Increased endothelin-like immunoreactivity in ibotenic acid-lesioned hippocampal formation of the rat brain

6-Mercaptopurine exerts an immunomodulatory and neuroprotective effect on permanent focal cerebral occlusion in rats

BACKGROUND

A bursting cascade of inflammation imposes progressive neurological deterioration after experimental stroke has been demonstrated. In our study, 6-mercaptopurine (6-mp) has been successful in alleviating cerebral infarct in a rodent permanent middle cerebral artery occlusion (pMCAO) model. The present study was aimed to examine the effect of 6-mp on cytokine levels in experimental stroke.

METHODS

The rodent pMCAO model was employed. A dose of 2 mg/kg 6-mp or vehicle (0.1 mol/L PBS) was administered intraperitoneally 30 min after the induction of pMCAO. Neurological score, serum, and cerebrospinal fluid (CSF) cytokines such as IL-1beta, IL-6, and TNF-alpha and infarct volume were determined 48 h after pMCAO.

RESULTS

Cerebral infarction volume was significantly decreased in animals treated with 6-mp (74.3%, p < 0.01), and the ratio of tissue edema was also decreased in 6-mp-treated groups (71%). Animals receiving 6-mp thus showed a significant decrease in IL-1 and TNF-alpha (18/43% and 48/64% in CSF/serum, respectively) when compared with the pMCAO groups (p < 0.01).

CONCLUSION

This study demonstrates that 6-mp interposes the production of IL-1 and TNF-alpha in CSF and serum, attenuates ischemic brain injury, and thus alleviates neurological deficits in the pMCAO animals. These findings also offer first evidence that 6-mp may attenuate TNF-alpha-related neuron apoptosis and also support the notion that 6-mp and other anti-inflammatory agents could potentially have therapeutic uses in cases of cerebral infarct.

6-Mercaptopurine attenuates adhesive molecules in experimental vasospasm

OBJECTIVE

Adhesion molecules, including intercellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule-1 (VCAM-1), and E-selectin, are important inflammatory mediators which are elevated in the serum of patients following aneurysmal subarachnoid hemorrhage (SAH). The authors previously found that 6-mercaptopurine (6-mp) was effective in preventing and reversing arterial narrowing in a rodent SAH model. The present study was to examine whether levels of adhesion molecules were altered after treatment with 6-mp in this animal model.

MATERIALS AND METHODS

Animals were each injected with autologous blood into the cisterna magna, and intraperitoneal treatment with 6-mp (2 mg/kg) was initiated 1 h before (prevention) or later (treatment). The compound was subsequently administered at 24 and 48 h post-SAH. Blood samples were collected at 72 h post-SAH to measure ICAM-1, VCAM-1, and E-selectin levels. The basilar arteries were harvested and sliced, and their cross-sectional areas were measured. Morphologically, convolution of the internal elastic lamina, distorted endothelial wall, and myonecrosis of the smooth muscle were prominently observed in the SAH only and vehicle-treated SAH groups, but not in the 6-mp-treated SAH group or in healthy controls. No significant differences were found in the levels of VCAM-1 among all groups. However, the levels of E-selectin were increased in all animals subjected to SAH (SAH only and SAH plus vehicle groups) compared with healthy controls (no SAH), but not in the 6-mp group (SAH plus 6-mp treatment and preventive treatment with 6-mp).Likewise, the levels of ICAM-1 in the SAH only and SAH plus vehicle groups were significantly elevated (p < 0.001), and pretreatment and treatment with 6-mp reduced ICAM-1 to control levels.

CONCLUSION

These results show that ICAM-1 and E-selectin may play a role in mediating SAH-induced vasospasm and that a reduction of both adhesive molecules after SAH may partly contribute to the antispastic effect of 6-mp.

Endothelin 1 protects HN33 cells from serum deprivation-induced neuronal apoptosis through Ca(2+)-PKCalpha-ERK pathway

Endothelins (ETs), which were originally found to be potent vasoactive transmitters, were known to be implicated in nervous system, but the mode of mechanism remains unclear. ETs (ET-1, ET-2, and ET-3) were added to HN33 (mouse hippocampal neuron chi neuroblastoma) cells. Among the three types of ET, only ET-1 increased the intracellular calcium levels in a PLC dependent manner with the induction of ERK 1/2 activation. As the result of ET-1 exposure, the survival rate of HN33 cells and the PKCalpha translocation into the plasma membrane were increased. We suggest that ET-1 participated in the neuroprotective effect involving the calcium-PKCalpha-ERK1/2 pathway.

Cortical expression of endothelin receptor subtypes A and B following middle cerebral artery occlusion in rats

This work aimed to define the spatial expression of endothelin A (ET(A)) and B (ET(B)) receptors in the cerebral cortex after permanent middle cerebral artery occlusion (MCAO) and to identify the phenotype of cells expressing ET(A) and ET(B) receptors. Cortical expression of ET(A) and ET(B) receptors was determined at the mRNA level by semi-quantitative reverse transcription-polymerase chain reaction and at the protein level by immunofluorescence staining, 12, 24 and 72 h after MCAO. Cells expressing endothelin receptors were phenotyped by double labelling with antibodies, anti-protein gene product (PGP9.5) and anti-ED1, towards neurons and activated microglia/macrophages, respectively. Both ET(A) and ET(B) receptor mRNA expressions increased significantly in the ipsilateral cortex in a time-dependent manner after MCAO. Robust expression of ET(A) receptors was noted in most neurons of the ischemic core and in several neurons in laminae 3 and 4 of the peri-infarct region 24 and 72 h after MCAO. ET(B) receptor immunoreactivity was observed in activated microglia/macrophages, beginning 24 h after MCAO. These results provide the first evidence that the action of endothelin during ischemia may be mediated by neuronal ET(A) receptors and activated microglia/macrophage ET(B) receptors. This differential localization of ET(A) and ET(B) receptors suggests that endothelin is involved in some complex neuron-glial interactions in addition to its vascular modulatory activity during ischemia.

Endothelins Inhibit Junctional Permeability in Cultured Mouse Astrocytes

Endothelins, a family of potent vasoconstrictor peptides initially characterized in peripheral tissues, have also been reported to be synthesized in the brain. In this structure several cell types, including astrocytes, endothelial cells and certain neurons, are potential targets for these peptides. In astrocytes, endothelins induce changes in the concentration of several second messengers (calcium, diacylglycerol, arachidonic acid, cAMP) known to be involved in the regulation of gap junction channels. Using the scrape loading/dye transfer technique we have observed that two isoforms of endothelin, endothelin-1 and endothelin-3, strongly inhibit the extent of dye-coupling between confluent astrocytes, suggesting that gap junction permeability was reduced. This inhibitory effect on dye coupling was reproduced by the snake venom sarafotoxin. When used at 10-7 M, these three compounds had inhibitory effects on gap junction channels which were comparable to those induced by the well known uncoupling agents octanol and halothane. In the absence of extracellular calcium, the effects of endothelins were largely prevented, suggesting that second messengers linked to the activation of phospholipases C and/or A2, which both are dependent on external calcium, could be involved in the uncoupling mechanism.

Endothelin and dopamine release

Endothelins and endothelin receptors are widespread in the brain. There is increasing evidence that endothelins play a role in brain mechanisms associated with behaviour and neuroendocrine regulation as well as cardiovascular control. We review the evidence for an interaction of endothelin with brain dopaminergic mechanisms. Our work has shown that particularly endothelin-1 and ET(B) receptors are present at significant levels in typical brain dopaminergic regions such as the striatum. Moreover, lesion studies showed that ET(B) receptors are present on dopaminergic neuronal terminals in striatum and studies with local administration of endothelins into the ventral striatum showed that activation of these receptors causes dopamine release, as measured both with in vivo voltammetry and behavioural methods. While several previous studies have focussed on the possible role of very high levels of endothelins in ischemic and pathological mechanisms in the brain, possibly mediated by ET(A) receptors, we propose that physiological levels of these peptides play an important role in normal brain function, at least partly by interacting with dopamine release through ET(B) receptors.

Distinct pharmacological properties of ET-1 and ET-3 on astroglial gap junctions and Ca(2+) signaling

Astrocytes represent a major target for endothelins (ETs), a family of peptides that have potent and multiple effects on signal transduction pathways and can be released by several cell types in the brain. In the present study we have investigated the involvement of different ET receptor subtypes on intercellular dye diffusion, intracellular Ca(2+) homeostasis, and intercellular Ca(2+) signaling in cultured rat astrocytes from hippocampus and striatum. Depending on the ET concentration and the receptor involved, ET-1- and ET-3-induced intracellular Ca(2+) increases with different response patterns. Both ET-1 and ET-3 are powerful inhibitors of gap junctional permeability and intercellular Ca(2+) signaling. The nonselective ET receptor agonist sarafotoxin S6b and the ET(B) receptor-selective agonist IRL 1620 mimicked these inhibitions. The ET-3 effects were only marginally affected by an ET(A) receptor antagonist but completely blocked by an ET(B) receptor antagonist. However, the ET-1-induced inhibition of gap junctional dye transfer and intercellular Ca(2+) signaling was only marginally blocked by ET(A) or ET(B) receptor-selective antagonists but fully prevented when these antagonists were applied together. The ET-induced inhibition of gap junction permeability and intercellular Ca(2+) signaling indicates that important changes in the function of astroglial communication might occur when the level of ETs in the brain is increased.

Endothelin converting enzyme-1-, endothelin-1-, and endothelin-3-like immunoreactivity in the rat brain

Neurons likely to use endothelin as a neurotransmitter/neurohormone were mapped in the rat brain using polyclonal antibodies directed against endothelin-converting enzyme-1, endothelin-1, and endothelin-3. Anti-endothelin-converting enzyme-1 antibodies produced the most robust staining, permitting the best visualization of the distribution and morphology of neurons. Labeled neurons were found in the dorsal thalamic nuclei and reticular thalamic nuclei, medial preoptic area, pontine nucleus, and locus coeruleus. Localization of endothelin-converting enzyme-like immunoreactivity in the locus coeruleus and in the reticular nucleus of the thalamus suggests that endothelin is co-localized with norepinephrine and GABA, respectively. Additionally, endothelin-converting enzyme-like immunoreactivity was found in the globus pallidus, septal nuclei, and in both the vertical and horizontal limbs of the nucleus of the diagonal band of Broca, and the ventrolateral area of the caudate-putamen. Strong endothelin-converting enzyme-like immunoreactivity was found in a continuous band of pyramidal neurons throughout the neocortex primarily in layer V, extending into the cingulate gyrus and piriform cortex. Motor nuclei, including oculomotor, facial, and trigeminal nuclei, were also endothelin-converting enzyme-immunoreactive. In the cerebellum, Purkinje cells were stained. Non-neuronal cells such as oligodendroglia, microglia, and astrocytes generally were not endothelin-converting enzyme-immunoreactive, although astrocytes were rarely stained. Endothelin-converting enzyme-, endothelin-1-, and endothelin-3-like immunoreactivities were generally found co-existing in given nuclei. The diversity of neurons immunostained for endothelin suggests multiple roles of endothelin in the CNS.

Endothelins promote the activation of astrocytes in rat neostriatum through ET(B) receptors

The effects of endothelin (ET)-3 and an ET(B) receptor agonist on astrocytic activation in rat caudate putamen were examined by an immunohistochemical staining of glial fibrillary acidic protein (GFAP), a marker of reactive astrocytes. A single injection of 40 pmol ET-3 into rat caudate putamen increased the number of GFAP positive cells compared to that in the contralateral saline-injected side. Ala(1,3,11,15)-ET-1 (40 pmol), an ET(B) receptor agonist, also increased the number of striatal GFAP positive cells. The increases in GFAP positive cells were maximum (about 150% of the control side) in 1-2 weeks after injections of the ETs, and then reduced in 4 weeks. A continuous infusion of BQ788, an ET(B) receptor antagonist (23 nmol/day), into the lateral ventricle of the cerebrum antagonized the effect of Ala(1,3,11,15)-ET-1, while BQ788 also reduced the number of GFAP positive cells in saline-injected caudate putamen. Intrastriatal injection of 40 pmol Ala(1,3,11,15)-ET-1 did not affect the number of cells stained by B4 isolectin from Griffonia simplicifolia, which labels activated microglia/macrophages. Intraperitoneal administration of 5 mg/kg per day chloroquine and 0.2 mg/kg per day colchicine did not affect the action of Ala(1,3,11,15)-ET-1. These results suggest that activation of ET(B) receptors is involved in the induction of reactive astrocytes.

Physiological role of brain endothelin in the central autonomic control: from neuron to knockout mouse

Although endothelin (ET) was discovered as a potent vascular endothelium-derived constricting peptide, its presumed physiological and pathophysiological roles are now considered much more diverse than originally though. Endothelin in the brain is thought to be deeply involved in the central autonomic control and consequent cardiorespiratory homeostasis, possibly as a neuromodulator or a hormone that functions locally in an autocrine/paracrine manner or widely through delivery by the cerebrospinal fluid (CSF). This notion is based on the following lines of evidence. (1) Mature ET, its precursors, converting enzymes, and receptors all are detected at strategic sites in the central nervous system (CNS), especially those controlling the autonomic functions. (2) The ET is present in the CSF at concentrations higher than in the plasma. (3) There is a topographical correspondence of ET and its receptors in the CNS. (4) The ET is released by primary cultures of hypothalamic neurons. (5) When ET binds to its receptors, intracellular calcium channels. (6) An intracerebroventricular or topical application of ET to CNS sites elicits a pattern of cardiorespiratory changes accompanied by responses of vasomotor and respiratory neurons. (7) Recently generated knockout mice with disrupted genes encoding ET-1 exhibited, along with malformations in a subset of the tissues of neural crest cell lineage, cardiorespiratory abnormalities including elevation of arterial pressure, sympathetic overactivity, and impairment of the respiratory reflex. Definitive evidence is expected from thorough analyses of knockout mice by applying conventional experimental methods.

Endothelin-induced cytoskeletal actin re-organization in cultured astrocytes: inhibition by C3 ADP-ribosyltransferase

We investigated signal transduction mechanisms of endothelin (ET) receptor-mediated actin re-organization of rat cultured astrocytes. Staining of filamentous actin (F-actin) showed that stress fibers were a prominent cytoskeletal actin structure in protoplasmic astrocytes. A treatment with 0.5 mM dibutyryl cAMP (DBcAMP) caused cytoplasmic retraction and disappearance of stress fibers of astrocytes. A subsequent addition of 1 nM ET-3 after the DBcAMP treatment expanded the cytoplasm and stimulated stress fiber formation. ET-1, sarafotoxin S6b, and [Ala1,3,11,16]-ET-1 had similar effects. Pre-treatment with 0.1 microgram/ml pertussis toxin (PTX) and chelation of cytosolic Ca2+ did not affect astrocytic stress fiber formation by ET-3. ET-3 stimulated stress fiber formation in stellate astrocytes induced by 50 microM ML-9, 20 microM W-7, and 5 microM cytochalasin B (CB). Cytoplasmic microinjection of C3ADP-ribosyltransferase of C. botulinum (C3 enzyme), which impairs the interaction between rho proteins and the effectors, prevented ET-3-induced stress fiber formation and cytoplasmic expansion in DBcAMP-and CB-treated cells. Effects of ET-1 and sarafotoxin on stress fiber formation were also prevented by C3 enzyme. On the other hand, injection of C3 enzyme did not affect increase in cytoplasmic Ca2+ levels induced by ET-3. These results suggest that rho proteins are involved in the ET receptor-mediated actin re-organization of astrocytes.

Endothelin-1 regulates glucose utilization in cultured astrocytes by controlling intercellular communication through gap junctions

The role played by endothelin-1 and intercellular communication mediated by gap junctions in the regulation of glucose disposal by astrocytes has been studied in primary culture. Endothelin-1 increased glucose uptake by astrocytes as did one of its putative messenger arachidonic acid and the non-physiological gap junction uncoupler alpha-glycyrrhetinic acid (AGA). None of these agents increased glucose uptake by C6 glioma cells, a cell line in which gap junction proteins are poorly expressed. In confluent astrocytes, the inhibition of gap junction permeability caused by AGA doubled the activity of the pentose phosphate shunt with minimal changes in the activity of the pyruvate dehydrogenase-catalyzed reaction and that of the tricarboxylic acid cycle. By contrast, these effects were not observed in dissociated astrocytes in which intercellular communication is lacking. The scraped loading dye transfer technique was modified to follow the passage of glucose and its metabolites through astrocyte gap junctions. The diffusion of glucose, the phosphorylated derivative glucose-6-phosphate, the phosphorylisable but not metabolisable derivative ortho-methyl-glucose, and the anaerobic glycolytic product L-lactate was much higher in astrocytes than in C6 glioma cells and was inhibited by the inhibition of gap junction permeability caused by endothelin-1, arachidonic acid, octanol, or AGA. It is concluded that gap junction permeability may regulate brain metabolism by controlling the uptake, utilization, and intercellular distribution of glucose and its metabolites in astrocytes.

Spinal endothelin content is elevated after moderate local trauma in the rat to levels associated with locomotor dysfunction after intrathecal injection

The role of endothelin (ET) in the pathophysiology of secondary neural damage after experimental spinal cord injury (SCI) was examined in a rat model of weight-drop contusion injury. Initial studies demonstrated a significant increase in spinal ET concentrations in a 7.5-mm segment of tissue (centered at the impact site) at 30 min, 4 h, and 24 h after a moderate (50 g-cm) contusion injury. Subsequent experiments were aimed at reproducing these elevations by the intrathecal (i.t.) administration of ET and observing the effect on locomotor function. These studies showed that i.t. dosage of 9.6 ng produced spinal cord elevations of ET similar to those seen 30 min after moderate SCI as well as mild locomotor deficits. A 48 ng dose of ET resulted in moderate to severe locomotor deficits that were associated with spinal ET elevations much greater than those seen after injury. The mild deficits attributable to the lower dose of ET could contribute to the pathophysiological actions of other purported secondary injury mediators. The more pronounced locomotor deficits associated with the higher dose could be of relevance for severe SCI.

Reactions of astrocytes and microglial cells around hematogenous metastases of the human brain. Expression of endothelin-like immunoreactivity in reactive astrocytes and activation of microglial cells

An immunohistochemical study was made on the peritumoral gliosis which is produced around hematogenous metastases of the human brain. The material was derived from 73 autopsy cases with different primary malignancies and six control cases without evidence of brain disease. Reactive astrocytes of hypertrophic and gemistocytic types were present around and within all the metastases. Eighty-five percent of the cases with metastases showed expression of endothelin-like immunoreactivity in the peritumoral astrocytes. Such immunoreactive astrocytes were not present in the normal controls but have previously been found in reactive astrocytes around infarcts, lacunas, inflammatory and degenerative brain diseases. The endothelin-like immunoreactivity probably reflects an increased intracellular content of endothelin. If this peptide is released from the reactive astrocytes it may act as a mitogen and influence microcirculation by inducing vasoconstriction. Activation of microglial cells, detected by ferritin immunohistochemistry, was another frequent and widespread glial cell alteration around the metastases. In conclusion, the reactive gliosis, which is formed around metastases involves activated astrocytes as well as activated microglial cells. Both cell types can release numerous biologically active compounds which may influence the structure and function of the brain tissue around the metastases.

The role of intracellular calcium and protein kinase C in endothelin-stimulated proliferation of rat type I astrocytes

The increased expression of immunoreactive endothelin-1 (ET-1) in reactive astrocytes and its mitogenic effects on astrocytes and glioma cell lines, have implicated endothelins in the development of reactive gliosis. In this study, an increase in DNA synthesis in rat type I astrocytes was observed after cultures were transiently exposed to ET-1 for 15 min, suggesting that early signal transduction events are essential and sufficient for the propagation of the ET-1-induced mitogenic signal. Prompt increases in inositol triphosphate (IP3) formation and [Ca2+]i were observed upon the addition of ET-1 to these cells. The ET-1-evoked increase in [Ca2+]i consisted of an initial peak which was preserved in Ca(2+)-free medium, and a sustained phase which was abolished in Ca(2+)-free medium and partly attenuated by nifedipine. ET-1 also increased the activity of membrane-associated protein kinase C (PKC) and induced the in vivo phosphorylation of the 85 kD MARCKS protein, an endogenous PKC-specific substrate. The ET-1-evoked increases in DNA synthesis, IP3, [Ca2+]i, membrane PKC, and 85 kD MARCKS protein phosphorylation in rat cortical astrocytes were prevented by either the selective endothelin ETA receptor antagonist, BQ-123, or the phospholipase C (PLC)-specific inhibitor, U-73122. However, the inhibition of PKC activity did not affect ET-1-induced DNA synthesis in rat cortical astrocytes. These results suggest that ET-1-induced IP3 and/or [CA2+]i responses, but not the activation of PKC, are essential for the growth-factor like actions of ET-1 in rat cortical astrocytes.

[125I]endothelin binding in rat cerebellum is increased following L-2-chloropropionic-acid-induced granule cell necrosis

The systemic administration of L-2-chloropropionic acid (L-CPA) to rats produced a marked depletion of cerebellar granule cells (> 80% of the total) when administered in a single oral dose of 750 or 250 mg/kg/day for 3 days. The nature of the L-CPA-induced neurotoxicity is currently unknown but it exhibits a number of features in common with excitatory amino acid-induced neuronal cell death. We observed an increase in [125I]endothelin-1 (ET-1) binding in the cerebellar cortex, as measured by quantitative receptor autoradiography, which occurs at 48 h, but not 24 h, following the 750 mg/kg L-CPA dosing regimen. The increase in [125I]ET-1 binding did not parallel the cellular damage and resultant astrocyte proliferation, as measured by GFAP immunoreactivity, which was primarily confined to the granular layer of the cerebellar cortex. The increased [125I]ET-1 binding occurred in the molecular layer of the cerebellar cortex (controls 2.03 +/- 0.26 fmol/mg tissue; L-CPA-treated 6.69 +/- 0.45 fmol/mg tissue, n = 6; p < 0.01, Student's t-test) which appeared to contain astrocytic processes originating from the large increase in astrocyte number situated in the granular layer. Pretreatment of the rats with the irreversible NMDA receptor antagonist, MK-801, protected the cerebellar granule cells against the L-CPA neurotoxicity and also prevented the increase in [125I]ET-1 binding in the cerebellar cortex. The increased [125I]ET-1 binding in rat cerebellum appears to be linked to the reactive gliosis that occurs in association with neuronal cell injury.

Reactive astrocytes in viral infections of the human brain express endothelin-like immunoreactivity

In order to investigate the expression of endothelin-like immunoreactivity in astrocytes of viral infections of the human brain the avidin-biotin peroxidase complex method and a polyclonal antiserum were used. Autopsy material was obtained from 5 cases of herpes simplex encephalitis, two of progressive multifocal leukoencephalopathy (PML) and two of subacute sclerosing panencephalitis (SSPE). All the 5 herpes simplex encephalitis cases presented groups of immunoreactive astrocytes around necrotic, inflammatory lesions. The PML cases exhibited a large number of immunoreactive astrocytes in and around lesions of the white matter. The cases of SSPE disclosed numerous, markedly stained fibrillary immunoreactive astrocytes; they were most abundant in degenerated regions of the white matter. The processes and peripheral cytoplasm of giant astrocytes in the PML cases contained immunoreactive material but the perinuclear region was devoid of such material. In the herpes simplex and the SSPE cases immunoreactivity was present throughout the cytoplasm and processes of reactive fibrillary astrocytes. Many nerve cells in the cerebral cortex, hippocampus, cerebellum and pons of control cases exhibited endothelin-like immunoreactivity but this occurred in only exceptional astrocytes of control cases. Endothelin-like immunoreactivity was not present in the oligodendrocytes and vascular endothelial cells of controls and cases of virus infection. The expression of endothelin-like immunoreactivity in astrocytes in human viral diseases reflects probably an increased intracellular content of endothelin. If this peptide is released from such astrocytes, it may act as a mitogen and by inducing constriction of arterioles it may influence the microcirculation.

Cerebral microvascular responses to endothelins: the role of ETA receptors

The regulatory role of endothelins in cerebral microvessels was investigated in a recently developed model system which allows the study of small cerebral vessels in their normal microenvironment. Using brain slices of the rat neocortex, it was shown that the isopeptide endothelin-3 (ET-3) had no effect on cerebral microvessels, while the isopeptide endothelin-1 (ET-1) produced a potent, dose-dependent vasoconstriction. When a recently developed antagonist of ETA receptors (cyclo-[D-Asp-L-Pro-D-Val-L-Leu-D-Trp]; ETant) was administered prior to treatment with ET-1, the vasoconstrictor response to ET-1 was inhibited in a dose-dependent manner. When ETant was administered after the establishment of a constriction by ET-1, the constrictor response to ET-1 was partially reversed, and this effect was weaker than that seen in the pre-treatment paradigm. These findings indicate that constrictor responses to ET-1 in cerebral microvessels are mediated by ETA receptors. Inasmuch as endothelins have been implicated in pathological forms of vasoconstriction in the CNS, the present findings also suggest that endothelin antagonists may be useful in the treatment of cerebral ischemia.

Receptor-mediated regulation of neuropeptide gene expression in astrocytes

One of the functions of glial receptors is to regulate synthesis and release of a variety of neuropeptides and growth factor peptides, which in turn act on neurons or other glia. Because of the potential importance of these interactions in injured brain, we have examined the role of two different receptors in the regulation of astrocyte neuropeptide synthesis. Stimulation of beta-adrenergic receptors on type 1 astrocytes resulted in increased mRNA and protein for the proenkephalin (PE) and somatostatin genes. This receptor also increased expression of nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF). The potential role of opiate receptors was examined in several ways. Treatment of newborn rats for 7 days with the opiate antagonist naltrexone, prior to preparation of astrocytes, had no effect on PE mRNA or met-enkephalin content but resulted in a significant increase in NGF content. However, treatment of astrocytes in culture with met-enkephalin, morphine, or naltrexone had no effect on any of these parameters. No opiate binding could be detected, using either etorphine or bremazocine, to membranes of astrocytes prepared from cortex, cerebellum, striatum, or hippocampus of 1-day, 7-day, or 14-day postnatal rats. Thus we conclude that type 1 astrocytes do not express opiate receptors and that the in vivo effects of naltrexone are mediated indirectly via some other cell type/receptor.

Endothelin levels increase in rat focal and global ischemia

Endothelin-1, a peptide exhibiting extremely potent cerebral vasoactive properties, is elevated in the cerebrospinal fluid after hemorrhagic stroke and implicated in cerebral vasospasm. The purpose of this study was to determine changes in endothelin in ischemic rat brain by assaying endothelin tissue and extracellular levels. Immunoreactive endothelin levels in ischemic brain tissue following permanent or transient focal ischemia produced by middle cerebral artery occlusion was determined. In addition, endothelin levels were assayed in striatal extracellular fluid collected by microdialysis before, during, and after global ischemia produced by two-vessel occlusion combined with hypotension. Twenty-four hours after the onset of permanent middle cerebral artery occlusion, the ischemic cortex level (0.58 +/- 0.27 fmol/mg protein) of immunoreactive endothelin was significantly (p < 0.05) increased, by 100%, over that in the nonischemic cortex (0.29 +/- 0.13 fmol/mg protein). Transient artery occlusion for 80 min with reperfusion for 24 h also resulted in a similar significant (p < 0.05) increase, 78%, in immunoreactive endothelin in the ischemic zone. Global forebrain ischemia significantly (p < 0.05) increased the level of immunoreactive endothelin collected in striatal microdialysis perfusate, from a basal level of 14.6 +/- 6.7 to 26.5 +/- 7.7 and 26.2 +/- 7.4 amol/microliters (i.e. 82 and 79%). These changes reflect the relative picomolar extracellular concentration increases during ischemia and following reperfusion, respectively. This is the first demonstration of elevated levels of endothelin in focal ischemic tissue and in the extracellular fluid in global ischemia and suggests a role of the peptide in ischemic and postischemic derangements of cerebral vascular function and tissue injury.

Astrocytes in Alzheimer's disease express immunoreactivity to the vaso-constrictor endothelin-1

The avidin-biotin peroxidase complex method and a polyclonal antiserum were used to investigate the distribution of endothelin-1-like immunoreactivity of cerebral astrocytes in autopsy cases of Alzheimer's disease compared with controls. The cases of Alzheimer's disease presented numerous astrocytes with intense endothelin-1-like immunoreactivity of the cell body often extending into the finest ramifications of the cell processes. Absorption of the antiserum by the corresponding antigen eliminated this immunostaining. The immunoreactive astrocytes were most consistently present in the subcortical white matter of the cerebral hemispheres and the folia of the cerebellum. The immunoreactive cells were often located in small clusters close to blood vessels. Five of the seven cases showed immunoreactive astrocytes in the molecular layer of the cerebral cortex and three of the seven cases presented regions in which immunoreactive astrocytes appeared to be located in the periphery of plaques. The pons contained small groups of immunoreactive astrocytes in five of the cases. The cerebellum had such cells in six of the seven investigated patients. Immunoreactive astrocytes were very rare in control cases without cerebral disease. Many nerve cells in the cerebral neocortex, hippocampus, cerebellum and pons of Alzheimer cases and controls exhibited endothelin-1-like immunoreactivity. Oligodendrocytes and endothelial cells of blood vessels of controls and Alzheimer cases did not show such immunoreactivity. The expression of endothelin-1-like immunoreactivity in astrocytes of Alzheimer's disease probably reflects an increased content of endothelin-1. If endothelin-1 is released from such astrocytes it may reach smooth muscle cells of the intracerebral blood vessels and disturb micro-circulation since this compound is a most powerful vasoconstrictor peptide.

Endothelin-1-induced reductions in cerebral blood flow: dose dependency, time course, and neuropathological consequences

The capacity of endothelin-1 to induce severe reductions in cerebral blood flow and ischaemic neuronal damage was assessed in anaesthetised rats. Endothelin-1 (25 microliters of 10(-7)-10(-4) M) was applied to the adventitial surface of an exposed middle cerebral artery and striatal blood flow assessed by the hydrogen clearance technique. Endothelin-1 induced severe dose-dependent reductions in cerebral blood flow (e.g., minimum CBF at 10(-5) M of 9 +/- 11 ml 100 g-1 min-1 compared to 104 +/- 22 ml 100 g-1 min-1 with vehicle, p < 0.05), which persisted for at least 60 min at each concentration of endothelin-1. Application of endothelin-1 to the middle cerebral artery produced dose-dependent ischaemic brain damage (e.g., volume of damage of 65 +/- 34 mm3 at 10(-5) M compared to 0.22 +/- 0.57 mm3 for vehicle, p < 0.01). These data demonstrate that endothelin-1 is capable of reducing blood flow to pathologically low levels and provide a new model of controlled focal ischaemia followed by reperfusion.

Increased production of endothelins in the hippocampus of stroke-prone spontaneously hypertensive rats following transient forebrain ischemia: histochemical evidence

1. The effect of transient forebrain ischemia on endothelin-1 (ET-1) and endothelin-3 (ET-3) production in the hippocampus of stroke-prone spontaneously hypertensive rats (SHRSPs) was investigated using immunohistochemical techniques. 2. In SHRSPs subjected to 10-min bilateral carotid occlusion, neuronal degeneration in the CA1 pyramidal cell layer of the hippocampus was detectable at 4 days and remarkable at 7 days after reperfusion. 3. Coinciding with neuronal degeneration, ET-1- and ET-3-like immunoreactivities were intense in the CA1 pyramidal-cell layer, the stratum lacunosum moleculare, and the CA4 subfield of the hippocampus. Almost all of the immunostained cells had morphological characteristics of astrocytes. 4. The possibility that ET has a role in the development of neuronal cell death following transient forebrain ischemia warrants further attention.

Endothelin antagonizes the hypotension and potentiates the hypertension induced by clonidine

Modification of clonidine-induced cardiovascular effects by endothelin-1 (ET-1) was studied in male Sprague-Dawley rats. A dose-dependent decrease in blood pressure and heart rate was produced by clonidine (100, 250 and 500 micrograms/kg i.v.). Lower doses produced only a fall in blood pressure (through central alpha-adrenoceptors) while higher doses of clonidine produced an initial hypertensive response (through peripheral alpha-adrenoceptors) and subsequent longer lasting hypotension and bradycardia. The hypotension and bradycardia induced by 100 and 250 micrograms/kg i.v. dose of clonidine were completely blocked by ET-1 (100 ng/kg i.v.) pretreatment. Conversely, the hypertensive response induced by high dose of clonidine (500 micrograms/kg i.v.) was significantly potentiated by ET-1 pretreatment. In cervical sectioned rats, i.v. administered clonidine failed to produce any hypotensive effect, indicating lack of central effect of clonidine. ET-1 significantly (P < 0.0005) potentiated the hypertensive response of a low dose (50 micrograms/kg i.v.) of clonidine in cervical-sectioned rats. I.c.v. administration of clonidine (1, 2, 4 and 6 micrograms) produced a dose-dependent decrease in blood pressure and heart rate. ET-1 pretreatment (25 ng i.c.v.) transiently blocked the clonidine-induced decrease in blood pressure and heart rate for about 10 min but the hypotension and bradycardia was observed subsequently. Since the major site of action of clonidine is the ventral surface of medulla, clonidine was applied directly to the ventral surface of medulla and produced a decrease in blood pressure and heart rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative neurotoxic potential of glutamate, endothelins, and plateletactivating factor in cerebral cortical cultures

The excitatory amino acid glutamate, the peptides endothelin-1 and -3, and the phospholipid platelet-activating factor have been implicated in ischemic injury to the nervous system. To determine if, like glutamate, endothelins and platelet-activating factor are directly toxic to neurons, we examined their effects on lactate dehydrogenase release and trypan blue dye exclusion in rat cerebral cortical cultures. Glutamate (1 mM) increased lactate dehydrogenase release by approximately 75% and reduced the number of cells excluding trypan blue dye by approximately 50%. In contrast, endothelins (0.5 and 100 nM) and platelet-activating factor (0.1 and 10 microM) had no effect on these indices of cell injury. Endothelins and platelet-activating factor appear more likely to act on blood vessels than on neurons or glia as mediators of ischemic injury.

Involvement of local ischemia in endothelin-1 induced lesions of the neostriatum of the anaesthetized rat

The present study examines the possibility that lesions induced by intrastriatal injections of endothelin-1 (ET-1, 0.43 nmol/0.5 microliter) are ischemic in nature due to a vasoconstriction of the cerebral microvessels. In time course and dose-response experiments with ET-1 and in comparisons with ET-3, the volume of the lesions has been determined based mainly on the disappearance of striatal nerve cells, using a computer assisted morphometrical analysis. The blood flow in the neostriatum close to the site of injection of ET-1 was determined acutely by Laser-Doppler flowmetry. The acute metabolic effects of ET-1 were also studied on striatal superfusate levels of lactate, pyruvate, dopamine and its metabolites DOPAC (3,4-dihydroxyphenylacetic acid) and homovanillic acid (HVA) using an intrastriatal microdialysis probe. Dose related striatal lesions were observed with ET-1 (0.043-0.43 nmol) with a peak lesion volume after 24-48 h and the possible existence of a penumbra area. ET-3 showed a reduced potency to produce striatal lesions compared to ET-1. The lesions induced by ET-1 were prevented by coinjection with dihydralazine, a vasodilator drug. Acutely ET-1 (0.43 nmol/0.5 microliter) produced a prolonged reduction of the cerebral blood flow down to 40% of control values and temporary increases of striatal lactate and DA efflux, the latter change being very marked. Also a significant reduction of DOPAC and HVA was observed. These neurochemical changes were all prevented by treatment with dihydralazine.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurotransmitters as Neurotrophic Factors: a New Set of Functions

At the start of this review, factors were deemed trophic if they stimulated mitosis, permitted neural cell survival, promoted neurite sprouting and growth cone motility, or turned on a specific neuronal phenotype. The in vitro evidence from cell cultures is overwhelming that both neurotransmitters and neuropeptides can have such actions. Furthermore, the same chemical can exert several of these effects, either on the same or on different cell populations. Perhaps the most striking example is that of VIP, which can stimulate not only mitosis, but also survival and neurite sprouting of sympathetic ganglion neuroblasts (Pincus et al., 1990a,b). The in vivo data to support the in vitro experiments are starting to appear. A role for VIP in neurodevelopment is supported by in vivo studies that show behavioral deficits produced in neonatal rats by treatment with a VIP antagonist (Hill et al., 1991). The work of Shatz' laboratory (Chun et al., 1987; Ghosh et al., 1990) suggests that neuropeptide-containing neurons, transiently present, serve as guideposts for thalamocortical axons coming in to innervate specific cortical areas. Along similar lines, Wolff et al. (1979) demonstrated gamma-aminobutyric acid-accumulating glia in embryonic cortex that appeared to form axoglial synapses and suggested the possibility that gamma-aminobutyric acid released from the glia might play a role in synaptogenesis by increasing the number of postsynaptic thickenings. Meshul et al. (1987) have provided evidence that astrocytes can regulate synaptic density in the developing cerebellum. The work of Zagon and McLaughlin (1986a,b, 1987) has shown that naltrexone, an antagonist of the endogenous opioid peptides, affects both cell number and neuronal sprouting. Lauder's laboratory (Lauder et al., 1982) has shown a role for 5-HT in regulation of the proliferation of numerous cell types. These studies illustrate another important point, that neurotransmitters and neuropeptides function in communication not only between neurons, but also between neurons and glial cells, and between glial cells. Given that astrocytes can express virtually all of the neural receptors and can produce at least some of the neurotransmitters and neuropeptides, they must now be considered equal partners in the processes of intercellular communication in the nervous system, including the trophic responses. The actions of neurotransmitters and neuropeptides have to be considered in terms of a broad spectrum of actions that range from the trophic actions described in this review, to the classic transmitter actions, to potential roles in neurotoxicity.(ABSTRACT TRUNCATED AT 400 WORDS)

Ontogeny of endothelin and its receptors in rat brain

The ontogeny of endothelin (ET) system in rats was studied in preterm (18 days of gestation), term (21 days of gestation) and 1 week post term rats. Brains were dissected out and (1) processed for the estimation of endogenous ET-1 by RIA and (2) membranes were prepared for radioreceptor binding. Receptor characteristics, affinity (Kd) and density (Bmax) were determined using [125I] ET-1 and [125I] SRT 6b (which is structurally similar to ET) and cold ET-1 or SRT 6b as displacer. ET levels were found to be 25.66 +/- 3.18 pg/g protein in preterm, 47.37 +/- 5.31 pg/g protein in term and 48.30 +/- 1.90 pg/g protein in post term rats. ET levels were significantly lower in preterm as compared to term and post term rats. Preterm, term and post term rats showed single high affinity binding site for both [125I] ET-1 and [125I] SRT 6b. The Kd values for [125I] ET-1 and [125I] SRT 6b binding were similar in preterm, term and post term rats. The Bmax values of both [125I] ET-1 and [125I] SRT 6b binding were found to be similar in preterm and term rats while they were significantly higher in post term rats. In adult (4 month old) rats the Kd values were similar to neonatal rats while the Bmax values were significantly lower than the post term neonatal rats. It is concluded that ET and its receptors are developmentally regulated and there is a possibility that endogenous ET is involved in the regulation of ET receptor density.

Presence of endothelin-1 and endothelin-3 in peripheral tissues and central nervous system of the pig

The distribution of endothelin (ET) peptides in the pig was studied in a variety of tissues using selective radioimmunoassays combined with reverse-phase high performance liquid chromatography (HPLC). The levels of ET-like immunoreactivity (LI) were overall relatively low. The highest levels of ET-LI were found in blood vessels, cerebral and coronary arteries containing 3190 +/- 910 and 1330 +/- 450 fmol/g, respectively. Veins generally contained higher levels of ET-LI per tissue weight than corresponding arteries. Peripheral sympathetic and sensory ganglia contained a higher concentration of ET-LI than the studied central nervous system (CNS) areas. In the CNS the highest concentration of ET-LI was found in a non-neuronal structure, the choroid plexus. The levels of ET-LI were also relatively high in the respiratory tract (100-400 fmol/g). In the heart, the endocardium contained the highest levels (190 +/- 44 fmol/g). In the kidney, the concentration of ET-LI was 3-fold higher in the medulla than in the cortex. In the gastrointestinal tract all levels were below 100 fmol/g, except for the colon which contained 120 +/- 50 fmol/g. The characterization of ET-LI in extracts of some of these tissues revealed that ET-1 dominated in the lung, spleen and hypothalamus while ET-3 and ET-1 were present in approximately equal amounts in renal medulla and thoracic spinal cord. The HPLC analysis provided no clear-cut evidence for significant presence of vasoactive intestinal contractor, ET-2 or big ET-1(1-39) in the lung, spleen, kidney, spinal cord or hypothalamus. It is concluded that mature ET-1 and ET-3 are the predominant ET peptides in peripheral tissues and CNS.

Binding sites for 125I ET-1, ET-2, ET-3 and vasoactive intestinal contractor are present in adult rat brain and neurone-enriched primary cultures of embryonic brain cells

Binding sites for iodinated endothelin (ET)-2, ET-3 and vasoactive intestinal contractor (VIC) were visualised in the adult rat brain using quantitative autoradiography and have a similar anatomical distribution to that of ET-1 and sarafotoxin S6b. Highest densities of binding sites for all 5 labelled peptides were present in the granular layer of the cerebellum. Cross-competition experiments show that at a concentration of 1 microM, unlabelled ET-1, ET-2, ET-3, VIC and sarafotoxin S6b were able to compete for the binding sites detected by each of the iodinated peptides. Binding sites for the ET isoforms were also present after 7-14 days in vitro in neurone-enriched primary cultures derived from embryonic rat cerebellum (16-18 days gestation) in which more than 90% of cells stained with an anti-neurofilament antibody. Using micro-autoradiography to detect the binding sites, an average of 14% of cells in these cultures with a diameter of 9.2 +/- 0.6 microns were associated with high silver grain densities (greater than 400 grains/100 microns). With some of these cells, silver grains were localised over cell bodies and branching processes characteristic of a neuronal phenotype. A second group of cells with high grain densities were more difficult to classify using morphological criteria and may be non-neuronal. The density of silver grains over the remaining cells was low (less than 20 grains/100 microns 2) and was similar to that measured in nuclear emulsion overlying cultures used to assess non-specific binding. These results indicate that binding sites for all ET peptides are present in both adult rat brain and embryonic cerebellar cultures.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelin as a putative sensory neuropeptide in the guinea-pig: different properties in comparison with calcitonin gene-related peptide

Both endothelin-(ET) and calcitonin gene-related peptide- (CGRP) like immunoreactivity (-LI) were present in a variety of organs and neuronal tissue of the guinea-pig as determined by radioimmunoassay (RIA). Neuronal tissues like dorsal root ganglia (DRG) contained by far the highest levels of both ET- (65 +/- 11 pmol/g) and CGRP-LI (34 +/- 5 pmol/g). The tissue levels of ET-LI remained unchanged after 6-hydroxydopamine and capsaicin-pretreatment, while CGRP-LI was markedly reduced after capsaicin. Chromatographic characterization revealed that the main portion of ET-LI in the DRG, right atrium and lung corresponded to synthetic ET-1. Immunohistochemical studies showed the presence of ET-LI in a few neurons of intact DRG and many neurons in DRG cell-cultures, partly co-existing with CGRP-LI. In the neuronal cells of DRG cultures the ratio between the ET- and CGRP-LI was 1:27 compared to 2:1 in intact DRG. 24 h after ligation of the sciatic or vagal nerves no accumulation of ET-LI was observed above the ligation, while CGRP-LI was increased 4-5-fold. Transection (10 days) of the sciatic nerve caused a 85-95% depletion of CGRP-LI in the distal skin, gastrocnemius muscle and trunk below the transection site, while in the proximal portion of the nerve CGRP-LI increased. No effects on ET-LI in these tissues were observed after sciatic nerve transfection. In release experiments on DRG cell cultures. Langendorff heart preparations or perfused guinea-pig lungs, potassium (60 mM), capsaicin or antidromic nerve stimulation evoked a clear-cut increase in the supernatant levels of CGRP-LI, suggesting release, while no effect on the ET-LI concentration was observed in the effluent. Furthermore, anoxia failed to influence the outflow of ET-LI from the heart and lung. It is concluded that ET-1-LI is present in high levels in spinal ganglia and ET-LI occurs in afferent cell-bodies, but in comparison with CGRP, ET shows remarkable inertness upon various experimental conditions including no evidence for axonal transport, loss after denervation or release. The neuronal ET-LI seems to increase under culture conditions, however. The possible function for the high content of ET-LI in the intact guinea-pig peripheral nervous system remains to be elucidated and may mainly be related to a non-neuronal pool considering the relatively low content of ET-LI compared to CGRP in cultured DRG cells.

Endothelin induces a sustained rise in intracellular calcium in hippocampal astrocytes

We report that the endothelins, a newly described family of vasoactive peptides, have a profound effect on intracellular calcium levels of cultured rat hippocampal astrocytes that resembles the effect of endothelin (ET) on vascular smooth muscle cells (VSMCs) in many respects. The astrocyte's response has two components that can be distinguished by their extracellular calcium requirement and time course. Within seconds of application, ET induces a transient calcium spike that corresponds to a release of calcium from internal stores. The second component follows immediately, is dependent upon extracellular calcium, and maintains an elevated intracellular calcium level for many minutes. Sustained elevations of intracellular calcium can dramatically alter astrocyte morphology and induce cell division in many other cell types. ET may serve these functions, and thus form a communication link between blood vessels and neurons through astrocytes.

Demonstration and nature of endothelin-3-like immunoreactivity in somatostatin and choline acetyltransferase-immunoreactive nerve cells of the neostriatum of the rat

Using a rabbit endothelin-1 (ET-1) antiserum together with a goat-anticholineacetylase antiserum or a mouse anti-somatostatin antiserum it was possible by means of double immunolabelling procedures to demonstrate ET-like immunoreactivity in striatal nerve cell bodies of the rat, which were shown to contain either cholineacetylase or somatostatin immunoreactivity. Absorption studies with ET-3, ET-1 or big ET-1 indicated that the ET-like immunoreactivity was ET-3 like. In agreement the radioimmunoassay showed that ET-3-like immunoreactivity was present in higher concentrations than ET-1-like immunoreactivity in the neostriatum and other brain areas. Characterization by reversed phase HPLC revealed, however, that a major portion of the neostriatal ET-3-like immunoreactivity was not identical to ET-3. The nature of neuronal ET in the rat may thus be more complicated than hitherto assumed.

Endothelin receptor binding and cellular signal transduction in neurohybrid NG108-15 cells

Endothelins are a novel group of potent vasoconstrictor peptides originally isolated from cultured porcine endothelial cells. We and others have previously reported the presence of endothelin receptors in the central nervous system, and this study was designed to further characterize endothelin receptors and their transduction mechanism in cultured neurohybrid NG108-15 cells. Specific binding of [125I]endothelin-1 to NG108-15 cells reached saturation within 60 min at 22 degrees C and was only partially reversible. Scatchard analysis of the saturation binding revealed the presence of one class of high-affinity binding sites with an apparent dissociation constant of 160 pM and a maximal binding capacity of 3.3 x 10(4) sites/cell. Unlabeled endothelin analogues competitively inhibited [125I]endothelin-1 binding to NG108-15 cells and the apparent dissociation constant values obtained from the competition curves correlated well with the EC50 values obtained for inducing elevation of intracellular free Ca2+ level. Endothelin stimulated phosphoinositide metabolism in a dose-dependent manner with an EC50 value of 5.4 nM for inositol trisphosphate formation. The protein kinase C-activator phorbol ester dose-dependently inhibited endothelin-induced phosphoinositide turnover and intracellular free Ca2+ increase, suggesting the involvement of protein kinase C in the regulation of endothelin-induced responses. Neither endothelin-induced phosphoinositide hydrolysis nor endothelin-induced increase in intracellular free Ca2+ were affected by pertussis toxin. These data indicate that endothelin receptors are present on NG108-15 cells and the G protein coupled to endothelin receptor for inducing activation of phospholipase C and increase of free intracellular Ca2+ is insensitive to pertussis toxin.

Topographical localisation of endothelin mRNA and peptide immunoreactivity in neurones of the human brain

The distribution of endothelin mRNA and immunoreactivity in the human brain was investigated using the technique of in situ hybridization and immunocytochemistry. Cryostat sections from 22 cases of neurologically normal adult human brain, collected 3-7 h post-mortem were hybridized with 35S-labelled complementary (c)RNA probes prepared from the 3' non-coding region of endothelin-1 cDNA, and the chromosomal genes encoding endothelin-2 and -3. In situ hybridization with all three cRNA probes revealed labelled neuronal cell bodies in laminae III-VI of the parietal, temporal and frontal cortices. Labelled cells were also seen, scattered throughout the para- and periventricular, supraoptic and lateral hypothalamic nuclei, the caudate nucleus, amygdala, hippocampus, basal nucleus of Meynert, substantia nigra, raphe nuclei, Purkinje cell layer of the cerebellum and in the dorsal motor nuclei of the vagus of the medulla oblongata. The distribution of neurones immunoreactive to endothelin was similar to that of endothelin mRNA, although fewer immunoreactive cells throughout the brain, were noted. Immunoreactive fibres were present mainly in the cortex and hypothalamus, and to a lesser extent in the brain stem. Combined in situ hybridization and immunocytochemistry on the same section revealed the presence of endothelin-1 mRNA and immunoreactivity in the same cortical neuronal cell. Colocalisation studies in the cortex revealed endothelin-1 mRNA and immunoreactivity in a number of cells which also expressed neuropeptide Y mRNA and immunoreactivity. In the hypothalamus and basal nucleus of Meynert endothelin immunoreactivity was colocalised to a subset of neurophysin- and galanin-immunoreactive cell bodies respectively. Endothelin mRNA and immunoreactivity was also seen in some blood vessel endothelial cells. The findings of endothelin mRNAs and immunoreactivity in heterogenous neuronal populations further emphasises the potential role of endothelin as a neuropeptide, probably having diverse actions in the nervous system of man.

Effect of endothelins on cytosolic free calcium concentration in neuroblastoma NG108-15 and NCB-20 cells

The effects of endothelin(ET)-1, ET-2, ET-3 and Big ET on intracellular level of Ca2+ ([Ca2+]i) were studied in neuroblastoma NG108-15 and NCB-20 cells. All ETs, except Big ET, induced an increase in [CA2+]i in NG108-15 cells in a dose-dependent manner, with EC50: 6.7, 11.2 and 71 nM, respectively. However, none of the ET increased [Ca2+]i in NCB-20 cells. Calcium channel blockers diltiazem or nicardipine had no effect on ET-induced increase in [CA2+]i, but extracellular Ca2(+)-depletion significantly reduced the response of NG108-15 cells to ETs. NG108-15 cells exhibited a homologous desensitization to sequential addition of ETs, but no heterologous desensitization among ET, bradykinin and PAF was observed. These data suggest that ET-induced receptor activation results in increased intracellular Ca2+ via a non voltage calcium channel mechanism and intracellular Ca2+ release.