visual responses to increments and decrements in intensity across the visual scene (Fig. 2). The outer half of the IPL encodes responses to decrements (the OFF sublaminae), and the inner half encodes responses to increments in illumination (the ON sublaminae) [15, 16]. In the ON sublaminae, synaptic contacts are made between bipolar and ganglion cells that are excited by increments of illumination of their receptive field centers. In the OFF sublaminae, synaptic connections are made between bipolar and ganglion cells that are excited by decrements of illumination of their receptive field centers. The ON and OFF sublaminae are further divided into ten morphological strata (see Fig. 2) that are functionally distinct and encode different representations of the visual scene [14].

Sustained and Transient Response Stratification

Some retinal neurons generate brisk, transient responses to continuous visual stimulation, while others maintain their activity for the duration of the stimulus. These transient and sustained cell classes are believed to play crucial roles in encoding the temporal and spatial features of the visual world and represent additional parallel sensory channels in the retina. A major functional subdivision of the ON and OFF sublaminae is the sustained and transient response stratification that occurs in each sublaminae, suggesting that there are transient and sustained responding subtypes of the major ON and OFF classes of retinal neurons. Sustained and transient strata were first described in turtle retina [17] and subsequently confirmed in other species [10, 12, 14]. Sustained and transient bipolar cells provide the inputs to these IPL strata [10]. Transient bipolar cell axon terminals contact transient amacrine and ganglion cell processes in the mid-IPL, and sustained bipolar cells axon terminals contact the sustained responding amacrine and ganglion cells near the inner and outer margins of the IPL [10, 12, 18].

SYNAPTIC MECHANISMS SHAPE EXCITATORY SIGNALS IN THE IPL

The functional stratification of the IPL suggests that distinct bipolar cells generate sustained and transient responses. Wunk and Werblin [19] suggest that the separation of sustained and transient visual signals occurs in the IPL, where synaptic interactions generate sustained and transient bipolar cell outputs. However, subsequent work suggests that sustained and transient visual signals are generated in the OPL at the dendrites of different bipolar cell subtypes, attributable to the filtering of distinct glutamate receptors [9, 10]. Although the main signal separation occurs in the OPL, additional refinement of sustained and transient signals takes place in the IPL. Glutamate release from bipolar cells is truncated by inhibitory amacrine cell input to shape the time course of transient excitatory responses in ganglion cells [20–22]. As noted in section, Synaptic Mechanisms Shape Excitatory Signals in the IPL, transient ganglion cell responses are also shaped by other IPL synaptic mechanisms (see Fig. 4B), such as glutamate uptake by transporters [23] and desensitizing glutamate receptors [24].

Glutamate Release Is Tonic and Graded

Since bipolar cells do not use action potentials, but use slow graded depolarizations to signal, glutamate release is graded and tonic [25, 26]. The release machinery in bipolar cells is optimized for sustained glutamate release. The L-type calcium channels, present

Fig. 4. A A bipolar cell terminal illustrating the processes that contribute to tonic glutamate release. Tonic release is attributed to sustained, graded depolarizations, prolonged calcium influx through L-type calcium channels, and a large pool of ribbon-associated vesicles. B A bipolar cell terminal illustrating the synaptic mechanisms that shape excitatory signaling to ganglion cells. Transporters limit excitatory signaling by removing glutamate from the synapse. Desensitizing postsynaptic glutamate receptors (X) limit responses to sustained glutamate release. Glutamate release may be reduced by the activation of presynaptic metabotropic glutamate receptors (mGluR) or the activation of presynaptic γ-aminobutyric acid A (GABAA) and GABAC receptors.

in bipolar terminals, open in response to graded depolarization and mediate a sustained calcium influx that elicits tonic glutamate release (Fig. 4A). Specialized structures called synaptic ribbons are located at release zones and contain large numbers of tethered, glutamate-filled vesicles (Fig. 4A) that mediate sustained signaling (for review, see [27]). While tonic glutamate release is tailored for graded signaling to ganglion cells, there are several challenges that this signaling poses. How is the sustained signal rapidly terminated? Do postsynaptic glutamate receptors desensitize to tonic glutamate release? These mechanisms are considered in more detail.

Transporters Terminate Excitatory Signaling to Ganglion Cells

In most parts of the central nervous system (CNS), synaptic responses are terminated by either the chemical degradation of transmitter or, in case of glutamate, by its rapid diffusion from the synaptic cleft [28, 29]. However, at bipolar-to-ganglion cell synapses [23, 30], and other specialized synapses in the CNS [31–33], excitation is terminated by the active clearance of glutamate by transporters into surrounding neurons and glia. Glutamate transporters shape ganglion cell excitatory responses (Fig. 4B). Blockade of glutamate transporters in the IPL enhances and prolongs glutamate signaling to ganglion cells, indicating that transporters limit the amplitude and time course of ganglion cell excitation [23].

Postsynaptic Glutamate Receptor Properties Shape Ganglion Cell Excitation

Glutamate receptors on ganglion cells are exposed to sustained, elevated glutamate concentrations, attributable to sustained light-evoked release and slow clearance by

transporters (compared to diffusion). Glutamate activates both AMPA and NMDA (N-methyl-D-aspartate) receptors on ganglion cells. AMPA receptors desensitize to sustained activation by glutamate (Fig. 4B), responding transiently to maintained stimuli [34]. When AMPA receptor desensitization is reduced in ganglion cells, either pharmacologically or by holding the cells at positive potentials, excitatory light responses are enhanced, suggesting that AMPA receptor desensitization shapes ganglion cell excitatory responses [23, 24]. Both AMPA receptor desensitization and glutamate uptake limit ganglion cell excitation, but they do so in distinct ways. Glutamate uptake limits the time course of excitation, and receptor desensitization limits the amplitude of the late phase of excitation [23].

Modulating Glutamate Release Shapes Excitatory Responses

Several mechanisms that modulate glutamate release from bipolar cells control excitatory signaling to ganglion cells. These mechanisms include autoinhibitory mechanisms by which glutamate and protons that are released from exocytosed vesicles feed back to limit glutamate release [35–37]. Glutamate limits release by activating metabotropic glutamate receptors, and protons limit release by inhibiting calcium influx (Fig. 4B). Also, as described next, presynaptic inhibition by amacrine cells also limits the probability of glutamate release [20, 38].

Amacrine Cells Mediate Inhibition in the IPL

While excitatory input to ganglion cells comes almost exclusively from bipolar cells, inhibitory signals in the IPL are mediated by amacrine cells. Amacrine cells are the most diverse class of retinal interneurons (Fig. 5). They are morphologically and functionally distinct, with different sets of neurotransmitters and receptors. Amacrine cells, like ganglion cells, are excited by bipolar cells. They mediate inhibition by releasing either γ-aminobutyric acid (GABA) or glycine onto their postsynaptic targets, which include ganglion cell dendrites, bipolar cell axon terminals, and other amacrine cells. Approximately 50% of amacrine cells are GABAergic, and 50% are glycinergic. Ganglion cells receive mainly amacrine cell input (Masland 2001), underscoring the importance of inhibition in shaping the output of the retina. Bipolar cell terminals are inhibited by presynaptic amacrine cell inputs, controlling glutamate release and excitatory signaling to ganglion cells. Amacrine cells also inhibit other amacrine cells, resulting in complex serial, inhibitory synaptic interactions [39–41]. There is also a population of excitatory amacrine cells called starburst amacrine cells, but their role in visual signal processing in adult animals remains unclear (reviewed in [42]) and is considered in the section, Directional-Selective Ganglion Cells.

There are two fundamental, morphological classes of amacrine cells, narrow field and wide field, named for the extent of their processes (Fig. 5). Wide-field amacrine cells are composed of many functional and neurochemical (~15) subtypes [6]. However, they share the common attribute of signaling over relatively long distances, usually confined to specific IPL strata. The narrow-field amacrine cell class is also composed of many subtypes that mediate local signaling. Another trait of narrow-field amacrine cells is that they often signal between strata, mediating vertical interaction across different layers within the IPL. Thus, communication between strata that represent distinct functional channels is mediated by narrow-field amacrine cells [43].