Angelika Varga

Effects of the novel TRPV1 receptor antagonist SB366791 in vitro and in vivo in the rat

The TRPV1 capsaicin receptor is a non-selective cation channel localized in the cell membrane of a subset of primary sensory neurons and functions as an integrator molecule in nociceptive/inflammatory processes. The present paper characterizes the effects of SB366791, a novel TRPV1 antagonist, on capsaicin-evoked responses both in vitro and in vivo using rat models. SB366791 (100 and 500 nM) significantly inhibited capsaicin-evoked release of the pro-inflammatory sensory neuropeptide substance P from isolated tracheae, while it did not influence electrically induced neuropeptide release. It also decreased capsaicin-induced Ca2+ influx in cultured trigeminal ganglion cells in a concentration-dependent manner (0.5-10 microM) with an IC50 of 651.9 nM. In vivo 500 microg/kg i.p. dose of SB366791 significantly inhibited capsaicin-induced hypothermia, wiping movements and vasodilatation in the knee joint, while 2 mg/kg capsazepine was ineffective, its effect lasted for 1h. However, neither antagonist was able to inhibit capsaicin-evoked hypothermia in Balb/c mice. Based on these data SB366791 is a more selective and in vivo also a more potent TRPV1 receptor antagonist than capsazepine in the rat therefore, it may promote the assessment of the therapeutic utility of TRPV1 channel blockers.

Relative roles of protein kinase A and protein kinase C in modulation of transient receptor potential vanilloid type 1 receptor responsiveness in rat sensory neurons in vitro and peripheral nociceptors in vivo.

The function of the transient receptor potential vanilloid type 1 capsaicin receptor is subject to modulation by phosphorylation catalyzed by various enzymes including protein kinase C and cAMP-dependent protein kinase. The aim of this study was to compare the significance of the basal and stimulated activity of protein kinase C and cAMP-dependent protein kinase in transient receptor potential vanilloid type 1 receptor responsiveness in the rat in vitro by measurement of the intracellular calcium concentration in cultured trigeminal ganglion neurons and in vivo by determination of the behavioral noxious heat threshold. KT5720, a selective inhibitor of cAMP-dependent protein kinase, reduced the calcium transients induced by capsaicin or the other, much more potent transient receptor potential vanilloid type 1 receptor agonist resiniferatoxin in trigeminal sensory neurons and diminished the drop of the noxious heat threshold (heat allodynia) evoked by intraplantar resiniferatoxin injection. Chelerythrine chloride, a selective inhibitor of protein kinase C, failed to alter either of these responses, although it inhibited the effect of phorbol 12-myristate 13-acetate in the in vitro assay. Staurosporine, a rather nonselective protein kinase inhibitor, failed to reduce the capsaicin- and resiniferatoxin-induced calcium transients but inhibited the resiniferatoxin-evoked heat allodynia. Dibutyryl-cAMP and phorbol 12-myristate 13-acetate, activator(s) of cAMP-dependent protein kinase and protein kinase C, respectively, enhanced the effect of capsaicin in the calcium uptake assay while forskolin, an activator of adenylyl cyclase, augmented that of resiniferatoxin in the heat allodynia model. None of the protein kinase inhibitors or activators altered the calcium transients evoked by high potassium, a nonspecific depolarizing stimulus. It is concluded that basal activity of cAMP-dependent protein kinase, unlike protein kinase C, is involved in the maintenance of transient receptor potential vanilloid type 1 receptor function in somata of trigeminal sensory neurons but stimulation of either cAMP-dependent protein kinase or protein kinase C above the resting level can lead to an enhanced transient receptor potential vanilloid type 1 receptor responsiveness. Similar mechanisms are likely to operate in vivo in peripheral terminals of nociceptive dorsal root ganglion neurons.

Actions of 3-methyl-N-oleoyldopamine, 4-methyl-N-oleoyldopamine and N-oleoylethanolamide on the rat TRPV1 receptor in vitro and in vivo.

N-oleoyldopamine (OLDA) has been identified as an agonist of the transient receptor potential vanilloid type 1 (TRPV1) receptor. A related fatty acid amide, N-oleoylethanolamide (OEA), was found to excite sensory neurons and produce visceral hyperalgesia via activation of the TRPV1 receptor, however, a recent study described this agent as an antinociceptive one. The aim of the present paper was to characterize two newly synthesized derivatives of N-oleoyldopamine, 3-methyl-N-oleoyldopamine (3-MOLDA) and 4-methyl-N-oleoyldopamine (4-MOLDA) as well as OEA with regard to their effects on the TRPV1 receptor. Radioactive 45Ca2+ uptake was measured in HT5-1 cells transfected with the rat TRPV1 receptor and intracellular Ca2+ concentration was monitored by fura-2 microfluorimetry in cultured trigeminal sensory neurons. Thermonociception was assessed by determining the behavioral noxious heat threshold in rats. 3-MOLDA induced 45Ca2+ uptake in a concentration-dependent manner, whereas 4-MOLDA and OEA were without effect. 4-MOLDA and OEA, however, concentration-dependently reduced the 45Ca2+ uptake-inducing effect of capsaicin. In trigeminal sensory neurons, 3-MOLDA caused an increase in intracellular Ca2+ concentration and this effect exhibited tachyphylaxis upon repeated application. Again, 4-MOLDA and OEA failed to alter intracellular Ca2+ levels. Upon intraplantar injection, 3-MOLDA caused an 8-10 degrees C drop of the noxious heat threshold in rats which was inhibited by the TRPV1 receptor antagonist iodo-resiniferatoxin. 4-MOLDA and OEA failed to alter the heat threshold but inhibited the threshold drop induced by the TRPV1 receptor agonist resiniferatoxin. These data show that 3-MOLDA behaves as an agonist, whereas 4-MOLDA and OEA appear to be antagonists, at the rat TRPV1 receptor.

Direct evidence for activation and desensitization of the capsaicin receptor by N-oleoyldopamine on TRPV1-transfected cell, line in gene deleted mice and in the rat

Effects of the endogenous lipid N-oleoyldopamine (OLDA) were analyzed on the rTRPV1-expressing HT1080 human fibrosarcoma cell line (HT5-1), on cultured rat trigeminal neurons, on the noxious heat threshold of rats and on nocifensive behavior of TRPV1 knockout mice. The EC(50) of capsaicin and OLDA on (45)Ca accumulation of rTRPV1-expressing HT5-1 cells was 36 nM and 1.8 microM, respectively. The efficacy of OLDA was 60% as compared to the maximum response of capsaicin. OLDA (330 nM to 3.3 microM) caused a transient increase in fluorescence of fura-2 loaded cultured small trigeminal neurons of the rat and rTRPV1-transfected HT5-1 cells measured with a ratiometric technique. Repeated application of OLDA and capsaicin caused similar desensitization in the Ca(2+) transients both in cultured neurons and rTRPV1-transfected HT5-1 cells. In the rat intraplantar injection of OLDA (5 nmol) decreased the noxious heat threshold by 6-9 degrees C and this response was strongly inhibited by the TRPV1 antagonist iodoresiniferatoxin (0.05 nmol intraplantarly (i.pl.)). In wild-type mice OLDA (50 nmol i.pl.) evoked paw lifting/licking which was significantly less sustained in TRPV1 knockout mice. It is concluded that on TRPV1 capsaicin receptors OLDA is 50 times less potent than capsaicin and it might serve as an endogenous ligand for TRPV1 in the rat, but more likely in humans.

PACAP Immunoreactivity in Human Malignant Tumor Samples and Cardiac Diseases

Pituitary adenylate cyclase activating polypeptide (PACAP) is a pleiotropic and multifunctional neuropeptide having important roles in various physiological processes. Recent trends in PACAP research point to the clinical introduction of PACAP or its analogs/fragments possibly in the near future. Recently, we have shown the presence of PACAP in human plasma, milk, placenta, and follicular fluid samples. However, relatively few data are available on PACAP in human tissues from patients with different disorders. The aim of the present study was to determine, by radioimmunoassay, the tissue level of PACAP38-like immunoreactivity (LI) and PACAP27-LI in different primary non-small cell lung cancer, colon tumor samples, and in cardiac muscle samples from patients suffering from ischemic heart disease and valvular disorders. We also labeled the PAC1 receptors in human cardiac cells. All samples showed significantly higher PACAP38-LI compared with PACAP27-LI. We found significantly lower levels of PACAP38-LI and PACAP27-LI in tumoral and peripheral samples compared with normal healthy tissue in both lung and colon cancers. Further investigations are necessary to describe the exact function of PACAP in oncogenesis. We showed that PACAP38-LI and PACAP27-LI are significantly higher in ischemic heart diseases compared with valvular abnormalities, suggesting that PACAP might play a role in ischemic heart disorders.

Anandamide in primary sensory neurons: too much of a good thing?

The quest for possible targets for the development of novel analgesics has identified the activation of the cannabinoid type 1 (CB1) receptor outside the CNS as a potential means of providing relief from persistent pain, which currently constitutes an unmet medical need. Increasing tissue levels of the CB1 receptor endogenous ligand N‐arachidonoylethanolamine (anandamide), by inhibiting anandamide degradation through blocking the anandamide‐hydrolysing enzyme fatty acid amide hydrolase, has been suggested to be used to activate the CB1 receptor. However, recent clinical trials revealed that this approach does not deliver the expected relief from pain. Here, we discuss one of the possible reasons, the activation of the transient receptor potential vanilloid type 1 ion channel (TRPV1) on nociceptive primary sensory neurons (PSNs) by anandamide, which may compromise the beneficial effects of increased tissue levels of anandamide. We conclude that better design such as concomitant blocking of anandamide hydrolysis and anandamide uptake into PSNs, to inhibit TRPV1 activation, could overcome these problems.

Involvement of cholecystokinin in baseline and post-prandial whole body insulin sensitivity in rats

The objective of the study was to investigate the role of cholecystokinin (CCK) on the food-induced insulin sensitization phenomenon in healthy Long Evans Tokushima Otsuka (LETO) and Otsuka Long Evans Tokushima Fatty (OLETF) rats. Whole body insulin sensitivity determined by hyperinsulinaemic euglycaemic glucose clamping and the rapid insulin sensitivity test served as endpoints. Determinations were done in both fasted and re-fed animals. The involvement of CCK in post-prandial insulin sensitization was assessed by using proglumide, a CCK receptor blocker, by assessment of hypothalamic CCK-1/CCK-2 receptor expression by rt-PCR technique and by plasma insulin immunoreactivity determinations by means of radioimmunoassay as pharmacological, genetic and analytical approaches, respectively. The body weight of the OLETF rats and the amount of food consumed much exceeded those seen with LETO rats. The post-prandial increase in insulin sensitivity was marked in LETO, but not in OLETF rats. Intravenous proglumide attenuated post-prandial insulin sensitivity in LETO rats, with no effect in OLETF rats. Nevertheless, baseline insulin sensitivity was much lower in OLETF than in LETO rats. Treatment with rosiglitazone increased baseline insulin sensitivity of OLETF rats and evoked an increase in CCK-1 receptor gene expression in LETO rats. The results provide evidence for the involvement of CCK receptors in adjustment of both fasting and post-prandial insulin sensitivity. The data obtained with OLETF rats strongly suggest the predominant role of CCK-1 receptors.

Ischemia/reperfusion-induced Kidney Injury in Heterozygous PACAP-deficient Mice

Pituitary adenylate cyclase activating polypeptide (PACAP) is a neuropeptide with very diverse distribution and functions. Among others, PACAP is a potent cytoprotective peptide due to its antiapoptotic, anti-inflammatory, and antioxidant actions. This also has been shown in different kidney pathologies, including ischemia/reperfusion-induced kidney injury. Similar protective effects of the endogenous PACAP are confirmed by the increased vulnerability of PACAP-deficient mice to different harmful stimuli. Kidneys of homozygous PACAP-deficient mice have more severe damages in renal ischemia/reperfusion and kidney cell cultures isolated from these mice show increased sensitivity to renal oxidative stress. In our present study we raised the question of whether the partial lack of the PACAP gene is also deleterious, i.e. whether heterozygous PACAP-deficient mice also display more severe damage after renal ischemia/reperfusion. Mice underwent 45 or 60 minutes of ischemia followed by 2 weeks reperfusion. Histological evaluation of the kidneys was performed and individual histopathological parameters were graded. Furthermore, we investigated apoptotic markers, cytokine expression, and the activity of superoxide dismutase (SOD) enzyme 24 hours after 60 minutes of renal ischemia/reperfusion. We found no difference between the intact kidneys of wild-type and heterozygous mice, but marked differences could be observed following ischemia/reperfusion. Heterozygous PACAP-deficient mice had more severe histological alterations, with significantly higher histopathological scores for most of the tested parameters. Higher level of the proapoptotic pp38 MAPK and of some proinflammatory cytokines, as well as lower activity of the antioxidant SOD could be found in these mice. In conclusion, the partial lack of the PACAP gene results in worse outcomes in cases of renal ischemia/reperfusion, confirming that PACAP functions as an endogenous protective factor in the kidney.

Diet-induced obesity alters dural CGRP release and potentiates TRPA1-mediated trigeminovascular responses

Background Clinical studies suggest a link between obesity and the primary headache disorder migraine. In our study we aimed to reveal the effect of obesity on meningeal nociceptor function in rats receiving a high-fat, high-sucrose diet. Methods Transient receptor potential ankyrin 1 (TRPA1) receptor activation-induced changes in meningeal blood flow, release of calcitonin gene-related peptide (CGRP) from trigeminal afferents and TRPA1 protein expression in the trigeminal ganglia were measured in control and obese rats. Metabolic parameters of the animals were assessed by measuring glucose and insulin homeostasis as well as plasma cytokine concentrations. Results The present experiments revealed an enhanced basal and TRPA1 receptor agonist-induced CGRP release from meningeal afferents of obese insulin-resistant rats and an attenuated CGRP release to potassium chloride. Obesity was also associated with an augmented vasodilatation in meningeal arteries after dural application of the TRPA1 agonist acrolein, a reduction in TRPA1 protein expression in the trigeminal ganglia and elevations in circulating proinflammatory cytokines IL-1β and IL-6 in addition to increased fasting blood glucose and insulin concentrations. Conclusions Our results suggest trigeminal sensitisation as a mechanism for enhanced headache susceptibility in obese individuals after chemical exposure of trigeminal nociceptors.

Meal-induced enhancement in insulin sensitivity is not triggered by hyperinsulinemia in rats

Several reports confirmed the phenomenon of postprandial increase in whole-body insulin sensitivity. Although the initial step of this process is unknown, the pivotal role of postprandial hyperinsulinemia has strongly been suggested. The aim of the present study was to determine whether hyperinsulinemia per se induces insulin sensitization in healthy male Wistar rats. Rapid insulin sensitivity test (RIST) were performed in fasted, anesthetized rats before and during stable hyperinsulinemia achieved by hyperinsulinemic euglycemic glucose clamping (HEGC) with insulin infused either through the jugular vein (systemic HEGC) or into the portal circulation (portal HEGC) at a rate of 3 mU/(kg min). Insulin sensitivity expressed by the rapid insulin sensitivity (RIST) index (in milligrams per kilogram) was characterized by the total amount of glucose needed to maintain prestudy blood glucose level succeeding an intravenous bolus infusion of 50 mU/kg insulin over 5 minutes. In fasted animals, the RIST index was 37.4 +/- 3.1 mg/kg. When hyperinsulinemia mimicking the postprandial state was achieved by systemic HEGC, the RIST index (39.7 +/- 10.6 mg/kg) showed no significant changes as compared with the pre-HEGC values. Hyperinsulinemia achieved by portal insulin infusion also failed to modify the RIST index (35.7 +/- 4.3 mg/kg). The results demonstrate that acute hyperinsulinemia, no matter how induced, does not yield any sensitization to the hypoglycemic effect of insulin.