Involvement of the spino-parabrachio -amygdaloid and -hypothalamic pathways in the autonomic and affective emotional aspects of pain

Activation of CGRP neurons in the parabrachial nucleus suppresses addictive behavior

Punishment such as electric shock or physical discipline employs a mixture of physical pain and emotional distress to induce behavior modification. However, a neural circuit that produces behavior modification by selectively focusing the emotional component, while bypassing the pain typically induced by peripheral nociceptor activation, is not well studied. Here, we show that genetically silencing the activity of neurons expressing calcitonin gene-related peptide (CGRP) in the parabrachial nucleus blocks the suppression of addictive-like behavior induced by footshock. Furthermore, activating CGRP neurons suppresses not only addictive behavior induced by self-stimulating dopamine neurons but also behavior resulting from self-administering cocaine, without eliciting nocifensive reactions. Moreover, among multiple downstream targets of CGRP neurons, terminal activation of CGRP in the central amygdala is effective, mimicking the results of cell body stimulation. Our results indicate that unlike conventional electric footshock, stimulation of CGRP neurons does not activate peripheral nociceptors but effectively curb addictive behavior.

Structural and Physiological Modeling (SAPM) for the Analysis of Functional MRI Data Applied to a Study of Human Nociceptive Processing

A novel method has been developed for analyzing connectivity between regions based on functional magnetic resonance imaging (fMRI) data. This method, termed structural and physiological modeling (SAPM), combines information about blood oxygenation-level dependent (BOLD) responses, anatomy, and physiology to model coordinated signaling across networks of regions, including input and output signaling from each region and whether signaling is predominantly inhibitory or excitatory. The present study builds on a prior proof-of-concept demonstration of the SAPM method by providing evidence for the choice of network model and anatomical sub-regions, demonstrating the reproducibility of the results and identifying statistical thresholds needed to infer significance. The method is further validated by applying it to investigate human nociceptive processing in the brainstem and spinal cord and comparing the results to the known neuroanatomy, including anatomical regions and inhibitory and excitatory signaling. The results of this analysis demonstrate that it is possible to obtain reliable information about input and output signaling from anatomical regions and to identify whether this signaling has predominantly inhibitory or excitatory effects. SAPM provides much more detailed information about neuroanatomy than was previously possible based on fMRI data.

Fifty years of pain research and clinical advances: highlights and key trends

ABSTRACT

This article highlights advances in basic science preclinical pain research, clinical research, and psychological research occurring over the 50 years since the International Association for the Study of Pain was founded. It presents important findings and key trends in these 3 areas of pain science: basic science preclinical research, clinical research, and psychological research.

Stimulation of lateral parabrachial (LPB) to central amygdala (CeA) pituitary adenylate cyclase-activating polypeptide (PACAP) neurons induces anxiety-like behavior and mechanical allodynia

BACKGROUND

Anxiety disorders are the most prevalent psychiatric disorders, and they are highly comorbid with chronic pain conditions. The central nucleus of the amygdala (CeA) is known not only for its role in the regulation of anxiety but also as an important site for the negative affective dimension of pain. Pituitary adenylate cyclase activating polypeptide (PACAP), a neuropeptide whose terminals are abundant in the CeA, is strongly implicated in the stress response as well as in pain processing. Here, using Cre-dependent viral vectors, we explored in greater detail the role of the PACAP projection to the CeA that originates in the lateral parabrachial nucleus (LPB).

METHODS

We first performed a circuit mapping experiment by injecting an anterograde Cre-dependent virus expressing a fluorescent reporter in the LPB of PACAP-Cre mice and observing their projections. Then, we used a chemogenetic approach (a Cre-dependent Designer Receptors Activated by Designer Drugs, DREADDs) to assess the effects of the direct stimulation of the PACAP LPB to CeA projection on general locomotor activity, anxiety-like behavior (using a defensive withdrawal test), and mechanical pain sensitivity (using the von Frey test).

RESULTS

We found that the CeA, together with other areas, is one of the major downstream projection targets of PACAP neurons originating in the lateral parabrachial nucleus (LPB). In the DREADD experiment, we then found that the selective activation of this neuronal pathway is sufficient to increase both anxiety-like behavior and mechanical pain sensitivity in mice, without affecting general locomotor activity.

CONCLUSION

In conclusion, our data suggest that the dysregulation of this circuit may contribute to a variety of anxiety disorders and chronic pain states, and that PACAP may represent an important therapeutic target for the treatment of these conditions.

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.

Medication overuse headache and substance use disorder: A comparison based on basic research and neuroimaging

It has yet to be determined whether medication overuse headache (MOH) is an independent disorder or a combination of primary headache and substance addiction. To further explore the causes of MOH, we compared MOH with substance use disorder (SUD) in terms of the brain regions involved to draw more targeted conclusions. In this review, we selected alcohol use disorder (AUD) as a representative SUD and compared MOH and AUD from two aspects of neuroimaging and basic research. We found that in neuroimaging studies, there were many overlaps between AUD and MOH in the reward circuit, but the extensive cerebral cortex damage in AUD was more serious than that in MOH. This difference was considered to reflect the sensitivity of the cortex structure to alcohol damage. In future research, we will focus on the central amygdala (CeA), prefrontal cortex (PFC), orbital-frontal cortex (OFC), hippocampus, and other brain regions for interventions, which may have unexpected benefits for addiction and headache symptoms in MOH patients.

Somatotopy of Mouse Spinothalamic Innervation and the Localization of a Noxious Stimulus Requires Deleted in Colorectal Carcinoma Expression by Phox2a Neurons

Anterolateral system (AS) neurons transmit pain signals from the spinal cord to the brain. Their morphology, anatomy, and physiological properties have been extensively characterized and suggest that specific AS neurons and their brain targets are concerned with the discriminatory aspects of noxious stimuli, such as their location or intensity, and their motivational/emotive dimension. Among the recently unraveled molecular markers of AS neurons is the developmentally expressed transcription factor Phox2a, providing us with the opportunity to selectively disrupt the embryonic wiring of AS neurons to gain insights into the logic of their adult function. As mice with a spinal-cord-specific loss of the netrin-1 receptor deleted in colorectal carcinoma (DCC) have increased AS neuron innervation of ipsilateral brain targets and defective noxious stimulus localization or topognosis, we generated mice of either sex carrying a deletion of Dcc in Phox2a neurons. Such DccPhox2a mice displayed impaired topognosis along the rostrocaudal axis but with little effect on left-right discrimination and normal aversive responses. Anatomical tracing experiments in DccPhox2a mice revealed defective targeting of cervical and lumbar AS axons within the thalamus. Furthermore, genetic labeling of AS axons revealed their expression of DCC on their arrival in the brain, at a time when many of their target neurons are being born and express Ntn1 Our experiments suggest a postcommissural crossing function for netrin-1:DCC signaling during the formation of somatotopically ordered maps and are consistent with a discriminatory function of some of the Phox2a AS neurons.SIGNIFICANCE STATEMENT How nociceptive (pain) signals are relayed from the body to the brain remains an important question relevant to our understanding of the basic physiology of pain perception. Previous studies have demonstrated that the AS is a main effector of this function. It is composed of AS neurons located in the spinal cord that receive signals from nociceptive sensory neurons that detect noxious stimuli. In this study, we generate a genetic miswiring of mouse AS neurons that results in a decreased ability to perceive the location of a painful stimulus. The precise nature of this defect sheds light on the function of different kinds of AS neurons and how pain information may be organized.

Epigenetic modulation of visceral nociception

Epigenetics is a process that alters gene activity or phenotype without any changes in the underlying DNA sequence or genotype. These biological changes may have deleterious effects and can lead to various human diseases. Ongoing research is continuing to illuminate the role of epigenetics in a variety of pathophysiologic processes. Several categories of epigenetic mechanisms have been studied including chromatin remodeling, DNA methylation, histone modification, and non-coding RNA mechanisms. These epigenetic changes can have a long-term effect on gene expression without any underlying changes in the DNA sequences. The underlying pathophysiology of disorders of brain-gut interaction and stress-induced visceral pain are not fully understood and the role of epigenetic mechanisms in these disorders are starting to be better understood. Current work is underway to determine how epigenetics plays a role in the neurobiology of patients with chronic visceral pain and heightened visceral nociception. More recently, both animal models and human studies have shown how epigenetic regulation modulates stress-induced visceral pain. While much more work is needed to fully delineate the mechanistic role of epigenetics in the neurobiology of chronic visceral nociception, the current study by Louwies et al., in Neurogastroenterology and Motility provides additional evidence supporting the involvement of epigenetic alterations in the central nucleus of the amygdala in stress-induced visceral hypersensitivity in rodents.

Topographic organization underlies intrinsic and morphological heterogeneity of central amygdala neurons expressing corticotropin-releasing hormone

The central nucleus of the amygdala (CeA) network consists of a heterogeneous population of inhibitory GABAergic neurons distributed across distinct subregions. While the specific roles for molecularly defined CeA neurons have been extensively studied, our understanding of functional heterogeneity within classes of molecularly distinct CeA neurons remains incomplete. In addition, manipulation of genetically defined CeA neurons has produced inconsistent behavioral results potentially due to broad targeting across CeA subregions. Therefore, elucidating heterogeneity within molecularly defined neurons in subdivisions of the CeA is pivotal for gaining a complete understanding of how CeA circuits function. Here, we used a multifaceted approach involving transgenic reporter mice, brain slice electrophysiology, and neuronal morphology to dissect the heterogeneity of corticotropin‐releasing hormone (CRH) neurons in topographically distinct subregions of the CeA. Our results revealed that intrinsic and morphological properties of CRH‐expressing (CRH+) neurons in the lateral (CeL) and medial (CeM) subdivisions of the CeA were significantly different. We found that CeL‐CRH+ neurons are relatively homogeneous in morphology and firing profile. Conversely, CeM‐CRH+ neurons displayed heterogeneous electrophysiological and morphological phenotypes. Overall, these results show phenotypic differences between CRH+ neurons in CeL and CeM.

A multistudy analysis reveals that evoked pain intensity representation is distributed across brain systems

Information is coded in the brain at multiple anatomical scales: locally, distributed across regions and networks, and globally. For pain, the scale of representation has not been formally tested, and quantitative comparisons of pain representations across regions and networks are lacking. In this multistudy analysis of 376 participants across 11 studies, we compared multivariate predictive models to investigate the spatial scale and location of evoked heat pain intensity representation. We compared models based on (a) a single most pain-predictive region or resting-state network; (b) pain-associated cortical-subcortical systems developed from prior literature ("multisystem models"); and (c) a model spanning the full brain. We estimated model accuracy using leave-one-study-out cross-validation (CV; 7 studies) and subsequently validated in 4 independent holdout studies. All spatial scales conveyed information about pain intensity, but distributed, multisystem models predicted pain 20% more accurately than any individual region or network and were more generalizable to multimodal pain (thermal, visceral, and mechanical) and specific to pain. Full brain models showed no predictive advantage over multisystem models. These findings show that multiple cortical and subcortical systems are needed to decode pain intensity, especially heat pain, and that representation of pain experience may not be circumscribed by any elementary region or canonical network. Finally, the learner generalization methods we employ provide a blueprint for evaluating the spatial scale of information in other domains.

An Investigation of Descending Pain Modulation in Women With Provoked Vestibulodynia: Alterations of Brain Connectivity

Provoked Vestibulodynia (PVD) is the most common vulvodynia subtype (idiopathic chronic vulvar pain). Functional magnetic resonance imaging (fMRI) studies indicate that women with PVD exhibit altered function in a number of pain modulatory regions in response to noxious stimulation, such as in the secondary somatosensory cortex, insula, dorsal midcingulate, posterior cingulate, and thalamus. However, previous neuroimaging studies of PVD have not examined periods of time before and after noxious stimulation or investigated functional connectivity among pain modulatory regions. Fourteen women with PVD and 14 matched Control participants underwent five fMRI runs with no painful stimuli interleaved randomly with five runs with calibrated, moderately painful heat stimuli applied to the thenar eminence. As recent findings indicate that pain processing begins before and continues after painful stimulation, 2-min periods were included in each run before and after the stimulus. Functional brain connectivity was assessed during both trials of Pain and No Pain stimulation for each group using structural equation modeling (SEM). Analyses of variance (ANOVAs) on connectivity values demonstrated significant main effects of study condition, and group, for connectivity among pain modulatory regions. Most of the differences between the Pain and No Pain conditions found only in the PVD group take place before (i.e., thalamus to INS, ACC to S1, thalamus to S1, and thalamus to S2) and after pain stimulation (i.e., INS to amygdala, PPC to S1, and thalamus to S2). Such differences were not observed in the Control group. These findings further support previous results indicating that women with PVD have altered pain processing compared to pain-free women.

Chronic Pain in the Elderly: Mechanisms and Perspectives

Chronic pain affects a large part of the population causing functional disability, being often associated with coexisting psychological disorders, such as depression and anxiety, besides cognitive deficits, and sleep disturbance. The world elderly population has been growing over the last decades and the negative consequences of chronic pain for these individuals represent a current clinical challenge. The main painful complaints in the elderly are related to neurodegenerative and musculoskeletal conditions, peripheral vascular diseases, arthritis, and osteoarthritis, contributing toward poorly life quality, social isolation, impaired physical activity, and dependence to carry out daily activities. Organ dysfunction and other existing diseases can significantly affect the perception and responses to chronic pain in this group. It has been proposed that elderly people have an altered pain experience, with changes in pain processing mechanisms, which might be associated with the degeneration of circuits that modulate the descending inhibitory pathways of pain. Aging has also been linked to an increase in the pain threshold, a decline of painful sensations, and a decrease in pain tolerance. Still, elderly patients with chronic pain show an increased risk for dementia and cognitive impairment. The present review article is aimed to provide the state-of-art of pre-clinical and clinical research about chronic pain in elderly, emphasizing the altered mechanisms, comorbidities, challenges, and potential therapeutic alternatives.

Dim the Lights: A Narrative Review of Photophobia in Migraine

A preference for darkness is one of the main associated features in people with migraine, the cause remaining a mystery until some decades ago. In this article, we describe the epidemiology of photophobia in migraine and explain the pathophysiological mechanisms following an anatomical structure. In addition, we review the current management of migraine and photophobia. Ongoing characterization of patients with photophobia and its different manifestations continues to increase our understanding of the intricate pathophysiology of migraine and vice versa. Detailed phenotyping of the patient with photophobia is encouraged.

Glutamatergic Neurons in the Amygdala Are Involved in Paclitaxel-Induced Pain and Anxiety

Paclitaxel is widely used as a first-line chemotherapy agent to treat malignant tumors. However, paclitaxel causes peripheral nerve fiber damage and neuropathic pain in some patients. In addition, patients received paclitaxel chemotherapy are often accompanied by negative emotions such as anxiety. The amygdala is critically involved in regulating pain signals, as well as anxiety. The purpose of this study is to clarify the role of Ca²⁺/calmodulin-dependent protein kinase II (CaMKII)-positive glutamatergic neurons in the amygdala in paclitaxel-induced pain and negative affective symptoms. Intraperitoneal injection of paclitaxel into mice caused mechanical and thermal allodynia, as measured by Von Frey test and Hargreaves test, and anxiety, as measured by open field test and elevated plus maze test. Immunofluorescence staining revealed that c-fos-positive neurons were significantly more in the basolateral amygdala (BLA) and central amygdala (CeA) in paclitaxel-treated mice than untreated mice. Furthermore, part of c-fos-positive neurons in the BLA were immunoreactive of CaMKII. Engineered Designer receptors exclusively activated by designer drugs (DREADD) receptor hM4Di or hM3Dq was selectively expressed on CaMKII neurons by injection of adeno-associated virus (AAV) vectors containing CaMKII and hM4Di or hM3Dq. Administration of DREADD agonist CNO to selectively inhibit the CaMKII neurons in the BLA significantly increased the paw withdrawal thresholds and paw withdrawal latencies. In addition, selectively inhibition of CaMKII neurons in the BLA alleviated anxiety behavior without affecting the motor activity. In summary, our findings suggest that CaMKII neurons in the amygdala are critical for neuropathic pain and anxiety behaviors induced by paclitaxel chemotherapy.

Early-life exposure to selective serotonin reuptake inhibitors: Long-term effects on pain and affective comorbidities

A growing body of evidence indicates that early‐life exposure to selective serotonin reuptake inhibitor has long‐term consequences on the offspring's pain in addition to affective disorders like anxiety disorder and major depression. Serotonin, besides its role in regulating pain and emotions, promotes neuronal network formation. The prefrontal cortex and the amygdala are two key brain regions involved in the modulation of pain and its affective comorbidities. Thus, the aim of this review is to understand how early‐life selective serotonin reuptake inhibitor exposure alters the developing prefrontal cortex and amygdala and thereby underlies the long‐term changes in pain and its affective comorbidities in later life. While there is still limited data on the effects of early‐life selective serotonin reuptake inhibitor exposure on pain, there is a substantial body of evidence on its affective comorbidities. From this perspective paper, four conclusions emerged. First, early‐life selective serotonin reuptake inhibitor exposure results in long‐term nociceptive effects, which needs to be consistently studied to clarify. Second, it results in enhanced depressive‐like behaviour and diminished exploratory behaviour in adult rodents. Third, early‐life selective serotonin reuptake inhibitor exposure alters serotonergic levels, transcription factors expression, and brain‐derived neurotrophic factor levels, resulting in hyperconnectivity within the amygdala and the prefrontal cortex. Finally, it affects antinociceptive inputs of the prefrontal cortex and the amygdala in the spinal cord. We conclude that early‐life selective serotonin reuptake inhibitor exposure affects the maturation of prefrontal cortex and amygdala circuits and thereby enhances their antinociceptive inputs in the spinal cord.

Activation of parabrachial nucleus - ventral tegmental area pathway underlies the comorbid depression in chronic neuropathic pain in mice

Depression symptoms are often found in patients suffering from chronic pain, a phenomenon that is yet to be understood mechanistically. Here, we systematically investigate the cellular mechanisms and circuits underlying the chronic-pain-induced depression behavior. We show that the development of chronic pain is accompanied by depressive-like behaviors in a mouse model of trigeminal neuralgia. In parallel, we observe increased activity of the dopaminergic (DA) neuron in the midbrain ventral tegmental area (VTA), and inhibition of this elevated VTA DA neuron activity reverses the behavioral manifestations of depression. Further studies establish a pathway of glutamatergic projections from the spinal trigeminal subnucleus caudalis (Sp5C) to the lateral parabrachial nucleus (LPBN) and then to the VTA. These glutamatergic projections form a direct circuit that controls the development of the depression-like behavior under the state of the chronic neuropathic pain.

Pain modulates dopamine neurons via a spinal-parabrachial-mesencephalic circuit

Pain decreases the activity of many ventral tegmental area (VTA) dopamine (DA) neurons, yet the underlying neural circuitry connecting nociception and the DA system is not understood. Here we show that a subpopulation of lateral parabrachial (LPB) neurons is critical for relaying nociceptive signals from the spinal cord to the substantia nigra pars reticulata (SNR). SNR-projecting LPB neurons are activated by noxious stimuli and silencing them blocks pain responses in two different models of pain. LPB-targeted and nociception-recipient SNR neurons regulate VTA DA activity directly through feed-forward inhibition and indirectly by inhibiting a distinct subpopulation of VTA-projecting LPB neurons thereby reducing excitatory drive onto VTA DA neurons. Correspondingly, ablation of SNR-projecting LPB neurons is sufficient to reduce pain-mediated inhibition of DA release in vivo. The identification of a neural circuit conveying nociceptive input to DA neurons is critical to our understanding of how pain influences learning and behavior.

Role of the central amygdala in acupuncture inhibition of methamphetamine-induced behaviors in rats

Methamphetamine (METH) enhances dopamine (DA) transmission in the mesolimbic system implicated in its reinforcing effects. Our previous studies have shown that acupuncture attenuates drug-seeking behaviors by modulating GABAergic transmission in the ventral tegmental area and DA release in the nucleus accumbens (NAc) of the striatum. The effects of acupuncture on METH-induced behaviors and its mediation by neural pathways remain a relatively understudied area of research. The central amygdala (CeA) plays a critical role in physiological and behavioral responses to somatosensory and drug stimuli and has been implicated in negative reinforcement. Thus, we evaluated the role of the CeA in acupuncture effects on locomotor activity, positive affective states, and DA release in the NAc following acute administration of METH. Acupuncture at acupoint HT7 reduced locomotor activity, 50-kHz ultrasonic vocalizations (USVs), and NAc DA release following systemic injection of METH, which was prevented by electrolytic lesions or optogenetic inhibition of the CeA. Acupuncture alone excited CeA neurons and reversed the suppression of CeA neurons induced by METH. These results suggest that acupuncture can relieve psychomotor responses and positive affective states following METH by inhibiting NAc DA release and this effect is mediated by activation of CeA neurons.

Abnormal Spontaneous Brain Activities of Limbic-Cortical Circuits in Patients With Dry Eye Disease

Whether brain function is altered in patients with dry eye disease (DED) remains unclear. Twenty patients with DED and 23 healthy controls (HCs) were scanned using resting-state functional magnetic resonance imaging. Regional homogeneity (ReHo) and support vector machine (SVM) were used to analyze the imaging data. Relative to the HCs, the patients with DED showed significantly increased ReHo values in the left inferior occipital gyrus (IOG), left superior temporal gyrus, and right superior medial prefrontal cortex, and significantly decreased ReHo values in the right superior frontal gyrus/middle frontal gyrus and bilateral middle cingulum (MC). SVM results indicated that the combination of ReHo values in the left MC and the left IOG in distinguishing patients with DED from HCs had a sensitivity of 95.00%, a specificity of 91.30%, and an accuracy of 93.02%. The present study found that the patients with DED had abnormal ReHo values in the limbic-cortical circuits. A combination of ReHo values in the left MC and the left IOG could be applied as a potential imaging biomarker to distinguish patients with DED from HCs. The dysfunction of limbic-cortical circuits may play an important role in the pathophysiology of DED.

Parabrachial nucleus circuit governs neuropathic pain-like behavior

The lateral parabrachial nucleus (LPBN) is known to relay noxious information to the amygdala for processing affective responses. However, it is unclear whether the LPBN actively processes neuropathic pain characterized by persistent hyperalgesia with aversive emotional responses. Here we report that neuropathic pain-like hypersensitivity induced by common peroneal nerve (CPN) ligation increases nociceptive stimulation-induced responses in glutamatergic LPBN neurons. Optogenetic activation of GABAergic LPBN neurons does not affect basal nociception, but alleviates neuropathic pain-like behavior. Optogenetic activation of glutamatergic or inhibition of GABAergic LPBN neurons induces neuropathic pain-like behavior in naïve mice. Inhibition of glutamatergic LPBN neurons alleviates both basal nociception and neuropathic pain-like hypersensitivity. Repetitive pharmacogenetic activation of glutamatergic or GABAergic LPBN neurons respectively mimics or prevents the development of CPN ligation-induced neuropathic pain-like hypersensitivity. These findings indicate that a delicate balance between excitatory and inhibitory LPBN neuronal activity governs the development and maintenance of neuropathic pain.

The parabrachial nucleus (PBN) projects to the amygdala, and contributes to affective aspects of neuropathic pain. Here the authors demonstrate that the lateral parabrachial nucleus (LPBN) contributes to hypersensitivity in a mouse model of neuropathic pain.

Cognitive self-regulation influences pain-related physiology

Cognitive self-regulation can shape pain experience, but its effects on autonomic responses to painful events are unclear. In this study, participants (N = 41) deployed a cognitive strategy based on reappraisal and imagination to regulate pain up or down on different trials while skin conductance responses (SCRs) and electrocardiogram activity were recorded. Using a machine learning approach, we first developed stimulus-locked SCR and electrocardiogram physiological markers predictive of pain ratings. The physiological markers demonstrated high sensitivity and moderate specificity in predicting pain across 2 data sets, including an independent test data set (N = 84). When we tested the markers on the cognitive self-regulation data, we found that cognitive self-regulation had significant impacts on both pain ratings and pain-related physiology in accordance with regulatory goals. These findings suggest that self-regulation can impact autonomic nervous system responses to painful stimuli and provide pain-related autonomic profiles for future studies.

Comparing Gene Expression in the Parabrachial and Amygdala of Diestrus and Proestrus Female Rats after Orofacial Varicella Zoster Injection

The orofacial pain pathway projects to the parabrachial and amygdala, and sex steroids have been shown to affect neuronal activity in these regions. GABA positive cells in the amygdala are influenced by sex steroid metabolites to affect pain, and sex steroids have been shown to alter the expression of genes in the parabrachial, changing neuronal excitability. Mechanisms by which sex steroids affect amygdala and parabrachial signaling are unclear. The expression of genes in the parabrachial and amygdala in diestrus (low estradiol) and proestrus (high estradiol) female rats were evaluated in this study. First, varicella zoster virus was injected into the whisker pad of female rats to induce a pain response. Second, gene expression was quantitated using RNA-seq one week after injection. Genes that had the greatest change in expression and known to function in pain signaling were selected for the quantitation of protein content. Protein expression of four genes in the parabrachial and seven genes in the amygdala were quantitated by ELISA. In the parabrachial, neurexin 3 (Nrnx3) was elevated at proestrus. Nrnx3 has a role in AMPA receptor and GABA signaling. Neuronatin (Nnat) and protein phosphatase, Mg²⁺/Mn²⁺ dependent 1E (Ppm1e) were elevated in the parabrachial of diestrus animals both genes having a role in pain signaling. Epoxide hydroxylase (Ephx2) was elevated in the parabrachial at proestrus and the vitamin D receptor (Vdr) was elevated in the amygdala. Ephx2 antagonists and vitamin D have been used to treat neuropathic pain. In conclusion, sex steroids regulate genes in the parabrachial and amygdala that might result in the greater pain response observed during diestrus.

The Parabrachial Nucleus Directly Channels Spinal Nociceptive Signals to the Intralaminar Thalamic Nuclei, but Not the Amygdala

The parabrachial nucleus (PBN) is one of the major targets of spinal projection neurons and plays important roles in pain. However, the architecture of the spinoparabrachial pathway underlying its functional role in nociceptive information processing remains elusive. Here, we report that the PBN directly relays nociceptive signals from the spinal cord to the intralaminar thalamic nuclei (ILN). We demonstrate that the spinal cord connects with the PBN in a bilateral manner and that the ipsilateral spinoparabrachial pathway is critical for nocifensive behavior. We identify Tacr1-expressing neurons as the major neuronal subtype in the PBN that receives direct spinal input and show that these neurons are critical for processing nociceptive information. Furthermore, PBN neurons receiving spinal input form functional monosynaptic excitatory connections with neurons in the ILN, but not the amygdala. Together, our results delineate the neural circuit underlying nocifensive behavior, providing crucial insight into the circuit mechanism underlying nociceptive information processing.

Fatigue-induced Fos immunoreactivity within the lumbar cord and amygdala decreases after С60 fullerene pretreatment

The fundamental aspects related to the mechanisms of action of C₆₀ fullerene nanoparticles on the level of the central nervous system in different experimental conditions are still unclear. Electrophysiological investigation and immunohistochemical techniques of c-fos expression were combined to determine which neural elements within the lumbar segments and in the central nucleus of the amygdala (CeA) are activated under skeletal muscle fatigue development with prior application of C₆₀ fullerenes (dissolved in dimethyl sulfoxide and in distilled water, FDS). After high-frequency electrical stimulation of the triceps surae muscle, the main fatigue-related increases in the c-Fos expression level were registered ipsilaterally within lamina 1 and 5 of the lumbar segments and within the contralateral capsular part of the CeA. C₆₀ fullerene pretreatment in animals with subsequent electrical stimulation induced a distinct (2–4 times) decrease in the level of Fos immunoreactivity in the observed structures in comparison with only fatigue-induced rats. It can be supposed that FDS, as antioxidant compound, can decrease the concentration of free radicals in fatigued tissue and reduce the transmission intensity of nociceptive information from muscles to the spinal cord and amygdala, thereby changing the level of c-Fos expression within the lumbar segments and CeA.

The Medial Prefrontal Cortex as a Central Hub for Mental Comorbidities Associated with Chronic Pain

Chronic pain patients frequently develop and suffer from mental comorbidities such as depressive mood, impaired cognition, and other significant constraints of daily life, which can only insufficiently be overcome by medication. The emotional and cognitive components of pain are processed by the medial prefrontal cortex, which comprises the anterior cingulate cortex, the prelimbic, and the infralimbic cortex. All three subregions are significantly affected by chronic pain: magnetic resonance imaging has revealed gray matter loss in all these areas in chronic pain conditions. While the anterior cingulate cortex appears hyperactive, prelimbic, and infralimbic regions show reduced activity. The medial prefrontal cortex receives ascending, nociceptive input, but also exerts important top-down control of pain sensation: its projections are the main cortical input of the periaqueductal gray, which is part of the descending inhibitory pain control system at the spinal level. A multitude of neurotransmitter systems contributes to the fine-tuning of the local circuitry, of which cholinergic and GABAergic signaling are particularly emerging as relevant components of affective pain processing within the prefrontal cortex. Accordingly, factors such as distraction, positive mood, and anticipation of pain relief such as placebo can ameliorate pain by affecting mPFC function, making this cortical area a promising target region for medical as well as psychosocial interventions for pain therapy.

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.

Differential rewarding effects of electrical stimulation of the lateral hypothalamus and parabrachial complex: Functional characterization and the relevance of opioid systems and dopamine

BACKGROUND

Since the discovery of rewarding intracranial self-stimulation by Olds and Milner, extensive data have been published on the biological basis of reward. Although participation of the mesolimbic dopaminergic system is well documented, its precise role has not been fully elucidated, and some authors have proposed the involvement of other neural systems in processing specific aspects of reinforced behaviour.

AIMS AND METHODS

We reviewed published data, including our own findings, on the rewarding effects induced by electrical stimulation of the lateral hypothalamus (LH) and of the external lateral parabrachial area (LPBe) - a brainstem region involved in processing the rewarding properties of natural and artificial substances - and compared its functional characteristics as observed in operant and non-operant behavioural procedures.

RESULTS

Brain circuits involved in the induction of preferences for stimuli associated with electrical stimulation of the LBPe appear to functionally and neurochemically differ from those activated by electrical stimulation of the LH.

INTERPRETATION

We discuss the possible involvement of the LPBe in processing emotional-affective aspects of the brain reward system.

Spinal cord stimulation in chronic pain: evidence and theory for mechanisms of action

Well-established in the field of bioelectronic medicine, Spinal Cord Stimulation (SCS) offers an implantable, non-pharmacologic treatment for patients with intractable chronic pain conditions. Chronic pain is a widely heterogenous syndrome with regard to both pathophysiology and the resultant phenotype. Despite advances in our understanding of SCS-mediated antinociception, there still exists limited evidence clarifying the pathways recruited when patterned electric pulses are applied to the epidural space. The rapid clinical implementation of novel SCS methods including burst, high frequency and dorsal root ganglion SCS has provided the clinician with multiple options to treat refractory chronic pain. While compelling evidence for safety and efficacy exists in support of these novel paradigms, our understanding of their mechanisms of action (MOA) dramatically lags behind clinical data. In this review, we reconstruct the available basic science and clinical literature that offers support for mechanisms of both paresthesia spinal cord stimulation (P-SCS) and paresthesia-free spinal cord stimulation (PF-SCS). While P-SCS has been heavily examined since its inception, PF-SCS paradigms have recently been clinically approved with the support of limited preclinical research. Thus, wide knowledge gaps exist between their clinical efficacy and MOA. To close this gap, many rich investigative avenues for both P-SCS and PF-SCS are underway, which will further open the door for paradigm optimization, adjunctive therapies and new indications for SCS. As our understanding of these mechanisms evolves, clinicians will be empowered with the possibility of improving patient care using SCS to selectively target specific pathophysiological processes in chronic pain.

Parabrachial Interleukin-6 Reduces Body Weight and Food Intake and Increases Thermogenesis to Regulate Energy Metabolism

Chronic low-grade inflammation and increased serum levels of the cytokine IL-6 accompany obesity. For brain-produced IL-6, the mechanisms by which it controls energy balance and its role in obesity remain unclear. Here, we show that brain-produced IL-6 is decreased in obese mice and rats in a neuroanatomically and sex-specific manner. Reduced IL-6 mRNA localized to lateral parabrachial nucleus (lPBN) astrocytes, microglia, and neurons, including paraventricular hypothalamus-innervating lPBN neurons. IL-6 microinjection into lPBN reduced food intake and increased brown adipose tissue (BAT) thermogenesis in male lean and obese rats by increasing thyroid and sympathetic outflow to BAT. Parabrachial IL-6 interacted with leptin to reduce feeding. siRNA-mediated reduction of lPBN IL-6 leads to increased weight gain and adiposity, reduced BAT thermogenesis, and increased food intake. Ambient cold exposure partly normalizes the obesity-induced suppression of lPBN IL-6. These results indicate that lPBN-produced IL-6 regulates feeding and metabolism and pinpoints (patho)physiological contexts interacting with lPBN IL-6.

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.

Distinct patterns of brain activity mediate perceptual and motor and autonomic responses to noxious stimuli

Pain is a complex phenomenon involving perceptual, motor, and autonomic responses, but how the brain translates noxious stimuli into these different dimensions of pain is unclear. Here, we assessed perceptual, motor, and autonomic responses to brief noxious heat stimuli and recorded brain activity using electroencephalography (EEG) in humans. Multilevel mediation analysis reveals that each pain dimension is subserved by a distinct pattern of EEG responses and, conversely, that each EEG response differentially contributes to the different dimensions of pain. In particular, the translation of noxious stimuli into autonomic and motor responses involved the earliest N1 wave, whereas pain perception was mediated by later N2 and P2 waves. Gamma oscillations mediated motor responses rather than pain perception. These findings represent progress towards a mechanistic understanding of the brain processes translating noxious stimuli into pain and suggest that perceptual, motor, and autonomic dimensions of pain are partially independent rather than serial processes.

Pain is a complex phenomenon involving not just the perception of pain, but also autonomic and motor responses. Here, the authors show that these different dimensions of pain are associated with distinct patterns of neural responses to noxious stimuli as measured using EEG.

Insular-limbic dissociation to intra-epidermal electrical Aδ activation: A comparative study with thermo-nociceptive laser stimulation

Intra-epidermal electrical stimulation (IEES) has been shown to activate selectively Aδ fibers subserving spinothalamic-mediated sensations. Owing to electrically induced highly synchronous afferent volleys, IEES induces Aδ-mediated evoked potentials at nonpainful intensities, contrasting with thermo-nociceptive laser pulses which entail painful pricking sensations. Here, we recorded intracortical responses from sensory and limbic-cognitive regions of human subjects in response to IEE and laser stimuli, in order to test the hypothesis that IEES could dissociate the sensory from nonsensory networks of nociceptive processing. Intracortical evoked potentials were obtained in 11 epileptic patients with stereotactically implanted electrodes in sensory regions receiving spinothalamic afferents (posterior insula), limbic regions receiving spino-parabrachial input (amygdalar nucleus), and high-order affective-cognitive regions (anteromedial frontal cortex, including perigenual anterior cingulate and rostromedial prefrontal areas). Responses in the sensory posterior insula were of similar amplitude and latency to IEE and laser stimuli (after accounting for heat-transduction time of laser), and consistent in both cases with spinothalamic activation. However, responses to IEES in the amygdala and the anteromedial frontal regions were inconsistent and significantly smaller compared to those evoked to the laser stimulation. Thus, IEES can effectively activate the spinothalamic-sensory system with little recruitment of affective-motivational networks, including those triggered by spino-parabrachio-amygdalar projections. The fact that identical sensory responses were associated to either painful or nonpainful percepts underscores that subjective pain perception is not solely dependent on the sensory recruitment, but rather on the combined activation of sensory, limbic and cognitive areas with precise spatiotemporal relations.

VGLUT1 or VGLUT2 mRNA-positive neurons in spinal trigeminal nucleus provide collateral projections to both the thalamus and the parabrachial nucleus in rats

The trigemino-thalamic (T-T) and trigemino-parabrachial (T-P) pathways are strongly implicated in the sensory-discriminative and affective/emotional aspects of orofacial pain, respectively. These T-T and T-P projection fibers originate from the spinal trigeminal nucleus (Vsp). We previously determined that many vesicular glutamate transporter (VGLUT1 and/or VGLUT2) mRNA-positive neurons were distributed in the Vsp of the adult rat, and most of these neurons sent their axons to the thalamus or cerebellum. However, whether VGLUT1 or VGLUT2 mRNA-positive projection neurons exist that send their axons to both the thalamus and the parabrachial nucleus (PBN) has not been reported. Thus, in the present study, dual retrograde tract tracing was used in combination with fluorescence in situ hybridization (FISH) for VGLUT1 or VGLUT2 mRNA to identify the existence of VGLUT1 or VGLUT2 mRNA neurons that send collateral projections to both the thalamus and the PBN. Neurons in the Vsp that send collateral projections to both the thalamus and the PBN were mainly VGLUT2 mRNA-positive, with a proportion of 90.3%, 93.0% and 85.4% in the oral (Vo), interpolar (Vi) and caudal (Vc) subnucleus of the Vsp, respectively. Moreover, approximately 34.0% of the collateral projection neurons in the Vc showed Fos immunopositivity after injection of formalin into the lip, and parts of calcitonin gene-related peptide (CGRP)-immunopositive axonal varicosities were in direct contact with the Vc collateral projection neurons. These results indicate that most collateral projection neurons in the Vsp, particularly in the Vc, which express mainly VGLUT2, may relay orofacial nociceptive information directly to the thalamus and PBN via axon collaterals.

Chronic stress exacerbates neuropathic pain via the integration of stress-affect-related information with nociceptive information in the central nucleus of the amygdala

Exacerbation of pain by chronic stress and comorbidity of pain with stress-related psychiatric disorders, including anxiety and depression, represent significant clinical challenges. However, the underlying mechanisms still remain unclear. Here, we investigated whether chronic forced swim stress (CFSS)-induced exacerbation of neuropathic pain is mediated by the integration of stress-affect-related information with nociceptive information in the central nucleus of the amygdala (CeA). We first demonstrated that CFSS indeed produces both depressive-like behaviors and exacerbation of spared nerve injury (SNI)-induced mechanical allodynia in rats. Moreover, we revealed that CFSS induces both sensitization of basolateral amygdala (BLA) neurons and augmentation of long-term potentiation (LTP) at the BLA-CeA synapse and meanwhile, exaggerates both SNI-induced sensitization of CeA neurons and LTP at the parabrachial (PB)-CeA synapse. In addition, we discovered that CFSS elevates SNI-induced functional up-regulation of GluN2B-containing NMDA (GluN2B-NMDA) receptors in the CeA, which is proved to be necessary for CFSS-induced augmentation of LTP at the PB-CeA synapse and exacerbation of pain hypersensitivity in SNI rats. Suppression of CFSS-elicited depressive-like behaviors by antidepressants imipramine or ifenprodil inhibits the CFSS-induced exacerbation of neuropathic pain. Collectively, our findings suggest that CFSS potentiates synaptic efficiency of the BLA-CeA pathway, leading to the activation of GluN2B-NMDA receptors and sensitization of CeA neurons, which subsequently facilitate pain-related synaptic plasticity of the PB-CeA pathway, thereby exacerbating SNI-induced neuropathic pain. We conclude that chronic stress exacerbates neuropathic pain via the integration of stress-affect-related information with nociceptive information in the CeA.

TFOS DEWS II pain and sensation report

Pain associated with mechanical, chemical, and thermal heat stimulation of the ocular surface is mediated by trigeminal ganglion neurons, while cold thermoreceptors detect wetness and reflexly maintain basal tear production and blinking rate. These neurons project into two regions of the trigeminal brain stem nuclear complex: ViVc, activated by changes in the moisture of the ocular surface and VcC1, mediating sensory-discriminative aspects of ocular pain and reflex blinking. ViVc ocular neurons project to brain regions that control lacrimation and spontaneous blinking and to the sensory thalamus. Secretion of the main lacrimal gland is regulated dominantly by autonomic parasympathetic nerves, reflexly activated by eye surface sensory nerves. These also evoke goblet cell secretion through unidentified efferent fibers. Neural pathways involved in the regulation of meibomian gland secretion or mucin release have not been identified. In dry eye disease, reduced tear secretion leads to inflammation and peripheral nerve damage. Inflammation causes sensitization of polymodal and mechano-nociceptor nerve endings and an abnormal increase in cold thermoreceptor activity, altogether evoking dryness sensations and pain. Long-term inflammation and nerve injury alter gene expression of ion channels and receptors at terminals and cell bodies of trigeminal ganglion and brainstem neurons, changing their excitability, connectivity and impulse firing. Perpetuation of molecular, structural and functional disturbances in ocular sensory pathways ultimately leads to dysestesias and neuropathic pain referred to the eye surface. Pain can be assessed with a variety of questionaires while the status of corneal nerves is evaluated with esthesiometry and with in vivo confocal microscopy.

Parabrachial Pituitary Adenylate Cyclase-Activating Polypeptide Activation of Amygdala Endosomal Extracellular Signal-Regulated Kinase Signaling Regulates the Emotional Component of Pain

BACKGROUND

Chronic pain and stress-related psychopathologies, such as depression and anxiety-associated abnormalities, are mutually reinforcing; however, the neuronal circuits and mechanisms that underlie this reinforcement are still not well understood. Pituitary adenylate cyclase-activating polypeptide (PACAP; Adcyap1) and its cognate PAC1 receptor (Adcyap1r1) are expressed in peripheral nociceptive pathways, participate in anxiety-related responses and have been have been linked to posttraumatic stress disorder and other mental health afflictions.

METHODS

Using immunocytochemistry, pharmacological treatments and behavioral testing techniques, we have used a rodent partial sciatic nerve chronic constriction injury model (n = 5-8 per group per experiment) to evaluate PACAP plasticity and signaling in nociceptive and stress-related behaviors.

RESULTS

We show that chronic neuropathic pain increases PACAP expression at multiple tiers along the spinoparabrachioamygdaloid tract. Furthermore, chronic constriction injury bilaterally augments nociceptive amygdala (in the central nucleus of the amygdala [CeA]) PACAP immunoreactivity, extracellular signal-regulated kinase phosphorylation, and c-Fos activation, in parallel with heightened anxiety-like behavior and nociceptive hypersensitivity. Acute CeA infusions with the PACAP receptor antagonist PACAP(6-38) blocked chronic constriction injury-induced behavioral responses. Additionally, pretreatments with inhibitors of mitogen-activated protein kinase enzymes or endocytosis to block endosomal PACAP receptor extracellular signal-regulated kinase signaling attenuated PACAP-induced CeA neuronal activation and nociceptive responses.

CONCLUSIONS

Our data suggest that chronic pain-induced PACAP neuroplasticity and signaling in spinoparabrachioamygdaloid projections have an impact on CeA stress- and nociception-associated maladaptive responses, which can be ameliorated upon receptor antagonism even during injury progression. Thus, the PACAP pathway provides for an important mechanism underlying the intersection of stress and chronic pain pathways via the amygdala.

Analysis of alcohol use disorders from the Nathan Kline Institute-Rockland Sample: Correlation of brain cortical thickness with neuroticism

BACKGROUND

Although differences in both neuroanatomical measures and personality traits, in particular neuroticism, have been associated with alcohol use disorders (AUD), whether lifetime AUD diagnosis alters the relationship between neuroticism and neuroanatomical structures remains to be determined.

METHODS

Data from 65 patients with lifetime AUD diagnoses and 65 healthy comparisons (HC) group-matched on age, sex and race were extracted from the Nathan Kline Institute - Rockland Sample data set. Each subject completed personality trait measures and underwent MRI scanning. Cortical thickness measures at 68 Desikan-Killiany Atlas regions were obtained using FreeSurfer 5.3.0. Regression analyses were performed to identify brain regions at which the neuroticism-cortical thickness relationship was altered by lifetime AUD status.

RESULTS

As expected, AUDs had higher neuroticism scores than HCs. Correlations between neuroticism and cortical thickness in the left insula and right fusiform differed significantly across groups. Higher neuroticism score in AUD and the interaction between the insular cortical thickness-neuroticism correlation and AUD status were confirmed in a replication study using the Human Connectome Project data set.

CONCLUSIONS

Results confirmed the relationship between neuroticism and AUD and suggests that specific cortical regions, particularly the left insula, represent anatomic substrates underlying this association in AUD.

Specific Targeting of the Basolateral Amygdala to Projectionally Defined Pyramidal Neurons in Prelimbic and Infralimbic Cortex

Adjacent prelimbic (PL) and infralimbic (IL) regions in the medial prefrontal cortex have distinct roles in emotional learning. A complete mechanistic understanding underlying this dichotomy remains unclear. Here we explored targeting of specific PL and IL neurons by the basolateral amygdala (BLA), a limbic structure pivotal in pain and fear processing. In mice, we used retrograde labeling, brain-slice recordings, and adenoviral optogenetics to dissect connectivity of ascending BLA input onto PL and IL neurons projecting to the periaqueductal gray (PAG) or the amygdala. We found differential targeting of BLA projections to PL and IL cortex. Activating BLA projections evoked excitatory and inhibitory responses in cortico-PAG (CP) neurons in layer 5 (L5) of both PL and IL cortex. However, all inhibitory responses were polysynaptic and monosynaptic BLA input was stronger to CP neurons in IL cortex. Conversely, the BLA preferentially targeted corticoamygdalar (CA) neurons in layer 2 (L2) of PL over IL cortex. We also reveal that BLA input is projection specific by showing preferential targeting of L5 CP neurons over neighboring L3/5 CA neurons in IL cortex. We conclude by showing that BLA input is laminar-specific by producing stronger excitatory responses CA neurons in L3/5 compared with L2 in IL cortex. Collectively, this study reveals differential targeting of the BLA to PL and IL cortex, which depends both on laminar location and projection target of cortical neurons. Overall, our findings should have important implications for understanding the processing of pain and fear input by the PL and IL cortex.

Urine Trouble: Alterations in Brain Function Associated with Bladder Pain

PURPOSE

Chronic bladder pain is a debilitating condition often accompanied by alterations in affective and autonomic function. Many symptoms associated with chronic bladder pain are mediated by the central nervous system. In this review data from preclinical animal models and human neuroimaging studies were analyzed and a theoretical supraspinal bladder pain network was generated.

MATERIALS AND METHODS

We comprehensively reviewed the literature using PubMed® and Google Scholar™. Relevant reviews and original research articles, and the cited references were summarized and then organized on a neuroanatomical basis.

RESULTS

The brain loci the most predominant in the bladder pain literature are the thalamus, parabrachial nucleus, cerebral cortex, amygdala, hypothalamus, periaqueductal gray and rostral ventromedial medulla. This review highlights each of these regions, discussing the molecular and physiological changes that occur in each in the context of bladder pain.

CONCLUSIONS

A complex network of brain loci is involved in bladder pain modulation. Studying these brain regions and the changes that they undergo during the transition from acute to chronic bladder pain will provide novel therapeutic strategies for patients with chronic bladder pain diseases such as interstitial cystitis/bladder pain syndrome and chronic prostatitis/chronic pelvic pain syndrome.

Comorbidity Factors and Brain Mechanisms Linking Chronic Stress and Systemic Illness

Neuropsychiatric symptoms and mental illness are commonly present in patients with chronic systemic diseases. Mood disorders, such as depression, are present in up to 50% of these patients, resulting in impaired physical recovery and more intricate treatment regimen. Stress associated with both physical and emotional aspects of systemic illness is thought to elicit detrimental effects to initiate comorbid mental disorders. However, clinical reports also indicate that the relationship between systemic and psychiatric illnesses is bidirectional, further increasing the complexity of the underlying pathophysiological processes. In this review, we discuss the recent evidence linking chronic stress and systemic illness, such as activation of the immune response system and release of common proinflammatory mediators. Altogether, discovery of new targets is needed for development of better treatments for stress-related psychiatric illnesses as well as improvement of mental health aspects of different systemic diseases.

Neurotransmitters in the vestibular system

Neuronal networks that are linked to the peripheral vestibular system contribute to gravitoinertial sensation, balance control, eye movement control, and autonomic function. Ascending connections to the limbic system and cerebral cortex are also important for motion perception and threat recognition, and play a role in comorbid balance and anxiety disorders. The vestibular system also shows remarkable plasticity, termed vestibular compensation. Activity in these networks is regulated by an interaction between: (1) intrinsic neurotransmitters of the inner ear, vestibular nerve, and vestibular nuclei; (2) neurotransmitters associated with thalamocortical and limbic pathways that receive projections originating in the vestibular nuclei; and (3) locus coeruleus and raphe (serotonergic and nonserotonergic) projections that influence the latter components. Because the ascending vestibular interoceptive and thalamocortical pathways include networks that influence a broad range of stress responses (endocrine and autonomic), memory consolidation, and cognitive functions, common transmitter substrates provide a basis for understanding features of acute and chronic vestibular disorders.

Elucidating an Affective Pain Circuit that Creates a Threat Memory

Animals learn to avoid harmful situations by associating a neutral stimulus with a painful one, resulting in a stable threat memory. In mammals, this form of learning requires the amygdala. Although pain is the main driver of aversive learning, the mechanism that transmits pain signals to the amygdala is not well resolved. Here, we show that neurons expressing calcitonin gene-related peptide (CGRP) in the parabrachial nucleus are critical for relaying pain signals to the central nucleus of amygdala and that this pathway may transduce the affective motivational aspects of pain. Genetic silencing of CGRP neurons blocks pain responses and memory formation, whereas their optogenetic stimulation produces defensive responses and a threat memory. The pain-recipient neurons in the central amygdala expressing CGRP receptors are also critical for establishing a threat memory. The identification of the neural circuit conveying affective pain signals may be pertinent for treating pain conditions with psychiatric comorbidities.

Cross-frequency coupling in deep brain structures upon processing the painful sensory inputs

Cross-frequency coupling has been shown to be functionally significant in cortical information processing, potentially serving as a mechanism for integrating functionally relevant regions in the brain. In this study, we evaluate the hypothesis that pain-related gamma oscillatory responses are coupled with low-frequency oscillations in the frontal lobe, amygdala and hippocampus, areas known to have roles in pain processing. We delivered painful laser pulses to random locations on the dorsal hand of five patients with uncontrolled epilepsy requiring depth electrode implantation for seizure monitoring. Two blocks of 40 laser stimulations were delivered to each subject and the pain-intensity was controlled at five in a 0-10 scale by adjusting the energy level of the laser pulses. Local-field-potentials (LFPs) were recorded through bilaterally implanted depth electrode contacts to study the oscillatory responses upon processing the painful laser stimulations. Our results show that painful laser stimulations enhanced low-gamma (LH, 40-70 Hz) and high-gamma (HG, 70-110 Hz) oscillatory responses in the amygdala and hippocampal regions on the right hemisphere and these gamma responses were significantly coupled with the phases of theta (4-7 Hz) and alpha (8-1 2 Hz) rhythms during pain processing. Given the roles of these deep brain structures in emotion, these findings suggest that the oscillatory responses in these regions may play a role in integrating the affective component of pain, which may contribute to our understanding of the mechanisms underlying the affective information processing in humans.

Ventral hippocampal nicotinic acetylcholine receptors mediate stress-induced analgesia in mice

Evidence suggests that various stressful procedures induce an analgesic effect in laboratory animals commonly referred to as stress-induced analgesia (SIA). The aim of the present study was to assess the role of ventral hippocampal (VH) nicotinic acetylcholine receptors (nAChRs) in SIA in adult male NMRI mice. The VHs of animals were bilaterally cannulated and nociceptive threshold was measured using infrared source in a tail-flick apparatus. Acute stress was evoked by placing the animals on an elevated platform for 10, 20 and 30 min. The results showed that exposure to 20 and 30 min acute stress produced analgesia, while exposure to 10 min stress had no effect on the pain response. Intra-VH microinjection of nicotine (0.001-0.1 μg/mouse), 5 min before an ineffective stress (10 min stress), induced analgesia, suggesting the potentiative effect of nicotine on SIA. It is important to note that bilateral intra-VH microinjections of the same doses of nicotine without stress had no effect on the tail-flick test. On the other hand, intra-VH microinjection of mecamylamine (0.5-1 μg/mouse) 5 min before 20-min stress inhibited SIA. However, bilateral intra-VH microinjections of the same doses of mecamylamine without stress had no effect on the tail-flick response. In addition, the microinjection of mecamylamine into the VH reversed the potentiative effect of nicotine on SIA. Taken together, it can be concluded that exposure to acute stress induces SIA in a time-dependent manner and the ventral hippocampal cholinergic system may be involved in SIA via nAChRs.