can reduce the risk of opioid-induced addiction. Finally, co-administration of TRPV1 antagonists and opioids in the primary action sites of the periphery can significantly relieve neuropathic pain. In general, the regulation of TRPV1 may potentially ameliorate the side effects of opioids and enhance their analgesic efficacy in neuropathic pain.

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.

Impact of acute and chronic nicotine administration on midbrain dopaminergic neuron activity and related behaviors in TRPV1 knock-out juvenile mice

The addictive properties of nicotine, the main alkaloid in tobacco and tobacco-derived products, largely depend on its action on the activity of midbrain dopamine (DA) neurons. The Transient Receptor Potential Vanilloid 1 (TRPV1) channel has also been examined as an emerging contributor to addiction-related symptoms due to its ability to modulate midbrain neurons. Thus, the objective of our study was to explore the role of TRPV1 receptors (TRPV1Rs) on nicotine-induced behaviors and associated response of DA neuron activity. Both wild type juvenile mice and juvenile mice with invalidation of the TRPV1R gene were exposed to acute or chronic nicotine 0.3 mg/kg administration. We analyzed locomotor activity in response to the drug. In addition, we performed cell-attached and whole-cell recordings from ventral tegmental area (VTA) neurons after nicotine exposure. Our results showed that the genetic deletion of TRPV1Rs reduced nicotine-induced locomotor sensitization. In addition, it provided evidence in support of TRPV1Rs being regulators of inhibitory synaptic transmission in the VTA. However, TRPV1Rs did not seem to modulate either nicotine-induced conditioning place preference or nicotine-evoked electrical activity of DA neurons. In conclusion, TRPV1Rs modulate nicotine-induced psychomotor sensitization in mice independently of a control on VTA DA neuron activity. Thus, TRPV1R control may depend on another key player of the mesolimbic circuit.

Midbrain Dopamine Neurons Defined by TrpV1 Modulate Psychomotor Behavior

Dopamine (DA) neurons of the ventral tegmental area (VTA) continue to gain attention as far more heterogeneous than previously realized. Within the medial aspect of the VTA, the unexpected presence of TrpV1 mRNA has been identified. TrpV1 encodes the Transient Receptor Potential cation channel subfamily V member 1, TRPV1, also known as the capsaicin receptor, well recognized for its role in heat and pain processing by peripheral neurons. In contrast, the brain distribution of TrpV1 has been debated. Here, we hypothesized that the TrpV1+ identity defines a distinct subpopulation of VTA DA neurons. To explore these brain TrpV1+ neurons, histological analyses and Cre-driven mouse genetics were employed. TrpV1 mRNA was most strongly detected at the perinatal stage forming a band of scattered neurons throughout the medial VTA, reaching into the posterior hypothalamus. Within the VTA, the majority of TrpV1 co-localized with both Tyrosine hydroxylase (Th) and Vesicular monoamine transporter 2 (Vmat2), confirming a DA phenotype. However, TrpV1 also co-localized substantially with Vesicular glutamate transporter 2 (Vglut2), representing the capacity for glutamate (GLU) release. These TrpV1+/Th+/Vglut2+/Vmat2+ neurons thus constitute a molecularly and anatomically distinct subpopulation of DA-GLU co-releasing neurons. To assess behavioral impact, a TrpV1Cre -driven strategy targeting the Vmat2 gene in mice was implemented. This manipulation was sufficient to alter psychomotor behavior induced by amphetamine. The acute effect of the drug was accentuated above control levels, suggesting super-sensitivity in the drug-na ve state resembling a "pre-sensitized" phenotype. However, no progressive increase with repeated injections was observed. This study identifies a distinct TrpV1+ VTA subpopulation as a critical modulatory component in responsiveness to amphetamine. Moreover, expression of the gene encoding TRPV1 in selected VTA neurons opens up for new possibilities in pharmacological intervention of this heterogeneous, but clinically important, brain area.

Social disruption-induced stress pre-exposure aggravates, while the presence of conspecifics diminishes, acetic acid-induced writhing

RATIONALE AND OBJECTIVE

This study was undertaken to assess the modulating effects of (1) pre-exposure to repeated social disruption and (2) group testing on writhing associated with visceral pain induced by intraperitoneal administration of acetic acid.

MATERIALS AND METHODS

Six consecutive days of social disruption were used to prime for stress, while group testing referred to 3 mouse cage-mates receiving the acetic acid-induced writhing test as a group.

RESULTS

Social disruption-induced stress-pre-exposed mice displayed a greater number acid-induced writhes compared to mice not receiving the pre-exposure. However, mice displayed fewer acid-induced writhes in a triad group vs. individually, suggesting group-mediated writhing-reducing effects. Likewise, group testing prevented the stress pre-exposure escalation in acid-induced writhes. Additional studies revealed that the stress-pre-exposed mice had increased expression in accumbal TRPV1 receptors. Systemic (0.25 mg/kg) and bilateral intra-accumbal (0.2 ng/0.2 µl/side) administration of SB366791, a TRPV1 receptor antagonist, reliably prevented the stress pre-exposure escalation in acid-induced writhing; SB366791 treatment alone did not affect acid-induced writhing, stress pre-exposure anxiety-like behavior, or the group testing effects. Furthermore, lower neuronal activation was found in the medial septal nucleus in group vs. individual tested mice. Intra-medial septum (0.2 µg/0.5 µl) infusion with bicuculline, a GABAA receptor antagonist, effectively prevented group-mediated writhing-reducing effects, but not individual acid-induced writhing effects.

CONCLUSIONS

These findings suggest that social disruption-induced stress pre-exposure may upregulate accumbal TRPV1 receptor expression and consequently aggravate acid-induced writhing. Group testing prevents such stress pre-exposure escalation of acid-induced writhing most likely by strengthening the GABAergic inhibition on local neural activity in the medial septum.

Magnetothermal nanoparticle technology alleviates parkinsonian-like symptoms in mice

Deep brain stimulation (DBS) has long been used to alleviate symptoms in patients suffering from psychiatric and neurological disorders through stereotactically implanted electrodes that deliver current to subcortical structures via wired pacemakers. The application of DBS to modulate neural circuits is, however, hampered by its mechanical invasiveness and the use of chronically implanted leads, which poses a risk for hardware failure, hemorrhage, and infection. Here, we demonstrate that a wireless magnetothermal approach to DBS (mDBS) can provide similar therapeutic benefits in two mouse models of Parkinson's disease, the bilateral 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and in the unilateral 6-hydroxydopamine (6-OHDA) model. We show magnetothermal neuromodulation in untethered moving mice through the activation of the heat-sensitive capsaicin receptor (transient receptor potential cation channel subfamily V member 1, TRPV1) by synthetic magnetic nanoparticles. When exposed to an alternating magnetic field, the nanoparticles dissipate heat, which triggers reversible firing of TRPV1-expressing neurons. We found that mDBS in the subthalamic nucleus (STN) enables remote modulation of motor behavior in healthy mice. Moreover, mDBS of the STN reversed the motor deficits in a mild and severe parkinsonian model. Consequently, this approach is able to activate deep-brain circuits without the need for permanently implanted hardware and connectors.

The cross-talk between dopaminergic and nitric oxide systems in the medial septal nucleus, and their distinct effects on anxiety-like behaviors in male rats

Anxiety disorders, which have a noticeable global prevalence and may be caused by many factors, include a spectrum of disorders that share features of excessive fear- and anxiety-related behavioral disturbances. Different brain areas and neurotransmitter systems have been under investigation for anxiety-related disorders. In this study, we investigated the possible interaction between the dopaminergic and nitric oxide (NO) neurotransmitter systems in the medial septal nucleus and their roles in anxiety-like behaviors using elevated plus-maze (EPM) test in male rats. Our results showed that: (i) both D1-and D2-like receptor agonists, SKF-38393 and quinpirole, augmented anxiety-like behaviors at their two highest applied doses in the EPM test; (ii) both D1-and D2-like receptor antagonists, SCH- 23390 and sulpiride, reduced anxiety-like behaviors at their two highest applied doses in the EPM test; (iii) L-Arginine, a NO precursor, increased anxiety-like behaviors, but L-NAME, a non-specific nitric oxide synthase (NOS) inhibitor, reduced them in the EPM test; (iv) L-NAME could not reverse the anxiety-like parameters produced by SKF-38393, but it significantly reduced the anxiety-like behaviors induced by quinpirole; (v) Neither SCH- 23390 nor sulpiride changed anxiety-related behaviors induced by L-Arginine. It can be concluded that both dopaminergic and nitric oxide systems in the medial septal nucleus are involved in modulating anxiety-like behaviors. While NO has an involvement in the exerted effects by the D2-like agonist, such effects were not observed at the applied range of the doses for D1-and D2-like antagonists.

Insights into Potential Targets for Therapeutic Intervention in Epilepsy

Epilepsy is a chronic brain disease that affects approximately 65 million people worldwide. However, despite the continuous development of antiepileptic drugs, over 30% patients with epilepsy progress to drug-resistant epilepsy. For this reason, it is a high priority objective in preclinical research to find novel therapeutic targets and to develop effective drugs that prevent or reverse the molecular mechanisms underlying epilepsy progression. Among these potential therapeutic targets, we highlight currently available information involving signaling pathways (Wnt/β-catenin, Mammalian Target of Rapamycin (mTOR) signaling and zinc signaling), enzymes (carbonic anhydrase), proteins (erythropoietin, copine 6 and complement system), channels (Transient Receptor Potential Vanilloid Type 1 (TRPV1) channel) and receptors (galanin and melatonin receptors). All of them have demonstrated a certain degree of efficacy not only in controlling seizures but also in displaying neuroprotective activity and in modifying the progression of epilepsy. Although some research with these specific targets has been done in relation with epilepsy, they have not been fully explored as potential therapeutic targets that could help address the unsolved issue of drug-resistant epilepsy and develop new antiseizure therapies for the treatment of epilepsy.

Ultra-low doses of the transient receptor potential vanilloid 1 agonist, resiniferatoxin, prevents vomiting evoked by diverse emetogens in the least shrew (Cryptotis parva)

Published studies have shown that the transient receptor potential vanilloid 1 (TRPV1) receptor agonist, resiniferatoxin (RTX), has pro and antiemetic effects. RTX can suppress vomiting evoked by a variety of nonselective emetogens such as copper sulfate and cisplatin in several vomit-competent species. In the least shrew, we have already demonstrated that combinations of ultra-low doses of RTX and low doses of the cannabinoid CB1/2 receptor agonist delta-9-tetrahydrocannabinol (Δ-THC) produce additive antiemetic effects against cisplatin-evoked vomiting. In the current study, we investigated the broad-spectrum antiemetic potential of very low nonemetic doses of RTX against a diverse group of specific emetogens including selective and nonselective agonists of serotonergic 5-hydroxytrptamine (5-HT3) receptor (5-HT and 2-Me-5-HT), dopaminergic D2 receptor (apomorphine and quinpirole), cholinergic M1 receptor (pilocarpine and McN-A-343), as well as the selective substance P neurokinin NK1 receptor agonist GR73632, the selective L-Type calcium channel agonist FPL64176, and the sarcoplasmic endoplasmic reticulum calcium ATPase (SERCA) inhibitor thapsigargin. When administered subcutaneously, ultra-low (0.01 µg/kg) to low (5.0 µg/kg) doses of RTX suppressed vomiting induced by the aforementioned emetogens in a dose-dependent fashion with 50% inhibitory dose values ranging from 0.01 to 1.26 µg/kg. This study is the first to demonstrate that low nanomolar nonemetic doses of RTX have the capacity to completely abolish vomiting caused by diverse receptor specific emetogens in the least shrew model of emesis.

Role of TRPV1/TRPV3 channels in olanzapine-induced metabolic alteration: Possible involvement in hypothalamic energy-sensing, appetite regulation, inflammation and mesolimbic pathway

Atypical antipsychotics (AAPs) have the tendency of inducing severe metabolic alterations like obesity, diabetes mellitus, insulin resistance, dyslipidemia and cardiovascular complications. These alterations have been attributed to altered hypothalamic appetite regulation, energy sensing, insulin/leptin signaling, inflammatory reactions and active reward anticipation. Line of evidence suggests that transient receptor potential vanilloid type 1 and 3 (TRPV1 and TRPV3) channels are emerging targets in treatment of obesity, diabetes mellitus and could modulate feed intake. The present study was aimed to investigate the putative role TRPV1/TRPV3 in olanzapine-induced metabolic alterations in mice. Female BALB/c mice were treated with olanzapine for six weeks to induce metabolic alterations. Non-selective TRPV1/TRPV3 antagonist (ruthenium red) and selective TRPV1 (capsazepine) and TRPV3 antagonists (2,2-diphenyltetrahydrofuran or DPTHF) were used to investigate the involvement of TRPV1/TRPV3 in chronic olanzapine-induced metabolic alterations. These metabolic alterations were differentially reversed by ruthenium red and capsazepine, while DPTHF didn't show any significant effect. Olanzapine treatment also altered the mRNA expression of hypothalamic appetite-regulating and nutrient-sensing factors, inflammatory genes and TRPV1/TRPV3, which were reversed with ruthenium red and capsazepine treatment. Furthermore, olanzapine treatment also increased expression of TRPV1/TRPV3 in nucleus accumbens (NAc), TRPV3 expression in ventral tegmental area (VTA), which were reversed by the respective antagonists. However, DPTHF treatment showed reduced feed intake in olanzapine treated mice, which might be due to TRPV3 specific antagonism and reduced hedonic feed intake. In conclusion, our results suggested the putative role TRPV1 in hypothalamic dysregulations and TRPV3 in the mesolimbic pathway; both regulate feeding in olanzapine treated mice.

Blockade of the endovanilloid receptor, TRPV1, and of the endocannabinoid enzyme, FAAH, within the nucleus accumbens shell elicits anxiolytic-like effects in male rats

RATIONALE

The functional role of the endocannabinoid system (ECS) and Transient Receptor Potential Vanilloid type-1 (TRPV1) within the Nucleus Accumbens shell (NAc shell) remains unknown. Preclinical studies in rodents have reported that the ECS modulates emotional responses such as anxiety. The NAc shell has a high density of synaptically co-localized cannabinoid receptor type-1 (CB1R) and TRPV1, suggesting a potential involvement in the modulation of anxiety.

OBJECTIVES

The present study aims to establish the role of ECS-TRPV1 interactions within the NAc shell and its effects on anxiety. It is hypothesized that the neurochemical regulation elicited by ECS within the NAc shell mediates anxiety-like behaviors in rodents.

METHODS

In this study, male Sprague Dawley rats were implanted with bilateral brain cannula targeting the NAc shell. Following recovery from surgery, animals received microinfusion pretreatments (0, 0.125, 0.5 nmol/0.4 μl) of N-arachidonoyl-serotonin (AA-5-HT), a dual blocker of the endocannabinoid-inactivating enzyme, fatty acid amide hydrolase (FAAH) and a TRPV1 antagonist in the NAc shell. Following treatment, animals were tested in an elevated plus maze (EPM) paradigm for a period of 5 minutes. At the end of the experiment, animals were sacrificed and their brains collected for histological and biochemical analysis.

RESULTS

Results showed that animals treated with AA-5-HT in a dose dependent manner spent significantly more time in the open arms than vehicle-treated animals. In addition, AA-5-HT administration induced a significant downregulation of CB1R expression in the NAc shell.

CONCLUSIONS

The present findings suggest that the ECS within the NAc shell modulates anxiety-like behaviors via FAAH and CB1R activity.

Transient receptor potential vanilloid 1 mediates cocaine reinstatement via the D1 dopamine receptor in the nucleus accumbens

PURPOSE

The transient receptor potential vanilloid 1 (TRPV1) is a nonselective cation channel that mediates synaptic modification in the nucleus accumbens (NAc). However, no study has yet examined the mechanism of TRPV1 in the NAc on cocaine reinstatement. We investigated the mechanism of TRPV1 in NAc on cocaine reinstatement using the conditioned place preference (CPP) test in mice.

METHODS

We examined the effect of capsazepine (5 mg/kg, a TRPV1 antagonist, administered intraperitoneally (i.p.)), capsaicin (0.3 mg/kg, a TRPV1 agonist, administered i.p.), and genetic deletion of TRPV1 on the reinstatement of cocaine-induced CPP (15 mg/kg, administered i.p.). The expression of TRPV1 and Ca2⁺/calmodulin-mediated kinase II (CaMKII) in the NAc were determined after cocaine reinstatement. Microinjection of SB366791 (0.2 ng, a selective TRPV1 antagonist) in the NAc was assessed on SKF-81297 (1 µg, D1-like dopamine (DA) receptor agonist) primed cocaine reinstatement.

RESULTS

Capsazepine suppressed and capsaicin potentiated cocaine CPP in the reinstatement phase. In addition, genetic deletion of TRPV1 inhibited cocaine-priming reinstatement. Cocaine reinstatement was mediated by increased TRPV1 expression in the NAc, which involves CaMKII. Microinjection of SB366791 in the NAc prevented the cocaine reinstatement evoked by microinjection of SKF-81297 in the NAc.

CONCLUSIONS

These findings suggest that activation of TRPV1 mediates the stimulation of D1-like DA receptors and CaMKII in the NAc, resulting in the facilitation of cocaine reinstatement behaviors. Thus, our findings reveal a previously unknown TRPV1 mechanism in the reinstatement to drugs of abuse.

Neuropsychiatric implications of transient receptor potential vanilloid (TRPV) channels in the reward system

Neuropsychiatric disorders (NPDs) exert a devastating impact on an individual's personal and social well-being, encompassing various conditions and brain anomalies that influence affect, cognition, and behavior. Because the pathophysiology of NPDs is multifactorial, the precise mechanisms underlying the development of such disorders remain unclear, representing a unique challenge in current neuropsychopharmacotherapy. Transient receptor potential vanilloid (TRPV) type channels are a family of ligand-gated ion channels that mainly include sensory receptors that respond to thermal, mechanical and chemical stimuli. TRPV channels are abundantly present in dopaminergic neurons, thus playing a pivotal role in the modulation of the reward system and in pathophysiology of diseases such as stress, anxiety, depression, schizophrenia, neurodegenerative disorders and substance abuse/addiction. Recent evidence has highlighted TRPV channels as potential targets for understanding modulation of the reward system and various forms of addiction (opioids, cocaine, amphetamines, alcohol, nicotine, cannabis). In this review, we discuss the distribution, physiological roles, ligands and therapeutic importance of TRPV channels with regard to NPDs and addiction biology.

Brain-derived neurotrophic factor-mediated projection-specific regulation of depressive-like and nociceptive behaviors in the mesolimbic reward circuitry

Increasing evidence suggests that the mesolimbic reward system plays critical roles in the regulation of depression and nociception; however, its circuitry and cellular mechanisms remain unclear. In this study, we investigated the output-specific regulatory roles of dopaminergic (DA) neurons within the ventral tegmental area (VTA) in depressive-like and nociceptive behaviors in mice subjected to unpredictable chronic mild stress (CMS), using the projection-specific electrophysiological recording, pharmacological manipulation, behavioral test, and molecular biology technologies. We demonstrated that CMS decreased the firing activity in VTA projecting to medial prefrontal cortex (VTA → mPFC), but not in VTA to nucleus accumbens (VTA → NAc), DA neurons. However, both VTA → mPFC and VTA → NAc DA neurons showed increased firing activity in response to morphine perfusion in CMS mice. Behavioral results showed that intra-VTA microinjection of morphine (25.5 ng/0.15 μL) relieved depressive-like behaviors, intriguingly, accompanied by a thermal hyperalgesia. Furthermore, the relief of depressive-like behaviors induced by intra-VTA injection of morphine in CMS mice could be prevented by blocking brain-derived neurotrophic factor (BDNF) signaling and mimicked by the administration of exogenous BDNF in mPFC rather than in NAc shell. Nociceptive responses induced by the activation of VTA DA neurons with morphine in CMS mice could be prevented by blocking BDNF signaling or mimicked by administration of exogenous BDNF in NAc shell, but not in mPFC. These results reveal projection-specific regulatory mechanisms of depression and nociception in the mesolimbic reward circuitry and provide new insights into the neural circuits involved in the processing of depressive and nociceptive information.

Progress in neuromodulation of the brain: A role for magnetic nanoparticles?

The field of neuromodulation is developing rapidly. Current techniques, however, are still limited as they i) either depend on permanent implants, ii) require invasive procedures, iii) are not cell-type specific, iv) involve slow pharmacokinetics or v) have a restricted penetration depth making it difficult to stimulate regions deep within the brain. Refinements into the different fields of neuromodulation are thus needed. In this review, we will provide background information on the different techniques of neuromodulation discussing their latest refinements and future potentials including the implementation of nanoparticles (NPs). In particular we will highlight the usage of magnetic nanoparticles (MNPs) as transducers in advanced neuromodulation. When exposed to an alternating magnetic field (AMF), certain MNPs can generate heat through hysteresis. This MNP heating has been promising in the field of cancer therapy and has recently been introduced as a method for remote and wireless neuromodulation. This indicates that MNPs may aid in the exploration of brain functions via neuromodulation and may eventually be applied for treatment of neuropsychiatric disorders. We will address the materials chemistry of MNPs, their biomedical applications, their delivery into the brain, their mechanisms of stimulation with emphasis on MNP heating and their remote control in living tissue. The final section compares and discusses the parameters used for MNP heating in brain cancer treatment and neuromodulation. Concluding, using MNPs for nanomaterial-mediated neuromodulation seem promising in a variety of techniques and could be applied for different neuropsychiatric disorders when more extensively investigated.

Immune aspects of the bi-directional neuroimmune facilitator TRPV1

A rapidly growing area of interest in biomedical science involves the reciprocal crosstalk between the sensory nervous and immune systems. Both of these systems are highly integrated, detecting potential environmental harms and restoring homeostasis. Many different cytokines, receptors, neuropeptides, and other proteins are involved in this bidirectional communication that are common to both systems. One such family of proteins includes the transient receptor potential vanilloid (TRPV) proteins. Though much progress has been made in understanding TRPV proteins in the nervous system, their functions in the immune system are not well elucidated. Hence, further understanding their role in the peripheral immune system and as regulators of neuroimmunity is critical for evaluating their potential as therapeutic targets for numerous inflammatory disorders, cancers, and other disease states. Here, we focus on the latest advancements in understanding TRPV1 and TRPV2's roles in the immune system, TRPV1 in neuroimmunity, and TRPV1's potential involvement in anti-tumor therapy.

CB1-Dependent Long-Term Depression in Ventral Tegmental Area GABA Neurons: A Novel Target for Marijuana

The VTA is necessary for reward behavior with dopamine cells critically involved in reward signaling. Dopamine cells in turn are innervated and regulated by neighboring inhibitory GABA cells. Using whole-cell electrophysiology in juvenile-adolescent GAD67-GFP male mice, we examined excitatory plasticity in fluorescent VTA GABA cells. A novel CB1-dependent LTD was induced in GABA cells that was dependent on metabotropic glutamate receptor 5, and cannabinoid receptor 1 (CB1). LTD was absent in CB1 knock-out mice but preserved in heterozygous littermates. Bath applied Δ⁹-tetrahydrocannabinol depressed GABA cell activity, therefore downstream dopamine cells will be disinhibited; and thus, this could potentially result in increased reward. Chronic injections of Δ⁹-tetrahydrocannabinol occluded LTD compared with vehicle injections; however, a single exposure was insufficient to do so. As synaptic modifications by drugs of abuse are often tied to addiction, these data suggest a possible mechanism for the addictive effects of Δ⁹-tetrahydrocannabinol in juvenile-adolescents, by potentially altering reward behavioral outcomes.SIGNIFICANCE STATEMENT The present study identifies a novel form of glutamatergic synaptic plasticity in VTA GABA neurons, a currently understudied cell type that is critical for the brain's reward circuit, and how Δ⁹-tetrahydrocannabinol occludes this plasticity. This study specifically addresses a potential unifying mechanism whereby marijuana could exert rewarding and addictive/withdrawal effects. Marijuana use and legalization are a pressing issue for many states in the United States. Although marijuana is the most commonly abused illicit drug, the implications of legalized, widespread, or continued usage are speculative. This study in juvenile-adolescent aged mice identifies a novel form of synaptic plasticity in VTA GABA cells, and the synaptic remodeling that can occur after Δ⁹-tetrahydrocannabinol use.

TRPV1 modulates morphine-induced conditioned place preference via p38 MAPK in the nucleus accumbens

Emerging evidence suggests that the transient receptor potential vanilloid type 1 channel (TRPV1) is a novel target for the treatment of drug addiction, such as cocaine and morphine. Previously we reported that TRPV1 inhibition reduced morphine reward in the dorsal striatum (DSt) of mice and morphine self-administration through a decrease in accumbal activity in rats. However, the role of TRPV1 on morphine-conditioned reward in addiction-related brain regions, such as the nucleus accumbens (NAc), has not been previously established. Here, we investigated the effects of TRPV1 on morphine conditioned place preference (CPP) and intracellular mechanisms of TRPV1 using Western blot analysis and immunohistochemistry (IHC) in morphine-administered mice. TRPV1 knockout mice did not exhibit morphine reward responses, and both i.p. and intra-NAc injections of SB366791, a selective TRPV1 antagonist, reduced morphine-induced CPP in wild-type mice. Furthermore, i.p. injection of SB203580, a selective p38 MAPK inhibitor, also dampened morphine-induced CPP. To determine the molecular mechanisms of the TRPV1/p38 MAPK pathway in morphine CPP, we investigated the expression of adenylyl cyclase type 1 (AC1) and phospho-p38 mitogen-activated protein kinase (MAPK) and nuclear factor-kappa B (NF-κB) in the NAc. Either SB366791 or SB203580 decreased the protein expression levels of phospho-p38 MAPK, phosphor-NF-κB, and AC1 in the NAc of morphine CPP mice. Taken together, our findings suggest that TRPV1 may modulate morphine-induced conditioned reward effects via the p38 MAPK signaling pathway in the NAc. Therefore, blockade of TRPV1 may provide a novel therapeutic approach for the prevention and treatment of opioid addiction.

Spinal pathways involved in somatosensory inhibition of the psychomotor actions of cocaine

Previous studies have demonstrated that somatosensory stimuli influence dopamine transmission in the mesolimbic reward system and can reduce drug-induced motor behaviors, craving and dependence. Until now, the central links between somatosensory and brain reward systems are not known. Here, we show that the dorsal column (DC) somatosensory pathway contains projections that convey an inhibitory input from the periphery to mesolimbic reward circuits. Stimulation of the ulnar nerve under HT7 acupoint suppressed psychomotor response to cocaine, which was abolished by disruption of the DC pathway, but not the spinothalamic tract (STT). Low-threshold or wide-dynamic range neurons in the cuneate nucleus (CN) were excited by peripheral stimulation. Lesions of dorsal column or lateral habenula (LHb) prevented the inhibitory effects of peripheral stimulation on cocaine-induced neuronal activation in the nucleus accumbens (NAc). LHb neurons projecting to the ventral tegmental area (VTA)/rostromedial tegmental nucleus (RMTg) regions were activated by peripheral stimulation and LHb lesions reversed the inhibitory effects on cocaine locomotion produced by peripheral stimulation. These findings suggest that there exists a pathway in spinal cord that ascends from periphery to mesolimbic reward circuits (spino-mesolimbic pathway) and the activation of somatosensory input transmitted via the DC pathway can inhibit the psychomotor response to cocaine.

Cannabinoids and Vanilloids in Schizophrenia: Neurophysiological Evidence and Directions for Basic Research

Much of our knowledge of the endocannabinoid system in schizophrenia comes from behavioral measures in rodents, like prepulse inhibition of the acoustic startle and open-field locomotion, which are commonly used along with neurochemical approaches or drug challenge designs. Such methods continue to map fundamental mechanisms of sensorimotor gating, hyperlocomotion, social interaction, and underlying monoaminergic, glutamatergic, and GABAergic disturbances. These strategies will require, however, a greater use of neurophysiological tools to better inform clinical research. In this sense, electrophysiology and viral vector-based circuit dissection, like optogenetics, can further elucidate how exogenous cannabinoids worsen (e.g., tetrahydrocannabinol, THC) or ameliorate (e.g., cannabidiol, CBD) schizophrenia symptoms, like hallucinations, delusions, and cognitive deficits. Also, recent studies point to a complex endocannabinoid-endovanilloid interplay, including the influence of anandamide (endogenous CB₁ and TRPV₁ agonist) on cognitive variables, such as aversive memory extinction. In fact, growing interest has been devoted to TRPV₁ receptors as promising therapeutic targets. Here, these issues are reviewed with an emphasis on the neurophysiological evidence. First, we contextualize imaging and electrographic findings in humans. Then, we present a comprehensive review on rodent electrophysiology. Finally, we discuss how basic research will benefit from further combining psychopharmacological and neurophysiological tools.

Role of N-Arachidonoyl-Serotonin (AA-5-HT) in Sleep-Wake Cycle Architecture, Sleep Homeostasis, and Neurotransmitters Regulation

The endocannabinoid system comprises several molecular entities such as endogenous ligands [anandamide (AEA) and 2-arachidonoylglycerol (2-AG)], receptors (CB₁ and CB₂), enzymes such as [fatty acid amide hydrolase (FAHH) and monoacylglycerol lipase (MAGL)], as well as the anandamide membrane transporter. Although the role of this complex neurobiological system in the sleep–wake cycle modulation has been studied, the contribution of the blocker of FAAH/transient receptor potential cation channel subfamily V member 1 (TRPV1), N-arachidonoyl-serotonin (AA-5-HT) in sleep has not been investigated. Thus, in the present study, varying doses of AA-5-HT (5, 10, or 20 mg/Kg, i.p.) injected at the beginning of the lights-on period of rats, caused no statistical changes in sleep patterns. However, similar pharmacological treatment given to animals at the beginning of the dark period decreased wakefulness (W) and increased slow wave sleep (SWS) as well as rapid eye movement sleep (REMS). Power spectra analysis of states of vigilance showed that injection of AA-5-HT during the lights-off period diminished alpha spectrum across alertness in a dose-dependent fashion. In opposition, delta power spectra was enhanced as well as theta spectrum, during SWS and REMS, respectively. Moreover, the highest dose of AA-5-HT decreased wake-related contents of neurotransmitters such as dopamine (DA), norepinephrine (NE), epinephrine (EP), serotonin (5-HT) whereas the levels of adenosine (AD) were enhanced. In addition, the sleep-inducing properties of AA-5-HT were confirmed since this compound blocked the increase in W caused by stimulants such as cannabidiol (CBD) or modafinil (MOD) during the lights-on period. Additionally, administration of AA-5-HT also prevented the enhancement in contents of DA, NE, EP, 5-HT and AD after CBD of MOD injection. Lastly, the role of AA-5-HT in sleep homeostasis was tested in animals that received either CBD or MOD after total sleep deprivation (TSD). The injection of CBD or MOD increased alertness during sleep rebound period after TSD. However, AA-5-HT blocked this effect by allowing animals to display an enhancement in sleep across sleep rebound period. Overall, our findings provide evidence that AA-5-HT is an important modulator of sleep, sleep homeostasis and neurotransmitter contents.

N-oleoyldopamine modulates activity of midbrain dopaminergic neurons through multiple mechanisms

N-oleoyl-dopamine (OLDA) is an amide of dopamine and oleic acid, synthesized in catecholaminergic neurons. The present study investigates OLDA targets in midbrain dopaminergic (DA) neurons. Substantia Nigra compacta (SNc) DA neurons recorded in brain slices were excited by OLDA in wild type mice. In transient receptor potential vanilloid 1 (TRPV1) knockout (KO) mice, however, SNc DA neurons displayed sustained inhibition of firing. In the presence of the dopamine type 2 receptor (D2R) antagonist sulpiride or the dopamine transporter blocker nomifensine no such inhibition was observed. Under sulpiride OLDA slightly excited SNc DA neurons, an action abolished upon combined application of the cannabinoid1 and 2 receptor antagonists AM251 and AM630. In ventral tegmental area (VTA) DA neurons from TRPV1 KO mice a transient inhibition of firing by OLDA was observed. Thus OLDA modulates the firing of nigrostriatal DA neurons through interactions with TRPV1, cannabinoid receptors and dopamine uptake. These findings suggest further development of OLDA-like tandem molecules for the treatment of movement disorders including Parkinson's disease.

Dopamine D3 receptor knockout mice exhibit abnormal nociception in a sex-different manner

Pain is a complex and subjective experience. Previous studies have shown that mice lacking the dopamine D3 receptor (D3RKO) exhibit hypoalgesia, indicating a role of the D3 receptor in modulation of nociception. Given that there are sex differences in pain perception, there may be differences in responses to nociceptive stimuli between male and female D3RKO mice. In the current study, we examined the role of the D3 receptor in modulating nociception in male and female D3RKO mice. Acute thermal pain was modeled by hot-plate test. This test was performed at different temperatures including 52°C, 55°C, and 58°C. The von Frey hair test was applied to evaluate mechanical pain. And persistent pain produced by peripheral tissue injury and inflammation was modeled by formalin test. In the hot-plate test, compared with wild-type (WT) mice, D3RKO mice generally exhibited longer latencies at each of the three temperatures. Specially, male D3RKO mice showed hypoalgesia compared with male WT mice when the temperature was 55°C, while for the female mice, there was a statistical difference between genotypes when the test condition was 52°C. In the von Frey hair test, both male and female D3RKO mice exhibited hypoalgesia. In the formalin test, the male D3RKO mice displayed a similar nociceptive behavior as their sex-matched WT littermates, whereas significantly depressed late-phase formalin-induced nociceptive behaviors were observed in the female mutants. These findings indicated that the D3 receptor affects nociceptive behaviors in a sex-specific manner and that its absence induces more analgesic behavior in the female knockout mice. © 2016 Wiley Periodicals, Inc.

Orexins contribute to restraint stress-induced cocaine relapse by endocannabinoid-mediated disinhibition of dopaminergic neurons

Orexins are associated with drug relapse in rodents. Here, we show that acute restraint stress in mice activates lateral hypothalamic (LH) orexin neurons, increases levels of orexin A and 2-arachidonoylglycerol (2-AG) in the ventral tegmental area (VTA), and reinstates extinguished cocaine-conditioned place preference (CPP). This stress-induced reinstatement of cocaine CPP depends on type 1 orexin receptors (OX1Rs), type 1 cannabinoid receptors (CB1Rs) and diacylglycerol lipase (DAGL) in the VTA. In dopaminergic neurons of VTA slices, orexin A presynaptically inhibits GABAergic transmission. This effect is prevented by internal GDP-β-S or inhibiting OX1Rs, CB1Rs, phospholipase C or DAGL, and potentiated by inhibiting 2-AG degradation. These results suggest that restraint stress activates LH orexin neurons, releasing orexins into the VTA to activate postsynaptic OX1Rs of dopaminergic neurons and generate 2-AG through a G_q-protein-phospholipase C-DAGL cascade. 2-AG retrogradely inhibits GABA release through presynaptic CB1Rs, leading to VTA dopaminergic disinhibition and reinstatement of cocaine CPP.

Stress is a major cause of relapse to cocaine seeking behaviour. Tung et al. show that orexin mediates stress-induced reinstatement of cocaine seeking behaviour in mice by endocannabinoid-dependent disinhibition in the ventral tegmental area.

TRPV currents and their role in the nociception and neuroplasticity

Transient receptor potential channels sensitive to vanilloids (TRPVs) are group of ion channels which are sensitive to various tissue damaging signals and their activation is generally perceived as pain. Therefore, they are generally named as nociceptors. Understanding their activation and function as well as their interaction with intracellular pathways is crucial for the development of pharmacological interference in order to reduce pain perception. The current review summarizes basic facts in regard to TRPV and discusses their relevance in the sensing of (pain-) signals and their intracellular processing, focussing on their modulation of the intracellular calcium ([Ca(2+)]i) signal. Furthermore we discuss the basic mechanisms how the modification of [Ca(2+)]i through TRPV might induce long-term-potentiation (LTP) or long-term- depression (LTD) and from "memories" of pain. Understanding of these mechanisms is needed to localize the best point of interference for pharmacological treatment. Therefore, high attention is given to highlight physiological and pathological processes and their interaction with significant modulators and their roles in neuroplasticity and pain modulation.

Modulation of pain, nociception, and analgesia by the brain reward center

The midbrain dopamine center comprises a key network for reward, salience, motivation, and mood. Evidence from various clinical and preclinical settings points to the midbrain dopamine circuit as an important modulator of pain perception and pain-induced anxiety and depression. This review summarizes recent findings that shed light to the neuroanatomical, electrophysiological and molecular adaptations that chronic pain conditions promote in the mesolimbic dopamine system. Chronic pain states induce changes in neuronal plasticity and functional connectivity in several parts of the brain reward center, including nucleus accumbens, the ventral tegmental area and the prefrontal cortex. Here, we discuss recent findings on the mechanisms involved in the perception of chronic pain, in pain-induced anxiety and depression, as well as in pain-killer addiction vulnerability. Several new studies also show that the mesolimbic dopamine circuit potently modulates responsiveness to opioids and antidepressants used for the treatment of chronic pain. We discuss recent data supporting a role of the brain reward pathway in treatment efficacy and we summarize novel findings on intracellular adaptations in the brain reward circuit under chronic pain states.

Transient receptor potential vanilloid 3 (TRPV3) in the ventral tegmental area of rat: Role in modulation of the mesolimbic-dopamine reward pathway

While dopamine (DA) neurons in the ventral tegmental area (VTA) drive the mesolimbic-reward pathway, confluent lines of evidence underscore the importance of transient receptor potential vanilloid (TRPV) channels as novel regulators of these neurons. Among the TRPV-subfamily, TRPV3 is of particular interest in reward, since active ingredients of flavour-enhancing spices in food serve as TRPV3 agonists and modulate DAergic neurotransmission. The nature of TRPV3 elements in the VTA and their role in driving the mesolimbic-DA-reward pathway has however, remained unexplored. We observed TRPV3 mRNA as well as TRPV3-immunoreactive neurons in the VTA of Wistar rats. We therefore explored whether these ion channels participate in modulating mesolimbic-DA reward pathway. In the posterior VTA (pVTA), 82 ± 2.6% of the TRPV3 neurons co-express tyrosine hydroxylase and 68 ± 5.5% of these neurons project to the nucleus accumbens shell (Acb shell). While ex vivo treatment of midbrain slices with TRPV3-agonist, thymol increased [Ca(2+)]i-activity in pVTA neurons, intra-pVTA injections of thymol in freely-moving, satiated rats enhanced positive reinforcement for active lever pressings in an operant chamber to self-administer sweet pellets. This behavior was attenuated by prior treatment with intra-Acb shell DA D1- and D2-like receptor antagonists. These results demonstrate a role for TRPV3 in driving mesolimbic-DA food-reward pathway, and underscores the importance of these channels in the VTA as key components processing reward.

Using In Vitro Electrophysiology to Screen Medications: Accumbal Plasticity as an Engram of Alcohol Dependence

The nucleus accumbens (NAc) is a central component of the mesocorticolimbic reward system. Increasing evidence strongly implicates long-term synaptic neuroadaptations in glutamatergic excitatory activity of the NAc shell and/or core medium spiny neurons in response to chronic drug and alcohol exposure. Such neuroadaptations likely play a critical role in the development and expression of drug-seeking behaviors. We have observed unique cell-type-specific bidirectional changes in NAc synaptic plasticity (metaplasticity) following acute and chronic intermittent ethanol exposure. Other investigators have also previously observed similar metaplasticity in the NAc following exposure to psychostimulants, opiates, and amazingly, even following an anhedonia-inducing experience. Considering that the proteome of the postsynaptic density likely contains hundreds of biochemicals, proteins and other components and regulators, we believe that there is a large number of potential molecular sites through which accumbal metaplasticity may be involved in chronic alcohol abuse. Many of our companion laboratories are now engaged in identifying and screening medications targeting candidate genes and its products previously linked to maladaptive alcohol phenotypes. We hypothesize that if manipulation of such target genes and their products change NAc plasticity, then that observation constitutes an important validation step for the development of novel therapeutics to treat alcohol dependence.

The pharmacology of effort-related choice behavior: Dopamine, depression, and individual differences

This review paper is focused upon the involvement of mesolimbic dopamine (DA) and related brain systems in effort-based processes. Interference with DA transmission affects instrumental behavior in a manner that interacts with the response requirements of the task, such that rats with impaired DA transmission show a heightened sensitivity to ratio requirements. Impaired DA transmission also affects effort-related choice behavior, which is assessed by tasks that offer a choice between a preferred reinforcer that has a high work requirement vs. less preferred reinforcer that can be obtained with minimal effort. Rats and mice with impaired DA transmission reallocate instrumental behavior away from food-reinforced tasks with high response costs, and show increased selection of low reinforcement/low cost options. Tests of effort-related choice have been developed into models of pathological symptoms of motivation that are seen in disorders such as depression and schizophrenia. These models are being employed to explore the effects of conditions associated with various psychopathologies, and to assess drugs for their potential utility as treatments for effort-related symptoms. Studies of the pharmacology of effort-based choice may contribute to the development of treatments for symptoms such as psychomotor slowing, fatigue or anergia, which are seen in depression and other disorders.

Perspectives of TRPV1 Function on the Neurogenesis and Neural Plasticity

The development of new strategies to renew and repair neuronal networks using neural plasticity induced by stem cell graft could enable new therapies to cure diseases that were considered lethal until now. In adequate microenvironment a neuronal progenitor must receive molecular signal of a specific cellular context to determine fate, differentiation, and location. TRPV1, a nonselective calcium channel, is expressed in neurogenic regions of the brain like the subgranular zone of the hippocampal dentate gyrus and the telencephalic subventricular zone, being valuable for neural differentiation and neural plasticity. Current data show that TRPV1 is involved in several neuronal functions as cytoskeleton dynamics, cell migration, survival, and regeneration of injured neurons, incorporating several stimuli in neurogenesis and network integration. The function of TRPV1 in the brain is under intensive investigation, due to multiple places where it has been detected and its sensitivity for different chemical and physical agonists, and a new role of TRPV1 in brain function is now emerging as a molecular tool for survival and control of neural stem cells.

Ventral tegmental area dopamine and GABA neurons: Physiological properties and expression of mRNA for endocannabinoid biosynthetic elements

The ventral tegmental area (VTA) is involved in adaptive reward and motivation processing and is composed of dopamine (DA) and GABA neurons. Defining the elements regulating activity and synaptic plasticity of these cells is critical to understanding mechanisms of reward and addiction. While endocannabinoids (eCBs) that potentially contribute to addiction are known to be involved in synaptic plasticity mechanisms in the VTA, where they are produced is poorly understood. In this study, DA and GABAergic cells were identified using electrophysiology, cellular markers, and a transgenic mouse model that specifically labels GABA cells. Using single-cell RT-qPCR and immunohistochemistry, we investigated mRNA and proteins involved in eCB signaling such as diacylglycerol lipase α, N-acyl-phosphatidylethanolamine-specific phospholipase D, and 12-lipoxygenase, as well as type I metabotropic glutamate receptors (mGluRs). Our results demonstrate the first molecular evidence of colocalization of eCB biosynthetic enzyme and type I mGluR mRNA in VTA neurons. Further, these data reveal higher expression of mGluR1 in DA neurons, suggesting potential differences in eCB synthesis between DA and GABA neurons. These data collectively suggest that VTA GABAergic and DAergic cells have the potential to produce various eCBs implicated in altering neuronal activity or plasticity in adaptive motivational reward or addiction.

Drug design and development through the vanilloid receptor

The vanilloid receptor (TRPV1) has attracted a great expectation in pain therapeutics for the treatment of chronic inflammatory conditions. As a result, several drug discovery programmes were launched in the past years that yielded a large number of receptor agonists and antagonists. However, despite the claimed therapeutic potential of TRPV1 modulators, a disappointing number of candidates have progressed into clinical trials and those were only for dental pain and migraine, indicating that our understanding of the role of TRPV1 in pain is still very limited. The widespread distribution of TRPV1 in different tissues suggests an involvement in body functions other than pain. Indeed, new findings indicate that TRPV1 is tonically active in physiological conditions and its pharmacological blockade leads to hyperthermia. Furthermore, the full abrogation of TRPV1 in some models of chronic pain results in enhanced pain. Therefore, a remaining challenge is the development of drugs that preserve the physiological activity of TRPV1 and downregulate the function of overactive receptors.

Ethanol attenuation of long-term depression in the nucleus accumbens can be overcome by activation of TRPV1 receptors

BACKGROUND

Altered expression of synaptic plasticity within the nucleus accumbens (NAc) constitutes a critical neuroadaptive response to ethanol (EtOH) and other drugs of abuse. We have previously reported that N-methyl-D-aspartate receptor (NMDAR)-dependent long-term depression (LTD) is markedly affected by chronic intermittent ethanol exposure in vivo; however, endocannabinoid (eCB)-dependent synaptic depression, despite being very well-documented in the dorsal striatum, is much less well understood in the NAc.

METHODS

Whole cell patch clamp electrophysiology was used to investigate interactions between these different plasticity-induction systems. Excitatory postsynaptic currents (EPSCs) were measured in the NAc shell and NMDAR-LTD was induced by a pairing protocol (500 stimuli at 1 Hz stimulation [low-frequency stimulation (LFS)] paired with postsynaptic depolarization to -50 mV). AM251, a CB1 receptor antagonist, was used to determine whether this form of LTD is modulated by eCBs. To determine the effect of EtOH on a purely eCB-dependent response in the NAc, depolarization-induced suppression of excitation (DSE) was used in the presence of 40 mM EtOH. Finally, we determined whether the enhancement of eCB signaling with URB597, a fatty acid amide hydrolase inhibitor, and AM404, an anandamide re-uptake inhibitor would also modulate LFS LTD in the presence of NMDAR blockade or EtOH.

RESULTS

In the presence of AM251, the LFS pairing protocol resulted in NMDAR-dependent long-term potentiation that was blocked with either EtOH or DL-APV. We also found that DSE in the NAc shell was blocked by AM251 and suppressed by EtOH. Enhanced eCB signaling rescued NAc-LTD expression in the presence of EtOH through a distinct mechanism requiring activation of TRPV1 receptors.

CONCLUSIONS

EtOH modulation of synaptic plasticity in the NAc is dependent upon a complex interplay between NMDARs, eCBs, and TRPV1 receptors. These findings demonstrate a novel form of TRPV1-dependent LTD in the NAc shell that may be critical for EtOH dependence.

Short-term increases in transient receptor potential vanilloid-1 mediate stress-induced enhancement of neuronal excitation

Progression of neurodegeneration in disease and injury is influenced by the response of individual neurons to stressful stimuli and whether this response includes mechanisms to counter declining function. Transient receptor potential (TRP) cation channels transduce a variety of disease-relevant stimuli and can mediate diverse stress-dependent changes in physiology, both presynaptic and postsynaptic. Recently, we demonstrated that knock-out or pharmacological inhibition of the TRP vanilloid-1 (TRPV1) capsaicin-sensitive subunit accelerates degeneration of retinal ganglion cell neurons and their axons with elevated ocular pressure, the critical stressor in the most common optic neuropathy, glaucoma. Here we probed the mechanism of the influence of TRPV1 on ganglion cell survival in mouse models of glaucoma. We found that induced elevations of ocular pressure increased TRPV1 in ganglion cells and its colocalization at excitatory synapses to their dendrites, whereas chronic elevation progressively increased ganglion cell Trpv1 mRNA. Enhanced TRPV1 expression in ganglion cells was transient and supported a reversal of the effect of TRPV1 on ganglion cells from hyperpolarizing to depolarizing, which was also transient. Short-term enhancement of TRPV1-mediated activity led to a delayed increase in axonal spontaneous excitation that was absent in ganglion cells from Trpv1(-/-) retina. In isolated ganglion cells, pharmacologically activated TRPV1 mobilized to discrete nodes along ganglion cell dendrites that corresponded to sites of elevated Ca(2+). These results suggest that TRPV1 may promote retinal ganglion cell survival through transient enhancement of local excitation and axonal activity in response to ocular stress.

Dopamine D2-like receptors modulate unconditioned fear: role of the inferior colliculus

Background

A reduction of dopamine release or D₂ receptor blockade in the terminal fields of the mesolimbic system clearly reduces conditioned fear. Injections of haloperidol, a preferential D₂ receptor antagonist, into the inferior colliculus (IC) enhance the processing of unconditioned aversive information. However, a clear characterization of the interplay of D₂ receptors in the mediation of unconditioned and conditioned fear is still lacking.

Methods

The present study investigated the effects of intra-IC injections of the D₂ receptor-selective antagonist sulpiride on behavior in the elevated plus maze (EPM), auditory-evoked potentials (AEPs) to loud sounds recorded from the IC, fear-potentiated startle (FPS), and conditioned freezing.

Results

Intra-IC injections of sulpiride caused clear proaversive effects in the EPM and enhanced AEPs induced by loud auditory stimuli. Intra-IC sulpiride administration did not affect FPS or conditioned freezing.

Conclusions

Dopamine D₂-like receptors of the inferior colliculus play a role in the modulation of unconditioned aversive information but not in the fear-potentiated startle response.

Blocking TRPV1 in nucleus accumbens inhibits persistent morphine conditioned place preference expression in rats

The function of TRPV1 (transient receptor potential vanilloid subfamily, member 1) in the central nervous system is gradually elucidated. It has been recently proved to be expressed in nucleus accumbens (NAc), a region playing an essential role in mediating opioid craving and taking behaviors. Based on the general role of TRPV1 antagonist in blocking neural over-excitability by both pre- and post-synaptic mechanisms, TRPV1 antagonist capsazepine (CPZ) was tested for its ability to prohibit persistent opioid craving in rats. In the present study, we assessed the expression of TRPV1 in nucleus accumbens and investigated the effect of CPZ in bilateral nucleus accumbens on persistent morphine conditioned place preference (mCPP) in rats. We also evaluated the side-effect of CPZ on activity by comparing cross-beam times between groups. We found that morphine conditioned place preference increased the TRPV1 expression and CPZ attenuated morphine conditioned place preference in a dose-dependent and target-specific manner after both short- and long-term spontaneous withdrawal, reflected by the reduction of the increased time in morphine-paired side. CPZ (10 nM) could induce prolonged and stable inhibition of morphine conditioned place preference expression. More importantly, CPZ did not cause dysfunction of activity in the subjects tested, which indicates the inhibitory effect was not obtained at the sacrifice of regular movement. Collectively, these results indicated that injection of TRPV1 antagonist in nucleus accumbens is capable of attenuating persistent morphine conditioned place preference without affecting normal activity. Thus, TRPV1 antagonist is one of the promising therapeutic drugs for the treatment of opioid addiction.