Neonatal Maternal Deprivation Followed by Adult Stress Enhances Adrenergic Signaling to Advance Visceral Hypersensitivity
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The pathophysiology of visceral pain in patients with irritable bowel syndrome remains largely unknown. Our previous study showed that neonatal maternal deprivation (NMD) does not induce visceral hypersensitivity at the age of 6 weeks in rats. The aim of this study was to determine whether NMD followed by adult stress at the age of 6 weeks induces visceral pain in rats and to investigate the roles of adrenergic signaling in visceral pain. Here we showed that NMD rats exhibited visceral hypersensitivity 6 h and 24 h after the termination of adult multiple stressors (AMSs). The plasma level of norepinephrine was significantly increased in NMD rats after AMSs. Whole-cell patch-clamp recording showed that the excitability of dorsal root ganglion (DRG) neurons from NMD rats with AMSs was remarkably increased. The expression of β2 adrenergic receptors at the protein and mRNA levels was markedly higher in NMD rats with AMSs than in rats with NMD alone. Inhibition of β2 adrenergic receptors with propranolol or butoxamine enhanced the colorectal distention threshold and application of butoxamine also reversed the enhanced hypersensitivity of DRG neurons. Overall, our data demonstrate that AMS induces visceral hypersensitivity in NMD rats, in part due to enhanced NE-β2 adrenergic signaling in DRGs.
KeywordsIrritable bowel syndrome Dorsal root ganglion Norepinephrine Visceral pain Stress
Irritable bowel syndrome (IBS) is a common gastrointestinal disease characterized by disorders of intestinal motility and accompanied by chronic abdominal pain [1, 2, 3]. The treatment of chronic abdominal pain is difficult . Research on this disease is progressing slowly due to the lack of suitable animal models. Although the currently available research provides insights into the processing and regulation of chronic pain [5, 6], the precise pathophysiology of IBS has not been fully elucidated and effective strategies for treating the primary symptoms are not available [1, 7, 8]. Our previous studies have shown that neonatal maternal deprivation (NMD) can induce chronic visceral pain in adult rats at the age of 7 weeks but not at 6 weeks [9, 10]. It seems that the age of 7 weeks is an important time point for the development of visceral hypersensitivity in rats with NMD. However, it is unknown whether 6-week-old NMD rats are more sensitive to environmental stimuli than age-matched controls.
A growing body of evidence has demonstrated that severe adverse environmental factors such as stress might be a stimulus to generate visceral hypersensitivity at the adult age in humans [11, 12] and animals [13, 14, 15]. For example, repetitive water-avoidance stressors for 10 days induce visceral hypersensitivity in rats and mice . Heterotypic intermittent stress for 9 consecutive days induces visceral hypersensitivity immediately after termination of the last stressors [14, 17]. However, single or mild stressors do not induce visceral hypersensitivity in healthy adult animals. In the present study, we designed a stress protocol to determine whether one stressor or mild stressors could induce visceral pain in NMD rats at the age of 6 weeks.
Adrenergic signaling plays many important roles in the nervous system to regulate stress responses [18, 19, 20]. Adrenergic receptors (ARs) are classically divided into two main groups: α- and β-adrenoceptors. The α-adrenoceptors include α1A, α1B, α1D, α2A, α2B, and α2C subtypes, and β-adrenoceptors into β1, β2, and β3 subtypes . They have been reported to be expressed in primary sensory neurons with their cell bodies located in the dorsal root ganglia (DRGs) [22, 23]. Previous studies have suggested that the β1 and β2 subtypes are involved in the adrenergic activation  that may play a role in colonic transit. The β2 ARs are reported to produce a hyperalgesic state in rats [22, 25, 26]. The β3 ARs, mainly expressed in brown and white adipose tissue, regulate energy metabolism and thermogenesis . Besides, other studies have shown that the adrenergic system plays a role in the visceral pain caused by chronic stress . Recently, we have reported that adrenergic β2 receptors mediate visceral pain evoked by heterotypic intermittent stress in rats . However, whether adrenergic signaling in primary sensory neurons participates in visceral pain of NMD rats at the age of 6 weeks after additional adult stress is unknown.
Thus, we designed this study to test the hypothesis that adrenergic activation plays a crucial role in the switch of NMD rats at the age of 6 weeks from no pain status to pain hypersensitivity induced by stress in adulthood.
Materials and Methods
We used male Sprague-Dawley rats in the present experiments. The experimental protocol was approved by the Institutional Animal Care and Use Committee of Soochow University. Animal care and handling were strictly in accordance with the regulations and guidelines of the International Association for the Study of Pain.
Adult Stress Protocol to Induce Visceral Pain
Measurement of Behavioral Responses to Graded Colorectal Distention (CRD)
Chronic visceral hyperalgesia was assessed by grading the behavioral response of rats to CRD at the age of 6 weeks based on previous publications [9, 10, 20]. All behavioral tests were performed in a blinded manner.
In the behavioral experiments, 5 mg/kg butoxamine (BUTO, a β2 antagonist; Sigma, St. Louis, MO), 3 mg/kg propranolol (PROP, a norepinephrine β receptor antagonist; Sigma) or 3 mg/kg phentolamine (PHEN, an α receptor antagonist; Sigma) dissolved in 0.9% normal saline (NS) was intraperitoneally injected into AMS rats once for behavioral experiments and once daily for 7 consecutive days for patch-clamp recordings and western blotting. The drug concentrations used were based on our previous study and reports from other groups [14, 20].
Measurement of Norepinephrine (NE) in Blood Plasma
Blood samples were collected from the trunk into centrifuge tubes containing 0.45% citric acid and 2.5% sodium citrate at euthanasia by decapitation. After refrigerated centrifugation, the supernatant was quickly aliquoted and stored at − 80 °C for experiments. NE levels in the plasma were determined using an enzyme immunoassay kit from Abnova (Norepinephrine ELISA Kit), as previously described .
DRGs at T13–L2 from AMS-treated control or NMD rats were collected to measure the protein levels of β1, β2, and β3 receptors. DRGs at T13–L2 from control, NMD, and ASS-treated control or NMD rats were collected to measure the protein levels of β2 receptors. The antibodies used were anti-β1 (1:500, Santa Cruz Biotechnology, Inc.), anti-β2 (1:500, Santa Cruz Biotechnology, Inc.), anti-β3 (1:500, Santa Cruz Biotechnology, Inc.), and anti-GAPDH (1:2000, Hangzhou Goodhere Biotechnology). Band density was measured using ImageJ software. The β1, β2, and β3 expression were normalized to GAPDH as described previously [17, 20, 29, 30].
Total RNAs were extracted from DRGs (T13–L2) from control, AMS, and NMD rats with TRIzol (Ambion, Shanghai, China). cDNA was synthesized from total RNA using a reverse transcription kit (Transgen Biotech, Beijing, China) according to the supplier’s instructions. The primers are as follows: β2 forward 5′-GGTTGGGCTATGTCAACTCTG-3′, reverse 5′-GTCTGTCCTACCGTTGCTGTT-3′; gapdh (internal control) forward 5′-TGGAGTCTACTGGCGTCTT-3′, reverse 5′-TGTCATATTTCTCGTGGTTCA-3′. Control reactions were carried out without cDNA templates.
Dissociation of DRG Neurons and Whole-Cell Patch-Clamp Recordings
Rats from AMS-treated NMD or control (~6 weeks) were sacrificed by decapitation. The detailed procedures for the acute isolation of DRG neurons and patch clamp recordings were as previously reported [14, 20, 31]. The dissecting solution contained (in mmol/L): 130 NaCl, 5 KCl, 2 KH2PO4, 1.5 CaCl2, 6 MgSO4, 10 glucose, and 10 HEPES, pH 7.2 with osmolarity 305 mOsm. For the patch-clamp recordings, normal external solution contained (in mmol/L): 130 NaCl, 5 KCl, 2 KH2PO4, 2.5 CaCl2, 1 MgCl2, 10 HEPES, 10 glucose, pH adjusted to 7.2 with NaOH, osmolarity 295–300 mOsm. The pipette solution contained (in mmol/L): 140 K-gluconate, 10 NaCl, 10 HEPES, 10 glucose, 5 EGTA, 1 CaCl2, pH 7.25 adjusted with KOH; osmolarity 292 mOsm.
All data are presented as mean ± SEM. Statistical testing was performed using OriginPro 8 (OriginLab, Northampton, MA). Normality was first checked for all data before analysis. Significance was determined using the two-sample t test, Mann-Whitney test, Mann-Whitney test following Friedman ANOVA, or Tukey’s post-hoc test following two-way repeated measures ANOVA. P < 0.05 was considered to be statistically significant.
AMS Induces Visceral Hypersensitivity in NMD Rats
AMS Increases NE Concentration in Blood Plasma of NMD Rats
AMS Enhances Excitability of Colon DRG Neurons in NMD Rats
AMS Upregulates the Expression of β2 Adrenergic Receptors in DRGs
Treatment with β2 Receptor Antagonist Reverses Visceral Hypersensitivity
Treatment with β2 Receptor Antagonist Reverses Neuronal Hyperexcitability
In the present experiments, we demonstrated that NMD rats exhibited reduced thresholds and increased abdominal withdrawal reflex scores to CRD when compared with age-matched control rats after exposure to multiple stressors as adults. This indicates that a combination of NMD and AMS exacerbated the symptoms by enhancing visceral hypersensitivity in NMD rats at the age of 6 weeks. This also supports an idea that NMD puts such rats at risk when they grew to the age of 6 weeks although these rats do not show any visceral hypersensitivity as reported previously . Although these rats are more sensitive to multiple stressors at the age of 6 weeks than age-matched control rats, this does not mean that these NMD rats are sensitive to any environmental stimulus, since the CRS alone did not induce any visceral response. Environmental stimuli have to reach a minimal threshold to induce visceral hypersensitivity. Although we did not define the minimal threshold for NMD rats, our data might have clinic relevance in that an adverse neonatal stimulus followed by adult stressors aggravates the symptoms of IBS or/and shortens the time window to induce visceral hypersensitivity.
The finding of enhanced visceral sensitivity was strongly supported by the enhanced neuronal excitability in NMD rats followed by AMS. By whole-cell patch clamp recording, we showed that the excitability of DRG neurons was remarkably enhanced in NMD rats followed by AMS when compared with that of age-matched control rats followed by AMS. Our electrophysiological data provide a cellular mechanism underlying the enhanced visceral pain behaviors. Of note is that the ionic basis for this enhanced cellular excitability needs further investigation.
In the present experiments, we focused on the mechanism by which AMS produced visceral hypersensitivity and neuronal hyperexcitability in NMD rats at the age of 6 weeks. A recent study has shown that chronic stress involves NE release, AR expression, and/or the activation of intermediates in AR-induced signaling, thus contributing to the pathology of many immune-mediated diseases . Adrenergic activation is involved in neuropathic and inflammatory pain states [34, 35]. Our previous studies showed that neonatal colonic inflammation or heterotypic intermittent stress increases the NE concentration in blood plasma without alterations in β2 AR expression in DRGs [14, 20]. A new finding in the present study was that AMS not only enhanced the plasma NE levels but also increased the expression of β2 ARs in DRGs at 6 weeks. This discrepancy might be due to the different stimulus protocols used. The neonatal colonic inflammation model was established only by one colonic stimulus in the neonate. The heterotypic intermittent stress model was induced by stressors only in adulthood [14, 17]. However, the present model was established by a combined stress protocol in both neonates and adults. This combination of stimuli might be more relevant to the clinic situation. Therefore, our study also provides a good animal model to better mimic the clinic situation in patients with IBS, thus providing a better basis on which to investigate the mechanisms of visceral pain.
We showed that β2 ARs played an important role in the peripheral nervous system. The expression of β2 ARs at the protein and mRNA levels was remarkably upregulated in NMD rats with AMS while the expression of β1 and β3 ARs was not altered greatly. Inhibition of β2 ARs by PROP or BUTO enhanced the CRD threshold in a time-dependent manner while inhibition of α adrenergic receptors by PHEN did not affect the CRD threshold. Furthermore, application of BUTO also reversed the enhanced hypersensitivity of DRG neurons. This anti-nociceptive effect was specific since BUTO did not have any effect on age-matched control rats with AMS. These data demonstrated that enhanced NE and β2 adrenergic signaling plays an important role in increasing the visceral hypersensitivity in rats with NMD and AMS. Of note is that the protein expression of β2 ARs in NMD rats followed by ASS was also significantly increased. However, the NE level in the plasma was not altered remarkably. This may explain why the NMD rats followed by ASS did not show enhanced visceral sensitivity. In addition, we do not know whether AMS can exacerbate the visceral hypersensitivity of NMD rats at the age of 7 weeks, although NMD rats at this age already exhibit enhanced visceral hypersensitivity. In the present study, we only focused on the peripheral mechanisms; the central mechanisms, such as spinal synaptic plasticity , deserve further investigation.
In summary, we demonstrated that NMD followed by AMS increased visceral hypersensitivity in association with an elevation of the NE concentration in plasma, the expression of β2 ARs in DRGs, and the neuronal excitability of colonic DRG neurons. Blockade of β2 ARs attenuated visceral hypersensitivity to colorectal distension and neuronal hyperexcitability. Together with our previous studies, our data provide additional evidence to support the idea that the NE–β2 signaling pathway plays an important role in the development of visceral hypersensitivity. This study might shed light on the pathogenesis of visceral hypersensitivity imposed by environmental stress during early and adult life. AR inhibitors might serve as alternates to relieve abdominal pain in patients with IBS.
This work was supported by grants from the National Natural Science Foundation of China (81471137, 31730040, and 81500952) and the Priority Academic Program Development of Jiangsu Higher Education Institutions of China.
Conflict of interest
The authors declare that they have no conflict of interest.
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