Endothelin-sensitive areas in the ventral surface of the rat medulla

Elevated renal afferent nerve activity in a rat model of endothelin B receptor deficiency

Renal nerves have been an attractive target for interventions aimed at lowering blood pressure; however, the specific roles of renal afferent (sensory) versus efferent sympathetic nerves in mediating hypertension are poorly characterized. A number of studies have suggested that a sympathoexcitatory signal conveyed by renal afferents elicits increases in blood pressure, whereas other studies identified sympathoinhibitory afferent pathways. These sympathoinhibitory pathways have been identified as protective against salt-sensitive increases in blood pressure through endothelin B (ETB) receptor activation. We hypothesized that ETB-deficient (ETB-def) rats, which are devoid of functional ETB receptors except in adrenergic tissues, lack appropriate sympathoinhibition and have lower renal afferent nerve activity following a high-salt diet compared with transgenic controls. We found that isolated renal pelvises from high salt-fed ETB-def animals lack a response to a physiological stimulus, prostaglandin E2, compared with transgenic controls but respond equally to a noxious stimulus, capsaicin. Surprisingly, we observed elevated renal afferent nerve activity in intact ETB-def rats compared with transgenic controls under both normal- and high-salt diets. ETB-def rats have been previously shown to have heightened global sympathetic tone, and we also observed higher total renal sympathetic nerve activity in ETB-def rats compared with transgenic controls under both normal- and high-salt diets. These data indicate that ETB receptors are integral mediators of the sympathoinhibitory renal afferent reflex (renorenal reflex), and, in a genetic rat model of ETB deficiency, the preponderance of sympathoexcitatory renal afferent nerve activity prevails and may contribute to hypertension.NEW & NOTEWORTHY Here, we found that endothelin B receptors are an important contributor to renal afferent nerve responsiveness to a high-salt diet. Rats lacking endothelin B receptors have increased afferent nerve activity that is not responsive to a high-salt diet.

A new liposome-based gene delivery system targeting lung epithelial cells using endothelin antagonist

We formulated a new gene delivery system based on targeted liposomes. The efficacy of the delivery system was demonstrated in in vitro and in vivo models. The targeting moiety consists of a high-affinity 7-amino-acid peptide, covalently and evenly conjugated to the liposome surface. The targeting peptide acts as an endothelin antagonist, and accelerates liposome binding and internalization. It is devoid of other biological activity. Liposomes with high phosphatidyl serine (PS) were specially formulated to help their fusion with the endosomal membrane at low pH and enable release of the liposome payload into the cytoplasm. A DNA payload, pre-compressed by protamine, was encapsulated into the liposomes, which directed the plasmid into the cell's nucleus. Upon exposure to epithelial cells, binding of the liposomes occurred within 5-10 min, followed by facilitated internalization of the complex. Endosomal escape was complete within 30 min, followed by DNA accumulation in the nucleus 2h post-transfection. A549 lung epithelial cells transfected with plasmid encoding for GFP encapsulated in targeted liposomes expressed significantly more protein than those transfected with plasmid complexed with Lipofectamine. The intra-tracheal instillation of plasmid encoding for GFP encapsulated in targeted liposomes into rat lungs resulted in the expression of GFP in bronchioles and alveoli within 5 days. These results suggest that this delivery system has great potential in targeting genes to lungs.

Regulation of blood pressure and salt homeostasis by endothelin

Endothelin (ET) peptides and their receptors are intimately involved in the physiological control of systemic blood pressure and body Na homeostasis, exerting these effects through alterations in a host of circulating and local factors. Hormonal systems affected by ET include natriuretic peptides, aldosterone, catecholamines, and angiotensin. ET also directly regulates cardiac output, central and peripheral nervous system activity, renal Na and water excretion, systemic vascular resistance, and venous capacitance. ET regulation of these systems is often complex, sometimes involving opposing actions depending on which receptor isoform is activated, which cells are affected, and what other prevailing factors exist. A detailed understanding of this system is important; disordered regulation of the ET system is strongly associated with hypertension and dysregulated extracellular fluid volume homeostasis. In addition, ET receptor antagonists are being increasingly used for the treatment of a variety of diseases; while demonstrating benefit, these agents also have adverse effects on fluid retention that may substantially limit their clinical utility. This review provides a detailed analysis of how the ET system is involved in the control of blood pressure and Na homeostasis, focusing primarily on physiological regulation with some discussion of the role of the ET system in hypertension.

Expression of a neuropeptide, endothelin-1 in pons and medulla of prenatal and perinatal mouse brains

Endothelin-1 (ET-1), a potent vasoconstrictor, is widely distributed in the central nervous system. This article demonstrates the spatio-temporal expression of mouse preproendothelin-1 (mPPET-1) gene in pre- and perinatal mouse brain by in situ hybridization using a probe specific for mPPET-1. mPPET-1 mRNA expression was first observed in medulla at embryonic age 11.5 (E11.5) and the level became increasingly stronger toward later stages of development. At E18.5 and postnatal day 0.5 (D0.5), mPPET-1 mRNA was found in discrete nucleus group in ventrolateral medulla. mPPET-1 mRNA was also detected in thalamic reticular nucleus at E16.5, E18.5, and D0.5. These results showed that mPPET-1 mRNA is present in neurons of central cardiorespiratory region and drastically increased during the transition from episodic fetal breathing to continuous postnatal respiration (E18.5 to D0.5), implicating the important role of ET-1 in central cardiorespiratory control regulating the onset of respiration during this critical period.

L-proline microinjected into the rat ventrolateral medulla induces a depressor response distinct from L-glutamate

The neurotransmitter candidate L-proline elicits changes in the cardiovascular system via actions in the brainstem. However, its action have not yet been determined in the ventrolateral medulla (VLM), a brain region critical in mediating vasomotor sympathetic nervous system responses. Microinjections of L-glutamate produce depressor responses in the caudal (C) VLM, but pressor responses in the rostral (R) VLM and the caudal pressor area (CPA) in the far caudal CVLM. The present study tested whether microinjections of l-proline in the VLM produce a pattern of hemodynamic responses distinct from that of l- glutamate. Urethane-anesthetized rats received arterial catheters and were implanted with flow probes around the abdominal aorta (supplies hindquarters). The surface of each rat's VLM was then exposed. L-Proline induced dose- dependent depressor responses in the CVLM (0.003-1.0 M, 34 nl), but did not induce hemodynamic responses in sites of the RVLM (0.01-1.0 M, 34 nl) that responded to L-glutamate (0.01 M, 34 nl). L-Proline injections (0.1 M, 34 nl) induced rapid and consistent depressor responses correlated with coincident decreases in hindquarter resistance (arterial blood pressure/flow) in the CVLM and CPA, but only inconsistent responses in a few sites in the RVLM. In summary, L-proline induced a distinct pattern of depressor responses preferentially in caudal regions of the VLM, and these depressor effects were associated with decreases in hindquarter resistance. These findings indicate that L-proline may have unique roles including cardiovascular regulation independently from L-glutamate, especially in caudal region of the VLM, via a mechanism that involves altering hindquarter resistance.

Endothelin system: the double-edged sword in health and disease

The endothelin system consists of two G-protein-coupled receptors, three peptide ligands, and two activating peptidases. Its pharmacological complexity is reflected by the diverse expression pattern of endothelin system components, which have a variety of physiological and pathophysiological roles. In the vessels, the endothelin system has a basal vasoconstricting role and participates in the development of diseases such as hypertension, atherosclerosis, and vasospasm after subarachnoid hemorrhage. In the heart, the endothelin system affects inotropy and chronotropy, and it mediates cardiac hypertrophy and remodeling in congestive heart failure. In the lungs, the endothelin system regulates the tone of airways and blood vessels, and it is involved in the development of pulmonary hypertension. In the kidney, it controls water and sodium excretion and acid-base balance, and it participates in acute and chronic renal failure. In the brain, the endothelin system modulates cardiorespiratory centers and the release of hormones. More advanced functional analysis of the endothelin system awaits not only additional pharmacological studies using highly specific endothelin antagonists but also the generation of genetically altered rodent models with conditional loss-of-function and gain-of-function manipulations.

Role of endothelin-1 in stress response in the central nervous system

Endothelin (ET)-1 is a 21-amino acid peptide that induces a variety of biological activities, including vasoconstriction and cell proliferation, and its likely involvement in cardiovascular and other diseases has recently led to broad clinical trials of ET receptor antagonists. ET-1 is widely distributed in the central nervous system (CNS), where it is thought to regulate hormone and neurotransmitter release. Here we show that CNS responses to emotional and physical stressors are differentially affected in heterozygous ET-1-knockout mice, which exhibited diminished aggressive and autonomic responses toward intruders (emotional stressors) but responded to restraint-induced (physical) stress more intensely than wild-type mice. This suggests differing roles of ET-1 in the central pathways mediating responses to different types of stress. Hypothalamic levels of ET-1 and the catecholamine metabolite 3-methoxy-4-hydroxyphenylglycol (MHPG) were both increased in wild-type mice subjected to intruder stress, whereas MHPG levels were not significantly affected in ET-1-knockout mice. Furthermore, immunohistochemical analysis showed that ET-1 and tyrosine hydroxylase, an enzyme in the catecholamine synthesis pathway, were colocalized within certain neurons of the hypothalamus and amygdala. Our findings suggest that ET-1 modulates central coordination of stress responses in close association with catecholamine metabolism.

Endothelin in the central control of cardiovascular and respiratory functions

1. Exogenously administered endothelin (ET) modulates the activity of cardiovascular and respiratory neurons in the central nervous system (CNS) and, thus, affects arterial blood pressure (ABP) and ventilation. However, a physiological role(s) for endogenous ET in the CNS has not been elucidated. To address this question, we examined ABP and ventilation in mutant mice deficient in ET-1, ETA and ETB receptors and endothelin-converting enzyme-1, which were made by gene targeting. 2. Respiratory frequency and volume was measured in mice by whole body plethysmography when animals breathed normal room air and hypoxic and hypercapnic gas mixtures. A few days after respiratory measurements, a catheter was implanted into the femoral artery under halothane anaesthesia. On the following day, the ABP of awake mice was measured through the indwelling catheter and heart rate was calculated from the ABP signal. After 2 h ABP measurement, arterial blood was collected through the catheter and pH and the partial pressures of O2 and CO2 were measured by a blood gas analyser. 3. Compared with corresponding controls, the mean (+/- SEM) ABP in ET-1+/- and ETB-deficient mice was significantly higher (118 +/- 2 vs 106 +/- 3 mmHg for ET-1+/- (n = 22) and ET-1+/+ (n = 17) mice, respectively; 127 +/- 3 vs 109 +/- 4 mmHg for ETB-/s (n = 9) and ETB+/s (n = 9) mice, respectively; P < 0.05 for both). In ET-1+/- mice, PCO2 tended to be higher and PO2 was significantly lower than corresponding values in ET-1+/+ mice. Under resting conditions, there was no significant difference in respiratory parameters between mutants and their corresponding controls. However, reflex increases of ventilation to hypoxia and hypercapnia were significantly attenuated in ET-1+/-, ET-1-/- and ETA-/- mice. 4. In another series of experiments in ET-1+/- mice, we found that sympathetic nerve activity (SNA) was augmented and reflex excitation of phrenic nerve activity (PNA) in response to hypoxia and hypercapnia was blunted. Attenuation of the reflex PNA response to hypercapnia was also observed in the medulla-spinal cord preparation from ET-1-/- mice. 5. Elevation of ABP in ETB-deficient mice was most likely due to a peripheral mechanism, because SNA and respiratory reflexes were not different from those in control animals. 6. We conclude that endogenous ET-1 plays an important role in the central neural control of circulation and respiration and that ETA receptors mediate this mechanism.

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.

Endothelins and rat carotid body: autoradiographic and functional pharmacological studies

The effects of ET-1 and ET-3 on ventilation and carotid chemosensory discharge have been studied in rats anaesthetised with pentotarbitone. Autoradiographic studies were also performed in vitro to investigate the binding of [125I]ET-1 to rat carotid body, nodose ganglion and brain stem. ET-1 caused a dose-related hyperventilation that was abolished by cutting both carotid sinus nerves. Recordings of chemosensory discharge from the carotid sinus nerve confirmed that ET-1 caused chemoexcitation. ET-3 had only slight effects. The hyperventilation evoked by ET-1 was antagonised by the ETA receptor antagonist FR139317, but responses to hypoxia (10% oxygen) and to cyanide were unaffected. [125I]ET-1 bound to the carotid body, the nodose ganglion and to the brain stem, particularly in the region of the nucleus tractus solitarii. ET-1 binding in the carotid body was displaceable by FR139317, which is consistent with the functional evidence for ETA receptors in the carotid body. The effects of ET-1 on ventilation, coupled with the presence of ET binding sites in areas involved in respiratory and cardiovascular regulation, is consistent with a physiological role for ET in the control of respiration, but our evidence suggests that ET is not crucial for chemotransduction in acute hypoxia.

Prostaglandin D2-sensitive, sleep-promoting zone defined in the ventral surface of the rostral basal forebrain

The site of action for the sleep-promoting effect of prostaglandin (PG) D2 was extensively examined in the brain of adult male rats (n = 231). PGD2 was administered at 100 pmol/0.2 microliter per min for 6 hr (2300-0500 hr) through chronically implanted microdialysis probes or infusion cannulae. Among the administrations of PDG2 by dialysis probes (n = 176), only those (n = 8) to a ventro-rostral part of the basal forebrain by the probes implanted on the midline consistently increased slow-wave sleep (SWS), by 51 +/- 6 min (mean +/- SEM) above the baseline value (111 +/- 11 min). Since this area is separated by a cleft into right and left regions, the results were interpreted to mean that, through this cleft, PGD2 diffused in the subarachnoid space over the adjacent ventral surface, where it had the effect of promoting sleep. When PGD2 was directly infused into the subarachnoid space (n = 55), extraordinary increases exceeding 90 min were consistently attained for the SWS at sites located between 0.5 and 2 mm rostral to the bregma and between 0 and 1.2 mm lateral to the midline defined according to the stereotaxic coordinates adopted from the brain atlas of Paxinos and Watson [Paxinos, G. & Watson, C. (1986) The Rat Brain in Stereotaxic Coordinates (Academic, San Diego)]. Thus, we demarcated a "PGD2-sensitive, sleep-promoting zone" within this region in the ventral surface of the rostral basal forebrain. During the bilateral infusion of PGD2 into the subarachnoid space of this zone, the hourly mean SWS level of the nocturnal animals (n = 6) in the night reached the maximum at the second hour of the infusion period; this maximum hourly SWS level, corresponding to the daytime level of the same animals, lasted until the end of PGD2 infusion.

Elevated blood pressure and craniofacial abnormalities in mice deficient in endothelin-1

The endothelin-1 (ET-1) gene was disrupted in mouse embryonic stem cells by homologous recombination to generate mice deficient in ET-1. These ET-1-/- homozygous mice die of respiratory failure at birth and have morphological abnormalities of the pharyngeal-arch-derived craniofacial tissues and organs. ET-1+/- heterozygous mice, which produce lower levels of ET-1 than wild-type mice, develop elevated blood pressure. These results suggest that ET-1 is essential for normal mouse development and may also play a physiological role in cardiovascular homeostasis.

Respiratory suppression by focal cooling of ventral medullary surface in anesthetized rats; functional and neuroanatomical correlate

Bilateral cooling of the parapyramidal region in the rostral ventral medullary surface (VMS) elicited a reduction in respiratory frequency and phrenic inspiratory activity in halothane anesthetized rats. A distinct cluster of neurons (nucleus parapyramidalis superficialis) was found in a superficial layer (10-15 microns from the surface) just beneath the area where cooling produced suppression of respiration. The rat VMS layer contains neural substrates which regulate the respiratory rhythm generation and inspiratory neural output.

An endothelin ETA receptor antagonist, FR139317, ameliorates cerebral vasospasm in dogs

The role of endothelin in the pathogenesis of cerebral vasospasm after subarachinoid hemorrhage was investigated by evaluating the effect of FR139317, a novel potent ETA receptor antagonist, on the vasospasm in a canine two-hemorrhage model. Intracisternal administration of FR139317 (0.1 mg) significantly reduced the vasoconstriction of the basilar artery at day 7 (control group, n = 6, 61.6 +/- 4.0%, FR139317 treated group, n = 6, 75.9 +/- 1.5% of basal diameter, p < 0.01). In normal anesthetized dogs, the intracisternal administration of FR139317 did not affect the basal diameter of the basilar artery, blood pressure or heart rate. These results suggest that endothelin plays an important role in the pathogenesis of cerebral vasospasm after subarachnoid hemorrhage, and that FR139317 could be a valuable tool for preventing vasospasm after subarachnoid hemorrhage.