VIP and PACAP: very important in pain?

Pharmacology of PACAP and VIP receptors in the spinal cord highlights the importance of the PAC1 receptor

BACKGROUND AND PURPOSE

The spinal cord is a key structure involved in the transmission and modulation of pain. Pituitary adenylate cyclase-activating peptide (PACAP) and vasoactive intestinal peptide (VIP), are expressed in the spinal cord. These peptides activate G protein-coupled receptors (PAC1, VPAC1 and VPAC2) that could provide targets for the development of novel pain treatments. However, it is not clear which of these receptors are expressed within the spinal cord and how these receptors signal.

EXPERIMENTAL APPROACH

Dissociated rat spinal cord cultures were used to examine agonist and antagonist receptor pharmacology. Signalling profiles were determined for five signalling pathways. The expression of different PACAP and VIP receptors was then investigated in mouse, rat and human spinal cords using immunoblotting and immunofluorescence.

KEY RESULTS

PACAP, but not VIP, potently stimulated cAMP, IP1 accumulation and ERK and cAMP response element-binding protein (CREB) but not Akt phosphorylation in spinal cord cultures. Signalling was antagonised by M65 and PACAP6-38. PACAP-27 was more effectively antagonised than either PACAP-38 or VIP. The patterns of PAC1 and VPAC2 receptor-like immunoreactivity appeared to be distinct in the spinal cord.

CONCLUSIONS AND IMPLICATIONS

The pharmacological profile in the spinal cord suggested that a PAC1 receptor is the major functional receptor subtype present and thus likely mediates the nociceptive effects of the PACAP family of peptides in the spinal cord. However, the potential expression of both PAC1 and VPAC2 receptors in the spinal cord highlights that these receptors may play differential roles and are both possible therapeutic targets.

Early Alterations of PACAP and VIP Expression in the Female Rat Brain Following Spinal Cord Injury

Previous evidence shows that rapid changes occur in the brain following spinal cord injury (SCI). Here, we interrogated the expression of the neuropeptides pituitary adenylyl cyclase-activating peptide (PACAP), vasoactive intestinal peptides (VIP), and their binding receptors in the rat brain 24 h following SCI. Female Sprague-Dawley rats underwent thoracic laminectomy; half of the rats received a mild contusion injury at the level of the T10 vertebrate (SCI group); the other half underwent sham surgery (sham group). Twenty-four hours post-surgery, the hypothalamus, thalamus, amygdala, hippocampus (dorsal and ventral), prefrontal cortex, and periaqueductal gray were collected. PACAP, VIP, PAC1, VPAC1, and VPAC2 mRNA and protein levels were measured by real-time quantitative polymerase chain reaction and Western blot. In SCI rats, PACAP expression was increased in the hypothalamus (104-141% vs sham) and amygdala (138-350%), but downregulated in the thalamus (35-95%) and periaqueductal gray (58-68%). VIP expression was increased only in the thalamus (175-385%), with a reduction in the amygdala (51-68%), hippocampus (40-75%), and periaqueductal gray (74-76%). The expression of the PAC1 receptor was the least disturbed by SCI, with decrease expression in the ventral hippocampus (63-68%) only. The expression levels of VPAC1 and VPAC2 receptors were globally reduced, with more prominent reductions of VPAC1 vs VPAC2 in the amygdala (21-70%) and ventral hippocampus (72-75%). In addition, VPAC1 downregulation also extended to the dorsal hippocampus (69-70%). These findings demonstrate that as early as 24 h post-SCI, there are region-specific disruptions of PACAP, VIP, and related receptor transcript and protein levels in supraspinal regions controlling higher cognitive functions.

Major Differences in Transcriptional Alterations in Dorsal Root Ganglia Between Spinal Cord Injury and Peripheral Neuropathic Pain Models

Chronic, often intractable, pain is caused by neuropathic conditions such as traumatic peripheral nerve injury (PNI) and spinal cord injury (SCI). These conditions are associated with alterations in gene and protein expression correlated with functional changes in somatosensory neurons having cell bodies in dorsal root ganglia (DRGs). Most studies of DRG transcriptional alterations have utilized PNI models where axotomy-induced changes important for neural regeneration may overshadow changes that drive neuropathic pain. Both PNI and SCI produce DRG neuron hyperexcitability linked to pain, but contusive SCI produces little peripheral axotomy or peripheral nerve inflammation. Thus, comparison of transcriptional signatures of DRGs across PNI and SCI models may highlight pain-associated transcriptional alterations in sensory ganglia that do not depend on peripheral axotomy or associated effects such as peripheral Wallerian degeneration. Data from our rat thoracic SCI experiments were combined with meta-analysis of published whole-DRG RNA-seq datasets from prominent rat PNI models. Striking differences were found between transcriptional responses to PNI and SCI, especially in regeneration-associated genes (RAGs) and long noncoding RNAs (lncRNAs). Many transcriptomic changes after SCI also were found after corresponding sham surgery, indicating they were caused by injury to surrounding tissue, including bone and muscle, rather than to the spinal cord itself. Another unexpected finding was of few transcriptomic similarities between rat neuropathic pain models and the only reported transcriptional analysis of human DRGs linked to neuropathic pain. These findings show that DRGs exhibit complex transcriptional responses to central and peripheral neural injury and associated tissue damage. Although only a few genes in DRG cells exhibited similar changes in expression across all the painful conditions examined here, these genes may represent a core set whose transcription in various DRG cell types is sensitive to significant bodily injury, and which may play a fundamental role in promoting neuropathic pain.

Estrogen modulation of the pronociceptive effects of serotonin on female rat trigeminal sensory neurons is timing dependent and dosage dependent and requires estrogen receptor alpha

ABSTRACT

The role of the major estrogen estradiol (E2) on orofacial pain conditions remains controversial with studies reporting both a pronociceptive and antinociceptive role of E2. E2 modulation of peripheral serotonergic activity may be one mechanism underlying the female prevalence of orofacial pain disorders. We recently reported that female rats in proestrus and estrus exhibit greater serotonin (5HT)-evoked orofacial nocifensive behaviors compared with diestrus and male rats. Further coexpression of 5HT 2A receptor mRNA in nociceptive trigeminal sensory neurons that express transient receptor potential vanilloid 1 ion channels contributes to pain sensitization. E2 may exacerbate orofacial pain through 5HT-sensitive trigeminal nociceptors, but whether low or high E2 contributes to orofacial pain and by what mechanism remains unclear. We hypothesized that steady-state exposure to a proestrus level of E2 exacerbates 5HT-evoked orofacial nocifensive behaviors in female rats, explored the transcriptome of E2-treated female rats, and determined which E2 receptor contributes to sensitization of female trigeminal sensory neurons. We report that a diestrus level of E2 is protective against 5HT-evoked orofacial pain behaviors, which increase with increasing E2 concentrations, and that E2 differentially alters several pain genes in the trigeminal ganglia. Furthermore, E2 receptors coexpressed with 5HT 2A and transient receptor potential vanilloid 1 and enhanced capsaicin-evoked signaling in the trigeminal ganglia through estrogen receptor α. Overall, our data indicate that low, but not high, physiological levels of E2 protect against orofacial pain, and we provide evidence that estrogen receptor α receptor activation, but not others, contributes to sensitization of nociceptive signaling in trigeminal sensory neurons.

Pituitary Adenylate Cyclase-Activating Polypeptide in Learning and Memory

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a highly conserved neuropeptide that regulates neuronal physiology and transcription through Gs/Gq-coupled receptors. Its actions within hypothalamic, limbic, and mnemonic systems underlie its roles in stress regulation, affective processing, neuroprotection, and cognition. Recently, elevated PACAP levels and genetic disruption of PAC1 receptor signaling in humans has been linked to maladaptive threat learning and pathological stress and fear in post-traumatic stress disorder (PTSD). PACAP is positioned to integrate stress and memory in PTSD for which memory of the traumatic experience is central to the disorder. However, PACAP's role in memory has received comparatively less attention than its role in stress. In this review, we consider the evidence for PACAP-PAC1 receptor signaling in learning and plasticity, discuss emerging data on sex differences in PACAP signaling, and raise key questions for further study toward elucidating the contribution of PACAP to adaptive and maladaptive fear learning.

Regulators of G protein signalling as pharmacological targets for the treatment of neuropathic pain

Neuropathic pain, a specific type of chronic pain resulting from persistent nervous tissue lesions, is a debilitating condition that affects about 7% of the population. This condition remains particularly difficult to treat because of the poor understanding of its underlying mechanisms. Drugs currently used to alleviate this chronic pain syndrome are of limited benefit due to their lack of efficacy and the elevated risk of side effects, especially after a prolonged period of treatment. Although drugs targeting G protein-coupled receptors (GPCR) also have several limitations, such as progressive loss of efficacy due to receptor desensitization or unavoidable side effects due to wide receptor distribution, the identification of several molecular partners that contribute to the fine-tuning of receptor activity has raised new opportunities for the development of alternative therapeutic approaches. Regulators of G protein signalling (RGS) act intracellularly by influencing the coupling process and activity of G proteins, and are amongst the best-characterized physiological modulators of GPCR. Changes in RGS expression have been documented in a range of models of neuropathic pain, or after prolonged treatment with diverse analgesics, and could participate in altered pain processing as well as impaired physiological or pharmacological control of nociceptive signals. The present review summarizes the experimental data that implicates RGS in the development of pain with focus on the pathological mechanisms of neuropathic pain, including the impact of neuropathic lesions on RGS expression and, reciprocally, the influence of modifying RGS on GPCRs involved in the modulation of nociception as well as on the outcome of pain. In this context, we address the question of the relevance of RGS as promising targets in the treatment of neuropathic pain.

Tissue-type plasminogen activator-primed human iPSC-derived neural progenitor cells promote motor recovery after severe spinal cord injury

The goal of stem cell therapy for spinal cord injury (SCI) is to restore motor function without exacerbating pain. Induced pluripotent stem cells (iPSC) may be administered by autologous transplantation, avoiding immunologic challenges. Identifying strategies to optimize iPSC-derived neural progenitor cells (hiNPC) for cell transplantation is an important objective. Herein, we report a method that takes advantage of the growth factor-like and anti-inflammatory activities of the fibrinolysis protease, tissue plasminogen activator tPA, without effects on hemostasis. We demonstrate that conditioning hiNPC with enzymatically-inactive tissue-type plasminogen activator (EI-tPA), prior to grafting into a T3 lesion site in a clinically relevant severe SCI model, significantly improves motor outcomes. EI-tPA-primed hiNPC grafted into lesion sites survived, differentiated, acquired markers of motor neuron maturation, and extended βIII-tubulin-positive axons several spinal segments below the lesion. Importantly, only SCI rats that received EI-tPA primed hiNPC demonstrated significantly improved motor function, without exacerbating pain. When hiNPC were treated with EI-tPA in culture, NMDA-R-dependent cell signaling was initiated, expression of genes associated with stemness (Nestin, Sox2) was regulated, and thrombin-induced cell death was prevented. EI-tPA emerges as a novel agent capable of improving the efficacy of stem cell therapy in SCI.

Subclinical lipopolysaccharide from Salmonella Enteritidis induces neuropeptide dysregulation in the spinal cord and the dorsal root ganglia

Background

Despite increasing evidence that lipopolysaccharide (LPS) affects the biological active substances of dorsal root ganglia (DRG) we have limited knowledge of the influence of a single low dose of LPS, which does not result in any clinical symptoms of disease (subclinical LPS) on neuropeptides connected with the sensory pathway. Accordingly, in this work, we investigated the influence of subclinical LPS from Salmonella Enteritidis on selected neuropeptides: substance P (SP), galanin (GAL), neuropeptide Y (NPY), vasoactive intestinal peptide (VIP) and somatostatin (SOM) in the cervical, thoracic, lumbar and sacral regions of the DRG and spinal cord.

Methods

This study was performed on immature female pigs of the Pietrain × Duroc breed. Seven days after the intravenous injection of saline solution for control animals (n = 5) and 5 μg/kg b.w. LPS from S. Enteritidis for the experimental group (n = 5), the DRG and the spinal cord were collected to extract the neuropeptides using solid-phase extraction technology.

Results

Our results demonstrated that subclinical LPS in DRG was able to change the levels of all studied neuropeptides except SOM, whereas in the spinal cord it down-regulated all studied neuropeptides in the sacral spinal cord, maintaining the concentration of all studied neuropeptides in other regions similar to that observed in the control animals. The significant differences in the intensity and character of observed changes between particular regions of the DRG suggest that the exact functions of the studied neuropeptides and mechanisms of responses to subclinical LPS action depend on specific characteristics and functions of each examination region of DRG.

Conclusions

The mechanisms of observed changes are not fully understood and require further study of the molecular interactions between subclinical LPS from S. Enteritidis and neuronal and non-neuronal cells of DRG and spinal cord. The peripheral and central pain pathways must be analysed with the aspect of unknown long-term consequences of the influence of subclinical LPS from S. Enteritidis on neuropeptides in the spinal cord and the dorsal root ganglia.

Vasoactive intestinal peptide is upregulated in women with endometriosis and chronic pelvic pain

PROBLEM

Chronic pelvic pain (CPP) causes compromised the quality of life in women with endometriosis and is often attributed to local inflammation and ingrowth of nerve fibers. In this pilot study, we aimed to investigate whether the inflammation-related vasoactive intestinal peptide (VIP) and interleukin (IL)-6 were increased in affected patients.

METHOD OF STUDY

Endometrial and endometriotic tissue biopsy specimens, and serum and peritoneal fluid (PF) samples, were obtained from 85 endometriosis patients and 53 controls. VIP and IL-6 analysis and measurement of microvessel density in tissue were performed using immunohistochemistry, Western blotting, RT-qPCR, and ELISA.

RESULTS

Compared with controls, VIP transcript and protein levels were increased in endometrium from endometriosis patients and further elevated in patients with CPP. In addition, microvessel density, a measurement of angiogenic activity, was increased in the endometrium and in endometriosis lesions in the same subset of patients. Serum and PF levels of VIP and IL-6 were higher in women with endometriosis and CPP compared with endometriosis patients who reported no chronic pain.

CONCLUSION

Vasoactive intestinal peptide is upregulated in endometriosis patients reporting chronic pain. Increased microvessel density in tissue and peritoneal fluid concentrations of IL-6 indicate an elevated inflammation in the pelvic microenvironment of these patients.

PACAP and migraine headache: immunomodulation of neural circuits in autonomic ganglia and brain parenchyma

The discovery that intravenous (IV) infusions of the neuropeptide PACAP-38 (pituitary adenylyl cyclase activating peptide-38) induced delayed migraine-like headaches in a large majority of migraine patients has resulted in considerable excitement in headache research. In addition to suggesting potential therapeutic targets for migraine, the finding provides an opportunity to better understand the pathological events from early events (aura) to the headache itself. Although PACAP-38 and the closely related peptide VIP (vasoactive intestinal peptide) are well-known as vasoactive molecules, the dilation of cranial blood vessels per se is no longer felt to underlie migraine headaches. Thus, more recent research has focused on other possible PACAP-mediated mechanisms, and has raised some important questions. For example, (1) are endogenous sources of PACAP (or VIP) involved in the triggering and/or propagation of migraine headaches?; (2) which receptor subtypes are involved in migraine pathophysiology?; (3) can we identify specific anatomical circuit(s) where PACAP signaling is involved in the features of migraine? The purpose of this review is to discuss the possibility, and supportive evidence, that PACAP acts to induce migraine-like symptoms not only by directly modulating nociceptive neural circuits, but also by indirectly regulating the production of inflammatory mediators. We focus here primarily on postulated extra-dural sites because potential mechanisms of PACAP action in the dura are discussed in detail elsewhere (see X, this edition).

Neuropeptide imaging in rat spinal cord with MALDI-TOF MS: Method development for the application in pain-related disease studies

Spinal cord as a connection between brain and peripheral nervous system is an essential material for studying neural transmission, especially in pain-related research. This study was the first to investigate pain-related neuropeptide distribution in rat spinal cord using a matrix-assisted laser desorption ionization-time of flight imaging mass spectrometry (MALDI TOF MS) approach. The imaging workflow was evaluated and showed that MALDI TOF MS provides efficient resolution and robustness for neuropeptide imaging in rat spinal cord tissue. The imaging result showed that in naive rat spinal cord the molecular distribution of haeme, phosphatidylcholine, substance P and thymosin beta 4 were well in line with histological features. Three groups of pain-related neuropeptides, which are cleaved from prodynorphin, proenkephalin and protachykinin-1 proteins were detected. All these neuropeptides were found predominantly localized in the dorsal spinal cord and each group had unique distribution pattern. This study set the stage for future MALDI TOF MS application to elucidate signalling mechanism of pain-related diseases in small animal models.

Neuropeptides and Microglial Activation in Inflammation, Pain, and Neurodegenerative Diseases

Microglial cells are responsible for immune surveillance within the CNS. They respond to noxious stimuli by releasing inflammatory mediators and mounting an effective inflammatory response. This is followed by release of anti-inflammatory mediators and resolution of the inflammatory response. Alterations to this delicate process may lead to tissue damage, neuroinflammation, and neurodegeneration. Chronic pain, such as inflammatory or neuropathic pain, is accompanied by neuroimmune activation, and the role of glial cells in the initiation and maintenance of chronic pain has been the subject of increasing research over the last two decades. Neuropeptides are small amino acidic molecules with the ability to regulate neuronal activity and thereby affect various functions such as thermoregulation, reproductive behavior, food and water intake, and circadian rhythms. Neuropeptides can also affect inflammatory responses and pain sensitivity by modulating the activity of glial cells. The last decade has witnessed growing interest in the study of microglial activation and its modulation by neuropeptides in the hope of developing new therapeutics for treating neurodegenerative diseases and chronic pain. This review summarizes the current literature on the way in which several neuropeptides modulate microglial activity and response to tissue damage and how this modulation may affect pain sensitivity.

Pituitary adenylate cyclase-activating polypeptide is regulated by alternative splicing of transcriptional repressor REST/NRSF in nerve injury

AIMS

The pathophysiological mechanism for neuropathic pain (NP), one of the most common types of intractable pain, remains largely unknown. We previously reported that pituitary adenylate-cyclase activating polypeptide (PACAP) is required for the development of spinal sensitization and induction of NP. Previous in vitro studies suggest that PACAP transcription unit has two RE1-like elements and that the transcriptional repressor REST controls expression of PACAP gene. However the regulation of PACAP gene through its RE1 sites in vivo has not been studied. We have analyzed the functional role of PACAP gene RE1 element following nerve injury.

MAIN METHODS

An L5-spinal nerve transection (L5-SNT) in mice was used as a model of spinal injury. DRGs after the L5-SNT were studied.

KEY FINDINGS

PACAP mRNA increased in the DRG following spinal nerve injury. REST4, an alternatively spliced isoform of REST was shown to be regulated by the splicing activator (nSR100) and nSR100 itself also increased. Overexpression of either REST4 or nSR100 in vitro increased PACAP expression, while overexpression of REST repressed PACAP mRNA production. Reporter gene analysis showed that a novel RE1 previously predicted by in silico analysis was indeed functional. ChIP analysis showed that REST bound significantly to this RE1 in the DRG of naïve mice, while REST binding to this RE1 was decreased following spinal nerve injury. The expression of REST was decreased by nSR100-dependent alternative splicing of the REST gene, leading to derepression of PACAP.

SIGNIFICANCE

PACAP expression in the DRG following spinal nerve injury is controlled through a novel RE1 by REST.

Extreme thermal noxious stimuli induce pain responses in zebrafish larvae

Exposing tissues to extreme high or low temperature leads to burns. Burned animals sustain several types of damage, from the disruption of the tissue to degeneration of axons projecting through muscle and skin. Such damage causes pain due to both inflammation and axonal degeneration (neuropathic-like pain). Thus, the approach to cure and alleviate the symptoms of burns must be twofold: rebuilding the tissue that has been destroyed and alleviating the pain derived from the burns. While tissue regeneration techniques have been developed, less is known on the treatment of the induced pain. Thus, appropriate animal models are necessary for the development of the best treatment for pain induced in burned tissues. We have developed a methodology in the zebrafish aimed to produce a new animal model for the study of pain induced by burns. Here, we show that two events linked to the onset of burn-induced inflammation and neuropathic-like pain in mammals, degeneration of axons innervating the affected tissues and over-expression of specific genes in sensory tissues, are conserved from zebrafish to mammals.

Role of Pituitary Adenylate-Cyclase Activating Polypeptide and Tac1 gene derived tachykinins in sensory, motor and vascular functions under normal and neuropathic conditions

Pituitary Adenylate-Cyclase Activating Polypeptide (PACAP) and Tac1 gene-encoded tachykinins (substance P: SP, neurokinin A: NKA) are expressed in capsaicin-sensitive nerves, but their role in nociception, inflammation and vasoregulation is unclear. Therefore, we investigated the function of these neuropeptides and the NK1 tachykinin receptor (from Tacr1 gene) in the partial sciatic nerve ligation-induced traumatic mononeuropathy model using gene deficient (PACAP(-/-), Tac1(-/-), and Tacr1(-/-)) mice. Mechanonociceptive threshold of the paw was measured with dynamic plantar aesthesiometry, motor coordination with Rota-Rod and cutaneous microcirculation with laser Doppler imaging. Neurogenic vasodilation was evoked by mustard oil stimulating sensory nerves. In wildtype mice 30-40% mechanical hyperalgesia developed one week after nerve ligation, which was not altered in Tac1(-/-) and Tacr1(-/-) mice, but was absent in PACAP(-/-) animals. Motor coordination of the PACAP(-/-) and Tac1(-/-) groups was significantly worse both before and after nerve ligation compared to their wildtypes, but it did not change in Tacr1(-/-) mice. Basal postoperative microcirculation on the plantar skin of PACAP(-/-) mice did not differ from the wildtypes, but was significantly lower in Tac1(-/-) and Tacr1(-/-) ones. In contrast, mustard oil-induced neurogenic vasodilation was significantly smaller in PACAP(-/-) mice, but not in Tacr1(-/-) and Tac1(-/-) animals. Both PACAP and SP/NKA, but not NK1 receptors participate in normal motor coordination. Tachykinins maintain basal cutaneous microcirculation. PACAP is a crucial mediator of neuropathic mechanical hyperalgesia and neurogenic vasodilation. Therefore identifying its target and developing selective, potent antagonists, might open promising new perspectives for the treatment of neuropathic pain and vascular complications.

Dural mast cell degranulation is a putative mechanism for headache induced by PACAP-38

BACKGROUND

Pituitary adenylate cyclase activating peptide-38 (PACAP-38) has been shown to induce migraine in migraineurs, whereas the related peptide vasoactive intestinal peptide (VIP) does not. In the present study we examine the hypothesis that PACAP-38 and its truncated version PACAP-27 but not VIP cause degranulation of mast cells in peritoneum and in dura mater.

METHODS

The degranulatory effects of PACAP-38, PACAP-27 and VIP were investigated by measuring the amount of N-acetyl-β-hexosaminidase released from isolated peritoneal mast cells and from dura mater attached to the skull of the rat in vitro. In peritoneal mast cells N-truncated fragments of PACAP-38 (PACAP(6-38), PACAP(16-38) and PACAP(28-38)) were also studied. To investigate transduction pathways involved in mast cell degranulation induced by PACAP-38, PACAP-27 and VIP, the phospholipase C inhibitor U-73122 and the adenylate cyclase inhibitor SQ 22536 were used.

RESULTS

The peptides induced degranulation of isolated peritoneal mast cells of the rat with the following order of potency: PACAP-38 = PACAP(6-38) = PACAP(16-38) » PACAP-27 = VIP = PACAP(28-38). In the dura mater we found that 10(-5) M PACAP-38 was significantly more potent in inducing mast cell degranulation than the same concentration of PACAP-27 or VIP. Inhibition of intracellular mechanisms demonstrated that PACAP-38-induced degranulation is mediated by the phospholipase C pathway. Selective blockade of the PAC(1) receptor did not attenuate degranulation.

CONCLUSION

These findings correlate with clinical studies and support the hypothesis that mast cell degranulation is involved in PACAP-induced migraine. PACAP-38 has a much stronger degranulatory effect on rat peritoneal and dural mast cells than VIP and PACAP-27. The difference in potency between PACAP-38- and PACAP-27/VIP-induced peritoneal mast cell degranulation is probably not related to the PAC(1) receptor but is caused by a difference in efficacy on phospholipase C.

Changes in pituitary adenylate cyclase-activating Peptide 27-like immunoreactive nervous structures in the porcine descending colon during selected pathological processes

This study reports on changes in the pituitary adenylate cyclase-activating peptide 27-like immunoreactive (PACAP-27-LI) nerve structures of the enteric nervous system (ENS) in the porcine descending colon, caused by chemically induced inflammation, nerve injury, and proliferative enteropathy (PE), which is a "natural" inflammation of the porcine digestive tract. The distribution pattern of PACAP-27-LI structures was studied using the immunofluorescence technique in the circular muscle layer, enteric plexuses (i.e., myenteric plexus (MP), outer submucous plexus (OSP), and inner submucous plexus (ISP)), and in the mucosal layer. Under physiological conditions, PACAP-27-LI perikarya have been shown to constitute 4.04 ± 0.66, 6.66 ± 0.77, and 11.19 ± 0.74 % in the MP, OSP, and ISP, respectively. Changes in PACAP-27 immunoreactivity depended on the pathological factor studied. The numbers of the PACAP-27-LI perikarya amounted to 12.26 ± 1.43, 12.28 ± 0.79, and 21.13 ± 1.19 % in chemically induced colitis, 17.83 ± 0.88, 9.03 ± 1.05, and 20.72 ± 1.35 % during PE and 10.65 ± 0.82, 6.88 ± 1.04, and 14.04 ± 1.09 % after axotomy in MP, OSP, and ISP, respectively. All of the studied processes generally resulted in an increase in the number of PACAP-27-LI nerve fibers in the circular muscle and mucosal layers. The obtained results suggest that PACAP-27-LI nerve structures of ENS may participate in various pathological states within the porcine descending colon, and their functions probably depend on the type of pathological factor.

Mechanisms of VIP-induced inhibition of the lymphatic vessel pump

Lymphatic vessels serve as a route by which interstitial fluid, protein and other macromolecules are returned to the blood circulation and immune cells and antigens gain access to lymph nodes. Lymph flow is an active process promoted by rhythmical contraction-relaxation events occurring in the collecting lymphatic vessels. This lymphatic pumping is an intrinsic property of the lymphatic muscles in the vessel wall and consequent to action potentials. Compromised lymphatic pumping may affect lymph and immune cell transport, an action which could be particularly detrimental during inflammation. Importantly, many inflammatory mediators alter lymphatic pumping. Vasoactive intestinal peptide (VIP) is a neuro- and immuno-modulator thought to be released by nerve terminals and immune cells in close proximity to lymphatic vessels. We demonstrated the presence of the peptide in lymphatic vessels and in the lymph and examined the effects of VIP on mesenteric collecting lymphatic vessels of the guinea pig using pharmacological bioassays, intracellular microelectrode electrophysiology, immunofluorescence and quantitative real-time PCR. We showed that VIP alters lymphatic pumping by decreasing the frequency of lymphatic contractions and hyperpolarizing the lymphatic muscle membrane potential in a concentration-dependent manner. Our data further suggest that these effects are mainly mediated by stimulation of the VIP receptor VPAC2 located on the lymphatic muscle and the downstream involvement of protein kinase A (PKA) and ATP-sensitive K⁺ (KATP) channels. Inhibition of lymphatic pumping by VIP may compromise lymph drainage, oedema resolution and immune cell trafficking to the draining lymph nodes.

Crystal structure of the PAC1R extracellular domain unifies a consensus fold for hormone recognition by class B G-protein coupled receptors

Pituitary adenylate cyclase activating polypeptide (PACAP) is a member of the PACAP/glucagon family of peptide hormones, which controls many physiological functions in the immune, nervous, endocrine, and muscular systems. It activates adenylate cyclase by binding to its receptor, PAC1R, a member of class B G-protein coupled receptors (GPCR). Crystal structures of a number of Class B GPCR extracellular domains (ECD) bound to their respective peptide hormones have revealed a consensus mechanism of hormone binding. However, the mechanism of how PACAP binds to its receptor remains controversial as an NMR structure of the PAC1R ECD/PACAP complex reveals a different topology of the ECD and a distinct mode of ligand recognition. Here we report a 1.9 Å crystal structure of the PAC1R ECD, which adopts the same fold as commonly observed for other members of Class B GPCR. Binding studies and cell-based assays with alanine-scanned peptides and mutated receptor support a model that PAC1R uses the same conserved fold of Class B GPCR ECD for PACAP binding, thus unifying the consensus mechanism of hormone binding for this family of receptors.

Additive effects of orexin B and vasoactive intestinal polypeptide on LL-37-mediated antimicrobial activities

The present study examined the bactericidal effects of orexin B (ORXB) and vasoactive intestinal peptide (VIP) alone or combined with cationic antimicrobial peptides, such as LL-37, on Escherichia coli, Pseudomonas aeruginosa, Streptococcus mutans and Staphylococcus aureus. The bactericidal effect of ORXB or VIP alone was detected in low NaCl concentration, but attenuated in physiological NaCl concentration (150 mM). However, such attenuated bactericidal activities of ORXB and VIP in 150 mM NaCl were regained by adding LL-37. Therefore, our results indicate that VIP and ORXB appear to mediate bactericidal effects in concert with LL-37 in the physiological context of mucosal tissue.

Regulation of spinal dynorphin 1-17 release by endogenous pituitary adenylyl cyclase-activating polypeptide in the male rat: relevance of excitation via disinhibition

Opioids inhibit release of primary afferent transmitters but it is unclear whether the converse occurs. To test the hypothesis that primary afferent transmitters influence opioid-ergic tone, we studied the functional and anatomical relationships between pituitary adenylyl cyclase-activating polypeptide (PACAP) and dynorphin 1-17 (Dyn) in spinal cord. We found that activation of the PACAP-specific receptor PAC(1) (PAC(1)R) inhibited, whereas PAC(1)R blockade augmented, spinal release of Dyn. It is noteworthy that in the formalin-induced pain model PAC(1)R blockade (via PACAP6-38) also resulted in antinociception that was abolished by spinal κ-opioid receptor blockade. These findings indicate that Dyn release is tonically inhibited by PACAP and that blocking this inhibition, which increases the spinal release of Dyn, results in antinociception. Consistent with this conclusion, we found in the spinal dorsal horn that Dyn-immunoreactive neurons 1) expressed PAC(1)R and 2) were apposed by PACAP terminals. Present results, in combination with the previous demonstration that the release of spinal Dyn is tonically inhibited by opioid- and nociceptin/orphanin FQ-coupled pathways (J Pharmacol Exp Ther 298:1213-1220, 2001), indicate that spinal Dyn-ergic neurons integrate multiple inhibitory inputs, the interruption of any one of which (i.e., disinhibition) is sufficient to enhance spinal Dyn release and generate antinociception. Gaining a better understanding of the role of primary afferent neurotransmitters in negatively modulating the spinal release of Dyn and the physiological use of disinhibition to increase spinal Dyn activity could suggest novel clinically useful approaches for harnessing endogenous Dyn for pain control.

Huangshu Enema down-regulates VIP expression in the colonic mucosa of rats with diarrhea-predominant irritable bowel syndrome

AIM: To investigate the effect of Chinese medicine Huangshu Enema (HSE) on the expression of vasoactive intestinal polypeptide (VIP) in the colonic mucosa of rats with diarrhea-predominant irritable bowel syndrome (D-IBS) and to explore possible mechanisms involved.

METHODS: Forty-eight female Sprague-Dawley rats were randomly divided into normal control group, model group, HSE treatment group and dicetel treatment group (n = 12 for each group). D-IBS was induced in rats by intragastric administration of senna decoction and restraint stress. The expression of VIP in the colonic mucosa of rats was determined by immunohistochemistry.

RESULTS: The number of VIP-positive cells and VIP staining intensity in the colonic mucosa of rats were significantly higher in the model group than in the normal control group (21.06 ± 2.69 vs 6.25 ± 2.08, t = 17.4216, P < 0.01; 2.43 ± 0.63 vs 0.63 ± 0.62, t = 6.5223, P < 0.01), in the model group than in the two treatment groups (t = 11.6697 and 5.5119; t = 6.0910 and 2.1575, respectively; both P < 0.01), and in the dicetel treatment group than in the HSE treatment group (10.62 ± 2.36 vs 15.31 ± 2.65, t = 5.2876, P < 0.01; 1.25 ± 0.58 vs 1.93 ± 0.68, t = 2.9272, P < 0.01).

CONCLUSION: HSE can reduce the number of VIP-positive cells and VIP staining intensity in the colonic mucosa of D-IBS rats and thereby exerts therapeutic effects against D-IBS.

Impaired nocifensive behaviours and mechanical hyperalgesia, but enhanced thermal allodynia in pituitary adenylate cyclase-activating polypeptide deficient mice

Pituitary adenylate cyclase-activating polypeptide-38 (PACAP-38) and its receptors (PAC1 and VPAC) have been shown in the spinal dorsal horn, dorsal root ganglia and sensory nerve terminals. Data concerning the role of PACAP in central pain transmission are controversial and we have recently published its divergent peripheral effects on nociceptive processes. The aim of the present study was to investigate acute somatic and visceral nocifensive behaviours, partial sciatic nerve ligation-evoked chronic neuropathic, as well as resiniferatoxin-induced inflammatory thermal and mechanical hyperalgesia in PACAP deficient (PACAP(-/-)) mice to elucidate its overall function in pain transmission. Neuronal activation was investigated with c-Fos immunohistochemistry. Paw lickings in the early (0-5 min) and late (20-45 min) phases of the formalin test were markedly reduced in PACAP(-/-) mice. Acetic acid-evoked abdominal contractions referring to acute visceral chemonociception was also significantly attenuated in PACAP knockout animals. In both models, the excitatory role of PACAP was supported by markedly greater c-Fos expression in the periaqueductal grey and the somatosensory cortex. In PACAP-deficient animals neuropathic mechanical hyperalgesia was absent, while c-Fos immunopositivity 20 days after the operation was significantly higher. In this chronic model, these neurons are likely to indicate the activation of secondary inhibitory pathways. Intraplantarly injected resiniferatoxin-evoked mechanical hyperalgesia involving both peripheral and central processes was decreased, but thermal allodynia mediated by only peripheral mechanisms was increased in PACAP(-/-) mice. These data clearly demonstrate an overall excitatory role of PACAP in pain transmission originating from both exteroceptive and interoceptive areas, it is also involved in central sensitization. This can be explained by the signal transduction mechanisms of its identified receptors, both PAC1 and VPAC activation leads to neuronal excitation. In contrast, it is an inhibitory mediator at the level of the peripheral sensory nerve endings and decreases their sensitization to heat with presently unknown mechanisms.

Multiple chronic pain states are associated with a common amino acid-changing allele in KCNS1

Not all patients with nerve injury develop neuropathic pain. The extent of nerve damage and age at the time of injury are two of the few risk factors identified to date. In addition, preclinical studies show that neuropathic pain variance is heritable. To define such factors further, we performed a large-scale gene profiling experiment which plotted global expression changes in the rat dorsal root ganglion in three peripheral neuropathic pain models. This resulted in the discovery that the potassium channel alpha subunit KCNS1, involved in neuronal excitability, is constitutively expressed in sensory neurons and markedly downregulated following nerve injury. KCNS1 was then characterized by an unbiased network analysis as a putative pain gene, a result confirmed by single nucleotide polymorphism association studies in humans. A common amino acid changing allele, the 'valine risk allele', was significantly associated with higher pain scores in five of six independent patient cohorts assayed (total of 1359 subjects). Risk allele prevalence is high, with 18-22% of the population homozygous, and an additional 50% heterozygous. At lower levels of nerve damage (lumbar back pain with disc herniation) association with greater pain outcome in homozygote patients is P = 0.003, increasing to P = 0.0001 for higher levels of nerve injury (limb amputation). The combined P-value for pain association in all six cohorts tested is 1.14 E-08. The risk profile of this marker is additive: two copies confer the most, one intermediate and none the least risk. Relative degrees of enhanced risk vary between cohorts, but for patients with lumbar back pain, they range between 2- and 3-fold. Although work still remains to define the potential role of this protein in the pathogenic process, here we present the KCNS1 allele rs734784 as one of the first prognostic indicators of chronic pain risk. Screening for this allele could help define those individuals prone to a transition to persistent pain, and thus requiring therapeutic strategies or lifestyle changes that minimize nerve injury.

Cutaneous nociception and neurogenic inflammation evoked by PACAP38 and VIP

Pituitary adenylate cyclase-activating peptide-38 (PACAP38) and vasoactive intestinal peptide (VIP) belong to the same secretin–glucagon superfamily and are present in nerve fibers in dura and skin. Using a model of acute cutaneous pain we explored differences in pain perception and vasomotor responses between PACAP38 and VIP in 16 healthy volunteers in a double-blind, placebo-controlled, crossover study. All participants received intradermal injections of 200 pmol PACAP38, 200 pmol VIP and placebo into the volar forearm. Measurements included pain intensity on a visual analog scale (VAS), blood flow by laser Doppler flowmetry, visual flare and wheal. Pain intensities after PACAP38 and VIP were mild and limited to a short time of about 100 s after injection. The area under the VAS-time curve was larger following PACAP38 (P = 0.004) and VIP (P = 0.01) compared to placebo. We found no statistical difference in pain perception between PACAP38 and VIP. Skin blood flow increase, flare and wheal were larger after both PACAP38 (P = 0.011) and VIP (P = 0.001) compared to placebo. VIP induced a considerably larger increase in skin blood flow, flare and wheal than PACAP38 (P = 0.002). In conclusion, we found that peripheral nociceptive cutaneous responses elicited by PACAP38 and VIP are similar in healthy volunteers. This suggests that acute pain and vasomotor responses following intradermal injections of PACAP38 and VIP are primarily mediated by VPAC receptors.

The PACAP receptor: a novel target for migraine treatment

The origin of migraine pain has not yet been clarified, but accumulating data point to neuropeptides present in the perivascular space of cranial vessels as important mediators of nociceptive input during migraine attacks. Pituitary adenylate cyclase-activating polypeptide (PACAP) is present in sensory trigeminal neurons and may modulate nociception at different levels of the nervous system. Human experimental studies have shown that PACAP-38 infusion induces marked dilatation of extracerebral vessels and delayed migraine-like attacks in migraine patients. PACAP selectively activates the PAC(1) receptor, which suggests a possible signaling pathway implicated in migraine pain. This review summarizes the current evidence supporting the involvement of PACAP in migraine pathophysiology and the PAC(1) receptor as a possible novel target for migraine treatment.

PACAP and VIP affect NF1 expression in rat malignant peripheral nerve sheath tumor (MPNST) cells

In our previous study we have identified PACAP, VIP and their receptors in rat malignant peripheral nerve sheath tumor (MPNST) cells, thus showing anti-apoptotic roles. Recently it has been shown that the tumor suppressor neurofibromin, encoded by the Neurofibromatosis type I (NF1) gene, promotes MPNST cells sensitivity to apoptosis after serum withdrawal. In the present study we investigated whether PACAP or VIP negatively regulate NF1 expression under normal or serum-dependent pro-apoptotic culture conditions. Results indicated that serum itself significantly influenced gene and protein levels. In fact, the low NF1 levels of cells cultured in normal serum-containing medium were remarkably increased in cells switched to low- or no-serum after 24h and 48 h. Treatment with 100 nM PACAP or VIP did not affect NF1 expression when using normal amounts of serum, whereas it significantly inhibited transcript and protein levels both in low- or no-serum cultured cells. In particular, PACAP reduced NF1 levels already after 24h in low-serum cultured cells, while VIP showed a similar effect only after serum deprivation. However, both PACAP and VIP downregulated gene and protein levels within 48 h either in low-dose and serum-starved cells. Results were confirmed by fluorescence microscopy, showing that 100 nM PACAP or VIP attenuated neurofibromin cytoplasmic localization only in low- or no-serum cultured cells. The present study provides a comprehensive analysis of both neuropeptides effect on NF1 expression in normal, low- or serum-starved MPNST cells, ameliorating the hypothesis that resistance to apoptosis in serum-deprived cells might be correlated to PACAP-/VIP-induced NF1 inhibition.

In search of analgesia: emerging roles of GPCRs in pain

Of all clinically marketed drugs, greater than thirty percent are modulators of G protein-coupled receptors (GPCRs). Nearly 400 GPCRs (i.e., excluding odorant and light receptors) are encoded within the human genome, but only a small fraction of these seven-transmembrane proteins have been identified as drug targets. Chronic pain affects more than one-third of the population, representing a substantial societal burden in use of health care resources and lost productivity. Furthermore, currently available treatments are often inadequate, underscoring the significant need for better therapeutic strategies. The expansion of the identified human GPCR repertoire, coupled with recent insights into the function and structure of GPCRs, offers new opportunities for the development of novel analgesic therapeutics.

The in vivo Effect of VIP, PACAP-38 and PACAP-27 and mRNA Expression of Their Receptors in Rat Middle Meningeal Artery

The parasympathetic nervous system is probably involved in migraine pathogenesis. Its activation releases a mixture of signalling molecules including vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP), which subsequently stimulate VPAC1, VPAC2 and PAC1 receptors. The objective of the present study was to investigate the in vivo effect of VIP, PACAP-27, PACAP-38, the selective VPAC1 agonist ([Lys15, Arg16, Leu27]-VIP(1–7)-GRF(8–27)) and a PAC1 agonist, maxadilan on rat middle meningeal artery (MMA) diameter using the closed cranial window model. Selective antagonists were used for further characterization of the responses. Reverse transcriptase-polymerase chain reaction experiments were also conducted to determine expression of mRNA of PACAP receptors in the MMA. The results showed that VIP, PACAP-38, PACAP-27 and the VPAC1 specific agonist evoked significant dilations with the rank order of potency; VIP = PACAP-38 > PACAP-27 = [Lys15, Arg16, Leu27]-VIP(1–7)-GRF(8–27). Significant inhibition of dilation was only observed for the VPAC1 antagonist PG97–269 on PACAP-38-induced dilation of MMA. The VPAC2 antagonist PG99–465 and PAC1 antagonist PACAP(6–38) did not significantly block VIP- or PACAP-induced dilation. Expression of mRNA of all three receptors was detected in the MMA. In conclusion, the VPAC1 receptor seems to be predominant in mediating MMA dilation. A selective VPAC1 antagonist may be a candidate molecule in the treatment of migraine headache.

Differential involvement of hippocampal vasoactive intestinal peptide in nociception of rats with a model of depression

The effects of VIP microinjected unilaterally (left or right) into the hippocampal CA1 area at a dose of 10 and 100 ng or bilaterally (10 ng), on nociception of male Wistar rats with a model of depression (bilateral olfactory bulbectomy-OBX) were studied. Nociception was examined applying mechanical pressure on the left hind paw of the rat (analgesy-meter test). It was found that in OBX rats the pain threshold is increased. VIP showed differential effects depending on the side and dose of administration. The pain threshold after left-side microinjections of VIP into the hippocampal CA1 area of OBX rats was significantly higher than that after injections into right-side. There are no significant differences between right-side VIP-treated and OBX rats. Bilateral microinjections of VIP also exerted antinociceptive effect. These findings suggest that the hippocampal lateralized antinociceptive effect of VIP in OBX rats depends on the hemisphere of injection and suggest that VIP-ergic neurons in the hippocampal CA1 area may play differential role in nociception of rats with a model of depression.

VPAC and PAC receptors: From ligands to function

Vasoactive intestinal peptide (VIP) and the pituitary adenylate cyclase activating polypeptides (PACAPs) share 68% identity at the amino acid level and belong to the secretin peptide family. Following the initial discovery of VIP almost four decades ago a substantial amount of knowledge has been presented describing the mechanisms of action, distribution and pleiotropic functions of these related peptides. It is now known that the physiological actions of these widely distributed peptides are produced through activation of three common G-protein coupled receptors (VPAC(1), VPAC(2) and PAC(1)R) which preferentially stimulate adenylate cyclase and increase intracellular cAMP, although stimulation of other intracellular messengers, including calcium and phospholipase D, has been reported. Using a range of in vitro and in vivo approaches, including cell-based functional assays, transgenic animals and rodent models of disease, VPAC/PAC receptor activation has been associated with numerous physiological processes (e.g. control of circadian rhythms) and clinical conditions (e.g. pulmonary hypertension), which underlies on-going research efforts and makes these peptides and their cognate receptors attractive targets for the pharmaceutical industry. However, despite the considerable interest in VPAC/PAC receptors and the processes which they mediate, there is still a paucity of selective and available, non-peptide ligands, which has hindered further advances in this field both at the basic research and clinical level. This review summarises the current knowledge of VIP/PACAP and the VPAC/PAC receptors with regard to their distribution, pharmacology, signalling pathways, splice variants and finally, the utility of animal models in exploring their physiological roles.

Persistent changes in spinal cord gene expression after recovery from inflammatory hyperalgesia: a preliminary study on pain memory

BACKGROUND

Previous studies found that rats subjected to carrageenan injection develop hyperalgesia, and despite complete recovery in several days, they continue to have an enhanced hyperalgesic response to a new noxious challenge for more than 28d. The study's aim was to identify candidate genes that have a role in the formation of the long-term hyperalgesia-related imprint in the spinal cord. This objective was undertaken with the understanding that the long-lasting imprint of acute pain in the central nervous system may contribute to the transition of acute pain to chronicity.

RESULTS

To analyze changes in gene expression when carrageenan-induced hyperalgesia has disappeared but propensity for the enhanced hyperalgesic response is still present, we determined the gene expression profile using oligo microarray in the lumbar part of the spinal cord in three groups of rats: 28d after carrageenan injection, 24h after injection (the peak of inflammation), and with no injection (control group). Out of 17,000 annotated genes, 356 were found to be differentially expressed compared with the control group at 28d, and 329 at 24h after carrageenan injection (both groups at p < 0.01). Among differentially expressed genes, 67 (39 in 28d group) were identified as being part of pain-related pathways, altered in different models of pain, or interacting with proteins involved in pain-related pathways. Using gene ontology (GO) classification, we have identified 3 functional classes deserving attention for possible association with pain memory: They are related to cell-to-cell interaction, synaptogenesis, and neurogenesis.

CONCLUSION

Despite recovery from inflammatory hyperalgesia, persistent changes in spinal cord gene expression may underlie the propensity for the enhanced hyperalgesic response. We suggest that lasting changes in expression of genes involved in the formation of new synapses and neurogenesis may contribute to the transition of acute pain to chronicity.

Meta-analysis of microarray-derived data from PACAP-deficient adrenal gland in vivo and PACAP-treated chromaffin cells identifies distinct classes of PACAP-regulated genes

Initial PACAP-regulated transcriptomes of PACAP-treated cultured chromaffin cells, and the adrenal gland of wild-type versus PACAP-deficient mice, have been assembled using microarray analysis. These were compared to previously acquired PACAP-regulated transcriptome sets from PC12 cells and mouse central nervous system, using the same microarray platform. The Ingenuity Pathways Knowledge Base was then employed to group regulated transcripts into common first and second messenger regulatory clusters. The purpose of our meta-analysis was to identify sets of genes regulated distinctly or in common by the neurotransmitter/neurotrophin PACAP in specific physiological contexts. Results suggest that PACAP participates in both the basal differentiated expression, and the induction upon physiological stimulation, of distinct sets of transcripts in neuronal and endocrine cells. PACAP in both developmental and acute regulatory paradigms acts on target genes also regulated by either TNFalpha or TGFbeta, two first messengers acting on transcription mainly through NFkappaB and Smads, respectively.

Solution structure and mutational analysis of pituitary adenylate cyclase-activating polypeptide binding to the extracellular domain of PAC1-RS

The pituitary adenylate cyclase-activating polypeptide (PACAP) receptor is a class II G protein-coupled receptor that contributes to many different cellular functions including neurotransmission, neuronal survival, and synaptic plasticity. The solution structure of the potent antagonist PACAP (residues 6'-38') complexed to the N-terminal extracellular (EC) domain of the human splice variant hPAC1-R-short (hPAC1-R(S)) was determined by NMR. The PACAP peptide adopts a helical conformation when bound to hPAC1-R(S) with a bend at residue A18' and makes extensive hydrophobic and electrostatic interactions along the exposed beta-sheet and interconnecting loops of the N-terminal EC domain. Mutagenesis data on both the peptide and the receptor delineate the critical interactions between the C terminus of the peptide and the C terminus of the EC domain that define the high affinity and specificity of hormone binding to hPAC1-R(S). These results present a structural basis for hPAC1-R(S) selectivity for PACAP versus the vasoactive intestinal peptide and also differentiate PACAP residues involved in binding to the N-terminal extracellular domain versus other parts of the full-length hPAC1-R(S) receptor. The structural, mutational, and binding data are consistent with a model for peptide binding in which the C terminus of the peptide hormone interacts almost exclusively with the N-terminal EC domain, whereas the central region makes contacts to both the N-terminal and other extracellular parts of the receptor, ultimately positioning the N terminus of the peptide to contact the transmembrane region and result in receptor activation.

Tóth Z, House SB, Gainer H. Depolarization and neurotransmitter regulation of vasopressin gene expression in the rat suprachiasmatic nucleus in vitro

Vasopressin (VP) transcription in the rat suprachiasmatic nucleus (SCN) in organotypic culture was studied by in situ hybridization histochemistry using an intron-specific VP heteronuclear RNA probe. The circadian peak of VP gene transcription in the SCN in vitro is completely blocked by a 2 h exposure to tetrodotoxin (TTX) in the culture medium, and this TTX inhibition of VP gene transcription is reversed by exposure of the SCN to either forskolin or potassium depolarization. This suggests that an intrinsic, spontaneously active neuronal mechanism in the SCN is responsible for the cAMP- and depolarization-dependent pathways involved in maintaining peak VP gene transcription. In this paper, we evaluate a variety of neurotransmitter candidates, membrane receptors, and signal-transduction cascades that might constitute the mechanisms responsible for the peak of VP gene transcription. We find that vasoactive intestinal peptide (VIP) and a VPAC2 (VIP receptor subtype 2) receptor-specific agonist, Ro-25-1553, are the most effective ligands tested in evoking a cAMP-mitogen-activated protein kinase signal transduction cascade leading to an increase in VP gene transcription in the SCN. In addition, a second independent pathway involving depolarization activating L-type voltage-gated calcium channels and a Ca-dependent kinase pathway [inhibited by KN62 (1-[N,O-bis(5-isoquinolinesulphonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazine)] rescues VP gene transcription in the presence of TTX. In the absence of TTX, these independent pathways appear to act in a cooperative manner to generate the circadian peak of VP gene transcription in the SCN.

Electrophysiological evidence that the vasoactive intestinal peptide receptor antagonist VIP6-28 reduces nociception in an animal model of osteoarthritis

OBJECTIVE

The present study examined whether local administration of the neuropeptide vasoactive intestinal polypeptide (VIP) could modulate joint nociception in normal rat knee joints and if the VIP antagonist VIP(6-28) could ameliorate joint mechanosensitivity in an animal model of osteoarthritis (OA).

METHODS

OA was induced in male Wistar rats by intra-articular injection of 3mg sodium monoiodo-acetate with a recovery period of 14 days. Electrophysiological recordings were made from knee joint primary afferents in response to normal rotation and noxious hyper-rotation of the joint both before and following close intra-arterial injection of different doses of VIP and VIP(6-28).

RESULTS

Local application of VIP to normal knees caused afferent firing rate to be significantly enhanced during normal rotation (up to 180% P<0.01; n=17) and during hyper-rotation (up to 37% P<0.01; n=17) of the knee. VIP-induced sensitization was blocked by pre-administration of the VIP receptor antagonist VIP(6-28). In the OA group, application of VIP(6-28) caused afferent firing rate to be significantly reduced during normal rotation (up to 45% P<0.05; n=17) and during hyper-rotation (up to 34% P<0.01; n=15) of the knee joint.

CONCLUSION

These findings indicate that VIP is involved in peripheral sensitization of knee joint afferents especially in response to normal joint movements. OA-induced sensitization of knee joint afferents was inhibited by local administration of VIP(6-28), indicating that VIP is released into OA knee joints, potentially contributing to joint pain. As such, VIP(6-28) may prove to be a beneficial agent for the treatment of arthritis pain.

Activation of transcription factor c-jun in dorsal root ganglia induces VIP and NPY upregulation and contributes to the pathogenesis of neuropathic pain

Vasoactive intestinal peptide (VIP) and neuropeptide Y (NPY) in dorsal root ganglia (DRGs) are known to be upregulated and to contribute to the mechanisms of neuropathic pain following peripheral nerve injury. Moreover, transcription factor c-Jun regulates the expressions of both VIP and NPY in cultured DRG neurons. To elucidate the role of c-Jun in the induction of neuropathic pain hypersensitivity, we examined whether activated c-Jun affects pain behavior and the expressions of VIP and NPY following chronic constriction injury (CCI) of rat sciatic nerve. Intrathecal treatment with c-jun antisense oligodeoxynucleotides (AS-ODN) significantly reduced mechanical allodynia, but not thermal hyperalgesia following CCI. In addition, c-jun AS-ODN also suppressed the remarkable elevations of VIP and NPY mRNAs and the percentages of phosphorylated c-Jun-, VIP-, and NPY-immunoreactive neurons observed in DRGs following CCI. These results show that the activation of c-Jun in DRGs induces VIP and NPY upregulation and contributes to the pathogenesis of neuropathic pain following CCI.

Noncompensation in peptide/receptor gene expression and distinct behavioral phenotypes in VIP- and PACAP-deficient mice

Pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP) are closely related neurotrophic peptides of the secretin/glucagon family. The two peptides are derived from a common ancestral gene and share many functional attributes in neuronal development/regeneration which occur not only from overlapping receptor subtype signaling but also through common mechanisms regulating their expression. Although PACAP or VIP null mice have been generated for study, it is unclear whether the expression of the complementary peptide or their receptor systems are altered in a compensatory manner during nervous system development. By radioimmunoassay and quantitative PCR measurements, we first show that PACAP and VIP have very different temporal patterns of expression in developing postnatal mouse brain. In wild-type animals, PACAP transcript and peptide levels increased rapidly 2- and 5-fold, respectively, within 1 week of age. These levels at 1 week of age were maintained through adulthood. VIP transcript and peptide levels, by contrast, increased 25- and 50-fold, respectively, over a later time course. In parallel studies of development, there were no apparent compensatory increases in brain VIP expression in the PACAP knockout animals, PACAP expression in the VIP-deficient animals, or receptor mRNA levels in either genotype. To the contrary, there was evidence for developmental delays in the expression of peptide and receptor transcripts in the knockout animals. A series of behavioral and neurological tests demonstrated differences between the knockout genotypes, revealing some functional distinctions between the two genes. These results suggest that the PACAP and VIP have evolved to possess distinct biological activities and intimate that the respective knockout phenotypes represent deficits unmitigated by the actions of the complementary related peptide.

A systematic comparison of intracellular cyclic AMP and calcium signalling highlights complexities in human VPAC/PAC receptor pharmacology

VPAC/PAC receptor activation classically results in cyclic-AMP production, with limited reports evaluating calcium signalling. These studies systematically characterise intracellular cyclic-AMP ([cAMP](i)) and calcium ([Ca(2+)](i)) responses in CHO-cells expressing recombinant human (h) VPAC/PAC receptors (hVPAC(1)R, hVPAC(2)R, hPAC(1)R), using two simple, non-radioactive, HT-amenable assays. The rank order of potency (ROP) of the agonists VIP, PACAP-27 and PACAP-38 was similar in both assays for each individual receptor subtype, although potencies (EC(50)) in the [Ca(2+)](i) assay were approximately 100-fold lower. Importantly, this shift was also evident in SHSY-5Y cells endogenously expressing hPAC(1)R. Furthermore, [Ala(11,22,28)]VIP and maxadilan were selective hVPAC(1)R and hPAC(1)R agonists, respectively, and although R3P65 had no demonstrable hVPAC(2)R selectivity, these compounds exhibited comparable reductions in [Ca(2+)](i) EC(50) values. In contrast, PG97-269 and PG99-465, putatively selective hVPAC(1)R and hVPAC(2)R antagonists, respectively, were marginally less potent in [cAMP](i) studies, whereas M65 was equipotent at hPAC(1)R. Moreover, PG99-465 alone increased [cAMP](i) at all three hVPAC/PAC receptor subtypes, with full hVPAC(1)R and hPAC(1)R agonism. With equivalent agonist ROPs generated in both assays, [Ca(2+)](i) signalling provides an alternative approach to examine hVPAC/PAC receptor pharmacology. However, these studies underscore the paucity of receptor selective compounds, complexities in comparing drug potencies across assays, and the pleiotropic nature of VPAC/PAC-receptor signalling.

Neuronal and smooth muscle receptors involved in the PACAP- and VIP-induced relaxations of the pig urinary bladder neck

BACKGROUND AND PURPOSE

As pituitary adenylate cyclase-activating polypeptide 38 (PACAP 38)- and vasoactive intestinal peptide (VIP) are widely distributed in the urinary tract, the current study investigated the receptors and mechanisms involved in relaxations induced by these peptides in the pig bladder neck.

EXPERIMENTAL APPROACH

Urothelium-denuded strips were suspended in organ baths for isometric force recordings and the relaxations to VIP and PACAP analogues were investigated.

KEY RESULTS

VIP, PACAP 38, PACAP 27 and [Ala(11,22,28)]-VIP produced similar relaxations. Inhibition of neuronal voltage-gated Ca(2+) channels reduced relaxations to PACAP 38 and increased those induced by VIP. Blockade of capsaicin-sensitive primary afferents (CSPA), nitric oxide (NO)-synthase or guanylate cyclase reduced the PACAP 38 relaxations but failed to modify the VIP responses. Inhibition of VIP/PACAP receptors and of voltage-gated K(+) channels reduced PACAP 38 and VIP relaxations, which were not modified by the K(+) channel blockers iberiotoxin, charybdotoxin, apamin or glibenclamide. The phosphodiesterase 4 inhibitor rolipram and the adenylate cyclase activator forskolin produced potent relaxations. Blockade of protein kinase A (PKA) reduced PACAP 38- and VIP-induced relaxations.

CONCLUSIONS AND IMPLICATIONS

PACAP 38 and VIP relax the pig urinary bladder neck through muscle VPAC(2) receptors linked to the cAMP-PKA pathway and involve activation of voltage-gated K(+) channels. Facilitatory PAC(1) receptors located at CSPA and coupled to NO release, and inhibitory VPAC receptors at motor endings are also involved in the relaxations to PACAP 38 and VIP, respectively. VIP/PACAP receptor antagonists could be useful in the therapy of urinary incontinence produced by intrinsic sphincter deficiency.