Effects of intra-basolateral amygdala administration of rimonabant on nociceptive behaviour and neuronal activity in the presence or absence of contextual fear

Basolateral amygdala cannabinoid CB1 receptors mediate the antinociceptive activity of harmaline in adolescent male mice

Evidence suggests a clear role for the amygdala endocannabinoid system in pain processing. Harmaline has been also known as the main nociceptive agent extracted from the Peganum harmala plant. In this study, the role of basolateral amygdala (BLA) cannabinoid CB1 receptors in pain sensitivity of harmaline-treated mice were assessed using tail-flick and hot plate methods in adolescent male NMRI mice. Intraperitoneal administration of two higher doses of harmaline (6 and 8 mg/kg) increased tail-flick latency, suggesting an antinociceptive activity. The same result was observed for the higher dose of harmaline in the hot plate test. Intra-BLA microinjection of CB1 receptor agonist ACPA (1 and 1.5 ng/mouse) or (1.5 ng/mouse) enhanced the ineffective dose-response of harmaline on pain threshold in the tail-flick or hot plate tests, respectively. Microinjection of two higher doses of CB1 receptor antagonist AM251 (0.5 and 1 ng/mouse) attenuated the antinociceptive activity of harmaline (8 ng/mouse) in both tail-flick and hot plate tests. Meanwhile, ACPA and AM251 did not alter latency to withdraw from the noxious stimulus in both tests, by themselves. It should be noted that the analgesic dose of the drugs alone or in combination did not affect locomotor activity. The obtained results highlight that BLA CB1 receptors mediate the antinociceptive activity of harmaline.

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

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

Antinociceptive Effects of the GPR55 Antagonist CID16020046 Injected into the Rat Anterior Cingulate Cortex

The G-protein coupled receptor, GPR55, modulates nociceptive processing. Given the expression of GPR55 in the anterior cingulate cortex (ACC), a key brain region involved in the cognitive and affective dimensions of pain, the present study tested the hypothesis that GPR55 signalling in the ACC facilitates inflammatory pain behaviour in rats. The expression of GPR55 in the ACC was confirmed by both western blotting and immunostaining, with evidence for neuronal localisation. Microinjection of the selective GPR55 antagonist CID16020046 into the ACC of adult male Sprague-Dawley rats significantly reduced second phase formalin-evoked nociceptive behaviour compared with vehicle-treated controls. CID16020046 administration was associated with a reduction in phosphorylation of extracellular signal-regulated kinase (ERK), a downstream target of GPR55 activation, in the ACC. Intra-ACC administration of CID16020046 prevented the formalin-induced increases in expression of mRNA coding for the immediate early gene and marker of neuronal activity, c-Fos, in the ipsilateral dorsal horn of the spinal cord. Intra-plantar injection of formalin reduced tissue levels of the endogenous GPR55 ligand 2-arachidonoyl-sn-glycero-3-phosphoinositol (2-AGPI) in the ACC, likely reflecting its increased release/utilisation. These data suggest that endogenous activation of GPR55 signalling and increased ERK phosphorylation in the ACC facilitates inflammatory pain via top-down modulation of descending pain control.

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

BACKGROUND AND PURPOSE

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

EXPERIMENTAL APPROACH

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

KEY RESULTS

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

CONCLUSION AND IMPLICATIONS

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

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

BACKGROUND AND PURPOSE

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

EXPERIMENTAL APPROACH

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

KEY RESULTS

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

CONCLUSIONS AND IMPLICATIONS

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

LINKED ARTICLES

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

Cannabinoid Receptor Type 1 in the Brain Regulates the Affective Component of Visceral Pain in Mice

Endocannabinoids acting through cannabinoid receptor type 1 (CB1) are major modulators of peripheral somatic and visceral nociception. Although only partially studied, some evidence suggests a particular role of CB1 within the brain in nociceptive processes. As the endocannabinoid system regulates affect and emotional behaviors, we hypothesized that cerebral CB1 influences affective processing of visceral pain-related behaviors in laboratory animals. To study nocifensive responses modulated by supraspinal CB1, we used conditional knock-out mice lacking CB1 either in cortical glutamatergic neurons (Glu-CB1-KO), or in forebrain GABAergic neurons (GABA-CB1-KO), or in principal neurons of the forebrain (CaMK-CB1-KO). These mutant mice and mice treated with the CB1 antagonist SR141716 were tested for different pain-related behaviors. In an acetic acid-induced abdominal constriction test, supraspinal CB1 deletions did not affect nocifensive responses. In the cerulein-model of acute pancreatitis, mechanical allodynia or hyperalgesia were not changed, but Glu-CB1- and CaMK-CB1-KO mice showed significantly increased facial grimacing scores indicating increased affective responses to this noxious visceral stimulus. Similarly, these brain-specific CB1 KO mice also showed significantly changed thermal nociception in a hot-plate test. These results reveal a novel, and important role of CB1 expressed by cortical glutamatergic neurons in the affective component of visceral nociception.

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

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

The cannabinoid system and pain

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

Cannabinoids and Pain: Sites and Mechanisms of Action

The endocannabinoid system, consisting of the cannabinoid₁ receptor (CB₁R) and cannabinoid₂ receptor (CB₂R), endogenous cannabinoid ligands (endocannabinoids), and metabolizing enzymes, is present throughout the pain pathways. Endocannabinoids, phytocannabinoids, and synthetic cannabinoid receptor agonists have antinociceptive effects in animal models of acute, inflammatory, and neuropathic pain. CB₁R and CB₂R located at peripheral, spinal, or supraspinal sites are important targets mediating these antinociceptive effects. The mechanisms underlying the analgesic effects of cannabinoids likely include inhibition of presynaptic neurotransmitter and neuropeptide release, modulation of postsynaptic neuronal excitability, activation of the descending inhibitory pain pathway, and reductions in neuroinflammatory signaling. Strategies to dissociate the psychoactive effects of cannabinoids from their analgesic effects have focused on peripherally restricted CB₁R agonists, CB₂R agonists, inhibitors of endocannabinoid catabolism or uptake, and modulation of other non-CB₁R/non-CB₂R targets of cannabinoids including TRPV1, GPR55, and PPARs. The large body of preclinical evidence in support of cannabinoids as potential analgesic agents is supported by clinical studies demonstrating their efficacy across a variety of pain disorders.

Characterisation of peroxisome proliferator-activated receptor signalling in the midbrain periaqueductal grey of rats genetically prone to heightened stress, negative affect and hyperalgesia

The stress-hyperresponsive Wistar-Kyoto (WKY) rat strain exhibits a hyperalgesic phenotype and is a useful genetic model for studying stress-pain interactions. Peroxisome proliferator-activated receptor (PPAR) signalling in the midbrain periaqueductal grey (PAG) modulates pain. This study characterised PPAR signalling in the PAG of WKY rats exposed to the formalin test of inflammatory pain, versus Sprague-Dawley (SD) controls. Formalin injection reduced levels of the endogenous PPAR ligands N-palmitoylethanolamide (PEA) and N-oleoylethanolamide (OEA) in the lateral(l) PAG of SD rats, but not WKY rats which exhibited higher levels of these analytes compared with formalin-injected SD counterparts. Levels of mRNA coding for fatty acid amide hydrolase (FAAH; catabolises PEA and OEA) were lower in the lPAG of WKY versus SD rats. PPARγ mRNA and protein levels in the lPAG were higher in saline-treated WKY rats, with PPARγ protein levels reduced by formalin treatment in WKY rats only. In the dorsolateral(dl) or ventrolateral(vl) PAG, there were no effects of formalin injection on PEA or OEA levels but there were some differences in levels of these analytes between saline-treated WKY and SD rats and some formalin-evoked alterations in levels of PPARα, PPARγ or FAAH mRNA in WKY and/or SD rats. Pharmacological blockade of PPARγ in the lPAG enhanced formalin-evoked nociceptive behaviour in WKY, but not SD, rats. These data indicate differences in the PPAR signalling system in the PAG of WKY versus SD rats and suggest that enhanced PEA/OEA-mediated tone at PPARγ in the lPAG may represent an adaptive mechanism to lower hyperalgesia in WKY rats.

Development and characterization of a novel, anatomically relevant rat model of acute postoperative pain

Acute postoperative pain remains a significant health care issue. Development of anatomically relevant animal models of postoperative pain, with improved predictive validity, would advance understanding of postoperative pain mechanisms and improve treatment outcomes. This study aimed to develop, characterize, and validate a rat model of acute postoperative pain associated with inguinal hernia repair based on the Lichtenstein inguinal hernia repair procedure (without hernia induction). We hypothesized that the surgery would result in reduced spontaneous locomotor activity, which would represent a pain-related phenotype. Postsurgical characterization involved extensive monitoring of home cage and open field locomotor activity, as well as mechanical hypersensitivity and assessment of c-Fos expression in the dorsal horn of the spinal cord. In pharmacologic validation studies, rats received morphine, carprofen, or paracetamol 1 hour before, and/or immediately after, surgery. Rats that underwent hernia repair surgery exhibited significantly lower horizontal and vertical activities in the home cage and open field in the early postsurgical period, compared with sham rats or rats that underwent skin incision only. Morphine, carprofen, and paracetamol attenuated the surgery-induced reductions in locomotor activity, to varying degrees. Surgery was associated with significantly increased c-Fos expression in the ipsilateral dorsal horn of the spinal cord, an effect attenuated by carprofen treatment. These results support the development and characterization of a novel, anatomically relevant animal model of acute postoperative pain that may facilitate development of improved treatment regimens.

PERSPECTIVE

Acute pain following inguinal hernia repair can be difficult to treat. Here we report, for the first time, the development of a novel, anatomically relevant rat model to facilitate improved understanding and treatment of acute postoperative pain following inguinal hernia repair.

A role for PPARα in the medial prefrontal cortex in formalin-evoked nociceptive responding in rats

BACKGROUND AND PURPOSE

The nuclear hormone receptor, PPARα, and its endogenous ligands, are involved in pain modulation. PPARα is expressed in the medial prefrontal cortex (mPFC), a key brain region involved in both the cognitive-affective component of pain and in descending modulation of pain. However, the role of PPARα in the mPFC in pain responding has not been investigated. Here, we investigated the effects of pharmacological modulation of PPARα in the rat mPFC on formalin-evoked nociceptive behaviour and the impact of formalin-induced nociception on components of PPARα signalling in the mPFC.

EXPERIMENTAL APPROACH

The effects of intra-mPFC microinjection of a PPARα agonist (GW7647) or a PPARα antagonist (GW6471) on formalin-evoked nociceptive behaviour in rats were studied. Quantitative real-time PCR and LC-MS/MS were used to study the effects of intraplantar injection of formalin on PPARα mRNA expression and levels of endogenous ligands, respectively, in the mPFC.

KEY RESULTS

Intra-mPFC administration of GW6471, but not GW7647, resulted in delayed onset of the early second phase of formalin-evoked nociceptive behaviour. Furthermore, formalin-evoked nociceptive behaviour was associated with significant reductions in mPFC levels of endogenous PPARα ligands (N-palmitoylethanolamide and N-oleoylethanolamide) and a 70% reduction in PPARα mRNA but not protein expression.

CONCLUSIONS AND IMPLICATIONS

These data suggest that endogenous ligands may act at PPARα in the mPFC to play a facilitatory/permissive role in second phase formalin-evoked nociceptive behaviour in rats.

LINKED ARTICLES

This article is part of a themed section on Cannabinoids 2013. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2014.171.issue-6.

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

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

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

BACKGROUND AND PURPOSE

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

EXPERIMENTAL APPROACH

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

KEY RESULTS

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

CONCLUSIONS AND IMPLICATIONS

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

LINKED ARTICLES

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

N-arachidonoyl-serotonin in the basolateral amygdala increases anxiolytic behavior in the elevated plus maze

CB(1) receptors in the amygdala have been shown to mediate learned and unlearned anxiety states, however, the role of amygdalar TRPV1 receptors remains unclear. In the present study we investigated the potential anxiolytic action of intra-basolateral amygdala (BLA) infusion of N-arachidonoyl-serotonin (AA-5-HT), a dual blocker of the endocannabinoid-inactivating enzyme, fatty acid amide hydrolase (FAAH), and a TRPV1 antagonist. Varying doses of AA-5-HT (0-0.5 nmol) were administered into the BLA prior to elevated plus maze testing. AA-5-HT significantly increased both time spent and number of entries into the open arms. Next, to determine whether the anxiolytic effects were the result of blocking FAAH, TRPV1, or whether a combined action was required, rats were given intra-BLA infusions of either 0.25 nmol AA-5-HT, 1.0 nmol capsazepine (CZP, a TRPV1 antagonist), 0.01 μg URB597 (a selective FAAH inhibitor), or vehicle. Again, AA-5-HT increased the time spent in the open arms as well as the number of open arm entries. In contrast, CZP and URB597 did not reliably alter plus maze performance. We then investigated the effects of co-administration of CZP (1.0 or 10.0 nmol) and URB597 (0.01 or 0.1 μg). At lower doses, co-injections significantly increased both open arm entries as well as the time spent in the open arms, compared to vehicle or either compound alone. While co-administration of the higher doses had no significant effect when compared to either vehicle or CZP treatment, we did observe that open arm activity was elevated in rats receiving combined CZP-URB597 treatment compared to URB597 alone. Overall, our findings indicate that simultaneous FAAH activity and TRPV1 activation are important with respect to the expression of unconditioned fear as mediated within the BLA.

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

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

Neurokinin-1 receptor deletion modulates behavioural and neurochemical alterations in an animal model of depression

The substance P/NK1 receptor system plays an important role in the regulation of stress and emotional responding and as such had been implicated in the pathophysiology of anxiety and depression. The present study investigated whether alterations in the substance P/NK1 receptor system in brain areas which regulate emotional responding accompany the depressive behavioural phenotype observed in the olfactory bulbectomised (OB) mouse. The effect of NK1 receptor deletion on behavioural responding and monoamine levels in discrete brain regions of the OB model, were also examined. Substance P levels in the frontal cortex and NK1 receptor expression in the amygdala and hippocampus were enhanced following olfactory bulbectomy. Although NK1 receptor knockout (NK1-/-) mice did not exhibit altered behavioural responding in the open field test, noradrenaline levels were enhanced in the frontal cortex, amygdala and hippocampus, as were serotonin levels in the frontal cortex. Locomotor activity and exploratory behaviour were enhanced in wild type OB mice, indicative of a depressive-like phenotype, an effect attenuated in NK1-/- mice. Bulbectomy induced a decrease in noradrenaline and 5-HIAA in the frontal cortex and an increase in serotonin in the amygdala, effects attenuated in OB NK1-/- mice. The present studies indicate that alterations in substance P/NK1 receptor system underlie, at least in part, the behavioural and monoaminergic changes in this animal model of depression.

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

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

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

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

PERSPECTIVE

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