Spiking and nonspiking models of starburst amacrine cells in the rabbit retina

Voltage- and calcium-gated ion channels of neurons in the vertebrate retina

In this review, we summarize studies investigating the types and distribution of voltage- and calcium-gated ion channels in the different classes of retinal neurons: rods, cones, horizontal cells, bipolar cells, amacrine cells, interplexiform cells, and ganglion cells. We discuss differences among cell subtypes within these major cell classes, as well as differences among species, and consider how different ion channels shape the responses of different neurons. For example, even though second-order bipolar and horizontal cells do not typically generate fast sodium-dependent action potentials, many of these cells nevertheless possess fast sodium currents that can enhance their kinetic response capabilities. Ca2+ channel activity can also shape response kinetics as well as regulating synaptic release. The L-type Ca2+ channel subtype, CaV1.4, expressed in photoreceptor cells exhibits specific properties matching the particular needs of these cells such as limited inactivation which allows sustained channel activity and maintained synaptic release in darkness. The particular properties of K+ and Cl- channels in different retinal neurons shape resting membrane potentials, response kinetics and spiking behavior. A remaining challenge is to characterize the specific distributions of ion channels in the more than 100 individual cell types that have been identified in the retina and to describe how these particular ion channels sculpt neuronal responses to assist in the processing of visual information by the retina.

Stimulus-dependent engagement of neural mechanisms for reliable motion detection in the mouse retina

Direction selectivity is a fundamental computation in the visual system and is first computed by the direction-selective circuit in the mammalian retina. Although landmark discoveries on the neural basis of direction selectivity have been made in the rabbit, many technological advances designed for the mouse have emerged, making this organism a favored model for investigating the direction-selective circuit at the molecular, synaptic, and network levels. Studies using diverse motion stimuli in the mouse retina demonstrate that retinal direction selectivity is implemented by multilayered mechanisms. This review begins with a set of central mechanisms that are engaged under a wide range of visual conditions and then focuses on additional layers of mechanisms that are dynamically recruited under different visual stimulus conditions. Together, recent findings allude to an emerging theme: robust motion detection in the natural environment requires flexible neural mechanisms.

Cross-compartmental Modulation of Dendritic Signals for Retinal Direction Selectivity

Compartmentalized signaling in dendritic subdomains is critical for the function of many central neurons. In the retina, individual dendritic sectors of a starburst amacrine cell (SAC) are preferentially activated by different directions of linear motion, indicating limited signal propagation between the sectors. However, the mechanism that regulates this propagation is poorly understood. Here, we find that metabotropic glutamate receptor 2 (mGluR2) signaling, which acts on voltage-gated calcium channels in SACs, selectively restricts cross-sector signal propagation in SACs, but does not affect local dendritic computation within individual sectors. mGluR2 signaling ensures sufficient electrotonic isolation of dendritic sectors to prevent their depolarization during non-preferred motion, yet enables controlled multicompartmental signal integration that enhances responses to preferred motion. Furthermore, mGluR2-mediated dendritic compartmentalization in SACs is important for the functional output of direction-selective ganglion cells (DSGCs). Therefore, our results directly link modulation of dendritic compartmentalization to circuit-level encoding of motion direction in the retina.

Computation of motion direction in the vertebrate retina

How direction of image motion is detected as early as at the level of the vertebrate eye has been intensively studied in retina research. Although the first direction-selective (DS) ret­inal ganglion cells were already described in the 1960s and have since then been in the fo­cus of many studies, scientists are still puz­zled by the intricacy of the neuronal circuits and computational mechanisms underlying retinal direction selectivity. The fact that the retina can be easily isolated and studied in a Petri dish-by presenting light stimuli while recording from the various cell types in the retinal circuits-in combination with the ex­tensive anatomical, molecular and physiolog­ical knowledge about this part of the brain presents a unique opportunity for studying this intriguing visual circuit in detail. This ar­ticle provides a brief overview of the histo­ry of research on retinal direction selectivi­ty, but then focuses on the past decade and the progress achieved, in particular driven by methodological advances in optical record­ing techniques, molecular genetics approach­es and large-scale ultrastructural reconstruc­tions. As it turns out, retinal direction selec­tivity is a complex, multi-tiered computation, involving dendrite-intrinsic mechanisms as well as several types of network interactions on the basis of highly selective, likely genet­ically predetermined synaptic connectivi­ty. Moreover, DS ganglion cell types appear to be more diverse than previously thought, differing not only in their preferred direction and response polarity, but also in physiology, DS mechanism, dendritic morphology and, importantly, the target area of their projec­tions in the brain.

Does heterogeneity of intracellular Ca[Formula: see text] dynamics underlie speed tuning of direction-selective responses in starburst amacrine cells?

The starburst amacrine cell (SAC) plays a fundamental role in retinal motion perception. In the vertebrate retina, SAC dendrites have been shown to be directionally selective in terms of their Ca[Formula: see text] responses for stimuli that move centrifugally from the soma. The mechanism by which SACs show Ca[Formula: see text] bias for centrifugal motion is yet to be determined with precision. Recent morphological studies support a presynaptic delay in glutamate receptor activation induced Ca[Formula: see text] release from bipolar cells preferentially contacting SACs. However, bipolar cells are known to be electrotonically coupled so time delays between the bipolar cells that provide input to SACs seem unlikely. Using fluorescent microscopy and imunnostaining, we found that the endoplasmic reticulum (ER) is omnipresent in the soma extending to the distal processes of SACs. Consequently, a working hypothesis on heterogeneity of intracellular Ca[Formula: see text] dynamics from ER is proposed as a possible explanation for the cause of speed tuning of direction-selective Ca[Formula: see text] responses in dendrites of SACs.

Mathematical analysis and modeling of motion direction selectivity in the retina

Motion detection is one of the most important and primitive computations performed by our visual system. Specifically in the retina, ganglion cells producing motion direction-selective responses have been addressed by different disciplines, such as mathematics, neurophysiology and computational modeling, since the beginnings of vision science. Although a number of studies have analyzed theoretical and mathematical considerations for such responses, a clear picture of the underlying cellular mechanisms is only recently emerging. In general, motion direction selectivity is based on a non-linear asymmetric computation inside a receptive field differentiating cell responses between preferred and null direction stimuli. To what extent can biological findings match these considerations? In this review, we outline theoretical and mathematical studies of motion direction selectivity, aiming to map the properties of the models onto the neural circuitry and synaptic connectivity found in the retina. Additionally, we review several compartmental models that have tried to fill this gap. Finally, we discuss the remaining challenges that computational models will have to tackle in order to fully understand the retinal motion direction-selective circuitry.

The role of starburst amacrine cells in visual signal processing

Starburst amacrine cells (SBACs) within the adult mammalian retina provide the critical inhibition that underlies the receptive field properties of direction-selective ganglion cells (DSGCs). The SBACs generate direction-selective output of GABA that differentially inhibits the DSGCs. We review the biophysical mechanisms that produce directional GABA release from SBACs and test a network model that predicts the effects of reciprocal inhibition between adjacent SBACs. The results of the model simulations suggest that reciprocal inhibitory connections between closely spaced SBACs should be spatially selective, while connections between more widely spaced cells could be indiscriminate. SBACs were initially identified as cholinergic neurons and were subsequently shown to contain release both acetylcholine and GABA. While the role of the GABAergic transmission is well established, the role of the cholinergic transmission remains unclear.

Direction selectivity in the retina: symmetry and asymmetry in structure and function

Visual information is processed in the retina to a remarkable degree before it is transmitted to higher visual centres. Several types of retinal ganglion cells (the output neurons of the retina) respond preferentially to image motion in a particular direction, and each type of direction-selective ganglion cell (DSGC) is comprised of multiple subtypes with different preferred directions. The direction selectivity of the cells is generated by diverse mechanisms operating within microcircuits that rely on independent neuronal processing in individual dendrites of both the DSGCs and the presynaptic neurons that innervate them.

Amacrine cell-mediated input to bipolar cells: Variations on a common mechanistic theme

Feedback is a ubiquitous feature of neural circuits in the mammalian central nervous system (CNS). Analogous to pure electronic circuits, neuronal feedback provides either a positive or negative influence on the output of upstream components/neurons. Although the particulars (i.e., connectivity, physiological encoding/processing/signaling) of circuits in higher areas of the brain are often unclear, the inner retina proves an excellent model for studying both the anatomy and physiology of feedback circuits within the functional context of visual processing. Inner retinal feedback to bipolar cells is almost entirely mediated by a single class of interneurons, the amacrine cells. Although this might sound like a simple circuit arrangement with an equally simple function, anatomical, molecular, and functional evidence suggest that amacrine cells represent an extremely diverse class of CNS interneurons that contribute to a variety of retinal processes. In this review, I classify the amacrine cells according to their anatomical output synapses and target cell(s) (i.e., bipolar cells, ganglion cells, and/or amacrine cells) and discuss specifically our current understandings of amacrine cell-mediated feedback and output to bipolar cells on the synaptic, cellular, and circuit levels, while drawing connections to visual processing.

GABA-mediated spatial and temporal asymmetries that contribute to the directionally selective light responses of starburst amacrine cells in retina

Starburst amacrine cells (SACs) are an essential component of the mechanism that generates direction selectivity in the retina. SACs exhibit opposite polarity, directionally selective (DS) light responses, depolarizing to stimuli that move centrifugally away from the cell through the receptive field surround, but hyperpolarizing to stimuli that move centripetally towards the cell through the surround.Recent findings suggest that (1) the intracellular chloride concentration ([Cl(−)](i)) is high in SAC proximal, but low in SAC distal dendritic compartments, so that GABA depolarizes and hyperpolarizes the proximal and distal compartments, respectively, and (2) this [Cl(−)](i) gradient plays an essential role in generating SAC DS light responses. Employing a biophysically realistic, computational model of SACs, which incorporated experimental measurements of SAC electrical properties and GABA and glutamate responses, we further investigated whether and how a [Cl(−)](i) gradient along SAC dendrites produces their DS responses. Our computational analysis suggests that robust DS light responses would be generated in both the SAC soma and distal dendrites if (1) the Cl(−) equilibrium potential is more positive in the proximal dendrite and more negative in the distal dendrite than the resting membrane potential, so that GABA depolarizes and hyperpolarizes the proximal and distal compartments, respectively, and (2) the GABA-evoked increase in the Cl(−) conductance lasts longer than the glutamate-evoked increase in cation conductance. The combination of these two specific GABA-associated spatial and temporal asymmetries, in conjunction with symmetric glutamate excitation, may underlie the opposite polarity, DS light responses of SACs.

Immunocytochemical study of NR1, NR2A and NR2B subunits of NMDA receptor in frog retina (Rana ridibunda)

The NMDA receptors are ionotropic glutamate receptors that are involved in a variety of functions in the nervous system and in particular in the retina. They are composed of NR1 and NR2 subunits. The NMDA receptors have been fairly well studied in the retina of mammals, however, there is only limited information concerning these receptors in the retinas of lower vertebrates. The aim of the present study was to investigate immunocytochemically the NR1, NR2A and NR2B subunits of the NMDA receptors in the frog retina. Six primary antibodies were used. Three of them were directed to different splice variants of the NR1 subunit and the remaining three variants directed to NR2 subunits. All antibodies showed well expressed labeling in the frog retina. The labels had a punctate character and were located mainly in the inner and the outer plexiform layers. The results obtained indicate that the NR1, NR2A and NR2B subunits of NMDA receptor may participate in the glutamatergic neurotransmission from photoreceptors to second order retinal neurons, as well as from bipolar cells to third order retinal neurons. It has been proposed that in the frog retina, several subtypes of NMDA receptors exist each involved with different functions.

Tetrodotoxin-resistant sodium channels contribute to directional responses in starburst amacrine cells

The biophysical mechanisms that give rise to direction selectivity in the retina remain uncertain. Current evidence suggests that the directional signal first arises within the dendrites of starburst amacrine cells (SBACs). Two models have been proposed to explain this phenomenon, one based on mutual inhibitory interactions between SBACs, and the other positing an intrinsic dendritic mechanism requiring a voltage-gradient depolarizing towards the dendritic tips. We tested these models by recording current and voltage responses to visual stimuli in SBACs. In agreement with previous work, we found that the excitatory currents in the SBACs were directional, and remained directional when GABA receptors were blocked. Contrary to the mutual-inhibitory model, stimuli that produce strong directional signals in ganglion cells failed to reveal a significant inhibitory input to SBACs. Suppression of the tonic excitatory conductance, proposed to generate the dendritic voltage-gradient required for the dendrite autonomous model, failed to eliminate the directional signal in SBACs. However, selective block of tetrodotoxin-resistant sodium channels did reduce the strength of the directional excitatory signal in the SBACs. These results indicate that current models of direction-selectivity in the SBACs are inadequate, and suggest that voltage-gated excitatory channels, specifically tetrodotoxin-resistant sodium channels, are important elements in directional signaling. This is the first physiological evidence that tetrodotoxin-resistant sodium channels play a role in retinal information processing.

Cellular mechanisms for direction selectivity in the retina

Direction selectivity represents a fundamental computation found across multiple sensory systems. In the mammalian visual system, direction selectivity appears first in the retina, where excitatory and inhibitory interneurons release neurotransmitter most rapidly during movement in a preferred direction. Two parallel sets of interneuron signals are integrated by a direction-selective ganglion cell, which creates a direction preference for both bright and dark moving objects. Direction selectivity of synaptic input becomes amplified by action potentials in the ganglion cell dendrites. Recent work has elucidated direction-selective mechanisms in inhibitory circuitry, but mechanisms in excitatory circuitry remain unexplained.

A dendrite-autonomous mechanism for direction selectivity in retinal starburst amacrine cells

Detection of image motion direction begins in the retina, with starburst amacrine cells (SACs) playing a major role. SACs generate larger dendritic Ca(2+) signals when motion is from their somata towards their dendritic tips than for motion in the opposite direction. To study the mechanisms underlying the computation of direction selectivity (DS) in SAC dendrites, electrical responses to expanding and contracting circular wave visual stimuli were measured via somatic whole-cell recordings and quantified using Fourier analysis. Fundamental and, especially, harmonic frequency components were larger for expanding stimuli. This DS persists in the presence of GABA and glycine receptor antagonists, suggesting that inhibitory network interactions are not essential. The presence of harmonics indicates nonlinearity, which, as the relationship between harmonic amplitudes and holding potential indicates, is likely due to the activation of voltage-gated channels. [Ca(2+)] changes in SAC dendrites evoked by voltage steps and monitored by two-photon microscopy suggest that the distal dendrite is tonically depolarized relative to the soma, due in part to resting currents mediated by tonic glutamatergic synaptic input, and that high-voltage-activated Ca(2+) channels are active at rest. Supported by compartmental modeling, we conclude that dendritic DS in SACs can be computed by the dendrites themselves, relying on voltage-gated channels and a dendritic voltage gradient, which provides the spatial asymmetry necessary for direction discrimination.

Form and Function of on-off Amacrine Cells in the Amphibian Retina

on-off amacrine cells were studied with whole cell recording techniques and intracellular staining methods using intact retina-eyecup preparations of the tiger salamander ( Ambystoma tigrinum) and the mudpuppy ( Necturus maculosus). Morphological characterization of these cells included three-dimensional reconstruction methods based on serial optical sections obtained with a confocal microscope. Some cells had their detailed morphology digitized with a computer-assisted tracing system and converted to compartmental models for computer simulations. The dendrites of on-off amacrine cells have spines and numerous varicosities. Physiological recordings confirmed that on-off amacrine cells generate both large- and small-amplitude impulses attributed, respectively, to somatic and dendritic generation sites. Using a multichannel model for impulse generation, computer simulations were carried out to evaluate how impulses are likely to propagate throughout these structures. We conclude that the on-off amacrine cell is organized with multifocal dendritic impulse generating sites and that both dendritic and somatic impulse activity contribute to the functional repertoire of these interneurons: locally generated dendritic impulses can provide regional activation, while somatic impulse activity results in rapid activation of the entire dendritic tree.

The interdependence and independence of amacrine cell dendrites: patch-clamp recordings and simulation studies on cultured GABAergic amacrine cells

Previously we reported that cultured rat GABAergic amacrine cells can evoke subthreshold graded depolarization and action potentials. Both types of electrical signals are thought to contribute to neurotransmitter release from their dendrites, because Ca(2+) channels in amacrine cells can be activated at a subthreshold level (around -50 mV). The aim of the present study is to describe the spatiotemporal pattern of the spread of these electrical signals in an amacrine cell, using a computer simulation study. The simulation is based on physiological data, obtained by dual whole-cell patch-clamp recordings on the soma and the dendrites of cultured rat GABAergic amacrine cells. We determined passive and active properties of amacrine cells from the physiological recordings. Then, using the NEURON simulator, we conducted computer simulations on a reconstructed model of amacrine cells. We show that graded potentials and action potentials spread through amacrine cells with distinct patterns, and discuss the electrical interrelationship among the dendrites of an amacrine cell. Subthreshold graded potentials applied to a distal dendrite were sufficiently localized, so that each dendrite could behave independently (dendritic independence). However, at a suprathreshold level, once action potentials were triggered, they propagated into every dendrite, exciting the entire cell (dendritic interdependence). We also showed that GABAergic inhibitory inputs on the dendrites suppress the dendritic interdependence of amacrine cells. These results suggest that an inhibitory amacrine cell can mediate both local and wide-field lateral inhibition, regulated by the spatiotemporal pattern of excitatory and inhibitory synaptic inputs on its dendrites.

BIOPHYSICAL MECHANISMS AND ESSENTIAL TOPOGRAPHY OF DIRECTIONALLY SELECTIVE SUBUNITS IN RABBIT'S RETINA

We commemorate the 40th anniversary of the classical study undertaken by Barlow-Levick with a new challenge: to show how direction selectivity in the dendritic plexus of starburst amacrine cells is being computed. In the rabbit retina, although the cellular locus of direction selectivity is known to occur predominantly in the dendrites of starburst amacrine cells, the biophysical mechanism by which this takes place and its essential topography are yet to be specified with precision. A cotransmission model, involving a conjoint release of excitation/inhibition (i.e., a bisynaptic relay of endogenous ACh and GABA) from the distal varicosities of individual starburst amacrines, will be non-diphasic when the vesicular release of Ach and the non-vesicular, carrier-mediated release of GABA by transporters in the anterograde direction are preferentially suppressed by a negative feedback mechanism involving autoreceptors. Such biophysical mechanisms, including the asymmetric distribution of chloride cotransporters, explain somatofugal motion bias in starburst amacrine cells leading to autonomous functioning "subunits" that underlie the formation of directional selectivity. However, the functional independence of starburst amacrine cell "subunits" is partly a question of their network organization. The topography of directionally selective "subunits" resides in the plexus of crisscrossing dendrites of juxtaposed starburst amacrines, consisting of (i) serial synapses of three or more starburst amacrines and a ON-OFF directionally selective ganglion cell; (ii) a synaptic couplet between two starburst amacrines; and (iii) a conventional synapse between a starburst amacrine and a ON-OFF directionally selective ganglion cell. Cholinergic and GABAergic monosynaptic interactions between starburst amacrine cells, including glutamatergic interactions with cone bipolar cells, are involved in the primary circuit underlying directional selectivity. Furthermore, the secondary circuit underlying directional selectivity, consists of starburst amacrine cells and cone bipolar cells arranged in a "push-pull" configuration, interacting synaptically onto ON-OFF directionally selective ganglion cells.

A unique role for Kv3 voltage-gated potassium channels in starburst amacrine cell signaling in mouse retina

Direction-selective retinal ganglion cells show an increased activity evoked by light stimuli moving in the preferred direction. This selectivity is governed by direction-selective inhibition from starburst amacrine cells occurring during stimulus movement in the opposite or null direction. To understand the intrinsic membrane properties of starburst cells responsible for direction-selective GABA release, we performed whole-cell recordings from starburst cells in mouse retina. Voltage-clamp recordings revealed prominent voltage-dependent K(+) currents. The currents were mostly blocked by 1 mm TEA, activated rapidly at voltages more positive than -20 mV, and deactivated quickly, properties reminiscent of the currents carried by the Kv3 subfamily of K+ channels. Immunoblots confirmed the presence of Kv3.1 and Kv3.2 proteins in retina and immunohistochemistry revealed their expression in starburst cell somata and dendrites. The Kv3-like current in starburst cells was absent in Kv3.1-Kv3.2 knock-out mice. Current-clamp recordings showed that the fast activation of the Kv3 channels provides a voltage-dependent shunt that limits depolarization of the soma to potentials more positive than -20 mV. This provides a mechanism likely to contribute to the electrical isolation of individual starburst cell dendrites, a property thought essential for direction selectivity. This function of Kv3 channels differs from that in other neurons where they facilitate high-frequency repetitive firing. Moreover, we found a gradient in the intensity of Kv3.1b immunolabeling favoring proximal regions of starburst cells. We hypothesize that this Kv3 channel gradient contributes to the preference for centrifugal signal flow in dendrites underlying direction-selective GABA release from starburst amacrine cells

Direction selectivity in a model of the starburst amacrine cell

The starburst amacrine cell (SBAC), found in all mammalian retinas, is thought to provide the directional inhibitory input recorded in On–Off direction-selective ganglion cells (DSGCs). While voltage recordings from the somas of SBACs have not shown robust direction selectivity (DS), the dendritic tips of these cells display direction-selective calcium signals, even when γ-aminobutyric acid (GABAa,c) channels are blocked, implying that inhibition is not necessary to generate DS. This suggested that the distinctive morphology of the SBAC could generate a DS signal at the dendritic tips, where most of its synaptic output is located. To explore this possibility, we constructed a compartmental model incorporating realistic morphological structure, passive membrane properties, and excitatory inputs. We found robust DS at the dendritic tips but not at the soma. Two-spot apparent motion and annulus radial motion produced weak DS, but thin bars produced robust DS. For these stimuli, DS was caused by the interaction of a local synaptic input signal with a temporally delayed “global” signal, that is, an excitatory postsynaptic potential (EPSP) that spread from the activated inputs into the soma and throughout the dendritic tree. In the preferred direction the signals in the dendritic tips coincided, allowing summation, whereas in the null direction the local signal preceded the global signal, preventing summation. Sine-wave grating stimuli produced the greatest amount of DS, especially at high velocities and low spatial frequencies. The sine-wave DS responses could be accounted for by a simple mathematical model, which summed phase-shifted signals from soma and dendritic tip. By testing different artificial morphologies, we discovered DS was relatively independent of the morphological details, but depended on having a sufficient number of inputs at the distal tips and a limited electrotonic isolation. Adding voltage-gated calcium channels to the model showed that their threshold effect can amplify DS in the intracellular calcium signal.

New directions in retinal research

Direction-selective retinal ganglion cells (DSGCs) respond to image motion in a "preferred" direction but not the opposite "null" direction. Extracellular spike recordings from rabbit DSGCs suggested that the key mechanism underlying the directional responses is spatially offset inhibition projecting in the null direction. Recent patch-clamp recordings have shown that this inhibition, which acts directly on the DSGC, is already direction selective. Dual recordings established that the inhibition arises from starburst amacrine cells (SBACs) located on the null side of the DSGC but not from those on the preferred side. Thus, for each radially symmetric SBAC, processes pointing in different directions would provide the null-direction inhibition to subtypes of DSGCs with different preferred directions. Ca2+ imaging revealed that the SBAC terminal processes respond more strongly to image motion away from the soma than towards the soma, therefore accounting for the direction selectivity of the inhibitory input to the DSGCs.

Directionally selective calcium signals in dendrites of starburst amacrine cells

The detection of image motion is fundamental to vision. In many species, unique classes of retinal ganglion cells selectively respond to visual stimuli that move in specific directions. It is not known which retinal cell first performs the neural computations that give rise to directional selectivity in the ganglion cell. A prominent candidate has been an interneuron called the 'starburst amacrine cell'. Using two-photon optical recordings of intracellular calcium concentration, here we find that individual dendritic branches of starburst cells act as independent computation modules. Dendritic calcium signals, but not somatic membrane voltage, are directionally selective for stimuli that move centrifugally from the cell soma. This demonstrates that direction selectivity is computed locally in dendritic branches at a stage before ganglion cells.

GABA-Mediated inhibition between amacrine cells in the goldfish retina

Retinal amacrine cells have abundant dendro-dendritic synapses between neighboring amacrine cells. Therefore an amacrine cell has both presynaptic and postsynaptic aspects. To understand these synaptic interactions in the amacrine cell, we recorded from amacrine cells in the goldfish retinal slice preparation with perforated- and whole cell-patch clamp techniques. As the presynaptic element, 19% of the cells recorded (15 of 78 cells) showed spontaneous tetrodotoxin (TTX)-sensitive action potentials. As the postsynaptic element, all amacrine cells (n = 9) were found to have GABA-evoked responses and, under perforated patch clamp, 50 microM GABA hyperpolarized amacrine cells by activating a Cl(-) conductance. Bicuculline-sensitive spontaneous postsynaptic currents, carried by Cl(-), were observed in 82% of the cells (64 of 78 cells). Since the source of GABA in the inner plexiform layer is amacrine cells alone, these events are likely to be inhibitory postsynaptic currents (IPSCs) caused by GABA spontaneously released from neighboring amacrine cells. IPSCs were classified into three groups. Large amplitude IPSCs were suppressed by TTX (1 microM), indicating that presynaptic action potentials triggered GABA release. Medium amplitude IPSCs were also TTX sensitive. Small amplitude IPSCs were TTX insensitive (miniature IPSCs; n = 26). All of spike-induced, medium amplitude, and miniature IPSCs were Ca(2+) dependent and blocked by Co(2+). Blocking of glutamatergic inputs by DL-2-amino-phosphonoheptanoate (AP7; 30 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 2 microM) had almost no effect on spontaneous GABA release from presynaptic amacrine cells. We suggest that these dendro-dendrotic inhibitory networks contribute to receptive field spatiotemporal properties.

Corelease of two functionally opposite neurotransmitters by retinal amacrine cells: experimental evidence and functional significance

The Dale's law postulates that a neuron releases the same neurotransmitter from all its branches. In the case of multiple neurotransmitters it would require all transmitters to be released from all branches. The retinal cholinergic amacrine cells contain and release gamma-aminobutyric (GABA) and, therefore, if GABA and acetylcholine (ACh) are released at the same sites, this could mean that amacrine cells simultaneously excite and inhibit postsynaptic cells. Conversely, if the two neurotransmitters are released at different synapses, or if their release is regulated in a distinct manner, they may play different physiological roles. Recent studies carried out in cultured cholinergic amacrine-like neurons showed that Ca(2+)-dependent release of ACh and GABA have a different sensitivity to membrane depolarization, to the effect of blockers of voltage gated Ca(2+) channels (VGCC) and to the effect of presynaptic A(1) adenosine receptors. Therefore, it is proposed that in retinal amacrine cells the Ca(2+)-dependent release of ACh and GABA occurs at distinct cellular locations. The possible nature of these release sites and the physiological significance of this model are discussed in this review.