Neurons are nerve cells involved in receiving or sending signals. Excitatory and inhibitory neurons in the CNS are distinguished by several features, including morphology, transmitter content, and synapse architecture. Such distinctions are exemplified in the vertebrate retina, with the retina well-mapped, and for the past 100 years scientists placing retinal interneurons squarely into one of two boxes. Now, a study from researchers at University of Washington identifies a previously unknown type of neuron, that falls outside century-old classifications, in the retina of mice. The team state that the new cell, which they have named GluMI (pronounced gloomy), acts like one class of neurons but anatomically resembles another. The opensource study is published in the journal Current Biology.
Previous studies show that bipolar neurons relay information from the retina’s photoreceptors, which capture light, to the specialized cells that process those signals into vision for the brain, called ganglion cells. Monopolar neurons, on the other hand, typically aren’t contacted directly by photoreceptors. They also provide inhibition, meaning they hit the brakes to keep nerve cell signaling traffic in check. Earlier studies from the lab identified the GluMIs in 2010, while studying the retina of transgenic mice. These animals were engineered to manufacture a fluorescent protein to help illuminate different cells in different colors. The group observed a cell type that looked monopolar but, puzzlingly, didn’t have any of the markers of an inhibitory retinal cell. The current study clarifies the appearance and function of the GluMI cell, whose structure is monopolar, yet acts like a bipolar cell by exciting the ganglion cells.
The current study utilises 3-D images to show that the glutamatergic monopolar interneuron, or GluMI, was relaying light information and showed that its light responses differed from those of bipolar cells. The researchers note that since the new cell isn’t contacted by the photoreceptors, the source of these light responses is a mystery.
Data findings show that the GluMIs do not receive direct photoreceptor input, and their light responses are strongly shaped by both ON and OFF pathway-derived inhibitory input. Results show that GluMIs contact and make almost as many synapses as bipolar cells onto retinal ganglion cells. The lab state that, however, GluMIs and bipolar cells possess functionally distinct light-driven responses and may therefore mediate separate components of the excitatory synaptic input to retinal ganglion cells.
The team surmise that the identification of GluMIs unveils a novel cellular component of excitatory circuits in the vertebrate retina, underscoring the complexity in defining cell types even in this well-characterized region of the CNS. For the future, the researchers state that although they didn’t intend to find it, they look forward to exploring the role of the GluMI cell in visual function, in conjunction with the rest of the global medical community.