12 results
AN INTERCOMPARISON PROJECT ON 14C FROM SINGLE-YEAR TREE RINGS
- Sabrina G K Kudsk, Jesper Olsen, Gregory W L Hodgins, Mihály Molnár, Todd E Lange, Jessica A Nordby, A J Timothy Jull, Tamás Varga, Christoffer Karoff, Mads F Knudsen
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- Journal:
- Radiocarbon / Volume 63 / Issue 5 / October 2021
- Published online by Cambridge University Press:
- 07 September 2021, pp. 1445-1452
- Print publication:
- October 2021
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A laboratory intercomparison project was carried out on 20 annually resolved late-wood samples from the Danish oak record. The project included the following three laboratories: (1) the University of Arizona AMS Laboratory, University of Arizona, USA (AA); (2) HEKAL AMS Laboratory, MTA Atomki, Hungary (DeA); and (3) Aarhus AMS Centre (AARAMS), Aarhus University, Denmark (AAR). The large majority of individual data points (96%) lie within ±2σ of the weighted mean. Further assessment of the accuracy associated with the individual laboratories showed good agreement, indicating that consistent and reliable 14C measurements well in agreement with each other are produced at the three laboratories. However, the quoted analytical uncertainties appear to be underestimated when compared to the observed variance of differences from the geometric mean of the samples. This study provides a general quality check of the single-year tree-ring 14C measurements that are included in the new calibration curve.
Marine20—The Marine Radiocarbon Age Calibration Curve (0–55,000 cal BP)
- Part of
- Timothy J Heaton, Peter Köhler, Martin Butzin, Edouard Bard, Ron W Reimer, William E N Austin, Christopher Bronk Ramsey, Pieter M Grootes, Konrad A Hughen, Bernd Kromer, Paula J Reimer, Jess Adkins, Andrea Burke, Mea S Cook, Jesper Olsen, Luke C Skinner
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- Journal:
- Radiocarbon / Volume 62 / Issue 4 / August 2020
- Published online by Cambridge University Press:
- 12 August 2020, pp. 779-820
- Print publication:
- August 2020
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The concentration of radiocarbon (14C) differs between ocean and atmosphere. Radiocarbon determinations from samples which obtained their 14C in the marine environment therefore need a marine-specific calibration curve and cannot be calibrated directly against the atmospheric-based IntCal20 curve. This paper presents Marine20, an update to the internationally agreed marine radiocarbon age calibration curve that provides a non-polar global-average marine record of radiocarbon from 0–55 cal kBP and serves as a baseline for regional oceanic variation. Marine20 is intended for calibration of marine radiocarbon samples from non-polar regions; it is not suitable for calibration in polar regions where variability in sea ice extent, ocean upwelling and air-sea gas exchange may have caused larger changes to concentrations of marine radiocarbon. The Marine20 curve is based upon 500 simulations with an ocean/atmosphere/biosphere box-model of the global carbon cycle that has been forced by posterior realizations of our Northern Hemispheric atmospheric IntCal20 14C curve and reconstructed changes in CO2 obtained from ice core data. These forcings enable us to incorporate carbon cycle dynamics and temporal changes in the atmospheric 14C level. The box-model simulations of the global-average marine radiocarbon reservoir age are similar to those of a more complex three-dimensional ocean general circulation model. However, simplicity and speed of the box model allow us to use a Monte Carlo approach to rigorously propagate the uncertainty in both the historic concentration of atmospheric 14C and other key parameters of the carbon cycle through to our final Marine20 calibration curve. This robust propagation of uncertainty is fundamental to providing reliable precision for the radiocarbon age calibration of marine based samples. We make a first step towards deconvolving the contributions of different processes to the total uncertainty; discuss the main differences of Marine20 from the previous age calibration curve Marine13; and identify the limitations of our approach together with key areas for further work. The updated values for ΔR, the regional marine radiocarbon reservoir age corrections required to calibrate against Marine20, can be found at the data base http://calib.org/marine/.
The IntCal20 Northern Hemisphere Radiocarbon Age Calibration Curve (0–55 cal kBP)
- Part of
- Paula J Reimer, William E N Austin, Edouard Bard, Alex Bayliss, Paul G Blackwell, Christopher Bronk Ramsey, Martin Butzin, Hai Cheng, R Lawrence Edwards, Michael Friedrich, Pieter M Grootes, Thomas P Guilderson, Irka Hajdas, Timothy J Heaton, Alan G Hogg, Konrad A Hughen, Bernd Kromer, Sturt W Manning, Raimund Muscheler, Jonathan G Palmer, Charlotte Pearson, Johannes van der Plicht, Ron W Reimer, David A Richards, E Marian Scott, John R Southon, Christian S M Turney, Lukas Wacker, Florian Adolphi, Ulf Büntgen, Manuela Capano, Simon M Fahrni, Alexandra Fogtmann-Schulz, Ronny Friedrich, Peter Köhler, Sabrina Kudsk, Fusa Miyake, Jesper Olsen, Frederick Reinig, Minoru Sakamoto, Adam Sookdeo, Sahra Talamo
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- Journal:
- Radiocarbon / Volume 62 / Issue 4 / August 2020
- Published online by Cambridge University Press:
- 12 August 2020, pp. 725-757
- Print publication:
- August 2020
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Radiocarbon (14C) ages cannot provide absolutely dated chronologies for archaeological or paleoenvironmental studies directly but must be converted to calendar age equivalents using a calibration curve compensating for fluctuations in atmospheric 14C concentration. Although calibration curves are constructed from independently dated archives, they invariably require revision as new data become available and our understanding of the Earth system improves. In this volume the international 14C calibration curves for both the Northern and Southern Hemispheres, as well as for the ocean surface layer, have been updated to include a wealth of new data and extended to 55,000 cal BP. Based on tree rings, IntCal20 now extends as a fully atmospheric record to ca. 13,900 cal BP. For the older part of the timescale, IntCal20 comprises statistically integrated evidence from floating tree-ring chronologies, lacustrine and marine sediments, speleothems, and corals. We utilized improved evaluation of the timescales and location variable 14C offsets from the atmosphere (reservoir age, dead carbon fraction) for each dataset. New statistical methods have refined the structure of the calibration curves while maintaining a robust treatment of uncertainties in the 14C ages, the calendar ages and other corrections. The inclusion of modeled marine reservoir ages derived from a three-dimensional ocean circulation model has allowed us to apply more appropriate reservoir corrections to the marine 14C data rather than the previous use of constant regional offsets from the atmosphere. Here we provide an overview of the new and revised datasets and the associated methods used for the construction of the IntCal20 curve and explore potential regional offsets for tree-ring data. We discuss the main differences with respect to the previous calibration curve, IntCal13, and some of the implications for archaeology and geosciences ranging from the recent past to the time of the extinction of the Neanderthals.
Chapter 111 - Vitreoretinal surgery
- from Section 23 - Ophthalmic Surgery
- Edited by Michael F. Lubin, Emory University, Atlanta, Thomas F. Dodson, Emory University, Atlanta, Neil H. Winawer, Emory University, Atlanta
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- Medical Management of the Surgical Patient
- Published online:
- 05 September 2013
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- 15 August 2013, pp 700-701
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Summary
Vitreoretinal surgical techniques are used to approach disorders of the posterior segment of the eye. Over the past 30 years, great strides have been made in the ability to safely and effectively operate in this segment. The spectrum of disorders menable to operative intervention has broadened significantly with the evolution of advanced, smaller-gauge microsurgical instruments, computer-controlled infusion and aspiration systems, endolaser probes, perfluorocarbon heavy liquid for manipulation of detached retinal tissue, implantable slow-release pharmacological devices, wide-angle optical viewing systems, and long-acting gases and silicone oil for intraocular tamponade. The treatment of intraocular tumors with radioactive episcleral plaques has also become well-characterized and “evidence-based” through large-scale, prospective, randomized clinical trial data. The advent and sophistication of the pars plana approach with microsurgical vitrectomy instrumentation has allowed for the repair of most simple and complex primary and recurrent retinal detachments. The pars plana is the section of the eye located approximately at the junction of the iris and the sclera and is a safe place to insert intraocular instruments without damage to internal structures. However, in certain cases of primary retinal detachment, the most appropriate treatment remains scleral buckling surgery, as has been performed for over 60 years.
Scleral buckling surgery involves the placement of a strip of silicone around the outside of the globe to cause a slight indentation or buckle of the eye wall and support the intraocular retinal breaks and vitreous base. The procedure is effective because the external support helps close the causative retinal tear inside the eye. The retinal tear is repaired by a combination of support from the buckle and the formation of a chorioretinal scar induced by a thermal modality such as cryotherapy (freezing) or laser (heating). The usual procedure for addressing complex retinal detachments with very large or posteriorly located retinal tears, significant retinal scarring, vitreous hemorrhage, or severe cataract formation is to combine scleral buckle surgery with the more advanced intraocular vitrectomy techniques.
Chapter 109 - Cataract surgery
- from Section 23 - Ophthalmic Surgery
- Edited by Michael F. Lubin, Emory University, Atlanta, Thomas F. Dodson, Emory University, Atlanta, Neil H. Winawer, Emory University, Atlanta
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- Medical Management of the Surgical Patient
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- 05 September 2013
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- 15 August 2013, pp 696-697
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Summary
Cataracts are characterized by opacity of the crystalline lens of the eye. They represent the primary cause of treatable blindness in the world. Cataracts are generally categorized as congenital or age related; however, they may also result from exposure to drugs, toxins, or radiation; or be the product of various metabolic diseases. Visually significant cataracts are a major public health issue: they are found in 50% of persons 65–74 years of age and 70% of persons 75 years of age or older.
Modern cataract extraction is accompanied by insertion of an intraocular lens (IOL). It is a highly effective and efficient operation that restores visual acuity and contrast sensitivity in patients with visually significant cataracts. Presently, the operation employs a systematic, minimally invasive approach which involves creating a small (2.5–3.5 mm) wound at the edge of the cornea. The incision is carefully created in a beveled manner that minimizes leakage through the wound without sutures. A viscoelastic material is injected into the anterior chamber to protect the cornea and to maintain a working chamber in the eye for instrumentation. Next, a portion of the anterior capsule of the lens is removed to allow access to the lens cortex and nucleus, creating a circular opening in the capsular bag (capsulotomy). An ultrasonic probe (phacoemulsification tip) is then inserted through the anterior chamber and capsulotomy, into the lens. The energy generated at the tip of the probe is used to fragment and remove the cataractous lens nucleus and cortex. The remaining capsule of the lens is left intact (referred to commonly as the capsular “bag”). A custom IOL is selected, with appropriate focusing power to neutralize the refractive error. The measurements are based on the axial length of the eye as well as the corneal curvature. Most lenses are folded and inserted through the incision into the “bag,” where the lens can then unfold and rest in the location of the original native lens. After the instruments are removed, the wound is self-sealing and watertight. Occasionally, one or more sutures are required to secure the wound. Despite the highly technical aspects of cataract surgery, experienced surgeons can perform the operation in 30 minutes or less.
Chapter 112 - Glaucoma surgery
- from Section 23 - Ophthalmic Surgery
- Edited by Michael F. Lubin, Emory University, Atlanta, Thomas F. Dodson, Emory University, Atlanta, Neil H. Winawer, Emory University, Atlanta
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- Medical Management of the Surgical Patient
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- 05 September 2013
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- 15 August 2013, pp 702-703
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Glaucoma is the most common cause of optic neuropathy. Many ocular conditions may lead to the development of glaucomatous nerve damage. In general, therapeutic interventions are directed towards lowering intraocular pressure, a key risk factor for disease progression.
Typically, therapy begins with topical medications, the first and simplest option. These include the prostaglandin analogs, beta adrenergic receptor blockers, carbonic anhydrase inhibitors, alpha adrenergic agonists, and miotics. These agents are used alone or in combination, and are often sufficient to control intraocular pressure. In cases of open-angle glaucoma, laser trabeculoplasty may also be used to lower intraocular pressure. Laser interventions are performed in the clinic either alone or in combination with medical therapy. For cases of angle-closure glaucoma, laser iridotomy may be performed to either treat or prevent pupillary block (iris-lens diaphragm obstruction), an anatomic predisposition that is responsible for the majority of cases. Cyclodestructive surgery (intentional destruction of the ciliary body tissues) is another laser procedure that may be used when other interventions have failed, including incisional surgery. These procedures, which are usually performed in the clinic under local anesthesia, are commonly performed with lasers and, less commonly, cryotherapy.
Chapter 113 - Refractive surgery
- from Section 23 - Ophthalmic Surgery
- Edited by Michael F. Lubin, Emory University, Atlanta, Thomas F. Dodson, Emory University, Atlanta, Neil H. Winawer, Emory University, Atlanta
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- Medical Management of the Surgical Patient
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- 05 September 2013
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- 15 August 2013, pp 704-705
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Refractive surgery is performed to reduce dependence on glasses or contact lenses. Refractive surgical techniques reshape the cornea using incisions, heat, various forms of laser, or implantation of intraocular lenses to decrease myopia (nearsightedness), astigmatism, or hyperopia (farsightedness). The excimer laser is currently the technology of choice for keratorefractive surgeons. The laser can reshape the cornea by ablating the anterior corneal surface in procedures such as photorefractive keratectomy (PRK), and more commonly in laser-assisted in situ keratomileusis (LASIK). In LASIK, the surgeon performs corneal stromal ablation using the excimer laser directed to the corneal tissue under a thin lamellar flap. This flap is created by either a microkeratome or a laser.
In order to select the best candidates for refractive surgery, a thorough preoperative history, assessment, and complete eye exam is required. Parameters such as corneal topography, central corneal thickness, degree of refractive error, ocular surface health and patient expectations are all carefully considered in order to determine whether the individual is a good refractive surgical candidate, as well as for selecting the most appropriate procedure. Absolute contraindications to laser vision correction include diagnoses of keratoconus or ectatic corneal dystrophies. Relative systemic contraindications include poorly controlled rheumatoid arthritis and diabetes, pregnancy, and AIDS.
Chapter 115 - Enucleation, evisceration, and exenteration
- from Section 23 - Ophthalmic Surgery
- Edited by Michael F. Lubin, Emory University, Atlanta, Thomas F. Dodson, Emory University, Atlanta, Neil H. Winawer, Emory University, Atlanta
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- Medical Management of the Surgical Patient
- Published online:
- 05 September 2013
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- 15 August 2013, pp 708-710
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Removal of an eye or the contents of an orbit may be indicated when the eye is affected by neoplasia or a severe infectious process, or when an end-stage ocular disease in a blind eye causes pain. These ophthalmic interventions are usually classified as:
Enucleation: the removal of the entire globe, including the sclera, intraocular contents, and the cornea. The stump of the optic nerve as well as the extraocular muscles are left behind.
Evisceration: the removal of intraocular contents including the lens, uvea, retina, vitreous humor, and in some cases the cornea. Only the sclera and extraocular muscles remain intact.
Exenteration: the removal of the globe and all of the orbital contents. This procedure may include removal of selective sections of orbital bone.
Following enucleations and eviscerations, an orbital implant is used to replace the globe and restore the lost orbital volume. The implant or sphere serves to maintain the structure of the orbit and to provide motility to the overlying prosthesis. For children, it additionally serves to maintain more normal growth of the surrounding orbital bones. In cases of exenteration, an osseointegrated prosthesis may be attached within the orbit, secured with metal support elements or magnets that are attached to bone.
Chapter 110 - Corneal transplantation
- from Section 23 - Ophthalmic Surgery
- Edited by Michael F. Lubin, Emory University, Atlanta, Thomas F. Dodson, Emory University, Atlanta, Neil H. Winawer, Emory University, Atlanta
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- Medical Management of the Surgical Patient
- Published online:
- 05 September 2013
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- 15 August 2013, pp 698-699
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Summary
Traditional full-thickness corneal transplantation, known as penetrating keratoplasty (PK), was first performed over a century ago and remains the gold standard of corneal transplantation. Penetrating keratoplasty involves replacement of all layers of the central 7–9 mm of the host cornea with allograft tissue, usually derived from eye banks. Donor corneas are harvested from 1–2 weeks prior to transplantation. Tissue or blood typing is not routinely done. The overall optical clarity, integrity of donor tissue, and donor endothelial cell density are evaluated at the eye bank. The tissue is screened for multiple infectious diseases of the donor, a procedure commonly performed for other human allograft tissues.
Penetrating keratoplasty
Penetrating keratoplasty usually begins with the removal of the diseased central host cornea by use of various forms of trephine. Next, the donor tissue is trephinated from the donor corneal tissue and is usually slightly oversized. The donor tissue is secured in the recipient bed using either interrupted or continuous nylon sutures. When necessary, a cataract extraction may be performed in combination with intraocular lens implantation. Monitored anesthesia care in these cases usually includes brief, intravenous sedation combined with a retrobulbar block. General anesthesia may be required for selected patients unable to be cooperative, such as children or anxious adult patients. Total surgical time is around 30–45 minutes for an experienced surgeon. Corneal graft survival at 1 year is 90% for PK. Indications for PK include haze, ectatic disease, opacities in the cornea, and corneal edema causing decreased vision or pain. In addition, infections, scars, trauma, congenital dystrophies, and corneal decompensation or injury from prior intraocular surgery are also indications.
Chapter 114 - Strabismus surgery
- from Section 23 - Ophthalmic Surgery
- Edited by Michael F. Lubin, Emory University, Atlanta, Thomas F. Dodson, Emory University, Atlanta, Neil H. Winawer, Emory University, Atlanta
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- Medical Management of the Surgical Patient
- Published online:
- 05 September 2013
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- 15 August 2013, pp 706-707
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Extraocular muscle surgery is performed to correct strabismus. Strabismus includes any horizontal, vertical, or torsional misalignment of the eyes and can affect either children or adults. The disease can be categorized as congenital, acquired, restrictive, or paralytic. The goal of surgery is to restore the eyes to their normal anatomical position and to maximize the potential for binocularity. Other indications include eliminating diplopia, relieving mechanical restriction or restoring normal head position. In cases of nystagmus, surgery has the potential to improve vision. Either individual or multiple extroacular muscles may be operated upon during surgery; bilateral procedures are common. In selective cases, adjustable suture surgery may be performed.
Strabismus surgery is most commonly performed under general anesthesia. However, in selected cases, local anesthesia may be preferred. Topical anesthesia may be used for standard “muscle weakening” procedures for surgical patients who are good candidates for conscious sedation. Retrobulbar or peribulbar anesthesia may be useful for strabismus correction if strabismus correction surgery is only being performed in one eye under monitored anesthesia.
Chapter 108 - General considerations in ophthalmic surgery
- from Section 23 - Ophthalmic Surgery
- Edited by Michael F. Lubin, Emory University, Atlanta, Thomas F. Dodson, Emory University, Atlanta, Neil H. Winawer, Emory University, Atlanta
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- Medical Management of the Surgical Patient
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- 05 September 2013
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- 15 August 2013, pp 693-695
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A vast array of surgical interventions may be performed in the treatment of ocular and periorbital disease. Because of the high technical difficulty, the subspecialist often performs a significant portion of the ophthalmic surgeries. Most procedures in ophthalmology involve microsurgery and are usually limited to the eye and orbit. Thus, typically there is minimal risk to other organs. Ophthalmic surgery offers a high probability of success, with a major positive impact on quality of life. Nevertheless, many patients with eye pathology are elderly, and some have significant systemic illness. Therefore, the risk of elective intervention must be balanced against the expected benefits, and appropriate counseling should be performed prior to surgery. Optimizing the management of medical problems preoperatively can make the surgery safer and minimize patient discomfort.
Anesthesia
The large majority of ophthalmic interventions can be performed under local anesthesia with intravenous sedation. In some cases, even topical anesthetics are sufficient. But there are ophthalmic surgeries that require general anesthesia, such as those that involve significant extraocular manipulation, for which the local anesthetic may not be as effective, or those that may be prolonged, as is often the case in many vitreoretinal and orbital procedures. Some periorbital or facial cosmetic interventions often necessitate general anesthesia as well. General anesthesia is also indicated in younger patients and those who may not be cooperative enough to remain motionless during surgery. In addition, general anesthetics are required in trauma cases with significant ocular laceration, where administration of local anesthetics may raise intraorbital pressure, necessitating subsequent extrusion of intraocular contents. Several choices exist in the route of administration of local ophthalmic anesthesia for intraocular surgery. The most widely used approach is injection of 3–7 mL of a mixture of lidocaine 2% and marcaine 0.75% through a retrobulbar approach using a blunted needle (Atkinson needle). This is often performed with a regional seventh nerve block to paralyze eyelid closure. The risks of local ophthalmic anesthesia are remote, but they may be significant. They include local damage through retrobulbar hemorrhage, extraocular muscle damage, and penetration of the globe or optic nerve. Systemic exposure to the injected medication through intravascular or subarachnoid injection of the anesthetic has been known to cause hypertension, seizures, apnea, or even death.
2 - Epilepsy and movement disorders in the GABAA receptor β3 subunit knockout mouse: model of Angelman syndrome
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- By Richard W. Olsen, Department of Molecular and Medical Pharmacology, UCLA School of Medicine, Los Angeles, CA, USA, Timothy M. DeLorey, Molecular Research Institute, Mountain View, CA, USA
- Edited by Renzo Guerrini, University of London, Jean Aicardi, Hôpital Robert-Debré, Paris, Frederick Andermann, Montreal Neurological Institute & Hospital, Mark Hallett, National Institutes of Health, Baltimore
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- Epilepsy and Movement Disorders
- Published online:
- 03 May 2010
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- 13 December 2001, pp 15-28
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Summary
GABAA receptor structure and function: multiple subunit genes, and implications for epilepsy and movement disorders?
γ-Aminobutyric acid type A (GABAA) receptors (GABAR) mediate the bulk of rapid inhibitory synaptic transmission in the central nervous system (Olsen & DeLorey, 1999). The GABAR belong to the superfamily of ligand-gated ion channel receptors, i.e. they are ion channel proteins whose opening is controlled by the binding of the neurotransmitter (DeLorey & Olsen, 1992). These GABAR are a family of heteropentamers formed from a family of at least 19 related subunits in mammals, named α(1–6), β(1–4), γ(1–3), δ, ε, π, and ρ(1–3) (Tyndale et al., 1995; Davies et al., 1997; Hedblom & Kirkness, 1997). Splicing variants exist for some subunits, primarily related to phosphorylation substrates in the intracellular loop, e.g. the γ2 subunit longer version (γ2L) contains an 8 amino acid insert in the cytoplasmic loop that contains a consensus substrate site for phosphorylation by protein kinase C that is missing in γ2S (Burt & Kamatchi, 1991; McKernan & Whiting, 1996). Important CNS drug targets are present on GABAR, notably sites for the benzodiazepines, barbiturates, neurosteroids, other general anesthetics, and picrotoxin-like convulsants (Macdonald & Olsen, 1994). The individual subunits show variable regional and temporal expression. A dozen or more heteropentameric isoforms of the GABAR occur naturally with reasonable abundance; these exhibit various pharmacological properties and presumably biological properties as well (Lüddens et al., 1995; McKernan & Whiting, 1996; Barnard et al., 1998).