Postinjury administration of pituitary adenylate cyclase activating polypeptide (PACAP) attenuates traumatically induced axonal injury in rats

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.

The Roles of Neurotrophins in Traumatic Brain Injury

Neurotrophins are a collection of structurally and functionally related proteins. They play important roles in many aspects of neural development, survival, and plasticity. Traumatic brain injury (TBI) leads to different levels of central nervous tissue destruction and cellular repair through various compensatory mechanisms promoted by the injured brain. Many studies have shown that neurotrophins are key modulators of neuroinflammation, apoptosis, blood–brain barrier permeability, memory capacity, and neurite regeneration. The expression of neurotrophins following TBI is affected by the severity of injury, genetic polymorphism, and different post-traumatic time points. Emerging research is focused on the potential therapeutic applications of neurotrophins in managing TBI. We conducted a comprehensive review by organizing the studies that demonstrate the role of neurotrophins in the management of TBI.

The Neuroprotective and Biomarker Potential of PACAP in Human Traumatic Brain Injury

Traumatic brain injury remains a growing public health concern and represents the greatest contributor to death and disability globally among all trauma-related injuries. There are limited clinical data regarding biomarkers in the diagnosis and outcome prediction of TBI. The lack of real effective treatment for recovery calls for research of TBI to be shifted into the area of prevention, treatment of secondary brain injury and neurorehabilitation. The neuropeptide pituitary adenylate cyclase activating polypeptide (PACAP) has been reported to act as a hormone, a neuromodulator, a neurotransmitter and a trophic factor, and has been implicated in a variety of developmental and regenerative processes. The importance of PACAP in neuronal regeneration lies in the upregulation of endogenous PACAP and its receptors and the protective effect of exogenous PACAP after different central nervous system injury. The aim of this minireview is to summarize both the therapeutic and biomarker potential of the neuropeptide PACAP, as a novel possible target molecule presently being investigated in several human conditions including TBI, and with encouraging results in animal models of TBI.

A Systematic Review of Closed Head Injury Models of Mild Traumatic Brain Injury in Mice and Rats

Mild TBI (mTBI) is a significant health concern. Animal models of mTBI are essential for understanding mechanisms, and pathological outcomes, as well as to test therapeutic interventions. A variety of closed head models of mTBI that incorporate different aspects (i.e., biomechanics) of the mTBI have been reported. The aim of the current review was to compile a comprehensive list of the closed head mTBI rodent models, along with the common data elements, and outcomes, with the goal to summarize the current state of the field. Publications were identified from a search of PubMed and Web of Science and screened for eligibility following PRISMA guidelines. Articles were included that were closed head injuries in which the authors classified the injury as mild in rats or mice. Injury model and animal-specific common data elements, as well as behavioral and histological outcomes, were collected and compiled from a total of 402 articles. Our results outline the wide variety of methods used to model mTBI. We also discovered that female rodents and both young and aged animals are under-represented in experimental mTBI studies. Our findings will aid in providing context comparing the injury models and provide a starting point for the selection of the most appropriate model of mTBI to address a specific hypothesis. We believe this review will be a useful starting place for determining what has been done and what knowledge is missing in the field to reduce the burden of mTBI.

Effect of controlled cortical impact on the passage of pituitary adenylate cyclase activating polypeptide (PACAP) across the blood-brain barrier

Injuries to the central nervous system can affect the blood-brain barrier (BBB), including disruption and influencing peptide transport across the BBB. Pituitary adenylate cyclase-activating polypeptide 38 (PACAP38) is a potent neurotrophic and neuroprotective peptide currently being investigated for its therapeutic role following injury to the central nervous system and can cross the BBB in a saturable manner. The goal of the current study was to investigate for the first time PACAP38 uptake by the brain following traumatic brain injury (TBI). Using radioactively labeled PACAP38, we measured the levels of PACAP38 present in the injured, ipsilateral cortex in Sham-treated mice compared to mice receiving a controlled cortical impact (CCI), a model of TBI. Experiments were conducted at 6 different time points (from 2h up to 4 weeks) following CCI to determine temporal changes in PACAP38 transport. PACAP38 uptake was increased at 2 and 72h post-CCI compared to Sham. We did not detect changes in PACAP38 uptake in the contralateral cortex and cerebellum between Sham and CCI-treatment. The rate of PACAP38 transport into the ipsilateral cortex following CCI was increased 3.6-fold 72h after compared to 2h post-CCI. In addition, the rate of transport into the cerebellum was greater than that of the cortices. The data presented here shows PACAP38 transport is temporally altered following CCI-treatment and PACAP38 uptake is greater in the cerebellum compared to the cortices.

Peptide Pharmacological Approaches to Treating Traumatic Brain Injury: a Case for Arginine-Rich Peptides

Traumatic brain injury (TBI) has a devastating effect on victims and their families, and has profound negative societal and economic impacts, a situation that is further compounded by the lack of effective treatments to minimise injury after TBI. The current strategy for managing TBI is partly through preventative measures and partly through surgical and rehabilitative interventions. Secondary brain damage remains the principal focus for the development of a neuroprotective therapeutic. However, the complexity of TBI pathophysiology has meant that single-action pharmacological agents have been largely unsuccessful in combatting the associated brain injury cascades, while combination therapies to date have proved equally ineffective. Peptides have recently emerged as promising lead agents for the treatment of TBI, especially those rich in the cationic amino acid, arginine. Having been shown to lessen the impact of ischaemic stroke in animal models, there are reasonable grounds to believe that arginine-rich peptides may have neuroprotective therapeutic potential in TBI. Here, we review a range of peptides previously examined as therapeutic agents for TBI. In particular, we focus on cationic arginine-rich peptides -- a new class of agents that growing evidence suggests acts through multiple neuroprotective mechanisms.

Acute Response of the Hippocampal Transcriptome Following Mild Traumatic Brain Injury After Controlled Cortical Impact in the Rat

We have previously demonstrated that mild controlled cortical impact (mCCI) injury to rat cortex causes indirect, concussive injury to underlying hippocampus and other brain regions, providing a reproducible model for mild traumatic brain injury (mTBI) and its neurochemical, synaptic, and behavioral sequelae. Here, we extend a preliminary gene expression study of the hippocampus-specific events occurring after mCCI and identify 193 transcripts significantly upregulated, and 21 transcripts significantly downregulated, 24 h after mCCI. Fifty-three percent of genes altered by mCCI within 24 h of injury are predicted to be expressed only in the non-neuronal/glial cellular compartment, with only 13% predicted to be expressed only in neurons. The set of upregulated genes following mCCI was interrogated using Ingenuity Pathway Analysis (IPA) augmented with manual curation of the literature (190 transcripts accepted for analysis), revealing a core group of 15 first messengers, mostly inflammatory cytokines, predicted to account for >99% of the transcript upregulation occurring 24 h after mCCI. Convergent analysis of predicted transcription factors (TFs) regulating the mCCI target genes, carried out in IPA relative to the entire Affymetrix-curated transcriptome, revealed a high concordance with TFs regulated by the cohort of 15 cytokines/cytokine-like messengers independently accounting for upregulation of the mCCI transcript cohort. TFs predicted to regulate transcription of the 193-gene mCCI cohort also displayed a high degree of overlap with TFs predicted to regulate glia-, rather than neuron-specific genes in cortical tissue. We conclude that mCCI predominantly affects transcription of non-neuronal genes within the first 24 h after insult. This finding suggests that early non-neuronal events trigger later permanent neuronal changes after mTBI, and that early intervention after mTBI could potentially affect the neurochemical cascade leading to later reported synaptic and behavioral dysfunction.

Changes of PACAP level in cerebrospinal fluid and plasma of patients with severe traumatic brain injury

PACAP has well-known neuroprotective potential including traumatic brain injury (TBI). Its level is up-regulated following various insults of the CNS in animal models. A few studies have documented alterations of PACAP levels in human serum. The time course of post-ictal PACAP levels, for example, show correlation with migraine severity. Very little is known about the course of PACAP levels following CNS injury in humans and the presence of PACAP has not yet been detected in cerebrospinal fluid (CSF) of subjects with severe TBI (sTBI). The aim of the present study was to determine whether PACAP occurs in the CSF and plasma (Pl) of patients that suffered sTBI and to establish a time course of PACAP levels in the CSF and Pl. Thirty eight subjects with sTBI were enrolled with a Glasgow Coma Scale ≤8 on admission. Samples were taken daily, until the time of death or for maximum 10 days. Our results demonstrated that PACAP was detectable in the CSF, with higher concentrations in patients with TBI. PACAP concentrations markedly increased in both Pl and CSF in the majority of patients 24-48h after the injury stayed high thereafter. In cases of surviving patients, Pl and CSF levels displayed parallel patterns, which may imply the damage of the blood-brain barrier. However, in patients, who died within the first week, Pl levels were markedly higher than CSF levels, possibly indicating the prognostic value of high Pl PACAP levels.

VIP and PACAP: neuropeptide modulators of CNS inflammation, injury, and repair

Inflammatory processes play both regenerative and destructive roles in multiple sclerosis, stroke, CNS trauma, amyotrophic lateral sclerosis and aging-related neurodegenerative diseases such as Alzheimer's, Parkinson's and Huntington's. Endogenous defence mechanisms against these pathologies include those that are directly neuroprotective, and those that modulate the expression of inflammatory mediators in microglia, astrocytes, and invading inflammatory cells. While a number of mechanisms and molecules have been identified that can directly promote neuronal survival, less is known about how the brain protects itself from harmful inflammation, and further, how it co-opts the healing function of the immune system to promote CNS repair. The two closely related neuroprotective peptides, vasoactive intestinal peptide (VIP) and pituitary adenylyl cyclase-activating peptide (PACAP), which are up-regulated in neurons and immune cells after injury and/or inflammation, are known to protect neurons, but also exert powerful in vivo immunomodulatory actions, which are primarily anti-inflammatory. These peptide actions are mediated by high-affinity receptors expressed not only on neurons, but also astrocytes, microglia and peripheral inflammatory cells. Well-established immunomodulatory actions of these peptides are to inhibit macrophage and microglia production and release of inflammatory mediators such as TNF-α and IFN-γ, and polarization of T-cell responses away from Th1 and Th17, and towards a Th2 phenotype. More recent studies have revealed that these peptides can also promote the production of both natural and inducible subsets of regulatory T-cells. The neuroprotective and immunomodulatory actions of VIP and PACAP suggest that receptors for these peptides may be therapeutic targets for neurodegenerative and neuroinflammatory diseases and other forms of CNS injury.

Subacute intranasal administration of tissue plasminogen activator promotes neuroplasticity and improves functional recovery following traumatic brain injury in rats

Traumatic brain injury (TBI) is a major cause of death and long-term disability worldwide. To date, there are no effective pharmacological treatments for TBI. Recombinant human tissue plasminogen activator (tPA) is the effective drug for the treatment of acute ischemic stroke. In addition to its thrombolytic effect, tPA is also involved in neuroplasticity in the central nervous system. However, tPA has potential adverse side effects when administered intravenously including brain edema and hemorrhage. Here we report that tPA, administered by intranasal delivery during the subacute phase after TBI, provides therapeutic benefit. Animals with TBI were treated intranasally with saline or tPA initiated 7 days after TBI. Compared with saline treatment, subacute intranasal tPA treatment significantly 1) improved cognitive (Morris water maze test) and sensorimotor (footfault and modified neurological severity score) functional recovery in rats after TBI, 2) reduced the cortical stimulation threshold evoking ipsilateral forelimb movement, 3) enhanced neurogenesis in the dentate gyrus and axonal sprouting of the corticospinal tract originating from the contralesional cortex into the denervated side of the cervical gray matter, and 4) increased the level of mature brain-derived neurotrophic factor. Our data suggest that subacute intranasal tPA treatment improves functional recovery and promotes brain neurogenesis and spinal cord axonal sprouting after TBI, which may be mediated, at least in part, by tPA/plasmin-dependent maturation of brain-derived neurotrophic factor.

PACAP38 suppresses cortical damage in mice with traumatic brain injury by enhancing antioxidant activity

The production of reactive oxygen species (ROS) and the resulting oxidative stress in mice in response to a controlled cortical impact (CCI) are typical exacerbating factors associated with traumatic brain injury (TBI). Pituitary adenylate cyclase-activating polypeptide 38 (PACAP38) is a multifunctional peptide that has been shown to exhibit neuroprotective effects in response to a diverse range of injuries to neuronal cells. We recently reported that PACAP38 might regulate oxidative stress in mice. The aim of the present study was to determine whether PACAP38 exerts neuroprotective effects by regulating oxidative stress in mice with TBI. Reactive oxidative metabolites (ROMs) and biological antioxidant potential (BAP) were measured in male C57Bl/6 mice before and 3, 4, and 24 h after CCI. PACAP38 was administered intravenously immediately following CCI, and immunostaining for the oxidative stress indicator nitrotyrosine (NT), and for neuronal death as an indicator of the area affected by TBI, was measured 24 h later. Western blot experiments to determine antioxidant activity [as indicated by superoxide dismutase-2 (SOD-2) and glutathione peroxidase 1 (GPx-1)] in the neocortical region were also performed 3 h post-CCI. Results showed that plasma BAP and ROM levels were dramatically increased 3 h after CCI. PACAP38 suppressed the extent of TBI and NT-positive regions 24 h after CCI, and increased SOD-2 and GPx-1 levels in both hemispheres. Taken together, these results suggest that increasing antioxidant might be involving in the neuroprotective effect of PACAP38 in mice subjected to a CCI.

Exogenous administration of PACAP alleviates traumatic brain injury in rats through a mechanism involving the TLR4/MyD88/NF-κB pathway

Pituitary adenylate cyclase-activating polypeptide (PACAP) is effective in reducing axonal damage associated with traumatic brain injury (TBI), and has immunomodulatory properties. Toll-like receptor 4 (TLR4) is an important mediator of the innate immune response. It significantly contributes to neuroinflammation induced by brain injury. However, it remains unknown whether exogenous PACAP can modulate TBI through the TLR4/adapter protein myeloid differentiation factor 88 (MyD88)/nuclear factor-κB (NF-κB) signaling pathway. In this study, we investigated the potential neuroprotective mechanisms of PACAP pretreatment in a weight-drop model of TBI. PACAP38 was microinjected intracerebroventricularly before TBI. Brain samples were extracted from the pericontusional area in the cortex and hippocampus. We found that TBI induced significant upregulation of TLR4, with peak expression occurring 24 h post-trauma, and that pretreatment with PACAP significantly improved motor and cognitive dysfunction, attenuated neuronal apoptosis, and decreased brain edema. Pretreatment with PACAP inhibited upregulation of TLR4 and its downstream signaling molecules MyD88, p-IκB, and NF-κB, and suppressed increases in the levels of the downstream inflammatory agents interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α), in the brain tissue around the injured cortex and in the hippocampus. Administration of PACAP both in vitro and in vivo attenuated the ability of the TLR4 agonist lipopolysaccharide (LPS) to increase TLR4 protein levels. Therefore, PACAP exerts a neuroprotective effect in this rat model of TBI, by inhibiting a secondary inflammatory response mediated by the TLR4/MyD88/NF-κB signaling pathway in microglia and neurons, thereby reducing neuronal death and improving the outcome following TBI.

Therapy development for diffuse axonal injury

Diffuse axonal injury (DAI) remains a prominent feature of human traumatic brain injury (TBI) and a major player in its subsequent morbidity. The importance of this widespread axonal damage has been confirmed by multiple approaches including routine postmortem neuropathology as well as advanced imaging, which is now capable of detecting the signatures of traumatically induced axonal injury across a spectrum of traumatically brain-injured persons. Despite the increased interest in DAI and its overall implications for brain-injured patients, many questions remain about this component of TBI and its potential therapeutic targeting. To address these deficiencies and to identify future directions needed to fill critical gaps in our understanding of this component of TBI, the National Institute of Neurological Disorders and Stroke hosted a workshop in May 2011. This workshop sought to determine what is known regarding the pathogenesis of DAI in animal models of injury as well as in the human clinical setting. The workshop also addressed new tools to aid in the identification of this axonal injury while also identifying more rational therapeutic targets linked to DAI for continued preclinical investigation and, ultimately, clinical translation. This report encapsulates the oral and written components of this workshop addressing key features regarding the pathobiology of DAI, the biomechanics implicated in its initiating pathology, and those experimental animal modeling considerations that bear relevance to the biomechanical features of human TBI. Parallel considerations of alternate forms of DAI detection including, but not limited to, advanced neuroimaging, electrophysiological, biomarker, and neurobehavioral evaluations are included, together with recommendations for how these technologies can be better used and integrated for a more comprehensive appreciation of the pathobiology of DAI and its overall structural and functional implications. Lastly, the document closes with a thorough review of the targets linked to the pathogenesis of DAI, while also presenting a detailed report of those target-based therapies that have been used, to date, with a consideration of their overall implications for future preclinical discovery and subsequent translation to the clinic. Although all participants realize that various research gaps remained in our understanding and treatment of this complex component of TBI, this workshop refines these issues providing, for the first time, a comprehensive appreciation of what has been done and what critical needs remain unfulfilled.

Inflammatory response following diffuse axonal injury

DAI is a leading cause of the patient's death or lasting vegetable state following severe TBI, and up to now the detailed mechanism of axonal injury after head trauma is still unclear. Inflammatory responses have been proved to be an important mechanism of neural injury after TBI. However, most of these studies are concerned with focal cerebral injury following head trauma. In contrast to focal injury, studies on the inflammatory reaction following DAI are only beginning. And in this article, we aimed to review such studies. From the studies reviewed, immune response cells would become reactive around the sites of axonal injury after DAI. Besides, the concentrations of several important inflammatory factors, such as IL-1 family, IL-6 and TNF-ɑ, increased after DAI as well, which implies the participation of inflammatory responses. It can be concluded that inflammatory responses probably participate in the neural injury in DAI, but at present the study of inflammatory responses following DAI is still limited and the clear effects of inflammatory response on axonal injury remain to be more explored.

Effect of PACAP in central and peripheral nerve injuries

Pituitary adenylate cyclase activating polypeptide (PACAP) is a bioactive peptide with diverse effects in the nervous system. In addition to its more classic role as a neuromodulator, PACAP functions as a neurotrophic factor. Several neurotrophic factors have been shown to play an important role in the endogenous response following both cerebral ischemia and traumatic brain injury and to be effective when given exogenously. A number of studies have shown the neuroprotective effect of PACAP in different models of ischemia, neurodegenerative diseases and retinal degeneration. The aim of this review is to summarize the findings on the neuroprotective potential of PACAP in models of different traumatic nerve injuries. Expression of endogenous PACAP and its specific PAC1 receptor is elevated in different parts of the central and peripheral nervous system after traumatic injuries. Some experiments demonstrate the protective effect of exogenous PACAP treatment in different traumatic brain injury models, in facial nerve and optic nerve trauma. The upregulation of endogenous PACAP and its receptors and the protective effect of exogenous PACAP after different central and peripheral nerve injuries show the important function of PACAP in neuronal regeneration indicating that PACAP may also be a promising therapeutic agent in injuries of the nervous system.

Neuroprotective effect of endogenous pituitary adenylate cyclase-activating polypeptide on spinal cord injury

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuroprotective peptide expressed in the central nervous system. To date, changes in the expression and effect of endogenous PACAP have not been clarified with respect to spinal cord injury (SCI). The aim of this study was to elucidate the expression pattern and function of endogenous PACAP on the contusion model of SCI using heterozygous PACAP knockout (PACAP(+/-)) and wild-type mice. Real-time polymerase chain reaction methods revealed that the level of PACAP mRNA increased gradually for 14 days after SCI and that PAC1R mRNA levels also increased for 7 days compared with intact control mice. PACAP and PAC1R immunoreactivities colabeled with a neuronal marker in the intact spinal cord. Seven days after SCI, PAC1R immunoreactivity was additionally co-expressed with an astrocyte marker. Wild-type mice gradually recovered motor function after 14 days, but PACAP(+/-) mice showed significantly impaired recovery from 3 days compared with wild-type mice. The injury volume at day 7 in PACAP(+/-) mice, and the number of single-stranded DNA-immunopositive cells as a marker of neuronal cell death at day 3 were significantly higher than values measured in wild-type mice. These data suggest that endogenous PACAP is upregulated by SCI and has a neuroprotective effect on the damaged spinal cord.

Pituitary adenylate cyclase-activating peptide induces long-lasting neuroprotection through the induction of activity-dependent signaling via the cyclic AMP response element-binding protein-regulated transcription co-activator 1

Pituitary adenylate cyclase-activating peptide (PACAP) is a neuroprotective peptide which exerts its effects mainly through the cAMP-protein kinase A (PKA) pathway. Here, we show that in cortical neurons, PACAP-induced PKA signaling exerts a major part of its neuroprotective effects indirectly, by triggering action potential (AP) firing. Treatment of cortical neurons with PACAP induces a rapid and sustained PKA-dependent increase in AP firing and associated intracellular Ca²⁺ transients, which are essential for the anti-apoptotic actions of PACAP. Transient exposure to PACAP induces long-lasting neuroprotection in the face of apoptotic insults which is reliant on AP firing and the activation of cAMP response element (CRE) binding protein (CREB)-mediated gene expression. Although direct, activity-independent PKA signaling is sufficient to trigger phosphorylation on CREB’s activating serine-133 site, this is insufficient for activation of CREB-mediated gene expression. Full activation is dependent on CREB-regulated transcription co-activator 1 (CRTC1), whose PACAP-induced nuclear import is dependent on firing activity-dependent calcineurin signaling. Over-expression of CRTC1 is sufficient to rescue PACAP-induced CRE-mediated gene expression in the face of activity-blockade, while dominant negative CRTC1 interferes with PACAP-induced, CREB-mediated neuroprotection. Thus, the enhancement of AP firing may play a significant role in the neuroprotective actions of PACAP and other adenylate cyclase-coupled ligands.

Traumatic brain injury and recovery mechanisms: peptide modulation of periventricular neurogenic regions by the choroid plexus-CSF nexus

In traumatic brain injury (TBI), severe disruptions occur in the choroid plexus (CP)-cerebrospinal fluid (CSF) nexus that destabilize the nearby hippocampal and subventricular neurogenic regions. Following invasive and non-invasive injuries to cortex, several adverse sequelae harm the brain interior: (i) structural damage to CP epithelium that opens the blood-CSF barrier (BCSFB) to protein, (ii) altered CSF dynamics and intracranial pressure (ICP), (iii) augmentation of leukocyte traffic across CP into the CSF-brain, (iv) reduction in CSF sink action and clearance of debris from ventricles, and (v) less efficient provision of micronutritional and hormonal support for the CNS. However, gradual post-TBI restitution of the injured CP epithelium and ependyma, and CSF homeostatic mechanisms, help to restore subventricular/subgranular neurogenesis and the cognitive abilities diminished by CNS damage. Recovery from TBI is facilitated by upregulated choroidal/ependymal growth factors and neurotrophins, and their secretion into ventricular CSF. There, by an endocrine-like mechanism, CSF bulk flow convects the neuropeptides to target cells in injured cortex for aiding repair processes; and to neurogenic niches for enhancing conversion of stem cells to new neurons. In the recovery from TBI and associated ischemia, the modulating neuropeptides include FGF2, EGF, VEGF, NGF, IGF, GDNF, BDNF, and PACAP. Homeostatic correction of TBI-induced neuropathology can be accelerated or amplified by exogenously boosting the CSF concentration of these growth factors and neurotrophins. Such intraventricular supplementation via the CSF route promotes neural restoration through enhanced neurogenesis, angiogenesis, and neuroprotective effects. CSF translational research presents opportunities that involve CP and ependymal manipulations to expedite recovery from TBI.

Pituitary adenylate cyclase-activating polypeptide (PACAP) stimulates proliferation of reactive astrocytes in vitro

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide originally isolated from ovine hypothalamus. Recently, we have shown that the PACAP receptor (PAC1-R) is expressed in reactive astrocytes following an in vivo stub wound brain injury. However, the functional role of PACAP has not yet been clarified. In order to investigate the effect of PACAP on the proliferation of reactive astrocytes, a scratch wound paradigm was applied to astrocytic monolayers. Following injury, there was an increase in PAC1-R and glial fibrillary acidic protein (GFAP) immunoreactivity in the astrocytes surrounding the scratch line. PACAP at concentrations of 10(-15) to 10(-7) M was applied immediately after scratching, and the proliferating astrocytes were visualized by multiple immunofluorescence labeling. The percentage of cells that colabeled for Ki67 (a marker of proliferating cells) and GFAP increased in the 10(-11)- and 10(-13)-M PACAP-treated groups. The proliferating astrocytes induced by PACAP treatment mainly occurred in the proximal wound area where many reactive astrocytes were observed. Pretreatment with the PACAP receptor antagonist PACAP6-38 significantly suppressed the PACAP-induced effects. These results strongly suggest that PACAP plays an important role in the proliferation of reactive astrocytes following nerve injury.

Expression of PACAP-like compounds during the caudal regeneration of the earthworm Eisenia fetida

The regeneration of the ventral nerve cord ganglion and peripheral tissues was investigated by radioimmunoassay and immunohistochemistry in the model animal, Eisenia fetida (Annelida, Oligochaeta). It is now well-established that pituitary adenylate cyclase-activating polypeptide (PACAP) is a neurotrophic factor, playing important roles in the development of the nervous system in vertebrate animals. Based on the apparent evolutionary conservation of PACAP and on the several common mechanisms of vertebrate and invertebrate nervous regeneration, the question was raised whether PACAP has any role in the regeneration of the earthworm nervous system. As a first step, we studied the distribution, concentration, and time-course of PACAP-like immunoreactivity during caudal regeneration of both lost segments and the ventral nerve cord ganglia in E. fetida. A strong upregulation of PACAP-like immunoreactivity was observed in most tissues following injury as determined by radioimmunoassay and immunohistochemistry. Significant increases in the concentration of PACAP-like compounds were found in the body wall, alimentary canal, and in coelomocytes. The most characteristic morphological feature was the accumulation of immunolabeled neoblasts in the injured tissues, especially in the ventral nerve cord ganglion that initiates and mediates regeneration processes. Our present results show that PACAP/PACAP-like peptides accumulate in the regenerating tissues of the earthworm, suggesting trophic functions of these compounds in earthworm tissues similarly to vertebrate species.

Neuroprotective effects of PACAP27 in mice model of Parkinson's disease involved in the modulation of K(ATP) subunits and D2 receptors in the striatum

Pituitary adenylate cyclase activating polypeptide (PACAP) exhibits a protective effect against neural injury in vitro and in vivo. However, it has not been reported whether peripheral intravenous administration of PACAP could confer benefits in animal models of Parkinson's disease (PD). Furthermore, the underlying molecular mechanisms responsible for these effects are poorly understood. In the present experiments, we determined the effects and mechanism of action of intravenously administered PACAP27 in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mice. Our results indicate that intravenous injection of PACAP27 offers neuroprotective effects in the MPTP-induced PD mouse model which may not be directly associated with the expression levels of the monoamine transporters. However, this effect may be correlated with its ability to selectively regulate not only K(ATP) subunits, but D2 receptors in the striatum. Our findings suggest that the benefit of PACAP may accompany with changes not only in dopaminergic neurotransmission, but also in cholinergic neurotransmission that are relatively associated with the K(ATP) subunits and D2 receptors in the striatum.

Inhibitory effect of PACAP on caspase activity in neuronal apoptosis: a better understanding towards therapeutic applications in neurodegenerative diseases

Programmed cell death, which is part of the normal development of the central nervous system, is also implicated in various neurodegenerative disorders. Cysteine-dependent aspartate-specific proteases (caspases) play a pivotal role in the cascade of events leading to apoptosis. Many factors that inhibit cell death have now been identified, but the underlying mechanisms are not fully understood. Pituitary adenylate cyclase-activating polypeptide (PACAP) has been shown to exert neurotrophic activities during development and to prevent neuronal apoptosis induced by various insults such as ischemia. Most of the neuroprotective effects of PACAP are mediated through the PAC1 receptor. This receptor activates a transduction cascade of second messengers to stimulate Bcl-2 expression, which inhibits cytochrome c release and blocks the activation of caspases. The inhibitory effect of PACAP on the apoptotic cascade suggests that selective, stable, and potent PACAP derivatives could potentially be of therapeutic value for the treatment of post-traumatic and/or chronic neurodegenerative processes.

PACAP-38 induces neuronal differentiation of human SH-SY5Y neuroblastoma cells via cAMP-mediated activation of ERK and p38 MAP kinases

The intracellular signaling pathways mediating the neurotrophic actions of pituitary adenylate cyclase-activating polypeptide (PACAP) were investigated in human neuroblastoma SH-SY5Y cells. Previously, we showed that SH-SY5Y cells express the PAC₁ and VIP/PACAP receptor type 2 (VPAC₂) receptors, and that the robust cAMP production in response to PACAP and vasoactive intestinal peptide (VIP) was mediated by PAC₁ receptors (Lutz et al. 2006). Here, we investigated the ability of PACAP-38 to differentiate SH-SY5Y cells by measuring morphological changes and the expression of neuronal markers. PACAP-38 caused a concentration-dependent increase in the number of neurite-bearing cells and an up-regulation in the expression of the neuronal proteins Bcl-2, growth-associated protein-43 (GAP-43) and choline acetyltransferase: VIP was less effective than PACAP-38 and the VPAC₂ receptor-specific agonist, Ro 25-1553, had no effect. The effects of PACAP-38 and VIP were blocked by the PAC₁ receptor antagonist, PACAP6-38. As observed with PACAP-38, the adenylyl cyclase activator, forskolin, also induced an increase in the number of neurite-bearing cells and an up-regulation in the expression of Bcl-2 and GAP-43. PACAP-induced differentiation was prevented by the adenylyl cyclase inhibitor, 2′,5′-dideoxyadenosine (DDA), but not the protein kinase A (PKA) inhibitor, H89, or by siRNA-mediated knock-down of the PKA catalytic subunit. PACAP-38 and forskolin stimulated the activation of extracellular signal-regulated kinase (ERK), mitogen-activated protein kinase (MAP; p38 MAP kinase) and c-Jun N-terminal kinase (JNK). PACAP-induced neuritogenesis was blocked by the MEK1 inhibitor PD98059 and partially by the p38 MAP kinase inhibitor SB203580. Activation of exchange protein directly activated by cAMP (Epac) partially mimicked the effects of PACAP-38, and led to the phosphorylation of ERK but not p38 MAP kinase. These results provide evidence that the neurotrophic effects of PACAP-38 on human SH-SY5Y neuroblastoma cells are mediated by the PAC₁ receptor through a cAMP-dependent but PKA-independent mechanism, and furthermore suggest that this involves Epac-dependent activation of ERK as well as activation of the p38 MAP kinase signaling pathway.

Rescuing neurons and glia: is inhibition of apoptosis useful?

Traumatic brain injury (TBI) represents a leading cause of death in western countries. Despite all research efforts we still lack any pharmacological agent that could effectively be utilized in the clinical treatment of TBI. Detailed unraveling of the pathobiological processes initiated by/operant in TBI is a prerequisite to the development of rational therapeutic interventions. In this review we provide a summary of those therapeutic interventions purported to inhibit the cell death (CD) cascades ignited in TBI. On noxious stimuli three major forms of CD, apoptosis, autophagia and necrosis may occur. Apoptosis can be induced either via the mitochondrial (intrinsic) or the receptor mediated (extrinsic) pathway; endoplasmic reticular stress is the third trigger of caspase-mediated apoptotic processes. Although, theoretically pan-caspase inhibition could be an efficient tool to limit apoptosis and thereby the extent of TBI, potential cross-talk between various avenues of CD suggests that more upstream events, particularly the preservation of the cellular energy homeostasis (cyclosporine-A, poly ADP ribose polymerase (PARP) inhibition, hypothermia treatment) may represent more efficient therapeutic targets hopefully also translated to the clinical care of the severely head injured.

Cellular localization of pituitary adenylate cyclase-activating peptide (PACAP) following traumatic brain injury in humans

The pituitary adenylate cyclase-activating peptide (PACAP) is involved in many processes of the developing and mature central nervous system, such as proliferation, differentiation, apoptosis, neurotransmission, inflammation and neuroprotection. Alternative posttranslational processing of PACAP results in two biologically active, amidated 27- and 38-amino acid peptides termed PACAP27 and PACAP38. In the present study, we examined whether traumatic brain injury (TBI) affects cellular immunopositivity for PACAP27 and PACAP38. Patients (n = 55) were classified into three groups dependent on their survival time (under 24 h, between 24 h and 7 days and between 7 days and 99 days postinjury). PACAP27 and PACAP38 were expressed by neurons and glial cells in normal human neocortex (n = 10). Following TBI, the total number of PACAP27- and PACAP38-positive cells was significantly decreased for a prolonged survival period within the traumatized neocortex. In the pericontusional cortex, the number of cells expressing PACAP27 and PACAP38 was significantly increased at all survival times examined. Triple immunofluorescence examinations revealed a significant increase in the absolute numbers of GFAP-positive reactive astrocytes as well as a decrease in the CNP-positive oligodendrocytes, each coexpressing PACAP27 or PACAP38 in the contusional and pericontusional cortex. We hypothesize that the increase of glial PACAP immunoreactivity may be interpreted as part of a complex endogenous neuroprotective response in the pericontusional regions, but the precise role of PACAP following TBI is yet to be determined.

Expression of PACAP and glutamate transporter proteins in satellite oligodendrocytes of the human CNS

White matter oligodendrocytes have been shown to actively regulate extracellular glutamate levels in the CNS. Such function has yet not been examined in satellite oligodendrocytes of gray matter. Similar to those in white matter, satellite oligodendrocytes are involved in myelination. In addition, they modulate the activity of surrounding neurons. This study examined whether satellite oligodendrocytes express PACAP and glutamate transporter proteins and whether this expression is influenced by global ischemia. We demonstrated expression of PACAP27 and PACAP38 in a major fraction of satellite oligodendrocytes in normal neocortex and hippocampus of human brain tissues obtained post-mortem. All three glutamate transporters EAAT1, EAAT2 and EAAT3 were expressed in satellite oligodendrocytes from these tissues. Thus, satellite oligodendrocytes may participate in the perineuronal glutamate homeostasis. Following transient global ischemia, the total number of satellite oligodendrocytes expressing PACAP or glutamate transporter proteins was significantly decreased in cerebral neocortex and hippocampus. However, alterations of PACAP and glutamate transporter protein expression were region and time specific. In satellite oligodendrocytes of CA1 an early strong reduction of PACAP and glutamate transporter expression was observed. This contrasted with late reduction of PACAP27, PACAP38 and glutamate transporters EAAT1, EAAT2 and EAAT3 in satellite oligodendrocytes of neocortex. Further studies should clarify whether these alterations in protein expression are primary or secondary to neuronal cell death.