Search PubMed⌕ Search

Biomedical subjects

R Sellar

Publications and source records attributed to R Sellar.

At least 19 recordsLinked to original sources
PubMed2002

Interobserver variability of magnetic resonance angiography in the diagnosis of carotid stenosis--effect of observer experience.

Magnetic resonance angiography (MRA) for determining the degree of carotid stenosis prior to carotid endarterectomy is attractive because it does not have the high morbidity associated with conventional intra-arterial angiography. We assessed the interobserver variability in the estimation of the degree of stenosis amongst observers of different experience. In a prospective study, consecutive patients with transient ischaemic attacks and symptomatic tight carotid stenosis shown by Doppler ultrasound underwent conventional intra-arterial angiography and 2-D and 3-D time-of-flight MRA of the carotid bifurcations. The films of the processed MRA images were reviewed blind to other clinical and imaging data by eight observers of different levels of experience and coded for the presence and degree of stenosis. The stenosis on intra-arterial angiography was used as the reference standard. There was considerable variability between observers for estimation of the degree of stenosis on MRA. The observers' accuracy ranged from 41% (student) to 79% (experienced). From these estimations, excluding those of the student, it could be seen that up to 23% of patients who would have had an endarterectomy based on MRA should not have done so according to angiography, and up to 33% of patients who should have had an endarterectomy according to angiography would have inappropriately missed having an endarterectomy based on MRA results. Observer reliability of MRA processed images is not good, even in experienced hands. Using the source images and views of the circle of Willis might improve the accuracy, but a further study is required to assess this.

Carotid Artery, Internal↗
PubMed2002

Impact of early endovascular aneurysmal occlusion on outcome of patients in poor grade after subarachnoid haemorrhage: a prospective, consecutive study.

Patients in poor grade (WFNS IV and V) after aneurysmal subarachnoid hemorrhage (SAH) often have a bad outcome. To evaluate early GDC embolisation on such patients a prospective observational study, with comparison to a historical cohort was performed. From January 1996 to December 1998 113 patients were admitted to the Department of Clinical Neurosciences in poor grade after SAH (45 WFNS IV and 68 WFNS V). Eighty-one patients were managed actively with endovascular occlusion of the aneurysm (n = 42) where possible and delayed clipping (n = 16) where not. On an intention to treat basis, 46% had a favourable outcome (Glasgow Outcome Score IV or V) and 48% had died by 3 months. Compared to an historical cohort managed in the same unit between 1992 and 1995 (n = 62, 52% favourable outcome) these results suggest that early GDC aneurysmal occlusion has had a minimal impact on overall outcome.

Adult↗
PubMed2002

Magnetic resonance brain imaging in people with dizziness: a comparison with non-dizzy people.

BACKGROUND: Dizziness is a common symptom affecting about 30% of people over the age of 65. OBJECTIVE: To investigate the hypothesis that structural abnormalities of the brain and cervical cord are more common in dizzy than in non-dizzy subjects. METHODS: A case-control study of subjects over the age of 65 with and without dizziness: 125 dizzy subjects and 86 non-dizzy subjects were recruited from the community through articles in the local press. Magnetic resonance imaging (MRI) of the brain and neck was performed, and was read by a consultant neuroradiologist blind to the clinical details. RESULTS: All dizzy subjects and all controls had at least one structural abnormality. Cerebral atrophy was found in 86% of dizzy subjects and 85% controls (p = 1.0) At least one white matter lesion was found in 69% of dizzy subjects and 78% controls (p = 0.21). White matter lesions in the midbrain were more common in dizzy than in non-dizzy subjects (22% v 4%, p < 0.001). There were no significant differences in the prevalence of cord compression, cervical subluxation, facet joint degeneration, vertebral artery compression, or vertebral artery occlusion between dizzy and non-dizzy subjects. CONCLUSIONS: Structural abnormalities of the brain and neck are common in both dizzy and non-dizzy subjects. "Routine" MRI is unlikely to reveal a specific cause for dizziness. The observation of more frequent white matter lesions in the midbrain in dizzy subjects requires further study to determine whether small vessel changes could cause dizziness in older people.

Aged↗
PubMed2001

A novel mammalian receptor for the evolutionarily conserved type II GnRH.

Mammalian gonadotropin-releasing hormone (GnRH I: pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2) stimulates pituitary gonadotropin secretion, which in turn stimulates the gonads. Whereas a hypothalamic form of GnRH of variable structure (designated type I) had been shown to regulate reproduction through a cognate type I receptor, it has recently become evident that most vertebrates have one or two other forms of GnRH. One of these, designated type II GnRH (GnRH II: pGlu-His-Ser-His-Gly-Trp-Tyr-Pro-Gly-NH2), is conserved from fish to man and is widely distributed in the brain, suggesting important neuromodulatory functions such as regulating K+ channels and stimulating sexual arousal. We now report the cloning of a type II GnRH receptor from marmoset cDNA. The receptor has only 41% identity with the type I receptor and, unlike the type I receptor, has a carboxyl-terminal tail. The receptor is highly selective for GnRH II. As with the type I receptor, it couples to G(alpha)q/11 and also activates extracellular signal-regulated kinase (ERK1/2) but differs in activating p38 mitogen activated protein (MAP) kinase. The type II receptor is more widely distributed than the type I receptor and is expressed throughout the brain, including areas associated with sexual arousal, and in diverse non-neural and reproductive tissues, suggesting a variety of functions. Surprisingly, the type II receptor is expressed in the majority of gonadotropes. The presence of two GnRH receptors in gonadotropes, together with the differences in their signaling, suggests different roles in gonadotrope functioning.

Amino Acid Sequence↗
PubMed2000

A chicken gonadotropin-releasing hormone receptor that confers agonist activity to mammalian antagonists. Identification of D-Lys(6) in the ligand and extracellular loop two of the receptor as determinants.

Mammalian receptors for gonadotropin-releasing hormone (GnRH) have over 85% sequence homology and similar ligand selectivity. Biological studies indicated that the chicken GnRH receptor has a distinct pharmacology, and certain antagonists of mammalian GnRH receptors function as agonists. To explore the structural determinants of this, we have cloned a chicken pituitary GnRH receptor and demonstrated that it has marked differences in primary amino acid sequence (59% homology) and in its interactions with GnRH analogs. The chicken GnRH receptor had high affinity for mammalian GnRH (K(i) 4.1 +/- 1.2 nM), similar to the human receptor (K(i) 4.8 +/- 1.2 nM). But, in contrast to the human receptor, it also had high affinity for chicken GnRH ([Gln(8)]GnRH) and GnRH II ([His(5),Trp(7),Tyr(8)]GnRH) (K(i) 5.3 +/- 0.5 and 0.6 +/- 0.01 nM). Three mammalian receptor antagonists were also pure antagonists in the chicken GnRH receptor. Another three, characterized by D-Lys(6) or D-isopropyl-Lys(6) moieties, functioned as pure antagonists in the human receptor but were full or partial agonists in the chicken receptor. This suggests that the Lys side chain interacts with functional groups of the chicken GnRH receptor to stabilize it in the active conformation and that these groups are not available in the activated human GnRH receptor. Substitution of the human receptor extracellular loop two with the chicken extracellular loop two identified this domain as capable of conferring agonist activity to mammalian antagonists. Although functioning of antagonists as agonists has been shown to be species-dependent for several GPCRs, the dependence of this on an extracellular domain has not been described.

Amino Acid Sequence↗
PubMed2000

The rat gonadotropin-releasing hormone receptor internalizes via a beta-arrestin-independent, but dynamin-dependent, pathway: addition of a carboxyl-terminal tail confers beta-arrestin dependency.

This study examined the mechanism underlying the rat GnRH receptor (GnRH-R) internalization pathway by investigating the role of added/extended C-terminal tails and the effect of beta-arrestins and dynamin. The internalization of the wild-type (WT) rat GnRH-R, stop codon mutants, GnRH-R/TRH receptor (TRH-R) chimera, rat TRH-R, and catfish GnRH-R was examined using radioligand binding assay. Overexpression of beta-arrestin in COS-7 cells expressing each of the receptor constructs substantially increased endocytosis rate constants (k(e)) of the TRH-R, catfish GnRH-R, and GnRH-R/TRH-R chimera, but not of the WT rat GnRH-R and stop codon mutants. The beta-arrestin-promoted increase in the k(e) value was diminished by cotransfecting cells with the dominant negative beta-arrestin-(319-418) mutant, whereas WT GnRH-R and stop codon mutant internalization were unaffected. Additionally, confocal microscopy showed that activated GnRH-Rs failed to induce time-dependent redistribution of either beta-arrestin-1- or beta-arrestin-2-green fluorescent protein conjugate to the plasma membrane. However, the dominant negative dynamin (DynK44A) mutant impaired internalization of all of the receptors regardless of their beta-arrestin dependency, indicating that they internalize via a clathrin-mediated pathway. We conclude that the mammalian GnRH-R uses a beta-arrestin-independent, dynamin-dependent internalization mechanism distinct from that employed by the other receptors studied.

Amino Acid Sequence↗
PubMed1999

Lack of a C-terminal tail in the mammalian gonadotropin-releasing hormone receptor confers resistance to agonist-dependent phosphorylation and rapid desensitization.

The mammalian gonadotropin-releasing hormone receptor (GnRH-R) is, at present, the only G-protein-coupled receptor that activates phospholipase C and lacks a C-terminal tail. We have previously demonstrated that this unique structural feature is associated with resistance to rapid desensitization of phosphoinositide signaling in COS-7 and HEK-293 cells (Heding, A., Vrecl, M., Bogerd, J., McGregor, A., Sellar, R., Taylor, P. L., and Eidne, K. A. (1998) J. Biol. Chem. 273, 11472-11477). Using receptors tagged with a nonapeptide of the influenza hemagglutinin protein to enable immunoprecipitation, we now demonstrate that the mammalian GnRH-R is not phosphorylated in an agonist-dependent manner. In contrast, the mammalian thyrotropin-releasing hormone receptor and the African catfish GnRH-R, both of which have a C-terminal tail, are phosphorylated in response to agonist challenge. Furthermore, chimeras of the mammalian GnRH-R with the C-terminal tail of either the mammalian thyrotropin-releasing hormone receptor or the catfish GnRH-R are also phosphorylated in an agonist-dependent manner. Only those receptors having C-terminal tails showed desensitization of phosphoinositide responses within 5-10 min of agonist challenge. We also show that the internalization of all these receptors when expressed transiently in COS-7 cells is similar. This dissociates receptor internalization from rapid desensitization and demonstrates that the lack of a C-terminal tail in the mammalian GnRH-R results in an inability of the receptor to undergo agonist-dependent phosphorylation and that this results directly in a resistance to rapid desensitization.

Animals↗
PubMed1999

Pivotal role for the cytoplasmic carboxyl-terminal tail of a nonmammalian gonadotropin-releasing hormone receptor in cell surface expression, ligand binding, and receptor phosphorylation and internalization.

The gonadotropin-releasing hormone receptor (GnRH-R) of the African catfish couples to phospholipase C and belongs to the large family of G protein-coupled receptors. We recently demonstrated that removal of the carboxyl-terminal tail (S331-Q379) from the catfish GnRH-R results in a loss of agonist binding; the current study sought to define more precisely the role of this region in receptor function. Progressive truncations of the carboxyl-terminal tail decreased cell surface expression detected by either enzyme-linked immunosorbent assay or agonist-binding. The two most truncated receptors (stop331 and stop337) showed no binding but were detected at the cell surface by enzyme-linked immunosorbent assay. All receptors able to bind agonist were also able to activate phospholipase C. The catfish GnRH-R was phosphorylated after agonist-occupation and use of truncated mutants showed this phosphorylation to be within the carboxyl-terminal tail. Furthermore, studies with S356A, S363A and SS356,363AA mutant receptors demonstrated that Ser363 is a major site of agonist-induced phosphorylation. The absence of this phospho-acceptor site markedly impaired agonist-mediated receptor internalization. In addition, both, Ser363 and the last 12 residues of the tail (not containing Ser363) were shown to be important for beta-arrestin-dependent internalization. These observations are relevant to the regulatory function of the carboxyl-terminal tail of G protein-coupled receptors in general and are particularly intriguing given the absence of this region in mammalian GnRH-Rs.

Animals↗
PubMed1999

Isolation and characterisation of the marmoset gonadotrophin releasing hormone receptor: Ser(140) of the DRS motif is substituted by Phe.

In order to facilitate the understanding of gonadotrophin-releasing hormone (GnRH) agonist and antagonist action in the primate animal model, the marmoset GnRH receptor (GnRH-R) was cloned and characterised. It was shown to have 95% and 85% sequence identity with the human and rat GnRH-Rs, respectively, and, when transiently expressed in COS-7 cells, it exhibited high-affinity des-Gly(10), [d-Trp(6)]-GnRH binding, with a K(d) value similar to those of both the rat and human forms, but with a greatly reduced B(max) value. The ED(50) for production of GnRH-induced total inositol phosphate (IP) for the marmoset GnRH-R was also similar to those of the rat and the human, but the maximal response compared with the rat receptor was markedly reduced. In all mammalian forms of the GnRH-R cloned to date, the conserved DRY region of G-protein-coupled receptors is substituted with DRS. The most interesting feature of the marmoset GnRH-R was the substitution of this motif with DRF. In order to investigate the DRS to DRF substitution, a Ser(140)Phe rat GnRH-R mutant was generated. The mutant had a K(d) value similar to that of the wild-type rat receptor, although the B(max) value was slightly lower, indicating that expression of functional mutant receptor at the cell surface was reduced. The ED(50) value for IP production was also similar to that of the wild-type receptor, with a reduction in maximal response. The level of internalisation for the rat wild-type and mutant GnRH-R constructs was also assessed and the Ser(140)Phe mutant was shown to have an increased rate of receptor internalisation, suggesting a role for this residue in regulating internalisation. These results show that the marmoset GnRH-R exhibits a substitution in the DRS motif and that this substitution may play a part in desensitisation and internalisation events.

Amino Acid Sequence↗
PubMed1998

Gonadotropin-releasing hormone receptors with intracellular carboxyl-terminal tails undergo acute desensitization of total inositol phosphate production and exhibit accelerated internalization kinetics.

The mammalian gonadotropin-releasing hormone receptor (GnRH-R) is the only G-protein-coupled receptor (GPCR) in which the intracellular C-terminal tail is completely absent. In contrast to other GPCRs, the GnRH-R does not show rapid desensitization of total inositol (IP) production, and the rates of internalization are exceptionally slow. We investigated whether the incorporation of a cytoplasmic tail into the C terminus of the GnRH-R affects desensitization events and receptor internalization rates. A GnRH-R/TRH-R chimera was created where the intracellular tail of the rat thyrotropin-releasing hormone receptor (TRH-R) was engineered into the C terminus of the rat GnRH-R. Three different rat GnRH-R cDNA stop codon mutations (one for each reading frame) were also made. The GnRH-stimulated IP production of the wild-type rat GnRH-R expressed in either COS-7 or HEK 293 cells did not desensitize even after prolonged stimulation with GnRH. In contrast, the catfish GnRH-R (which does possess an intracellular tail) and the TRH-R rapidly (<10 min) desensitized following agonist stimulation. The GnRH-R/TRH-R chimera also desensitized following treatment with GnRH, resembling the pattern shown by the TRH-R and the catfish GnRH-R. Two of the stop codon mutants did not show desensitization of IP production, and the third mutant with the longest tail was not functional. Internalization experiments showed that the rat GnRH-R had the slowest endocytosis and recycling rates compared with the TRH-R, the catfish GnRH-R, and the chimeric GnRH/TRH-R. This study demonstrates that the addition of a functional intracellular C-terminal tail to the GnRH-R produces rapid desensitization of IP production and significantly increases internalization rates.

Amino Acid Sequence↗
PubMed1998

The effect of angiographic technique and image quality on the reproducibility of measurement of carotid stenosis and assessment of plaque surface morphology.

We studied the reproducibility of measurement of carotid stenosis and assessment of plaque surface morphology on 1001 angiograms from a consecutive series of patients entered into the European Carotid Surgery Trial. Inter-observer agreement (Kappa statistic, 95% confidence interval (CI)) for categorization of carotid stenosis, as 0-29%, 30-69% or 70-99% was good (0.68, 0.63-0.73) on 789 conventional or digitally subtracted selective angiograms, and good (0.64, 0.54-0.75) on 174 conventionally and digitally subtracted aortic arch injection angiograms, but was poor (0.29, 0.02-0.80) on 29 intravenous digital subtraction angiograms. Inter-observer agreement did not vary with the method of image acquisition of arterial angiograms, but was dependent on the quality of visualization of the stenosis: kappa = 0.73 (0.67-0.79) for good quality angiograms vs. 0.54 (0.44-0.64) for poor quality angiograms. Inter-observer agreement for assessment of plaque surface morphology was moderate (kappa 0.4-0.6) and did not vary with type of angiography or method of image acquisition. However, ulceration was reported most frequently on selective angiograms and on those angiograms on which the quality of visualization of the stenosis was good. We conclude that the reproducibility of measurement of carotid stenosis and the assessment of plaque surface morphology vary depending on the type of angiography and the quality of visualization of the stenosis. This should be taken into account when validating non-invasive methods of imaging the carotid bifurcation.

Aged↗
PubMed1998

Traumatic vertebral arterial dissection and vertebrobasilar arterial thrombosis successfully treated with endovascular thrombolysis and stenting.

A case of traumatic extracranial vertebral arterial dissection leading to vertebrobasilar thrombosis and respiratory compromise requiring mechanical ventilation was managed with intraarterial thrombolysis and stenting of the vertebral intimal dissection. In contrast to similar, previously reported cases, this critically ill patient made a full recovery, returning to his job as a secondary school teacher.

Adult↗
PubMed1996

Management of poor grade patients after subarachnoid haemorrhage: the importance of neuroradiological findings on clinical outcome.

The prognosis for patients in poor neurological grade (WFNS grades IV and V) after subarachnoid haemorrhage (SAH) is grave. Previous reports of such patients have analysed outcome without defining either the cause or the course of the depressed level of consciousness. We report a retrospective study of the outcome of 62 consecutive patients in poor grade after SAH analysed with respect to their clinical course and the predominant computed tomographic feature. Neuroradiological findings were (1) intracranial haematoma, (2) hydrocephalus with or without intraventricular haemorrhage (IVH) and (3) SAH alone. Sixteen patients (25.8%) had a Glasgow Coma Score (GCS) < or = 12 on admission to hospital and subsequently deteriorated. The predominant computed tomographic feature of these patients was hydrocephalus/IVH. Twelve patients (19.4%) had a GCS < or = 12 on admission and subsequently improved without intervention; all had SAH on computed tomography (CT) on admission. Thirty-four patients (54.8%) had a GCS < or = 12 on admission and did not improve or improved only after emergency surgical intervention. Haematoma (44%) and hydrocephalus/IVH (47%) were the predominant CT features in this group. The overall mortality in the 62 patients was 44%. Fifty-two per cent of patients achieved a good outcome or were moderately disabled. Patients harbouring an intracerebral haematoma had a significantly poorer prognosis when compared with the other groups. Patients in poor neurological grade after SAH are a heterogenous group both clinically and neuroradiologically. Management approaches must consider the cause of clinical deterioration and the related CT findings.

Adult↗
PubMed1995

Gonadotropin-releasing hormone receptor messenger ribonucleic acid expression in rat ovary.

Rat GnRH pituitary receptor complementary DNA was used to isolate a truncated clone from a rat corpus luteum complementary DNA library that proved to be identical in sequence to the rat anterior pituitary GnRH receptor. The distribution of the GnRH receptor messenger RNA (mRNA) was then determined in rat ovary using in situ hybridization. GnRH receptor expression was investigated in cyclic female rats and in hypophysectomized immature female rats treated with either recombinant human FSH or human menopausal gonadotropin. The expression of LH receptor mRNA was determined in serial sections as an index of the health and differentiation of the follicles. In cyclic animals, GnRH receptor mRNA was detected in granulosa cells at varying stages of follicular development and in the corpus luteum. Ovaries from hypophysectomized animals expressed GnRH receptor mRNA in the granulosa cells of most follicles. The administration of FSH or human menopausal gonadotropin to hypophysectomized animals altered the distribution of GnRH receptor mRNA. During antral development, the signal was most abundant in medium-sized follicles not expressing LH receptor mRNA and showing signs of follicular atresia. However, healthy preovulatory follicles also expressed GnRH receptor mRNA. We conclude that the expression of the GnRH receptor gene in granulosa cells is 1) individually regulated for each follicle, 2) persists in the corpus luteum, and 3) expressed in atretic follicles.

Animals↗
PubMed1994

Single photon emission computed tomography in long-term survivors of adult brain tumours.

Sixteen patients with primary brain tumours were examined on average eight years after treatment with surgery or whole brain irradiation using standard clinical assessment, CT, a neuropsychological test battery, and single photon emission CT (SPECT) with 99mTc-exametazime. Seventeen lesions were discovered on inspection of SPECT images, 11 with x-ray CT. Quantitative assessment of tracer uptake compared with 16 matched healthy volunteers was consistent with the presence of lesions. Measurement of uptake in brain regions of the hemisphere not containing the primary tumour still showed significant reductions in patients. This may be due to remote direct effects of the tumour or, more likely, to the whole brain irradiation received. Psychometric performance on most tests was significantly impaired in the patient group and was correlated with abnormalities of tracer uptake to relevant brain regions.

Adult↗
PubMed1994

An experimental study to evaluate the accuracy of diencephalic and pallidal target localization using the Brown-Roberts-Wells stereotactic system and unreformatted axial GE8800 CT scanning.

An experimental study was undertaken to evaluate the accuracy with which the intercommissural line (IL) length and anterior-posterior commissural (AC-PC) plane could be selected using unreformatted axial CT8800 imaging. Using these reference points, the Schaltenbrandt and Bailey stereotactic atlas and the Brown-Roberts-Wells (BRW) stereotactic system 1.82 mm ball bearings were deposited at selected diencephalic and pallidal targets in cadaver brains. There were errors in both IL length (median 1.5 mm) and AC-PC plane (median forward angulation of 9 degrees) estimation. Ball bearing deposition into large nuclei such as the pulvinar and globus pallidus and those in proximity to the mid IL such as the nuclei ventro oralis anterior (Voa), ventro oralis posterior (Vop) and ventro intermedius (Vim) was generally good; however, localization of the relatively small intralaminar nucleus was poor. This study suggests that when the AC and PC are not well imaged on sagittal reformations of axial diencephalic CT scans estimation of the AC-PC plane and IL from axial CT scans may be accurate enough for anatomical localization of certain functional stereotactic targets. The implications of this experimental study to operative functional stereotaxy are discussed.

Biopsy↗