Ephaptic communication in the vertebrate retina

Abstract

Read online

In the vertebrate retina, cones project to the horizontal cells (HCs) and bipolar cells (BCs). The communication between cones and horizontal cells uses both chemical and ephaptic mechanisms. Photoreceptors release glutamate in a Ca2+-dependent manner, while HCs feed back to cones via an ephaptic mechanism. Hyperpolarization of HCs leads to an increased current through connexin hemichannels located on the tips of HC dendrites invaginating the cone synaptic terminals. This current makes the extracellular synaptic space slightly negative. The result is that the Ca2+-channels in the cone pre-synaptic membrane experience a slightly depolarized membrane potential and therefore more glutamate is released. This ephaptic mechanism forms a very fast and noise free negative feedback pathway. These characteristics are crucial, since the retina has to perform well in demanding conditions such as low light levels and detecting fast events. In this mini-review we will discuss the critical components of such an ephaptic mechanism. Furthermore, we will address the question whether such communication appears in other systems as well and indicate some fundamental features to look for when attempting to identify an ephaptic mechanism.

Keywords

• inhibition
• Cones
• ephaptic communication
• vertebrate retina
• horizontal cells