In vitro and in vivo analysis of the effects of corticotropin releasing factor on rat dorsal vagal complex

Magnetic Resonance Imaging of Gastric Motility in Conscious Rats

Introduction

Gastrointestinal (GI) magnetic resonance imaging (MRI) can simultaneously capture gastric peristalsis, emptying, and intestinal filling and transit. Performing GI MRI with animals requires anesthesia, which complicates physiology and confounds interpretation and translation from animals to humans. This study aims to enable MRI in conscious rats, and for the first time, characterize GI motor functions in awake versus anesthetized conditions.

Methods

We acclimated rats to remain awake, still, and minimally stressed during MRI. We scanned 14 Sprague-Dawley rats in both awake and anesthetized conditions after voluntarily consuming a contrast-enhanced test meal.

Results

Awake rats remained physiologically stable during MRI, showed gastric emptying of 23.7±1.4% after 48 minutes, and exhibited strong peristaltic contractions propagating through the antrum with a velocity of 0.72±0.04 mm/s, a relative amplitude of 40.7±2.3%, and a frequency of 5.1±0.1 cycles per minute. In the anesthetized condition, gastric emptying was about half of that in the awake condition, likely due to the effect of anesthesia in halving the amplitudes of peristaltic contractions rather than their frequency (not significantly changed) or velocity. In awake rats, the intestine filled more quickly and propulsive contractions were more occlusive.

Conclusion

We demonstrated the effective acquisition and analysis of GI MRI in awake rats. Awake rats show faster gastric emptying, stronger gastric contraction with a faster propagation speed, and more effective intestinal filling and transit, compared to anesthetized rats. Our protocol is expected to benefit future preclinical studies of GI physiology and pathophysiology.

Oxytocin and corticotropin-releasing hormone exaggerate nucleus tractus solitarii neuronal and synaptic activity following chronic intermittent hypoxia

Chronic intermittent hypoxia (CIH) in rodents mimics the hypoxia-induced elevation of blood pressure seen in individuals experiencing episodic breathing. The brainstem nucleus tractus solitarii (nTS) is the first site of visceral sensory afferent integration, and thus is critical for cardiorespiratory homeostasis and its adaptation during a variety of stressors. In addition, the paraventricular nucleus of the hypothalamus (PVN), in part through its nTS projections that contain oxytocin (OT) and/or corticotropin-releasing hormone (CRH), contributes to cardiorespiratory regulation. Within the nTS, these PVN-derived neuropeptides alter nTS activity and the cardiorespiratory response to hypoxia. Nevertheless, their contribution to nTS activity after CIH is not fully understood. We hypothesized that OT and CRH would increase nTS activity to a greater extent following CIH, and co-activation of OT+CRH receptors would further magnify nTS activity. Our data show that compared to their normoxic controls, 10 days' CIH exaggerated nTS discharge, excitatory synaptic currents and Ca2+ influx in response to CRH, which were further enhanced by the addition of OT. CIH increased the tonic functional contribution of CRH receptors, which occurred with elevation of mRNA and protein. Together, our data demonstrate that intermittent hypoxia exaggerates the expression and function of neuropeptides on nTS activity. KEY POINTS: Episodic breathing and chronic intermittent hypoxia (CIH) are associated with autonomic dysregulation, including elevated sympathetic nervous system activity. Altered nucleus tractus solitarii (nTS) activity contributes to this response. Neurons originating in the paraventricular nucleus (PVN), including those containing oxytocin (OT) and corticotropin-releasing hormone (CRH), project to the nTS, and modulate the cardiorespiratory system. Their role in CIH is unknown. In this study, we focused on OT and CRH individually and together on nTS activity from rats exposed to either CIH or normoxia control. We show that after CIH, CRH alone and with OT increased to a greater extent overall nTS discharge, neuronal calcium influx, synaptic transmission to second-order nTS neurons, and OT and CRH receptor expression. These results provide insights into the underlying circuits and mechanisms contributing to autonomic dysfunction during periods of episodic breathing.

Cdk5-dependent rapid formation and stabilization of dendritic spines by corticotropin-releasing factor

The neuropeptide corticotropin-releasing factor (CRF) exerts a pivotal role in modulating neuronal activity in the mammalian brain. The effects of CRF exhibit notable variations, depending on factors such as duration of exposure, concentration, and anatomical location. In the CA1 region of the hippocampus, the impact of CRF is dichotomous: chronic exposure to CRF impairs synapse formation and dendritic integrity, whereas brief exposure enhances synapse formation and plasticity. In the current study, we demonstrate long-term effects of acute CRF on the density and stability of mature mushroom spines ex vivo. We establish that both CRF receptors are present in this hippocampal region, and we pinpoint their precise subcellular localization within synapses by electron microscopy. Furthermore, both in vivo and ex vivo data collectively demonstrate that a transient surge of CRF in the CA1 activates the cyclin-dependent kinase 5 (Cdk5)-pathway. This activation leads to a notable augmentation in CRF-dependent spine formation. Overall, these data suggest that upon acute release of CRF in the CA1-SR synapse, both CRF-Rs can be activated and promote synaptic plasticity via activating different downstream signaling pathways, such as the Cdk5-pathway.

Perinatal high-fat diet exposure alters oxytocin and corticotropin releasing factor inputs onto vagal neurocircuits controlling gastric motility

Perinatal high-fat diet (pHFD) exposure alters the development of vagal neurocircuits that control gastrointestinal (GI) motility and reduce stress resiliency in offspring. Descending oxytocin (OXT; prototypical anti-stress peptide) and corticotropin releasing factor (CRF; prototypical stress peptide) inputs from the paraventricular nucleus (PVN) of the hypothalamus to the dorsal motor nucleus of the vagus (DMV) modulate the GI stress response. How these descending inputs, and their associated changes to GI motility and stress responses, are altered following pHFD exposure are, however, unknown. The present study used retrograde neuronal tracing experiments, cerebrospinal fluid extraction, in vivo recordings of gastric tone, motility and gastric emptying rates, and in vitro electrophysiological recordings from brainstem slice preparations to investigate the hypothesis that pHFD alters descending PVN-DMV inputs and dysregulates vagal brain-gut responses to stress. Compared to controls, rats exposed to pHFD had slower gastric emptying rates and did not respond to acute stress with the expected delay in gastric emptying. Neuronal tracing experiments demonstrated that pHFD reduced the number of PVNOXT neurons that project to the DMV, but increased PVNCRF neurons. Both in vitro electrophysiology recordings of DMV neurons and in vivo recordings of gastric motility and tone demonstrated that, following pHFD, PVNCRF -DMV projections were tonically active, and that pharmacological antagonism of brainstem CRF1 receptors restored the appropriate gastric response to brainstem OXT application. These results suggest that pHFD exposure disrupts descending PVN-DMV inputs, leading to a dysregulated vagal brain-gut response to stress. KEY POINTS: Maternal high-fat diet exposure is associated with gastric dysregulation and stress sensitivity in offspring. The present study demonstrates that perinatal high-fat diet exposure downregulates hypothalamic-vagal oxytocin (OXT) inputs but upregulates hypothalamic-vagal corticotropin releasing factor (CRF) inputs. Both in vitro and in vivo studies demonstrated that, following perinatal high-fat diet, CRF receptors were tonically active at NTS-DMV synapses, and that pharmacological antagonism of these receptors restored the appropriate gastric response to OXT. The current study suggests that perinatal high-fat diet exposure disrupts descending PVN-DMV inputs, leading to a dysregulated vagal brain-gut response to stress.

Stress-induced neuroplasticity in the gastric response to brainstem oxytocin in male rats

Previous studies have shown that pharmacological manipulations with stress-related hormones such as corticotropin-releasing factor and thyrotropin-releasing hormone induce neuroplasticity in brainstem vagal neurocircuits, which modulate gastric tone and motility. The prototypical antistress hormone oxytocin (OXT) has been shown to modulate gastric tone and motility via vagal pathways, and descending hypothalamic oxytocinergic inputs play a major role in the vagally dependent gastric-related adaptations to stress. The aim of this study was to investigate the possible cellular mechanisms through which OXT modulates central vagal brainstem and peripheral enteric neurocircuits of male Sprague-Dawley rats in response to chronic repetitive stress. After chronic (5 consecutive days) of homotypic or heterotypic stress load, the response to exogenous brainstem administration of OXT was examined using whole cell patch-clamp recordings from gastric-projecting vagal motoneurons and in vivo recordings of gastric tone and motility. GABAergic currents onto vagal motoneurons were decreased by OXT in stressed, but not in naïve rats. In naïve rats, microinjections of OXT in vagal brainstem nuclei-induced gastroinhibition via peripheral release of nitric oxide (NO). In stressed rats, however, the OXT-induced gastroinhibition was determined by the release of both NO and vasoactive intestinal peptide (VIP). Taken together, our data indicate that stress induces neuroplasticity in the response to OXT in the neurocircuits, which modulate gastric tone and motility. In particular, stress uncovers the OXT-mediated modulation of brainstem GABAergic currents and alters the peripheral gastric response to vagal stimulation.NEW & NOTEWORTHY The prototypical antistress hormone, oxytocin (OXT), modulates gastric tone and motility via vagal pathways, and descending hypothalamic-brainstem OXT neurocircuits play a major role in the vagally dependent adaptation of gastric motility and tone to stress. The current study suggests that in the neurocircuits, which modulate gastric tone and motility, stress induces neuroplasticity in the response to OXT and may reflect the dysregulation observed in stress-exacerbated functional motility disorders.

Brainstem Neuronal Circuitries Controlling Gastric Tonic and Phasic Contractions: A Review

This review is on how current knowledge of brainstem control of gastric mechanical function unfolded over nearly four decades from the perspective of our research group. It describes data from a multitude of different types of studies involving retrograde neuronal tracing, microinjection of drugs, whole-cell recordings from rodent brain slices, receptive relaxation reflex, accommodation reflex, c-Fos experiments, immunohistochemical methods, electron microscopy, transgenic mice, optogenetics, and GABAergic signaling. Data obtained indicate the following: (1) nucleus tractus solitarius (NTS)-dorsal motor nucleus of the vagus (DMV) noradrenergic connection is required for reflex control of the fundus; (2) second-order nitrergic neurons in the NTS are also required for reflex control of the fundus; (3) a NTS GABAergic connection is required for reflex control of the antrum; (4) a single DMV efferent pathway is involved in brainstem control of gastric mechanical function under most experimental conditions excluding the accommodation reflex. Dual-vagal effectors controlling cholinergic and non-adrenergic and non-cholinergic (NANC) input to the stomach may be part of the circuitry of this reflex. (5) GABAergic signaling within the NTS via Sst-GABA interneurons determine the basal (resting) state of gastric tone and phasic contractions. (6) For the vagal-vagal reflex to become operational, an endogenous opioid in the NTS is released and the activity of Sst-GABA interneurons is suppressed. From the data, we suggest that the CNS has the capacity to provide region-specific control over the proximal (fundus) and distal (antrum) stomach through engaging phenotypically different efferent inputs to the DMV.

Central Neurocircuits Regulating Food Intake in Response to Gut Inputs-Preclinical Evidence

The regulation of energy balance requires the complex integration of homeostatic and hedonic pathways, but sensory inputs from the gastrointestinal (GI) tract are increasingly recognized as playing critical roles. The stomach and small intestine relay sensory information to the central nervous system (CNS) via the sensory afferent vagus nerve. This vast volume of complex sensory information is received by neurons of the nucleus of the tractus solitarius (NTS) and is integrated with responses to circulating factors as well as descending inputs from the brainstem, midbrain, and forebrain nuclei involved in autonomic regulation. The integrated signal is relayed to the adjacent dorsal motor nucleus of the vagus (DMV), which supplies the motor output response via the efferent vagus nerve to regulate and modulate gastric motility, tone, secretion, and emptying, as well as intestinal motility and transit; the precise coordination of these responses is essential for the control of meal size, meal termination, and nutrient absorption. The interconnectivity of the NTS implies that many other CNS areas are capable of modulating vagal efferent output, emphasized by the many CNS disorders associated with dysregulated GI functions including feeding. This review will summarize the role of major CNS centers to gut-related inputs in the regulation of gastric function with specific reference to the regulation of food intake.

Hypothalamic-vagal oxytocinergic neurocircuitry modulates gastric emptying and motility following stress

KEY POINTS

Stress triggers and exacerbates the symptoms of functional gastrointestinal disorders, such as delayed gastric emptying and impaired gastric motility. Understanding the mechanisms by which the neural circuits, impaired by stress, are restored may help to identify potential targets for more effective therapeutic interventions. Oxytocin administration or release ameliorates the stress-induced delayed gastric emptying and motility. However, is it unclear whether the effects are mediated via the hypothalamic-pituitary-adrenocortical axis or the oxytocinergic projections from the paraventricular nucleus of the hypothalamus to brainstem neurones of the dorsal vagal complex. We used Cre-inducible designer receptors exclusively activated by designer drugs to demonstrate the fundamental role of the oxytocinergic hypothalamic-vagal projections in the gastric adaptation to stress.

ABSTRACT

Stress triggers and exacerbates the symptoms of functional gastrointestinal (GI) disorders, such as delayed gastric emptying and impaired gastric motility. The prototypical anti-stress hormone, oxytocin (OXT), plays a major role in the modulation of gastric emptying and motility following stress. It is not clear, however, whether the amelioration of dysregulated GI functions by OXT is mediated via an effect on the hypothalamic-pituitary-adrenocortical axis or the oxytocinergic projections from the paraventricular nucleus of the hypothalamus (PVN) to neurones of the dorsal vagal complex (DVC). In the present study we tested the hypothesis that the activity of hypothalamic-vagal oxytocinergic neurocircuits plays a major role in the gastric adaptation to stress. Cre-inducible designer receptors exclusively activated by designer drugs (DREADDs) were injected into the DVC of rats and retrogradely transported to allow selective expression in OXT neurones in the PVN. Following acute stress and either chronic heterotypic (CHe) or chronic homotypic (CHo) stress, gastric emptying was assessed via the [13 C]-octanoic acid breath test, and gastric tone and motility were assessed via strain gauges sewn on the surface of the stomach. Activation of the hypothalamic-vagal oxytocinergic neurocircuitry, by DREADD agonist clozapine-N-oxide (CNO), prevented the delayed gastric emptying observed following acute or CHe stress, and 4th ventricular administration of CNO increased gastric tone and motility. Conversely, CNO-mediated inhibition of the hypothalamic-vagal oxytocinergic neurocircuitry prevented the CHo-induced adaptation in gastric emptying, and an increase in gastric tone and motility. Taken together, the data support the hypothesis that hypothalamic-vagal oxytocinergic neurocircuits play a major role in the modulation of gastric emptying and motility following stress.

Electroacupuncture ameliorates corticotrophin-releasing factor-induced jejunal dysmotility in a rat model of stress

BACKGROUND

Central injection of corticotrophin-releasing factor (CRF) mimics the effect of stress on gastrointestinal (GI) responses, including inhibition of GI motility. This study was designed to explore the effects of electroacupuncture (EA) on disordered jejunal motility in a rat model of stress induced by intracisternal (IC) injection of CRF.

METHODS

A stress model was established by IC injection of CRF in Sprague-Dawley rats. GI motility was evaluated by assessing gastric emptying (GE), gastrointestinal transit (GIT) and jejunal motility in vivo. EA was performed at ST36. The functional roles of CRF receptor subtype 1 and subtype 2 (CRFr1 and CRFr2) were examined by IC administration of the corresponding selective CRF antagonists. Protein expression of CRFr1 and CRFr2 in the hypothalamus and jejunum was detected by Western blotting.

RESULTS

IC injection of CRF significantly inhibited GE, GIT and jejunal motility. EA treatment remarkably improved the disturbed GI motility. Intriguingly, the disordered jejunal motility induced by central CRF was abolished by IC injection of a selective CRFr2 antagonist, indicating the essential role of central CRFr2 in mediating the stress-induced jejunal motor disorder. EA at ST36 decreased central and peripheral expression of CRFr2, which might be one of the potential mechanisms underlying the beneficial effect of EA on jejunal dysmotility in this rat model of stress.

CONCLUSION

This study suggested that EA at ST36 could ameliorate disordered jejunal motility induced by stress, and that this might be associated with the down-regulation of CRFr2.

Neuromechanism of acupuncture regulating gastrointestinal motility

Acupuncture has been used in China for thousands of years and has become more widely accepted by doctors and patients around the world. A large number of clinical studies and animal experiments have confirmed that acupuncture has a benign adjustment effect on gastrointestinal (GI) movement; however, the mechanism of this effect is unclear, especially in terms of neural mechanisms, and there are still many areas that require further exploration. This article reviews the recent data on the neural mechanism of acupuncture on GI movements. We summarize the neural mechanism of acupuncture on GI movement from four aspects: acupuncture signal transmission, the sympathetic and parasympathetic nervous system, the enteric nervous system, and the central nervous system.

Activation of CRF2 receptor increases gastric vagal afferent mechanosensitivity

Gastric vagal afferent (GVA) sensing of food-related mechanical stimuli is a crucial mechanism in the control of feeding behavior and gastric function. Stress is an important factor contributing to eating disorders and gastric diseases. Chronic stress has been shown to increase the mechanosensitivity of GVAs in mice and to reduce food intake and body weight. Whether the mechanosensitivity of GVAs is modulated by stress hormones is not known. This study aimed to determine the effect of stress hormones on GVA mechanosensitivity. The expression of stress hormone receptors in GVA cell bodies was determined in 8-wk-old male C57BL/6 mice using quantitative RT-PCR combined with laser capture microdissection. The mechanosensitivity of GVAs was determined in the absence and presence of stress hormones using an in vitro single-fiber recording preparation. NR3C1 and CRHR2 (mRNA isoforms of glucocorticoid receptor and CRF2 receptor, respectively) were expressed in GVA neurons. The glucocorticoid receptor agonist corticosterone had no effect on the mechanosensitivity of either tension or mucosal GVAs. Activation of CRF2 receptor by its specific analog, urocortin 3, significantly increased the mechanosensitivity of both tension and mucosal GVAs, an effect prevented by the CRF2 receptor antagonist astressin 2B. In conclusion, activation of CRF2 receptor increases the mechanosensitivity of GVAs. This may contribute to the stress- and CRF2 receptor-associated changes in feeding behavior and gastric function, possibly contributing to the hypersensitivity of GVAs in chronic stress conditions.

NEW & NOTEWORTHY Gastric vagal afferents (GVAs) relay food-related signals to the central nervous system, where they are processed, eventually leading to modulation of food intake and gastric function. GVA signaling can be modulated by an array of hormones. Stress has been shown to induce GVA hypersensitivity. This study demonstrates that GVA neurons express subtypes of stress hormone receptors, specifically CRF2. Furthermore, activation of CRF2 receptor increases GVA mechanosensitivity, which could have implications for food intake and gastric function.

GABAB Receptor Signaling in the Dorsal Motor Nucleus of the Vagus Stimulates Gastric Motility via a Cholinergic Pathway

Central nervous system regulation of the gastric tone and motility is primarily mediated via preganglionic neurons of the dorsal motor nucleus of the vagus (DMV). This is thought to occur by simultaneous engagement of both independent excitatory and inhibitory pathways from the DMV and has been proposed to underlie the opposing effects seen on gastric tone and motility in a number of in vivo models. Contrary to this view, we have been unable to find any evidence for this "dual effector" pathway. Since this possibility is so fundamental to how the brain-gut axis may interact in light of both peripheral and central demands, we decided to explore it further in two separate animal models previously used in conjunction with GABAB signaling to report the existence of a "dual effector" pathway. Using anesthetized rats or ferrets, we microinjected baclofen (7.5 pmol; n = 6), a GABAB agonist into the DMV of rats or intravenously administered it (0.5 mg/kg; n = 4) in ferrets. In rats, unilateral microinjection of baclofen into the DMV caused a robust dose-dependent increase in gastric tone and motility that was abolished by ipsilateral vagotomy and counteracted by pretreatment with atropine (0.1 mg/kg; IV). Similarly, as microinjection in the rats, IV administration of baclofen (0.5 mg/kg) in the ferrets induced its characteristic excitatory effects on gastric tone and motility, which were blocked by either pre- or post-treatment with atropine (0.1 mg/kg; IV). Altogether, our data provide evidence that the gastric musculature (other than the gastric sphincters) is regulated by a "single effector" DMV pathway using acetylcholine.

Role of estrogen and stress on the brain-gut axis

Symptoms of functional gastrointestinal disorders (FGIDs), including fullness, bloating, abdominal pain, and altered gastrointestinal (GI) motility, present a significant clinical problem, with a reported prevalence of 25%-40% within the general population. More than 60% of those affected seek and require healthcare, and affected individuals report a significantly decreased quality of life. FGIDs are highly correlated with episodes of acute and chronic stress and are increased in prevalence and reported severity in women compared with men. Although there is evidence that sex and stress interact to exacerbate FGID symptoms, the physiological mechanisms that mediate these sex-dependent disparities are incompletely understood, although hormonal-related differences in GI motility and visceral sensitivity have been purported to play a significant role in the etiology. In this mini review, we will discuss brain-gut axis control of GI motility and sensitivity, the influence of estrogen on GI motility and sensitivity, and stress modulation of the brain-gut axis.

Brain and Gut CRF Signaling: Biological Actions and Role in the Gastrointestinal Tract

BACKGROUND

Corticotropin-releasing factor (CRF) pathways coordinate behavioral, endocrine, autonomic and visceral responses to stress. Convergent anatomical, molecular, pharmacological and functional experimental evidence supports a key role of brain CRF receptor (CRF-R) signaling in stress-related alterations of gastrointestinal functions. These include the inhibition of gastric acid secretion and gastric-small intestinal transit, stimulation of colonic enteric nervous system and secretorymotor function, increase intestinal permeability, and visceral hypersensitivity. Brain sites of CRF actions to alter gut motility encompass the paraventricular nucleus of the hypothalamus, locus coeruleus complex and the dorsal motor nucleus while those modulating visceral pain are localized in the hippocampus and central amygdala. Brain CRF actions are mediated through the autonomic nervous system (decreased gastric vagal and increased sacral parasympathetic and sympathetic activities). The activation of brain CRF-R2 subtype inhibits gastric motor function while CRF-R1 stimulates colonic secretomotor function and induces visceral hypersensitivity. CRF signaling is also located within the gut where CRF-R1 activates colonic myenteric neurons, mucosal cells secreting serotonin, mucus, prostaglandin E2, induces mast cell degranulation, enhances mucosal permeability and propulsive motor functions and induces visceral hyperalgesia in animals and humans. CRF-R1 antagonists prevent CRF- and stressrelated gut alterations in rodents while not influencing basal state.

DISCUSSION

These preclinical studies contrast with the limited clinical positive outcome of CRF-R1 antagonists to alleviate stress-sensitive functional bowel diseases such as irritable bowel syndrome.

CONCLUSION

The translational potential of CRF-R1 antagonists in gut diseases will require additional studies directed to novel anti-CRF therapies and the neurobiology of brain-gut interactions under chronic stress.

Stress Adaptation Upregulates Oxytocin within Hypothalamo-Vagal Neurocircuits

Stress plays a pivotal role in the development and/or exacerbation of functional gastrointestinal (GI) disorders. The paraventricular nucleus of the hypothalamus (PVN) contains neurons that are part of the hypothalamic-pituitary-adrenal axis as well as preautonomic neurons innervating, among other areas, gastric-projecting preganglionic neurons of the dorsal vagal complex (DVC). The aim of the present study was to test the hypothesis that stress adaptation upregulates oxytocin (OXT) within PVN-brainstem vagal neurocircuitry. The retrograde tracer cholera toxin B (CTB) was injected into the DVC of rats which, after post-surgical recovery, were pair-housed and exposed to either homo- or heterotypic stress for five consecutive days. Fecal pellets were counted at the end of each stress load. Two hours after the last stressor, the whole brain was excised. Brainstem and hypothalamic nuclei were analyzed immunohistochemically for the presence of both OXT-immunopositive cells in identified preautonomic PVN neurons as well as OXT fibers in the DVC. Rats exposed to chronic homotypic, but not chronic heterotypic stress, had a significant increase in both number of CTB+ OXT co-localized neurons in the PVN as well as density of OXT-positive fibers in the DVC compared to control rats. These data suggest that preautonomic OXT PVN neurons and their projections to the DVC increase following adaptation to stress, and suggest that the possible up-regulation of OXT within PVN-brainstem vagal neurocircuitry may play a role in the adaptation of GI responses to stress.

Corticotrophin-Releasing Factor Modulates Cerebellar Purkinje Cells Simple Spike Activity in Vivo in Mice

Corticotropin-releasing factor (CRF) is a major neuromodulator that modulates cerebellar neuronal activity via CRF receptors during stress responses. In the cerebellar cortex, CRF dose-dependently increases the simple spike (SS) firing rate of Purkinje cells (PCs), while the synaptic mechanisms of this are still unclear. We here investigated the effect of CRF on the spontaneous SS activity of cerebellar PCs in urethane-anesthetized mice by in vivo electrophysiological recording and pharmacological methods. Cell-attached recordings from PCs showed that micro-application of CRF in cerebellar cortical molecular layer induced a dose-dependent increase in SS firing rate in the absence of GABA_A receptor activity. The CRF-induced increase in SS firing rate was completely blocked by a nonselective CRF receptor antagonist, α-helical CRF-(9–14). Nevertheless, application of either a selective CRF-R1 antagonist, BMS-763534 (BMS, 200 nM) or a selective CRF-R2 antagonist, antisauvagine-30 (200 nM) significantly attenuated, but failed to abolished the CRF-induced increase in PCs SS firing rate. In vivo whole-cell patch-clamp recordings from PCs showed that molecular layer application of CRF significantly increased the frequency, but not amplitude, of miniature postsynaptic currents (mEPSCs). The CRF-induced increase in the frequency of mEPSCs was abolished by a CRF-R2 antagonist, as well as protein kinase A (PKA) inhibitors. These results suggested that CRF acted on presynaptic CRF-R2 of cerebellar PCs resulting in an increase of glutamate release through PKA signaling pathway, which contributed to modulation of the cerebellar PCs outputs in Vivo in mice.

Acute high-fat diet upregulates glutamatergic signaling in the dorsal motor nucleus of the vagus

Obesity is associated with dysregulation of vagal neurocircuits controlling gastric functions, including food intake and energy balance. In the short term, however, caloric intake is regulated homeostatically although the precise mechanisms responsible are unknown. The present study examined the effects of acute high-fat diet (HFD) on glutamatergic neurotransmission within central vagal neurocircuits and its effects on gastric motility. Sprague-Dawley rats were fed a control or HFD diet (14% or 60% kcal from fat, respectively) for 3-5 days. Whole cell patch-clamp recordings and brainstem application of antagonists were used to assess the effects of acute HFD on glutamatergic transmission to dorsal motor nucleus of the vagus (DMV) neurons and subsequent alterations in gastric tone and motility. After becoming hyperphagic initially, caloric balance was restored after 3 days following HFD exposure. In control rats, the non- N-methyl-d-aspartate (NMDA) receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX), but not the NMDA receptor antagonist, amino-5-phosphonopentanoate (AP5), significantly decreased excitatory synaptic currents and action potential firing rate in gastric-projecting DMV neurons. In contrast, both AP5 and DNQX decreased excitatory synaptic transmission and action potential firing in acute HFD neurons. When microinjected into the brainstem, AP5, but not DNQX, decreased gastric motility and tone in acute HFD rats only. These results suggest that acute HFD upregulates NMDA receptor-mediated currents, increasing DMV neuronal excitability and activating the vagal efferent cholinergic pathway, thus increasing gastric tone and motility. Although such neuroplasticity may be a persistent adaptation to the initial exposure to HFD, it may also be an important mechanism in homeostatic regulation of energy balance. NEW & NOTEWORTHY Vagal neurocircuits are critical to the regulation of gastric functions, including satiation and food intake. Acute high-fat diet upregulates glutamatergic signaling within central vagal neurocircuits via activation of N-methyl-d-aspartate receptors, increasing vagal efferent drive to the stomach. Although it is possible that such neuroplasticity is a persistent adaptation to initial exposure to the high-fat diet, it may also play a role in the homeostatic control of feeding.

Vagally mediated gastric effects of brain stem α2-adrenoceptor activation in stressed rats

Chronic stress exerts vagally dependent effects to disrupt gastric motility; previous studies have shown that, among other nuclei, A2 neurons are involved in mediating these effects. Several studies have also shown robust in vitro and in vivo effects of α2-adrenoceptor agonists on vagal motoneurons. We have demonstrated previously that brainstem vagal neurocircuits undergo remodeling following acute stress; however, the effects following brief periods of chronic stress have not been investigated. Our aim, therefore, was to test the hypothesis that different types of chronic stress influence gastric tone and motility by inducing plasticity in the response of vagal neurocircuits to α2-adrenoreceptor agonists. In rats that underwent 5 days of either homotypic or heterotypic stress loading, we applied the α2-adrenoceptor agonist, UK14304, either by in vitro brainstem perfusion to examine its ability to modulate GABAergic synaptic inputs to vagal motoneurons or in vivo brainstem microinjection to observe actions to modulate antral tone and motility. In neurons from naïve rats, GABAergic currents were unresponsive to exogenous application of UK14304. In contrast, GABAergic currents were inhibited by UK14304 in all neurons from homotypic and, in a subpopulation of neurons, heterotypic stressed rats. In control rats, UK14304 microinjection inhibited gastric tone and motility via withdrawal of vagal cholinergic tone; in heterotypic stressed rats, the larger inhibition of antrum tone was due to a concomitant activation of peripheral nonadrenergic, noncholinergic pathways. These data suggest that stress induces plasticity in brainstem vagal neurocircuits, leading to an upregulation of α2-mediated responses. NEW & NOTEWORTHY Catecholaminergic neurons of the A2 area play a relevant role in stress-related dysfunction of the gastric antrum. Brief periods of chronic stress load induce plastic changes in the actions of adrenoceptors on vagal brainstem neurocircuits.

Inhibitory neurotransmission regulates vagal efferent activity and gastric motility

The gastrointestinal tract receives extrinsic innervation from both the sympathetic and parasympathetic nervous systems, which regulate and modulate the function of the intrinsic (enteric) nervous system. The stomach and upper gastrointestinal tract in particular are heavily influenced by the parasympathetic nervous system, supplied by the vagus nerve, and disruption of vagal sensory or motor functions results in disorganized motility patterns, disrupted receptive relaxation and accommodation, and delayed gastric emptying, amongst others. Studies from several laboratories have shown that the activity of vagal efferent motoneurons innervating the upper GI tract is inhibited tonically by GABAergic synaptic inputs from the adjacent nucleus tractus solitarius. Disruption of this influential central GABA input impacts vagal efferent output, hence gastric functions, significantly. The purpose of this review is to describe the development, physiology, and pathophysiology of this functionally dominant inhibitory synapse and its role in regulating vagally determined gastric functions.

Nutritional status-dependent endocannabinoid signalling regulates the integration of rat visceral information

KEY POINTS

Vagal sensory inputs transmit information from the viscera to brainstem neurones located in the nucleus tractus solitarii to set physiological parameters. These excitatory synapses exhibit a CB1 endocannabinoid-induced long-term depression (LTD) triggered by vagal fibre stimulation. We investigated the impact of nutritional status on long-term changes in this long-term synaptic plasticity. Food deprivation prevents LTD induction by disrupting CB1 receptor signalling. Short-term refeeding restores the capacity of vagal synapses to express LTD. Ghrelin and cholecystokinin, respectively released during fasting and refeeding, play a key role in the control of LTD via the activation of energy sensing pathways such as AMPK and the mTOR and ERK pathways.

ABSTRACT

Communication form the viscera to the brain is essential to set physiological homoeostatic parameters but also to drive more complex behaviours such as mood, memory and emotional states. Here we investigated the impact of the nutritional status on long-term changes in excitatory synaptic transmission in the nucleus tractus solitarii, a neural hub integrating visceral signals. These excitatory synapses exhibit a CB1 endocannabinoid (eCB)-induced long-term depression (LTD) triggered by vagal fibre stimulation. Since eCB signalling is known to be an important component of homoeostatic regulation of the body and is regulated during various stressful conditions, we tested the hypothesis that food deprivation alters eCB signalling in central visceral afferent fibres. Food deprivation prevents eCB-LTD induction due to the absence of eCB signalling. This loss was reversed by blockade of ghrelin receptors. Activation of the cellular fuel sensor AMP-activated protein kinase or inhibition of the mechanistic target of rapamycin pathway abolished eCB-LTD in free-fed rats. Signals associated with energy surfeit, such as short-term refeeding, restore eCB-LTD induction, which in turn requires activation of cholecystokinin receptors and the extracellular signal-regulated kinase pathway. These data suggest a tight link between eCB-LTD in the NTS and nutritional status and shed light on the key role of eCB in the integration of visceral information.

Vagal neurocircuitry and its influence on gastric motility

A large body of research has been dedicated to the effects of gastrointestinal peptides on vagal afferent fibres, yet multiple lines of evidence indicate that gastrointestinal peptides also modulate brainstem vagal neurocircuitry, and that this modulation has a fundamental role in the physiology and pathophysiology of the upper gastrointestinal tract. In fact, brainstem vagovagal neurocircuits comprise highly plastic neurons and synapses connecting afferent vagal fibres, second order neurons of the nucleus tractus solitarius (NTS), and efferent fibres originating in the dorsal motor nucleus of the vagus (DMV). Neuronal communication between the NTS and DMV is regulated by the presence of a variety of inputs, both from within the brainstem itself as well as from higher centres, which utilize an array of neurotransmitters and neuromodulators. Because of the circumventricular nature of these brainstem areas, circulating hormones can also modulate the vagal output to the upper gastrointestinal tract. This Review summarizes the organization and function of vagovagal reflex control of the upper gastrointestinal tract, presents data on the plasticity within these neurocircuits after stress, and discusses the gastrointestinal dysfunctions observed in Parkinson disease as examples of physiological adjustment and maladaptation of these reflexes.

Corticotrophin-releasing factor receptor 2 mediates the enhanced activation of satiety-related responses through oxytocin neurons in the paraventricular nucleus of the hypothalamus after adrenalectomy

Adrenalectomy (ADX)-induced hypophagia is associated with increased activation of corticotrophin-releasing factor (CRF) and oxytocin (OT) neurons in the paraventricular nucleus of the hypothalamus (PVN) after refeeding. CRF2- and OT-receptor antagonists abolish the hypophagia and the augmented activation of the nucleus of the solitary tract neurons induced by feeding after ADX. In addition, OT-receptor antagonist reversed CRF-induced anorexia. We evaluated the effect of intracerebroventricular pretreatment with CRF2-receptor antagonist, antisauvagine-30 (AS30), on the activation of OT neurons of the PVN in response to refeeding of sham, adrenalectomized (ADX) and ADX rats replaced with corticosterone (ADX+B). In vehicle-pretreated animals, refeeding increased the number of Fos+OT double labeled neurons in the posterior parvocellular subdivision of the PVN (PaPo) of sham, ADX and ADX+B animals, with higher Fos expression and OT neuronal activation in the ADX group. AS30 reversed refeeding-induced increased activation of OT and non-OT neurons in the PaPo in the ADX group. In the medial parvocellular subdivision of the PVN (PaMP) of vehicle-pretreated animals, the number of Fos- and Fos+OT-immunoreactive neurons was increased after refeeding in ADX group. AS30 in the ADX group attenuated the enhanced Fos expression but not the number of Fos+OT double labeled neurons in the PaMP. In conclusion, CRF2-receptor antagonist reverses the increased activation of OT neurons in the PaPo induced by feeding in ADX animals, suggesting that OT neurons might be downstream mediators of CRF effects on satiety-related responses after ADX.

Central nervous system control of gastrointestinal motility and secretion and modulation of gastrointestinal functions

Although the gastrointestinal (GI) tract possesses intrinsic neural plexuses that allow a significant degree of autonomy over GI functions, the central nervous system (CNS) provides extrinsic neural inputs that regulate, modulate, and control these functions. While the intestines are capable of functioning in the absence of extrinsic inputs, the stomach and esophagus are much more dependent upon extrinsic neural inputs, particularly from parasympathetic and sympathetic pathways. The sympathetic nervous system exerts a predominantly inhibitory effect upon GI muscle and provides a tonic inhibitory influence over mucosal secretion while, at the same time, regulates GI blood flow via neurally mediated vasoconstriction. The parasympathetic nervous system, in contrast, exerts both excitatory and inhibitory control over gastric and intestinal tone and motility. Although GI functions are controlled by the autonomic nervous system and occur, by and large, independently of conscious perception, it is clear that the higher CNS centers influence homeostatic control as well as cognitive and behavioral functions. This review will describe the basic neural circuitry of extrinsic inputs to the GI tract as well as the major CNS nuclei that innervate and modulate the activity of these pathways. The role of CNS-centered reflexes in the regulation of GI functions will be discussed as will modulation of these reflexes under both physiological and pathophysiological conditions. Finally, future directions within the field will be discussed in terms of important questions that remain to be resolved and advances in technology that may help provide these answers.

Plasticity in the brainstem vagal circuits controlling gastric motor function triggered by corticotropin releasing factor

Stress impairs gastric emptying, reduces stomach compliance and induces early satiety via vagal actions. We have shown recently that the ability of the anti-stress neuropeptide oxytocin (OXT) to modulate vagal brainstem circuits undergoes short-term plasticity via alterations in cAMP levels subsequent to vagal afferent fibre-dependent activation of metabotropic glutamate receptors. The aim of the present study was to test the hypothesis that the OXT-induced gastric response undergoes plastic changes in the presence of the prototypical stress hormone, corticotropin releasing factor (CRF). Whole cell patch clamp recordings showed that CRF increased inhibitory GABAergic synaptic transmission to identified corpus-projecting dorsal motor nucleus of the vagus (DMV) neurones. In naive brainstem slices, OXT perfusion had no effect on inhibitory synaptic transmission; following exposure to CRF (and recovery from its actions), however, re-application of OXT inhibited GABAergic transmission in the majority of neurones tested. This uncovering of the OXT response was antagonized by pretreatment with protein kinase A or adenylate cyclase inhibitors, H89 and di-deoxyadenosine, respectively, indicating a cAMP-mediated mechanism. In naive animals, OXT microinjection in the dorsal vagal complex induced a NO-mediated corpus relaxation. Following CRF pretreatment, however, microinjection of OXT attenuated or, at times reversed, the gastric relaxation which was insensitive to l-NAME but was antagonized by pretreatment with a VIP antagonist. Immunohistochemical analyses of vagal motoneurones showed an increased number of oxytocin receptors present on GABAergic terminals of CRF-treated or stressed vs. naive rats. These results indicate that CRF alters vagal inhibitory circuits that uncover the ability of OXT to modulate GABAergic currents and modifies the gastric corpus motility response to OXT.

Vagal afferent fibres determine the oxytocin-induced modulation of gastric tone

Oxytocin (OXT) inputs to the dorsal vagal complex (DVC; nucleus of the tractus solitarius (NTS) dorsal motor nucleus of the vagus (DMV) and area postrema) decrease gastric tone and motility. Our first aim was to investigate the mechanism(s) of OXT-induced gastric relaxation. We demonstrated recently that vagal afferent inputs modulate NTS-DMV synapses involved in gastric and pancreatic reflexes via group II metabotropic glutamate receptors (mGluRs). Our second aim was to investigate whether group II mGluRs similarly influence the response of vagal motoneurons to OXT. Microinjection of OXT in the DVC decreased gastric tone in a dose-dependent manner. The OXT-induced gastric relaxation was enhanced following bethanechol and reduced by l-NAME administration, suggesting a nitrergic mechanism of gastroinhibition. DVC application of the group II mGluR antagonist EGLU induced a gastroinhibition that was not dose dependent and shifted the gastric effects of OXT to a cholinergic-mediated mechanism. Evoked and miniature GABAergic synaptic currents between NTS and identified gastric-projecting DMV neurones were not affected by OXT in any neurones tested, unless the brainstem slice was (a) pretreated with EGLU or (b) derived from rats that had earlier received a surgical vagal deafferentation. Conversely, OXT inhibited glutamatergic currents even in naive slices, but their responses were unaffected by EGLU pretreatment. These results suggest that the OXT-induced gastroinhibition is mediated by activation of the NANC pathway. Inhibition of brainstem group II mGluRs, however, uncovers the ability of OXT to modulate GABAergic transmission between the NTS and DMV, resulting in the engagement of an otherwise silent cholinergic vagal neurocircuit.

Alteration of antral and proximal colonic motility induced by chronic psychological stress involves central urocortin 3 and vasopressin in rats

Because of the difficulties in developing suitable animal models, the pathogenesis of stress-induced functional gastrointestinal disorders is not well known. Here we applied the communication box technique to induce psychological stress in rats and then examined their gastrointestinal motility. We measured upper and lower gastrointestinal motility induced by acute and chronic psychological stress and examined the mRNA expression of various neuropeptides in the hypothalamus. Chronic psychological stress disrupted the fasted motility in the antrum and accelerated motility in the proximal colon. mRNA expression of AVP, oxytocin, and urocortin 3 was increased by chronic psychological stress. Intracerebroventricular (ICV) injection of urocortin 3 disrupted the fasted motility in the antrum, while ICV injection of Ucn3 antiserum prevented alteration in antral motility induced by chronic psychological stress. ICV injection of AVP accelerated colonic motility, while ICV injection of SSR 149415, a selective AVP V1b receptor antagonist, prevented alteration in proximal colonic motility induced by chronic psychological stress. Oxytocin and its receptor antagonist L 371257 had no effect on colonic motility in either the normal or chronic psychological stress model. These results suggest that chronic psychological stress induced by the communication box technique might disrupt fasted motility in the antrum via urocortin 3 pathways and accelerates proximal colonic motility via the AVP V1b receptor in the brain.

CRH-R1 and CRH-R2 differentially modulate dendritic outgrowth of hippocampal neurons

Corticotropin-releasing hormone (CRH) has been implicated to be involved in the development of dendrites in brain. In the present study, we examined the effect of CRH on dendrite outgrowth in primary cultured hippocampal neurons and defined the specific CRH receptor subtype involved. Treatment of neurons with increasing concentration of CRH resulted in an increase in the total dendritic branch length (TDBL) of neurons compared with untreated neurons over 2-4 days period of treatment. These effects can be reversed by the specific CRH-R1 antagonist antalarmin but not by the CRH-R2 antagonist astressin 2B. Treatment of neurons with urocortin II, the exclusive CRH-R2 agonist, significantly decreased TDBL of the cultured neurons. These effects can be reversed by the CRH-R2 antagonist astressin 2B. Our results suggest that CRH-R1 and CRH-R2 differentially modulate the dendritic growth of hippocampal neurons in culture.

Oxytocin-immunoreactive innervation of identified neurons in the rat dorsal vagal complex

BACKGROUND

Oxytocin (OXT) has been implicated in reproduction and social interactions and in the control of digestion and blood pressure. OXT-immunoreactive axons occur in the dorsal vagal complex (DVC; nucleus tractus solitarius, NTS, dorsal motor nucleus of the vagus, DMV, and area postrema, AP), which contains neurons that regulate autonomic homeostasis. The aim of the present work is to provide a systematic investigation of the OXT-immunoreactive innervation of dorsal motor nucleus of the vagus (DMV) neurons involved in the control of gastrointestinal (GI) function.

METHODS

We studied DMV neurons identified by (i) prior injection of retrograde tracers in the stomach, ileum, or cervical vagus or (ii) induction of c-fos expression by glucoprivation with 2-deoxyglucose. Another subgroup of DMV neurons was identified electrophysiologically by stimulation of the cervical vagus and then juxtacellularly labeled with biotinamide. We used two- or three-color immunoperoxidase labeling for studies at the light microscopic level.

KEY RESULTS

Close appositions from OXT-immunoreactive varicosities were found on the cell bodies, dendrites, and axons of DMV neurons that projected to the GI tract and that responded to 2-deoxyglucose and juxtacellularly labeled DMV neurons. Double staining for OXT and choline acetyltransferase revealed that OXT innervation was heavier in the caudal and lateral DMV than in other regions. OXT-immunoreactive varicosities also closely apposed a small subset of tyrosine hydroxylase-immunoreactive NTS and DMV neurons.

CONCLUSIONS & INFERENCES

Our results provide the first anatomical evidence for direct OXT-immunoreactive innervation of GI-related neurons in the DMV.

Corticotrophin-releasing factor mediates hypophagia after adrenalectomy, increasing meal-related satiety responses

Adrenalectomy-induced hypophagia is associated with increased satiety-related responses, which involve neuronal activation of the nucleus of the solitary tract (NTS). Besides its effects on the pituitary-adrenal axis, corticotrophin-releasing factor (CRF) has been shown to play an important role in feeding behaviour, as it possesses anorexigenic effects. We evaluated feeding-induced CRF mRNA expression in the paraventricular nucleus (PVN) and the effects of pretreatment with CRF(2) receptor antagonist (Antisauvagine-30, AS30) on food intake and activation of NTS neurons in response to feeding in adrenalectomised (ADX) rats. Compared to the sham group, ADX increased CRF mRNA levels in the PVN of fasted animals, which was further augmented by refeeding. AS30 treatment did not affect food intake in the sham and ADX+corticosterone (B) groups; however, it reversed hypophagia in the ADX group. In vehicle-pretreated animals, refeeding increased the number of Fos and Fos/TH-immunoreactive neurons in the NTS in the sham, ADX and ADX+B groups, with the highest number of neurons in the ADX animals. Similarly to its effect on food intake, pretreatment with AS30 in the ADX group also reversed the increased activation of NTS neurons induced by refeeding while having no effect in the sham and ADX+B animals. The present results show that adrenalectomy induces an increase in CRF mRNA expression in the PVN potentiated by feeding and that CRF(2) receptor antagonist abolishes the anorexigenic effect and the increased activation of NTS induced by feeding in the ADX animals. These data indicate that increased activity of PVN CRF neurons modulates brainstem satiety-related responses, contributing to hypophagia after adrenalectomy.

[Effects of L-glutamate microinjection into the ventral horn of spinal cord gray matter on gastrocnemius contractility in toad]

A diagram of motor neuron pool of ventral horn of spinal cord gray matter in toad was first delineated. Different concentrations (1, 0.5, 0.1, 0.01 mol/L) of excitatory amino acid L-Glu or physiological saline (0.65% NaCl) were then microinjected into the motor neuron pool in a urethane-anaesthetized toad. The contraction curve of the gastrocnemius was then recorded by the BL-420 Physiological Signal Recording. We took the maximal tension, the duration of rising phase, the velocity of tension variation, and the duration of descending phase as the parameters to study the characteristic of gastrocnemius contractility. It was found that the gastrocnemius contractility of all the 4 groups was tetanus but differed in degree, especially the maximal tension, and velocity of tension variation. In contrast to physiological saline, gastrocnemius contracted by the stimulation of L-Glu, and the contraction parameter showed dose-effect relationships except for the duration of descending phase, which was caused by the combination rate of L-Glu and the receptor.

Comparison of somatostatin and corticotrophin-releasing hormone immunoreactivity in forebrain neurons projecting to taste-responsive and non-responsive regions of the parabrachial nucleus in rat

Several forebrain areas have been shown to project to the parabrachial nucleus (PBN) and exert inhibitory and excitatory influences on taste processing. The neurochemicals by which descending forebrain inputs modulate neural taste-evoked responses remain to be established. This study investigated the existence of somatostatin (SS) and corticotrophin-releasing factor (CRF) in forebrain neurons that project to caudal regions of the PBN responsive to chemical stimulation of the anterior tongue as well as more rostral unresponsive regions. Retrograde tracer was iontophoretically or pressure ejected from glass micropipettes, and 7 days later the animals were euthanized for subsequent immunohistochemical processing for co-localization of tracer with SS and CRF in tissue sections containing the lateral hypothalamus (LH), central nucleus of the amygdala (CeA), bed nucleus of the stria terminalis (BNST), and insular cortex (IC). In each forebrain site, robust labeling of cells with distinguishable nuclei and short processes was observed for SS and CRF. The results indicate that CRF neurons in each forebrain site send projections throughout the rostral caudal extent of the PBN with a greater percentage terminating in regions rostral to the anterior tongue-responsive area. For SS, the percentage of double-labeled neurons was more forebrain site specific in that only BNST and CeA exhibited significant numbers of double-labeled neurons. Few retrogradely labeled cells in LH co-expressed SS, while no double-labeled cells were observed in IC. Again, tracer injections into rostral PBN resulted in a greater percentage of double-labeled neurons in BNST and CeA compared to caudal injections. The present results suggest that some sources of descending forebrain input might utilize somatostatin and/or CRF to exert a broad influence on sensory information processing in the PBN.

Neuroendocrine control of the gut during stress: corticotropin-releasing factor signaling pathways in the spotlight

Stress affects the gastrointestinal tract as part of the visceral response. Various stressors induce similar profiles of gut motor function alterations, including inhibition of gastric emptying, stimulation of colonic propulsive motility, and hypersensitivity to colorectal distension. In recent years, substantial progress has been made in our understanding of the underlying mechanisms of stress's impact on gut function. Activation of corticotropin-releasing factor (CRF) signaling pathways mediates both the inhibition of upper gastrointestinal (GI) and the stimulation of lower GI motor function through interaction with different CRF receptor subtypes. Here, we review how various stressors affect the gut, with special emphasis on the central and peripheral CRF signaling systems.

Vagally mediated effects of glucagon-like peptide 1: in vitro and in vivo gastric actions

Glucagon-like peptide-1 (GLP-1) is a neuropeptide released following meal ingestion that, among other effects, decreases gastric tone and motility. The central targets and mechanism of action of GLP-1 on gastric neurocircuits have not, however, been fully investigated. A high density of GLP-1 containing neurones and receptors are present in brainstem vagal circuits, suggesting that the gastroinhibition may be vagally mediated. We aimed to investigate: (1) the response of identified gastric-projecting neurones of the dorsal motor nucleus of the vagus (DMV) to GLP-1 and its analogues; (2) the effects of brainstem application of GLP-1 on gastric tone; and (3) the vagal pathway utilized by GLP-1 to induce gastroinhibition. We conducted our experiments using whole-cell recordings from identified gastric-projecting DMV neurones and microinjection in the dorsal vagal complex (DVC) of anaesthetized rats while monitoring gastric tone. Perfusion with GLP-1 induced a concentration-dependent excitation of a subpopulation of gastric-projecting DMV neurones. The GLP-1 effects were mimicked by exendin-4 and antagonized by exendin-9-39. In an anaesthetized rat preparation, application of exendin-4 to the DVC decreased gastric tone in a concentration-dependent manner. The gastroinhibitory effects of exendin-4 were unaffected by systemic pretreatment with the pro-motility muscarinic agonist bethanechol, but were abolished by systemic administration of the nitric oxide synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME), or by bilateral vagotomy. Our data indicate that GLP-1 activates selective receptors to excite DMV neurones mainly and that the gastroinhibition observed following application of GLP-1 in the DVC is due to the activation of an inhibitory non-adrenergic, non-cholinergic input to the stomach.

Effects of brain stem cholecystokinin-8s on gastric tone and esophageal-gastric reflex

The actions of cholecystokinin (CCK) on gastrointestinal functions occur mainly via paracrine effects on peripheral sensory vagal fibers, which engage vago-vagal brain stem circuits to convey effector responses back to the gastrointestinal tract. Recent evidence suggests, however, that CCK also affects brain stem structures directly. Many electrophysiological studies, including our own, have shown that brain stem vagal circuits are excited by sulfated CCK (CCK-8s) directly, and we have further demonstrated that CCK-8s induces a remarkable degree of plasticity in GABAergic brain stem synapses. In the present study, we used fasted, anesthetized Sprague-Dawley rats to investigate the effects of brain stem administration of CCK-8s on gastric tone before and after activation of the esophageal-gastric reflex. CCK-8s microinjected in the dorsal vagal complex (DVC) or applied on the floor of the fourth ventricle induced an immediate and transient decrease in gastric tone. Upon recovery of gastric tone to baseline values, the gastric relaxation induced by esophageal distension was attenuated or even reversed. The effects of CCK-8s were antagonized by vagotomy or fourth ventricular, but not intravenous, administration of the CCK-A antagonist lorglumide, suggesting a central, not peripheral, site of action. The gastric relaxation induced by DVC microinjection of CCK-8s was unaffected by pretreatment with systemic bethanecol but was completely blocked by NG-nitro-L-arginine methyl ester, suggesting a nitrergic mechanism of action. These data suggest that 1) brain stem application of CCK-8s induces a vagally mediated gastric relaxation; 2) the CCK-8s-induced gastric relaxation is mediated via activation of nonadrenergic, noncholinergic pathways; and 3) CCK-8s reverses the esophageal-gastric reflex transiently.

Hindbrain Effects of L-Glutamate on Gastric Motility in Rats

BACKGROUND

There are no unanimous standpoints about the dorsal motor nucleus of the vagus (DMV) and nucleus of solitary tract (NTS) involving in the regulation of gastric motility up to now.

METHODS

In this study, we injected L-Glutamate (L-Glu), an incitant neurotransmitter in the central neural system, into DMV and NTS to further investigate the effects of the two nuclei on gastric motility. A latex balloon connected with a pressure transducer was inserted into the pylorus through the fundus for continuous recording of the change of gastric smooth muscle contractile curves.

RESULTS

L-Glu (10 nmol in 0.1 µl) microinjected into right DMV and NTS significantly inhibited gastric motility. We compared the effects of L-Glu (10 nmol) microinjected into the two nuclei, the L-Glu microinjected into right NTS had the greater inhibitory effect on gastric motility than microinjected into the right DMV. The physiological saline microinjection evoked no significant effect on gastric motility.

CONCLUSIONS

L-Glu microinjected into right DMV and NTS evoked significant inhibition on gastric motility in rats. At equal dose of L-Glu, NTS had the greater inhibitory effect than DMV.

Type 1 corticotropin-releasing factor receptor expression reported in BAC transgenic mice: implications for reconciling ligand-receptor mismatch in the central corticotropin-releasing factor system

In addition to its established role in initiating the endocrine arm of the stress response, corticotropin-releasing factor (CRF) can act in the brain to modulate neural pathways that effect coordinated physiological and behavioral adjustments to stress. Although CRF is expressed in a set of interconnected limbic and autonomic cell groups implicated as primary sites of stress-related peptide action, most of these are lacking or impoverished in CRF receptor (CRFR) expression. Understanding the distribution of functional receptor expression has been hindered by the low resolution of ligand binding approaches and the lack of specific antisera, which have supported immunolocalizations at odds with analyses at the mRNA level. We have generated a transgenic mouse that shows expression of the principal, or type 1, CRFR (CRFR1). This mouse expresses GFP in a cellular distribution that largely mimics that of CRFR1 mRNA and is extensively colocalized with it in individual neurons. GFP-labeled cells display indices of activation (Fos induction) in response to central CRF injection. At the cellular level, GFP labeling marks somatic and proximal dendritic morphology with high resolution and is also localized to axonal projections of at least some labeled cell groups. This includes a presence in synaptic inputs to central autonomic structures such as the central amygdalar nucleus, which is implicated as a stress-related site of CRF action, but lacks cellular CRFR1 expression. These findings validate a new tool for pursuing the role of central CRFR signaling in stress adaptation and suggest means by which the pervasive ligand-receptor mismatch in this system may be reconciled.

Regulative effects of essential oil from Atractylodes lancea on delayed gastric emptying in stress-induced rats

Gastric motor dysfunction induced by psychological stress results in many symptoms of functional dyspepsia (FD). There are a number of herbal medicines that are reported to improve gastrointestinal motor. However, the mechanisms of considerable herbal medicines are not explicit. In the present study, the effects of an essential oil (EO) extracted from Atractylodes lancea on delayed gastric emptying, gastrointestinal hormone and hypothalamic corticotropin-releasing factor (CRF) abnormalities induced by restraint stress in rats were investigated and the mechanism of the EO is also explored. Oral administration of EO for 7 days did not affect normal gastric emptying, but accelerated delayed gastric emptying induced by restraint stress in rats. The EO significantly increased the levels of motilin (MTL) and gastrin (GAS) and decreased the levels of somatostatin (SS) and CRF. The EO did not modify gastric emptying in vagotomized rats that underwent restraint stress, but antagonized delayed gastric emptying induced by intracisternal injection of CRF. These results suggest that the regulative effects of the EO on delayed gastric emptying are preformed mainly via inhibition of the release of central CRF and activation of vagal pathway, which are also involved in the release of gastrointestinal hormones such as MTL, GAS and SS.

Daily intake of high dietary fiber slows accelerated colonic transit induced by restrain stress in rats

BACKGROUND

Stress and central corticotropin releasing factor (CRF) are contributing factors to the pathogenesis of irritable bowel syndrome (IBS). It has been shown that restraint stress and central CRF stimulate colonic motility in rats. Short-chain fatty acids (SCFAs) are produced by bacterial fermentation from dietary fibers. It is controversial whether daily intake of dietary fiber is beneficial for patients with diarrhea-predominant IBS. We studied whether dietary fiber intake affects colonic transit and motility stimulated by restraint stress and central CRF in rats.

METHODS

Corn starch is digested almost completely in the small intestine, while potato starch shows substantial resistance to alpha-amylase and its consumption leads to stimulation of large-bowel fermentation. Four weeks after a corn starch, potato starch, or usual diet (rat chow), colonic transit and motility stimulated by restraint stress and intracisternal (ic) injection of CRF were studied. Fecal concentration of SCFAs in the cecum was measured by high-performance liquid chromatography (HPLC).

RESULTS

Four-week treatment with potato starch diet significantly increased cecal concentrations of SCFAs, compared to treatment with corn starch diet. Accelerated colonic transit induced by stress and ic injection of CRF (1 microg) were significantly attenuated in rats receiving potato starch diet compared to rats receiving corn starch diet. The incidence of unformed stool (diarrhea) induced by stress and CRF was also reduced in rats receiving potato starch diet compared to rats receiving corn starch diet and usual diet.

CONCLUSION

It is suggested that daily intake of a high-fiber diet may prevent stress- and CRF-induced acceleration of colonic transit and diarrhea. This study may contribute to treatment for the patients of diarrhea-predominant IBS.

Synaptic physiology of central CRH system

Corticotropin-Releasing Hormone (CRH) or Corticotropin-Releasing Factor (CRF) and its family of related naturally occurring endogenous peptides and receptors are becoming recognized for their actions within central (CNS) and peripheral (PNS) nervous systems. It should be recognized that the term 'CRH' has been displaced by 'CRF' [Guillemin, R., 2005. Hypothalamic hormones a.k.a. hypothalamic releasing factors. J. Endocrinol. 184, 11-28]. However, to maintain uniformity among contributions to this special issue we have used the original term, CRH. The term 'CRF' has been associated recently with CRH receptors and designated with subscripts by the IUPHAR nomenclature committee [Hauger, R.L., Grigoriadis, D.E., Dallman, M.F., Plotsky, P.M., Vale, W.W., Dautzenberg, F.M., 2003. International Union of Pharmacology. XXXVI. Corticotrophin-releasing factor and their ligands. Pharmacol. Rev. 55, 21-26] to denote the type and subtype of receptors activated or antagonized by CRH ligands. CRH, as a hormone, has long been identified as the regulator of basal and stress-induced ACTH release within the hypothalamo-pituitary-adrenal axis (HPA axis). But the concept, that CRH and its related endogenous peptides and receptor ligands have non-HPA axis actions to regulate CNS synaptic transmission outside the HPA axis, is just beginning to be recognized and identified [Orozco-Cabal, L., Pollandt, S., Liu, J., Shinnick-Gallagher, P., Gallagher, J.P., 2006a. Regulation of Synaptic Transmission by CRF Receptors. Rev. Neurosci. 17, 279-307; Orozco-Cabal, L., Pollandt, S., Liu, J., Vergara, L., Shinnick-Gallagher, P., Gallagher, J.P., 2006b. A novel rat medial prefrontal cortical slice preparation to investigate synaptic transmission from amygdala to layer V prelimbic pyramidal neurons. J. Neurosci. Methods 151, 148-158] is especially noteworthy since this synapse has become a prime focus for a variety of mental diseases, e.g. schizophrenia [Fischbach, G.D., 2007. NRG1 and synaptic function in the CNS. Neuron 54, 497-497], and neurological disorders, e.g., Alzheimer's disease [Bell, K.F., Cuello, C.A., 2006. Altered synaptic function in Alzheimer's disease. Eur. J. Pharmacol. 545, 11-21]. We suggest that "The Stressed Synapse" has been overlooked [c.f., Kim, J.J., Diamond, D.M. 2002. The stressed hippocampus, synaptic plasticity and lost memories. Nat. Rev., Neurosci. 3, 453-462; Radley, J.J., Morrison, J.H., 2005. Repeated stress and structural plasticity in the brain. Ageing Res. Rev. 4, 271-287] as a major contributor to many CNS disorders. We present data demonstrating CRH neuroregulatory and neuromodulatory actions at three limbic synapses, the basolateral amygdala to central amygdala synapse; the basolateral amygdala to medial prefrontal cortex synapse, and the lateral septum mediolateral nucleus synapse. A novel stress circuit is presented involving these three synapses. We suggest that CRH ligands and their receptors are significant etiological factors that need to be considered in the pharmacotherapy of mental diseases associated with CNS synaptic transmission.

Characterization of noradrenergic transmission at the dorsal motor nucleus of the vagus involved in reflex control of fundus tone

Quantitative analysis of innervation to dorsal motor nucleus of the vagus (DMV) fundus-projecting neurons indicates that approximately 17% of input neurons are noradrenergic. To determine whether this small percentage of neurons innervating DMV output to the stomach is physiologically relevant, we evaluated the role of norepinephrine at the DMV in mediating a vagovagal reflex controlling the fundus. A strain gauge was sutured onto the fundus of isoflurane-anesthetized rats to monitor changes in tone evoked by esophageal distension (ED). ED produced a decrease in fundus tone of 0.31 +/- 0.02 g (P < 0.05), which could be reproduced after a 30-min interval between distensions. Bilateral cervical vagotomy and/or pretreatment with intravenous atropine methylbromide prevented the reflex-induced fundus relaxation. In contrast, intravenous N(G)-nitro-L-arginine methyl ester had no effect. Bilateral microinjection of alpha2-adrenoreceptor antagonists (yohimbine and RS-79948) into the DMV also prevented the response. Before microinjection of alpha2-adrenoreceptor antagonists, ED decreased fundus tone by 0.33 +/- 0.05 g (P < 0.05). After antagonist microinjection, ED decreased fundus tone by only 0.05 +/- 0.06 g (P > 0.05). Bilateral microinjection of prazosin into the DMV had no effect on the response. Microinjection of norepinephrine into the DMV mimicked the effect of ED and was also prevented by prior microinjection of an alpha2-adrenoreceptor antagonist. Our results indicate that noradrenergic innervation of DMV fundus-projecting neurons is physiologically important and suggest that norepinephrine released at the DMV acts on alpha2-adrenoreceptors to inhibit activity in a cholinergic-cholinergic excitatory pathway to the fundus.

Corticotropin-releasing factor receptor-1: a therapeutic target for cardiac autonomic disturbances

Corticotropin-releasing factor (CRF), a neuropeptide involved in triggering a myriad of responses to fear and stress, is favourably positioned in the CNS to modulate the sympathetic and parasympathetic branches of the cardiac autonomic nervous system. In vivo studies suggest that central CRF inhibits vagal output and stimulates sympathetic activity. Therefore, CRF may function to inhibit exaggerated vagal activation that results in severe bradycardia or even vasovagal syncope. On the other hand, CRF receptor-1 (CRF(1)) antagonists increase cardiac vagal and decrease sympathetic activity, thereby also implicating CRF(1) as a therapeutic target for autonomic disturbances resulting in elevated sympathetic activity, such as hypertension and coronary heart disease. The central distribution of CRF(1) and the cardiovascular effects of CRF(1) agonists and antagonists, suggest it mediates CRF-induced autonomic changes. However, there is insufficient information regarding the autonomic effects of CRF(2)-selective compounds to rule out CRF(2) contribution. This review provides an update on the autonomic effects of CRF and the neuronal projections thought to mediate these cardiovascular responses.

Dorsal motor nucleus of the vagus: a site for evoking simultaneous changes in crural diaphragm activity, lower esophageal sphincter pressure, and fundus tone

The sphincter mechanism at the esophagogastric junction includes smooth muscle of the lower esophagus and skeletal muscle of the crural diaphragm (CD). Smooth muscle is known to be under the control of the dorsal motor nucleus of the vagus (DMV), while central nervous system (CNS) control of the CD is unknown. The main purposes of our study were to determine the CNS site that controls the CD and whether simultaneous changes in lower esophageal sphincter (LES) pressure and CD activity occur when this site is activated. Experiments were performed on anesthetized male ferrets whose LES pressure, CD activity, and fundus tone were monitored. To activate DMV neurons, L-glutamate was microinjected unilaterally into the DMV at three areas: intermediate, rostral, and caudal. Stimulation of the intermediate DMV decreased CD activity (-4.8 +/- 0.1 bursts/min and -0.3 +/- 0.01 mV) and LES pressure (-13.2 +/- 2.0 mmHg; n = 9). Stimulation of this brain site also produced an increase in fundus tone. Stimulation of the rostral DMV elicited increases in the activity of all three target organs (n = 5). Stimulation of the caudal DMV had no effect on the CD but did decrease both LES pressure and fundus tone (n = 5). All changes in LES pressure, fundus tone, and some DMV-induced changes in CD activity (i.e., bursts/min) were prevented by ipsilateral vagotomy. Our data indicate that simultaneous changes in activity of esophagogastric sphincters and fundus tone occur from rostral and intermediate areas of the DMV and that these changes are largely mediated by efferent vagus nerves.

Effects of chronic spinal cord injury on body weight and body composition in rats fed a standard chow diet

The inability to maintain body weight within prescribed ranges occurs in a significant portion of the human spinal cord injury (SCI) population. Using a rodent model of long-term high thoracic (spinal level T3) spinal cord transection (TX), we aimed to identify derangements in body weight, body composition, plasma insulin, glucose tolerance, and metabolic function, as measured by uncoupling protein 1 (UCP1) expression in interscapular brown adipose tissue (IBAT). Sixteen weeks after SCI, body weights of injured female rats stabilized and were significantly lower than surgical control animals. At the same time point, SCI rats had a significantly lower whole body fat:lean tissue mass ratio than controls, as measured indirectly by NMR. Despite lower body weight and fat mass, the cumulative consumption of standard laboratory chow (4.0 kcal/g) and mean energy intake (kcal.day(-1).100 g body wt(-1)) of chronic SCI rats was significantly more than controls. Glucose tolerance tests indicated a significant enhancement in glucose handling in 16-wk SCI rats, which were coupled with lower serum insulin levels. The post mortem weight of gonadal and retroperitoneal fat pads was significantly reduced after SCI and IBAT displayed significantly lower real-time PCR expression of UCP1 mRNA. The reduced fat mass and IBAT UCP1 mRNA expression are contraindicative of the cumulative caloric intake by the SCI rats. The prolonged postinjury loss of body weight, including fat mass, is not due to hypophagia but possibly to permanent changes in gastrointestinal transit and absorption, as well as whole body homeostatic mechanisms.

Gastric Vagal Efferent Inhibition Evoked by Intravenous CRF Is Unrelated to Simultaneously Recorded Vagal Afferent Activity in Urethane-Anesthetized Rats

Corticotropin-releasing factor (CRF) injected peripherally or released in response to stressful challenges to the organism reduces gastric tone and contractility, in part by vagal pathways. However, information on the changes in gastric vagal impulse activity evoked by peripheral CRF administration is entirely lacking. Using a novel “dual recording” method in urethane-anesthetized rats, vagal efferent (VE) and afferent (VA) impulse activities were recorded simultaneously from separate, fine bundles dissected from the ventral gastric vagus nerve branch innervating the glandular stomach. Activity records for 38 VA single units (SUs) and 33 VE SUs were sorted from multiunit records obtained from 13 preparations. Intravenous (iv) administration of saline had no effect on multiunit VE activity, whereas CRF (1 μg/kg, iv) immediately inhibited VE activity, reaching a nadir of 54 ± 8.0% of preinjection levels at 3.0 min postinjection. CRF (1 μg/kg, iv) inhibited 25/33 (75.8%) VE SUs and excited three of 33 (9.1%) VE SUs. In contrast to potent effects on VE activity, iv CRF did not alter multiunit VA activity. Single-unit analysis, however, revealed five of 38 (13.1%) VA SUs excited by iv CRF at widely varying latencies (suggesting an indirect mode of action) and one inhibited VA SU. VA SUs excited after iv CRF did not respond during gastric distention and vice versa. These experiments are the first to use simultaneous recording of gastric VA and VE units. The data demonstrate a predominantly inhibitory influence of iv CRF on VE outflow to the hindstomach, not driven by gastric vagovagal reflex activity.

Corticotropin-releasing factor receptors and stress-related alterations of gut motor function

Over the past few decades, corticotropin-releasing factor (CRF) signaling pathways have been shown to be the main coordinators of the endocrine, behavioral, and immune responses to stress. Emerging evidence also links the activation of CRF receptors type 1 and type 2 with stress-related alterations of gut motor function. Here, we review the role of CRF receptors in both the brain and the gut as part of key mechanisms through which various stressors impact propulsive activity of the gastrointestinal system. We also examine how these mechanisms translate into the development of new approaches for irritable bowel syndrome, a multifactorial disorder for which stress has been implicated in the pathophysiology.

Historical reflections on the afterhyperpolarization--firing rate relation of vertebrate spinal neurons

In mammalian spinal motoneurons (MNs), the slow component of the afterhyperpolarization (AHP) that follows the spike of each action potential is a major but not the sole determinant of the cells' firing rate. In this brief historical review, we emphasize four points about the AHP-firing rate relation. (1) There is a relatively sparse literature across vertebrates that directly addresses this topic. (2) After the advent of intracellular recording in the early 1950s, there was evidence from mammals to the contrary of an idea that subsequently became prevalent: that the high-firing rates attainable by spinal interneurons (INs) and low-threshold MNs was attributable to their small AHP at rheobase. (3) Further work is needed to determine whether our present findings on the AHP-firing rate relation of turtle cells generalize to the spinal neurons of other vertebrate species. (4) Relevant to point 3, substantial in vivo and in vitro work is potentially available in raw data used in reports on several mammalian and non-mammalian vertebrates. In summary, the factors in addition to the slow AHP that help determine spinal INs and MN firing rate deserve further evaluation across vertebrates, with relevant data already potentially available in several laboratories.

A reevaluation of the effects of stimulation of the dorsal motor nucleus of the vagus on gastric motility in the rat

Our primary purpose was to characterize vagal pathways controlling gastric motility by microinjecting l-glutamate into the dorsal motor nucleus of the vagus (DMV) in the rat. An intragastric balloon was used to monitor motility. In 39 out of 43 experiments, microinjection of l-glutamate into different areas of the DMV rostral to calamus scriptorius (CS) resulted in vagally mediated excitatory effects on motility. We observed little evidence for inhibitory effects, even with intravenous atropine or with activation of gastric muscle muscarinic receptors by intravenous bethanechol. Inhibition of nitric oxide synthase with N(omega)-nitro-l-arginine methyl ester (l-NAME) HCl did not augment DMV-evoked excitatory effects on gastric motility. Microinjection of l-glutamate into the DMV caudal to CS produced vagally mediated gastric inhibition that was resistant to l-NAME. l-Glutamate microinjected into the medial subnucleus of the tractus solitarius (mNTS) also produced vagally mediated inhibition of gastric motility. Motility responses evoked from the DMV were always blocked by ipsilateral vagotomy, while responses evoked from the mNTS required bilateral vagotomy to be blocked. Microinjection of oxytocin into the DMV inhibited gastric motility, but the effect was never blocked by ipsilateral vagotomy, suggesting that the effect may have been due to diffusion of oxytocin to the mNTS. Microinjection of substance P and N-methyl-d-aspartate into the DMV also produced inhibitory effects attributable to excitation of nearby mNTS neurons. Our results do not support previous studies indicating parallel vagal excitatory and inhibitory pathways originating in the DMV rostral to CS. Our results do support previous findings of vagal inhibitory pathways originating in the DMV caudal to CS.