cerebral ischemia. Stroke 5: 1-7, 1974. 3. Hoff JT, Smith A, Hankinson H, et al: Barbiturate protection from cerebral infarction in primates. Stroke 6: 28-33, 1975. 4. Black KL, Weidler DJ .... minate the ischemic episode. The frozen brain was .....
Jun 19, 2008 - In both situations, the brain localizes the self based on four different lines of sensory ... I thank Antonio Damasio for his mentorship and support. .... One problem with this view is that it goes against what we know about the RHI.
Fast-learning VIEWNET architectures for recognizing three-dimensional objects from multiple two-dimensional views. Neural Networks 8, 1053â1080. Brunel, N.
to examine brain anatomy and function in health and disease, and the ... is complex but coherent, and moves beyond older ideas of ..... (shown with overlying cortex removed); MTG = middle temporal gyrus; PL = parietal lobe;. PT = planum temporale; ST
sensory perception. Processing of the sensory informa- et al., 1993). In addition, it has been demonstrated in tion requires the coding of sensory inputs into specific ... stimuli are presented to the distant olfactory sensory thousand glomeruli; fis
Feb 9, 2012 - (e.g., how to build a chassis). In this analogy, the middle line workers are abstracted away from the job description of the early line workers.
shown in Figure 7-1. Consider how formidable this insect's development is. First, the egg must develop a body, including a nervous system. This nervous system has to ..... One reason may be that, when neurogenesis has stopped, it is very hard to star
Movements. Investigating Neural Control of Skilled Movements. The Control of Skilled Movements in Other Species. How Motor Cortex Damage Affects Skilled .... including playing the piano, painting pictures, and playing basketball, would require .... S
Mar 16, 2016 - nition in IT (section 3), and we conclude by discussing key open directions (section 4). 1. What Is Object .... is a domain where biological visual systems excel, it is experi- mentally tractable, and it engages the crux ... Considerin
Based on âRateâ hypothesis of Parkinson's disease, i.e. motor symptoms occur as a result of excessive activity in the STN and GPi. â¢ Deep brain stimulation ...
Procedure 200 [49, 50] ). The functional ... measuring and their linkage to public procedure. He sub- ...... Brain Res Brain Res Rev 2003;43:224â230. 94 Wicker ...
Sarah-Jayne BlakemoreCA and Uta Frith. Institute of ...... Some features of autism, such as stereotyped movements ..... Imitation of facial and manual gestures by.
Sep 25, 2013 - III-V of the MEC contain cells responsive to the animal's head ... The spatial fields of MEC grid cells can rotate when salient external cues are ...
âThe more that you read, the more things you will know. The more that you learn, the more places you'll go.â Dr Seuss. Questions?Missing:
you remember what you were doing at the time? How did you feel when you ... found that flashbulb memories can sometimes be extraordinarily accurate and ... Collective memories are often relevant to the citizens of a particular nation ..... senses we
individual and social activities, are considered instrumental to human interaction ..... During the practice trials, incorrect answers were queried and participants ...
[email protected][email protected] Human Language Technology Center. HKUST. Department of Computer Science and Engineering. University of Science and Technology, Clear Water Bay, Hong Kong. Abstract ..... fined by their surface form. This again .... d
Photograph of the body patterns for typical territorial and nonterritorial males. .... tested the importance of the orientation of the eyebar relative to the body and ...
This sensoryâcognitive interac- tion is a reminder that the auditory system may be the conduit to the brain, but it is the brain that 'hears' . The connection between hearing acuity and cognition, however, may go beyond the effects of competitio
Apr 14, 2011 - Principle â The brain develops most rapidly and is most vulnerable in early life, by age ... support system in order to counter effect the relational.
Dec 11, 2003 - lower cash flow. Given that jet fuel costs are hedgeable, airlines with a desire for expansion may find value in hedging future purchases of jet fuel. Our results ... In this paper, we study the dual questions of âdoes hedging add va
Feb 12, 2016 - HIV-associated nephropathy (HIVAN) was the most common cause of kidney disease in HIV.  However, certain components of cART can ...
Studies in animals showed that stress results in damage to the hippocampus, a brain .... and reversible effects on memory and cognition that are at least partially ..... find evidence for hippocampal atrophy in depression in patients with current ...
Visual performance of the vertebrate eye requires large amounts of oxygen, and thus the retina is one of the highest oxygen-consuming tissues of the body. Here we show that neuroglobin, a neuron-specific respiratory protein distantly related to hemog
SCIENCE IN RENAL MEDICINE
How Does the Brain Sense Osmolality? Joseph G. Verbalis Professor of Medicine and Physiology, Georgetown University School of Medicine, Washington, DC
ABSTRACT For nearly 60 years, we have known that the brain plays a pivotal role in regulating the osmolality of body fluids. Over this time period, scientists have determined the structure and function of arginine vasopressin and its receptors, the role of the posterior pituitary as a storage site, and the determinants of vasopressin release. The cellular mechanisms by which the kidney responds to vasopressin are also well understood. One area that remains unclear is the neural mechanisms underlying osmoreception. New findings have implicated the TRPV family of cation channels as osmo-mechanoreceptors that may mediate the neuronal responses to changes in systemic tonicity. This topic is reviewed here. J Am Soc Nephrol 18: 3056 –3059, 2007. doi: 10.1681/ASN.2007070825
Body fluid homeostasis is directed at maintaining the stability of the osmolality of body fluids (osmotic homeostasis) and the intravascular blood volume (volume homeostasis). Osmotic regulation serves to minimize osmotically induced perturbations in cell volume, which has adverse effects on multiple cellular functions. Body fluid osmolality in humans is maintained between 280 and 295 mOsm/kg H2O, representing one of the most highly regulated parameters of body physiology. This is accomplished through an integration of thirst, arginine vasopressin (AVP) secretion, and renal responsiveness to AVP. To preserve plasma osmolality within such narrow tolerances, pituitary AVP secretion must vary in response to small changes in plasma osmolality, which is achieved through the activation or inhibition of central osmoreceptor cells. Understanding where and how the brain senses the osmolality of body fluids and transduces this information into mechanisms that regulate AVP secretion and thirst is the subject of this commentary. 3056
ISSN : 1046-6673/1812-3056
WHERE ARE OSMORECEPTORS LOCATED?
The pioneering investigations of Verney in the 1940s1 found infusion of hyperosmotic solutions into blood vessels that perfused the anterior hypothalamus produced an antidiuresis in dogs, thereby identifying this area as the site of osmoresponsive elements in the brain. The most parsimonious explanation for these findings would be that the AVP-secreting magnocellular neurons themselves are the osmoreceptors. Although these neurons do display osmoreceptive characteristics,2 their location inside the blood– brain barrier does not position them to respond quickly to small changes in osmolality in the circulation. Subsequent studies strongly implicated the circumventricular organ named the organum vasculosum of the lamina terminalis (OVLT), which lacks a blood– brain barrier, as well as areas of the adjacent hypothalamus near the anterior wall of the third cerebral ventricle as the site of the principle brain osmoreceptors. Destruction of this area of the brain abolishes both AVP secretion and thirst re-
Figure 1. Brain osmoreceptor pathways. The primary brain osmoreceptors lie outside the blood– brain barrier in the OVLT. Different neural projections connect the primary osmoreceptors to brain areas responsible for AVP secretion and thirst. Whether the same (A) or different subsets (B) of osmoreceptors project to both areas is presently unknown. Although osmoreceptors can both stimulate as well as inhibit AVP secretion and thirst in response to systemic hyper-and hypotonicity, respectively, it is also not known whether there are separate subsets of excitatory and inhibitory osmoreceptor cells, or whether this is a property of single osmoreceptive cells.
techniques to detect early gene products in rats, which serve as markers of cell activation after dehydration. Intense expression of the cFos protein in and around the OVLT confirms this area is strongly activated by induced dehydration, and retrograde tracing studies verify that a subset of the activated neurons send projections to the magnocellular AVP neurons in the hypothalamus.6 Although many of the neural pathways connecting the OVLT and other circumventricular organs with the magnocellular AVP-secreting cells in the SON and PVN have been identified, the neural circuits in the forebrain that stimulate thirst after osmoreceptor activation are still largely unknown. Recent studies using functional magnetic resonance imaging in humans have shown that the anterior cingulate area of the cortex is reliably activated in conjunction with the sensation of thirst.7 Although data from lesion studies in both animals and man support the concept of a single group of osmoreceptive neurons that control both AVP J Am Soc Nephrol 18: 3056 –3059, 2007
secretion and thirst, this has not been definitively confirmed. Separate but parallel pathways for these complementary functions remain possible (Figure 1B) and could account for the lower osmotic threshold for activation of AVP secretion compared with thirst.8
WHAT DO BRAIN OSMORECEPTORS RESPOND TO?
Plasma Vasopressin (pg/mL)
SCIENCE IN RENAL MEDICINE
7 6 5
urea 4 3 2 1
Neither AVP secretion nor thirst is equally sensitive to all plasma solutes. Sodium and its anions, which normally contribute ⬎95% of the osmotic pressure of plasma, are the most potent solutes in terms of their capacity to stimulate AVP secretion and thirst, although some sugars such as mannitol and sucrose are also equally effective when infused intravenously.8 In contrast, increases in plasma osmolality caused by solutes such as urea or glucose cause little or no increase in plasma AVP levels in humans or animals (Figure 2).8,9 These differences in response to various plasma solutes are independent of any recognized nonosmotic influence, which indicates they are an intrinsic property of the osmoregulatory mechanism itself. Thus, it is clear that osmoreceptor cells in the brain primarily respond to plasma tonicity rather than to total plasma osmolality. The physiological relevance of this finding is that osmoreceptors function primarily to preserve cell volume; elevations of solutes such as urea, unlike elevations of sodium, do not cause cellular dehydration and consequently do not activate the mechanisms that defend body fluid homeostasis by preserving or increasing body water stores.
WHAT ARE THE CELLULAR MECHANISMS UNDERLYING OSMORECEPTION?
“Effective” solutes are those that penetrate cells slowly, or not at all, thereby creating an osmotic gradient that causes an efflux of water from osmoreceptor cells. The resultant shrinkage of osmo-
glucose 0 285
Plasma Osmolality (mOsm/kg H2O)
Figure 2. Solute specificity of brain osmoreceptors. The lines represent the relationship of plasma AVP to plasma osmolality in healthy adults during intravenous infusion of hypertonic solutions of different solutes. Note that effective solutes, i.e., those compartmentalized to the extracellular fluid (NaCl and mannitol), are much more effective at eliciting AVP secretion than the noneffective solutes, urea and glucose, that distribute across cell membranes into the intracellular fluid as well (adapted from Zerbe and Robertson GL.8)
sensitive neurons has been found to activate membrane nonselective cationic conductances that generate inward current; if of sufficient magnitude, the resulting depolarization of the osmoreceptor neuron then produces an action potential.10 Conversely, “ineffective” solutes that penetrate cells readily create no osmotic gradient and thus have little to no effect on the cell volume of the osmoreceptors. Electrophysiological studies of neurons in the OVLT show they display changes in action potential firing rate that vary in proportion to the tonicity of extracellular fluid, supporting the likelihood that these cells represent osmosensory neurons.5 Osmotically evoked changes in the firing rate of the OVLT neurons in turn synaptically regulate the electrical activity of downstream effector neurons, importantly including the magnocellular AVP neurons in the SON and PVN, through graded changes in release of the excitatory neurotransmitter glutamate. This mechanism agrees well with How Does the Brain Sense Osmolality?
SCIENCE IN RENAL MEDICINE
the observed relationship between the effect of specific solutes such as sodium, mannitol, and glucose on AVP secretion (Figure 2). The presumption that the cell volume of the osmoreceptor cells represents the primary signaling event by which osmoreceptors detect changes in the tonicity of the extracellular fluid raises some interesting dilemmas. First, most cells in the body regulate their volume to prevent or minimize the detrimental effects of cell swelling or shrinkage on cellular functions. However, if osmoreceptors displayed volume-regulatory increases or decreases in response to changes in extracellular tonicity, this would not allow for an absolute plasma osmolality around which body fluid homeostasis is maintained; that is, chronic hyperosmolality would not elicit sustained stimuli to AVP secretion and thirst. Results using OVLT neurons in shortterm dispersed cultures indeed suggest that these cells do not volume-regulate, consistent with their putative function as the primary brain osmoreceptors.11 Whether this is also true after longer periods of sustained changes in tonicity has not been studied. Second, in response to chronic changes in tonicity, the magnocellular AVP neurons undergo effects opposite of those expected. These neurons enlarge in response to chronic hypertonicity12 and shrink in response to chronic hypotonicity.13 This is postulated to be a result of changes in cell synthetic machinery; upregulation of the many proteins required for increased AVP synthesis during chronic hypertonicity causes cell hypertrophy, and downregulation of these proteins during chronic hypotonicity produces the opposite effects. Thus, the true determinant of osmoreceptor activity must be the degree of stretch of the osmoreceptor cell membrane, with subsequent effects on stretch-activated or stretch-inactivated channels, rather than the absolute size of the neurons.10 In this sense, osmoreceptors function as mechanoreceptors that detect the degree of membrane stretch at the cellular level, similar to 3058
the function of baroreceptors at the vascular level. The cellular osmosensing mechanism utilized by the OVLT cells is an intrinsic depolarizing receptor potential, which these cells generate through a molecular transduction complex. Recent results suggest this likely includes members of the transient receptor potential vanilloid (TRPV) family of cation channel proteins. These channels are generally activated by cell membrane stretch to cause a nonselective conductance of cations, with a preference for Ca2⫹. Multiple studies have characterized various members of the TRPV family as cellular mechanoreceptors in different tissues.14 Both in vitro and in vivo studies of the TRPV family of cation channel proteins provides evidence supporting roles for TRPV1, TRPV2, and TRPV4 proteins in the transduction of osmotic stimuli in mammals.15 An N-terminal trpv1 variant is expressed in OVLT cells, and trpv1null mice have defects in osmotically stimulated AVP secretion and thirst.5 Heterologous expression of the trpv2 gene in Chinese hamster ovary (CHO) cells causes an activation of Ca2⫹ influx in response to hypotonicity, a response that can be mimicked by cell membrane stretch.15 trpv4-Transfected cells respond similarly to hypotonicity and mechanical stretch, and they display deficient volume-regulatory decreases in response to hypoosmolality.16 But in vivo studies have yielded inconsistent findings. trpv4-Null mice have a potentiated AVP response to a combined hypertonic and hypovolemic stimulus in one study17 but blunted responses of both AVP secretion and thirst to a selective hypertonic stimulus in another.18 These findings are not necessarily contradictory because both AVP secretion and thirst are likely under bimodal control; that is, they are stimulated by hypertonicity and inhibited by hypotonicity.19 In support of this possibility, treatment with desmopressin leads to hyponatremia in trpv4null mice but not wild-type controls, indicating a failure of osmotic inhibition of drinking.18 Thus, different channels and/or different sets of osmoreceptor cells may mediate opposite responses to
Journal of the American Society of Nephrology
cell membrane stretch, although osmosensitive inhibitory neurons have not yet been identified in the OVLT.5 The combined studies to date therefore strongly support the characterization of TRPV1, TRPV2, and TRPV4 as osmomechano-TRPs.15 However, despite the very promising nature of these findings, several dilemmas are evident with regard to their involvement in brain osmoreception. First, it is striking that animals with gene deletions of individual members of the TRPV family manifest blunted AVP secretion and thirst but have a normal basal plasma osmolality. These results stand in marked contrast to animals with lesions that destroy the OVLT and surrounding hypothalamus, in which osmotically stimulated AVP secretion and thirst are virtually abolished, leading to chronically elevated plasma osmolality. This raises the likelihood that different ion channels, or possibly combinations of subunits from different channels, mediate osmoresponsivity in the brain and compensate for the absence of individual ion channels. Second, it is surprising that all of the TRPV channels appear to be activated by membrane stretch, including the cell swelling induced by extracellular hypotonicity, whereas in vitro studies of putative OVLT osmoreceptors have indicated that the mechanism responsible for hyperosmolar activation of these cells is activation of a stretch-inactivated cationic conductance that responds to cell shrinkage.10 These and other questions remain to be answered before we fully understand brain osmoreceptors and how they function. UNRESOLVED QUESTIONS
Although the details of exactly how and where various members of the TRPV family of cation channel proteins participate in osmoregulation in different species remains to be ascertained by additional studies, a strong case can be made for their involvement in the transduction of osmotic stimuli in the neural cells in the OVLT and surrounding hypothalamus that regulate osmotic homeostasis, which appears to be highly conserved J Am Soc Nephrol 18: 3056 –3059, 2007
throughout evolution.15 Future studies will be necessary to address still unanswered questions, including the exact structure of the molecular transduction complex that regulates the opening of cationic channel(s) in response to changes in tonicity; whether different heteromultimeric combinations of TRPV1, TRPV2, and TRPV4 and possibly other cationic channels, mediate differential responses to changes in tonicity; whether separate excitatory and inhibitory osmoreceptors control AVP secretion and thirst; and the potential involvement of the TRPV family of ion channels in the responses of different tissues to changes in tonicity, particularly the kidney and the vasculature. But with promising candidate cells and gene products now clearly identified, answers to these questions should be forthcoming. DISCLOSURES
REFERENCES 9. 1. Verney EB: The antidiuretic hormone and the factors which determine its release. Proc R Soc London (Ser B) 136: 25–106, 1947 2. Mason WT, Hatton GI, Kato M, Bicknell RJ: Signal transduction in the neurohypophyseal
J Am Soc Nephrol 18: 3056 –3059, 2007
compartments. Prog Brain Res 92: 267–276, 1992 Johnson AK, Thunhorst RL: The neuroendocrinology of thirst and salt appetite: Visceral sensory signals and mechanisms of central integration. Front Neuroendocrinol 18: 292– 353, 1997 Baylis PH, Thompson CJ: Osmoregulation of vasopressin secretion and thirst in health and disease. Clin Endocrinol (Oxf) 29: 549 – 576, 1988 Bourque CW, Ciura S, Trudel E, Stachniak TJ, Sharif-Naeini R: Neurophysiological characterization of mammalian osmosensitive neurones. Exp Physiol 92: 499–505, 2007 McKinley MJ, Hards DK, Oldfield BJ: Identification of neural pathways activated in dehydrated rats by means of Fos-immunohistochemistry and neural tracing. Brain Res 653: 305–314, 1994 Egan G, Silk T, Zamarripa F, Williams J, Federico P, Cunnington R, Carabott L, BlairWest J, Shade R, McKinley M, Farrell M, Lancaster J, Jackson G, Fox P, Denton D: Neural correlates of the emergence of consciousness of thirst. Proc Natl Acad Sci U S A 100: 15241–15246, 2003 Zerbe RL, Robertson GL: Osmoregulation of thirst and vasopressin secretion in human subjects: effect of various solutes. Am J Physiol 244: E607–E614, 1983 Thrasher TN: Osmoreceptor mediation of thirst and vasopressin secretion in the dog. Fed Proc 41: 2528 –2532, 1982 Bourque CW, Voisin DL, Chakfe Y: Stretchinactivated cation channels: Cellular targets for modulation of osmosensitivity in supraop-
SCIENCE IN RENAL MEDICINE
tic neurons. Prog Brain Res 139: 85–94, 2002 11. Oliet SH, Bourque CW: Mechanosensitive channels transduce osmosensitivity in supraoptic neurons. Nature 364: 341–343, 1993 12. Armstrong WE, Gregory WA, Hatton GI: Nucleolar proliferation and cell size changes in rat supraoptic neurons following osmotic and volemic challenges. Brain Res Bull 2: 7–14, 1977 13. Zhang B, Glasgow E, Murase T, Verbalis JG, Gainer H: Chronic hypoosmolality induces a selective decrease in magnocellular neurone soma and nuclear size in the rat hypothalamic supraoptic nucleus. J Neuroendocrinol 13: 29 –36, 2001 14. Liedtke W, Kim C: Functionality of the TRPV subfamily of TRP ion channels: add mechano-TRP and osmo-TRP to the lexicon! Cell Mol Life Sci 62: 2985–3001, 2005 15. Liedtke W: Role of TRPV ion channels in sensory transduction of osmotic stimuli in mammals. Exp Physiol 92: 507–512, 2007 16. Becker D, Blase C, Bereiter-Hahn J, Jendrach M: TRPV4 exhibits a functional role in cell-volume regulation. J Cell Sci 118: 2435– 2440, 2005 17. Mizuno A, Matsumoto N, Imai M, Suzuki M: Impaired osmotic sensation in mice lacking TRPV4. Am J Physiol Cell Physiol 285: C96 – C101, 2003 18. Liedtke W, Friedman JM: Abnormal osmotic regulation in trpv4⫺/⫺ mice. Proc Natl Acad Sci U S A 100: 13698 –13703, 2003 19. Verbalis JG: Osmotic inhibition of neurohypophysial secretion. Ann N Y Acad Sci 689: 146 –160, 1993