Peter D. Lukasiewicz

Presynaptic inhibition modulates spillover, creating distinct dynamic response ranges of sensory output.

Sensory information is thought to be modulated by presynaptic inhibition. Although this form of inhibition is a well-studied phenomenon, it is still unclear what role it plays in shaping sensory signals in intact circuits. By visually stimulating the retinas of transgenic mice lacking GABAc receptor-mediated presynaptic inhibition, we found that this inhibition regulated the dynamic range of ganglion cell (GC) output to the brain. Presynaptic inhibition acted differentially upon two major retinal pathways; its elimination affected GC responses to increments, but not decrements, in light intensity across the visual scene. The GC dynamic response ranges were different because presynaptic inhibition limited glutamate release from ON, but not OFF, bipolar cells, which modulate the extent of glutamate spillover and activation of perisynaptic NMDA receptors at ON GCs. Our results establish a role for presynaptic inhibitory control of spillover in determining sensory output in the CNS.

GABAC receptors in the vertebrate retina.

In the central nervous system (CNS), the inhibitory transmitter GABA interacts with three subtypes of GABA receptors, type A, type B, and type C. Historically, GABA receptors have been classified as either the inotropic GABAA receptors or the metabotropic GABAB receptors. Over the past 10 yr, studies have shown that a third class, called the GABAC receptor, also exists. GABAC receptors are found primarily in the vertebrate retina and to some extent in other parts of the CNS. Although GABAA and GABAC receptors both gate chloride channels, they are pharmacologically, molecularly, and functionally distinct. The ρ subunit of the GABAC receptor, which has about 35% amino acid homology to GABAA receptor subunits, was cloned from the retina and, when expressed inXenopus oocytes, has properties similar to retinal GABAC receptors. There are probably distinct roles for GABAC receptors in the retina, because they are found on only a subset of neurons, whereas GABAA receptors are ubiquitous. This article reviews recent electrophysiological and molecular studies that have characterized the unique properties of GABAC receptors and describes the roles that these receptors may play in visual information processing in the retina.

GABAA, GABAC and glycine receptor‐mediated inhibition differentially affects light‐evoked signalling from mouse retinal rod bipolar cells

Rod bipolar cells relay visual signals evoked by dim illumination from the outer to the inner retina. GABAergic and glycinergic amacrine cells contact rod bipolar cell terminals, where they modulate transmitter release and contribute to the receptive field properties of third order neurones. However, it is not known how these distinct inhibitory inputs affect rod bipolar cell output and subsequent retinal processing. To determine whether GABAA, GABAC and glycine receptors made different contributions to light‐evoked inhibition, we recorded light‐evoked inhibitory postsynaptic currents (L‐IPSCs) from rod bipolar cells mediated by each pharmacologically isolated receptor. All three receptors contributed to L‐IPSCs, but their relative roles differed; GABAC receptors transferred significantly more charge than GABAA and glycine receptors. We determined how these distinct inhibitory inputs affected rod bipolar cell output by recording light‐evoked excitatory postsynaptic currents (L‐EPSCs) from postsynaptic AII and A17 amacrine cells. Consistent with their relative contributions to L‐IPSCs, GABAC receptor activation most effectively reduced the L‐EPSCs, while glycine and GABAA receptor activation reduced the L‐EPSCs to a lesser extent. We also found that GABAergic L‐IPSCs in rod bipolar cells were limited by GABAA receptor‐mediated inhibition between amacrine cells. We show that GABAA, GABAC and glycine receptors mediate functionally distinct inhibition to rod bipolar cells, which differentially modulated light‐evoked rod bipolar cell output. Our findings suggest that modulating the relative proportions of these inhibitory inputs could change the characteristics of rod bipolar cell output.

GABAC receptor-mediated inhibition in the retina

Inhibition at bipolar cell axon terminals regulates excitatory signaling to ganglion cells and is mediated, in part, by GABAC receptors. We investigated GABAC receptor-mediated inhibition using pharmacological approaches and genetically altered mice that lack GABAC receptors. Responses to applied GABA showed distinct time courses in various bipolar cell classes, attributable to different proportions of GABAA and GABAC receptors. The elimination of GABAC receptors in GABAC null mice reduced and shortened GABA-activated currents and light-evoked inhibitory synaptic currents (L-IPSCs) in rod bipolar cells. ERG measurements and recordings from the optic nerve showed that inner retinal function was altered in GABAC null mice. These data suggest that GABAC receptors determine the time course and extent of inhibition at bipolar cell terminals that, in turn, modulates the magnitude of excitatory transmission from bipolar cells to ganglion cells.

Presynaptic inhibition differentially shapes transmission in distinct circuits in the mouse retina

Diverse retinal outputs are mediated by ganglion cells that receive excitatory input from distinct classes of bipolar cells (BCs). These classes of BCs separate visual signals into rod, ON and OFF cone pathways. Although BC signalling is a major determinant of the ganglion cell‐mediated retinal output, it is not fully understood how light‐evoked, presynaptic inhibition from amacrine cell inputs shapes BC outputs. To determine whether differences in presynaptic inhibition uniquely modulate BC synaptic output to specific ganglion cells, we assessed the inhibitory contributions of GABAA, GABAC and glycine receptors across the BC pathways. Here we show that different proportions of GABAA and GABAC receptor‐mediated inhibition determined the kinetics of GABAergic presynaptic inhibition across different BC classes. Large, slow GABAC and small, fast GABAA receptor‐mediated inputs to rod BCs prolonged light‐evoked inhibitory postsynaptic currents (L‐IPSCs), while smaller GABAC and larger GABAA receptor‐mediated contributions produced briefer L‐IPSCs in ON and OFF cone BCs. Glycinergic inhibition also varied across BC class. In the rod‐dominant conditions studied here, slow glycinergic inputs dominated L‐IPSCs in OFF cone BCs, attributable to inputs from the rod pathway via AII amacrine cells, while rod and ON cone BCs received little and no glycinergic input, respectively. As these large glycinergic inputs come from rod signalling pathways, in cone‐dominant conditions L‐IPSCs in OFF cone bipolar cells will probably be dominated by GABAA receptor‐mediated input. Thus, unique presynaptic receptor combinations mediate distinct forms of inhibition to selectively modulate BC outputs, enhancing the distinctions among parallel retinal signals.

Action Potentials Are Required for the Lateral Transmission of Glycinergic Transient Inhibition in the Amphibian Retina

Transient lateral inhibition (TLI), the suppression of responses of a ganglion cell to light stimuli in the receptive field center by changes in illumination in the receptive field surround, was studied in light-adapted mud puppy and tiger salamander retinas using both eyecup and retinal slice preparations. In the eyecup, TLI was measured in on–off ganglion cells as the ability of rotating, concentric windmill patterns of 500–1200 μm inner diameter to suppress the response to a small spot stimulus in the receptive field center. Both the suppression of the spot response and the hyperpolarization produced in ganglion cells by rotation of the windmill were blocked in the presence of 2 μm strychnine or 500 nm tetrodotoxin (TTX), but not by 150 μm picrotoxin. In the slice preparation in which GABA-mediated currents were blocked with picrotoxin, IPSCs elicited by diffuse illumination were blocked by strychnine and strongly reduced by TTX. The TTX-resistant component was probably attributable to illumination of the receptive field center. TTX had a much greater effect in reducing the glycinergic inhibition elicited by laterally displaced stimulation versus nearby focal electrical stimulation. Strychnine enhanced light-evoked excitatory currents in ganglion cells, but this was not mimicked by TTX. The results suggest that local glycinergic transient inhibition does not require action potentials and is mediated by synapses onto both ganglion cell dendrites and bipolar cell terminals. In contrast, the lateral spread of this inhibition (at least over distances >250 μm) requires action potentials and is mainly onto ganglion cell dendrites.

Receptor and Transmitter Release Properties Set the Time Course of Retinal Inhibition

Synaptic inhibition is determined by the properties of postsynaptic receptors, neurotransmitter release, and clearance, but little is known about how these factors shape sensation-evoked inhibition. The retina is an ideal system to investigate inhibition because it can be activated physiologically with light, and separate inhibitory pathways can be assayed by recording from rod bipolar cells that possess distinct glycine, GABAA, and GABAC receptors (R). We show that receptor properties differentially shape spontaneous IPSCs, whereas both transmitter release and receptor properties shape light-evoked (L) IPSCs. GABACR-mediated IPSCs decayed the slowest, whereas glycineR- and GABAAR-mediated IPSCs decayed more rapidly. Slow GABACRs determined the L-IPSC decay, whereas GABAARs and glycineRs, which mediated rapid onset responses, determined the start of the L-IPSC. Both fast and slow inhibitory inputs distinctly shaped the output of rod bipolar cells. The slow GABACRs truncated glutamate release, making the A17 amacrine cell L-EPSCs more transient, whereas the fast GABAAR and glycineRs reduced the initial phase of glutamate release, limiting the peak amplitude of the L-EPSC. Estimates of transmitter release time courses suggested that glycine release was more prolonged than GABA release. The time course of GABA release activating GABACRs was slower than that activating GABAARs, consistent with spillover activation of GABACRs. Thus, both postsynaptic receptor and transmitter release properties shape light-evoked inhibition in retina.

Multiple pathways of inhibition shape bipolar cell responses in the retina

Bipolar cells (BCs) are critical relay neurons in the retina that are organized into parallel signaling pathways. The three main signaling pathways in the mammalian retina are the rod, ON cone, and OFF cone BCs. Rod BCs mediate incrementing dim light signals from rods, and ON cone and OFF cone BCs mediate incrementing and decrementing brighter light signals from cones, respectively. The outputs of BCs are shaped by inhibitory inputs from GABAergic and glycinergic amacrine cells in the inner plexiform layer, mediated by three distinct types of inhibitory receptors: GABA(A), GABA(C), and glycine receptors. The three main BC pathways receive distinct forms of inhibition from these three receptors that shape their light-evoked inhibitory signals. Rod BC inhibition is dominated by slow GABA(C) receptor inhibition, while OFF cone BCs are dominated by glycinergic inhibition. The inhibitory inputs to BCs are also shaped by serial inhibitory connections between GABAergic amacrine cells that limit the spatial profile of BC inhibition. We discuss our recent studies on how inhibitory inputs to BCs are shaped by receptor expression, receptor properties, and neurotransmitter release properties and how these affect the output of BCs.

Sodium Channels in Transient Retinal Bipolar Cells Enhance Visual Responses in Ganglion Cells

Retinal bipolar cells are slow potential neurons that respond to photoreceptor inputs with graded potentials and do not fire action potentials. We found that transient ON bipolar cells recorded in retinal slices possess voltage-gated sodium channels located on either their dendrites or somas. The sodium currents in these neurons did not generate spikes but enhanced voltage responses evoked by visual stimulation, which selectively boosted transmission to transient ganglion cells. In contrast, sodium currents were not found in sustained ON bipolar cells, and light responses in sustained bipolar cells and ganglion cells were not affected by TTX. The presence of sodium channels in transient ON bipolar cells contributed to the separation of transient and sustained signals by selectively enhancing the responses of ON transient ganglion cells to light. Our results suggest that bipolar cell sodium channels augment transient signals and contribute to the temporal segregation of visual information.

Interneuron Circuits Tune Inhibition in Retinal Bipolar Cells

While connections between inhibitory interneurons are common circuit elements, it has been difficult to define their signal processing roles because of the inability to activate these circuits using natural stimuli. We overcame this limitation by studying connections between inhibitory amacrine cells in the retina. These interneurons form spatially extensive inhibitory networks that shape signaling between bipolar cell relay neurons to ganglion cell output neurons. We investigated how amacrine cell networks modulate these retinal signals by selectively activating the networks with spatially defined light stimuli. The roles of amacrine cell networks were assessed by recording their inhibitory synaptic outputs in bipolar cells that suppress bipolar cell output to ganglion cells. When the amacrine cell network was activated by large light stimuli, the inhibitory connections between amacrine cells unexpectedly depressed bipolar cell inhibition. Bipolar cell inhibition elicited by smaller light stimuli or electrically activated feedback inhibition was not suppressed because these stimuli did not activate the connections between amacrine cells. Thus the activation of amacrine cell circuits with large light stimuli can shape the spatial sensitivity of the retina by limiting the spatial extent of bipolar cell inhibition. Because inner retinal inhibition contributes to ganglion cell surround inhibition, in part, by controlling input from bipolar cells, these connections may refine the spatial properties of the retinal output. This functional role of interneuron connections may be repeated throughout the CNS.