An astrocyte bridge from synapse to blood flow

news and viewsAn astrocyte bridge from synapse toblood flowRheinallt Parri and Vincenzo Crunelli© 2003 Nature Publishing Group http://www.nature.com/natureneuroscienceNeural activity locally increases cerebral blood flow to deliver oxygen to working neurons. Anew paper shows that astrocytes can detect activity at glutamatergic synapses via calciumchanges in astrocytic endfeet and directly couple it to blood flow in vitro and in vivo.When neurons are active, blood flow andglucose uptake in that local brain regionincrease, supplying oxygen and energy tomeet the enhanced local metabolicdemand 1,2 . This tight physiological couplingbetween neuronal activity and bloodflow has fascinated neuroscientists for morethan a century 3 . Its understanding has takenon greater importance in recent years, aschanges in blood oxygenation (resultingfrom local vasodilation and increased bloodflow) form the basis for techniques such asfunctional magnetic resonance imaging(f MRI) that are widely used to map bothnormal and pathological brain function 1 .There are currently three hypotheses, notmutually exclusive, of how this increasedblood flow might occur. First, locally secretedsubstances such as adenosine or K + ,which are formed by the enhanced neuronalactivity, act on neighboring blood vesselsto cause vasodilation 4 . Second,neuronal afferents (containing, for example,acetylcholine, monoamines or peptides)innervate the brain vasculature tocontrol vasodilation 5–7 . Third, NMDAreceptor activation on postsynaptic neuronsleads to the stimulation of nitric oxide synthase(NOS) to produce nitric oxide (NO),which diffuses to elicit vascular smoothmuscle relaxation 8 .In this issue, Zonta et al. 9 proposean elegant new mechanism for cerebralvasodilation in which astrocytes are pivotalintermediaries between neurons and brainmicrocirculation. The authors report thatsome astrocytes detect the level ofglutamate-dependent synaptic activity andthen signal to adjacent cerebral vessels tocause vasodilation. This mechanism isattractive because it relies on glutamate, themost abundant CNS excitatory transmitter,and it provides a very specific vasodilationthat is tightly linked to local synapticThe authors are at the School of Biosciences,Cardiff University, Museum Avenue, Box 911,Cardiff, CF10 3US, Wales, UK.e-mail: parri@cardiff.ac.uk;crunelli@cardiff.ac.ukactivity. Indeed, this process is consistentwith the finding that fMRI signals dependmore strongly on synaptic input to a specificbrain area than on output (actionpotentials) of neurons in that area 10 .One of the proposed functions of astrocytesis coupling of neuronal activity toincreased energy demand 11 . Glutamatetransporters on the astrocytic processes thatensheath synaptic contacts take up synapticallyreleased glutamate, which is thenmetabolized to glutamine before beingrecycled back to the neurons. Removingglutamate from the synaptic cleft helps toterminate its postsynaptic action, and itsconversion to glutamine drives glucoseuptake from local blood vessels as a sourceof energy. This is achieved through glucosetransporters that are present on other astrocyticprocesses called ‘endfeet,’ which are incontact with blood vessels.PostsynapticterminalGluRGlutamateAstrocytemGluREndfootPresynapticterminalThe results from Zonta et al. 9 now suggestthat astrocytes also provide the linkbetween neuronal activation and vasodilation.Together with transporters, the astrocyticprocesses that ensheath synapticcontacts possess a variety of neurotransmitterreceptors, including both ionotropicand metabotropic glutamate receptors(mGluRs; Fig. 1). Activation of these receptorsby synaptically released transmitters isknown to lead to transient and gradedincreases in intracellular Ca 2+ concentration;the astrocyte is thus able to ‘sense’ thelevel of synaptic activity 12 . Now, by showingthe coupling of synaptically releasedglutamate to Ca 2+ changes in the endfeetand to local vasodilation (Fig. 1), the Zontaet al. 9 data indicate a clear physiologicalfunction for the astrocyte’s ability to sensesynaptic activity and suggest how these cellsmay act as a unit to couple brain activityBlood vesselProstanoidreleasemGluRFig. 1. Proposed mechanism for astrocyte-mediated coupling of synaptic activity to local vasodilation.Synaptic glutamate release activates postsynaptic neuronal ionotropic glutamate receptors(GluRs) and astrocytic metabotropic receptors (mGluRs). Activation of mGluRs induces a transientincrease in intracellular Ca 2+ , which propagates throughout the astrocyte and to its endfoot.The Ca 2+ increase elicits production of arachidonic acid (AA) from phosphatidylinositol (PI), mostlikely by activation of phospholipase A 2 (PLA 2 ). The action of cyclooxygenase (COX) producesprostanoid products that cause vasodilation, and agents such as aspirin or indomethacin can blockthis COX action.PICa 2+PLA 2 ?AAAspirinCOXIndomethacinProstanoidIvelisse Roblesnature neuroscience • volume 6 no 1 • january 2003 5

news and views© 2003 Nature Publishing Group http://www.nature.com/natureneurosciencenot only to energy demands but also toblood flow requirements.To investigate the mechanisms underlyingcerebral vasodilation, Zonta et al. 9 usedacutely isolated slices of cerebral cortex inwhich the structural and functional integrityof neurons, astrocytes and blood vesselswas maintained. Electrical stimulation ofglutamatergic afferents in the slices causedneuronal excitation and dilation of bloodvessels, as expected. Calcium imaging experimentsshowed that during synaptic stimulation,calcium concentration increased inthe astrocyte processes that ensheath synapticcontacts, and these calcium increasespropagated to the astrocyte soma and oninto the astrocyte endfeet (Fig. 1). Vasodilationwas only observed when this propagationoccurred, and the calcium increasesin the astrocyte endfeet were also correlatedwith the time course of the vasodilation.Perhaps surprisingly, the authors showlittle involvement of NO in this sequence ofevents. Incubation with the NOS inhibitorL-NAME caused constriction over time,indicating a role for constitutive NOSexpression in setting the vascular tone.However, neither L-NAME nor NMDAreceptor antagonists reduced the vasodilationelicited by afferent stimulation, excludingan effect of neuronal NO production inthis mechanism.On the other hand, Zonta et al. 9 foundthat antagonists of metabotropic glutamatereceptors (mGluRs) inhibited astrocytic calciumincreases and vasodilation, indicatinga role for these astrocytic receptors inmatching synaptic activation to thevasodilatory response. Eliciting astrocyticcalcium increases with the mGluR agonisttrans-ACPD also resulted in vasodilation,indicating that mGluR activation and astrocyticcalcium increases are involved inmediating vasodilation. Direct confirmationthat the astrocytic activation was ultimatelyresponsible for the vasodilation wasachieved using patch electrodes to stimulateindividual astrocytes. Mechanical stimulationduring seal formation, ordepolarization of the patched astrocytes, ledto the dilation of the adjacent vessel.Remarkably, and crucially, the authorswere also able to confirm their resultsin vivo using a standard somatosensorystimulation protocol. Using laser Dopplerflowmetry to directly measure blood flow,they showed that the cerebral blood flowincrease observed in the contralateralsomatosensory cortex after forepaw stimulationwas markedly inhibited (66%) byintravenously injected mGluR antagonists,whereas the evoked neuronal activityrecorded in the same cortical region wasunaffected. This finding, together with thein-vitro data, suggested that astrocyte-mediatedvasodilation is the major mechanismof coupling neuronal activity to blood flowin the brain, and a major component of thesignals that are recorded in fMRI studies.What is the nature of the astrocyticsignal that mediates vasodilation? Astrocytesare known to release, in a calciumdependentmanner, a number ofcompounds that have neuroactive or neuromodulatoryactions, including ATP andglutamate 12 . Zonta et al. 9 , however, foundthat vasodilation was blocked byindomethacin and aspirin, both inhibitorsof cyclooxygenase (COX), indicating thatthe compound(s) released from the astrocyteto cause blood vessel dilation is a COXproduct. The pathway elucidated by Zontaet al. 9 , therefore, seems to be that a rise incalcium in the astrocyte activates phospholipaseA 2 to produce arachidonic acid, fromwhich COX catalyzes the production of arange of products (and their metabolites)that have vasoactive actions, includingprostaglandins and prostacyclin (Fig. 1).This study raises a number of questions.What is the identity of the releasedfactor? The prime candidate seems to bePGE 2 , as it can be released from astrocytesin a calcium-dependent manner 13 . Indeed,application of PGE 2 caused vasodilation inthe same vessels dilated by synaptic stimulation,presumably indicating the presenceof functional prostaglandinreceptors 9 . The possibility remains, however,that other factors may mediate orcontribute to the vasodilation.The site and mechanism of action of thevasodilation also remain unknown.Notably, is it a direct action on smoothmuscle cells, or are the endothelial cellsinvolved? The vasodilation could be due toactivation of second messenger pathwayssuch as soluble guanylyl cyclase, as occurswith NO, or direct activation of K + channelsto cause membrane hyperpolarization,as proposed for endothelial- derived hyperpolarizingfactor (EDHF) 14 .There are great technical difficulties inaddressing such issues, but the preparationused by Zonta et al. 9 provides promisingpossibilities. One of the hurdles in thisstudy was to simultaneously measure astrocyticcalcium and vasodilation, as well assmooth muscle or endothelial calciumchanges. The use of transgenic techniques,such as the expression of fluorescent proteinsin smooth muscle cells, would allowthe concomitant monitoring of astrocyticcalcium changes and vasodilation and amore rigorous analysis of the temporalaspects of the mechanism. In addition,measuring calcium in endothelial andmuscle cells would greatly help in determiningthe vasodilatory mechanism, especiallywith respect to EDHF, which involvescharacteristic calcium changes in endothelialand smooth muscle cells 14 .As mentioned earlier, activation ofneurotransmitter receptors on the astrocytesleads to a transient increase in intracellularcalcium. As astrocytes in differentbrain areas express different neurotransmitterreceptors 12,15 , could the processdescribed by Zonta et al. 9 be a generalmechanism for vasodilation in regionswhere glutamate is not the main transmitter?Might astrocytes also serve an integrativefunction that could explain whythey have different transmitter receptors?In addition to describing a mechanismof cerebral vasodilation and extending theknown repertoire of astrocytic functions,the findings of Zonta et al. 9 are importantfor our knowledge of astrocytic roles, asthey define a functional subtype of astrocytesthat might represent a pivotal pointin brain physiology. Identifying the cellularmediator and biochemical pathways underlyingcerebral vasodilation is essential inunderstanding brain microcirculation andwill also aid in studying pathophysiologicalconditions with a new understanding ofthe central role of astrocytes.1. Raichle, M.E. Proc. Natl. Acad. Sci. USA 95,765–772 (1998).2. Bonvento, G., Sibson, N. & Pellerin, L. TrendsNeurosci. 25, 359–364 (2002).3. Roy, C.S. & Sherrington, C. J. Physiol. (Lond.)11, 85–108 (1890).4. Faraci, F.M. & Heistad, D.D. Physiol. Rev. 78,53–97 (1998).5. Krimer, L.S., Muly, E.C., Williams, G.V. &Goldman-Rakic, P.S. Nat. Neurosci. 1, 286–289(1998).6. Reinhard, J.F. Jr., Liebmann, J.E., Schlosberg,A.J. & Moskowitz, M.A. Science 206, 85–87(1979).7. Vaucher, E. & Hamel, E. J. Neurosci. 15,7427–7441 (1995).8. Iadecola, C. Trends Neurosci. 16, 206–214(1993).9. Zonta, M. et al. Nat. Neurosci. 6, 43–50(2003).10. Logothetis, N.K., Pauls, J., Augath, M., Trinath,T. & Oeltermann, A. Nature 412, 150–157(2001).11. Pellerin, L. & Magistretti, P.J. Proc. Natl. Acad.Sci. USA 91, 10625–10629 (1994).12. Haydon, P.G. Nat. Rev. Neurosci. 2, 185–193(2001).13. Bezzi, P. et al. Nature 391, 281–285 (1998).14. Busse, R. et al. Trends Pharmacol. Sci. 23,374–380 (2002).15. Verkhratsky, A., Orkand, R.K. & Kettenmann,H. Physiol. Rev. 78, 99–141 (1998).6 nature neuroscience • volume 6 no 1 • january 2003