Pituitary adenylate cyclase-activating polypeptide induces pial arteriolar vasodilation through cyclooxygenase-dependent and independent mechanisms in newborn pigs

Pituitary adenylate cyclase-activating peptide: Potential roles in the pathophysiology and complications of cirrhosis

Pituitary adenylate cyclase-activating peptide (PACAP) is a ubiquitous neuropeptide with diverse functions throughout the organism. Most abundantly investigated for its role in several neurological disorders as well as in circadian rhythms, other fields of medicine, including cardiology, have recently shown interest in the role of PACAP and its potential as a biomarker. Timely diagnosis and treatment of cirrhosis and its complications is a considerable challenge for health services world-wide and development of new areas of research is warranted. Direct and indirect evidence exists of PACAP involvement in the cascade of pathological events and processes ultimately leading to cirrhosis and its complications, but its exact role remains to be determined. Studies have documented PACAP involvement in immune function, metabolism, local vasoconstriction and dilatation and systemic vascular decompensation and there is ongoing research of a possible role in liver reperfusion injury. Considering these reports, PACAP could theoretically exude influence on the disease course of cirrhosis through the hypothalamus-pituitary-adrenal axis, chronic inflammation, fibrogenesis, vasodilation and reduced vascular resistance. The paucity of literature on the specific topic of PACAP and cirrhosis reflects complex mechanisms and difficulty in accurate measurements and sample taking. This does not detract from the need to further characterize and elucidate the role PACAP plays in the underdiagnosed and undertreated condition of cirrhosis.

Protective effects of PACAP in ischemia

Pituitary adenylate cyclase activating polypeptide (PACAP) is an ubiquitous peptide involved, among others, in neurodevelopment, neuromodulation, neuroprotection, neurogenic inflammation and nociception. Presence of PACAP and its specific receptor, PAC1, in the trigeminocervical complex, changes of PACAP levels in migraine patients and the migraine-inducing effect of PACAP injection strongly support the involvement of PACAP/PAC1 receptor in migraine pathogenesis. While antagonizing PAC1 receptor is a promising therapeutic target in migraine, the diverse array of PACAP's functions, including protection in ischemic events, requires that the cost-benefit of such an intervention is well investigated by taking all the beneficial effects of PACAP into account. In the present review we summarize the protective effects of PACAP in ischemia, especially in neuronal ischemic injuries, and discuss possible points to consider when developing strategies in migraine therapy interfering with the PACAP/PAC1 receptor system.

Examination of Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP) as a Potential Biomarker in Heart Failure Patients

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a pleiotropic and multifunctional neuropeptide having neurotrophic, neuroprotective, and general cytoprotective actions in a variety of tissues based on its anti-apoptotic, anti-inflammatory, and antioxidant effects. Several studies have demonstrated its cardioprotective effects in vitro and in various animal models. However, few data are available on the presence of PACAP in human cardiac tissues and its role in the pathomechanism and progression of different cardiac disorders, particularly heart failure. Earlier, our research group has shown PAC1 receptor immunoreactivity in human heart tissue samples and we have found significantly elevated PACAP27- and PACAP38-like immunoreactivity in ischemic cardiac samples compared to valvular abnormalities with radioimmunoassay. In the last few years, numerous studies examined the presence and the changes of PACAP levels in different human tissue samples and biological fluids to show alterations in different physiological and pathological conditions. Therefore, the aim of the present study was to measure the alterations of blood PACAP levels in chronic heart failure caused by primary dilated cardiomyopathy or ischemic cardiomyopathy and to examine the possible relationship between serum levels of PACAP, N-terminal prohormone of brain natriuretic peptide (NT-proBNP), and systolic left ventricular function, the most reliable biomarkers of heart failure. In the group of mild heart failure patients, a significant strong negative correlation was detected. Furthermore, in moderate heart failure, we found a significant moderate negative correlation between PACAP and NT-proBNP levels only in ischemic subgroup. Positive correlation was found between serum PACAP level and ejection fraction only in patients with heart failure due to ischemic cardiomyopathy but not in patients with primary dilated cardiomyopathy. In summary, remarkable differences were observed between the ischemic and non-ischemic heart failure suggesting that PACAP might play an important role in the pathomechanism and progression of ischemic heart failure and it might be a potential biomarker of cardiac diseases in the future.

Characterization of the thermoregulatory response to pituitary adenylate cyclase-activating polypeptide in rodents

Administration of the long form (38 amino acids) of pituitary adenylate cyclase-activating polypeptide (PACAP38) into the central nervous system causes hyperthermia, suggesting that PACAP38 plays a role in the regulation of deep body temperature (T b). In this study, we investigated the thermoregulatory role of PACAP38 in details. First, we infused PACAP38 intracerebroventricularly to rats and measured their T b and autonomic thermoeffector responses. We found that central PACAP38 infusion caused dose-dependent hyperthermia, which was brought about by increased thermogenesis and tail skin vasoconstriction. Compared to intracerebroventricular administration, systemic (intravenous) infusion of the same dose of PACAP38 caused significantly smaller hyperthermia, indicating a central site of action. We then investigated the thermoregulatory phenotype of mice lacking the Pacap gene (Pacap (-/-)). Freely moving Pacap (-/-) mice had higher locomotor activity throughout the day and elevated deep T b during the light phase. When the Pacap (-/-) mice were loosely restrained, their metabolic rate and T b were lower compared to their wild-type littermates. We conclude that PACAP38 causes hyperthermia via activation of the autonomic cold-defense thermoeffectors through central targets. Pacap (-/-) mice express hyperkinesis, which is presumably a compensatory mechanism, because under restrained conditions, these mice are hypometabolic and hypothermic compared to controls.

PACAP enhances barrier properties of cerebral microvessels

Cerebral microvascular endothelial cells-coming in contact with pericytes and astrocytes-constitute the structural basis of the blood-brain barrier (BBB). The continuous belt of interendothelial tight junctions (TJs) and the presence of specific transport systems, enzymes, and receptors in the brain endothelium regulate the molecular and cellular traffic into the central nervous system. Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide having several cellular protective effects. However, little is known about the effects of PACAP on the cerebral endothelium and BBB functions. Here, we show that PACAP has no significant pro-survival role in cerebral microvascular endothelial cells; however, it improves the barrier properties of the brain endothelium. PACAP induces an increase in the transendothelial electrical resistance, which is the most important marker of the tightness of the TJs. Moreover, PACAP has a protective role against glucose deprivation- and oxidative stress-induced junctional damage in microvascular brain endothelial cells.

VIP/PACAP receptors in cerebral arteries of rat: characterization, localization and relation to intracellular calcium

BACKGROUND

Vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase activating peptide (PACAP)-containing nerves surround cerebral blood vessels. The peptides have potent vasodilator properties via smooth muscle cell receptors and activation of adenylate cyclase. The purpose of this study was to describe the effects of two putative VIP/PACAP receptor antagonists and the distribution of the receptor protein in rat brain vessels.

METHODS

The vascular effects of VIP, PACAP-27 and PACAP-38 were investigated in segments of rat middle cerebral artery (MCA) by pressurized arteriography, and in a wire myograph. The antagonistic responses to PACAP6-38 and PG99-465 were evaluated. In addition, the receptor subtypes for VIP and PACAP (VPAC1, VPAC2 and PAC1) were visualized in the rat middle cerebral artery by immunohistochemistry and Western blotting.

RESULTS

In the perfusion model, abluminal but not luminal VIP, PACAP-27 and PACAP-38 caused concentration-dependent relaxations of the MCA (27.1±0.2%, 25.2±0.4% and 0.3±0.1%, respectively). In the wire myograph, there was no significant difference in potency of the peptides in the MCA. In both systems, PACAP6-38 and PG99-465 inhibited the VIP induced relaxation. Western blot showed the presence of the receptor proteins in cerebral vasculature and immunohistochemistry showed that all three receptors are present and located in the cytoplasm of smooth muscle cells.

CONCLUSION

In both systems, the two blockers antagonized the relaxant VIP effect; the potency order of agonists and the immunohistochemistry suggest the presence of the dilatory VPAC1 and VPAC2 receptors on the smooth muscle cells.

Effects of pituitary adenylate cyclase activating polypeptide in the urinary system, with special emphasis on its protective effects in the kidney

Pituitary adenylate cyclase activating polypeptide (PACAP) is a widespread neuropeptide with diverse effects in the nervous system and peripheral organs. One of the most well-studied effects of PACAP is its cytoprotective action, against different harmful stimuli in a wide variety of cells and tissues. PACAP occurs in the urinary system, from the kidney to the lower urinary tract. The present review focuses on the nephroprotective effects of PACAP and summarizes data obtained regarding the protective effects of PACAP in different models of kidney pathologies. In vitro data show that PACAP protects tubular cells against oxidative stress, myeloma light chain, cisplatin, cyclosporine-A and hypoxia. In vivo data provide evidence for its protective effects in ischemia/reperfusion, cisplatin, cyclosporine-A, myeloma kidney injury, diabetic nephropathy and gentamicin-induced kidney damage. Results accumulated on the renoprotective effects of PACAP suggest that PACAP is an emerging candidate for treatment of human kidney pathologies.

Peripheral and central alterations of pituitary adenylate cyclase activating polypeptide-like immunoreactivity in the rat in response to activation of the trigeminovascular system

Pituitary adenylate cyclase activating polypeptide (PACAP) is present in the cranial arteries and trigeminal sensory neurons. We therefore examined the alterations in PACAP-like immunoreactivity (PACAP-LI) in a time-dependent manner in two rat models of trigeminovascular system (TS) activation. In one group chemical stimulation (CS) was performed with i.p. nitroglycerol (NTG), and in the other one the trigeminal ganglia (TRG) were subjected to electrical stimulation (ES). The two biologically active forms, PACAP-38 and PACAP-27, were determined by means of radioimmunoassay (RIA) and mass spectrometry (MS) in the plasma, the cerebrospinal fluid (CSF), the trigeminal nucleus caudalis (TNC), the spinal cord (SC) and the TRG. The tissue concentrations of PACAP-27 were 10 times lower than those of PACAP-38 in the TNC and SC, but about half in the TRG. PACAP-38, but not PACAP-27, was present in the plasma. Neither form could be identified in the CSF. PACAP-38-LI in the plasma, SC and TRG remained unchanged after CS, but it was increased significantly in the TNC 90 and 180 min after NTG injection. In response to ES of the TRG, the level of PACAP-38 in the plasma and the TNC was significantly elevated 90 and 180 min later, but not in the SC or the TRG. The alterations in the levels of PACAP-27 in the tissue homogenates in response to both forms of stimulation were identical to those of PACAP-38. The selective increases in both forms of PACAP in the TNC suggest its important role in the central sensitization involved in migraine-like headache.

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.

PACAP and VIP differentially preserve neurovascular reactivity after global cerebral ischemia in newborn pigs

Pituitary adenylate cyclase activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP) are neuroprotective in numerous models. Impairment of cerebrovascular reactivity (CR) contributes to ischemia/reperfusion (I/R)-induced neuronal damage. We tested whether PACAP and/or VIP preserve CR to I/R-sensitive dilator responses dependent on endothelial and/or neuronal function. Accordingly, changes in pial arteriolar diameters in response to hypercapnia (5-10% CO(2) ventilation) or topical N-methyl-d-aspartate (NMDA, 10(-4) M) were determined before and after I/R via intravital microscopy in anesthetized/ventilated piglets. Local pretreatment with non-vasoactive doses of PACAP (10(-8) M) and VIP (10(-9) M) prevented the attenuation of postischemic CR to hypercapnia; to 10% CO(2), the CR values were 27+/-8% vs 92+/-5% vs 88+/-13% (vehicle vs PACAP38 vs VIP, CR expressed as a percentage of the response before I/R, mean+/-SEM, n=8-8, p<0.05). PACAP, but not VIP, preserved CR to NMDA after I/R, with CR values of 31+/-10% vs 87+/-8% vs 35+/-12% (vehicle vs PACAP38 vs VIP, n=6-6). Unlike PACAP, VIP-induced vasodilation has not yet been investigated in the piglet. We tested whether VIP-induced arteriolar dilation was sensitive to inhibitors of cyclooxygenase (COX)-1 (SC-560, 1 mg/kg), COX-2 (NS-398, 1 mg/kg), indomethacin (5 mg/kg), and nitric oxide synthase (L-NAME, 15 mg/kg). VIP (10(-8)-10(-7)-10(-6) M, n=8) induced reproducible, dose-dependent vasodilation of 16+/-3%, 33+/-6%, and 70+/-8%. The response was unaffected by all drugs, except that the vasodilation to 10(-8) M VIP was abolished by SC-560 and indomethacin. In conclusion, PACAP and VIP differentially preserve postischemic CR; independent of their vasodilatory effect.