Physiology and cell biology of the presynaptic terminal

Neurons communicate with one another by the release of neurotransmitters through exocytosis. Upon depolarization, presynaptic neurons open voltage-gated channels, which allow the entry of calcium, which in turn rapidly triggers the fusion of synaptic vesicles with the plasma membrane. Although all eukaryotic cells secrete molecules, a hallmark of neurons is the speed and spatial regulation of the secretion process. The main objective of the research in my laboratory is to understand how presynaptic terminals are specialized for these tasks. This work involves the study of several aspects of presynaptic function including vesicle transport, exocytosis and endocytosis. The primary model system in the laboratory is the retinal bipolar neuron, which has an unusually large synaptic terminal. These cells belong to a class of neurons that have specialized structures known as synaptic ribbons. One specific focus of the laboratory is to understand the role of these structures in synaptic transmission in sensory neurons of the retina and inner ear. In order to study presynaptic function my laboratory uses a combination of electrophysiological and optical approaches. One technique that has proven particularly useful for this purpose has been evanescent field fluorescence microscopy. Evanescent field fluorescence microscopy takes advantage of the sub-wavelength sized evanescent field of light created by light traveling at a supercritical angle, from a high refractive index coverslip to lower refractive index cell, to selectively illuminate fluorophores near the cell surface. This method enables us to directly image single 30-nm vesicles in living retinal neurons, which will improve our ability to study the mechanisms that control vesicle movement and capture in the synaptic terminal.

Three consecutive evanescent field fluorescence video images from an FM1-43 labeled vesicle fusing with the plasma membrane. Scale bar is 1 micron.