Corticotropin-releasing factor 1 receptor-mediated mechanisms inhibit colonic hypersensitivity in rats

The potential relationship between stress and irritable bowel syndrome (IBS) symptomatology suggests a possible role for stress-mediating hormones, such as corticotropin-releasing factor (CRF), in the altered perception of stimuli in IBS patients. In previous studies, Wistar-Kyoto (WKY) rats with genetic indices of high anxiety demonstrated colonic hypersensitivity coupled with a high basal level of CRF within the central nervous system. In the current study we tested the hypothesis that a selective, non-peptide CRF1 receptor antagonist, antalarmin, would inhibit hypersensitivity in the WKY rat colon. Colonic sensitivity was determined by monitoring a visceromotor behavioural response during innocuous levels of colorectal distention (30 mmHg). In high anxiety WKY rats we found that antalarmin (20 mg kg-1, i.p.) significantly decreased the visceromotor response induced by colorectal distention. In a second study central administration (i.c.v.) of CRF was used to induce colonic hypersensitivity in lower anxiety Fischer 344 (F-344) rats, and in this model, antalarmin significantly inhibited the CRF-induced colonic hypersensitivity. In summary, a selective CRF1 receptor antagonist, antalarmin, inhibits colonic hypersensitivity apparent in WKY rats or in F-344 rats given a central administration of CRF. Our findings suggest that CRF1 receptor antagonism may represent a novel therapeutic approach for the treatment of IBS.     

The CRF1 receptor antagonist, antalarmin, reverses isolation-induced up-regulation of dopamine D2 receptors in the amygdala and nucleus accumbens of fawn-hooded rats

We have previously shown that Fawn-Hooded (FH) rats reared in isolation display an anxiety-like phenotype and an enhanced acquisition of ethanol seeking behaviour. Furthermore, antalarmin, a selective corticotrophin-releasing factor type 1 (CRF1) receptor antagonist, reduces isolation-induced acquisition and maintenance of volitional ethanol consumption in this strain. The aim of this study was to investigate the ability of CRF1 receptor antagonism by antalarmin to impact upon brain chemistry in both isolated and group-housed FH rats. To achieve this, FH rats were reared, from weaning, in either group-housed or isolation-housed conditions and at 12 weeks of age were treated with antalarmin (20 mg/kg, i.p; n = 10 per group) or vehicle (1 mL/kg, i.p; n = 10 per group) bi-daily for ten consecutive days before being killed and their brains removed for neurochemical analyses. Autoradiography and in situ hybridization was employed to analyse changes in the dopaminergic and neurotrophin systems. Isolation rearing increased dopamine D2 receptor density in the central amygdala and nucleus accumbens, an effect reversed by antalarmin treatment. Conversely, treatment with antalarmin had no impact upon the isolation-induced alterations of the mRNA encoding brain-derived neurotrophic factor or the TrkB receptor. Collectively, these findings demonstrate that multiple signalling systems are susceptible to modulation by social isolation and that antalarmin can reverse some, but not all, isolation-induced alterations in brain chemistry.     

Analgesic action of suspended moxibustion in rats with chronic visceral hyperalgesia correlates with enkephalins in the spinal cord

Rats that modeled chronic visceral hyperalgesia received suspended moxibustion at bilateral Tianshu (ST25) and Shangjuxu (ST37) once daily over a period of 7 days. Results show that suspended moxibustion significantly depressed abdominal withdrawal reflex scores and increased enkephalin concentration in the spinal cord. The experimental findings suggest that spinal enkephalins contributed to the analgesic effect of suspended moxibustion in rats with chronic visceral hyperalgesia.     

A novel water-soluble selective CRF1 receptor antagonist,NBI 35965, blunts stress-induced visceral hyperalgesia and colonic motor function in rats

The stress response involves the activation of two corticotropin-releasing factor (CRF) receptor subtypes. We investigated the role of CRF1 in stress-related visceral responses. A novel water-soluble tricyclic CRF1 antagonist, NBI 35965 was developed that displayed a high affinity for CRF1 (Ki approximately 4 nM) while having no binding affinity to CRF2. This antagonist also inhibited the stimulation of cAMP induced by sauvagine in CRF1 transfected cells. NBI 35965 administered per orally (p.o.) in rats (1, 3, 10 or 30 mg/kg) inhibited dose-dependently [125I]sauvagine binding selectively at brain sites of CRF1 distribution as shown by ex vivo receptor autoradiography. At the highest doses, NBI 35965 completely prevented [125I]sauvagine labeling in the cortex. NBI 35965 (10 mg/kg) administered p.o. or subcutaneously (s.c.) 1 h before intravenous CRF completely blocked the 81% shortening of distal colonic transit time induced by CRF. NBI 35965 (20 mg/kg s.c.) significantly reduced the defecation in response to water avoidance stress but not that induced by s.c. carbachol. In adult male Long-Evans rats that had undergone maternal separation, acute water avoidance stress significantly increased the visceromotor response to colorectal distention (20-80 mmHg) by 42+/-19% compared with the response before stress. Stress-induced visceral hyperalgesia was abolished by NBI 35965 (20 mg/kg, s.c.). The data show that NBI 35965 is a novel water-soluble selective CRF1 antagonist with bioavailability to the brain upon peripheral administration and that CRF1 receptor signaling pathways are involved in water avoidance stress-induced hyperalgesia to colorectal distention and stimulation of colonic transit.     

Alleviation of the effects of endotoxin exposure on behavior and hippocampal IL-1beta by a selective non-peptide antagonist of corticotropin-releasing factor receptors

Previous research has shown that lipopolysaccharide (LPS) or interleukin-1beta (IL-1beta) administration produces learning/memory deficits in a variety of paradigms. In our laboratory, we have consistently observed LPS-induced behavioral alterations in a two-way active avoidance conditioning paradigm. Following LPS administration, one factor that affects cytokine production is corticotropin-releasing factor (CRF). CRF has well known anti-inflammatory effects, via stimulation of ACTH and corticosterone release. However, CRF acting directly on immune cells or within the CNS may potentiate proinflammatory effects. The current experiments explored the potential of antalarmin, a CRF-R1 non-peptide antagonist, to diminish or negate deficits observed with LPS administration. On the first day of testing, four-month-old male C57BL/6J mice received an intraperitoneal (i.p.) injection of antalarmin, followed 90min later by a second i.p. injection of LPS 4h prior to two-way active avoidance conditioning testing. As hypothesized, LPS administration altered performance. However, pretreatment with antalarmin attenuated the adverse effects of LPS administration. Moreover, evidence indicates that antalarmin attenuated hippocampal, but not peripheral, cytokine release. The behavioral results cannot be explained by alterations in the HPA axis, as antalarmin did not affect the LPS-induced rise in corticosterone. The current research contributes preliminary evidence that CRF may be an important factor in the development of LPS-induced behavioral effects, and that blocking the activity of CRF may be sufficient to alleviate some of the effects of endotoxin exposure, possibly due to diminished LPS-induced IL-1beta release in the dorsal hippocampus.     

5-HT3 receptors mediate the time-dependent vagal afferent modulation of nociception during chronic food allergen-sensitized visceral hyperalgesia in rats

Abstract  Converging lines of evidence demonstrate a vagally mediated antinociceptive pathway in animals undergoing acute visceral insults, the contribution of this system to visceral pain following chronic noxious stimuli is unknown. 5-HT₃ receptor (5-HT₃Rs) on spinal afferents are crucially involved in nociceptive processing, the role of 5-HT₃Rs on vagal afferents is unclear. The aim of the present study was to determine the contribution of vagal afferents to visceral nociception in rats undergoing chronic luminal allergen stimulation and whether it involves vagal 5-HT₃Rs. Sensitized rats received chicken egg albumin (EA, 1 mg mL⁻¹) in drinking water for 2 weeks (day 1–14). Visceromotor response (VMR) to colorectal distension [colorectal distension (CRD), 60 mmHg] and the levels of mRNA encoding 5-HT₃R (including 3A and 3B subunits) in the nodose ganglia (NG) were evaluated on day 2, 4, 8 and 15. Chronic EA challenge induced gradually increased visceral nociception, with a peak on day 15. Subdiaphragmatic vagotomy or functional deafferentation with capsaicin abolished this time-dependent manner, inducing hyperalgesia from day 2, lasting to day 15. Intraluminal infusion of a 5-HT₃R antagonist (granisetron), whether alone or infused after local mucosa anaesthetic with 1% lidocaine, mimicked the effects of vagotomy. The mRNA levels for 5-HT_3B or 5-HT_3A subunit in the NG showed an opposite time-course to that of visceral pain, which increased from day 2, then decreased gradually to levels lower than those of controls. Our results demonstrate a time-dependent vagal afferent modulation of chronic allergen-sensitized visceral hyperalgesia, which may involve a 5-HT₃R pathway.     

Peripheral α-helical CRF (9-41) does not reverse stress-induced mast cell dependent visceral hypersensitivity in maternally separated rats

Background Acute stress‐induced hypersensitivity to colorectal distention was shown to depend on corticotropin releasing factor (CRF)‐induced mast cell degranulation. At present it remains unclear whether CRF also induces chronic poststress activation of these cells. Accordingly, the objective of this study was to compare pre‐ and poststress CRF‐receptor antagonist treatment protocols for their ability to, respectively, prevent and reverse mast cell dependent visceral hypersensitivity in a rat model of neonatal maternal separation. Methods The visceromotor response to colonic distention was assessed in adult maternally separated and non‐handled rats before and at different time points after 1 h of water avoidance (WA). Rats were treated with the mast cell stabilizer doxantrazole and the CRF receptor‐antagonist α‐helical‐CRF (9‐41). Western blotting was used to assess mucosal protein levels of the mast cell protease RMCP‐2 and the tight junction protein occludin. Key Results In maternally separated, but not in non‐handled rats, WA induced chronic hypersensitivity (up to 30 days) to colorectal distention. Visceral hypersensitivity was prevented, but could not be reversed by administration of α‐helical‐CRF (9‐41). In contrast, however, the mast cell stabilizer doxantrazole reversed visceral hypersensitivity. Compared with vehicle‐treated rats, pre‐WA α‐helical‐CRF (9‐41) treated animals displayed higher mucosal RMCP‐2 and occludin levels. Conclusions & Inferences Water avoidance‐stress leads to persistent mast cell dependent visceral hypersensitivity in maternally separated rats, which can be prevented, but not reversed by blockade of peripheral CRF‐receptors. We conclude that persistent poststress mast cell activation and subsequent visceral hypersensitivity are not targeted by CRF‐receptor antagonists.     

Vagal modulation of nociception is mediated by adrenomedullary epinephrine in the rat

Vagal afferent activity modulates mechanical nociceptive threshold and inflammatory mediator-induced hyperalgesia, effects that are mediated by the adrenal medulla. To evaluate the role of epinephrine, the major hormone released from the adrenal medulla, the beta2-adrenergic receptor antagonist ICI 118,551 was chronically administered to vagotomized rats and epinephrine to normal rats. In vagotomized rats, chronic administration of ICI 118,551 markedly attenuated vagotomy-induced enhancement of bradykinin hyperalgesia but had no effect on nociceptive threshold. In normal rats, chronic epinephrine had the opposite effect, enhancing bradykinin hyperalgesia. Like vagotomy-, epinephrine-induced enhancement of hyperalgesia developed slowly, taking 14 days to reach its peak. Vagotomy induced a chronic elevation in plasma concentrations of epinephrine. We suggest that ongoing activity in vagal afferents inhibits the release of epinephrine from the adrenal medulla. Chronically elevated levels of epinephrine, occurring after vagotomy, desensitize peripheral beta2-adrenergic receptors and lead to enhancement of bradykinin hyperalgesia. The ability of prolonged elevated plasma levels of epinephrine to sensitize bradykinin receptors could contribute to chronic generalized pain syndromes.     

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.     

SYM 2081, an agonist that desensitizes kainate receptors,attenuates capsaicin and inflammatory hyperalgesia

Excitatory amino acids acting at non-NMDA receptors contribute to transmission of nociceptive information. SYM 2081 ((2S,4R)-4-methyl glutamic acid) desensitizes kainate receptors, one subtype of non-NMDA receptors, to subsequent release of excitatory amino acids and thus may attenuate transmission of nociceptive information. To determine if SYM 2081 can prevent development of hyperalgesia, SYM 2081 (10, 50 or 100 mg/kg, i.p.) was administered prior to injection of capsaicin into the hindpaw of rats, which produces mechanical and heat hyperalgesia. To determine if SYM 2081 can reduce ongoing inflammatory hyperalgesia, SYM 2081 (10 or 100 mg/kg, i.p.) was administered after development of carrageenan-evoked hyperalgesia. Intraplantar injection of capsaicin produced an increase in hindpaw withdrawal frequency to mechanical stimuli (from 4+/-2 to 41+/-7%; mean+/-S.E.M.) and a decrease in withdrawal latency to heat (from 12.3+/-0.3 to 5.9+/-0.4 s) in rats that received vehicle. In contrast, rats that received SYM 2081 (100 mg/kg) prior to injection of capsaicin exhibited a lower hindpaw withdrawal frequency (18+/-4%) and a longer withdrawal latency (7.7+/-0.5 s). Intrathecal (1-100 microg/5 microl), but not intraplantar (10 or 100 microg/50 microl), injection of SYM 2081 attenuated the development of capsaicin-evoked heat hyperalgesia suggesting that SYM 2081's antihyperalgesic effects were due to its central effects. Furthermore, SYM 2081 completely reversed ongoing carrageenan-evoked mechanical hyperalgesia and partially (approximately 50%) reversed ongoing heat hyperalgesia. The present study demonstrates that administration of a high-potency ligand that selectively desensitizes kainate receptors attenuates the development of mechanical and heat hyperalgesia and attenuates ongoing inflammatory hyperalgesia.     

Control by tachykinin NK(2) receptors of CRF(1) receptor-mediated activation of hippocampal acetylcholine release in the rat and guinea-pig

In vivo microdialysis was employed to explore the effects of different selective non-peptides NK(1),NK(2) and NK(3) receptor antagonists on the corticotropin releasing factor (CRF)-induced release of acetylcholine (ACh) in the hippocampus of rats and guinea-pigs. In both species, the intracerebroventricular (i.c.v.) administration of CRF produced a time- and dose-dependent increase in hippocampal ACh release that was totally suppressed by an intraperitoneally (i.p.) pretreatment with the selective non-peptide CRF(1) receptor antagonist antalarmin (30 mg/kg). Pretreatment with the selective NK(2) receptor antagonist SR48968 (1mg/kg, i.p.) significantly reduced the increase of ACh induced by CRF. In contrast, its low-affinity enantiomer SR48965 (1mg/kg, i.p.) or the NK(1) receptor antagonist, GR205171 (1mg/kg, i.p.) did not exert any antagonist effect. Moreover, administration of the selective NK(3) receptor antagonist SR142801 (1mg/kg, i.p.) did not significantly reduce the CRF-induced hippocampal ACh release in guinea-pigs (the only species studied). The selective activity of SR48968 versus GR205171 or SR142801 indicates that NK(2) receptors play a major role in the control of CRF-induced hippocampal ACh release. Moreover, in freely moving rats, two sessions of stroking of the neck and back of the rat for 30 min, at 90 min intervals, known to be a stressful stimulus, produced a marked and reproducible increase in hippocampal ACh release. This effect was prevented by the administration of the two selective non-peptide CRF1 and NK(2) receptor antagonists antalarmin (30 mg/kg, i.p.) and SR48968 (1mg/kg, i.p.), respectively. This suggests that stress-induced activation of the hippocampal ACh system may be under the control of both endogenously released CRF and NKA, and opens the possibility of the existence of a functional interplay between the pathways containing these peptides as we observed in our experiments on anaesthetized animals.     

Mild moxibustion at Tianshu (ST 25) decreases expression of prokineticin-1 and prokineticin receptor-1 in colon tissue of rats with chronic visceral hyperalgesia

Prokineticin-1 and prokineticin receptor-1 play important roles in visceral hypersensitivity and in-flammatory pain. Visceral hypersensitivity is closely associated with irritable bowel syndrome. Mild moxibustion can relieve chronic visceral hyperalgesia in rats with irritable bowel syndrome. We hypothesized that prokineticin-1 and prokineticin receptor-1 is the key target in the mechanism. This study established chronic visceral hyperalgesia rat models by colorectal distention. Protein and mRNA expression ...     

Stress and visceral pain: from animal models to clinical therapies

Epidemiological studies have implicated stress (psychosocial and physical) as a trigger of first onset or exacerbation of irritable bowel syndrome (IBS) symptoms of which visceral pain is an integrant landmark. A number of experimental acute or chronic exteroceptive or interoceptive stressors induce visceral hyperalgesia in rodents although recent evidence also points to stress-related visceral analgesia as established in the somatic pain field. Underlying mechanisms of stress-related visceral hypersensitivity may involve a combination of sensitization of primary afferents, central sensitization in response to input from the viscera and dysregulation of descending pathways that modulate spinal nociceptive transmission or analgesic response. Biochemical coding of stress involves the recruitment of corticotropin releasing factor (CRF) signaling pathways. Experimental studies established that activation of brain and peripheral CRF receptor subtype 1 plays a primary role in the development of stress-related delayed visceral hyperalgesia while subtype 2 activation induces analgesic response. In line with stress pathways playing a role in IBS, non-pharmacologic and pharmacologic treatment modalities aimed at reducing stress perception using a broad range of evidence-based mind-body interventions and centrally-targeted medications to reduce anxiety impact on brain patterns activated by visceral stimuli and dampen visceral pain.     

Knockdown of corticotropin-releasing factor in the central amygdala reverses persistent viscerosomatic hyperalgesia

Gastrointestinal nociception is exacerbated by chronic stress through an unknown mechanism. The amygdala is a key nucleus involved in the autonomic and neuroendocrine responses to stress. The goal of this study was to test the hypothesis that prolonged exposure of the central amygdala (CeA) to stress or the stress hormone cortisol (or corticosterone in rats) induces nociceptive behaviors mediated by corticotropin-releasing factor (CRF) within the CeA. We selectively knocked down CRF in the CeA via antisense oligodeoxynucleotides (ASO) in animals with targeted, stereotaxically placed corticosterone (CORT) micropellets or following repeated water avoidance stress (WAS). CRF expression in the CeA was analyzed concurrently with the assessment of visceral hypersensitivity to colonic distension and mechanical somatic withdrawal threshold. The responses were characterized at 7 or 28 days post implantation of the CORT micropellet or following 7 days of WAS. Exposure of the CeA to elevated CORT or WAS increased CRF expression and heightened visceral and somatic sensitivity. Infusion of CRF ASO into the CeA decreased CRF expression and attenuated visceral and somatic hypersensitivity in both models. Our study provides important evidence for a CRF-mediated mechanism specifically within the CeA that regulates stress-induced visceral and somatic nociception.     

Effect of E. coli Nissle 1917 on post-inflammatory visceral sensory function in a rat model

OBJECTIVE: Visceral hyperalgesia (VH) plays a key role for the manifestation of functional gastrointestinal (GI) disorders. In a subgroup of patients, the initial manifestation is preceded by GI inflammation. Recent studies have demonstrated an improvement of inflammation and symptoms during treatment with Escherichia coli Nissle 1917 (EcN). AIM: We aimed to characterize the effects of EcN on visceral sensitivity in a rat model of post-inflammatory VH. METHODS: Male Lewis rats underwent colorectal instillation of trinitrobenzenesulphonic acid (TNBS) plus an equal amount of ethanol (test group) or physiological saline solution (control group). After 28, 35 and 42 days, standardized colorectal distensions were performed and the visceromotor reflex (VMR) of abdominal wall muscles was quantified by electromyographic recording. From day 28 onwards, EcN was administered in drinking water. RESULTS: After TNBS, a significant increase of VMR was observed compared with saline controls over all study days. Administration of EcN reduced the TNBS-induced hyperalgesia [EcN: 863+/-125 microV vs placebo: 1258+/-157 microV (P<0.05)] at day 35, while there were no significant alterations at any other study day. CONCLUSION: The EcN administration caused a significant reduction of VH. Whether EcN might play a role in the treatment of post-infectious functional bowel disorders remains to be investigated in further studies.     

Urocortin shares the memory modulating effects of corticotropin-releasing factor (CRF): mediation by CRF1 receptors

Intracerebroventricular (i.c.v.) administration of corticotropin-releasing factor (CRF) biphasically affects performance in tests of learning and memory. In the present study, we used CRF, urocortin (Ucn), a recently cloned CRF homologue, and CRF receptor antagonists, to determine which CRF receptor subtype(s) mediate the memory modulating effects of CRF receptor agonists in male Wistar rats. Under difficult learning conditions (massed trials), i.c.v. pretreatment with CRF or Ucn facilitated the acquisition of spatial navigation in the Morris water maze in a non-dose-dependent fashion (optimal doses of 0.1 and 0.03 microg, respectively). Under less difficult learning conditions (spaced trials), both peptides impaired water maze performance. In addition, with i.c.v. posttraining treatment, the peptides were equipotent (1.0 microg) in facilitating the consolidation of passive avoidance learning. The performance-enhancing effects of Ucn in both water maze and passive avoidance paradigms were reversed by i.c.v. pretreatment with D-Phe CRF(12-41) (2.5, 5 microg), a broad CRF(1)/CRF(2) receptor antagonist, or antalarmin (10 microg), a potent, nonpeptide, CRF(1) selective receptor antagonist. Thus, Ucn shares CRF's memory-modulating effects, and these effects appear to be mediated via the CRF(1) receptor. These findings are consistent with the hypothesis that CRF receptor agonists affect performance in tests of learning and memory by increasing arousal.     

Pre-emptive analgesia in rats with artificial ureteric calculosis. Effects on visceral pain behavior in the post-operative period

'Pre-emptive' analgesia is a controversial issue in both the clinical and experimental literature on pain. This paper investigates the effect of chronic (4 days) administration of morphine or ketoprofen initiated pre- or post-operatively on behavioral indicators of visceral pain and referred hyperalgesia in an animal model of artificial ureteric calculosis. In the morphine experiment, female Sprague-Dawley rats were treated i.p. with saline or morphine sulphate (2.5 or 5 mg/kg/day) starting either 45 min before or 45 min after surgery (pentobarbital anesthesia) for stone implantation in the left ureter, until the 4th day after intervention. Behavioral crises of ureteric pain were recorded (video-tape) in all rats over 4 days post-operatively. Number, duration and complexity of crises of stone-rats were significantly and dose-dependently reduced by administration of morphine with respect to saline in an identical manner for the pre- and post-operative treatment. In the ketoprofen experiment, rats were given saline or ketoprofen (15 mg/kg/day, in 3 i.p. injections per day) starting either pre- or post-operatively with the same paradigm as for the morphine study. Vocalization thresholds to electrical stimulation of the left oblique musculature were measured daily for 3 days pre- and 4 days post-operatively. Muscle hyperalgesia (post-operative decrease in threshold with respect to pre-stone implantation) was significantly reduced in extent and duration in ketoprofen with respect to saline-injected animals but no difference was found between the pre- and post-operative treatment. It is concluded that pre-emptive administration of morphine or ketoprofen has no advantage in reducing behavioral indicators of visceral pain and referred hyperalgesia in this animal model.     

Activation of corticotropin-releasing factor neurons and microglia in paraventricular nucleus precipitates visceral hypersensitivity induced by colorectal distension in rats

Visceral hypersensitivity is a major contributor to irritable bowel syndrome and other disorders with visceral pain. Substantial evidence has established that glial activation and neuro-glial interaction play a key role in the establishment and maintenance of visceral hypersensitivity. We recently demonstrated that activation of spinal microglial toll-like receptor 4 (TLR4)/myeloid differentiation factor 88 (MyD88)/nuclear factor κB (NF-κB) signaling facilitated the development of visceral hypersensitivity in a rat model developed by neonatal and adult colorectal distensions (CRDs). Hypothalamic paraventricular nucleus (PVN) plays a pivotal role in the pathogenesis of chronic pain. In this study, we examined the mechanism by which microglia and neurons in PVN establish and maintain visceral hypersensitivity and the involvement of TLR4 signaling. Visceral hypersensitivity was precipitated by adult colorectal distension (CRD) only in rats that experienced neonatal CRDs. Visceral hypersensitivity was associated with an increase in the expression of c-fos, corticotropin-releasing factor (CRF) protein and mRNA in PVN, which could be prevented by intra-PVN infusion of lidocaine or small interfering RNA targeting the CRF gene. These results suggest PVN CRF neurons modulate visceral hypersensitivity. Adult CRD induced an increase in the expression of Iba-1 (a microglial marker), TLR4 protein, and its downstream effectors MyD88, NF-κB, as well as proinflammatory cytokines tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β) only in rats that experienced neonatal CRDs. Intra-PVN infusion of minocycline, a nonselective microglial inhibitor, attenuated the hyperactivity of TLR4 signaling cascade, microglial activation, and visceral hypersensitivity. Taken together, these data suggest that neonatal CRDs induce a glial activation in PVN. Adult CRD potentiates the glial and CRF neuronal activity, and precipitates visceral hypersensitivity and pain. TLR4 signaling and proinflammatory cytokines TNF-α and IL-1β may participate in neuro-glial interaction during the pathogenesis of visceral hypersensitivity.     

Anxiety states induced by post-weaning social isolation are mediated by CRF receptors in the dorsal raphe nucleus

Post-weaning social isolation of rats is utilized as a model of early life stress. We have previously demonstrated that rats exposed to post-weaning social isolation exhibit greater anxiety-like behaviors as adults. Furthermore, these rats exhibit greater density of corticotropin-releasing factor (CRF) type 2 receptors in the dorsal raphe nucleus. Therefore, we examined whether antagonism of CRF(2) receptors in the dorsal raphe nucleus reverses the effects of post-weaning social isolation on anxiety states. Male rats were reared in isolation or in groups from day of weaning (postnatal day [PND] 21) to mid-adolescence (PND42) and then allowed to develop to early adulthood housed in groups. At PND62, rats were either infused with vehicle, the CRF(1) receptor antagonist antalarmin (0.25-0.5 μg) or the CRF(2) receptor antagonist antisauvagine-30 (2 μg) into the dorsal raphe nucleus, 20 min prior to being introduced to the elevated plus maze. Isolation-reared rats showed reduced open arm behavior compared to group-reared rats, confirming the anxiogenic effects of post-weaning social isolation. Infusion of the CRF(2) receptor antagonist, but not the CRF(1) receptor antagonist, into the dorsal raphe nucleus of isolation-reared rats increased open arm behavior when compared to that of group-reared rats. Overall, the findings suggest that CRF(2) receptors within the dorsal raphe nucleus mediate anxiety-like states following post-weaning social isolation, and CRF(2) receptors may represent an important target for the treatment of anxiety disorders following early life stressors.     

Effects of the selective nonpeptide corticotropin-releasing factor receptor 1 antagonist antalarmin in the chronic mild stress model of depression in mice

Several recent studies on corticotropin-releasing factor (CRF) have suggested that this neuropeptide may play a role in depression. Consequently, CRF receptor antagonists have been proposed as potential new agents for the treatment of this condition. This study investigated the effects of a 4-week treatment with the well-known CRF(1) receptor antagonist, antalarmin, and the prototypical selective serotonin reuptake inhibitor (SSRI), fluoxetine, in the chronic mild stress (CMS) model in BALB/c mice. Animals were exposed to 9 weeks of CMS which rapidly (within 2 weeks) produced decrease of physical state (PS), body weight gain and blunted emotional response in the light/dark test. Chronic treatment with antalarmin (10 mg/kg ip) and fluoxetine (10 mg/kg ip) led to an improvement of CMS-induced modifications. These results suggest that CRF(1) receptor antagonists may represent potential antidepressants.