No abstract

Summary Starburst amacrine cells (SACs) process complex visual signals in the retina using both ACh and GABA, but the synaptic organization and function of ACh-GABA corelease remain unclear. Here, we show that SACs make cholinergic synapses onto On-Off direction-selective ganglion cells (DSGCs) from all directions, but make GABAergic synapses onto DSGCs only from the null direction. ACh and GABA were released differentially in a Ca2+ level-specific manner, suggesting the two transmitters were released from different vesicle populations. Despite the symmetric cholinergic connection, the light-evoked cholinergic input to a DSGC, detected at both light onset and offset, was motion- and direction-sensitive. This input was facilitated by two-spot apparent motion in the preferred direction, but supressed in the null direction, presumably by a GABAergic mechnism. The results revealed a new level of synaptic intricacy in the starburst circuit and suggest differential, yet synergistic, roles of ACh-GABA cotransmission in motion sensitivity and direction selectivity.

Pharmacologically isolated starburst amacrine cells (SACs) in perinatal rabbit retinas spontaneously generated semiperiodic calcium spikes and long-lasting after-hyperpolarizations (AHPs), mediated by calcium-activated, cyclic AMP-sensitive potassium currents. These AHPs, rather than a depletion of neurotransmitters (as was previously believed), produced the refractory period of spontaneous retinal waves and set the upper limit of the wave frequency. Each SAC received inputs from roughly 10-30 neighboring SACs during a wave. These inputs synchronized and reshaped the intrinsic bursts to produce network oscillations at a rhythm different from that of individual SACs. With maturation, the semiperiodic bursts in SACs disappeared, owing to reduced intrinsic excitability and increased network inhibition. Thus, retinal waves are generated by a transient and specific network of cell-autonomous oscillators synchronized by reciprocally excitatory connections.

SUMMARY Complex neural circuits in the mammalian brain develop through a combination of genetic instruction and activity-dependent refinement. The relative role of these factors and the form of neuronal activity responsible for circuit development is a matter of significant debate. In the mammalian visual system, retinal ganglion cell projections to the brain are mapped with respect to retinotopic location and eye of origin. We manipulated the pattern of spontaneous retinal waves present during development without changing overall activity levels through the transgenic expression of β2-nicotinic acetylcholine receptors in retinal ganglion cells of mice. We used this manipulation to demonstrate that spontaneous retinal activity is not just permissive, but instructive in the emergence of eye-specific segregation and retinotopic refinement in the mouse visual system. This suggests that specific patterns of spontaneous activity throughout the developing brain are essential in the emergence of specific and distinct patterns of neuronal connectivity.

We report here a systematic investigation of the dynamics, regulation and distribution of spontaneous waves in the rabbit retina during the course of wave development prior to eye opening. Three major findings were obtained in this longitudinal study.

Dual patch-clamp recording and Ca2+ uncaging revealed Ca2+-dependent corelease of ACh and GABA from, and the presence of reciprocal nicotinic and GABAergic synapses between, starburst cells in the perinatal rabbit retina. With maturation, the nicotinic synapses between starburst cells dramatically diminished, whereas the GABAergic synapses remained and changed from excitatory to inhibitory, indicating a coordinated conversion of the starburst network excitability from an early hyperexcitatory to a mature nonepileptic state. We show that this transition allows the starburst cells to use their neurotransmitters for two completely different functions. During early development, the starburst network mediates recurrent excitation and spontaneous retinal waves, which are important for visual system development. After vision begins, starburst cells release GABA in a prolonged and Ca2+-dependent manner and inhibit each other laterally via direct GABAergic synapses, which may be important for visual integration, such as the detection of motion direction.

SUMMARY The elaboration of nascent synaptic connections into highly ordered neural circuits is an integral feature of the developing vertebrate nervous system. In sensory systems, patterned spontaneous activity before the onset of sensation is thought to influence this process, but this conclusion remains controversial largely due to the inherent difficulty recording neural activity in early development. Here, we describe novel genetic and pharmacological manipulations of spontaneous retinal activity, assayed in vivo, that demonstrate a causal link between retinal waves and visual circuit refinement. We also report a de-coupling of downstream activity in retinorecipient regions of the developing brain after retinal wave disruption. Significantly, we show that the spatiotemporal characteristics of retinal waves affect the development of specific visual circuits. These results conclusively establish retinal waves as necessary and instructive for circuit refinement in the developing nervous system and reveal how neural circuits adjust to altered patterns of activity prior to experience.

SUMMARY In the vertebrate retina, glutamate is traditionally thought to be released only by photoreceptors and bipolar cells to transmit visual signals radially along parallel ON and OFF channels. Lateral interactions in the inner retina are mediated by amacrine cells, which are thought to be inhibitory neurons. Here, we report calcium-dependent glutamate release from vGluT3-expressing amacrine cells (GACs) in the mouse retina. GACs provide an excitatory glutamatergic input to ON-OFF and ON direction-selective ganglion cells and a subpopulation of W3 ganglion cells, but not to starburst amacrine cells. GACs receive excitatory inputs from both ON and OFF channels, generate ON-OFF light responses with a medium-center, wide-surround receptive field structure, and directly regulate ganglion cell activity. The results reveal a functional glutamatergic circuit that mediates non-canonical excitatory interactions in the retina and likely plays a role in generating ON-OFF responses, crossover excitation, and lateral excitation.