Pain is a salient "stressor" that is mediated by corticotropin-releasing factor-1 receptors

Research Progress of Central and Peripheral Corticotropin-Releasing Hormone in Irritable Bowel Syndrome with Comorbid Dysthymic Disorders

Irritable bowel syndrome (IBS) is considered a stress disorder characterized by psychological and gastrointestinal dysfunction. IBS patients not only suffer from intestinal symptoms such as abdominal pain, diarrhea, or constipation but also, experience dysthymic disorders such as anxiety and depression. Studies have found that corticotropin-releasing hormone plays a key role in IBS with comorbid dysthymic disorders. Next, we will summarize the effects of corticotropin-releasing hormone from the central nervous system and periphery on IBS with comorbid dysthymic disorders and relevant treatments based on published literatures in recent years.

Theoretical Frameworks and Mechanistic Aspects of Alcohol Addiction: Alcohol Addiction as a Reward Deficit/Stress Surfeit Disorder

Alcohol use disorder (AUD) can be defined by a compulsion to seek and take alcohol, the loss of control in limiting intake, and the emergence of a negative emotional state when access to alcohol is prevented. Alcohol use disorder impacts multiple motivational mechanisms and can be conceptualized as a disorder that includes a progression from impulsivity (positive reinforcement) to compulsivity (negative reinforcement). Compulsive drug seeking that is associated with AUD can be derived from multiple neuroadaptations, but the thesis argued herein is that a key component involves the construct of negative reinforcement. Negative reinforcement is defined as drug taking that alleviates a negative emotional state. The negative emotional state that drives such negative reinforcement is hypothesized to derive from the dysregulation of specific neurochemical elements that are involved in reward and stress within basal forebrain structures that involve the ventral striatum and extended amygdala, respectively. Specific neurochemical elements in these structures include decreases in reward neurotransmission (e.g., decreases in dopamine and opioid peptide function in the ventral striatum) and the recruitment of brain stress systems (e.g., corticotropin-releasing factor [CRF]) in the extended amygdala, which contributes to hyperkatifeia and greater alcohol intake that is associated with dependence. Glucocorticoids and mineralocorticoids may play a role in sensitizing the extended amygdala CRF system. Other components of brain stress systems in the extended amygdala that may contribute to the negative motivational state of withdrawal include norepinephrine in the bed nucleus of the stria terminalis, dynorphin in the nucleus accumbens, hypocretin and vasopressin in the central nucleus of the amygdala, and neuroimmune modulation. Decreases in the activity of neuropeptide Y, nociception, endocannabinoids, and oxytocin in the extended amygdala may also contribute to hyperkatifeia that is associated with alcohol withdrawal. Such dysregulation of emotional processing may also significantly contribute to pain that is associated with alcohol withdrawal and negative urgency (i.e., impulsivity that is associated with hyperkatifeia during hyperkatifeia). Thus, an overactive brain stress response system is hypothesized to be activated by acute excessive drug intake, to be sensitized during repeated withdrawal, to persist into protracted abstinence, and to contribute to the compulsivity of AUD. The combination of the loss of reward function and recruitment of brain stress systems provides a powerful neurochemical basis for a negative emotional state that is responsible for the negative reinforcement that at least partially drives the compulsivity of AUD.

Anhedonia, Hyperkatifeia, and Negative Reinforcement in Substance Use Disorders

Drug addiction has been defined as a chronically relapsing disorder that is characterized by a compulsion to seek and take a drug or stimulus, the loss of control in limiting intake, and the emergence of a negative emotional state when access to the drug or stimulus is prevented, a component of which is anhedonia. The present review explores a heuristic framework for understanding the role of anhedonia in addiction, in which anhedonia is a key component of hyperkatifeia (conceptualized as the potentiated intensity of negative emotional/motivational symptoms during drug withdrawal) and negative reinforcement in addiction. The neural substrates that mediate such anhedonia and crosstalk between elements of hyperkatifeia that contribute to anhedonia are then explored, including crosstalk between physical pain and emotional pain systems. The present review explores current knowledge of neurochemical neurocircuitry changes that are associated with conditioned hyperkatifeia/anhedonia. The overall hypothesis is that the shift in motivation toward negative reinforcement in addiction reflects the allostatic misregulation of hedonic tone, such that drug taking makes anhedonia worse during the process of seeking temporary relief by compulsive drug taking, thereby perpetuating the addiction cycle and hedonic comorbidities that are associated with addiction.

Drug Addiction: Hyperkatifeia/Negative Reinforcement as a Framework for Medications Development

Compulsive drug seeking that is associated with addiction is hypothesized to follow a heuristic framework that involves three stages (binge/intoxication, withdrawal/negative affect, and preoccupation/anticipation) and three domains of dysfunction (incentive salience/pathologic habits, negative emotional states, and executive function, respectively) via changes in the basal ganglia, extended amygdala/habenula, and frontal cortex, respectively. This review focuses on neurochemical/neurocircuitry dysregulations that contribute to hyperkatifeia, defined as a greater intensity of negative emotional/motivational signs and symptoms during withdrawal from drugs of abuse in the withdrawal/negative affect stage of the addiction cycle. Hyperkatifeia provides an additional source of motivation for compulsive drug seeking via negative reinforcement. Negative reinforcement reflects an increase in the probability of a response to remove an aversive stimulus or drug seeking to remove hyperkatifeia that is augmented by genetic/epigenetic vulnerability, environmental trauma, and psychiatric comorbidity. Neurobiological targets for hyperkatifeia in addiction involve neurocircuitry of the extended amygdala and its connections via within-system neuroadaptations in dopamine, enkephalin/endorphin opioid peptide, and γ-aminobutyric acid/glutamate systems and between-system neuroadaptations in prostress corticotropin-releasing factor, norepinephrine, glucocorticoid, dynorphin, hypocretin, and neuroimmune systems and antistress neuropeptide Y, nociceptin, endocannabinoid, and oxytocin systems. Such neurochemical/neurocircuitry dysregulations are hypothesized to mediate a negative hedonic set point that gradually gains allostatic load and shifts from a homeostatic hedonic state to an allostatic hedonic state. Based on preclinical studies and translational studies to date, medications and behavioral therapies that reset brain stress, antistress, and emotional pain systems and return them to homeostasis would be promising new targets for medication development. SIGNIFICANCE STATEMENT: The focus of this review is on neurochemical/neurocircuitry dysregulations that contribute to hyperkatifeia, defined as a greater intensity of negative emotional/motivational signs and symptoms during withdrawal from drugs of abuse in the withdrawal/negative affect stage of the drug addiction cycle and a driving force for negative reinforcement in addiction. Medications and behavioral therapies that reverse hyperkatifeia by resetting brain stress, antistress, and emotional pain systems and returning them to homeostasis would be promising new targets for medication development.

Neurobiological aspects of pain in the context of alcohol use disorder

Alcohol is an effective and widely utilized analgesic. However, the chronic use of alcohol can actually facilitate nociceptive sensitivity over time, a condition known as hyperalgesia. Excessive and uncontrollable alcohol drinking is also a hallmark feature of alcohol use disorder (AUD). Both AUD and chronic pain are typically accompanied by negative affective states that may underlie reinforcement mechanisms contributing to AUD maintenance or progression. Frequent utilization of alcohol to relieve pain in individuals suffering from AUD or other chronic pain conditions may thus represent a powerful negative reinforcement construct. This chapter will describe ties between alcohol-mediated pain relief and potential exacerbation of AUD. We describe neurobiological systems engaged in alcohol analgesia as well as systems recruited in the development and maintenance of AUD and hyperalgesia. Although few effective therapies exist for either chronic pain or AUD, the common interaction of these conditions will likely lead the way for promising new discoveries of more effective and even simultaneous treatment of AUD and co-morbid hyperalgesia. An abundance of neurobiological findings from multiple laboratories has implicated a potentiation of central amygdala (CeA) signaling in both pain and AUD, and these data also suggest that attenuation of stress-related systems (including corticotropin-releasing factor, vasopressin, and glucocorticoid receptor activity) would be particularly effective and comprehensive therapeutic strategies targeting the critical intersection of somatic and motivational mechanisms driving AUD, including alcohol-induced hyperalgesia.

Saporin from Saponaria officinalis as a Tool for Experimental Research, Modeling, and Therapy in Neuroscience

Saporin, which is extracted from Saponaria officinalis, is a protein toxin that inactivates ribosomes. Saporin itself is non-selective toxin but acquires high specificity after conjugation with different ligands such as signaling peptides or antibodies to some surface proteins expressed in a chosen cell subpopulation. The saporin-based conjugated toxins were widely adopted in neuroscience as a convenient tool to induce highly selective degeneration of desired cell subpopulation. Induction of selective cell death is one of approaches used to model neurodegenerative diseases, study functions of certain cell subpopulations in the brain, and therapy. Here, we review studies where saporin-based conjugates were used to analyze cell mechanisms of sleep, general anesthesia, epilepsy, pain, and development of Parkinson’s and Alzheimer’s diseases. Limitations and future perspectives of use of saporin-based toxins in neuroscience are discussed.

Research Recommendations on the Effects of Postpartum Depression and Pain on Infant Care and Development

Postpartum depression is one of the most common complications of childbirth, and another is pain. A growing body of research shows that the severity and duration of postpartum pain can increase a woman's risk for postpartum depression. Postpartum depression and pain negatively affect maternal well-being, and postpartum depression has been associated with adverse outcomes in children. However, there is a dearth of information about the effects of postpartum depression and pain on infant care and development. The objectives of this commentary were to highlight the need to address this gap in the literature, offer a preliminary conceptual model to advance the field, and ignite new lines of inquiry to inform infant care and development.

Evaluation of Antiulcer Activity of 80% Methanol Extract and Solvent Fractions of the Root of Croton macrostachyus Hocsht: Ex Del. (Euphorbiaceae) in Rodents

Background

Peptic ulcer disease causes significant mortality and morbidity. Plant kingdom provides a useful source for the development of new antiulcer agents. Croton macrostachyus is traditionally used to treat peptic ulcer in Ethiopia. This study aimed to evaluate the antiulcer activity of C. macrostachyus root extracts in rodents using different models.

Methods

The crude extract was obtained by cold maceration in 80% methanol and fractionated with chloroform, ethyl acetate, and distilled water. The antiulcer activity was evaluated using pylorus ligation-induced ulcer model in Sprague Dawley rats and acidified ethanol-induced ulcer model in Swiss albino mice. The test groups received three doses (100, 200, and 400 mg/kg) of the crude extract and fractions for 7 days before induction of ulcer. Positive controls received omeprazole 30 mg/kg for the pylorus ligation-induced ulcer model and sucralfate 100 mg/kg for the acidified ethanol-induced ulcer model. Negative controls received vehicle (2% tween 80).

Results

The crude hydromethanolic extract of C. macrostachyus showed significant (p < 0.05) antiulcer activity on both pyloric ligation and HCl/ethanol-induced ulcer in rats and mice. It has antisecretary effect (p < 0.001) as well. All three administered doses of chloroform fraction (p < 0.05) and only higher doses of ethyl acetate fraction (p < 0.05) possessed significant antiulcer activity. In contrast, the aqueous fraction did not have significant antiulcer effect at all tested doses.

Conclusion

The present study demonstrated that the crude extract, chloroform, and ethyl acetate fractions possessed significant dose-dependent antiulcer activity.

Pharmacological evaluation of NSAID-induced gastropathy as a "Translatable" model of referred visceral hypersensitivity

AIM

To evaluate whether non-steroidal anti-inflammatory drugs (NSAIDs)-induced gastropathy is a clinically predictive model of referred visceral hypersensitivity.

METHODS

Gastric ulcer pain was induced by the oral administration of indomethacin to male, CD1 mice (n = 10/group) and then assessed by measuring referred abdominal hypersensitivity to tactile application. A diverse range of pharmacological mechanisms contributing to the pain were subsequently investigated. These mechanisms included: transient receptor potential (TRP), sodium and acid-sensing ion channels (ASICs) as well as opioid receptors and guanylate cyclase C (GC-C).

RESULTS

Results showed that two opioids and a GC-C agonist, morphine, asimadoline and linaclotide, respectively, the TRP antagonists, AMG9810 and HC-030031 and the sodium channel blocker, carbamazepine, elicited a dose- and/or time-dependent attenuation of referred visceral hypersensitivity, while the ASIC blocker, amiloride, was ineffective at all doses tested.

CONCLUSION

Together, these findings implicate opioid receptors, GC-C, and sodium and TRP channel activation as possible mechanisms associated with visceral hypersensitivity. More importantly, these findings also validate NSAID-induced gastropathy as a sensitive and clinically predictive mouse model suitable for assessing novel molecules with potential pain-attenuating properties.

Discovery of 4-chloro-2-(2,4-dichloro-6-methylphenoxy)-1-methyl-7-(pentan-3-yl)-1H-benzimidazole, a novel CRF1 receptor antagonist

Compound 1 exhibits potent binding inhibition activity against a corticotropin-releasing factor 1 (CRF₁) receptor (IC₅₀=9.5nM) and in vitro antagonistic activity (IC₅₀=88nM) but is rapidly metabolized by human hepatic microsomes (182μL/min/mg). Here we identified metabolically stable compounds with potent CRF binding inhibitory activity. Structure-activity relationship (SAR) studies considering in vitro metabolic stability revealed that 4-chloro-2-(2,4-dichloro-6-methylphenoxy)-1-methyl-7-(pentan-3-yl)-1H-benzimidazole 24d was more stable in human microsomes (87μL/min/mg) than compound 1. Compound 24d demonstrated potent CRF binding inhibitory activity (IC₅₀=4.1nM), in vitro antagonistic activity (IC₅₀=44nM), and slow dissociation from the CRF₁ receptor. Orally administered compound 24d (6-24μmol/kg) showed ex vivo CRF₁ receptor binding in the rat pituitary, olfactory bulb, and frontal cortex and suppressed stress-induced adrenocorticotropic hormone (ACTH) secretion. In this report, we discuss SAR studies on the metabolic stability as well as CRF binding inhibitory activity of the benzimidazole series as CRF₁ receptor antagonists and the pharmacological profiles of compound 24d.

Activation of Extracellular Signal-Regulated Kinases (ERK 1/2) in the Locus Coeruleus Contributes to Pain-Related Anxiety in Arthritic Male Rats

BACKGROUND

There is increasing evidence suggesting that the Locus Coeruleus plays a role in pain-related anxiety. Indeed, we previously found that prolonged arthritis produces anxiety-like behavior in rats, along with enhanced expression of phosphorylated extracellular signal-regulated kinase 1/2 (a marker of plasticity) in the Locus Coeruleus. However, it is unknown how this effect correlates with the electrophysiological activity of Locus Coeruleus neurons or pain-related anxiety.

METHODS

Using the complete Freund's adjuvant model of monoarthritis in male Sprague-Dawley rats, we studied the behavioral attributes of pain and anxiety as well as Locus Coeruleus electrophysiology in vivo 1 (MA1W) and 4 weeks (MA4W) after disease induction.

RESULTS

The manifestation of anxiety in MA4W was accompanied by dampened tonic Locus Coeruleus activity, which was coupled to an exacerbated evoked Locus Coeruleus response to noxious stimulation of the inflamed and healthy paw. When a mitogen-activating extracellular kinase inhibitor was administered to the contralateral Locus Coeruleus of MA4W, the phosphorylated extracellular signal-regulated kinase 1/2 levels in the Locus Coeruleus were restored and the exaggerated evoked response was blocked, reversing the anxiogenic-like behavior while pain hypersensitivity remained unaltered.

CONCLUSION

As phosphorylated extracellular signal-regulated kinase 1/2 blockade in the Locus Coeruleus relieved anxiety and counteracted altered LC function, we propose that phosphorylated extracellular signal-regulated kinase 1/2 activation in the Locus Coeruleus plays a crucial role in pain-related anxiety.

Noradrenergic Locus Coeruleus pathways in pain modulation

The noradrenergic system is crucial for several activities in the body, including the modulation of pain. As the major producer of noradrenaline (NA) in the central nervous system (CNS), the Locus Coeruleus (LC) is a nucleus that has been studied in several pain conditions, mostly due to its strategic location. Indeed, apart from a well-known descending LC-spinal pathway that is important for pain control, an ascending pathway passing through this nucleus may be responsible for the noradrenergic inputs to higher centers of the pain processing, such as the limbic system and frontal cortices. Thus, the noradrenergic system appears to modulate different components of the pain experience and accordingly, its manipulation has distinct behavioral outcomes. The main goal of this review is to bring together the data available regarding the noradrenergic system in relation to pain, particularly focusing on the ascending and descending LC projections in different conditions. How such findings influence our understanding of these conditions is also discussed.

Intrathecal urocortin I in the spinal cord as a murine model of stress hormone-induced musculoskeletal and tactile hyperalgesia

Stress is antinociceptive in some models of pain, but enhances musculoskeletal nociceptive responses in mice and muscle pain in patients with fibromyalgia syndrome. To test the hypothesis that urocortins are stress hormones that are sufficient to enhance tactile and musculoskeletal hyperalgesia, von Frey fibre sensitivity and grip force after injection of corticotropin-releasing factor (CRF), urocortin I and urocortin II were measured in mice. Urocortin I (a CRF1 and CRF2 receptor ligand) produced hyperalgesia in both assays when injected intrathecally (i.t.) but not intracerebroventricularly, and only at a large dose when injected peripherally, suggesting a spinal action. Morphine inhibited urocortin I-induced changes in nociceptive responses in a dose-related fashion, confirming that changes in behaviour reflect hyperalgesia rather than weakness. No tolerance developed to the effect of urocortin I (i.t.) when injected repeatedly, consistent with a potential to enhance pain chronically. Tactile hyperalgesia was inhibited by NBI-35965, a CRF1 receptor antagonist, but not astressin 2B, a CRF2 receptor antagonist. However, while urocortin I-induced decreases in grip force were not observed when co-administered i.t. with either NBI-35965 or astressin 2B, they were even more sensitive to inhibition by astressin, a non-selective CRF receptor antagonist. Together these data indicate that urocortin I acts at CRF receptors in the mouse spinal cord to elicit a reproducible and persistent tactile (von Frey) and musculoskeletal (grip force) hyperalgesia. Urocortin I-induced hyperalgesia may serve as a screen for drugs that alleviate painful conditions that are exacerbated by stress.

Corticotropin-Releasing Factor Mediates Pain-Induced Anxiety through the ERK1/2 Signaling Cascade in Locus Coeruleus Neurons

BACKGROUND

The corticotropin-releasing factor is a stress-related neuropeptide that modulates locus coeruleus activity. As locus coeruleus has been involved in pain and stress-related patologies, we tested whether the pain-induced anxiety is a result of the corticotropin-releasing factor released in the locus coeruleus.

METHODS

Complete Freund's adjuvant-induced monoarthritis was used as inflammatory chronic pain model. α-Helical corticotropin-releasing factor receptor antagonist was microinjected into the contralateral locus coeruleus of 4-week-old monoarthritic animals. The nociceptive and anxiety-like behaviors, as well as phosphorylated extracellular signal-regulated kinases 1/2 and corticotropin-releasing factor receptors expression, were quantified in the paraventricular nucleus and locus coeruleus.

RESULTS

Monoarthritic rats manifested anxiety and increased phosphorylated extracellular signal-regulated kinases 1/2 levels in the locus coeruleus and paraventricular nucleus, although the expression of corticotropin-releasing factor receptors was unaltered. α-Helical corticotropin-releasing factor antagonist administration reversed both the anxiogenic-like behavior and the phosphorylated extracellular signal-regulated kinases 1/2 levels in the locus coeruleus.

CONCLUSIONS

Pain-induced anxiety is mediated by corticotropin-releasing factor neurotransmission in the locus coeruleus through extracellular signal-regulated kinases 1/2 signaling cascade.

Corticotrophin-releasing factor 1 activation in the central amygdale and visceral hyperalgesia

Corticotropin-releasing factor (CRF)-CRF1 receptor in the brain plays a key role in stress-related alterations of behavior including anxiety/depression, and autonomic and visceral functions. In particular, CRF1 signaling mediates hypersensitivity to colorectal distension (CRD) in various models (early life adverse events, repeated psychological stress, chronic high anxiety, postcolonic inflammation, or repeated nociceptive CRD). So far, knowledge of brain sites involved is limited. A recent article demonstrates in rats that CRF microinjected into the central amygdala (CeA) induces a hyperalgesic response to CRD and enhances the noradrenaline and dopamine levels at this site. The visceral and noradrenaline, unlike dopamine, responses were blocked by a CRF1 antagonist injected into the CeA. Here, we review the emerging role that CRF-CRF1 signaling plays in the CeA to induce visceral hypersensitivity. In the somatic pain field, CRF in the CeA was shown to induce pain sensitization. This is mediated by the activation of postsynaptic CRF1 receptors and protein kinase A signaling that increases N-methyl-d-aspartate receptor neurotransmission. In addition, the activation of tetraethylamonium-sensitive ion channels such as Kv3 accelerates repolarization and firing rate. Whether facilitation of pain transmission underlies CRF action in the CeA-induced visceral hypersensitivity will need to be delineated. CRF1 signaling in the CeA is also an important component of the neuronal circuitry inducing anxiety-like behavior and positioned at the interphase of the reciprocal relationship between pain and affective state. The hyperactivity of this system may represent the neuroanatomical and biochemical substrate contributing to the coexpression of hypersensitivity to CRD and mood disorders in subsets of irritable bowel syndrome patients.

Extended access to nicotine leads to a CRF1 receptor dependent increase in anxiety-like behavior and hyperalgesia in rats

Tobacco dependence is associated with the emergence of negative emotional states during withdrawal, including anxiety and nociceptive hypersensitivity. However, the current animal models of nicotine dependence have focused on the mechanisms that mediate the acute reinforcing effects of nicotine and failed to link increased anxiety and pain during abstinence with excessive nicotine self-administration. Here, we tested the hypothesis that the activation of corticotropin-releasing factor-1 (CRF1 ) receptors and emergence of the affective and motivational effects of nicotine abstinence only occur in rats with long access (>21 hours/day, LgA) and not short (1 hour/day, ShA) access to nicotine self-administration. ShA and LgA rats were tested for anxiety-like behavior, nociceptive thresholds, somatic signs of withdrawal and nicotine intake after 3 days of abstinence. The role of CRF1 receptors during abstinence was tested using systemic or intracerebral infusion of MPZP (N,N-bis(2-methoxyethyl)-3-(4-methoxy-2-methylphenyl)-2,5-dimethyl-pyrazolo(1,5α)pyrimidin-7-amine), a CRF1 receptor antagonist, in the central nucleus of the amygdala (CeA). LgA but not ShA rats exhibited abstinence-induced increases in anxiety-like behavior and nociceptive hypersensitivity, which both predicted subsequent excessive nicotine intake and were prevented by systemic administration of MPZP. Intra-CeA MPZP infusion prevented abstinence-induced increases in nicotine intake and nociceptive hypersensitivity. These findings demonstrate that the model of short access to nicotine self-administration has limited validity for tobacco dependence, highlight the translational relevance of the model of extended-intermittent access to nicotine self-administration for tobacco dependence and demonstrate that activation of CRF1 receptors is required for the emergence of abstinence-induced anxiety-like behavior, hyperalgesia and excessive nicotine intake.

Does the kappa opioid receptor system contribute to pain aversion?

The kappa opioid receptor (KOR) and the endogenous peptide-ligand dynorphin have received significant attention due the involvement in mediating a variety of behavioral and neurophysiological responses, including opposing the rewarding properties of drugs of abuse including opioids. Accumulating evidence indicates this system is involved in regulating states of motivation and emotion. Acute activation of the KOR produces an increase in motivational behavior to escape a threat, however, KOR activation associated with chronic stress leads to the expression of symptoms indicative of mood disorders. It is well accepted that KOR can produce analgesia and is engaged in chronic pain states including neuropathic pain. Spinal studies have revealed KOR-induced analgesia in reversing pain hypersensitivities associated with peripheral nerve injury. While systemic administration of KOR agonists attenuates nociceptive sensory transmission, this effect appears to be a stress-induced effect as anxiolytic agents, including delta opioid receptor agonists, mitigate KOR agonist-induced analgesia. Additionally, while the role of KOR and dynorphin in driving the dysphoric and aversive components of stress and drug withdrawal has been well characterized, how this system mediates the negative emotional states associated with chronic pain is relatively unexplored. This review provides evidence that dynorphin and the KOR system contribute to the negative affective component of pain and that this receptor system likely contributes to the high comorbidity of mood disorders associated with chronic neuropathic pain.

An intricate relationship between pain and depression: clinical correlates, coactivation factors and therapeutic targets

INTRODUCTION

An apparent clinical relationship between pain and depression has long been recognized, which makes an enormous impact on the individual health care. At present, the practical implication of such overlapping symptomatology between pain and depression is not clear, but the prevalence estimates for depression are substantially inflated among patients with chronic pain and vice versa. This interaction has been labeled as the depression-pain syndrome or depression-pain dyad.

AREAS COVERED

This article discusses the neurobiological substrates and neuroanatomical pathways involved in pain-depression dyad along with newer therapeutic targets.

EXPERT OPINION

Several key themes emerged from our review of the relationship between depression and pain. First, the diagnosis of depression in pain or vice versa is particularly challenging, and the development of better diagnostic framework that involves both pain and depression is particularly required. Secondly, the entwined relationship between pain and depression supports the possibility of common coactivating factors that results in their neurophysiological overlap. A broad understanding of the role played by the central nervous system (CNS) in the processing of pain and depression may eventually lead to the introduction of triple reuptake inhibitors, agomelatine, vilazodone and ketamine with novel mechanism of action, hence appear to be of promising potential for pain with depression.

Selective lesioning of nucleus incertus with corticotropin releasing factor-saporin conjugate

The nucleus incertus (NI), a brainstem nucleus found in the pontine periventricular grey, is the primary source of the neuropeptide relaxin-3 in the mammalian brain. The NI neurons have also been previously reported to express several receptors and neurotransmitters, including corticotropin releasing hormone receptor 1 (CRF₁) and gamma-aminobutyric acid (GABA). The NI projects widely to putative neural correlates of stress, anxiety, depression, feeding behaviour, arousal and cognition leading to speculation that it might be involved in several neuropsychiatric conditions. On the premise that relaxin-3 expressing neurons in the NI predominantly co-express CRF₁ receptors, a novel method for selective ablation of the rat brain NI neurons using corticotropin releasing factor (CRF)-saporin conjugate is described. In addition to a behavioural deficit in the fear conditioning paradigm, reverse transcriptase polymerase chain reaction (RT-PCR), western blotting (WB) and immunofluorescence labelling (IF) techniques were used to confirm the NI lesion. We observed a selective and significant loss of CRF₁ expressing cells, together with a consistent decrease in relaxin-3 and GAD65 expression. The significant ablation of relaxin-3 positive neurons of the NI achieved by this lesioning approach is a promising model to explore the neuropsychopharmacological implications of NI/relaxin-3 in behavioural neuroscience.

Predator odor stress alters corticotropin-releasing factor-1 receptor (CRF1R)-dependent behaviors in rats

Humans with stress-related anxiety disorders exhibit increases in arousal and alcohol drinking, as well as altered pain processing. Our lab has developed a predator odor stress model that produces reliable and lasting increases in alcohol drinking. Here, we utilize this predator odor stress model to examine stress-induced increases in arousal, nociceptive processing, and alcohol self-administration by rats, and also to determine the effects of corticotropin-releasing factor-1 receptors (CRF1Rs) in mediating these behavioral changes. In a series of separate experiments, rats were exposed to predator odor stress, then tested over subsequent days for thermal nociception in the Hargreaves test, acoustic startle reactivity, or operant alcohol self-administration. In each experiment, rats were systemically injected with R121919, a CRF1R antagonist, and/or vehicle. Predator odor stress increased thermal nociception (i.e., hyperalgesia) and acoustic startle reactivity. Systemic administration of R121919 reduced thermal nociception and hyperarousal in stressed rats but not unstressed controls, and reduced operant alcohol responding over days. Stressed rats exhibited increased sensitivity to the behavioral effects of R121919 in all three tests, suggesting up-regulation of brain CRF1Rs number and/or function in stressed rats. These results suggest that post-stress alcohol drinking may be driven by a high-nociception high-arousal state, and that brain CRF1R signaling mediates these stress effects.

Nicotine dependence produces hyperalgesia: role of corticotropin-releasing factor-1 receptors (CRF1Rs) in the central amygdala (CeA)

Because tobacco use has a large negative health and financial impact on society, it is critical to identify the factors that drive excessive use. These factors include the aversive withdrawal symptoms that manifest upon cessation of tobacco use, and may include increases in nociceptive processing. Corticotropin-releasing factor (CRF) signalling in the central amygdala (CeA) has been attributed an important role in: (1) central processing of pain, (2) excessive nicotine use that results in nicotine dependence, and (3) in mediating the aversive symptoms that manifest following cessation of tobacco exposure. Here, we describe three experiments in which the main hypothesis was that CRF/CRF1 receptor (CRF1R) signalling in the CeA mediates nicotine withdrawal-induced increases in nociceptive sensitivity in rats that are dependent on nicotine. In Experiment 1, nicotine-dependent rats withdrawn from chronic intermittent (14-h/day) nicotine vapor exhibited decreased hind paw withdrawal latencies in response to a painful thermal stimulus in the Hargreaves test, and this effect was attenuated by systemic administration of the CRF1R antagonist, R121919. In Experiment 2, nicotine-dependent rats withdrawn from nicotine vapor exhibited robust increases in mRNA for CRF and CRF1Rs in CeA. In Experiment 3, intra-CeA administration of R121919 reduced thermal nociception only in nicotine-dependent rats. Collectively, these results suggest that nicotine dependence increases CRF/CRF1R signalling in the CeA that mediates withdrawal-induced increases in sensitivity to a painful stimulus. Future studies will build on these findings by exploring the hypothesis that nicotine withdrawal-induced reduction in pain thresholds drive excessive nicotine use via CRF/CRF1R signalling pathways.

A Mechanism-Based Approach to Prevention of and Therapy for Fibromyalgia

Fibromyalgia syndrome (FMS) is characterized by pain referred to deep tissues. Diagnosis and treatment of FMS are complicated by a variable coexistence with regional pain, fatigue, sleep disruption, difficulty with mentation, and depression. The widespread, deep pain of FMS can be a consequence of chronic psychological stress with autonomic dysregulation. Stress acts centrally to facilitate pain and acts peripherally, via sympathetic vasoconstriction, to establish painful muscular ischemia. FMS pain, with or without a coexistent regional pain condition, is stressful, setting up a vicious circle of reciprocal interaction. Also, stress interacts reciprocally with systems of control over depression, mentation, and sleep, establishing FMS as a multiple-system disorder. Thus, stress and the ischemic pain it generates are fundamental to the multiple disorders of FMS, and a therapeutic procedure that attenuates stress and peripheral vasoconstriction should be highly beneficial for FMS. Physical exercise has been shown to counteract peripheral vasoconstriction and to attenuate stress, depression, and fatigue and improve mentation and sleep quality. Thus, exercise can interrupt the reciprocal interactions between psychological stress and each of the multiple-system disorders of FMS. The large literature supporting these conclusions indicates that exercise should be considered strongly as a first-line approach to FMS therapy.

Alcohol dependence as a chronic pain disorder

Dysregulation of pain neurocircuitry and neurochemistry has been increasingly recognized as playing a critical role in a diverse spectrum of diseases including migraine, fibromyalgia, depression, and PTSD. Evidence presented here supports the hypothesis that alcohol dependence is among the pathologies arising from aberrant neurobiological substrates of pain. In this review, we explore the possible influence of alcohol analgesia and hyperalgesia in promoting alcohol misuse and dependence. We examine evidence that neuroanatomical sites involved in the negative emotional states of alcohol dependence also play an important role in pain transmission and may be functionally altered under chronic pain conditions. We also consider possible genetic links between pain transmission and alcohol dependence. We propose an allostatic load model in which episodes of alcohol intoxication and withdrawal, traumatic stressors, and injury are each capable of dysregulating an overlapping set of neural substrates to engender sensory and affective pain states that are integral to alcohol dependence and comorbid conditions such as anxiety, depression, and chronic pain.

Experimental hypoglycemia is a human model of stress-induced hyperalgesia

Hypoglycemia is a physiological stress that leads to the release of stress hormones, such as catecholamines and glucocorticoids, and proinflammatory cytokines. These factors, in euglycemic animal models, are associated with stress-induced hyperalgesia. The primary aim of this study was to determine whether experimental hypoglycemia in humans would lead to a hyperalgesic state. In 2 separate 3-day admissions separated by 1 to 3 months, healthy study participants were exposed to two 2-hour euglycemic hyperinsulinemic clamps or two 2-hour hypoglycemic hyperinsulinemic clamps. Thermal quantitative sensory testing and thermal pain assessments were measured the day before and the day after euglycemia or hypoglycemia. In contrast to prior euglycemia exposure, prior hypoglycemia exposure resulted in enhanced pain sensitivity to hot and cold stimuli as well as enhanced temporal summation to repeated heat-pain stimuli. These findings suggest that prior exposure to hypoglycemia causes a state of enhanced pain sensitivity that is consistent with stress-induced hyperalgesia. This human model may provide a framework for hypothesis testing and targeted, mechanism-based pharmacological interventions to delineate the molecular basis of hyperalgesia and pain susceptibility.

Brain-specific inactivation of the Crhr1 gene inhibits post-dependent and stress-induced alcohol intake, but does not affect relapse-like drinking

Corticotropin-releasing hormone (CRH) and its receptor, CRH receptor-1 (CRHR1), have a key role in alcoholism. Especially, post-dependent and stress-induced alcohol intake involve CRH/CRHR1 signaling within extra-hypothalamic structures, but a contribution of the hypothalamic-pituitary-adrenal (HPA) axis activity might be involved as well. Here we examined the role of CRHR1 in various drinking conditions in relation to HPA and extra-HPA sites, and studied relapse-like drinking behavior in the alcohol deprivation model (ADE). To dissect CRH/CRHR1 extra-HPA and HPA signaling on a molecular level, a conditional brain-specific Crhr1-knockout (Crhr1(NestinCre)) and a global knockout mouse line were studied for basal alcohol drinking, stress-induced alcohol consumption, deprivation-induced intake, and escalated alcohol consumption in the post-dependent state. In a second set of experiments, we tested CRHR1 antagonists in the ADE model. Stress-induced augmentation of alcohol intake was lower in Crhr1(NestinCre) mice as compared with control animals. Crhr1(NestinCre) mice were also resistant to escalation of alcohol intake in the post-dependent state. Contrarily, global Crhr1 knockouts showed enhanced stress-induced alcohol consumption and a more pronounced escalation of intake in the post-dependent state than their control littermates. Basal intake and deprivation-induced intake were unaltered in both knockout models when compared with their respective controls. In line with these findings, CRHR1 antagonists did not affect relapse-like drinking after a deprivation period in rats. We conclude that CRH/CRHR1 extra-HPA and HPA signaling may have opposing effects on stress-related alcohol consumption. CRHR1 does not have a role in basal alcohol intake or relapse-like drinking situations with a low stress load.