GABA inhibits ACh release from the rabbit retina: A direct effect or feedback to bipolar cells?

Modeling Starburst cells' GABA(B) receptors and their putative role in motion sensitivity

Neal and Cunningham (Neal, M. J., and J. R. Cunningham. 1995. J. Physiol. (Lond.). 482:363-372) showed that GABA(B) agonists and glycinergic antagonists enhance the light-evoked release of retinal acetylcholine. They proposed that glycinergic cells inhibit the cholinergic Starburst amacrine cells and are in turn inhibited by GABA through GABA(B) receptors. However, as recently shown, glycinergic cells do not appear to have GABA(B) receptors. In contrast, the Starburst amacrine cell has GABA(B) receptors in a subpopulation of its varicosities. We thus propose an alternate model in which GABA(B)-receptor activation reduces the release of ACh from some dendritic compartments onto a glycinergic cell, which then feeds back and inhibits the Starburst cell. In this model, the GABA necessary to make these receptors active comes from the Starburst cell itself, making them autoreceptors. Computer simulations of this model show that it accounts quantitatively for the Neal and Cunningham data. We also argue that GABA(B) receptors could work to increase the sensitivity to motion over other stimuli.

Symmetric interactions within a homogeneous starburst cell network can lead to robust asymmetries in dendrites of starburst amacrine cells

Starburst amacrine cells in the mammalian retina respond asymmetrically to movement along their dendrites; centrifugal movement elicits stronger responses in each dendrite than centripetal movement. It has been suggested that the asymmetrical response can be attributed to intrinsic properties of the processes themselves. But starburst cells are known to release and have receptors for both GABA and acetylcholine. We tested whether interactions within the starburst cell network can contribute to their directional response properties. In a computational model of interacting starburst amacrine cells, we simulated the response of individual dendrites to moving light stimuli. By setting the model parameters for "synaptic connection strength" (cs) to positive or negative values, overlapping starburst dendrites could either excite or inhibit each other. For some values of cs, we observed a very robust inward/outward asymmetry of the starburst dendrites consistent with the reported physiological findings. This is the case, for example, if a starburst cell receives inhibition from other starburst cells located in its surround. For other values of cs, individual dendrites can respond best either to inward movement or respond symmetrically. A properly wired network of starburst cells can therefore account for the experimentally observed asymmetry of their response to movement, independent of any internal biophysical or biochemical properties of starburst cell 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.

Synaptic organization of complex ganglion cells in rabbit retina: type and arrangement of inputs to directionally selective and local-edge-detector cells

The type and topographic distribution of synaptic inputs to a directionally selective (DS) rabbit retinal ganglion cell (GC) were examined and were compared with those received by two other complex GC types. The percentage of cone bipolar cell (BC) input, presumably an index of sustained responses and simple receptive field properties, is much higher than expected for complex GCs in reference to previous reports in other species: approximately 20% for the type 1 bistratified ON-OFF DS GC and for a multistratified GC, and approximately 40% for the small-tufted local-edge-detector GC. Consistent with a previous study (Famiglietti [1991] J. Comp. Neurol. 309:40-70), no ultrastructural evidence is found for inhibitory synapses from starburst amacrine cells to the ON-OFF DS GC. The density of inputs to the ON-OFF DS GC is high and rather evenly distributed over the dendritic tree. Clustering of inputs brings excitatory and inhibitory inputs into proximity, but the strict on-path condition of more proximal inhibitory inputs, favoring shunting inhibition, is not satisfied. Prominent BC input and its regional variation suggest that BCs play key roles in DS neural circuitry, both pre- and postsynaptic to the ON-OFF DS GC, according to a bilayer model (Famiglietti [1993] Invest. Ophthalmol. Vis. Sci. 34:S985). Asymmetry of inhibitory amacrine cell input may signify a region on the preferred side of the receptive field, the inhibition-free zone (Barlow and Levick [1965] J. Physiol. (Lond.) 178:477-504), supporting a role for postsynaptic integration in the DS mechanism. Prominent BC input to the local-edge-detector, often without accompanying amacrine cell input, indicates presynaptic integration in forming its trigger feature.

Synaptic input to the on-off directionally selective ganglion cell in the rabbit retina

A physiologically identified on-off directionally selective (DS) ganglion cell with its preferred-null axis defined was stained with horseradish peroxidase (HRP) and prepared for electron microscopy. A continuous series of thin sections were used to examine the cell's synaptology. Although the DS cell dendrite received the majority of its synaptic input from a heterogeneous population of amacrine cell processes, a frequently observed synaptic profile consisted of a DS cell dendrite receiving synapses from a cluster of several amacrine cell processes. These clusters of processes were assumed to be from a fascicle of amacrine cells, most of which probably belonged to several different cholinergic starburst amacrine cells. The most frequently observed presynaptic profile within the clusters consisted of a synaptic couplet in which two processes synapsed with each other before one of them finally synapsed with the DS ganglion cell dendrite; occasionally, a chain of three serial synapses was seen. In addition, a specific microcircuit that has the potential to exert lateral feedforward inhibition was also observed. This microcircuit consisted of two cone bipolar cell terminal dyad synapses where one dyad contained an amacrine cell process making a reciprocal synapse and a DS ganglion cell dendrite receiving direct excitation; the other dyad synapse, found lateral to the first dyad, contained two amacrine cell processes that both made reciprocal synapses, but one fed forward to make a putative inhibitory synapse with the DS cell dendrite.

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.

Functional inhibition in direction-selective retinal ganglion cells: spatiotemporal extent and intralaminar interactions

We recorded from ON-OFF direction-selective ganglion cells (DS cells) in the rabbit retina to investigate in detail the inhibition that contributes to direction selectivity in these cells. Using paired stimuli moving sequentially across the cells' receptive fields in the preferred direction, we directly confirmed the prediction of that a wave of inhibition accompanies any moving excitatory stimulus on its null side, at a fixed spatial offset. Varying the interstimulus distance, stimulus size, luminance, and speed yielded a spatiotemporal map of the strength of inhibition within this region. This "null" inhibition was maximal at an intermediate distance behind a moving stimulus: 1/2 to 11/2 times the width of the receptive field. The strength of inhibition depended more on the distance behind the stimulus than on stimulus speed, and the inhibition often lasted 1-2 s. These spatial and temporal parameters appear to account for the known spatial frequency and velocity tuning of ON-OFF DS cells to drifting contrast gratings. Stimuli that elicit distinct ON and OFF responses to leading and trailing edges revealed that an excitatory response of either polarity could inhibit a subsequent response of either polarity. For example, an OFF response inhibited either an ON or OFF response of a subsequent stimulus. This inhibition apparently is conferred by a neural element or network spanning the ON and OFF sublayers of the inner plexiform layer, such as a multistratified amacrine cell. Trials using a stationary flashing spot as a probe demonstrated that the total amount of inhibition conferred on the DS cell was equivalent for stimuli moving in either the null or preferred direction. Apparently the cell does not act as a classic "integrate and fire" neuron, summing all inputs at the soma. Rather, computation of stimulus direction likely involves interactions between excitatory and inhibitory inputs in local regions of the dendrites.

Pattern of synaptic excitation and inhibition upon direction-selective retinal ganglion cells

The distributions of excitatory and inhibitory synapses upon the dendritic arbor of a direction-selective retinal ganglion cell were compared by triple-labeling techniques. The dendrites were visualized by confocal microscopy after injection of Lucifer yellow. Excitatory inputs from bipolar cells were located by using antibodies against kinesin II, a component of synaptic ribbons. Inhibitory inputs were identified by antibodies against gamma-aminobutyric acid-A receptors. The combined images were examined by visual inspection and by formal, automated analyses, in a search for anisotropies that might contribute to a directional preference of the ganglion cell. Within the limits of our analysis, none was found. If an anatomic specialization underlies direction selectivity, it appears to lie in the geometry and spatial positioning of the neurons afferent to the ganglion cell and/or the microcircuitry among its afferent synapses.

The computation of directional selectivity in the retina occurs presynaptic to the ganglion cell

Directional selectivity is a response that is greater for a visual stimulus moving in one (PREF) direction than for the opposite (NULL) direction, and its computation in the vertebrate retina is a classical issue in functional neurophysiology. To date, most quantitative experimental studies have relied on extracellular responses for identifying properties of the directionally selective circuit. Here I describe an intracellular analysis using whole-cell patch recordings of the synaptic events underlying the spike response in directionally selective ganglion cells of the turtle retina. These quantitative measurements allowed me to distinguish among various explicit classes of circuit models that can, in principle, account for ganglion cell directional selectivity. I found that ganglion cell directional selectivity is due to an excitatory input that itself is directionally selective, and that the crucial shunting inhibition implicated in this computation must act on cells presynaptic to the ganglion cell.

Contributions of GABAA receptors and GABAC receptors to acetylcholine release and directional selectivity in the rabbit retina

GABA is a major inhibitory neurotransmitter in the mammalian retina and it acts at many different sites via a variety of postsynaptic receptors. These include GABAA receptors and bicuculline-resistant GABAC receptors. The release of acetylcholine (ACh) is inhibited by GABA and strongly potentiated by GABA antagonists. In addition, GABA appears to mediate the null inhibition which is responsible for the mechanism of directional selectivity in certain ganglion cells. We have used these two well-known examples of GABA inhibition to compare three GABA antagonists and assess the contributions of GABAA and GABAC receptors. All three GABA antagonists stimulated ACh release by as much as ten-fold. By this measure, the ED50s for SR-95531, bicuculline, and picrotoxin were 0.8, 7.0, and 14 microM, respectively. Muscimol, a potent GABAA agonist, blocked the effects of SR-95531 and bicuculline, but not picrotoxin. This indicates that SR-95531 and bicuculline are competitive antagonists at the GABAA receptor, while picrotoxin blocks GABAA responses by acting at a different, nonreceptor site such as the chloride channel. In the presence of a saturating dose of SR-95531 to completely block GABAA receptors, picrotoxin caused a further increase in the release of ACh. This indicates that picrotoxin potentiates ACh release by a mechanism in addition to the block of GABAA responses, possibly by also blocking GABAC receptors, which have been associated with bipolar cells. All three GABA antagonists abolished directionally selective responses from ON/OFF directional-selective (DS) ganglion cells. In this system, the ED50S for SR-95531, bicuculline, and picrotoxin were 0.7 microM, 8 microM, and 94.6 microM, respectively. The results with SR-95531 and bicuculline indicate that GABAA receptors mediate the inhibition responsible for directional selectivity. The addition of picrotoxin to a high dose of SR-95531 caused no further increase in firing rate. The comparatively high dose required for picrotoxin also suggests that GABAC receptors do not contribute to directional selectivity. This in turn suggests that feedforward GABAA inhibition, as opposed to feedback at bipolar terminals, is responsible for the null inhibition underlying directional selectivity.

Is the input to a GABAergic or cholinergic synapse the sole asymmetry in rabbit's retinal directional selectivity?

We examined contrast, direction of motion, and concentration dependencies of the effects of GABAergic and cholinergic antagonists, and anticholinesterases on responses to movement of On-Off directionally selective (DS) ganglion cells of the rabbit's retina. The drugs tested were curare and hexamethonium bromide (cholinergic antagonists), physostigmine (anticholinesterase), and picrotoxin (GABAergic antagonist). They all reduced the cells' directional selectivity, while maintaining their preferred-null axis. However, cholinergic antagonists did not block directional selectivity completely even at saturating concentrations. The failure to eliminate directional selectivity was probably not due to an incomplete blockade of cholinergic receptors. In a extension of a Masland and Ames (1976) experiment, saturating concentrations of antagonists blocked the effects of exogenous acetylcholine or nicotine applied during synaptic blockade. Consequently, a noncholinergic pathway may be sufficient to account for at least some directional selectivity. This putative pathway interacts with the cholinergic pathway before spike generation, since physostigmine eliminated directional selectivity at contrasts lower than those saturating responses. This elimination apparently resulted from cholinergic-induced saturation, since reduction of contrast restored directional selectivity. Under picrotoxin, directional selectivity was lost in 33% of the cells regardless of contrast. However, 47% maintained their preferred direction despite saturating concentrations of picrotoxin, and 20% reversed the preferred and null directions. Therefore, models based solely on a GABAergic implementation of Barlow and Levick's asymmetric-inhibition model or solely on a cholinergic implementation of asymmetric-excitation models are not complete models of directional selectivity in the rabbit. We propose an alternate model for this retinal property.

Is the input to a GABAergic synapse the sole asymmetry in turtle's retinal directional selectivity?

We examined the effects of picrotoxin and pentylenetetrazol (PTZ) on the responses to motions of ON-OFF directionally selective (DS) ganglion cells of the turtle's retina. These drugs are antagonists of the inhibitory neurotransmitter GABA. For continuous motions, picrotoxin markedly reduced the overall directionality of the cells. In 21% of the cells, directional selectivity was lost regardless of speed and contrast. However, other cells maintained their preferred direction despite saturating concentrations of picrotoxin. And in most cells, loss, maintenance, or even reversal of preferred and null directions could occur as speed and contrast were modulated. In 50% of the cells, reversal of preferred and null directions occurred at some condition of visual stimuli. However, picrotoxin did not tend to alter the preferred-null axis for directional selectivity. For apparent motions, picrotoxin made motion facilitation, which normally occurs exclusively in preferred-direction responses, to become erratic and often occur during null-direction motions. Finally, PTZ had effects similar to picrotoxin but with less potency. The results in this paper indicated that models of directional selectivity based solely on a GABAergic implementation of Barlow and Levick's asymmetric-inhibition model do not apply to the turtle retina. Alternative models may comprise multiple directional mechanisms and/or a symmetric inhibitory one, but not asymmetric facilitation.

Cholinergic effects on cat retina In vitro: changes in rod- and cone-driven b-wave and optic nerve response

To identify cholinergically mediated components in the optic nerve response (ONR) we studied effects of cholinergic agonists and antagonists in the arterially perfused cat eye. Acetylcholine, carbachol, scopolamine, quinuclidinylbenzilate and mecamylamine were applied intra-arterially in micromolar concentrations. Recordings of rod- and cone-driven ERG accompanied those of the ONR and revealed: (i) cholinergic agonists enhanced the b-wave, particularly under photopic conditions, whereas scopolamine decreased the b-wave. Mecamylamine induced biphasic effects (decrease followed by increase) in the amplitudes of the rod- and cone-driven b-waves. The effects on the cone-driven ERG were more marked than those on the rod-driven ERG. (ii) The ON-component of the ONR was increased, then decreased by acetylcholine. The cholinergic antagonists exerted complex changes in the ONR-ON component depending on dosage and adaptation. Scopolamine increased, then decreased the rod-driven ON-component, but mainly increased the cone-driven ON-component. Mecamylamine tended to increase the cone-driven, but to decrease the rod-driven ON-component of the ONR. (iii) The configuration of the rod- as well as for the cone-driven ONR, in particular the early plateau and OFF-components, were consistently and reversibly changed by cholinergic agonists, as well as by both muscarinic and nicotinic antagonists. Agonists decreased, and antagonists increased the amplitude of the plateau-component. We conclude that the ERG b-wave was enhanced by acetylcholine, but decreased by cholinergic antagonists. Cholinergic agonists and antagonists affect the same specific components of the ONR in a dose-related and reversible fashion, indicating a major contribution of cholinergic mechanisms to information processing in the cat retina.

Baclofen enhancement of acetylcholine release from amacrine cells in the rabbit retina by reduction of glycinergic inhibition

1. The mechanism by which the GABAB-receptor agonist, baclofen, enhances the light-evoked release of [3H]acetylcholine (ACh) from cholinergic amacrine cells was studied using an eye-cup preparation in anaesthetized rabbits and isolated retinas. 2. When applied locally to the rabbit retina, baclofen increased the release of ACh evoked by a flickering light (3 Hz) by over 40%. 3. In isolated retinas, baclofen strikingly inhibited the K(+)-evoked release of glycine but had no effect on GABA release. 4. In the rabbit eye cup, strychnine enhanced the light-evoked release of ACh to a similar degree to that produced by baclofen. The effects of baclofen and strychnine on the light-evoked release of ACh were not additive. In contrast, bicuculline did not affect the enhancing action of baclofen on the light-evoked release of ACh. 5. In order to see whether the glycinergic amacrine cells might be stimulated by ACh, isolated rat and rabbit retinas were exposed to muscarine. This cholinergic agonist potentiated the K(+)-evoked release of glycine by 54%. 6. We suggest that baclofen enhances the light-evoked release of ACh from amacrine cells by inhibiting glycine release from glycinergic amacrine cells which are stimulated by ACh and form an inhibitory feedback loop to the cholinergic neurones.

Enhancement of retinal acetylcholine release by DAMGO: possibly a direct opioid receptor-mediated excitatory effect

1. An eye-cup preparation in anaesthetized rabbits was used to examine opioid modulation of acetylcholine (ACh) release from cholinergic neurones in the retina. 2. The mu-opioid receptor agonist, [D-Ala2, MePhe4, Gly-ol5]-enkephalin (DAMGO), when applied locally to the retina at concentrations between 1-30 microM significantly increased the light-evoked release of ACh. The effect of DAMGO was completely blocked by the selective mu-receptor antagonist CTOP but the kappa-receptor antagonist nor-binaltorphimine (norBNI) did not affect the action of DAMGO on ACh release indicating that the opioid produced its effect by activation of mu-receptors (the rabbit retina has negligible delta-receptors). 3. Blockade with bicuculline and strychnine of GABAergic and glycinergic inputs to the cholinergic neurones did not affect the action of DAMGO on ACh release. Also DAMGO did not reduce the potassium-evoked release of either GABA or glycine from rat isolated retinas. 4. Exposure of the rabbit retina to a combination of an A1-adenosine receptor antagonist, 8-cyclopentyl-1,3 dipropylxanthine (DPCPX), and adenosine deaminase did not affect the enhancing action of DAMGO on the light-evoked release of ACh. 5. When the retina in the rabbit eye-cup was exposed to kainate, the release of ACh was increased by approximately three times the resting release. In the presence of DAMGO the kainate-evoked release of ACh was enhanced by 44%. 6. These experiments show that activation of mu-opioid receptors by DAMGO increases the release of ACh elicited by physiological stimulation (flickering light). Since we could find no evidence thatDAMGO reduces inhibitory inputs to the cholinergic neurones, it seems that the enhancing action ofDAMGO on the light-evoked release of ACh involves a direct excitatory effect rather than disinhibition.This conclusion is supported by the enhancing action of DAMGO on the kainate-evoked release of ACh because kainate is thought to act directly on the cholinergic neurones.