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The many roles of starburst amacrine cells
Update
TRENDS in Neurosciences Vol.28 No.8 August 2005
Research Focus
The many roles of starburst amacrine cells Richard H. Masland Howard Hughes Medical Institute, Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114, USA
Starburst amacrine cells release two classical neurotransmitters, ACh and GABA. In a tour de force of pairedcell recording, Zheng et al. now show that the starburst cells are mutually excitatory during early development but mutually inhibitory in adult animals. The change occurs by remodeling of both the cholinergic and the GABAergic synapses between starburst cells. The finding gives a precise mechanistic basis for the developmental waves of activity in the retina. Introduction The rod amacrine excepted, starburst amacrine cells are the most numerous amacrine cells of the mammalian retina. They are found in species from turtles to macaques. Because they are both numerous and widely overlapping, they occupy a substantial fraction of all available volume of the inner synaptic layer of the retina. In the rabbit, where they have been most intensively studied, starburst amacrine cells deploy a total of six meters of dendrite per millimeter of retinal surface, for a total of more than two kilometers of starburst dendrite in the retina as a whole [1]. Only one other known retinal neuron is within an order of magnitude of this amount. A cell using so much precious ‘wire’ must do something important, and an incisive new paper by Jimmy Zhou and his colleagues [2] now shows that it does not one but several important things, changing its role dramatically during the course of development. Synapses that change during development In early postnatal days, the developing retina is repeatedly swept by waves of electrophysiological activity. These waves create correlated firing in the retinal ganglion cells, and the correlated activity is thought to instruct the topographical ordering of the central visual system [3]. The starburst cells are unusual in that they synthesize and release two classical neurotransmitters, ACh and GABA, and pharmacological evidence had suggested that the early developmental waves are mediated by both, because GABA can act in an excitatory way during early life [4,5]. How are the waves propagated? Anatomists have reported synapses between starburst cells [6,7], but their existence has always been somewhat controversial. The technical breakthrough was reliable recording from pairs of starburst cells, which allowed Zheng et al. to observe directly the starburst-to-starburst synapses. It was found that the cells communicate directly with each other, without intervention of any other retinal cells. Excitation Corresponding author: Masland, R.H. ([email protected]). Available online 23 June 2005 www.sciencedirect.com
of this sort is entirely adequate to explain the propagation of early developmental waves. A reason to doubt starburst-to-starburst synapses was that they would appear to create a positive feedback, in which excitation produces even more excitation, inevitably leading to epilepsy-like instability. It turns out that this does not happen because the starburst cell synapses are remodeled before the circuitry and excitability of the cells reaches the adult level. Nicotinic ACh receptor (nAChR)-mediated excitation between starburst cells drops to an almost undetectable level (cholinergic synapses from starburst to ganglion cells remain). At the same time, the GABAergic synapses among starburst cells switch from being excitatory to inhibitory. The cells are no longer at risk of overstimulation, because excitatory interactions between starburst cells have been reduced at one of their sources and reversed at the other.
Starburst function in the adult retina Zheng et al. convincingly resolve a long-standing ambiguity about starburst function – the mechanism of release of the two neurotransmitters. It was shown long ago that both neurotransmitters are released from the cells, but the mechanism of release has always been problematic [8], with some suggestion that part of the GABA release is transporter-mediated. In the new experiments, pairs of cells were studied in a medium in which Ca2C entry was blocked by a high concentration of Cd2C. Under normal ionic conditions, both ACh and GABA were released in a quantal fashion. In the high-Cd2C medium, synaptic potentials mediated by both neurotransmitters were entirely eliminated. These findings are hallmarks of standard vesicular release. This was confirmed by an elegant experiment in which caged Ca2C was injected into one of the cells via the recording pipette. When Ca2C was subsequently uncaged by a brief pulse of UV light, the injected cell released both cholinergic and GABAergic vesicles, as evidenced by quantal excitatory and inhibitory responses from the second starburst cell. What do these results say about the role of the starburst cells in adult retinas? A strong possibility is that they account for the unusually strong lateral inhibition observed for starburst responses to light [9,10]. Another is that they might participate in more complex circuits. It has now been shown that sectors of the starburst dendritic arbor (Figure 1) are capable of independent activity [9–12] and that each sector reports on only a particular direction of a moving stimulus [13]. If starburst cells obey a wiring postulate requiring that sectors having the same directional orientation all
396
Update
TRENDS in Neurosciences Vol.28 No.8 August 2005
suggest release from different vesicular pools. And what is the role of cholinergic synapses in the adult animal? They are known to excite the directionally selective ganglion cells, but this appears to occur in a nondirectional way. Do they have a modulatory function, entirely independent of directional selectivity? Or do they participate in more complex circuitry, perhaps involving local microcircuits presynaptic to the direction-selective ganglion cell [16–18]? Paired-cell recordings between starburst cells and other types of retinal neurons might also help to answer these questions. References
Figure 1. Starburst amacrine cells. One sector of the dendritic arbor is outlined in (a); it is defined by its origin at a single primary dendrite. First speculation and now direct evidence support the idea that these sectors can function in electrophysiological isolation from other sectors and that they ‘rectify’, releasing neurotransmitter in response to centrifugal motion (arrows) but not centripetal motion. This appears to be the fundamental asymmetry that underlies direction selectivity in retinal ganglion cells. Panels (b) and (c) show the starburst dendrites at postnatalday (P)0 and P17. The immature dendrites at P0 are relatively coarse and show numerous filopodia. At P17, the mature dendrites are thinner and are decorated with many varicosities, the sites of synaptic outputs. Scale bar, 50 mm. Panels (b) and (c) reproduced, with permission, from [2].
synapse on a single retinal ganglion cell (and not on others), then the directional preference of these starburst sectors will be passed on to the ganglion cell, which will itself become directionally selective [14,15]. Because the strongest determinant of direction selectivity is GABAmediated inhibition, removal of the uncertainty about the mechanism of GABA release brings the mechanism of direction selectivity one step closer to a definitive solution. Concluding remarks A couple of points remain to be understood. Are ACh and GABA packaged and secreted together, or at different sites? Zheng et al. intriguingly find differences in the Ca2C sensitivity of ACh and GABA release, which would
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1 Tauchi, M. and Masland, R.H. (1984) The shape and arrangement of the cholinergic neurons in the rabbit retina. Proc. R. Soc. Lond. B. Biol. Sci. 223, 101–119 2 Zheng, J. et al. (2004) A developmental switch in the excitability and function of the starburst network in the mammalian retina. Neuron 44, 851–864 3 Katz, L.C. and Shatz, C.J. (1996) Synaptic activity and the construction of cortical circuits. Science 274, 1133–1138 4 Sernagor, E. et al. (2003) Developmental modulation of retinal wave dynamics: shedding light on the GABA saga. J. Neurosci. 23, 7621–7629 5 Ben-Ari, Y. (2002) Excitatory actions of GABA during development: the nature of the nurture. Nat. Rev. Neurosci. 3, 728–739 6 Brandon, C. (1987) Cholinergic neurons in the rabbit retina: dendritic branching and ultrastructural connectivity. Brain Res. 426, 119–130 7 Millar, T.J. and Morgan, I.G. (1987) Cholinergic amacrine cells in the rabbit retina synapse onto other cholinergic amacrine cells. Neurosci. Lett. 74, 281–285 8 O’Malley, D.M. et al. (1992) Co-release of acetylcholine and GABA by the starburst amacrine cells. J. Neurosci. 12, 1394–1408 9 Taylor, W.R. and Wa¨ssle, H. (1995) Receptive field properties of starburst cholinergic amacrine cells in the rabbit retina. Eur. J. Neurosci. 7, 2308–2321 10 Peters, B.N. and Masland, R.H. (1996) Responses to light of starburst amacrine cells. J. Neurophysiol. 75, 469–480 11 Miller, R.F. and Bloomfield, S.A. (1983) Electroanatomy of a unique amacrine cell in the rabbit retina. Proc. Natl. Acad. Sci. U. S. A. 80, 3069–3073 12 Masland, R.H. et al. (1984) The functions of acetylcholine in the rabbit retina. Proc. R. Soc. Lond. B. Biol. Sci. 223, 121–139 13 Euler, T. et al. (2002) Directionally selective calcium signals in dendrites of starburst amacrine cells. Nature 418, 845–852 14 Vaney, D.I. et al. (1989) Dendritic relationships between cholinergic amacrine cells and directionally-selective retinal ganglion cells. In Neurobiology of the Inner Retina (Weiler, R. and Osborne, N.N., eds), pp. 157–168, Springer-Verlag 15 Borg-Graham, L.J. and Grzywacz, N.M. (1992) A model of the directional selectivity circuit in retina: transformations by neurons singly and in concert. In Single Neuron Computation (McKenna, T. et al., eds), pp. 347–376, Academic Press 16 Fried, S.I. et al. (2002) Mechanisms and circuitry underlying directional selectivity in the retina. Nature 420, 411–414 17 Taylor, W.R. and Vaney, D.I. (2002) Diverse synaptic mechanisms generate direction selectivity in the rabbit retina. J. Neurosci. 22, 7712–7720 18 Borg-Graham, L.J. (2001) The computation of directional selectivity in the retina occurs presynaptic to the ganglion cell. Nat. Neurosci. 4, 176–183 0166-2236/$ - see front matter Q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.tins.2005.06.002
TRENDS in Neurosciences Vol.28 No.8 August 2005
Research Focus
The many roles of starburst amacrine cells Richard H. Masland Howard Hughes Medical Institute, Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114, USA
Starburst amacrine cells release two classical neurotransmitters, ACh and GABA. In a tour de force of pairedcell recording, Zheng et al. now show that the starburst cells are mutually excitatory during early development but mutually inhibitory in adult animals. The change occurs by remodeling of both the cholinergic and the GABAergic synapses between starburst cells. The finding gives a precise mechanistic basis for the developmental waves of activity in the retina. Introduction The rod amacrine excepted, starburst amacrine cells are the most numerous amacrine cells of the mammalian retina. They are found in species from turtles to macaques. Because they are both numerous and widely overlapping, they occupy a substantial fraction of all available volume of the inner synaptic layer of the retina. In the rabbit, where they have been most intensively studied, starburst amacrine cells deploy a total of six meters of dendrite per millimeter of retinal surface, for a total of more than two kilometers of starburst dendrite in the retina as a whole [1]. Only one other known retinal neuron is within an order of magnitude of this amount. A cell using so much precious ‘wire’ must do something important, and an incisive new paper by Jimmy Zhou and his colleagues [2] now shows that it does not one but several important things, changing its role dramatically during the course of development. Synapses that change during development In early postnatal days, the developing retina is repeatedly swept by waves of electrophysiological activity. These waves create correlated firing in the retinal ganglion cells, and the correlated activity is thought to instruct the topographical ordering of the central visual system [3]. The starburst cells are unusual in that they synthesize and release two classical neurotransmitters, ACh and GABA, and pharmacological evidence had suggested that the early developmental waves are mediated by both, because GABA can act in an excitatory way during early life [4,5]. How are the waves propagated? Anatomists have reported synapses between starburst cells [6,7], but their existence has always been somewhat controversial. The technical breakthrough was reliable recording from pairs of starburst cells, which allowed Zheng et al. to observe directly the starburst-to-starburst synapses. It was found that the cells communicate directly with each other, without intervention of any other retinal cells. Excitation Corresponding author: Masland, R.H. ([email protected]). Available online 23 June 2005 www.sciencedirect.com
of this sort is entirely adequate to explain the propagation of early developmental waves. A reason to doubt starburst-to-starburst synapses was that they would appear to create a positive feedback, in which excitation produces even more excitation, inevitably leading to epilepsy-like instability. It turns out that this does not happen because the starburst cell synapses are remodeled before the circuitry and excitability of the cells reaches the adult level. Nicotinic ACh receptor (nAChR)-mediated excitation between starburst cells drops to an almost undetectable level (cholinergic synapses from starburst to ganglion cells remain). At the same time, the GABAergic synapses among starburst cells switch from being excitatory to inhibitory. The cells are no longer at risk of overstimulation, because excitatory interactions between starburst cells have been reduced at one of their sources and reversed at the other.
Starburst function in the adult retina Zheng et al. convincingly resolve a long-standing ambiguity about starburst function – the mechanism of release of the two neurotransmitters. It was shown long ago that both neurotransmitters are released from the cells, but the mechanism of release has always been problematic [8], with some suggestion that part of the GABA release is transporter-mediated. In the new experiments, pairs of cells were studied in a medium in which Ca2C entry was blocked by a high concentration of Cd2C. Under normal ionic conditions, both ACh and GABA were released in a quantal fashion. In the high-Cd2C medium, synaptic potentials mediated by both neurotransmitters were entirely eliminated. These findings are hallmarks of standard vesicular release. This was confirmed by an elegant experiment in which caged Ca2C was injected into one of the cells via the recording pipette. When Ca2C was subsequently uncaged by a brief pulse of UV light, the injected cell released both cholinergic and GABAergic vesicles, as evidenced by quantal excitatory and inhibitory responses from the second starburst cell. What do these results say about the role of the starburst cells in adult retinas? A strong possibility is that they account for the unusually strong lateral inhibition observed for starburst responses to light [9,10]. Another is that they might participate in more complex circuits. It has now been shown that sectors of the starburst dendritic arbor (Figure 1) are capable of independent activity [9–12] and that each sector reports on only a particular direction of a moving stimulus [13]. If starburst cells obey a wiring postulate requiring that sectors having the same directional orientation all
396
Update
TRENDS in Neurosciences Vol.28 No.8 August 2005
suggest release from different vesicular pools. And what is the role of cholinergic synapses in the adult animal? They are known to excite the directionally selective ganglion cells, but this appears to occur in a nondirectional way. Do they have a modulatory function, entirely independent of directional selectivity? Or do they participate in more complex circuitry, perhaps involving local microcircuits presynaptic to the direction-selective ganglion cell [16–18]? Paired-cell recordings between starburst cells and other types of retinal neurons might also help to answer these questions. References
Figure 1. Starburst amacrine cells. One sector of the dendritic arbor is outlined in (a); it is defined by its origin at a single primary dendrite. First speculation and now direct evidence support the idea that these sectors can function in electrophysiological isolation from other sectors and that they ‘rectify’, releasing neurotransmitter in response to centrifugal motion (arrows) but not centripetal motion. This appears to be the fundamental asymmetry that underlies direction selectivity in retinal ganglion cells. Panels (b) and (c) show the starburst dendrites at postnatalday (P)0 and P17. The immature dendrites at P0 are relatively coarse and show numerous filopodia. At P17, the mature dendrites are thinner and are decorated with many varicosities, the sites of synaptic outputs. Scale bar, 50 mm. Panels (b) and (c) reproduced, with permission, from [2].
synapse on a single retinal ganglion cell (and not on others), then the directional preference of these starburst sectors will be passed on to the ganglion cell, which will itself become directionally selective [14,15]. Because the strongest determinant of direction selectivity is GABAmediated inhibition, removal of the uncertainty about the mechanism of GABA release brings the mechanism of direction selectivity one step closer to a definitive solution. Concluding remarks A couple of points remain to be understood. Are ACh and GABA packaged and secreted together, or at different sites? Zheng et al. intriguingly find differences in the Ca2C sensitivity of ACh and GABA release, which would
www.sciencedirect.com
1 Tauchi, M. and Masland, R.H. (1984) The shape and arrangement of the cholinergic neurons in the rabbit retina. Proc. R. Soc. Lond. B. Biol. Sci. 223, 101–119 2 Zheng, J. et al. (2004) A developmental switch in the excitability and function of the starburst network in the mammalian retina. Neuron 44, 851–864 3 Katz, L.C. and Shatz, C.J. (1996) Synaptic activity and the construction of cortical circuits. Science 274, 1133–1138 4 Sernagor, E. et al. (2003) Developmental modulation of retinal wave dynamics: shedding light on the GABA saga. J. Neurosci. 23, 7621–7629 5 Ben-Ari, Y. (2002) Excitatory actions of GABA during development: the nature of the nurture. Nat. Rev. Neurosci. 3, 728–739 6 Brandon, C. (1987) Cholinergic neurons in the rabbit retina: dendritic branching and ultrastructural connectivity. Brain Res. 426, 119–130 7 Millar, T.J. and Morgan, I.G. (1987) Cholinergic amacrine cells in the rabbit retina synapse onto other cholinergic amacrine cells. Neurosci. Lett. 74, 281–285 8 O’Malley, D.M. et al. (1992) Co-release of acetylcholine and GABA by the starburst amacrine cells. J. Neurosci. 12, 1394–1408 9 Taylor, W.R. and Wa¨ssle, H. (1995) Receptive field properties of starburst cholinergic amacrine cells in the rabbit retina. Eur. J. Neurosci. 7, 2308–2321 10 Peters, B.N. and Masland, R.H. (1996) Responses to light of starburst amacrine cells. J. Neurophysiol. 75, 469–480 11 Miller, R.F. and Bloomfield, S.A. (1983) Electroanatomy of a unique amacrine cell in the rabbit retina. Proc. Natl. Acad. Sci. U. S. A. 80, 3069–3073 12 Masland, R.H. et al. (1984) The functions of acetylcholine in the rabbit retina. Proc. R. Soc. Lond. B. Biol. Sci. 223, 121–139 13 Euler, T. et al. (2002) Directionally selective calcium signals in dendrites of starburst amacrine cells. Nature 418, 845–852 14 Vaney, D.I. et al. (1989) Dendritic relationships between cholinergic amacrine cells and directionally-selective retinal ganglion cells. In Neurobiology of the Inner Retina (Weiler, R. and Osborne, N.N., eds), pp. 157–168, Springer-Verlag 15 Borg-Graham, L.J. and Grzywacz, N.M. (1992) A model of the directional selectivity circuit in retina: transformations by neurons singly and in concert. In Single Neuron Computation (McKenna, T. et al., eds), pp. 347–376, Academic Press 16 Fried, S.I. et al. (2002) Mechanisms and circuitry underlying directional selectivity in the retina. Nature 420, 411–414 17 Taylor, W.R. and Vaney, D.I. (2002) Diverse synaptic mechanisms generate direction selectivity in the rabbit retina. J. Neurosci. 22, 7712–7720 18 Borg-Graham, L.J. (2001) The computation of directional selectivity in the retina occurs presynaptic to the ganglion cell. Nat. Neurosci. 4, 176–183 0166-2236/$ - see front matter Q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.tins.2005.06.002