UC Berkeley

The development of neural circuits is perhaps one of the most fascinating phenomena of nature. Neural circuits combine an intrinsic program, that manifest itself in the form of spontaneous activity and molecular markers, with input from the environment to mature into a perfect arrangement of circuits that define how we interact with the world.

The immature mammalian retina is a perfect example of the intricacies of neural circuit development. The developing retina exhibits a robust pattern of spontaneous activity, known as retinal waves. Retinal waves are necessary for proper development of the neural connections in early parts of the visual pathway. Recent research on this topic establishes that specific spatiotemporal properties of waves instruct specific aspects of visual map formation. In addition, retinal waves have recently found roles in cell differentiation, cell migration, and synapse formation. Thus, understanding the underlying mechanisms of the spatiotemporal properties of waves and exploring new ways in which waves could influence developing neural circuits will contribute to our understanding the role of spontaneous activity in developmental processes of the nervous system.

In mice during postnatal days 1-9, retinal waves are generated by cholinergic starburst amacrine cells (SACs), which are excitatory retinal interneurons at this stage. Starburst amacrine cells exhibit slow afterhyperpolarizations that are thought to set a refractory period and thus the periodicity and frequency of waves. Using an array of electrophysiological methods such as patch-clamp and multi-electrode array recordings, and calcium imaging, we showed that the 2- pore potassium channel TREK1 underlies SACs slow afterhyperpolarization and thus wave frequency. First, we found that the SACs afterhyperpolarization is physiologically and pharmacologically consistent with a slow potassium current generated by a 2-pore potassium channel. Second, gene expression analysis of SACs revealed the expression of TREK1. Lastly, either silencing the putative TREK1-activated signaling pathways or knocking out TREK1 increases the frequency of retinal waves. Our work provides an example of how molecular mechanisms generate specific spatiotemporal properties of waves, which are important for the development of the visual system.

In addition to instructing the wiring of retinal inputs to the visual centers of the brain, retinal waves influence developmental aspects within the retina. Prior to the onset of vision, which happens after cessation of waves, the retina relies on intrinsically photosensitive retinal ganglion cells (ipRGCs) for light detection. We explored the influence of spontaneous retinal waves on the ipRGC-dependent light response of the developing retina. We combined patch- clamp recordings, calcium imaging, multi-electrode array recordings, and tracer coupling, among other methods to determine interactions between cholinergic waves and ipRGC circuits. First, we found that ipRGCs form gap junction connections with other retinal neurons, including other ipRGCs. Second, blocking cholinergic waves induces ipRGCs to form more gap junction connections. Third, blocking waves dramatically increases the number of light-responsive cells. Fourth, waves evoke dopamine release in the developing retina. Lastly, blocking dopamine signaling increases the number of light-responsive cells, similar to blocking waves. We concluded that retinal waves evoke dopamine release that act on ipRGC to down regulate their gap junction coupling. Thus, retinal waves modulate ipRGC gap junction coupling via dopaminergic signaling to regulate the overall light response of the developing retina. To further explore the properties of the light responses of developing ipRGCs, we characterized their light-evoked calcium transients. There are 5 different types of ipRGCs identified in adult mice, and 3 physiological types in developing mice. Pooling data from a large population of ipRGCs we analyzed amplitude, latency, and response decay of ipRGC light responses. We found that although amplitude and latency exhibited a large variation across cells, they could not be used to classify cells into different subtypes of ipRGCs. In contrast, response decay showed multimodal distributions and thus potential as a classifier for developing ipRGC types. This work contributes to our understanding of light-sensing in the developing retina.