Effects of pituitary adenylate cyclase-activating polypeptide on facial nerve recovery in the Guinea pig

Neuropeptides Involved in Facial Nerve Regeneration

Neuropeptides and neurotransmitters act as intermediaries to transmit impulses from one neuron to another via a synapse. These neuropeptides are also related to nerve degeneration and regeneration during nerve damage. Although there are various neuropeptides, three are associated with neural regeneration in facial nerve damage: calcitonin gene-related peptide (CGRP), galanin, and pituitary adenylyl cyclase-activating peptide (PACAP). Alpha CGRP in facial motoneurons is a signaling factor involved in neuroglial and neuromuscular interactions during regeneration. Thus, it may be a marker for facial nerve regeneration. Galanin is a marker of injured axons rather than nerve regeneration. PACAP has various effects on nerve regeneration by regulating the surrounding cells and providing neurotrophic factors. Thus, it may also be used as a marker for facial nerve regeneration. However, the precise roles of these substances in nerve generation are not yet fully understood. Animal studies have demonstrated that they may act as neuromodulators to promote neurotrophic factors involved in nerve regeneration as they appear early, before changes in the injured cells and their environment. Therefore, they may be markers of nerve regeneration.

Protective Effects of PACAP in a Rat Model of Diabetic Neuropathy

Pituitary adenylate cyclase-activating peptide (PACAP) is a neuropeptide with a widespread occurrence and diverse effects. PACAP has well-documented neuro- and cytoprotective effects, proven in numerous studies. Among others, PACAP is protective in models of diabetes-associated diseases, such as diabetic nephropathy and retinopathy. As the neuropeptide has strong neurotrophic and neuroprotective actions, we aimed at investigating the effects of PACAP in a rat model of streptozotocin-induced diabetic neuropathy, another common complication of diabetes. Rats were treated with PACAP1-38 every second day for 8 weeks starting simultaneously with the streptozotocin injection. Nerve fiber morphology was examined with electron microscopy, chronic neuronal activation in pain processing centers was studied with FosB immunohistochemistry, and functionality was assessed by determining the mechanical nociceptive threshold. PACAP treatment did not alter body weight or blood glucose levels during the 8-week observation period. However, PACAP attenuated the mechanical hyperalgesia, compared to vehicle-treated diabetic animals, and it markedly reduced the morphological signs characteristic for neuropathy: axon–myelin separation, mitochondrial fission, unmyelinated fiber atrophy, and basement membrane thickening of endoneurial vessels. Furthermore, PACAP attenuated the increase in FosB immunoreactivity in the dorsal spinal horn and periaqueductal grey matter. Our results show that PACAP is a promising therapeutic agent in diabetes-associated complications, including diabetic neuropathy.

PACAP and VIP increase the expression of myelin-related proteins in rat schwannoma cells: involvement of PAC1/VPAC2 receptor-mediated activation of PI3K/Akt signaling pathways

PACAP and its cognate peptide VIP participate in various biological functions, including myelin maturation and synthesis. However, defining whether these peptides affect peripheral expression of myelin proteins still remains unanswered. To address this issue, we assessed whether PACAP or VIP contribute to regulate the expression of three myelin proteins (MAG, MBP and MPZ, respectively) using the rat schwannoma cell line (RT4-P6D2T), a well-established model to study myelin gene expression. In addition, we endeavored to partly unravel the underlying molecular mechanisms involved. Expression of myelin-specific proteins was assessed in cells grown either in normal serum (10% FBS) or serum starved and treated with or without 100 nM PACAP or VIP. Furthermore, through pharmacological approach using the PACAP/VIP receptor antagonist (PACAP6-38) or specific pathway (MAPK or PI3K) inhibitors we defined the relative contribution of receptors and/or signaling pathways on the expression of myelin proteins. Our data show that serum starvation (24h) significantly increased both MAG, MBP and MPZ expression. Concurrently, we observed increased expression of endogenous PACAP and related receptors. Treatment with PACAP or VIP further exacerbated starvation-induced expression of myelin markers, suggesting that serum withdrawal might sensitize cells to peptide activity. Stimulation with either peptides increased phosphorylation of Akt at Ser473 residue but had no effect on phosphorylated Erk-1/2. PACAP6-38 (10 μM) impeded starvation- or peptide-induced expression of myelin markers. Similar effects were obtained after pretreatment with the PI3K inhibitor (wortmannin, 10 μM) but not the MAPKK inhibitor (PD98059, 50 μM). Together, the present finding corroborate the hypothesis that PACAP and VIP might contribute to the myelinating process preferentially via the canonical PI3K/Akt signaling pathway, providing the basis for future studies on the role of these peptides in demyelinating diseases.

Role of transcription factors in peripheral nerve regeneration

Following axotomy, the activation of multiple intracellular signaling cascades causes the expression of a cocktail of regeneration-associated transcription factors which interact with each other to determine the fate of the injured neurons. The nerve injury response is channeled through manifold and parallel pathways, integrating diverse inputs, and controlling a complex transcriptional output. Transcription factors form a vital link in the chain of regeneration, converting injury-induced stress signals into downstream protein expression via gene regulation. They can regulate the intrinsic ability of axons to grow, by controlling expression of whole cassettes of gene targets. In this review, we have investigated the functional roles of a number of different transcription factors - c-Jun, activating transcription factor 3, cAMP response element binding protein, signal transducer, and activator of transcription-3, CCAAT/enhancer binding proteins β and δ, Oct-6, Sox11, p53, nuclear factor kappa-light-chain-enhancer of activated B cell, and ELK3 - in peripheral nerve regeneration. Studies involving use of conditional mutants, microarrays, promoter region mapping, and different injury paradigms, have enabled us to understand their distinct as well as overlapping roles in achieving anatomical and functional regeneration after peripheral 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.

Disruption of the PACAP gene promotes medulloblastoma in ptc1 mutant mice

Hedgehog (Hh) proteins and cAMP-dependent protein kinase A (PKA) generally play opposing roles in developmental patterning events. Humans and mice heterozygous for mutations in the sonic hedgehog (Shh) receptor gene patched-1 (ptc1) have an increased incidence of certain types of cancer, including medulloblastoma (MB), a highly aggressive tumor of the cerebellum. Despite the importance of PKA in Hh signaling, little is known about how PKA activity is regulated in the context of Hh signaling, or the consequences of improper regulation. One molecule that can influence PKA activity is pituitary adenylyl cyclase-activating peptide (PACAP), which has been shown to regulate cerebellar granule precursor proliferation in vitro, a cell population thought to give rise to MB. To test for a PACAP/Hh interaction in the initiation or propagation of these tumors, we introduced a PACAP mutation into ptc1 mutant mice. Deletion of a single copy of PACAP increased MB incidence approximate 2.5-fold, to 66%, thereby demonstrating that PACAP exerts a powerful inhibitory action on the induction, growth or survival of these tumors. Tumors from PACAP/ptc1 mutant mice retained PACAP receptor gene expression, and exhibited superinduction of Hh target genes compared to those from ptc1+/- mice. Moreover, PACAP inhibited proliferation of cell lines derived from tumors in a PKA-dependent manner, and inhibited expression of the Hh target gene gli1. The results provide genetic evidence that PACAP acts as a physiological factor that regulates the pathogenesis of Hh pathway-associated MB tumors.

Impaired nerve regeneration and enhanced neuroinflammatory response in mice lacking pituitary adenylyl cyclase activating peptide

Peripheral nerve injury models are used to investigate processes that can potentially be exploited in CNS injury. A consistent change that occurs in injured peripheral neurons is an induction in expression of pituitary adenylyl cyclase activating peptide (PACAP), a neuropeptide with putative neuroprotective and neuritogenic actions. PACAP-deficient mice were used here to investigate actions of endogenous PACAP after facial nerve injury. Although motor neuron survival after axotomy was not significantly different in PACAP deficient vs. wild type mice, recovery of axon regeneration after crush injury was significantly delayed. The impaired regeneration was associated with 8- to 12-fold increases in gene expression of proinflammatory cytokines tumor necrosis factor-alpha, interferon-gamma, interleukin (IL) -6, and a 90% decrease in the anti-inflammatory cytokine IL-4 at the injury site. Similar cytokine changes and an increased microglial response were observed in the brainstem facial motor nucleus. Because immunocompromised animals such as SCID mice are known to exhibit peripheral nerve regeneration defects, the observations raise the novel hypothesis that PACAP is critically involved in a carefully controlled immune response that is necessary for proper nerve regeneration after injury.

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.

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

Pituitary adenylate cyclase activating polypeptide (PACAP) has several different actions in the nervous system. Numerous studies have shown its neuroprotective effects both in vitro and in vivo. Previously, it has been demonstrated that PACAP reduces brain damage in rat models of global and focal cerebral ischemia. Based on the protective effects of PACAP in cerebral ischemia and the presence of common pathogenic mechanisms in cerebral ischemia and traumatic brain injury (TBI), the aim of the present study was to investigate the possible protective effect of PACAP administered 30 min or 1 h postinjury in a rat model of diffuse axonal injury. Adult Wistar male rats were subjected to impact acceleration, and PACAP was administered intracerebroventricularly 30 min (n = 4), and 1 h after the injury (n = 5). Control animals received the same volume of vehicle at both time-points (n = 5). Two hours after the injury, brains were processed for immunohistochemical localization of damaged axonal profiles displaying either beta-amyloid precursor protein (beta-APP) or RMO-14 immunoreactivity, both considered markers of specific features of traumatic axonal injury. Our results show that treatment with PACAP (100 microg) 30 min or 1 h after the induction of TBI resulted in a significant reduction of the density of beta-APP-immunopositive axon profiles in the corticospinal tract (CSpT). There was no significant difference between the density of beta-APP-immunopositive axons in the medial longitudinal fascicle (MLF). PACAP treatment did not result in significantly different number of RMO-14-immunopositive axonal profiles in either brain areas 2 hours post-injury compared to normal animals. While the results of this study highlighted the complexity of the pathogenesis and manifestation of diffuse axonal injury, they also indicate that PACAP should be considered a potential therapeutic agent in TBI.

Effect of PACAP in 6-OHDA-induced injury of the substantia nigra in intact young and ovariectomized female rats

Pituitary adenylate cyclase activating polypeptide (PACAP) has neuroprotective effects in various neuronal cultures and in models of brain pathologies in vivo. Among others, it protects dopaminergic neurons in vitro, against 6-OHDA- and rotenone-induced injury. Recently, we have shown that PACAP reduces dopaminergic cell loss and ameliorates behavioral outcome following unilateral 6-OHDA-induced injury of the substantia nigra in male rats. However, after castration, PACAP led only to a slight amelioration of the behavioral symptoms. The aim of the present study was to investigate the degree of neuroprotection exerted by PACAP in female rats, using the same model. It was found that PACAP had no effect on the dopaminergic cell loss in intact female rats, only caused amelioration of certain acute behavioral signs. In contrast, PACAP effectively increased dopaminergic cell survival and decreased behavioral deficits in ovariectomized females. These results indicate that the neuroprotective effect of PACAP in a rat model of Parkinson's disease is gender-specific.

Effects of pituitary adenylate cyclase activating polypeptide in a rat model of traumatic brain injury

Pituitary adenylate cyclase activating polypeptide (PACAP) is a widely distributed neuropeptide that has numerous different actions. Recent studies have shown that PACAP exerts neuroprotective effects not only in vitro but also in vivo, in animal models of global and focal cerebral ischemia, Parkinson's disease and axonal injuries. Traumatic brain injury has an increasing mortality and morbidity and it evokes diffuse axonal injury which further contributes to its damaging effects. The aim of the present study was to examine the possible neuroprotective effect of PACAP in a rat model of diffuse axonal injury induced by impact acceleration. Axonal damage was assessed by immunohistochemistry using an antiserum against beta-amyloid precursor protein, a marker of altered axoplasmic transport considered as key feature in axonal injury. In these experiments, we have established the dose response curves for PACAP administration in traumatic axonal injury, demonstrating that a single post-injury intracerebroventricular injection of 100 microg PACAP significantly reduced the density of damaged, beta-amyloid precursor protein-immunoreactive axons in the corticospinal tract.

PACAP facilitate the nerve regeneration factors in the facial nerve injury

Pituitary adenylate cyclase-activating polypeptide (PACAP) has been reported as a strong neurotrophic factor in the various sites of nervous system. The facial nerve injury is one of the common problems in patients at the Otolaryngology since the nerve damage could occur easily due to the anatomical characteristics. Once it happens, the regeneration is little observed and functional recovery is poor. Thus, we investigated that PACAP might have some influence for regeneration after the facial nerve transaction in the guinea pig. PACAP treatment accelerated time for the appearance of compound muscle action potentials (CMAP) after the nerve transaction (first appeared at 1 versus 2 weeks in control) and shortened the latency at 4 weeks. The number of myelinated fibers increased at 4 weeks. Histochemical demonstration of GAP-43, a growth cone protein, was observed at the injury area at 2-4 days. PACAP increased the level of glial cell line-derived neurotrophic factor (GDNF), a neurotrophin, in facial target muscles at 1 day-4 weeks. These data indicated that PACAP promotes the regeneration factors and increases the possibility of functional recovery following the facial nerve injury.

Morphological and functional effects of PACAP in 6-hydroxydopamine-induced lesion of the substantia nigra in rats

Pituitary adenylate cyclase activating polypeptide (PACAP) has several different actions in the nervous system, including neuroprotective effects. In the present study, we investigated the effects of different doses of PACAP on the functional and morphological outcome in a rat model of Parkinson's disease. Rats were given unilateral injections of 6-hydroxydopamine (6-OHDA) into the substantia nigra. PACAP-treated animals received 1, 0.1 or 0.01 microg PACAP as a pretreatment. Control animals without PACAP treatment displayed severe hypokinesia at 1 and 10 days post-lesion when compared to normal animals or those receiving saline only. PACAP treatment resulted in less severe acute hypokinesia, and complete recovery by 10 days. Asymmetrical signs were observed in all lesioned animals 1 day post-lesion. PACAP-treated animals, however, showed better recovery as they ceased to display asymmetrical signs 10 days later and showed markedly less apomorphine-induced rotations. Best behavioral outcome was observed in animals treated with 0.1 microg PACAP. Tyrosine-hydroxylase (TH) immunohistochemistry revealed increased number of dopaminergic neurons in the substantia nigra pars compacta and in the ventral tegmental area in all PACAP-treated rats in contrast to the severe cell loss in control animals. These results indicate that PACAP may be a promising therapeutic agent in Parkinson's disease.

PACAP mRNA is expressed in rat spinal cord neurons

This study examines the expression of pituitary adenylate cyclase activating polypeptide (PACAP) mRNA in the rat spinal cord during normal conditions and in response to sciatic nerve transection. Previously, PACAP immunoreactivity has been found in fibers in the spinal cord dorsal horn and around the central canal and in neurons in the intermediolateral column (IML). Furthermore, in the dorsal root ganglia, PACAP immunoreactivity and PACAP mRNA expression have been observed preferentially in nerve cell bodies of smaller diameter terminating in the superficial laminae of the dorsal horn. However, neuronal expression of PACAP mRNA in adult rat spinal cord appeared limited to neurons of the IML. By using a refined in situ hybridization protocol, we now detect PACAP mRNA expression in neurons primarily in laminae I and II, but also in deeper laminae of the spinal cord dorsal horn and around the central canal. In addition, PACAP mRNA expression is observed in a few neurons in the ventral horn. PACAP expression in the ventral horn is increased in a population of large neurons, most likely motor neurons, both after distal and proximal sciatic nerve transection. The proposed role of PACAP in nociception is strengthened by our findings of PACAP mRNA-expressing neurons in the superficial laminae of the dorsal horn. Furthermore, increased expression of PACAP in ventral horn neurons, in response to nerve transection, suggests a role for PACAP in repair/regeneration of motor neurons.