Beyond the synapse: The role of extrasynaptic receptors in a retinal microcircuit

Our ability to see allows us to navigate and learn about our environment, as well as greet our friends and avoid our foes. Vision starts in the retina, a paper-thin network of neurons and connections at the back of our eyes. The retina is an outpost of our brain. These neurons are organized into microcircuits that turn photons of light into electrical and chemical signals that can be processed and interpreted by the brain. Neuroscientists think there are between 20 and 30 distinct microcircuits that process different aspects of our visual scene such as shadows, contrast, and colors. One important microcircuit allows for vision to take place in low light levels and in the dark. This is called the rod pathway microcircuit because it is the rod photoreceptors that are sensitive enough to detect single photons of light. The proper functioning of this microcircuit allows for our excellent night vision, and dysfunction can lead to diseases that cause night blindness. In this project, we have used a combination of sophisticated optical imaging and electrophysiological recording to explore the hypothesis that the neurons in this circuit do not only communicate at morphologically defined contact points called synapses, but that the chemicals used as messengers between neurons can spill out of the synapse and activate so called extrasynaptic receptors. We have found that there are two populations of extrasynaptic receptors within this circuit that are activated by the neurotransmitter glutamate. Each population of these extrasynaptic receptors is associated with a distinct type of interneuron, and is activated by a unique source of glutamate. One source of glutamate comes from neurons and the other source of glutamate comes from glial cells. Glial cells are generally thought to perform support functions for neurons. So, our results strengthen the hypothesis that these cells play an additional role in signaling information important for our sense of vision. We have also found that the required co-agonist for each of these receptor populations is different. Taken together, we have revealed a complexity within this circuit of neurons that can potentially be exploited to increase our knowledge of how this circuit processes visual information.

The aim of this project was to increase knowledge about the basic mechanisms involved in synaptic transmission in the central nervous system, with a focus on the location and function of extrasynaptic receptors that are involved in both normal physiological functions, such as long-lasting synaptic plasticity, as well as excitotoxicity and cell death. Although this project clearly falls under the category of basic research, there is potential for a long-term impact on human eye disease, as the neural microcircuit that we investigated here, that mediates night and low-light vision, is of considerable societal and clinical importance. Our results will also have an academic impact on the study of extrasynaptic receptors and their role in signal integration, with valuable information about the molecular mechanisms and spatial localization of such receptors in a microcircuit that utilizes circuit motifs common throughout the central nervous system, e.g. feedback and feedforward inhibition.

A basic goal of neuroscience research is to understand how the activity and interaction of neurons, the primary functional units of the nervous system, lead to behavior. Communication between neurons occurs at morphological junctions called synapses, at which the concerted action of molecular machines, comprising ion channels, receptor proteins, scaffolding proteins, down-stream effector systems and messengers, encode, transfer and decode information. In parallel to the transfer of information at synapses, however, it has become increasingly clear that the exchange of information also occurs outside of morphologically defined synapses. For example, neurotransmitters such as glutamate can spill out from a synapse and activate so-called extrasynaptic receptors that are involved in both normal physiological functions such as long-lasting synaptic plasticity, as well as, paradoxically, excitotoxicity and cell death. Thus, to understand the information processing that occurs in the central nervous system, we must look beyond the synapse to investigate the dynamics and functional properties of molecular elements operating extrasynaptically. Here, in an ambitious series of experiments, we will investigate the nanoscale organization and functional properties of extrasynaptic glutamate receptors in a well-defined retinal microcircuit. We will use an ensemble of complementary methods that our laboratory has implemented and refined over the years, including multi-electrode patch-clamp recording in intact tissue preparations, multi-photon microscopy for structural and functional imaging, as well as newer techniques such as super-resolution microscopy and membrane voltage imaging, relentlessly pushing the limits of spatial and temporal resolution. With this knowledge, the molecular computations that occur between neurons can be understood and exploited to provide new insights into the fundamental mechanisms of the nervous system that underlie the neural basis of behavior.