The central nucleus of the amygdala contributes to the production of morphine antinociception in the formalin test

Pain-related cortico-limbic plasticity and opioid signaling

Neuroplasticity in cortico-limbic circuits has been implicated in pain persistence and pain modulation in clinical and preclinical studies. The amygdala has emerged as a key player in the emotional-affective dimension of pain and pain modulation. Reciprocal interactions with medial prefrontal cortical regions undergo changes in pain conditions. Other limbic and paralimbic regions have been implicated in pain modulation as well. The cortico-limbic system is rich in opioids and opioid receptors. Preclinical evidence for their pain modulatory effects in different regions of this highly interactive system, potentially opposing functions of different opioid receptors, and knowledge gaps will be described here. There is little information about cell type- and circuit-specific functions of opioid receptor subtypes related to pain processing and pain-related plasticity in the cortico-limbic system. The important role of anterior cingulate cortex (ACC) and amygdala in MOR-dependent analgesia is most well-established, and MOR actions in the mesolimbic system appear to be similar but remain to be determined in mPFC regions other than ACC. Evidence also suggests that KOR signaling generally serves opposing functions whereas DOR signaling in the ACC has similar, if not synergistic effects, to MOR. A unifying picture of pain-related neuronal mechanisms of opioid signaling in different elements of the cortico-limbic circuitry has yet to emerge. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

Endogenous µ-opioid receptor activity in the lateral and capsular subdivisions of the right central nucleus of the amygdala prevents chronic postoperative pain

Tissue injury induces a long-lasting latent sensitization (LS) of spinal nociceptive signaling that is kept in remission by an opposing µ-opioid receptor (MOR) constitutive activity. To test the hypothesis that supraspinal sites become engaged, we induced hindpaw inflammation, waited 3 weeks for mechanical hypersensitivity to resolve, and then injected the opioid receptor inhibitors naltrexone, CTOP or β-funaltrexamine subcutaneously, and/or into the cerebral ventricles. Intracerebroventricular injection of each inhibitor reinstated hypersensitivity and produced somatic signs of withdrawal, indicative of LS and endogenous opioid dependence, respectively. In naïve or sham controls, systemic naloxone (3 mg/kg) produced conditioned place aversion, and systemic naltrexone (3 mg/kg) increased Fos expression in the central nucleus of the amygdala (CeA). In LS animals tested 3 weeks after plantar incision, systemic naltrexone reinstated mechanical hypersensitivity and produced an even greater increase in Fos than in sham controls, particularly in the capsular subdivision of the right CeA. One third of Fos+ profiles co-expressed protein kinase C delta (PKCδ), and 35% of PKCδ neurons co-expressed tdTomato+ in Oprm1^Cre ::tdTomato transgenic mice. CeA microinjection of naltrexone (1 µg) reinstated mechanical hypersensitivity only in male mice and did not produce signs of somatic withdrawal. Intra-CeA injection of the MOR-selective inhibitor CTAP (300 ng) reinstated hypersensitivity in both male and female mice. We conclude that MORs in the capsular subdivision of the right CeA prevent the transition from acute to chronic postoperative pain.

Simultaneous CMOS-Based Imaging of Calcium Signaling of the Central Amygdala and the Dorsal Raphe Nucleus During Nociception in Freely Moving Mice

Fluorescence imaging devices have been indispensable in elucidating the workings of the brain in living animals, including unrestrained, active ones. Various devices are available, each with their own strengths and weaknesses in terms of many factors. We have developed CMOS-based needle-type imaging devices that are small and lightweight enough to be doubly implanted in freely moving mice. The design also allowed angled implantations to avoid critical areas. We demonstrated the utility of the devices by using them on GCaMP6 mice in a formalin test experiment. Simultaneous implantations to the capsular-lateral central amygdala (CeLC) and dorsal raphe nucleus (DRN) were proven to be safe and did not hinder the execution of the study. Analysis of the collected calcium signaling data, supported by behavior data, showed increased activity in both regions as a result of pain stimulation. Thus, we have successfully demonstrated the various advantages of the device in its application in the pain experiment.

Dual and Opposing Functions of the Central Amygdala in the Modulation of Pain

Pain perception is essential for survival and can be amplified or suppressed by expectations, experiences, and context. The neural mechanisms underlying bidirectional modulation of pain remain largely unknown. Here, we demonstrate that the central nucleus of the amygdala (CeA) functions as a pain rheostat, decreasing or increasing pain-related behaviors in mice. This dual and opposing function of the CeA is encoded by opposing changes in the excitability of two distinct subpopulations of GABAergic neurons that receive excitatory inputs from the parabrachial nucleus (PB). Thus, cells expressing protein kinase C-delta (CeA-PKCδ) are sensitized by nerve injury and increase pain-related responses. In contrast, cells expressing somatostatin (CeA-Som) are inhibited by nerve injury and their activity drives antinociception. Together, these results demonstrate that the CeA can amplify or suppress pain in a cell-type-specific manner, uncovering a previously unknown mechanism underlying bidirectional control of pain in the brain.

The brain can bidirectionally influence behavioral responses to painful stimuli. Wilson et al identify a cellular mechanism underlying a pain rheostat system within the forebrain, with activation of CeA-Som neurons attenuating pain-related responses and increases in the activity of CeA-PKCδ neurons promoting amplification of pain-related behaviors following injury.

Amygdala, neuropeptides, and chronic pain-related affective behaviors

Neuropeptides play important modulatory roles throughout the nervous system, functioning as direct effectors or as interacting partners with other neuropeptide and neurotransmitter systems. Limbic brain areas involved in learning, memory and emotions are particularly rich in neuropeptides. This review will focus on the amygdala, a limbic region that plays a key role in emotional-affective behaviors and pain modulation. The amygdala is comprised of different nuclei; the basolateral (BLA) and central (CeA) nuclei and in between, the intercalated cells (ITC), have been linked to pain-related functions. A wide range of neuropeptides are found in the amygdala, particularly in the CeA, but this review will discuss those neuropeptides that have been explored for their role in pain modulation. Calcitonin gene-related peptide (CGRP) is a key peptide in the afferent nociceptive pathway from the parabrachial area and mediates excitatory drive of CeA neurons. CeA neurons containing corticotropin releasing factor (CRF) and/or somatostatin (SOM) are a source of long-range projections and serve major output functions, but CRF also acts locally to excite neurons in the CeA and BLA. Neuropeptide S (NPS) is associated with inhibitory ITC neurons that gate amygdala output. Oxytocin and vasopressin exert opposite (inhibitory and excitatory, respectively) effects on amygdala output. The opioid system of mu, delta and kappa receptors (MOR, DOR, KOR) and their peptide ligands (β-endorphin, enkephalin, dynorphin) have complex and partially opposing effects on amygdala function. Neuropeptides therefore serve as valuable targets to regulate amygdala function in pain conditions. This article is part of the special issue on Neuropeptides.

Exposure to workplace bullying, microRNAs and pain; evidence of a moderating effect of miR-30c rs928508 and miR-223 rs3848900

Prolonged exposure to bullying behaviors may give rise to symptoms such as anxiety, depression and chronic pain. Earlier data suggest that these symptoms often are associated with stress-induced low-grade systemic inflammation. Here, using data from both animals and humans, we examined the moderating role of microRNAs (miRNAs, miRs) in this process. In the present study, a resident-intruder paradigm, blood samples, tissue harvesting and subsequent qPCR analyses were used to screen for stress-induced changes in circulating miRNAs in rats. The negative acts questionnaire (NAQ), TaqMan assays and a numeric rating scale (NRS) for pain intensity were then used to examine the associations among bullying behaviors, relevant miRNA polymorphisms and pain in a probability sample of 996 Norwegian employees. In rats, inhibited weight gain, reduced pituitary POMC expression, adrenal Nr3c1 mRNA downregulation, as well as increased miR-146a, miR-30c and miR-223 in plasma were observed following 1 week of repeated exposure to social stress. When following up the miRNA findings from the animal study in the human working population, a stronger relationship between NAQ and NRS scores was observed in subjects with the miR-30c GG genotype (rs928508) compared to other subjects. A stronger relationship between NAQ and NRS scores was also seen in men with the miR-223 G genotype (rs3848900) as compared to other men. Our findings show that social stress may induce many physiological changes including changed expression of miRNAs. We conclude that the miR-30c GG genotype in men and women, and the miR-223 G genotype in men, amplify the association between exposure to bullying behaviors and pain.Lay summaryUsing an animal model of social stress, we identified miR-146a, miR-30c and miR-223 as potentially important gene regulatory molecules that may be involved in the stress response. Interestingly, human genotypes affecting the expression of mature miR-30c and miR-223 had a moderating effect on the association between exposure to bullying and pain. Subjects with the miR-30c rs928508 GG genotype had a significantly stronger association between exposure to bullying behaviors and pain than other subjects. The same was observed in men with the miR-223 rs3848900 G genotype, as compared to other men.

GluA1 in Central Amygdala Promotes Opioid Use and Reverses Inhibitory Effect of Pain

Increasing evidence suggests that long-term opioids and pain induce similar adaptive changes in the brain's reward circuits, however, how pain alters the addictive properties of opioids remains poorly understood. In this study using a rat model of morphine self-administration (MSA), we found that short-term pain, induced by an intraplantar injection of complete Freund's adjuvant (CFA), acutely decreased voluntary morphine intake, but not food intake, only at a morphine dose that did not affect pain itself. Pre-treatment with indomethacin, a non-opioid inhibitor of pain, before the pain induction blocked the decrease in morphine intake. In rats with steady MSA, the protein level of GluA1 subunits of glutamate AMPA receptors (AMPARs) was significantly increased, but that of GluA2 was decreased, resulting in an increased GluA1/GluA2 ratio in central nucleus of the amygdala (CeA). In contrast, pain decreased the GluA1/GluA2 ratio in the CeA of rats with MSA. Microinjection of NASPM, a selective inhibitor of homomeric GluA1-AMPARs, into CeA inhibited morphine intake. Furthermore, viral overexpression of GluA1 protein in CeA maintained morphine intake at a higher level than controls and reversed the pain-induced reduction in morphine intake. These findings suggest that CeA GluA1 promotes opioid use and its upregulation is sufficient to increase opioid consumption, which counteracts the acute inhibitory effect of pain on opioid intake. These results demonstrate that the CeA GluA1 is a shared target of opioid and pain in regulation of opioid use, which may aid in future development of therapeutic applications in opioid abuse.

GABAB receptors modulate morphine antinociception: Pharmacological and genetic approaches

Previous studies in our laboratory showed an interaction between the GABAergic and opioid systems involved in the analgesic effect of baclofen (BAC). Furthermore, it is known that sex differences exist regarding various pharmacological responses of morphine (MOR) and they are related to an increased sensitivity to MOR effects in males. The aims of the present study were to evaluate the possible involvement of the GABA_B receptors in the antinociceptive responses induced by MOR (1, 3 and 9 mg/kg, s.c.) administration using both pharmacological (BAC 2 mg/kg, i.p.; and 2-OH-saclofen, SAC 0.3 mg/kg, intra cisterna magna) and genetic approaches (GABA_B1 knockout mice; GABA_B1 KO) in mice of both sexes. In addition, we explored the alterations in c-Fos expression of different brain areas involved in the antinociceptive effect of MOR using both approaches. The pharmacological approach showed a higher dose-dependent antinociceptive effect of MOR in male mice compared to female mice. BAC and SAC pretreatment potentiated and attenuated the antinociceptive effect of MOR, respectively, in both sexes. The genetic approach revealed a dose-dependent antinociceptive effect of MOR in the wild type mice, but not in the GABA_B1 KO mice and no sex differences were observed. Additionally, BAC and SAC pretreatment and the lack of GABA_B1 subunit of the GABA_B receptor prevented the changes observed in c-Fos expression in the cingulate cortex and nucleus accumbens of male mice. Our results suggest that the GABA_B receptors are involved in the MOR antinociceptive effect of both male and female mice.

Involvement of GABAA receptors in the regulation of social preference and emotional behaviors by oxytocin in the central amygdala of female mandarin voles

The central nucleus of the amygdala (CeA) is the main output of the amygdala and plays an important role in behavioral and neuroendocrine responses to stress. Receptors for the neuropeptide oxytocin (OT) and GABA_A are found in high concentration in the CeA. The mechanisms underlying regulation of CeA OT in emotional and social behavior remain unclear. In this study we evaluated the effects of intra-CeA OT administration of different doses (0.1, 1 and 10ng/side), OT receptor antagonist (OTR-A) (1, 10 and 100ng/side) and OT plus OTR-A on social and emotional behavior using a social preference paradigm, open field test and elevated plus maze test in female monogamous mandarin voles (Microtus mandarinus). We then examined whether different doses of the GABA_A receptor antagonist bicuculline (5, 10 and 100ng/side) affected the behavioral changes induced by intra-CeA microinjection of OT (1ng/side). We found that administration of OT to the CeA increased social preference, central area investigation times in the open field test, and visits, transitions and time spent in the open arms in the elevated plus maze test; all responses were dose-dependent. Administration of OT plus OTR-A to the CeA produced no effects. Administration of bicuculline in combination with OT to the CeA decreased social preference, central area investigation times in the open field test, and visits, transitions and time spent in the open arms of the elevated plus maze test. These data suggest that OT in the CeA facilitates sociality and reduces levels of anxiety by interacting with local GABA_A receptors.

Distinct neuronal populations in the basolateral and central amygdala are activated with acute pain, conditioned fear, and fear-conditioned analgesia

Fear-conditioned analgesia (FCA) is modulated by brain areas involved in the descending inhibitory pain pathway such as the basolateral (BLA) and central amygdala (CEA). The BLA contains Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) and parvalbumin (PV) neurons. CEA neurons are primarily inhibitory (GABAergic) that comprise enkephalin (ENK) interneurons and corticotropin-releasing factor (CRF) - neurons that project to the periaqueductal grey. The purpose of our experiment was to determine the pattern of activation of CaMKII/PV and ENK/CRF neurons following the expression of acute pain, conditioned fear, and FCA. A significant reduction was observed in nociceptive behaviors in mice re-exposed to a contextually-aversive environment. Using NeuN and cFos as markers for activated neurons, CaMKII, PV, ENK, or CRF were used to identify neuronal subtypes. We find that mice expressing conditioned fear displayed an increase in c-Fos/CaMKII co-localization in the lateral amygdala and BLA compared to controls. Additionally a significant increase in cFos/CRF co-localization was observed in mice expressing FCA. These results show that amygdala processing of conditioned contextual aversive, nociceptive, and FCA behaviors involve different neuronal phenotypes and neural circuits between, within, and from various amygdala nuclei. This information will be important in developing novel therapies for treating pain and emotive disorders in humans.

Neuromodulation with percutaneous electrical nerve field stimulation is associated with reduction in signs and symptoms of opioid withdrawal: a multisite, retrospective assessment

BACKGROUND

Finding an effective, non-pharmacological approach to treat opioid withdrawal could remove some of the barriers associated with pharmacotherapy. The BRIDGE® is a noninvasive, percutaneous electrical nerve field stimulator developed to target pain.

OBJECTIVES

This pilot study aimed to determine (1) the effects of the BRIDGE on withdrawal scores during the induction phase of opioid withdrawal therapy, (2) the percentage of subjects who successfully transitioned to medication assisted therapy (MAT).

METHODS

Adult patients treated with the BRIDGE during medically supervised withdrawal were included in this open label, uncontrolled, and retrospective study. The clinical opioid withdrawal scale (COWS) scores were prospectively recorded at different intervals (20, 30, and 60 min) and analyzed retrospectively. A subset of patients had scores recorded 5-days post-BRIDGE. Those who returned to the clinic and received their first dose of maintenance medication were considered to be successfully transitioned.

RESULTS

In this cohort (n=73), 65% were male. The mean COWS score prior to BRIDGE placement was 20.1 (±6.1). Twenty minutes after BRIDGE placement, the mean score was reduced to 7.5 (±5.9) (62.7% reduction, p<0.001). The scores further decreased after 30 minutes 4.0 (±4.4) and 60 minutes 3.1 (±3.4) (84.6% reduction, p<0.001). No rescue medications were administered during this period. The mean withdrawal score on day 5 was 0.6 (97.1% reduction, p<0.001) (n=33). Overall, 64/73 patients (88.8%) successfully transitioned to MAT.

CONCLUSIONS

Neurostimulation with the BRIDGE is associated with a reduction in opioid withdrawal scores. This effect persisted during the induction period and allowed for effective transition to MAT.

Influences of Morphine on the Spontaneous and Evoked Excitatory Postsynaptic Currents in Lateral Amygdala of Rats

Acute morphine exposure induces antinociceptive activity, but the underlying mechanisms in the central nervous system are unclear. Using whole-cell patch clamp recordings, we explore the role of morphine in the modulation of excitatory synaptic transmission in lateral amygdala neurons of rats. The results demonstrate that perfusion of 10 μM of morphine to the lateral amygdala inhibits the discharge frequency significantly. We further find that there are no significant influences of morphine on the amplitude of spontaneous excitatory postsynaptic currents (sEPSCs). Interestingly, morphine shows no marked influence on the evoked excitatory postsynaptic currents (eEPSCs) in the lateral amygdala neurons. These results indicate that acute morphine treatment plays an important role in the modulation on the excitatory synaptic transmission in lateral amygdala neurons of rats.

Amygdala pain mechanisms

A limbic brain area, the amygdala plays a key role in emotional responses and affective states and disorders such as learned fear, anxiety, and depression. The amygdala has also emerged as an important brain center for the emotional-affective dimension of pain and for pain modulation. Hyperactivity in the laterocapsular division of the central nucleus of the amygdala (CeLC, also termed the "nociceptive amygdala") accounts for pain-related emotional responses and anxiety-like behavior. Abnormally enhanced output from the CeLC is the consequence of an imbalance between excitatory and inhibitory mechanisms. Impaired inhibitory control mediated by a cluster of GABAergic interneurons in the intercalated cell masses (ITC) allows the development of glutamate- and neuropeptide-driven synaptic plasticity of excitatory inputs from the brainstem (parabrachial area) and from the lateral-basolateral amygdala network (LA-BLA, site of integration of polymodal sensory information). BLA hyperactivity also generates abnormally enhanced feedforward inhibition of principal cells in the medial prefrontal cortex (mPFC), a limbic cortical area that is strongly interconnected with the amygdala. Pain-related mPFC deactivation results in cognitive deficits and failure to engage cortically driven ITC-mediated inhibitory control of amygdala processing. Impaired cortical control allows the uncontrolled persistence of amygdala pain mechanisms.

Monoarthritis-induced emotional and cognitive impairments in rats are sensitive to low systemic doses or intra-amygdala injections of morphine

Chronic pain is a multidimensional experience that not only includes changes in nociception but also impairments in emotional and cognitive functions, not often taken into account in preclinical research. The present study investigated emotional and cognitive impairments in an animal model of persistent inflammatory pain as well as the involvement of the basolateral complex (BLC) of the amygdala in these components. Monoarthritis was induced by intra-articular injection of complete Freund׳s adjuvant. Mechanical hypersensitivity, anxiety and depressive-like behaviours as well as cognitive capacities were assessed using several tests, such as von Frey, social interaction, open field, saccharin preference, spatial and social recognition memory tests. The effects of morphine administered systemically or into the BLC of the amygdala were also studied. Monoarthritic rats exhibited mechanical hypersensitivity, anxiety and depressive-like behaviours as well as cognitive impairments. Whereas low systemic doses and intra-BLC infusion of morphine failed to reduce mechanical hypersensitivity, they reversed monoarthritis-induced anxiety-like behaviours and cognitive impairments. Our findings further support a crucial role of amygdala in the effect of morphine on emotional/cognitive components of pain and not on mechanical hypersensitivity. Finally, our study highlights the interest of a multi-behavioural approach in the assessment of pain and the analgesic effect of drugs.

DAMGO in the central amygdala alleviates the affective dimension of pain in a rat model of inflammatory hyperalgesia

Pain has sensory-discriminative and emotional-affective dimensions. Recent studies show that the affective component can be assessed with a conditioned place avoidance (CPA) test. We hypothesized that systemic morphine before a post-conditioning test would more potently attenuate the affective aspect compared to the sensory component and that [d-Ala2-N-Me-Phe4, Gly-ol5]-enkephalin (DAMGO), a μ-selective opioid receptor agonist, injected into the central nucleus of the amygdala (CeA) would reduce established CPA. A rat model of inflammatory pain, produced by a complete Freund adjuvant (CFA) injection into the hind paw, was combined with a CPA test. Three experiments were performed on adult male Sprague-Dawley rats. Systemic morphine (0.5 or 1.0mg/kg) in Experiment 1, intrathecal (i.t.) morphine (2.5 μg/rat) in Experiment 2, and intra-CeA DAMGO (7.7-15.4 ng/0.4 μl) in Experiment 3 were given to CFA-injected rats (n=6-8/group) prior to a post-conditioning test. Saline-injected rats were used as control. Time spent in a pain-paired compartment was recorded twice, before conditioning and after a post-conditioning test. Paw withdrawal latency (PWL) to a noxious thermal stimulus was measured before experiment at day-1 and after the post-conditioning test; hyperalgesia was defined as a decrease in PWL. The data showed that CFA-injected rats had significantly negative CPA compared to those of saline-injected rats (P<0.05). Low-dosage systemic morphine significantly (P<0.05) reduced CFA-induced CPA but had no effect on PWL. I.t. morphine did not inhibit the display of CPA but significantly increased PWL, suppressing hyperalgesia (P<0.05). Intra-CeA DAMGO significantly inhibited the display of CPA compared to saline (P<0.05) but had no effect on PWL. The data demonstrate that morphine attenuates the affective component more powerfully than it does the sensory and suggests that the sensory and the emotional-affective dimensions are underpinned by different mechanisms.

The amygdala between sensation and affect: a role in pain

The amygdala is a structure of the temporal lobe thought to be involved in assigning emotional significance to environmental information and triggering adapted physiological, behavioral and affective responses. A large body of literature in animals and human implicates the amygdala in fear. Pain having a strong affective and emotional dimension, the amygdala, especially its central nucleus (CeA), has also emerged in the last twenty years as key element of the pain matrix. The CeA receives multiple nociceptive information from the brainstem, as well as highly processed polymodal information from the thalamus and the cerebral cortex. It also possesses the connections that allow influencing most of the descending pain control systems as well as higher centers involved in emotional, affective and cognitive functions. Preclinical studies indicate that the integration of nociceptive inputs in the CeA only marginally contributes to sensory-discriminative components of pain, but rather contributes to associated behavior and affective responses. The CeA doesn’t have a major influence on responses to acute nociception in basal condition, but it induces hypoalgesia during aversive situation, such as stress or fear. On the contrary, during persistent pain states (inflammatory, visceral, neuropathic), a long-lasting functional plasticity of CeA activity contributes to an enhancement of the pain experience, including hyperalgesia, aversive behavioral reactions and affective anxiety-like states.

The endogenous opioids related with antinociceptive effects induced by electrical stimulation into the amygdala

Pain relief is necessary and essential for dental treatments. Recently, the relationships of pain and emotion were studied, and electrical stimulation applied to the amygdala depressed the nociceptive response in the anterior cingulate cortex (ACC). Thus, the antinociceptive effects of the amygdala are elucidated, but its mechanism is not yet clarified. The present study was performed to investigate whether endogenous opioid system is related to the depression, and the quantitative changes of endogenous opioids induced by electrical stimulation to the amygdala. We investigated immunohistologically c-Fos expression to confirm the activated neurons, as well as the distribution and the amount of endogenous opioids (β-endorphin, enkephalin and dynorphin A) in the brain using male Wistar rats, when electrical stimulation was applied to the central nucleus of the amygdala (CeA) or noxious stimulation was delivered to the peripheral tissue. c-Fos expression in the ipsilateral ACC was increased by electrical stimulation to the CeA. However, only a small amount of endogenous opioids was observed in the ACC when noxious stimulation or electrical stimulation was applied. In contrast, the amount of dynorphin A in the periaqueductal gray (PAG) was increased by electrical stimulation to the CeA, and the amount of β-endorphin in the PAG was increased by noxious stimulation to the peripheral tissue. The results suggest that dynorphin A in the PAG induced by electrical stimulation to the CeA activate the descending antinociceptive system, and suggest that the nociceptive response in the ACC is depressed indirectly.

Long-term changes in reward-seeking following morphine withdrawal are associated with altered N-methyl-D-aspartate receptor 1 splice variants in the amygdala

The NR1 subunit of the NMDA receptor can be alternatively spliced by the insertion or removal of the N1, C1, C2, or C2' regions. Morphine dependence and withdrawal were previously demonstrated to lower N1 and C2' in the accumbens and lower N1, C1, and C2' in the amygdala (AMY). Withdrawal has also been demonstrated to increase motivational and anxiety/stress behaviors in rats. We tested the hypothesis that NR1 splicing would be associated with these behaviors during an extended withdrawal period of 2 months. Motivation was measured using an operant orofacial assay at non-aversive temperatures (37°C) while anxiety and stress were measured by examining this behavior at aversive temperatures (46°C). Lower C1 and C2 expression levels were observed in the AMY in a subset of the population of withdrawn rats even after 2 months of morphine withdrawal. These subsets were associated with a hypersensitivity to adverse conditions which may reflect long-term alterations in the withdrawn population.

Pain and post traumatic stress disorder - review of clinical and experimental evidence

Pain and Post Traumatic Stress Disorder (PTSD) are highly comorbid conditions. Patients with chronic pain have higher rates of PTSD. Likewise, patients with PTSD are often diagnosed with numerous chronic pain conditions. Despite the high pain-PTSD comorbidity, the neurobehavioral mechanisms underlying this phenomenon are incompletely understood and only recently researchers have started investigating it using experimental models. In this article, we systematically review the substantial clinical evidence on the co-occurrence of pain and PTSD, and the limited experimental evidence of pain processing in this disorder. We provide a detailed overview of the psychophysical and brain imaging experiments that compared somatosensory and pain processing in PTSD and non-PTSD populations. Based on the presented evidence, an extensive body of literature substantiates the clinical coexistence of pain and PTSD in patients but the limited experimental data show inconsistent results highlighting the need for well-controlled future studies. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Somatostatin and nociceptin inhibit neurons in the central nucleus of amygdala that project to the periaqueductal grey

The central nucleus of amygdala (CeA) plays an important role in modulation of the descending antinociceptive pathways. Using whole-cell patch clamp recordings from brain slices, we found that CeA neurons responded to the endogenous ligands somatostatin (SST) and nociceptin/orphanin FQ (OFQ) via an increased K-conductance. Co-application with selective antagonists suggested that SST and OFQ act on SSTR2 and ORL1 receptors, respectively. Taking account of anatomical localisation of recorded neurons, the present study showed that many responsive neurons were located within the medial subdivision of CeA and all CeA projection neurons to the midbrain periaqueductal grey invariably responded to these peptides. Randomly selected agonist-responsive neurons in CeA predominantly classified physiologically as low-threshold spiking neurons. The similarity of SST, OFQ and, as previously reported, opioid responsiveness in a sub-population of CeA neurons suggests converging roles of these peptides to inhibit the activity of projections from CeA to vlPAG, and potentially similar antinociceptive actions in this pathway.

Activation of metabotropic glutamate receptor 5 in the amygdala modulates pain-like behavior

The central nucleus of the amygdala (CeA) has been identified as a site of nociceptive processing important for sensitization induced by peripheral injury. However, the cellular signaling components underlying this function remain unknown. Here, we identify metabotropic glutamate receptor 5 (mGluR5) as an integral component of nociceptive processing in the CeA. Pharmacological activation of mGluRs with (R,S)-3,5-dihydroxyphenylglycine (DHPG) in the CeA of mice is sufficient to induce peripheral hypersensitivity in the absence of injury. DHPG-induced peripheral hypersensitivity is reduced via pharmacological blockade of mGluR5 or genetic disruption of mGluR5. Furthermore, pharmacological blockade or conditional deletion of mGluR5 in the CeA abrogates inflammation-induced hypersensitivity, demonstrating the necessity of mGluR5 in CeA-mediated pain modulation. Moreover, we demonstrate that phosphorylation of extracellular-signal regulated kinase 1/2 (ERK1/2) is downstream of mGluR5 activation in the CeA and is necessary for the full expression of peripheral inflammation-induced behavioral sensitization. Finally, we present evidence of right hemispheric lateralization of mGluR5 modulation of amygdalar nociceptive processing. We demonstrate that unilateral pharmacological activation of mGluR5 in the CeA produces distinct behavioral responses depending on whether the right or left amygdala is injected. We also demonstrate significantly higher levels of mGluR5 expression in the right amygdala compared with the left under baseline conditions, suggesting a potential mechanism for right hemispheric lateralization of amygdala function in pain processing. Together, these results establish an integral role for mGluR5 and ERK1/2 in nociceptive processing in the CeA.

Cognitive impairment of prefrontal-dependent decision-making in rats after the onset of chronic pain

Forced choice between alternative options of unpredictable outcome is a complex task that requires continual update of the value associated with each option. Prefrontal areas such as the orbitofrontal cortex (OFC) have been shown to play a major role in performance on ambiguous decision-making tasks with substantial risk component, broadly named as "gambling tasks." We have recently demonstrated that rats display complex decision-making behavior in a rodent gambling task based on serial choices between rewards of different value and probability. This rodent task retains many of the key characteristics of the human Iowa Gambling Task (IGT), and performance in this novel task is also disrupted by OFC or amygdalar lesioning. In the present study we addressed if rat models of chronic pain would have impaired performance in this gambling task, since it is already known that the IGT response patterns of human pain patients are comparable to individuals with OFC lesions. We found that animals with a monoarthritic inflammatory model of chronic pain systematically preferred the lever associated with larger but infrequent rewards. In addition, we measured the neurochemical content of the OFC, amygdala and nucleus accumbens using HPLC, and found that in prolonged chronic pain animals there was a decrease in the tonic levels of dopamine, DOPAC (3,4-hydroxyphenyl-acetic acid) and 5-HIAA (5-hydroxyindole-3-acetic acid) in the OFC. This is the first report of the effect of chronic pain in rat decision-making processes and supports the notion that pain may have profound effects on the functioning of the reward-aversion circuitry relevant to strategic planning.

Effects of morphine on rat behaviour in the elevated plus maze: the role of central amygdala dopamine receptors

The objective of the present study was to evaluate the possible role of dopamine D(1)/D(2) receptors of the central amygdala (CeA) on morphine-induced anxiolytic-like behaviour in adult male Wistar rats. The animals were bilaterally implanted with chronic cannulas the CeA and tested in an elevated plus maze (EPM) task. Intraperitoneal (i.p.) administration of morphine (5 and 6 mg/kg) increased the percentage of open arm time (%OAT) and open arm entries (%OAE), indicating an anxiolytic-like response. Intra-CeA administration of different doses of the dopaminergic agonist apomorphine (0.1-0.3 microg/rat) significantly increased %OAE, but not %OAT. Furthermore, co-administration of the same doses of apomorphine with an ineffective dose of morphine (4 mg/kg; i.p.) significantly increased %OAT and %OAE by the opioid. Single microinjection of the D(1) dopaminergic antagonist SCH23390 (0.5-1.5 microg/rat) or sulpiride (0.5-1.5 microg/rat) into the CeA caused no significant change for %OAT and %OAE. The obtained results also show that intra-CeA microinjection of the same doses of SCH23390 or sulpiride inhibits the anxiolytic-like effect of morphine (6 mg/kg; i.p.). Pre-treatment of animals with SCH23390 (intra-CeA) or sulpiride (intra-CeA) reversed the response induced by apomorphine (0.3 microg/rat) plus morphine (4 mg/kg; i.p.). It should be considered that the drugs also did not show any effect on locomotor activity in all experiments. In conclusion, these findings suggest that the central amygdala dopaminergic mechanisms, probably via D(1)/D(2) receptors, may be involved in the modulation of morphine-induced anxiolytic-like behaviour in rat.

Forebrain pain mechanisms

Emotional-affective and cognitive dimensions of pain are less well understood than nociceptive and nocifensive components, but the forebrain is believed to play an important role. Recent evidence suggests that subcortical and cortical brain areas outside the traditional pain processing network contribute critically to emotional-affective responses and cognitive deficits related to pain. These brain areas include different nuclei of the amygdala and certain prefrontal cortical areas. Their roles in various aspects of pain will be discussed. Biomarkers of cortical dysfunction are being identified that may evolve into therapeutic targets to modulate pain experience and improve pain-related cognitive impairment. Supporting data from preclinical studies in neuropathic pain models will be presented. Neuroimaging analysis provides evidence for plastic changes in the pain processing brain network. Results of clinical studies in neuropathic pain patients suggest that neuroimaging may help determine mechanisms of altered brain functions in pain as well as monitor the effects of pharmacologic interventions to optimize treatment in individual patients. Recent progress in the analysis of higher brain functions emphasizes the concept of pain as a multidimensional experience and the need for integrative approaches to determine the full spectrum of harmful or protective neurobiological changes in pain.

PKA and ERK, but not PKC, in the amygdala contribute to pain-related synaptic plasticity and behavior

The laterocapsular division of the central nucleus of the amygdala (CeLC) has emerged as an important site of pain-related plasticity and pain modulation. Glutamate and neuropeptide receptors in the CeLC contribute to synaptic and behavioral changes in the arthritis pain model, but the intracellular signaling pathways remain to be determined. This study addressed the role of PKA, PKC, and ERK in the CeLC. Adult male Sprague-Dawley rats were used in all experiments. Whole-cell patch-clamp recordings of CeLC neurons were made in brain slices from normal rats and from rats with a kaolin/carrageenan-induced monoarthritis in the knee (6 h postinduction). Membrane-permeable inhibitors of PKA (KT5720, 1 μM; cAMPS-Rp, 10 μM) and ERK (U0126, 1 μM) activation inhibited synaptic plasticity in slices from arthritic rats but had no effect on normal transmission in control slices. A PKC inhibitor (GF109203x, 1 μM) and an inactive structural analogue of U0126 (U0124, 1 μM) had no effect. The NMDA receptor-mediated synaptic component was inhibited by KT5720 or U0126; their combined application had additive effects. U0126 did not inhibit synaptic facilitation by forskolin-induced PKA-activation. Administration of KT5720 (100 μM, concentration in microdialysis probe) or U0126 (100 μM) into the CeLC, but not striatum (placement control), inhibited audible and ultrasonic vocalizations and spinal reflexes of arthritic rats but had no effect in normal animals. GF109203x (100 μM) and U0124 (100 μM) did not affect pain behavior. The data suggest that in the amygdala PKA and ERK, but not PKC, contribute to pain-related synaptic facilitation and behavior by increasing NMDA receptor function through independent signaling pathways.

Anxiolytic-like effects of morphine and buprenorphine in the rat model of fear-potentiated startle: tolerance, cross-tolerance, and blockade by naloxone

RATIONALE

Morphine and buprenorphine have analgesic and anxiolytic-like properties. While their analgesic effects have been well characterized, their anxiolytic-like properties have not.

OBJECTIVES

Effects of acute morphine and buprenorphine on the expression of acoustic fear-potentiated startle (FPS) and naloxone pretreatment were assessed. Effects of chronic morphine and buprenorphine on tolerance, cross-tolerance, and withdrawal were also examined.

MATERIALS AND METHODS

Fear-conditioned rats were given subcutaneous drug treatment immediately before testing for FPS. Experiment 1, rats were administered morphine (0.03, 0.25, 0.63, 2.5, or 10 mg/kg) or buprenorphine (0.004, 0.0075, 0.015, 0.03, or 0.25 mg/kg). Experiment 2, rats were given saline or naloxone (0.5 mg/kg) and 5 min later given saline, morphine (2.5 mg/kg), or buprenorphine (0.03 mg/kg). Experiment 3, rats received once-daily injections of saline, morphine (10 mg/kg), or buprenorphine (0.25 mg/kg) for 7 days. Immediately before testing, saline-treated rats were given saline, morphine (2.5 mg/kg), or buprenorphine (0.03 mg/kg), morphine-treated rats were given morphine (2.5 mg/kg) or buprenorphine (0.03 mg/kg), and buprenorphine-treated rats were given buprenorphine (0.03 mg/kg) or morphine (2.5 mg/kg). Tolerance and cross-tolerance in analgesia were assessed via the tail-flick test, as were naloxone-precipitated withdrawal.

RESULTS

Morphine and buprenorphine had parallel dose-response curves in blocking FPS, with buprenorphine 40 times more potent than morphine. Naloxone reversed these effects. Morphine and buprenorphine showed tolerance and cross-tolerance in their anxiolytic-like and analgesic effects. Chronic buprenorphine produced less withdrawal than chronic morphine.

CONCLUSIONS

Cross-tolerance between morphine and buprenorphine suggests a common receptor mediating their anxiolytic-like and analgesic effects.

Bradykinin into amygdala induces thermal hyperalgesia in rats

Bradykinin is one of the most potent endogenous algesic substances and its role in pain transmission has been intensively studied in the periphery. However, the action of this peptide in central structures involved in pain transmission remains unclear. Administration of bradykinin (0.25 nmol/site) into the right amygdala of adult male Wistar rats induced thermal hyperalgesia, evaluated in the paw-flick test. Bradykinin-induced hyperalgesia was abolished by co-administration with the B(2) receptor antagonist Hoe 140 (5 pmol/site), the NMDA antagonist MK-801 (5 nmol/site), the cyclooxygenase inhibitor indomethacin (10 nmol/site) and the glial metabolic inhibitor fluorocitrate (1 nmol/site). Since the intra-amygdala administration of bradykinin did not alter spontaneous locomotion in the open-field test, it is unlikely that the current described hyperalgesic effect of bradykinin is due to an unspecific action on motor activity. These findings provide evidence that bradykinin, through activation of amygdalar B(2) receptors induces hyperalgesia and that glutamatergic- and prostanoid-mediated mechanisms are involved in such effect.

Noradrenergic agonist administration into the central nucleus of the amygdala increases the tail-flick latency in lightly anesthetized rats

The amygdala is a medial forebrain structure with an established role in nociceptive modulation, including the expression of stress-induced hypoalgesia (SIH). Projections from the locus coeruleus increase levels of noradrenaline in the amygdala during acute stress. alpha(2)-Noradrenergic receptor agonists have significant clinical utility as analgesic agents. We therefore hypothesized that alpha(2)-noradrenergic activation of the amygdala may result in behaviorally measurable hypoalgesia. Lightly anesthetized rats underwent microinjection of the alpha(2)-noradrenergic agonist clonidine into the amygdala and intermittent measurement of thermal nociception using the tail-flick latency (TFL). Bilateral microinjection of clonidine into the central nucleus of the amygdala (CeA) resulted in a significant, dose-dependent increase in TFL. This effect was blocked by systemic pre-treatment with the alpha(2)-antagonist yohimbine or by local pre-injection of the alpha(2)-antagonist idazoxan but not by local pre-injection of the alpha(1)-antagonist WB-4101. When injected alone, no antagonist resulted in a significant change in TFL compared with baseline. Clonidine injection into the amygdala but outside the CeA, including the basolateral nucleus of the amygdala, did not significantly alter TFL. These results demonstrate that anatomically and pharmacologically specific activation of alpha(2)-receptors in the CeA in lightly anesthetized rats results in behaviorally measurable antinociception.

The endomorphin system and its evolving neurophysiological role

Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) are two endogenous opioid peptides with high affinity and remarkable selectivity for the mu-opioid receptor. The neuroanatomical distribution of endomorphins reflects their potential endogenous role in many major physiological processes, which include perception of pain, responses related to stress, and complex functions such as reward, arousal, and vigilance, as well as autonomic, cognitive, neuroendocrine, and limbic homeostasis. In this review we discuss the biological effects of endomorphin-1 and endomorphin-2 in relation to their distribution in the central and peripheral nervous systems. We describe the relationship between these two mu-opioid receptor-selective peptides and endogenous neurohormones and neurotransmitters. We also evaluate the role of endomorphins from the physiological point of view and report selectively on the most important findings in their pharmacology.

Induction of c-Fos and zif268 in the nociceptive amygdala parallel abstinence hyperalgesia in rats briefly exposed to morphine

Opioid-induced analgesia can be followed by spontaneous pain in humans, and hyperalgesia in rodents. In this study, opioid-induced hyperalgesia was measured by the tail-flick test when acute abstinence was precipitated by administering naloxone to drug naive rats that had experienced morphine analgesia for only 30 min. In a further experiment, the drug treatment that previously caused opioid-induced hyperalgesia was found to increase neurons expressing nuclear c-Fos or zif268 proteins in extended amygdalar regions targeted by projections of the ascending spino-parabrachio-amygdaloid nociceptive pathway. Transcription factor induction, however, was not detected in multiple brain regions known to respond in parallel with the same extended amygdalar structures when (1) rats are exposed to interoceptive/physical stressors, or (2) naloxone is used to precipitate abstinence in opioid dependent rats. Surprisingly, in many regions c-Fos induction by morphine was reduced or blocked by naloxone, even though these subjects had also experienced the effects of morphine for 30 min prior to antagonist administration. It is suggested transcription factor induction during opioid hyperalgesia in non-dependent rats could support the induction or consolidation of neural plasticity in nociceptive amygdaloid circuitry previously suggested to function in bi-directional control of pain and expression of pain-related behaviors.

Activation of the extracellular signal-regulated kinase in the amygdala modulates pain perception

The amygdala has been proposed to serve as a neural center for the modulation of pain perception. Numerous anatomical and behavioral studies demonstrate that exogenous manipulations of the amygdala (i.e., lesions, drug infusions) modulate behavioral responses to acute noxious stimuli; however, little is known about the endogenous molecular changes in the amygdala that contribute to alterations in nociceptive processing during persistent noxious stimuli that resemble pathological pain conditions. In the present study, we demonstrate that endogenous molecular changes in the amygdala play a crucial role in modulating long-lasting peripheral hypersensitivity associated with persistent inflammation and we further identify the extracellular signal-regulated kinase (ERK) as a molecular substrate underlying this behavioral sensitization. Using the formalin test as a mouse model of persistent inflammatory pain, we show that activation of ERK in the amygdala is both necessary for and sufficient to induce long-lasting peripheral hypersensitivity to tactile stimulation. Thus, blockade of inflammation-induced ERK activation in the amygdala significantly reduced long-lasting peripheral hypersensitivity associated with persistent inflammation, and pharmacological activation of ERK in the amygdala induced peripheral hypersensitivity in the absence of inflammation. Importantly, blockade of ERK activation in the amygdala did not affect responses to acute noxious stimuli in the absence of inflammation, indicating that modulation of nociceptive responses by amygdala ERK activation is specific to the persistent inflammatory state. Altogether, our results demonstrate a functional role of the ERK signaling cascade in the amygdala in inflammation-induced peripheral hypersensitivity.

Amygdala GABA-A receptor involvement in mediating sensory-discriminative and affective-motivational pain responses in a rat model of peripheral nerve injury

The contribution of the amygdala to neuropathic pain processing in animals has not been clearly acknowledged. To assess the relative contribution of amygdala GABA-A receptors in mediating sensory-discriminative and affective-motivational pain components, the GABA-A receptor agonist muscimol and the antagonist bicuculline (both 10-25 ng/microl) were administered by acute bilateral injection directly into the central amygdala in rats with a chronic constriction injury (CCI). Escape/avoidance behaviour reflecting the affective-motivational dimension of pain was measured using a light/dark chamber in combination with suprathreshold nociceptive stimulation, and was defined as a shift from the 'non-aversive' dark area of the chamber to the 'aversive' light area. Hindpaw mechanical allodynia and mechanical hyperalgesia thresholds reflecting the sensory-discriminative dimension of pain were determined prior to and following escape/avoidance testing. Muscimol administration into the amygdala attenuated escape/avoidance behaviour and reversed hindpaw mechanical hypersensitivity in CCI rats; the magnitude of reduction in escape/avoidance behaviour was 2- to 3-fold greater than mechanical allodynia. Surprisingly, administration of bicuculline also attenuated escape/avoidance behaviour but had no effect on nociceptive behaviours. The muscimol-induced reversal of hindpaw mechanical hypersensitivity was completely blocked by co-administration of bicuculline, in contrast to escape/avoidance behaviour. Motility behaviour was unaffected by injection of either drug as determined in the open field test. Thus, amygdala GABA-A receptors appear to play an important role in sensory and especially affective pain processing in neuropathic rats. Furthermore, after nerve injury reflex nociceptive behaviours appear to be under tonic control by descending inputs, which originate from or are modulated within the amygdala.

Affective Modulation of Pain in Substance-Dependent Veterans

OBJECTIVE

Prior work suggests that positive affect inhibits pain while negative affect facilitates it. The current study sought to determine whether: 1) affective modulation of pain extends to a patient population; 2) cocaine and alcohol dependence influences the pattern of modulation; and 3) affective modulation of pain is mediated by changes in arm temperature.

DESIGN

Thirty-seven participants with and without substance dependence (14 alcohol, 13 cocaine, 10 none) attended three experimental sessions intended to induce emotions (negative, neutral, positive) by picture-viewing. Following emotion-induction, participants were asked to submerge their arm in 33 degrees F water and keep it there until they reached tolerance. During submersion, pain ratings were made on a mechanical visual analog scale (M-VAS).

OUTCOME MEASURES

Latency from submersion to first movement of the M-VAS (pain threshold) and latency to arm removal (pain tolerance) were measured. Arm temperature and manipulation checks for emotion-induction (corrugator electromyogram, heart rate, skin conductance, self-report) were also recorded.

RESULTS

Manipulation checks confirmed that targeted affective states were achieved. Pain threshold and tolerance were higher after viewing pleasant pictures than after unpleasant ones. Although arm temperature did vary based on the affect induced, analyses suggested that temperature did not influence pain outcomes.

CONCLUSIONS

Affect modulates pain perception in patients and does not appear to be mediated by changes in arm temperature. Additionally, pain modulation was not significantly influenced by cocaine or alcohol dependence. These data are encouraging, because they suggest that nonpharmacological methods of pain modulation may be effective in substance-dependent individuals.

Characterization of neurons in the rat central nucleus of the amygdala: cellular physiology, morphology, and opioid sensitivity

The central nucleus of the amygdala (CeA) orchestrates autonomic and other behavioral and physiological responses to conditioned stimuli that are aversive or elicit fear. As a related CeA function is the expression of hypoalgesia induced by conditioned stimuli or systemic morphine administration, we examined postsynaptic opioid modulation of neurons in each major CeA subdivision. Following electrophysiological recording, biocytin-filled neurons were precisely located in CeA regions identified by chemoarchitecture (enkephalin-immunoreactivity) and cytoarchitecture (DAPI nuclear staining) in fixed adult rat brain slices. This revealed a striking distribution of physiological types, as 92% of neurons in capsular CeA were classified as late-firing, whereas no neurons in the medial CeA were of this class. In contrast, 60% or more of neurons in the lateral and medial CeA were low-threshold bursting neurons. Mu-opioid receptor (MOPR) agonists induced postsynaptic inhibitory potassium currents in 61% of CeA cells, and this ratio was maintained in each subdivision and for each physiological class of neuron. However, MOPR agonists more frequently inhibited bipolar/fusiform cells than triangular or multipolar neurons. A subpopulation of MOPR-expressing neurons were also inhibited by delta opioid receptor agonists, whereas a separate population were inhibited kappa opioid receptors (KOPR). The MOPR agonist DAMGO inhibited 9/9 CeM neurons with projections to the parabrachial nucleus identified by retrograde tracer injection. These data support models of striatopallidal organization that have identified striatal-like and pallidal-like CeA regions. Opioids can directly inhibit output from each subdivision by activating postsynaptic MOPRs or KOPRs on distinct subpopulations of opioid-sensitive neurons.

Gender differences in pain: do emotions play a role?

BACKGROUND

Research suggests that the influence of gender on the processing and experience of pain is a result of several mechanisms. One mediating variable is emotion, which may modulate pain through an interaction of valence (pleasant-unpleasant) and arousal (calm-excited).

OBJECTIVE

This review examines whether gender differences in the experience and processing of emotion contribute to differences in the modulation and perception of pain.

METHODS

An English-language search of MEDLINE and PsycINFO was conducted from 1887 to May 2005. Additional literature was obtained from reference lists of articles retained in the initial search.

RESULTS

Emotion appears to influence pain through a valence-by-arousal interaction. Specifically, negatively valenced emotions with low to moderate arousal (eg, anxiety) enhance pain, whereas negatively valenced emotions with high arousal (eg, fear) reduce pain. In contrast, positively valenced emotions always reduce pain, as long as minimal arousal is achieved. Some evidence suggests that women are more sensitive than men to threat-related stimuli and thus experience more negative affect than men. This would generally lead to enhanced pain perception in women. It is also possible that women are more likely than men to experience negative affect with high arousal (intense fear) and thus pain inhibition. However, the relatively lower base rate of intense negative emotions is not likely to contribute much to gender differences in pain. Evidence also suggests that men may be more sensitive to positive events, particularly sexual/erotic stimuli, which may lead to more positive emotion-induced pain reduction in men, relative to women.

CONCLUSIONS

This review suggests that gender differences in the experience of pain may arise from differences in the experience and processing of emotion that, in turn, differentially alter pain processing. Specifically, the system associated with negative affect may be more attuned to threatening stimuli in women, and the system associated with positive affect may be more attuned to pleasurable stimuli in men. However, there is a paucity of research directly addressing this issue; much of the research on this topic has failed to test a comprehensive model of emotion, failed to use adequate manipulation checks, or failed to use within-subject experimental designs that control for intra- and interindividual differences. Therefore, it is concluded that additional research is warranted.

Branching patterns of parabrachial neurons projecting to the central extended amgydala: single axonal reconstructions

Electrophysiological evidence suggests that the spinoparabrachioamygdaloid pathway carries nociceptive information that may be important for the elaboration of physiological and emotional responses to noxious events. The pontine parabrachial nucleus (pPB) sends a massive projection to the central nucleus of the amygdala (CeA) and lateral bed nucleus of the stria terminalis (BSTL), both regions belonging to a broader macrostructure, the central extended amygdala (EAc). The aim of this study was to examine whether different EAc components are targeted by a same pPB neuron, by reconstructing single axonal branching patterns after anterograde labelling. Small deposits of biotinylated dextran amine in the region of the external lateral pPB result in dense and specific labelling in the whole EAc. Reconstructed axons innervate either the lateral or the capsular part of the CeA with perisomatic or bushy terminals, respectively. A subset of axons enters the stria terminalis rostrally to follow its trajectory caudally toward the CeA. Individual axons targeting the CeA usually send collaterals to other EAc components, especially those projecting to the lateral CeA, which often coinnervate the BSTL. By contrast, only few branches were found outside the EAc. These results suggest that the noxious information travelling from the pPB to the CeA may also be transmitted to other EAc components. This pPB-EAc pathway, which appears distinct from the parabrachiohypothalamic and parabrachiothalamic projections, would be the anatomical basis through which the EAc elaborates the autonomic, endocrine, and emotional components of pain.

Enkephalinergic afferents of the centromedial amygdala in the rat

The connectivity of the amygdaloid complex has been extensively explored with both anterograde and retrograde tracers. Even though the afferents of the centromedial amygdala [comprising the central (CEA) and medial (MEA) amygdaloid nuclei] are well established, relatively little is known about the neuropeptide phenotype of these connections. In this study, we first examined the distribution of mu-opioid receptor (MOR) and delta-opioid receptor (DOR) in the amygdala via in situ hybridization and immunohistochemistry. We then investigated the distribution of Met-enkephalin (ENK) and Leu-ENK fibers with immunohistochemistry and examined the distribution of preproenkephalin mRNA in the amygdala by using in situ hybridization. Finally, we examined the ENK projections to the CEA and MEA by using stereotaxic injections of the retrograde tracer cholera toxin subunit B or fluorogold revealed by immunohistochemistry combined with in situ hybridization to identify ENKergic neurons. Our results indicate that the centromedial amygdala receives ENK afferents, as indicated by the presence of MOR, DOR, and ENK fibers in the CEA and MEA, originating primarily from the bed nucleus of the stria terminalis (BST) and from other amygdaloid nuclei. The posterior BST, the basomedial nucleus (BMA), and the cortical nucleus of the amygdala (COA) were found to be the major ENK afferents of the MEA, whereas the anterolateral BST, the COA, the MEA, and the BMA provided the main ENKergic innervation of the CEA. In addition, we found that the ventromedial nucleus of the hypothalamus and the pontine parabrachial nucleus provide a moderate ENK input to the CEA and MEA. The functional implications of these connections in stress, anxiety, and nociception are discussed.

Cannabinoid mechanisms of pain suppression

A large body of literature indicates that cannabinoids suppress behavioral responses to acute and persistent noxious stimulation in animals. This review examines neuroanatomical, behavioral, and neurophysiological evidence supporting a role for cannabinoids in suppressing pain at spinal, supraspinal, and peripheral levels. Localization studies employing receptor binding and quantitative autoradiography, immunocytochemistry, and in situ hybridization are reviewed to examine the distribution of cannabinoid receptors at these levels and provide a neuroanatomical framework with which to understand the roles of endogenous cannabinoids in sensory processing. Pharmacological and transgenic approaches that have been used to study cannabinoid antinociceptive mechanisms are described. These studies provide insight into the functional roles of cannabinoid CB1 (CB1R) and CB2 (CB2R) receptor subtypes in cannabinoid antinociceptive mechanisms, as revealed in animal models of acute and persistent pain. The role of endocannabinoids and related fatty acid amides that are implicated in endogenous mechanisms for pain suppression are discussed. Human studies evaluating therapeutic potential of cannabinoid pharmacotherapies in experimental and clinical pain syndromes are evaluated. The potential of exploiting cannabinoid antinociceptive mechanisms in novel pharmacotherapies for pain is discussed.

Functional magnetic resonance imaging studies of opioid receptor-mediated modulation of noxious-evoked BOLD contrast in rats

RATIONALE

Functional magnetic resonance imaging (fMRI) in rats can non-invasively identify brain regions activated by physiological stimuli and the effects of pharmacological intervention on these responses.

OBJECTIVES

This study was conducted to investigate the effects of systemic administration of the mu-opioid receptor agonist morphine on whole brain functional signal intensity in anaesthetised rats; to investigate whether pre-treatment with the opioid receptor antagonist naloxone blocks the effects of morphine; to determine whether pre-treatment with morphine attenuates noxious-evoked changes in whole brain functional signal intensity.

METHODS

Continuous whole brain fMRI scanning was used to study brain signal intensity prior to, and following, systemic administration of morphine (5 mg/kg, n=7), systemic administration of naloxone (1 mg/kg) and morphine (n=8). Effects of pre-treatment with saline (n=5) or morphine (5 mg/kg, n=5) on formalin (5%, intraplantar)-evoked changes in signal intensity were determined. Data were processed using SMP99 with fixed-effects analysis (p<0.05).

RESULTS

Morphine produced significant positive bilateral increases in signal intensity in the cingulate cortex, amygdala, thalamus, hypothalamus and PAG (p<0.05), and these effects were blocked by naloxone. Intraplantar injection of formalin produced a significant positive increase in signal intensity in the cingulate cortex, somatosensory cortex, amygdala, thalamus, hypothalamus and PAG (p<0.05). Morphine attenuated formalin-evoked increases in signal intensity in the PAG, amygdala, hypothalamus and cingulate cortex.

CONCLUSION

Our data demonstrate that morphine modulates noxious-evoked changes in signal intensity in discrete brain regions. fMRI studies in rats are able to identify specific brain regions involved in the pharmacological modification of physiologically evoked changes in regional brain activation.

Visceral hypersensitivity

Visceral hypersensitivity is considered one of the causes of functional gastrointestinal disorders. The objectives of this review are to provide a practical description of neuroanatomy and physiology of gut sensation, to describe the diverse tests of visceral sensation and the potential role of brain imaging to further our understanding of visceral sensitivity in health and disease. Changes in motor function in the gut may influence sensory levels, eg, during contractions or as a result of changes in viscus compliance. New insights on sensory end organs, such as intraganglionic laminar endings, and basic neurophysiologic studies showing afferent firing during changes in stretch rather than tension illustrate the importance of different types of stimuli, not just tension, to stimulate afferent sensation. These insights provide the basis for understanding visceral sensation in health and disease, which will be extensively discussed in subsequent articles.

Identification and synthesis of norhydromorphone, and determination of antinociceptive activities in the rat formalin test

The main objective of this paper is to report the identification and synthesis of norhydromorphone, a novel metabolite of hydromorphone, and its antinociceptive activities when tested in the formalin test as compared to other known analgesics. In addition, we are reporting for the first time the lack of antinociceptive activities of hydromorphone-3-glucuronide, dihydromorphine-3-glucuronide and dihydroisomorphine-3-glucuronide in the rat formalin test. Norhydromorphone was isolated and identified as a metabolite of hydromorphone in a cancer patient's urine. An authentic standard of norhydromorphone was synthesized. The identity of norhydromorphone in the urine sample was confirmed by comparing the LC retention time and MS ion fragmentation with the synthetic standard using a liquid chromatographic-mass spectrometric-mass spectrometric (LC-MS-MS) assay. Norhydromorphone was found to be a minor metabolite of hydromorphone in the urine. Additionally, the antinociceptive activities of norhydromorphone, hydromorphone, morphine, dihydromorphine, dihydroisomorphine, hydromorphone-3-glucuronide, dihydromorphine-3-glucuronide and dihydroisomorphine-3-glucuronide were determined in the rat formalin test following intraperitoneal (i.p.) administration. Only limited antinociception was observed and no significant increase in antinociception was detected at the three doses tested. The increased polarity of norhydromorphone as compared to hydromorphone due to the primary piperidine nitrogen may make it less favorable to cross the blood-brain-barrier (BBB), which may be partly responsible. In addition, lower intrinsic antinociceptive activity, which remains to be determined, could also contribute to the low antinociception. Our results also show that hydromorphone was five times as potent as morphine in the formalin test, while dihydromorphine and dihydroisomorphine were equipotent to and 36% as potent as morphine, respectively. Hydromorphone-3-glucuronide, dihydromorphine-3-glucuronide and dihydroisomorphine-3-glucuronide did not exhibit any antinociceptive effect at the doses tested. The results further underscore the importance of a free C3-OH to the analgesic effect of morphine alkaloids.

Contributions of the anterior cingulate cortex and amygdala to pain- and fear-conditioned place avoidance in rats

The pain experience includes a sensory-discriminative and an affective-emotional component. The sensory component of pain has been extensively studied, while data about the negative affective component of pain are quite limited. The anterior cingulate cortex (ACC), and amygdala are thought to be key neural substrates underlying emotional responses. Using formalin-induced conditioned place avoidance (F-CPA) and electric foot-shock conditioned place avoidance (S-CPA) models, the present study observed the effects of bilateral excitotoxic (quinolinic acid 200 nmol/microl) lesions of the ACC and amygdala on pain and fear induced negative emotion, as well as on sensory component of pain. In the place-conditioning paradigm, both intraplantar (i.pl.) injection of formalin and electric foot-shock produced conditioned place avoidance. Excitotoxin-induced lesion of either the ACC or amygdala significantly reduced the magnitude of F-CPA. However, the decrease in the magnitude of S-CPA occurred only in the amygdala, but not ACC lesioned animals. Neither ACC nor amygdala lesion significantly changed formalin-induced acute nociceptive behaviors. These results suggest that the amygdala is involved in both pain- and fear-related negative emotion, and the ACC might play a critical role in the expression of pain-related negative emotion.

Fear-induced hypoalgesia in humans: Effects on low intensity thermal stimulation and finger temperature

Prior research indicates that exposure to fear-inducing stimuli inhibits finger withdrawal to sudden onset and high intensity radiant heat in humans. Although withdrawal latencies to intense heat are thought to reflect changes in spinal nociceptive processing, supraspinal measures are needed to determine whether pain perception is altered. The present study used gradual onset and low intensity radiant heat to induce a finger withdrawal response that depends on supraspinal processes. After baseline pain threshold tests, 57 healthy human participants were randomly assigned to 1 of 2 groups. In the fear group, participants received 3 brief shocks. In the neutral group, participants did not receive shock. Results suggest that finger withdrawal latencies to low intensity heat were increased after shock presentation, providing additional evidence that fear reduces pain on a measure that is influenced by supraspinal processes. Both self-report and physiological (skin conductance level, heart rate, and blood pressure) measures of emotion confirmed that the intended affective states were induced. Finger temperature was unaffected by emotion manipulations; thus, skin cooling does not appear to mediate increased withdrawal latencies. These findings provide additional evidence that fear not only inhibits spinal nociceptive reflexes, it also inhibits supraspinal measures of pain.

PERSPECTIVE

From a clinical perspective, these data suggest that patients who experience intense fear in response to unpredictable threatening events will show a reduction in pain perception.

Effect of the μ Opioid on Excitatory and Inhibitory Synaptic Inputs to Periaqueductal Gray-Projecting Neurons in the Amygdala

Opioids are potent analgesics, but the sites of their action and cellular mechanisms are not fully understood. The central nucleus of the amygdala (CeA) is important for opioid analgesia through the projection to the periaquaductal gray (PAG). In this study, we examined the effects of mu opioid receptor stimulation on inhibitory and excitatory synaptic inputs to PAG-projecting CeA neurons retrogradely labeled with a fluorescent tracer injected into the ventrolateral PAG of rats. Whole-cell voltage-clamp recordings were performed on labeled CeA neurons in brain slices. The specific mu opioid receptor agonist, [d-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO, 1 microM), significantly reduced the frequency of miniature inhibitory postsynaptic currents (mIPSCs) without altering the amplitude and decay constant of mIPSCs in 47.6% (10 of 21) of cells tested. DAMGO also significantly decreased the peak amplitude of evoked IPSCs in 69% (9 of 13) of cells examined. However, DAMGO did not significantly alter the frequency of miniature excitatory postsynaptic currents (EPSCs) and the amplitude of evoked EPSCs in 69% (9 of 13) and 83% (10 of 12) of labeled cells, respectively. The IPSCs were blocked by the GABA(A) receptor antagonist bicuculline, whereas the EPSCs were largely abolished by the non-N-methyl-d-aspartate antagonist 6-cyano-7-nitroquinoxaline-2,3-dione. The immunoreactivity of mu opioid receptors was colocalized with synaptophysin, a presynaptic marker, in close appositions to labeled CeA neurons. These results suggest that activation of mu opioid receptors on presynaptic terminals primarily attenuates GABAergic synaptic inputs to PAG-projecting neurons in the CeA.