Synaptic plasticity in the amygdala in a model of arthritic pain: differential roles of metabotropic glutamate receptors 1 and 5

Differential effects of CRF1 and CRF2 receptor antagonists on pain-related sensitization of neurons in the central nucleus of the amygdala

As a hormone in the hypothalamic-pituitary-adrenocortical (HPA) axis corticotropin-releasing factor (CRF) mediates stress responses. CRF can also act as a neuromodulator of synaptic transmission outside the HPA axis. A major site of extrahypothalamic expression of CRF and its G-protein-coupled receptors is the amygdala, a key player in affect-related disorders such as anxiety. The laterocapsular division of the central nucleus of the amygdala (CeLC) is important for the modulation of pain affect. This study determined the effects of CRF1 and CRF2 receptor antagonists in CeLC neurons in an arthritis pain model. Extracellular single-unit recordings were made from CeLC neurons in anesthetized adult rats. All neurons responded more strongly to noxious than to innocuous mechanical stimulation (compression) of peripheral tissues, including the knee. Evoked responses and background activity were measured before and during the development of a kaolin/carrageenan-induced knee joint arthritis. Drugs were administered into the CeLC by microdialysis before and/or after arthritis induction. All CeLC neurons showed increased responses to mechanical stimuli ("sensitization") 5-6 h postinduction of arthritis. A selective CRF1 receptor antagonist (NBI27914; 1-100 microM, concentration in microdialysis probe; 15 min) inhibited evoked responses and background activity in arthritis (n = 9) but had no effect under normal conditions before arthritis (n = 9). In contrast, a selective CRF2 receptor antagonist (Astressin-2B; 1-100 microM, 15 min) had no effect in arthritis (n = 7) but increased the neurons' responses under normal conditions (n = 8). These data suggest that CRF1 receptors in the amygdala contribute to pain-related sensitization, whereas the normally inhibitory function of CRF2 receptors is lost in the arthritis pain model.

Bidirectional synaptic plasticity at nociceptive afferents in the rat central amygdala

Glutamatergic inputs arising from the parabrachial nucleus to neurons in the lateral sector of the central amygdala were studied in vitro. Tetanic stimulation of these inputs led to LTP that did not require activation of NMDA receptors or a rise of postsynaptic calcium. LTP was accompanied by a reduction in the paired-pulse ratio, indicating that LTP results from an increase in transmitter release probability. Activation of adenylyl cyclase with forskolin potentiated these inputs with a similar reduction in paired-pulse facilitation and occluded LTP induction. LTP was inhibited by the protein kinase A blocker H89. Low-frequency stimulation led to LTD that required activation of postsynaptic NMDA receptors and a rise in postsynaptic calcium. There was no change in paired-pulse facilitation with LTD. LTD was blocked by protein phosphatase blockers calyculin and okadaic acid. We conclude that parabrachial inputs to the lateral sector of the central amygdala show presynaptic LTP that requires activation of a presynaptic protein kinase A via a calcium-dependent adenylyl cyclase while LTD at the same synapses is postsynaptic and requires a rise in postsynaptic calcium and activation of protein phosphatase.

Glutamate receptor ligands

Glutamate acts through a variety of receptors to modulate neurotransmission and neuronal excitability. Glutamate plays a critical role in neuroplasticity as well as in nervous system dysfunctions and disorders. Hyperfunction or dysfunction of glutamatergic neurotransmission also represents a key mechanism of pain-related plastic changes in the central and peripheral nervous system. This chapter will review the classification of glutamate receptors and their role in peripheral and central nociceptive processing. Evidence from preclinical pain models and clinical studies for the therapeutic value of certain glutamate receptor ligands will be discussed.

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.

Glutamate receptors and pain

Pain is an important survival and protection mechanism for animals. However, chronic/persistent pain may be differentiated from normal physiological pain in that it confers no obvious advantage. An accumulating body of pharmacological, electrophysiological, and behavioral evidence is emerging in support of the notion that glutamate receptors play a crucial role in pain pathways and that modulation of glutamate receptors may have potential for therapeutic utility in several categories of persistent pain, including neuropathic pain resulting from injury and/or disease of central (e.g., spinal cord injury) or peripheral nerves (e.g., diabetic neuropathy, radiculopathy) and inflammatory or joint-related pain (e.g., rheumatoid arthritis, osteoarthritis). This review focuses on the role of glutamate receptors, including both ionotropic (AMPA, NMDA and kainate) and metabotropic (mGlu1-8) receptors in persistent pain states with particular emphasis on their expression patterns in nociceptive pathways and their potential as targets for pharmacological intervention strategies.

NMDA receptor-independent synaptic plasticity in the central amygdala in the rat model of neuropathic pain

Neurons in the latero-capsular part of the central nucleus of the amygdala (CeA), a region now called the "nociceptive amygdala", receive predominantly nociceptive information from the dorsal horn through afferent pathways relayed at the nucleus parabrachialis (PB). Excitatory synaptic transmission between the PB afferents and these neurons is reported to become potentiated within a few hours of the establishment of arthritic or visceral pain, making it a possible mechanism linking chronic pain and unpleasant negative emotional experiences. However, it remains unknown whether such synaptic potentiation is consolidated or becomes adaptively extinct in the longer-lasting form of chronic pain, such as neuropathic pain, an as yet serious and frequent type of pain of important clinical concern. To address this issue, we recorded postsynaptic currents in CeA neurons evoked by PB tract stimulation in acute brain slices from young rats with neuropathic pain, as evaluated by tactile allodynic responses, following unilateral spinal nerve ligature made 1 week earlier. CeA neurons contralateral to the nerve ligation showed significantly larger-amplitude postsynaptic currents than those in the ipsilateral CeA and sham- and non-operated groups. The degree of synaptic potentiation, as compared between two sides, was positively correlated to that of tactile allodynia responses. In addition, blockade of NMDA receptors did not affect this potentiation. We conclude that potentiation of the PB-CeA synapse is consolidated in long-lasting neuropathic pain but that this potentiation results from a molecular mechanism distinct from that in arthritic and visceral pain.

Metabotropic glutamate receptor subtype 5 antagonists MPEP and MTEP

Glutamate regulates the function of central nervous system (CNS), in part, through the cAMP and/or IP3/DAG second messenger-associated metabotropic glutamate receptors (mGluRs). The mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) has been extensively used to elucidate potential physiological and pathophysiological functions of mGluR5. Unfortunately, recent evidence indicates significant non-specific actions of MPEP, including inhibition of NMDA receptors. In contrast, in vivo and in vitro characterization of the newer mGluR5 antagonist 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) indicates that it is more highly selective for mGluR5 over mGluR1, has no effect on other mGluR subtypes, and has fewer off-target effects than MPEP. This article reviews literature on both of these mGluR5 antagonists, which suggests their possible utility in neurodegeneration, addiction, anxiety and pain management.

Pain-related synaptic plasticity in spinal dorsal horn neurons: role of CGRP

Background

The synaptic and cellular mechanisms of pain-related central sensitization in the spinal cord are not fully understood yet. Calcitonin gene-related peptide (CGRP) has been identified as an important molecule in spinal nociceptive processing and ensuing behavioral responses, but its contribution to synaptic plasticity, cellular mechanisms and site of action in the spinal cord remain to be determined. Here we address the role of CGRP in synaptic plasticity in the spinal dorsal horn in a model of arthritic pain.

Results

Whole-cell current- and voltage-clamp recordings were made from substantia gelatinosa (SG) neurons in spinal cord slices from control rats and arthritic rats (> 6 h postinjection of kaolin/carrageenan into the knee). Monosynaptic excitatory postsynaptic currents (EPSCs) were evoked by electrical stimulation of afferents in the dorsal root near the dorsal root entry zone. Neurons in slices from arthritic rats showed increased synaptic transmission and excitability compared to controls. A selective CGRP1 receptor antagonist (CGRP8-37) reversed synaptic plasticity in neurons from arthritic rats but had no significant effect on normal transmission. CGRP facilitated synaptic transmission in the arthritis pain model more strongly than under normal conditions where both facilitatory and inhibitory effects were observed. CGRP also increased neuronal excitability. Miniature EPSC analysis suggested a post- rather than pre-synaptic mechanism of CGRP action.

Conclusion

This study is the first to show synaptic plasticity in the spinal dorsal horn in a model of arthritic pain that involves a postsynaptic action of CGRP on SG neurons.

Differential changes of group II and group III mGluR function in central amygdala neurons in a model of arthritic pain

Metabotropic glutamate receptors (mGluRs) play important roles in neuroplasticity and disorders such as persistent pain. Group I mGluRs contribute to pain-related sensitization and synaptic plasticity of neurons in the laterocapsular division of the central nucleus of the amygdala (CeLC), although the roles of groups II and III mGluRs are not known. Extracellular single-unit recordings were made from 60 CeLC neurons in anesthetized adult rats. Background activity and evoked responses were measured before and during the development of the kaolin/carrageenan-induced knee-joint arthritis. Drugs were administered into the CeLC by microdialysis before and/or after arthritis induction. A selective group III mGluR agonist (LAP4) inhibited CeLC neurons' responses to stimulation of the knee and ankle in arthritis (n = 7) more potently than under normal conditions (n = 14). A selective group II agonist (LY354740) inhibited responses under normal conditions (n = 12) and became more potent in inhibiting responses to noxious stimulation of the knee in arthritis (n = 10). The effect of LY354740 on innocuous stimulation of the knee and stimulation of the ankle did not change in arthritis. Antagonists for groups II (EGLU, n = 9) and III (UBP1112, n = 8) had no effects under normal conditions. In arthritis, UPB1112 (n = 5) facilitated the responses to stimulation of knee and ankle, whereas EGLU (n = 5) selectively increased the responses to stimulation of the knee. These data suggest that mGluRs of groups II and III can inhibit nociceptive processing in CeLC neurons. The increased function and endogenous activation of group II mGluRs in the arthritis pain model appear more input-selective than the general changes of group III mGluRs.

Enhanced group II mGluR-mediated inhibition of pain-related synaptic plasticity in the amygdala

Background

The latero-capsular part of the central nucleus of the amygdala (CeLC) is the target of the spino-parabrachio-amygdaloid pain pathway. Our previous studies showed that CeLC neurons develop synaptic plasticity and increased neuronal excitability in the kaolin/carrageenan model of arthritic pain. These pain-related changes involve presynaptic group I metabotropic glutamate receptors (mGluRs) and postsynaptic NMDA and calcitonin gene-related peptide (CGRP1) receptors. Here we address the role of group II mGluRs.

Results

Whole-cell current- and voltage-clamp recordings were made from CeLC neurons in brain slices from control rats and arthritic rats (>6 h postinjection of kaolin/carrageenan into the knee). Monosynaptic excitatory postsynaptic currents (EPSCs) were evoked by electrical stimulation of afferents from the pontine parabrachial (PB) area. A selective group II mGluR agonist (LY354740) decreased the amplitude of EPSCs more potently in CeLC neurons from arthritic rats (IC₅₀ = 0.59 nM) than in control animals (IC₅₀ = 15.0 nM). The inhibitory effect of LY354740 was reversed by a group II mGluR antagonist (EGLU) but not a GABA_A receptor antagonist (bicuculline). LY354740 decreased frequency, but not amplitude, of miniature EPSCs in the presence of TTX. No significant changes of neuronal excitability measures (membrane slope conductance and action potential firing rate) were detected.

Conclusion

Our data suggest that group II mGluRs act presynaptically to modulate synaptic plasticity in the amygdala in a model of arthritic pain.

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.

Mu opioid receptor activation inhibits GABAergic inputs to basolateral amygdala neurons through Kv1.1/1.2 channels

The basolateral amygdala (BLA) is the major amygdaloid nucleus distributed with mu opioid receptors. The afferent input from the BLA to the central nucleus of the amygdala (CeA) is considered important for opioid analgesia. However, little is known about the effect of mu opioids on synaptic transmission in the BLA. In this study, we examined the effect of mu opioid receptor stimulation on the inhibitory and excitatory synaptic inputs to CeA-projecting BLA neurons. BLA neurons were retrogradely labeled with a fluorescent tracer injected into the CeA of rats. Whole cell voltage-clamp recordings were performed on labeled BLA neurons in brain slices. The specific mu opioid receptor agonist, (D-Ala2,N-Me-Phe4,Gly5-ol)-enkephalin (DAMGO, 1 microM), significantly reduced the frequency of miniature inhibitory postsynaptic currents (mIPSCs) in 77% of cells tested. DAMGO also significantly decreased the peak amplitude of evoked IPSCs in 75% of cells examined. However, DAMGO did not significantly alter the frequency of mEPSCs or the peak amplitude of evoked EPSCs in 90% and 75% of labeled cells, respectively. Bath application of the Kv channel blockers, 4-AP (Kv1.1, 1.2, 1.3, 1.5, 1.6, 3.1, 3.2), alpha-dendrotoxin (Kv1.1, 1.2, 1.6), dendrotoxin-K (Kv1.1), or tityustoxin-Kalpha (Kv1.2) each blocked the inhibitory effect of DAMGO on mIPSCs. Double immunofluorescence labeling showed that some of the immunoreactivities of Kv1.1 and Kv1.2 were colocalized with synaptophysin in the BLA. This study provides new information that activation of presynaptic mu opioid receptors primarily attenuates GABAergic synaptic inputs to CeA-projecting neurons in the BLA through a signaling mechanism involving Kv1.1 and Kv1.2 channels.

Mu-opioid-mediated inhibition of glutamate synaptic transmission in rat central amygdala neurons

The central nucleus of the amygdala (CeA) plays an important role both in stimulus-reward learning for the reinforcing effects of drugs of abuse and in environmental condition-induced analgesia. Both of these two CeA functions involve the opioid system within the CeA. However, the pharmacological profiles of its opioid receptor system have not been fully studied and the synaptic actions of opioid receptors in the CeA are largely unknown. In this study with whole-cell voltage-clamp recordings in brain slices in vitro, we examined actions of opioid agonists on glutamate-mediated excitatory postsynaptic currents (EPSCs) in CeA neurons. Opioid peptide methionine-enkephalin (ME; 10 microM) produced a significant inhibition (38%) in the amplitude of evoked EPSCs, an action mimicked by the mu-opioid receptor agonist [D-Ala(2),N-MePhe(4),Gly-ol(5)]-enkephalin (DAMGO; 1 microM, 44%). Both effects of ME and DAMGO were abolished by the mu receptor antagonist CTAP (1 microM), suggesting a mu receptor-mediated effect. Neither delta-opioid receptor agonist [D-Pen(2),D-Pen(5)]-enkephalin (1 microM) nor kappa-opioid receptor agonist U69593 (300 nM) had any effect on the glutamate EPSC. ME significantly increased the paired-pulse ratio of the evoked EPSCs and decreased the frequency of miniature EPSCs without altering the amplitude of miniature EPSCs. Furthermore, the mu-opioid inhibition of the EPSC was blocked by 4-aminopyridine (4AP; 100 microM), a voltage-dependent potassium channel blocker, and by phospholipase A(2) inhibitors AACOCF(3) (10 microM) and quinacrine (10 microM). These results indicate that only the mu-opioid receptor is functionally present on presynaptic glutamatergic terminals in normal CeA neurons, and its activation reduces the probability of glutamate release through a signaling pathway involving phospholipase A(2) and the presynaptic, 4AP-sensitive potassium channel. This study provides evidence for the presynaptic regulation of glutamate synaptic transmission by mu-opioid receptors in CeA neurons.

Alterations of membrane properties and effects of d-serine on NMDA-induced current in rat anterior cingulate cortex neurons after monoarthritis

Accumulated evidence implicates the anterior cingulate cortex (ACC) in pain processing. The activation of the NMDA receptor requires the occupation of both the glutamate site and the glycine site. d-Serine released by astrocytes is presumed to be an endogenous ligand for the glycine site of the NMDA receptor. Using whole-cell patch clamp recording, membrane characteristics and effects of exogenous d-serine on NMDA-evoked currents were examined in neurons in ACC slices from normal and complete Freund's adjuvant-induced monoarthritic rats. Neurons from rats with monoarthritis exhibited more depolarized membrane potential, lower firing threshold, lower input resistance and higher slope conductance compared with normal rats. The NMDA-evoked currents were enhanced by d-serine (20 microM) in both normal (135.3+/-4.3% of control, p < 0.01) and arthritic (157.9 +/- 9.7% of control, p < 0.01) rats, respectively. The effect of d-serine was greater in arthritic rats than control rats (p < 0.05). These results suggest that inflammatory pain increased the excitability of ACC neurons, and that the NMDA receptor glycine sites in the ACC neurons were not saturated in either normal or inflammatory pain states.

Computerized analysis of audible and ultrasonic vocalizations of rats as a standardized measure of pain-related behavior

The behavioral assessment of experimental pain is essential for the analysis of pain mechanisms and the validation of therapeutic targets. Arthritic pain, in particular, is significantly associated with negative affective states and disorders. Here we present a standardized method for the quantitative analysis of audible and ultrasonic (25 +/- 4 kHz) vocalizations in awake rats as a measure of higher integrated behavior in a model of arthritic pain. A bat detector and a condenser microphone were used to record ultrasonic and audible vocalizations, respectively, in response to innocuous and noxious mechanical stimulation of the knee before and after induction of acute arthritis in one knee. A computerized system was used to analyze number and duration of the filtered signals. For the behavioral tests, the animal was placed in a customized recording chamber to ensure consistent stimulus application and stable recordings and to eliminate any movement-induced noise. Noxious stimuli produced stronger vocalizations than innocuous stimuli. Both audible and ultrasonic vocalizations to innocuous (allodynia) and noxious (hyperalgesia) stimuli increased after the induction of acute arthritis. These changes were accompanied by increased knee joint circumference, lowered hind limb withdrawal thresholds and reduced exploratory behavior in the same animals. The computerized analysis of audible and ultrasonic vocalizations is a valid, quantitative, reliable and convenient method to measure pain-related behavior.

mGluR1 and mGluR5 antagonists in the amygdala inhibit different components of audible and ultrasonic vocalizations in a model of arthritic pain

Pain has a strong emotional component. The amygdala plays a key role in emotionality and is also involved in pain processing and pain modulation. Our previous studies showed an important role of group I metabotropic glutamate receptors (mGluRs) in pain-related synaptic plasticity and sensitization of neurons in the central nucleus of the amygdala (CeA). Here we address the roles of mGluR1 and mGluR5 subtypes in the CeA in the modulation of supraspinally organized behavioral responses in a model of arthritic pain. Audible and ultrasonic (25+/-4 kHz) vocalizations were measured in awake rats during and after innocuous and noxious stimulation (15 s) of the knee joint. Vocalizations were recorded in the same animals before arthritis, 6 h after arthritis induction and during administration of antagonists selective for mGluR1 (CPCCOEt) and mGluR5 (MPEP) into the CeA through stereotaxically implanted microdialysis probes. The duration of audible and ultrasonic vocalizations increased in the arthritic pain state. The duration of vocalizations during stimulation (VDS), which are organized at the brainstem level, was significantly reduced by CPCCOEt but not by MPEP. Vocalizations that continued after stimulation (VAS), which are organized in the limbic forebrain, particularly the amygdala, were inhibited by CPCCOEt and MPEP. These findings suggest differential roles of mGluR1 and mGluR5 in the CeA in pain-related vocalizations. Both mGluR1 and mGluR5 contribute to vocalizations generated in the amygdala whereas mGluR1, but not mGluR5, is involved in the amygdala-mediated modulation of vocalizations originating from activity in the brainstem.

The antinociceptive and anxiolytic-like effects of the metabotropic glutamate receptor 5 (mGluR5) antagonists, MPEP and MTEP, and the mGluR1 antagonist, LY456236, in rodents: a comparison of efficacy and side-effect profiles

RATIONALE

Modulation of metabotropic glutamate receptor (mGluR) subtypes represents a novel approach for the treatment of neurological and psychiatric disorders.

OBJECTIVES

This study was conducted to investigate the role of the mGluR5 and mGluR1 subtypes in the modulation of pain and anxiety.

METHODS

The mGluR5 antagonists, 2-methyl-6-(phenylethynyl)pyridine (MPEP) and 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP), and the mGluR1 antagonist, (4-methoxy-phenyl)-(6-methoxy-quinazolin-4-yl)-amine HCl (LY456236), were tested in models of pain [mouse formalin test, rat spinal nerve ligation (SNL)] and anxiety [Vogel conflict, conditioned lick suppression (CLS)], and their efficacious effects were compared to any associated side effects.

RESULTS

The systemic administration of MPEP, MTEP, and LY456236 reduced hyperalgesia induced by formalin and mechanical allodynia following SNL. However, only LY456236 completely reversed the allodynia. In the anxiety models, MPEP (3--30 mg/kg), MTEP (3--10 mg/kg), and LY456236 (10--30 mg/kg) produced anxiolytic-like effects similar to the benzodiazepine, chlordiazepoxide (CDP, 6 mg/kg). However, only MPEP and MTEP were able to produce a level of anxiolysis comparable to CDP. In a series of tests examining potential side effects, MPEP and MTEP reduced body temperature and locomotor activity and impaired operant responding for food and rotarod performance at doses of 3--30 and 1--30 mg/kg, respectively. LY456236 reduced operant responding at 30 mg/kg.

CONCLUSION

Both mGluR5 and mGluR1 antagonists are effective in models of pain and anxiety. However, an mGluR1 antagonist was more efficacious than the two mGluR5 antagonists in the pain models, which, conversely, appeared more efficacious in the anxiety models. These findings support the potential utility of mGluR5 and mGluR1 antagonists for both the treatment of chronic pain and as novel anxiolytics.

Protein kinase A-dependent enhanced NMDA receptor function in pain-related synaptic plasticity in rat amygdala neurones

Mechanisms of pain-related plasticity in the amygdala, a key player in emotionality, were studied at the cellular and molecular levels in a model of arthritic pain. The influence of the arthritis pain state induced in vivo on synaptic transmission and N-methyl-d-aspartate (NMDA) receptor function was examined in vitro using whole-cell voltage-clamp recordings of neurones in the latero-capsular part of the central nucleus of the amygdala (CeA), which is now defined as the 'nociceptive amygdala'. Synaptic transmission was evoked by electrical stimulation of afferents from the pontine parabrachial area (part of the spino-parabrachio-amygdaloid pain pathway) in brain slices from control rats and from arthritic rats. This study shows that pain-related synaptic plasticity is accompanied by protein kinase A (PKA)-mediated enhanced NMDA-receptor function and increased phosphorylation of NMDA-receptor 1 (NR1) subunits. Synaptic plasticity in the arthritis pain model, but not normal synaptic transmission in control neurones, was inhibited by a selective NMDA receptor antagonist. Accordingly, an NMDA receptor-mediated synaptic component was recorded in neurones from arthritic animals, but not in control neurones, and was blocked by inhibition of PKA but not protein kinase C (PKC). Exogenous NMDA evoked a larger inward current in neurones from arthritic animals than in control neurones, indicating a postsynaptic effect. Paired-pulse facilitation, a measure of presynaptic mechanisms, was not affected by an NMDA-receptor antagonist. Increased levels of phosphorylated NR1 protein, but not of total NR1, were measured in the CeA of arthritic rats compared to controls. Our results suggest that pain-related synaptic plasticity in the amygdala involves a critical switch of postsynaptic NMDA receptor function through PKA-dependent NR1 phosphorylation.

Anxiolytic effects of kindling role of anatomical location of the kindling electrode in response to kindling of the right basolateral amygdala

Study of effects of kindling on affect has been complicated by the fact that anxiogenic, anxiolytic or no effects may be observed following kindling of the amygdala. Factors affecting behavioral outcome include strain of rat, hemisphere kindled, amygdala nucleus kindled and location of the kindling electrodes within particular AP planes of a given nucleus. Previous work has suggested that kindling of the right basolateral amygdala (BLA) is predominantly anxiogenic. This conclusion was based on kindling of anterior or posterior parts of the BLA. The present study sought to clarify this conclusion by examining behavioral effects of right BLA kindling in a mid-range of AP planes not yet studied. A variety of measures of rodent anxiety-like behavior were examined, including behavior in the hole board, elevated plus maze, light/dark box, social interaction test and unconditioned acoustic startle. Anhedonic effects of kindling were assessed by a sucrose preference test with controls for fluid consumption and taste sensitivities. All effects were assessed shortly after kindling (1-2 days) and at a longer time interval (7-8 days). Kindling to four stage 5 seizures in the mid-right BLA altered behavior at all time points after kindling in all tests except the hole board and light/dark box tests. The effect of kindling was anxiolytic like in the plus maze, social interaction and startle tests. Kindling in mid-BLA also increased sucrose consumption. Effects on sucrose consumption are consistent with previous studies showing no depressive-like effects of amygdala kindling in rodents. It is hypothesized that the focal nature of the behavioral consequences of amygdala kindling are best understood in the context of the circuitry in which the cells stimulated are imbedded and the impact of kindling on functioning of those circuits.

Synaptic Plasticity in the Central Nucleus of the Amygdala

In recent years, the amygdala has emerged as a critical site of plasticity for the acquisition of various forms of Pavlovian learning, either aversive or appetitive. In most of these models, the critical site of plasticity has been localized to the basolateral complex of the amygdala (BLA). In contrast, the central nucleus of the amygdala has emerged as a passive relay of potentiated BLA outputs toward downstream effectors. At odds with this view, however, recent studies suggest that the central nucleus may also be a site of plasticity and play an active role in some forms of Pavlovian learning. The present review summarizes the evidence supporting this possibility.

Isoflurane modulates glutamatergic and GABAergic neurotransmission in the amygdala

Attempts have been made to attribute the particular features of general anaesthesia such as hypnosis, analgesia, amnesia and autonomic stability to certain brain regions. In the present study, we examined the effects of the commonplace volatile anaesthetic isoflurane on synaptic transmission in an in vitro slice preparation of the murine amygdala. Despite the established role of this limbic structure in the formation of aversive memories, conditioned fear and anxiety, as well as pain processing and regulation of sympathetic tone, the influence of volatile anaesthetics on synaptic signalling has not yet been investigated in this region of the brain. Evoked postsynaptic currents were monitored from principal neurons in the basolateral nucleus of the amygdala by means of patch-clamp recording. The mixed postsynaptic currents were mediated by non-NMDA, NMDA, GABA A and GABA B receptors. Isoflurane added to the perfusion medium reduced the strength of synaptic signalling following the activation of non-NMDA, NMDA, and GABA B receptors, whereas the GABA A receptor-mediated responses were enhanced. The overall reduction of neuronal excitability was also reflected in a reduction of field potential amplitudes. Isoflurane neither changed the membrane resting potential nor the input resistance of principal neurons in the amygdala. The present results may contribute to the understanding of how stress reactions and long-lasting neuroplastic processes are suppressed under general anaesthesia.

Inflammation persistently enhances nocifensive behaviors mediated by spinal group I mGluRs through sustained ERK activation

Group I metabotropic glutamate receptors (mGluRs) and their downstream signaling pathways, which involve the extracellular signal-regulated kinases (ERKs), have been implicated as mediators of plasticity in several pain models. In this study, we report that inflammation leads to a long-lasting enhancement of behavioral responses induced by activation of spinal group I mGluRs. Thus, the nocifensive response to intrathecal injection of the group I mGluR agonist (RS)-3,5-Dihydroxyphenylglycine (DHPG) is significantly potentiated seven days following Complete Freund's Adjuvant (CFA)-induced inflammation of the hind paw. This potentiation is not associated with increased mGlu1 or mGlu5 receptor expression but is associated with increased levels of phosphorylated ERK in dorsal horn neurons. We also tested whether the increased behavioral response to DHPG following inflammation may be explained by enhanced coupling of the group I mGluRs to ERK activation. DHPG-induced ERK phosphorylation in the dorsal horn is not potentiated following inflammation. However, inhibiting ERK activation using a MEK inhibitor, U0126, following inflammation attenuates the intrathecal DHPG-induced behavioral responses to a greater extent than in control animals. The results from this study indicate that persistent ERK activation is required for the enhanced behavioral responses to spinal group I mGluR activation following inflammation and suggest that tonic modulation of ERK activity may underlie a component of central sensitization in dorsal horn neurons.

Block of NMDA and non-NMDA receptor activation results in reduced background and evoked activity of central amygdala neurons in a model of arthritic pain

The latero-capsular division of the central nucleus of the amygdala (CeA) is now defined as the 'nociceptive amygdala' because of its high content of neurons activated exclusively or preferentially by noxious stimuli. Multireceptive (MR) neurons that respond to innocuous and, more strongly, to noxious stimuli become sensitized in arthritis pain. This form of nociceptive plasticity involves presynaptic group I metabotropic glutamate receptors, which increase glutamate release. Here we address the role of N-methyl-D-aspartate (NMDA) and non-NMDA receptors. Extracellular single-unit recordings were made from 25 CeA neurons in anesthetized rats. The neurons' responses to graded brief (15 s) mechanical stimuli, background activity, receptive field size and threshold were measured before and after the induction of kaolin/carrageenan arthritis in one knee and before and during drug applications into the CeA by microdialysis. All neurons examined received excitatory input from the knee(s) and were MR neurons. A selective NMDA receptor antagonist (AP5) inhibited responses to noxious stimuli more potently in the arthritic pain state (n = 6) than under control conditions before arthritis (n = 8) AP5 also inhibited the enhanced background activity and increased responses to normally innocuous stimuli in arthritis, but had no significant effects on these parameters under control conditions. A selective non-NMDA receptor antagonist (NBQX) inhibited background activity and evoked responses under normal control conditions (n = 6) and in arthritis (n = 8) These data suggest that activation of both NMDA and non-NMDA receptors contributes to pain-related sensitization of amygdala neurons.

The amygdala and persistent pain

A reciprocal relationship exists between persistent pain and negative affective states such as fear, anxiety, and depression. Accumulating evidence points to the amygdala as an important site of such interaction. Whereas a key role of the amygdala in the neuronal mechanisms of emotionality and affective disorders has been well established, the concept of the amygdala as an important contributor to pain and its emotional component is still emerging. This article will review and discuss evidence from anatomical, neuroimaging, behavioral, electrophysiological, pharmacological, and biochemical data that implicate the amygdala in pain modulation and emotional responses to pain. The latero-capsular division of the central nucleus of the amygdala is now defined as the "nociceptive amygdala" and integrates nociceptive information with poly-modal information about the internal and external bodily environment. Dependent on environmental conditions and affective states, the amygdala appears to play a dual facilitatory and inhibitory role in the modulation of pain behavior and nociceptive processing at different levels of the pain neuraxis. Only recently, electrophysiological, pharmacological, and biochemical neuroplastic changes were shown in the nociceptive amygdala in persistent pain. It is conceivable, however, that amygdala plasticity plays an important role in emotional pain behavior and its modulation by affective state.

Enhanced group III mGluR-mediated inhibition of pain-related synaptic plasticity in the amygdala

Pain has a strong emotional component. A key player in emotionality, the amygdala is also involved in pain processing. Our previous studies showed synaptic plasticity in the central nucleus of the amygdala (CeA) in a model of arthritic pain. Here, we address the role of group III metabotropic glutamate receptors (mGluRs) in the regulation of synaptic transmission in CeA neurons. Whole-cell current- and voltage-clamp recordings were made from neurons in the latero-capsular part of the CeA in brain slices from control rats and arthritic rats (>6 h postinduction). The latero-capsular part of the CeA is the target of the spino-parabrachio-amygdaloid pain pathway and is now designated as the "nociceptive amygdala". Monosynaptic excitatory postsynaptic currents (EPSCs) were evoked by electrical stimulation of afferents from the pontine parabrachial (PB) area. LAP4 decreased the amplitude of EPSCs more potently in CeA neurons from arthritic rats (EC(50)=1.2 nM) than in control animals (EC(50)=11.5 nM). The inhibitory effect of LAP4 was reversed by a selective group III mGluR antagonist (UBP1112). During the application of LAP4, paired-pulse facilitation was increased, while no significant changes in slope conductance and action potential firing rate of CeA neurons were observed. These data suggest that presynaptic group III mGluRs are involved in the regulation of synaptic plasticity in the amygdala in an arthritis pain model.

Firing properties and connectivity of neurons in the rat lateral central nucleus of the amygdala

Using whole cell recordings from acute slices of the rat amygdala, we have examined the physiological properties of and synaptic connectivity to neurons in the lateral sector of the central amygdala (CeA). Based on their response to depolarizing current injections, CeA neurons could be divided into three types. Adapting neurons fired action potentials at the start of the current injections at high frequency and then showed complete spike-frequency adaptation with only six to seven action potentials evoked with suprathreshold current injections. Late-firing neurons fired action potentials with a prolonged delay at threshold but then discharged continuously with larger current injections. Repetitive firers discharged at the start of the current injection at threshold and then discharged continuously with larger current injections. All three cells showed prolonged afterhyperpolarizations (AHPs) that followed trains of action potentials. The AHP was longer lasting with a larger slow component in adapting neurons. The AHP in all cell types contained a fast component that was inhibited by the SK channel blocker UCL1848. The slow component, not blocked by UCL1848, was blocked by isoprenaline and was significantly larger in adapting neurons. Blockade of SK channels increased the discharge frequency in late firers and regular-spiking neurons but had no effect on adapting neurons. Blockade of the slow AHP with isoprenaline had no effect on any cell type. All cells received a mixed glutamatergic and GABAergic input from a medial pathway. Electrical stimulation of the lateral (LA) and basolateral (BLA) nuclei evoked a large monosynaptic glutamatergic response followed by a disynaptic inhibitory postsynaptic potential. Activation of neurons in the LA and BLA by puffer application of glutamate evoked a small monosynaptic response in 13 of 55 CeA neurons. Local application of glutamate to the CeL evoked a GABAergic response in all cells. These results show that at least three types of neurons are present in the CeA that can be distinguished on their firing properties. The firing frequency of two of these cell types is determined by activation of SK channels. Cells receive a small input from the LA and BLA but may receive inputs that course through these nuclei en route to the CeA.

Activation of presynaptic group I metabotropic glutamate receptors enhances glutamate release in the rat spinal cord substantia gelatinosa

The activation of group I metabotropic glutamate receptors (mGluRs) produces a long-term potentiation of sensory transmission in the substantia gelatinosa (SG) region of the spinal cord (Prog. Brain Res. 129 (2000) 115). The mechanism(s) responsible for the induction of this potentiation is not known. Using rat spinal cord slice preparation and patch-clamp recordings, here we show, that the activation of the group I mGluRs by (S)-3,5-dihydroxyphenylglycine (DHPG, 1 microM), the mGluR1/5 agonist, increased the frequency of both activity-dependent spontaneous EPSCs, and activity-independent miniature EPSCs (mEPSCs). However, DHPG did not affect amplitude of mEPSCs. The effects of DHPG were not seen in the presence of the preferential mGluR1 antagonist CPCCOEt (10 microM). On the other hand, 2-methyl-6-(phenylethynyl)-pyridine (10 microM), a selective mGluR5 antagonist, blocked the DHPG facilitation present during the wash-out of the drug. This novel facilitating effect of the group I mGluR activation on glutamate release is the first report of a direct facilitatory action of both mGluR1 and mGluR5 subtypes on sensory transmission in the spinal cord SG region. These results indicate the potential contribution of synaptic activation of these facilitatory autoreceptors in plasticity of primary afferent neurotransmission.

Synaptic plasticity in the amygdala in a visceral pain model in rats

The amygdala plays a key role in the emotional-affective component of pain. This study is the first to analyze synaptic plasticity in the central nucleus of the amygdala (CeA) in a model of visceral pain. Whole-cell patch-clamp recordings were made from neurons in the latero-capsular part of the CeA in brain slices from control rats and rats with zymosan-induced colitis (>6 h postinduction). Monosynaptic responses were evoked by electrical stimulation of afferents from the pontine parabrachial area (PB) and from the basolateral amygdala (BLA). Enhanced synaptic transmission was observed at the nociceptive PB-CeA synapse, but not at the polymodal BLA-CeA synapse, in rats with colitis. The frequency of action potentials evoked by direct current injection was increased in CeA neurons from colitis rats, suggesting enhanced neuronal excitability. Our results provide novel evidence for an important role of the CeA in visceral pain.

Central sensitization and LTP: do pain and memory share similar mechanisms?

Synaptic plasticity is fundamental to many neurobiological functions, including memory and pain. Central sensitization refers to the increased synaptic efficacy established in somatosensory neurons in the dorsal horn of the spinal cord following intense peripheral noxious stimuli, tissue injury or nerve damage. This heightened synaptic transmission leads to a reduction in pain threshold, an amplification of pain responses and a spread of pain sensitivity to non-injured areas. In the cortex, LTP - a long-lasting highly localized increase in synaptic strength - is a synaptic substrate for memory and learning. Analysis of the molecular mechanisms underlying the generation and maintenance of central sensitization and LTP indicates that, although there are differences between the synaptic plasticity contributing to memory and pain, there are also striking similarities.

Differential roles of mGluR1 and mGluR5 in brief and prolonged nociceptive processing in central amygdala neurons

The laterocapsular division of the central nucleus of the amygdala (CeA) is now defined as the "nociceptive amygdala" because of its high content of neurons that respond to painful stimuli. The majority of these neurons become sensitized in a model of arthritis pain. Here we address the role of G protein-coupled group I metabotropic glutamate receptor subtypes mGluR1 and mGluR5 in nociceptive processing under normal conditions and in pain-related sensitization. Extracellular single-unit recordings were made from 65 CeA neurons in anesthetized rats. Each neuron's responses to brief mechanical stimuli, background activity, receptive field size, and threshold were measured before and after induction of the kaolin/carrageenan mono-arthritis in one knee and before and during applications of agonists and antagonists into the CeA by microdialysis. All neurons received excitatory input from the knee(s) and responded most strongly to noxious stimuli. Before arthritis, a group I mGluR1 and mGluR5 agonist (DHPG, n = 10) potentiated the responses to innocuous and noxious stimuli. This effect was mimicked by an mGluR5 agonist (CHPG, n = 15). In the arthritis pain state (>6 h after induction), the facilitatory effects of DHPG (n = 9), but not CHPG (n = 7), increased. An mGluR1 antagonist (CPCCOEt) had no effect before arthritis (n = 12) but inhibited the responses of sensitized neurons in the arthritis pain state (n = 8). An mGluR5 antagonist (MPEP) inhibited brief nociceptive responses under normal conditions (n = 19) and prolonged nociception in arthritis (n = 8). These data suggest a change of mGluR1 function and activation in the amygdala in pain-related sensitization, whereas mGluR5 is involved in brief as well as prolonged nociception.

The challenge of chronic pain

Chronic pain is a complex problem with staggering negative health and economic consequences. The complexity of chronic pain is presented within Cervero and Laird's model that describes three phases of pain, including pain without tissue damage, pain with tissue damage and inflammation, and neuropathic pain. The increased afferent input in phases 2 and 3 of chronic pain produces marked changes in primary afferents, dorsal root ganglia, and spinal cord dorsal horn. These changes promote the symptoms of chronic pain, including spontaneous pain, hyperalgesia, and allodynia. Increased afferent input also evokes supraspinal input to the dorsal horn, including biphasic innervation from the ventromedial medulla and A7 catecholamine cell group, that promotes hyperalgesia and allodynia. More rostral brain structures, such as the lateral hypothalamus, amygdala, and hippocampus, may also play a role in chronic pain. Although much has been discovered about the multiple pathological mechanisms involved in chronic pain, further research is needed to fully comprehend these mechanisms.