Fast neurotransmitter release triggered by Ca influx
through AMPA-type glutamate receptors
Andre ´s E. Cha ´vez1, Joshua H. Singer1† & Jeffrey S. Diamond1
Feedback inhibition at reciprocal synapses between A17 amacrine
the retina1–3. Glutamate-mediated excitation of A17 cells elicits
GABA (g-aminobutyric acid)-mediated inhibitory feedback onto
RBCs4–6,but the mechanisms thatunderlie GABArelease from the
dendrites of A17 cells are unknown. If, as observed at all other
synapses studied, voltage-gated calcium channels (VGCCs) couple
membrane depolarization to neurotransmitter release7, feed-
forward excitatory postsynaptic potentials could spread through
A17 dendritestoelicit‘surround’ feedbackinhibitory transmission
at neighbouring synapses. Here we show, however, that GABA
release from A17 cells in the rat retina does not depend on VGCCs
or membrane depolarization. Instead, calcium-permeable AMPA
(a-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) recep-
The AMPAR-mediated calcium signal is amplified by calcium-
induced calcium release (CICR) from intracellular calcium stores.
These results describe a fast synapse that operates independently
of VGCCs and membrane depolarization and reveal a previously
unknown form of feedback inhibition within a neural circuit.
In the mammalian retina, RBC synaptic terminals receive inhibi-
tory synapses from several types of amacrine cell8,9including GABA-
ergic A17 amacrine cells5,9–12, which receive excitatory input from
RBCs and make immediately adjacent synapses back onto the same
terminals. To investigate how A17 cells transduce RBC input into
reciprocal release of GABA, we recorded synaptic responses from
both cell types in acute retinal slices, beginning with RBCs (Fig. 1a).
Depolarization of voltage-clamped RBCs elicited sustained inward
calcium currents, upon which were superimposed transient, feed-
back inhibitory postsynaptic currents (IPSCs)5,6,13,14that were
reduced only slightly by the GABACreceptor (GABACR) antagonist
(1,2,5,6-tetrahydropyridin-4-yl) methylphosphinic acid (TPMPA,
50mM; P ¼ 0.012), but were blocked by the GABAA receptor
(GABAAR) antagonist SR95531 (10mM; P ¼ 5.3 £ 1025; Fig. 1b, d).
These voltage-step-evoked IPSCs (vIPSCs) were nearly completely
resistant to the sodium channel blocker tetrodotoxin (TTX, 0.5mM;
P ¼ 0.036; Fig. 1c, d), which eliminates feedback inhibition from
many amacrine cells15,16. TTX has been reported to exert variable
effects on reciprocal feedback in RBCs5,14, but it does not affect
voltage-gated conductancesinrat RBCsand A17cells5,17. ThevIPSCs
were abolished by 5,7-dihydroxytryptamine (DHT, 50mM;
P ¼ 0.0012; Fig. 1c, d), which kills indoleamine-accumulating cells
when taken up and oxidized by monoamine oxidase (MAO)18and
ablates A17 cells selectively when injected intravitreally several weeks
before enucleation1,3. Here, bath-applied DHT acted rapidly and
specifically (see Supplementary Fig. 1): DHT eliminated excitatory
postsynaptic currents (EPSCs) in A17 cells within 5min but barely
affected EPSCs in AII amacrine cells, which also receive synaptic
input from RBCs; DHT reduced calcium currents only slightly and
did not affect the responses of RBCs to GABA. The acute effects of
DHTon A17 EPSCs were blocked by the MAO inhibitor phenelzine
(10mM; see Supplementary Fig. 1d–f).
IPSCs were also elicited in RBCs by stimulating amacrine cell
to the inner plexiform layer (IPL), where RBCs, A17 cells and other
amacrine cells make synaptic contact (Fig. 1e, f). Glutamate-evoked
IPSCs (gIPSCs) reversed near to the calculated chloride equilibrium
potential (ECl¼ 240mV; Fig. 1f inset) and contained components
mediated by GABACRs and GABAARs that were blocked by TPMPA
(50mM) and SR95531 (10mM), respectively4(Fig. 1f, h). Glutamate
puffs in theouterplexiformlayer(OPL) elicitedno responsesinRBCs
(see Supplementary Fig. 2), indicating that gIPSCs reflected GABA
releaseintheIPL.TTXpartiallyblockedgIPSCs(P ¼ 0.0008;Fig.1g,h),
confirming that TTX-sensitive amacrine cells also provide (non-
reciprocal) inhibition to RBCs15,16. The TTX-insensitive component
DHT (P ¼ 4 £ 1025; Fig. 1g, h). DHT applied alone blocked only a
fraction of the gIPSC (P ¼ 0.002; Fig. 1h), indicating that it did not
cells. The TTX-insensitive component of the gIPSC was partially
sensitivetoTPMPA(56 ^ 6%ofcontrol,n ¼ 4,P ¼ 0.0015),consist-
ent with previous evidence that activation of A17 cells by multiple
RBCs3—or enhanced activation of A17 cells by a single RBC5,6—
recruits a GABACR component in the feedback IPSC. To isolate the
A17-mediated component of the gIPSC, all subsequent experiments
were performed in the presence of TTX, except where noted.
At reciprocal synapses in the olfactory bulb, NMDA (N-methyl-D-
aspartate) receptors (NMDARs) activated by glutamate released
from mitral cells help to trigger GABAergic feedback19. In contrast,
we found that feedback from A17 cellsto RBCs requires activation of
AMPARs but not NMDARs. Feedback IPSCs were blocked by
philanthotoxin 433 (PhTx, 1mM), a specific antagonist of calcium-
permeable AMPARs20(vIPSC: P ¼ 5 £ 1025; gIPSC: P ¼ 0.0003;
Fig. 2a, b, g). PhTx did not act by blocking nicotinic acetylcholine
receptors (nAChRs), because the nAChR antagonist d-tubocurarine
(dTC, 20mM) did not reduce vIPSCs (P ¼ 0.16; Fig. 2g). vIPSCs
also were eliminated by the AMPAR/kainate receptor antagonist
50mM; vIPSC: P ¼ 2.4 £ 1024; gIPSC: P ¼ 0.002; Fig. 2g) and the
AMPAR antagonist GYKI 53655 (25mM; vIPSC: P ¼ 8.1 £ 1025;
gIPSC: P ¼ 4.7 £ 1024; Fig. 2g), but the NMDAR antagonist 3-(2-
carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP, 10mM) did
not reduce feedback IPSCs (vIPSC: P ¼ 0.051; gIPSC: P ¼ 0.17;
Fig. 2c, d, g). EPSCs recorded in A17 cells, evoked by depolarizing a
population of ON bipolar cells (including RBCs) with puff applica-
tion of the group II/III mGluR antagonist (RS)-a-cyclopropyl-4-
phosphonophenylglycine (CPPG, 600mM, 100ms)21in the OPL,
1Synaptic Physiology Unit, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20892-3701, USA. †Present address:
Northwestern University, Departments of Ophthalmology and Physiology, 303 E. Chicago Avenue, Tarry Building, 5-727, Chicago, Illinois 60611, USA.
Vol 443|12 October 2006|doi:10.1038/nature05123
© 2006 Nature Publishing Group
were blocked by PhTx (P ¼ 0.009; Fig. 2e, g) and showed inward
permeable AMPARs. These results corroborate previous physiologi-
cal data indicating that AMPARs, not NMDARs, mediate synaptic
NMDARs contribute to reciprocal feedback onto RBCs14.
Calcium-permeable AMPARs could contribute to reciprocal
GABA release by depolarizing A17 dendrites to activate VGCCs
and/or by providing calcium directly. To distinguish between these
possibilities, recordings were made from synaptically coupled
RBC–A17 cell pairs (Fig. 3A, B). Depolarizing the RBC elicited an
EPSC in the A17 cell and quantal feedback IPSCs in the RBC
(Fig. 3A), but depolarizing the A17 cell evoked no detectable IPSC
in the RBC (n ¼ 7; Fig. 3B), indicating that membrane depolariza-
tion triggers release from RBCs but not A17 cells. Moreover, the
RBCs and abolished vIPSCs (P ¼ 9 £ 1026; Fig. 3C, I) but did not
affect gIPSCs (P ¼ 0.08; Fig. 3D, I). Similar effects on gIPSCs were
obtained with nimodipine (10mM; P ¼ 0.09; Fig. 3I) or kurtoxin
(350nM; Fig. 3I), which together inhibit voltage-activated confor-
mational changes in most subtypes of calcium channel22,23. Nimo-
dipine alone blocked calcium currents evoked by steps from 260 to
210mV in RBCs (P ¼ 0.0013; Fig. 3I). GABA release from A17 cells
does require calcium influx, because gIPSCs were reduced when
extracellular calcium was lowered from 2.5 mM to 0.5mM
(P ¼ 0.0013; Fig. 3E, I) and were abolished when calcium was
removed completely (0mM Ca, 1.5mM EGTA; P ¼ 2.4 £ 1025;
Fig. 3I). gIPSCs were also reduced after bath application of the
membrane-permeant calcium chelator BAPTA-AM (50mM;
P ¼ 0.0004; Fig. 3F, I). These results indicate that AMPARs, not
VGCCs, mediate thecalcium influx that is requiredfor GABA release
from A17 cells. In contrast, glycinergic gIPSCs in RBCs were
abolished by Cd (see Supplementary Fig. 3), indicating that other
amacrine cells use VGCCs to trigger transmitter release.
gIPSCs could appear insensitive to VGCC blockade if exogenous
glutamate activated AMPARs so strongly that consequent calcium
influxsaturated therelease machinery,renderingVGCCs unnecessary.
However, Cd did not reduce gIPSCs even in the presence of low
extracellularcalcium(94 ^ 5%oflowcalciumalone,n ¼ 5,P ¼ 0.1;
Fig. 3E), indicating that VGCCs do not contribute even when the
A17 amacrine cells. a, Diagram of experiments in which feedback
inhibition was elicited by depolarizing the RBC. A2 and A17 are amacrine
(50mM) and blocked by the GABAAR antagonist SR95531 (10mM).
c, vIPSCs are diminished slightly by TTX (0.5mM) and abolished by DHT
(50mM), which selectively ablates A17 cells. GABAR antagonists exerted no
of vIPSCs. e, Diagram of experiments in which A17 cells were activated by
exogenous application of glutamate. f, gIPSCs are sensitive to both TPMPA
(50mM) and SR95531 (10mM). Inset, current–voltage relationship of
component is blocked by DHT (50mM). h, Summarized pharmacological
profile of gIPSCs.
Figure 2 | Calcium-permeable AMPARs trigger GABA release from A17
cells. a, vIPSCs are blocked by the calcium-permeable AMPAR antagonist
PhTx (1mM). b, gIPSCs are also blocked by PhTx. c, vIPSCs are unaffected
by the NMDAR antagonist CPP (10mM). d, gIPSCs are also unaffected by
CPP. e, EPSCs recorded in A17 cells (Vhold¼ 260mV) and elicited by
depolarizing RBCs with the mGluR antagonist CPPG are reduced by
PhTx (1mM). f, Current–voltage plot indicating voltage dependence and
control response at þ60mV. g, Summarized drug effects.
© 2006 Nature Publishing Group
NATURE|Vol 443|12 October 2006
GABA release machinery is not activated maximally. Moreover,
IPSCs elicited by puffs of artificial cerebrospinal fluid (ACSF)
containing 110mM potassium (kIPSCs), which would depolarize
both RBC and A17 processes, were abolished by NBQX (P ¼ 0.002;
Fig. 3G, I). If GABA release were evoked by membrane depolariza-
Finally, direct depolarization of voltage-clamped A17 cells failed to
elicit IPSCs in synaptically coupled RBCs (Fig. 3B). EPSCs recorded
in A17 cells were reversed at positive holding potentials (Fig. 2f),
indicating that a patch electrode on an A17 cell soma could
depolarize the postsynaptic membrane potential sufficiently to
activate any VGCCs, if they were present at A17 GABA release sites.
Previous evidence indicates that GABA release from A17 cells is
vesicular: Postsynaptic processes on A17 cells contain small clear
synaptic vesicles9,24, indoleamine-accumulating (A17-like) post-
synaptic processes in the rabbit retina contain synaptic vesicle
proteins25, and vIPSCs in RBCs exhibit quantal components4–6
(Fig. 3A). In addition, gIPSCs were reduced significantly when
vacuolar proton-translocating ATPases (and, therefore, vesicular
filling) were inhibited by bath application of concanamycin A
(CmA, 10mM; P ¼ 0.004; Fig. 3H, I)26. Reversed uptake does not
contribute substantially to GABA release, as the neuronal GABA
transporter blocker NO-711 (10mM) reduced vIPSCs only slightly
(P ¼ 0.02; Fig. 3I) and enhanced gIPSCs (P ¼ 0.0008; Fig. 3I). This
latter effect probably reflected slowed removal of synaptically released
GABA, as NO-711 enhanced RBC responses to exogenous GABA to a
similar extent (135 ^ 21%, n ¼ 5, P ¼ 0.02; data not shown).
between AMPAR activation and GABA release by amplifying the
postsynaptic calcium signal27,28. Accordingly, activating the release of
receptor (RyR) agonist caffeine27(15mM, 50ms) in the IPL elicited
IPSCs in RBCs that were blocked by GABAR antagonists (P ¼ 0.002;
Fig. 4g), DHT (P ¼ 1.2 £ 1025; Fig. 4a, g), or the RyR antagonist
Figure 3 | Neither membrane depolarization nor VGCC activation
triggers GABA release from A17 cells. A, RBC depolarization elicits an
EPSC in a coupled A17 cell (Vhold¼ 270mV). Traces in a show single RBC
responses; trace in b shows a leak-subtracted (P/4) average RBC response.
B, Depolarizing the same A17 cell elicits no response in the same RBC
(Vhold¼ 0mV; 0.2mM EGTA in A17). Traces in a show single RBC
responses; trace in b shows average RBC response. C, Cd (200mM) reduces
VGCC-mediated current in RBCs and eliminates vIPSCs. D, Cd does not
affect gIPSCs. E, gIPSCs are reduced when extracellular calcium is lowered
from2.5mM(control) to 0.5mM. Cddoesnot reduce GABA releaseevenin
low extracellular calcium. F, gIPSCs are reduced by the calcium chelator
elicits a feedback IPSC (kIPSC) that is blocked completely by NBQX,
indicating that direct depolarization of A17 cells does not elicit GABA
release. H, gIPSCs are reduced when vesicle filling is inhibited with CmA.
I, Summarized drug effects.
Figure 4 | Calcium signalling in A17 amacrine cells is amplified by CICR.
a, Activation of RyRs by caffeine (15mM) elicits IPSCs in RBCs that are
blocked by DHT. b, Caffeine-evoked IPSCs are blocked by the RyR
antagonist RR (40mM). c, vIPSCs are reduced when intracellular calcium
stores are reduced by bath application of thapsigargin (1mM; thapsigargin
was also included in the patch pipette). d, Thapsigargin reduces the gIPSC.
e, RR partially reduces gIPSCs. f, Xestospongin C (Xe C, 3mM), an
Ins(1,4,5)P3receptor antagonist, does not affect glutamate-evoked IPSCs.
g, Summarized drug effects.
NATURE|Vol 443|12 October 2006
© 2006 Nature Publishing Group
ruthenium red27(RR, 40mM; P ¼ 0.0005; Fig. 4b, g). In addition, Download full-text
vIPSCs and gIPSCs were diminished when intracellular calcium
stores were reduced by bath application of 1mM thapsigargin
(vIPSC: P ¼ 0.004; gIPSC: P ¼ 0.006; Fig. 4c, d, g). Thapsigargin
did not affect the responses of RBCs to exogenously applied GABA
(100mM, 25ms; P ¼ 0.59; Fig. 4g). RR blocked the gIPSC only
partially (P ¼ 0.002; Fig. 4e, g), indicating that some of the calcium
that contributes to GABA release might arise from another source,
perhaps inositol-1,4,5-trisphosphate (Ins(1,4,5)P3)-receptor-sensitive
stores28or influx through calcium-permeable AMPARs. gIPSCs were
unaffected by the Ins(1,4,5)P3receptor antagonists xestospongin C
(3mM; P ¼ 0.06; Fig. 4f, g) or 2-APB (50mM; P ¼ 0.3; Fig. 4g),
arguing against a role for Ins(1,4,5)P3receptor-operated stores.
Thus, AMPAR-mediated calcium influx into A17 cells triggers
GABA release directly and through CICR.
Our results reveal a fast synapse at which AMPARs provide
calcium influx to trigger neurotransmitter release, and at which
release occurs independently of membrane depolarization. At most
synapses, VGCCs couple presynaptic membrane potential and neuro-
VGCCs tomediateassociativeinteractions between thesomaandthe
dendritic arborization29or,inthe olfactory bulb, totrigger reciprocal
GABA release19. Although specific physiological roles for VGCCs in
A17 cells remain unclear, they might replenish intracellular calcium
stores or trigger the release of other neurotransmitters.
If GABA release from A17 cells were triggered by VGCCs, the
spread of depolarization through the A17 dendrites could contribute
to surround inhibition byeliciting release atelectrotonically adjacent
synapses25. By using calcium-permeable AMPARs rather than
VGCCs to trigger GABA release, however, A17 dendrites might
compartmentalize reciprocal feedback to maintain synapse speci-
ficity. The rapid amplification of the signal by CICR probably occurs
in the immediate vicinity of the postsynaptic membrane and is
unlikely to compromise this specificity, particularly if, as in the
rabbit25, reciprocal synapses in rat A17 dendrites are separated by
20mm or more. Further experiments are required to determine the
spatial extent of synaptic calcium signalling in A17 dendrites.
At other reciprocal synapses in the olfactory bulb and goldfish
slower than that observed here14,19. Feedback in A17 cells might
require calcium-permeable AMPARs, which show faster kinetics
than NMDARs, to confer transience on the visual signal1–3and to
prevent rapid depletion of the readily-releasable vesicle pool in RBC
See Supplementary Information for a detailed description of methods. Retinal
slices were prepared from Sprague–Dawley rats (P17–21) as described pre-
viously6. Except where indicated, experiments were performed in solutions
supplemented with strychnine (1mM) and tetrodotoxin (TTX, 0.5mM) to
block glycine receptors and voltage-gated sodium channels, respectively.
During whole-cell recordings, RBCs and A17 cells were filled with Alexa-488
through the patch pipette and visualized by epifluorescent illumination to
confirm cell type. vIPSC amplitude was measured by fitting the last 30–50ms
the timepoint ofthe IPSCpeakand measuringthe difference (seeSupplementary
baseline before the stimulus. Unless indicated otherwise, statistical comparisons
concluded when P , 0.05. In the figures, * indicates P , 0.05, ** indicates
P , 0.01 and *** indicates P , 0.001. Data are reported as mean ^ s.d.
Received 10 May; accepted 31 July 2006.
Published online 1 October 2006.
1. Nakatsuka, K. & Hamasaki, D. I. Destruction of the indoleamine-accumulating
amacrine cells alters the ERG of rabbits. Invest. Ophthalmol. Vis. Sci. 26,
1109– -1116 (1985).
Euler, T. & Masland, R. H. Light-evoked responses of bipolar cells in a
mammalian retina. J. Neurophysiol. 83, 1817– -1829 (2000).
3.Dong, C.J.& Hare, W.A. Temporalmodulation ofscotopic visualsignalsbyA17
amacrine cells inmammalianretinainvivo. J.Neurophysiol. 89, 2159– -2166 (2003).
Hartveit, E. Membrane currents evoked by ionotropic glutamate receptor
agonists in rod bipolar cells in the rat retinal slice preparation. J. Neurophysiol.
76, 401– -422 (1996).
Hartveit, E. Reciprocal synaptic interactions between rod bipolar cells and
amacrine cells in the rat retina. J. Neurophysiol. 81, 2923– -2936 (1999).
Singer, J. H. & Diamond, J. S. Sustained Ca2þentry elicits transient postsynaptic
currents at a retinal ribbon synapse. J. Neurosci. 23, 10923– -10933 (2003).
Katz, B. & Miledi, R. The timing of calcium action during neuromuscular
transmission. J. Physiol. (Lond.) 189, 535– -544 (1967).
Kolb, H. & Nelson, R. Amacrine cells of the cat retina. Vision Res. 21, 1625– -1633
Sterling, P. & Lampson, L. A. Molecular specificity of defined types of amacrine
synapse in cat retina. J. Neurosci. 6, 1314– -1324 (1986).
10. Nelson, R. & Kolb, H. A17: a broad-field amacrine cell in the rod system of the
cat retina. J. Neurophysiol. 54, 592– -614 (1985).
11.Sandell, J. H. & Masland, R. H. A system of indoleamine-accumulating neurons
in the rabbit retina. J. Neurosci. 6, 3331– -3347 (1986).
12. Vaney, D. I. Morphological identification of serotonin-accumulating neurons in
the living retina. Science 233, 444– -446 (1986).
13. Singer, J. H., Lassova, L., Vardi, N. & Diamond, J. S. Coordinated multivesicular
release at a mammalian ribbon synapse. Nature Neurosci. 7, 826– -833 (2004).
14. Vigh, J. & von Gersdorff, H. Prolonged reciprocal signalling via NMDA and GABA
receptors at a retinal ribbon synapse. J. Neurosci. 25, 11412– -11423 (2005).
15. Bloomfield, S. A. & Xin, D. Surround inhibition of mammalian AII amacrine cells
is generated in the proximal retina. J. Physiol. (Lond.) 523, 771– -783 (2000).
16. Shields, C. R. & Lukasiewicz, P. D. Spike-dependent GABA inputs to bipolar cell
axon terminals contribute to lateral inhibition of retinal ganglion cells.
J. Neurophysiol. 89, 2449– -2458 (2003).
17. Menger, N. & Wassle, H. Morphological and physiological properties of the
A17 amacrine cell of the rat retina. Vis. Neurosci. 17, 769– -780 (2000).
18. Baumgarten, H. G. et al. Mode and mechanism of action of neurotoxic indole-
amines: a review and a progress report. Ann. NY Acad. Sci. 305, 3– -24 (1978).
19. Schoppa, N. E. & Urban, N. N. Dendritic processing within olfactory bulb
circuits. Trends Neurosci. 26, 501– -506 (2003).
20. Washburn, M. S. & Dingledine, R. Block of alpha-amino-3-hydroxy-5-methyl-4-
isoxazolepropionic acid (AMPA) receptors by polyamines and polyamine
toxins. J. Pharmacol. Exp. Ther. 278, 669– -678 (1996).
21. Nawy, S. Desensitization of the mGluR6 transduction current in tiger
salamander On bipolar cells. J. Physiol. (Lond.) 558, 137– -146 (2004).
22. Hess, P., Lansman, J. B. & Tsien, R. W. Different modes of Ca channel gating
behaviour favoured by dihydropyridine Ca agonists and antagonists. Nature 311,
538– -544 (1984).
23. Sidach, S. S. & Mintz, I. M. Kurtoxin, a gating modifier of neuronal high- and
low-threshold ca channels. J. Neurosci. 22, 2023– -2034 (2002).
24. Brandstatter, J. H., Koulen, P. & Wassle, H. Selective synaptic distribution of
kainate receptor subunits in the two plexiform layers of the rat retina.
J. Neurosci. 17, 9298– -9307 (1997).
25. Zhang, J., Li, W., Trexler, E. B. & Massey, S. C. Confocal analysis of reciprocal
feedback at rod bipolar terminals in the rabbit retina. J. Neurosci. 22,
10871– -10882 (2002).
26. Drose, S. & Altendorf, K. Bafilomycins and concanamycins as inhibitors of
V-ATPases and P-ATPases. J. Exp. Biol. 200, 1– -8 (1997).
27. Verkhratsky, A. & Petersen, O. H. The endoplasmic reticulum as an integrating
signalling organelle: from neuronal signalling to neuronal death. Eur.
J. Pharmacol. 447, 141– -154 (2002).
28. Warrier, A., Borges, S., Dalcino, D., Walters, C. & Wilson, M. Calcium from
internal stores triggers GABA release from retinal amacrine cells.
J. Neurophysiol. 94, 4196– -4208 (2005).
29. Sjostrom, P. J. & Nelson, S. B. Spike timing, calcium signals and synaptic
plasticity. Curr. Opin. Neurobiol. 12, 305– -314 (2002).
30. Singer, J. H. & Diamond, J. S. Vesicle depletion and synaptic depression at a
mammalian ribbon synapse. J. Neurophysiol. 95, 3191– -3198 (2006).
Supplementary Information is linked to the online version of the paper at
Acknowledgements We thank K. Swartz for his gift of kurtoxin, J. Isaac for his
gift of GYKI 53655, and J. Isaac, D. Copenhagen, C. Jahr and members of the
Diamond laboratory for comments on the manuscript. This research was
supported by the NINDS Intramural Research Program and a K22 award to
J.H.S. A.E.C. is a doctoral student in a graduate program partnership between
NIH and the University of Valparaı ´so, Chile.
Author Contributions A.E.C. and J.H.S. collected and analysed data and helped
to design experiments; J.S.D. directed the study, helped to design experiments
and wrote the manuscript.
Author Information Reprints and permissions information is available at
www.nature.com/reprints. The authors declare no competing financial interests.
Correspondence and requests for materials should be addressed to J.S.D.
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