Role of TRPV1 receptors on panic-like behaviors mediated by the dorsolateral periaqueductal gray in rats

Biochemical challenges for testing novel anti-panic drugs in humans

Current medications for panic disorder each carry significant limitations that indicate the need for novel anxiolytics. The high costs and low success rates of drug development demand that testing trials be efficient. Lab panicogenic challenges in humans allow for the rapid biochemical induction of panic symptoms and hence an efficient means of testing potential anti-panic drugs. This paper describes ideal characteristics of lab panicogens, reviews the validity and utility of various biochemical panicogenic agents, identifies key outcome measures for studies of novel anti-panic drugs, and makes broad recommendations for labs wishing to perform such studies. We conclude by presenting a four-tiered hierarchy of panicogens that matches each against ideal characteristics and reflects our recommendations for their laboratory use.

The subfornical organ regulates acidosis-evoked fear by engaging microglial acid-sensor TDAG8 and forebrain neurocircuits in male mice

An important role of pH homeostasis has been suggested in the physiology of panic disorder, with acidosis as an interoceptive trigger leading to fear and panic. Identification of novel mechanisms that can translate acidosis into fear will promote a better understanding of panic physiology. The current study explores a role of the subfornical organ (SFO), a blood-brain barrier compromised brain area, in translating acidosis to fear-relevant behaviors. We performed SFO-targeted acidification in male, wild-type mice and mice lacking microglial acid-sensing G protein-coupled receptor-T-cell death-associated gene 8 (TDAG8). Localized SFO acidification evoked significant freezing and reduced exploration that was dependent on the presence of acid-sensor TDAG8. Acidosis promoted the activation of SFO microglia and neurons that were absent in TDAG8-deficient mice. The assessment of regional neuronal activation in wild-type and TDAG8-deficient mice following SFO acidification revealed significant acidosis and genotype-dependent alterations in the hypothalamus, amygdala, prefrontal cortex, and periaqueductal gray nuclei. Furthermore, mapping of interregional co-activation patterns revealed that SFO acidosis promoted positive hypothalamic-cortex associations and desynchronized SFO-cortex and amygdala-cortex associations, suggesting an interplay of homeostatic and fear regulatory areas. Importantly, these alterations were not evident in TDAG8-deficient mice. Overall, our data support a regulatory role of subfornical organ microglial acid sensing in acidosis-evoked fear, highlighting a centralized role of blood-brain barrier compromised nodes in interoceptive sensing and behavioral regulation. Identification of pathways by which humoral information can modulate fear behavior is relevant to panic disorder, where aberrant interoceptive signaling has been reported.

Transient Receptor Potential Vanilloid 1 Function at Central Synapses in Health and Disease

The transient receptor potential vanilloid 1 (TRPV1), a ligand-gated nonselective cation channel, is well known for mediating heat and pain sensation in the periphery. Increasing evidence suggests that TRPV1 is also expressed at various central synapses, where it plays a role in different types of activity-dependent synaptic changes. Although its precise localizations remain a matter of debate, TRPV1 has been shown to modulate both neurotransmitter release at presynaptic terminals and synaptic efficacy in postsynaptic compartments. In addition to being required in these forms of synaptic plasticity, TRPV1 can also modify the inducibility of other types of plasticity. Here, we highlight current evidence of the potential roles for TRPV1 in regulating synaptic function in various brain regions, with an emphasis on principal mechanisms underlying TRPV1-mediated synaptic plasticity and metaplasticity. Finally, we discuss the putative contributions of TRPV1 in diverse brain disorders in order to expedite the development of next-generation therapeutic treatments.

Cannabidiol in the prelimbic cortex modulates the comorbid condition between the chronic neuropathic pain and depression-like behaviour in rats: The role of medial prefrontal cortex 5-HT1A and CB1 receptors

The prelimbic division (PrL) of the medial prefrontal cortex (mPFC) is a cerebral division that is putatively implicated in the chronic pain and depression. We investigated the activity of PrL cortex neurons in Wistar rats that underwent chronic constriction injury (CCI) of sciatic nerve and were further subjected to the forced swimming (FS) test and mechanical allodynia (by von Frey test). The effect of blockade of synapses with cobalt chloride (CoCl₂), and the treatment of the PrL cortex with cannabidiol (CBD), the CB₁ receptor antagonist AM251 and the 5-HT_1A receptor antagonist WAY-100635 were also investigated. Our results showed that CoCl₂ decreased the time spent immobile during the FS test but did not alter mechanical allodynia. CBD (at 15, 30 and 60 nmol) in the PrL cortex also decreased the frequency and duration of immobility; however, only the dose of 30 nmol of CBD attenuated mechanical allodynia in rats with chronic NP. AM251 and WAY-100635 in the PrL cortex attenuated the antidepressive and analgesic effect caused by CBD but did not alter the immobility and the mechanical allodynia when administered alone. These data show that the PrL cortex is part of the neural substrate underlying the comorbidity between NP and depression. Also, the previous blockade of CB₁ cannabinoid receptors and 5-HT_1A serotonergic receptors in the PrL cortex attenuated the antidepressive and analgesics effect of the CBD. They also suggest that CBD could be a potential medicine for the treatment of depressive and pain symptoms in patients with chronic NP/depression comorbidity.

Cannabinoid CB1 receptors mediate the anxiolytic effects induced by systemic alprazolam and intra-periaqueductal gray 5-HT1A receptor activation

The endocannabinoid system has been implicated in the modulation of behaviors related to anxiety and panic disorders. Accordingly, facilitation of CB₁ receptor signaling reduces the consequences of aversive stimuli in animal models. However, the role of the CB₁ receptor in the effects of anxiolytic drugs has remained unclear. Here, we tested the hypothesis that the anxiolytic and panicolytic responses to systemic alprazolam injection and local 5-HT_1A receptor activation in the dorsolateral periaqueductal gray (dlPAG) depend on CB₁ receptor activation. Systemic injection of alprazolam (4 mg/kg) induced an anxiolytic-like effect in the elevated T maze (ETM) model of panic and anxiety, which was prevented by the CB₁ antagonist AM251 (0.3 mg/kg). Likewise, intra-dlPAG injection of the 5-HT_1A receptor agonist 8-OH-DPAT (3.2 nmol/0.2 u L) also reduced anxiety-like behavior, a response prevented by intra-dlPAG injection of AM251 (100 pmol/0.2 µL). 8-OH-DPAT (8 nmol/0.2 µL) also presented a panicolytic-like activity in the escape reaction induced by chemical stimulation of the dlPAG, which was not prevented by AM251 (100 pmol/0.2 µL). These results suggest that CB₁ receptor signaling is involved in the effects of anxiolytic drugs, with potential implications for developing new treatments for anxiety disorders.

The role of acid-sensitive ion channels in panic disorder: a systematic review of animal studies and meta-analysis of human studies

Acid-sensitive ion channels, such as amiloride-sensitive cation channel (ACCN), transient receptor potential vanilloid-1 (TRPV1), and T-cell death-associated gene 8 (TDAG8) are highly related to the expression of fear and are expressed in several regions of the brain. These molecules can detect acidosis and maintain brain homeostasis. An important role of pH homeostasis has been suggested in the physiology of panic disorder (PD), with acidosis as an interoceptive trigger for panic attacks. To examine the effect of acid-sensitive channels on PD symptoms, we conducted a systematic review and meta-analysis of these chemosensors in rodents and humans. Following PRISMA guidelines, we systematically searched the Web of Science, Medline/Pubmed, Scopus, Science Direct, and SciELO databases. The review included original research in PD patients and animal models of PD that investigated acid-sensitive channels and PD symptoms. Studies without a control group, studies involving patients with a comorbid psychiatric diagnosis, and in vitro studies were excluded. Eleven articles met the inclusion criteria for the systematic review. The majority of the studies showed an association between panic symptoms and acid-sensitive channels. PD patients appear to display polymorphisms in the ACCN gene and elevated levels of TDAG8 mRNA. The results showed a decrease in panic-like symptoms after acid channel blockade in animal models. Despite the relatively limited data on this topic in the literature, our review identified evidence linking acid-sensitive channels to PD in humans and preclinical models. Future research should explore possible underlying mechanisms of this association, attempt to replicate the existing findings in larger populations, and develop new therapeutic strategies based on these biological features.

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.

The endocannabinoid system as a target for novel anxiolytic drugs

The endocannabinoid (eCB) system has attracted attention for its role in various behavioral and brain functions, and as a therapeutic target in neuropsychiatric disease states, including anxiety disorders and other conditions resulting from dysfunctional responses to stress. In this mini-review, we highlight components of the eCB system that offer potential 'druggable' targets for new anxiolytic medications, emphasizing some of the less well-discussed options. We discuss how selectively amplifying eCBs recruitment by interfering with eCB-degradation, via fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL), has been linked to reductions in anxiety-like behaviors in rodents and variation in human anxiety symptoms. We also discuss a non-canonical route to regulate eCB degradation that involves interfering with cyclooxygenase-2 (COX-2). Next, we discuss approaches to targeting eCB receptor-signaling in ways that do not involve the cannabinoid receptor subtype 1 (CB1R); by targeting the CB2R subtype and the transient receptor potential vanilloid type 1 (TRPV1). Finally, we review evidence that cannabidiol (CBD), while representing a less specific pharmacological approach, may be another way to modulate eCBs and interacting neurotransmitter systems to alleviate anxiety. Taken together, these various approaches provide a range of plausible paths to developing novel compounds that could prove useful for treating trauma-related and anxiety disorders.

Genotype-dependent responsivity to inflammatory pain: A role for TRPV1 in the periaqueductal grey

Negative affective state has a significant impact on pain, and genetic background is an important moderating influence on this interaction. The Wistar-Kyoto (WKY) inbred rat strain exhibits a stress-hyperresponsive, anxiety/depressive-like phenotype and also displays a hyperalgesic response to noxious stimuli. Transient receptor potential subfamily V member 1 (TRPV1) within the midbrain periaqueductal grey (PAG) plays a key role in regulating both aversive and nociceptive behaviour. In the present study, we investigated the role of TRPV1 in the sub-columns of the PAG in formalin-evoked nociceptive behaviour in WKY versus Sprague-Dawley (SD) rats. TRPV1 mRNA expression was significantly lower in the dorsolateral (DL) PAG and higher in the lateral (L) PAG of WKY rats, compared with SD counterparts. There were no significant differences in TRPV1 mRNA expression in the ventrolateral (VL) PAG between the two strains. TRPV1 mRNA expression significantly decreased in the DLPAG and increased in the VLPAG of SD, but not WKY rats upon intra-plantar formalin administration. Intra-DLPAG administration of either the TRPV1 agonist capsaicin, or the TRPV1 antagonist 5'-Iodoresiniferatoxin (5'-IRTX), significantly increased formalin-evoked nociceptive behaviour in SD rats, but not in WKY rats. The effects of capsaicin were likely due to TRPV1 desensitisation, given their similarity to the effects of 5'-IRTX. Intra-VLPAG administration of capsaicin or 5'-IRTX reduced nociceptive behaviour in a moderate and transient manner in SD rats, and similar effects were seen with 5'-IRTX in WKY rats. Intra-LPAG administration of 5'-IRTX reduced nociceptive behaviour in a moderate and transient manner in SD rats, but not in WKY rats. These results indicate that modulation of inflammatory pain by TRPV1 in the PAG occurs in a sub-column-specific manner. The data also provide evidence for differences in the expression of TRPV1, and differences in the effects of pharmacological modulation of TRPV1 in specific PAG sub-columns, between WKY and SD rats, suggesting that TRPV1 expression and/or functionality in the PAG plays a role in hyper-responsivity to noxious stimuli in a genetic background prone to negative affect.

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.

Role of the endocannabinoid 2-arachidonoylglycerol in aversive responses mediated by the dorsolateral periaqueductal grey

2-arachidonoylglycerol (2-AG) is an endogenous ligand of the cannabinoid CB1 receptor. This endocannabinoid and its hydrolyzing enzyme, monoacylglycerol lipase (MAGL), are present in encephalic regions related to psychiatric disorders, including the midbrain dorsolateral periaqueductal grey (dlPAG). The dlPAG is implicated in panic disorder and its stimulation results in defensive responses proposed as a model of panic attacks. The present work verified if facilitation of 2-AG signalling in the dlPAG counteracts panic-like responses induced by local chemical stimulation. Intra-dlPAG injection of 2-AG prevented panic-like response induced by the excitatory amino acid N-methyl-d-aspartate (NMDA). This effect was mimicked by the 2-AG hydrolysis inhibitor (MAGL preferring inhibitor) URB602. The anti-aversive effect of URB602 was reversed by the CB1 receptor antagonist, AM251. Additionally, a combination of sub-effective doses of 2-AG and URB602 also prevented NMDA-induced panic-like response. Finally, immunofluorescence assay showed a significant increase in c-Fos positive cells in the dlPAG after local administration of NMDA. This response was also prevented by URB602. These data support the hypothesis that 2-AG participates in anti-aversive mechanisms in the dlPAG and reinforce the proposal that facilitation of endocannabinoid signalling could be a putative target for developing additional treatments against panic and other anxiety-related disorders.

Anti-aversive role of the endocannabinoid system in the periaqueductal gray stimulation model of panic attacks in rats

RATIONALE

Direct activation of the cannabinoid CB1 receptor in the dorsolateral periaqueductal gray (dlPAG) inhibits anxiety- and panic-related behaviours in experimental animals. It has remained unclear, however, whether the local endocannabinoid signalling is recruited as a protective mechanism against aversive stimuli.

OBJECTIVES

The present study tested the hypothesis that the endocannabinoid system counteracts aversive responses in the dlPAG-stimulation model of panic attacks.

METHODS

All drugs were infused into the dlPAG of rats. Local chemical stimulation with N-methyl-D-aspartate (NMDA, 1 nmol) was employed to induce panic-like behavioural and cardiovascular responses in freely moving and anaesthetized animals, respectively. The neuronal activity in the dlPAG was investigated by c-Fos immunohistochemistry.

RESULTS

The selective CB1 receptor agonist, ACEA (0.005-0.5 pmol), prevented the NMDA-induced panic-like escape responses. More interestingly, increasing the local levels of endogenous anandamide with a fatty acid amide hydrolase (FAAH) inhibitor, URB597 (0.3-3 nmol), prevented both the behavioural response and the increase in blood pressure induced by NMDA. The effect of URB597 (3 nmol) was reversed by the CB1 receptor antagonist, AM251 (0.1 nmol). Moreover, an otherwise ineffective and sub-threshold dose of NMDA (0.5 nmol) was able to induce a panic-like response if local CB1 receptors were previously blocked by AM251 (0.1 nmol). Finally, URB597 prevented the NMDA-induced neuronal activation of the dlPAG.

CONCLUSIONS

The endocannabinoid system in the dlPAG attenuates the behavioural, cellular and cardiovascular consequences of aversive stimuli. This process may be considered for the development of additional treatments against panic and other anxiety-related disorders.

Role of endocannabinoid signalling in the dorsolateral periaqueductal grey in the modulation of distinct panic-like responses

Panic attacks, a major feature of panic disorder, can be modelled in rats by exposing animals to stimuli that induce escape reactions, such as the elevated T-maze or the activation of the dorsolateral periaqueductal grey. Since the cannabinoid CB1 receptor modulates various types of aversive responses, this study tested the hypothesis that enhancement of endocannabinoid signalling in the dorsolateral periaqueductal grey inhibits panic-like reactions in rats. Local injection of the CB1 agonist, arachidonoyl 2-Chloroethylamide (0.005-0.5 pmol), attenuated the escape response from the open arm of the elevated T-maze, a panicolytic effect. The anandamide hydrolysis inhibitor, URB597 (0.3-3 nmol), did not induce consistent results. In the test of dorsolateral periaqueductal grey stimulation with d,l-homocysteic acid, arachidonoyl 2-Chloroethylamide, at the lowest dose, attenuated the escape reaction. The highest dose of URB597 also inhibited this response, contrary to the result obtained in the elevated T-maze. This effect was reversed by the CB1 antagonist, AM251 (100 pmol). The present results confirm the anti-aversive property of direct CB1 receptor activation in the dorsolateral periaqueductal grey. The effect of the anandamide hydrolysis inhibitor, however, could be detected only in a model employing direct stimulation of this structure. Altogether, these results suggest that anandamide signalling is recruited only under certain types of aversive stimuli.

Effects of the fatty acid amide hydrolase inhibitor URB597 on coping behavior under challenging conditions in mice

RATIONALE

Recent evidence suggests that in addition to controlling emotional behavior in general, endocannabinoid signaling is engaged in shaping behavioral responses to challenges. This important function of endocannabinoids is still poorly understood.

OBJECTIVES

Here we investigated the impact of blockade of fatty acid amide hydrolase (FAAH), the degrading enzyme of anandamide on behavioral responses induced by challenges of different intensity.

METHODS

Mice treated with FAAH inhibitor URB597 were either manually restrained on their backs (back test) or received foot-shocks.

RESULTS

The behavior of mice showed bimodal distribution in the back test: they either predominantly showed escape attempts or equally distributed time between passivity and escape. URB597 increased escapes in animals with low escape scores. No effects were noticed in mice showing high escape scores, which is likely due to a ceiling effect. We hypothesized that stronger stressors would wash out individual differences in coping; therefore, we exposed mice to foot-shocks that decreased locomotion and increased freezing in all mice. URB597 ameliorated both responses. The re-exposure of mice to the shock cage 14 days later without delivering shocks or treatment was followed by reduced and fragmented sleep as shown by electrophysiological recordings. Surprisingly, sleep was more disturbed after the reminder than after shocks in rats receiving vehicle before foot-shocks. These reminder-induced disturbances were abolished by URB597 administered before shocks.

CONCLUSIONS

These findings suggest that FAAH blockade has an important role in the selection of behavioral responses under challenging conditions and-judging from its long-term effects-that it influences the cognitive appraisal of the challenge.