Visceromotor and spinal neuronal responses to colorectal distension in rats with aldosterone onto the amygdala

Electrophysiology as a Tool to Decipher the Network Mechanism of Visceral Pain in Functional Gastrointestinal Disorders

Abdominal pain, including visceral pain, is prevalent in functional gastrointestinal (GI) disorders (FGIDs), affecting the overall quality of a patient's life. Neural circuits in the brain encode, store, and transfer pain information across brain regions. Ascending pain signals actively shape brain dynamics; in turn, the descending system responds to the pain through neuronal inhibition. Pain processing mechanisms in patients are currently mainly studied with neuroimaging techniques; however, these techniques have a relatively poor temporal resolution. A high temporal resolution method is warranted to decode the dynamics of the pain processing mechanisms. Here, we reviewed crucial brain regions that exhibited pain-modulatory effects in an ascending and descending manner. Moreover, we discussed a uniquely well-suited method, namely extracellular electrophysiology, that captures natural language from the brain with high spatiotemporal resolution. This approach allows parallel recording of large populations of neurons in interconnected brain areas and permits the monitoring of neuronal firing patterns and comparative characterization of the brain oscillations. In addition, we discussed the contribution of these oscillations to pain states. In summary, using innovative, state-of-the-art methods, the large-scale recordings of multiple neurons will guide us to better understanding of pain mechanisms in FGIDs.

Interactions between gut permeability and brain structure and function in health and irritable bowel syndrome

Changes in brain-gut interactions have been implicated in the pathophysiology of chronic visceral pain in irritable bowel syndrome (IBS). Different mechanisms of sensitization of visceral afferent pathways may contribute to the chronic visceral pain reports and associated brain changes that characterize IBS. They include increased gut permeability and gut associated immune system activation, and an imbalance in descending pain inhibitory and facilitatory mechanisms. In order to study the involvement of these mechanisms, correlations between gut epithelial permeability and live bacterial passage, and structural and functional brain connectivity were measured in women with moderate-to-severe IBS and healthy women. The relationships between gut permeability and functional and anatomical connectivity were significantly altered in IBS compared with the healthy women. IBS participants with lower epithelial permeability reported increased IBS symptoms, which was associated with increased functional and structural connectivity in endogenous pain facilitation regions. The findings suggest that relationships between gut permeability and the brain are significantly altered in IBS and suggest the existence of IBS subtypes based on these interactions.

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Brain-gut interactions are significantly altered in irritable bowel syndrome.

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Brain-gut interactions engage pain modulation brain regions in healthy women.

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Normal levels of gut permeability in IBS are linked with endogenous pain facilitation.

Lesions of the central amygdala and ventromedial medulla reduce bladder hypersensitivity produced by acute but not chronic foot shock

Both acute and chronic stress has been shown to exacerbate symptoms of chronic visceral pain conditions such as interstitial cystitis. Studies using animal models support these findings in that both acute and chronic exposure to foot shock-induced stress (FS) augment nociceptive reflex responses to urinary bladder distension (UBD). Only a few studies have examined the neural substrates mediating these phenomena and it is not clear whether acute and chronic stress engage the same or different substrates to produce bladder hypersensitivity. The present studies examined the role of two important central nervous system structures - the amygdala (AMG) and the ventromedial medulla (VMM) - in mediating/modulating hypersensitivity evoked by acute versus chronic FS using responses to graded UBD in adult, female Sprague-Dawley rats. Bladder hypersensitivity produced by acute FS was significantly reduced by either bilateral central AMG or VMM lesions using measures generated by graded UBD, but these lesions had no significant effects using the same measures on bladder hyperalgesia produced by chronic FS. Our findings provide evidence that neural substrates underlying bladder hypersensitivity produced by chronic stress differ from those produced by acute stress. These findings suggest that while the AMG and VMM participate in pain processing during periods of limited exposure to stress, prolonged stress may recruit a new set of neural substrates not initially activated by acute exposure to stress.

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.

5-HT2CR blockade in the amygdala conveys analgesic efficacy to SSRIs in a rat model of arthritis pain

BACKGROUND

Pain, including arthritic pain, has a negative affective component and is often associated with anxiety and depression. However, selective serotonin reuptake inhibitor antidepressants (SSRIs) show limited effectiveness in pain. The amygdala plays a key role in the emotional-affective component of pain, pain modulation and affective disorders. Neuroplasticity in the basolateral and central amygdala (BLA and CeA, respectively) correlate positively with pain behaviors. Evidence suggests that serotonin receptor subtype 5-HT2CR in the amygdala contributes critically to anxiogenic behavior and anxiety disorders. In this study, we tested the hypothesis that 5-HT2CR in the amygdala accounts for the limited effectiveness of SSRIs in reducing pain behaviors and that 5-HT2CR blockade in the amygdala renders SSRIs effective.

RESULTS

Nocifensive reflexes, vocalizations and anxiety-like behavior were measured in adult male Sprague-Dawley rats. Behavioral experiments were done in sham controls and in rats with arthritis induced by kaolin/carrageenan injections into one knee joint. Rats received a systemic (i.p.) administration of an SSRI (fluvoxamine, 30 mg/kg) or vehicle (sterile saline) and stereotaxic application of a selective 5-HT2CR antagonist (SB242084, 10 μM) or vehicle (ACSF) into BLA or CeA by microdialysis. Compared to shams, arthritic rats showed decreased hindlimb withdrawal thresholds (increased reflexes), increased duration of audible and ultrasonic vocalizations, and decreased open-arm choices in the elevated plus maze test suggesting anxiety-like behavior. Fluvoxamine (i.p.) or SB242084 (intra-BLA) alone had no significant effect, but their combination inhibited the pain-related increase of vocalizations and anxiety-like behavior without affecting spinal reflexes. SB242084 applied into the CeA in combination with systemic fluvoxamine had no effect on vocalizations and spinal reflexes.

CONCLUSIONS

The data suggest that 5-HT2CR in the amygdala, especially in the BLA, limits the effectiveness of SSRIs to inhibit pain-related emotional-affective behaviors.

Importance of stress receptor-mediated mechanisms in the amygdala on visceral pain perception in an intrinsically anxious rat

BACKGROUND

Stress worsens abdominal pain experienced by patients with irritable bowel syndrome (IBS), a chronic disorder of unknown origin with comorbid anxiety. Previously, we have demonstrated colonic hypersensitivity in Wistar-Kyoto rats (WKYs), a high-anxiety strain, which models abdominal pain in IBS. In low-anxiety rats, we have demonstrated that the central nucleus of the amygdala (CeA) regulates colonic hypersensitivity and anxiety induced by selective activation of either glucocorticoid receptors (GR) or mineralocorticoid receptors (MR), which is also mediated by the corticotropin releasing factor (CRF) Type-1 receptor. The goal of the present study was to test the hypothesis that the CeA through GR, MR, and/or CRF-1R regulates colonic hypersensitivity in WKYs.

METHODS

One series of WKYs had micropellets of a GR antagonist, an MR antagonist or cholesterol (control) stereotaxically implanted onto the CeA. Another series were infused in the CeA with CRF-1R antagonist, or vehicle. Colonic sensitivity was measured as a visceromotor response (VMR) to graded colorectal distension (CRD).

KEY RESULTS

The exaggerated VMR to graded CRD in WKYs was unaffected by GR or MR antagonism in the CeA. In contrast, direct CeA infusion of CRF-1R antagonist significantly inhibited the VMR to CRD at noxious distension pressures.

CONCLUSIONS & INFERENCES

Stress hormones in the CeA regulate colonic hypersensitivity in the rat through strain-dependent parallel pathways. The colonic hypersensitivity in WKYs is mediated by a CRF-1R mechanism in the CeA, independent of GR and MR. These complementary pathways suggest multiple etiologies whereby stress hormones in the CeA may regulate abdominal pain in IBS patients.

Lateralized amygdala activation: importance in the regulation of anxiety and pain behavior

BACKGROUND

The amygdala is involved in the emotional responses to fear including anxiety and heightened pain reporting. In a rodent model, bilateral activation of the central amygdala (CeA) with corticosterone (CORT) produces anxiety-like behavior, somatic allodynia and visceral hypersensitivity. Although hemisphere-specific processing differences between the left and right amygdala have been reported, it remains unclear whether the right or left CeA is involved in the production of anxiety-like behavior, and abnormal somatic and visceral perception. The goal of the present study was to investigate the hypothesis that lateralized corticoid-mediated mechanisms in the CeA produce anxiety as well as abnormal pain perception.

METHODS

Anesthetized rats received stereotaxic implants of cholesterol (Chol; 30 μg) or CORT (30 μg) micropellets onto the left, right or both dorsal margins of the CeA. Following implantation (5-7 days), anxiety-like behavior was assessed on the elevated plus-maze (EPM), somatic allodynia was measured using Von Frey filaments, and visceral sensitivity was quantified as a visceromotor response (VMR) to colorectal distention (CRD) at 0-60 mmHg.

RESULTS

Unilateral implants of CORT onto either the left or right CeA produced anxiety-like behavior and somatic allodynia. However, our data illustrated that the bilateral placement of CORT onto the CeA was required to increase visceral sensitivity.

CONCLUSION

These results provide evidence that there is no hemispheric lateralization of the CeA involved in corticoid-mediated anxiety-like behavior and heightened pain reporting.

Differential effects of mGluR7 and mGluR8 activation on pain-related synaptic activity in the amygdala

Pain-related plasticity in the laterocapsular division of the central nucleus of the amygdala (CeLC) depends on the activation of group I metabotropic glutamate receptors (mGluRs) whereas groups II and III mGluRs generally serve inhibitory functions. Recent evidence suggests differential roles of group III subtypes mGluR7 (pain enhancing) and mGluR8 (pain inhibiting) in the amygdala (Palazzo et al., 2008). Here we addressed the underlying synaptic mechanisms of mGluR7 and mGluR8 function in the CeLC under normal conditions and in an arthritis pain model. Using patch-clamp recordings in rat brain slices, we measured monosynaptic excitatory post-synaptic currents (EPSCs), mono- and polysynaptic inhibitory synaptic currents (IPSCs), and synaptically evoked action potentials (E-S coupling) in CeLC neurons. Synaptic responses were evoked by electrical stimulation in the basolateral amygdala (BLA). A selective mGluR8 agonist (DCPG) inhibited evoked EPSCs and synaptic spiking more potently in slices from arthritic rats than in slices from normal rats. In contrast, a selective mGluR7 agonist (AMN082) increased EPSCs and E-S coupling in slices from normal rats but not in the pain model. The effects of AMN082 and DCPG were blocked by a group III antagonist (MAP4). AMN082 increased frequency, but not amplitude, of spontaneous EPSCs but had no effect on miniature EPSCs (in TTX). DCPG decreased frequency, but not amplitude, of spontaneous and miniature EPSCs. The data suggest that mGluR8 acts presynaptically to inhibit excitatory transmission whereas the facilitatory effects of mGluR7 are indirect through action potential-dependent network action. AMN082 decreased evoked IPSCs and frequency, but not amplitude, of spontaneous and miniature IPSCs in slices from normal rats. DCPG had no effect on inhibitory transmission. The results suggest that presynaptic mGluR7 inhibits inhibitory synaptic transmission to gate glutamatergic transmission to CeLC neurons under normal conditions but not in pain. Presynaptic mGluR8 inhibits pain-related enhanced excitatory transmission in the CeLC.

Footshock stress differentially affects responses of two subpopulations of spinal dorsal horn neurons to urinary bladder distension in rats

This investigation examined the effect of footshock on responses of 283 spinal dorsal horn neurons (DHNs) to urinary bladder distension (UBD). Female rats were treated with seven daily sessions of footshock (chronic footshock, CFS), six accommodation sessions followed by one exposure to footshock (acute footshock, AFS) or handled similarly without receiving any footshock (no footshock, NFS). After the final footshock or NFS session, rats were anesthetized, a laminectomy performed and extracellular single-unit recordings of L6-S1 DHNs obtained in intact or spinalized preparations. Neurons were classified as Type I-inhibited by heterotopic noxious conditioning stimuli (HNCS) or as Type II-not inhibited by HNCS-and characterized for spontaneous activity and for neuronal discharges evoked by graded UBD. A differential effect of footshock-induced stress was noted on neuronal subgroups. In intact preparations, Type I neurons were less responsive to UBD after either chronic or acute stress, while Type II neurons demonstrated significantly augmented responses to UBD. This enhanced neuronal responsiveness to UBD was present in spinalized preparations following exposure to CFS but not AFS. Type I neurons were still less responsive to stress in spinalized preparations following CFS and AFS. This study provides further evidence that (1) at least two populations of spinal neurons exist which encode for visceral stimuli and are likely to have distinct roles in visceral nociception, and that (2) the chronic stress-induced enhancement of DHN responses to UBD involves changes at the spinal level while the acute stress effects are dependent on a supraspinal substrate.

Reactive oxygen species are involved in group I mGluR-mediated facilitation of nociceptive processing in amygdala neurons

Recent biochemical and behavioral data implicate reactive oxygen species (ROS) in peripheral and spinal pain mechanisms. However, pain-related functions of ROS in the brain and mechanisms of pain-related ROS activation remain to be determined. Our previous studies showed that the amygdala plays a key role in emotional-affective pain responses and pain modulation. Hyperactivity of amygdala neurons in an animal pain model depends on group I metabotropic glutamate receptors (subtypes mGluR1 and mGluR5), but their signaling pathway remains to be determined. Here we tested the hypothesis that activation of group I mGluRs increases nociceptive processing in amygdala neurons through a mechanism that involves ROS. Extracellular single-unit recordings were made from neurons in the laterocapsular division of the central nucleus of the amygdala (CeLC) in anesthetized adult male rats. Administration of a group I mGluR agonist (DHPG) into the CeLC by microdialysis increased the responses to innocuous and noxious somatosensory (knee joint compression) and visceral (colorectal distention [CRD]) stimuli. A ROS scavenger (PBN) and a superoxide dismutase mimetic (TEMPOL) reversed the facilitatory effects of DHPG. An mGluR5 antagonist (MPEP) also inhibited the effects of DHPG on the responses to innocuous and noxious somatosensory and visceral stimuli, whereas an mGluR1 antagonist (LY367385) decreased only the responses to visceral stimulation. The results show for the first time that ROS mediate group I mGluR-induced facilitation of nociceptive processing in amygdala neurons. The antagonist data may suggest differential contributions of subtypes mGluR1 and mGluR5 to the processing of somatosensory and visceral nociceptive information in the amygdala.

Exposure of the amygdala to elevated levels of corticosterone alters colonic motility in response to acute psychological stress

The amygdala is important for integrating the emotional, endocrine and autonomic responses to stress. Exposure of the amygdala to elevated levels of corticosterone (CORT) induces anxiety-like behavior and a hypersensitive colon in rodents; however, effects on colonic transit are unknown. Micropellets releasing CORT alone or combined with a selective glucocorticoid (GR) or mineralocorticoid (MR) receptor antagonist were implanted bilaterally at the dorsal boundary of the central amygdala in male rats. Inactive cholesterol implants served as controls. Seven days later, rats received water avoidance stress (WAS) for 1 h and the fecal pellet output was measured. Colorectal transit was also evaluated following the stressor by recording the time for expulsion of a glass bead placed into the colorectum. Plasma CORT levels were evaluated before WAS, after 60 min of WAS and 90 min post-WAS. Exposure of the amygdala to elevated CORT did not alter daily fecal pellet production or the number of fecal pellets released in response to WAS. However, following WAS, rats with CORT implants on the amygdala showed a delay in colorectal transit compared to cholesterol-implanted controls. Plasma CORT measurements showed that basal and WAS-induced increases in plasma CORT were similar in all groups but a prolonged increase in plasma CORT was observed 90 after cessation of WAS in rats with CORT implants. The post-WAS changes in colonic motility and plasma CORT were prevented by antagonism of GR or MR in the amygdala, suggesting their importance in driving stress-associated changes in colonic motility.

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.

Mineralocorticoid and glucocorticoid receptors in the amygdala regulate distinct responses to colorectal distension

Previously we found that exposure of the amygdala to elevated levels of corticosterone (CORT) induces anxiety-like behavior coupled to colonic hypersensitivity to distension, however, the specific corticoid receptor mediating the CORT responses remains controversial. In this study we investigated, through the use of selective antagonists, the relative role of amygdaloid mineralocorticoid (MR) versus glucocorticoid receptors (GR) in CORT-mediated spinal and cardiovascular pseudoaffective reflex responses to colorectal distension (CRD). Micropellets containing, CORT and a selective MR antagonist (spironolactone) or GR antagonist (mifepristone) were implanted stereotaxically onto the dorsal margin of the amygdala in rats. On day 7 post-implantation in response to graded CRD we measured: (i) changes in the electrical activity of dorsal horn neurons in the L6-S1 spinal cord and (ii) the cardiovascular depressor responses. Exposure of the amygdala to CORT-releasing micropellets increased the proportion of spinal neurons showing high-threshold and/or long-lasting responses and potentiated the magnitude of excitation. Elevated levels of amygdala CORT also increased the magnitude of the cardiovascular depressor response to CRD. MR but not GR antagonism prevented the increase in spinal cord neuronal excitation, whereas either the MR or GR antagonist decreased the magnitude of the depressor cardiovascular response to CRD. We conclude that MR in the amygdala trigger descending pathways facilitating viscero-nociceptive processing in the spinal cord, whereas MR and GR have a non-redundant role in CORT-induced potentiation of the autonomic pseudoaffective responses to colorectal stimuli.

Regional brain activation in conscious, nonrestrained rats in response to noxious visceral stimulation

Preclinical drug development for visceral pain has largely relied on quantifying pseudoaffective responses to colorectal distension (CRD) in restrained rodents. However, the predictive value of changes in simple reflex responses in rodents for the complex human pain experience is not known. Male rats were implanted with venous cannulas and with telemetry transmitters for abdominal electromyographic (EMG) recordings. [(14)C]-iodoantipyrine was injected during noxious CRD (60 mmHg) in the awake, nonrestrained animal. Regional cerebral blood flow (rCBF)-related tissue radioactivity was quantified by autoradiography and analyzed in the three-dimensionally reconstructed brain by statistical parametric mapping. 60-mmHg CRD, compared with controls (0 mmHg) evoked significant increases in EMG activity (267+/-24% vs. 103+/-8%), as well as in behavioral pain score (77+/-6% vs. 3+/-3%). CRD elicited significant increases in rCBF as expected in sensory (insula, somatosensory cortex), and limbic and paralimbic regions (including anterior cingulate cortex and amygdala). Significant decreases in rCBF were seen in the thalamus, parabrachial nucleus, periaqueductal gray, hypothalamus and pons. Correlations of rCBF with EMG and with behavioral pain score were noted in the cingulate, insula, lateral amygdala, dorsal striatum, somatosensory and motor regions. Our findings support the validity of measurements of cerebral perfusion during CRD in the freely moving rat as a model of functional brain changes in human visceral pain. However, not all regions demonstrating significant group differences correlated with EMG or behavioral measures. This suggests that functional brain imaging captures more extensive responses of the central nervous system to noxious visceral distension than those identified by traditional measures.

Pro- and anti-nociceptive effects of corticotropin-releasing factor (CRF) in central amygdala neurons are mediated through different receptors

Corticotropin-releasing factor (CRF) is not only a stress hormone but also acts as a neuromodulator outside the hypothalamic-pituitary-adrenocortical axis, playing an important role in anxiety, depression, and pain modulation. The underlying mechanisms remain to be determined. A major site of extra-hypothalamic expression of CRF and its receptors is the amygdala, a key player in affect-related disorders such as anxiety. The latero-capsular division of the central nucleus of the amygdala (CeLC) is also important for pain modulation and pain affect. This study analyzed the effects of CRF on nociceptive processing in CeLC neurons and the contribution of CRF1 and CRF2 receptors and protein kinases A and C. Extracellular single-unit recordings were made from CeLC neurons in anesthetized adult rats. All neurons responded more strongly to noxious than innocuous mechanical stimulation of the knee. Evoked responses and background activity were measured before and during administration of CRF into the CeLC by microdialysis. CRF was administered alone or together with receptor antagonists or protein kinase inhibitors. CRF (0.01-1 microM; concentrations in microdialysis probe; 15 min) facilitated the evoked responses more strongly than background activity; a higher concentration (10 microM) had inhibitory effects. Facilitation by CRF (0.1 microM) was reversed by a selective CRF1 receptor antagonist (NBI27914, 10 microM) but not a CRF2 receptor antagonist (astressin-2B, 100 microM) and by a protein kinase A (PKA) inhibitor (KT5720, 100 microM) but not a protein kinase C inhibitor (GF109203X, 100 microM). Inhibitory effects of CRF (10 microM) were reversed by astressin-2B. These data suggest that CRF has dual effects on amygdala neurons: CRF1 receptor-mediated PKA-dependent facilitation and CRF2 receptor-mediated inhibition.

Neural mechanisms of pelvic organ cross-sensitization

Clinical observations of viscerovisceral referred pain in patients with gastrointestinal and genitourinary disorders suggest an overlap of neurohumoral mechanisms underlying both bowel and urinary bladder dysfunctions. Close proximity of visceral organs within the abdominal cavity complicates identification of the exact source of chronic pelvic pain, where it originates, and how it relocates with time. Cross-sensitization among pelvic structures may contribute to chronic pelvic pain of unknown etiology and involves convergent neural pathways of noxious stimulus transmission from two or more organs. Convergence of sensory information from discrete pelvic structures occurs at different levels of nervous system hierarchy including dorsal root ganglia, the spinal cord and the brain. The cell bodies of sensory neurons projecting to the colon, urinary bladder and male/female reproductive organs express a wide range of membrane receptors and synthesize many neurotransmitters and regulatory peptides. These substances are released from nerve terminals following enhanced neuronal excitability and may lead to the occurrence of neurogenic inflammation in the pelvis. Multiple factors including inflammation, nerve injury, ischemia, peripheral hyperalgesia, metabolic disorders and other pathological conditions dramatically alter the function of directly affected pelvic structures as well as organs located next to a damaged domain. Defining precise mechanisms of viscerovisceral cross-sensitization would have implications for the development of effective pharmacological therapies for the treatment of functional disorders with chronic pelvic pain such as irritable bowel syndrome and painful bladder syndrome. The complexity of overlapping neural pathways and possible mechanisms underlying pelvic organ crosstalk are analyzed in this review at both systemic and cellular levels.

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.

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.

Spinal neuronal responses to urinary bladder stimulation in rats with corticosterone or aldosterone onto the amygdala

Elevating glucocorticoids in the amygdala produces colorectal hypersensitivity through activation of lumbosacral spinal neurons. The aim of this study was to determine if descending modulation from the amygdala affects spinal processing of input from urinary bladder afferents. Fischer-344 rats received cholesterol (inactive control)-, corticosterone-, or aldosterone-containing micropellets placed stereotaxically on the dorsal margin of the left and right amygdala (n = 10 for each group). Seven days after amygdaloid implantation, extracellular potentials of single L6-S1 spinal neurons were examined for the responses to graded (0.5-2.0 ml, 20 s) urinary bladder distension (UBD). Spontaneous activity of neurons with excitatory responses to UBD in aldosterone-implanted rats [11.0 +/- 1.7 (SE) imp/s], but not in corticosterone-implanted rats, was higher than in the cholesterol-implanted group (6.6 +/- 1.1 imp/s, P < 0.05). Noxious UBD (1.5 ml) produced a greater excitatory response (21.6 +/- 2.6 imp/s) in aldosterone-implanted rats compared with cholesterol- or corticosterone-implanted rats (15.1 +/- 1.5 and 13.6 +/- 1.4 imp/s; P < 0.05). In contrast, the duration of excitatory responses to UBD in corticosterone-implanted rats (38.5 +/- 3.4 imp/s) was significantly longer than those in the aldosterone or control groups (26.8 +/- 1.8 and 24.7 +/- 1.5 imp/s). Neurons with low thresholds for excitatory responses to UBD were seen more frequently in aldosterone-implanted rats than in corticosterone or cholesterol treated rats (74 vs. 44% and 39%, P < 0.05). No difference in somatic field properties of spinal neurons responsive or nonresponsive to UBD was found among the three groups. These findings suggest that both mineralocorticoid- and glucocorticoid-mediated mechanisms in the amygdala are involved in descending modulation to lumbosacral spinal neurons receiving inputs from the urinary bladder; and this mechanism may play a role in the activation and maintenance of primary central sensitization to noxious visceral stimuli.