Optical and biophysical methods allow a closer examination of the nervous impulse.
Even after the link between an animals brain and its ability to perceive interactions with its environment was ascertained, decades of investigation and controversy ensued before a nervous system comprising discrete neurons rather than one long continuous nerve fiber was accepted. And many years of painstaking research passed before the chemical and electrical nature of the so-called nervous impulse was established. But even though our current understanding of the nervous system is accurate, it is still incomplete.
Nerves respond to electrical impulses by transmitting chemical messages from the axonal pole of the presynaptic neuron across a fluid-filled gap (the synapse) to the dendrites of the postsynaptic neuron. This chemical message is composed of
Technological advances have provided scientists with the means to measure changes in electrical potential between poles of neurons, gauge calcium concentrations both inside and outside of the cell, and design models of neurotransmitter release. However, technical limitations have relegated much of the synaptic junction to black box status. Numerous questions remain unanswered and even more remain unasked. Recently, technologies have been adapted for the neurosciences that shed light on the mechanisms of neurotransmission and provide some answers to the remaining questions.
The optical approach
Almers explains that the microscope objective lens is used both for observation and for generating the evanescent field, providing access to the cell on the other side of the objective lens for stimulation and for recording electrophysiologic measurements. Because synaptic terminals of neurons will stick to glass, the researchers used total internal reflection microscopy, an optical technique that precisely and instantaneously measures the distance between a flat surface and a microscopic sphere. This distance is determined by illuminating the sphere with an evanescent wave and then measuring the intensity of the scattered light (2). Because of the difference in refractive indices between the sphere and the glass and the resulting nonuniform illumination by the evanescent wave passing through the sphere, the sphere scatters light in direct proportion to its distance from the flat surface.
It was collaborator Jeurgen Steyer who first noticed the brightness increase as he watched the exocytosis of chromaffin granules. When a cell is stuck to and secretes material against a cover glass, it is illuminated more intensely and fluoresces more strongly, making evanescent field microscopes particularly well suited for filming exocytosis.
When Almers group built their first evanescent wave microscope in 1995 to image larger secretory vesicles, they were unable to see the much smaller synaptic vesicles. But the release of an objective lens with an exceptionally high numerical aperture (1.65), by Olympus Corp. (Melville, NY), enabled the group to build a microscope that looks <100 nm into a cell.
The investigators described vesicles held at small, discrete active zones near the synaptic terminal surface, which undergo rapid exocytosis after application of an electric stimulus. This event was followed by the movement of other vesicles, held in reserve about 20 nm from the plasma membrane, to exocytotic sites and that became release-ready 250 ms later. Our work opens a way to watch the supply of vesicles to sites of exocytosis, Almers indicates. It will also provide a means to answer the question of why some vesicles are release-ready, while others require priming.
The biophysical approach
Ralf Schneggenburger and Erwin Neher of Max Planck Institute for Biophysical Chemistry (Göttingen, Germany) used calcium-uncaging to explore the details of the calcium dependence of synaptic transmission rates in a large central nervous system synapse, the calyx of Held (4). It has been generally assumed that [Ca2+]i in presynaptic neurons must reach at least 100 µM near the sites of vesicle fusion, but Schneggenburgers team was able to show that step-wise elevations in [Ca2+]i to only 10 mM induced fast transmitter release, effectively releasing 80% of the pool of available vesicles in less than 3 ms. Biophysical measurements and subsequent calculations showed that the calcium sensors were far from saturated, making fast synaptic transmission very sensitive to regulation by changing [Ca2+]i.
At the synapse, two successive stimuli will, in most cases, not evoke similarly large amounts of neurotransmitter release. Rather, the second release event will either be larger (facilitated) or smaller(depressed). This short-term plasticity has a major influence on how a postsynaptic neuron can integrate the information flow from a presynaptic neuron, and thus, how information processing takes place in neuronal networks, Schneggenburger explains. He further stressed that without understanding the mechanism of neurotransmitter release at the molecular level, it will be hard to understand how neurotransmitter release is regulated.
Johann Bollmann and colleagues from Max Planck Institute for Medical Research (Heidelberg, Germany) and Swammerdam Institute for Life Sciences (Amsterdam) used calcium-uncaging with laser photolysis to measure the calcium sensitivity of glutamate release in a calyx-type terminal, concluding that a rise in [Ca2+]i to just 1 µM readily evoked glutamate release, and an increase greater than 30 µM depleted the releasable vesicle pool within 0.5 ms (5). This suggested that the sensitivity of these neurons to neurotransmitter release was greater than previously thought and predicted that the calcium sensor, which triggers the two waves of neurotransmitter release, may be the same. Thus, the delayed release is a consequence of delayed triggering rather than the triggering of a different sensor with a higher affinity.
Thanks to calcium uncaging, more is understood about the sensitivity of neurotransmitter release to intracellular calcium levels. For example, explains Schneggenburger, since we now know the range of relevant [Ca2+]i, it will be easier for molecular biologists to identify the Ca2+-binding proteins that initiate the fusion reaction upon Ca2+-binding.
Calcium uncaging also lends itself to other intriguing questions about the modulation of synaptic vesicle fusion. Because the amplitude of the Ca2+ signal produced by calcium uncaging is now known, investigators can apply stimuli and determine whether the relationship between [Ca2+] and neurotransmitter release changes. One question that might be answered is whether synaptic facilitationthe increase in neurotransmitter release sometimes observed with repeated neuronal stimulationis caused by an increase in Ca2+ directly dictating neurotransmitter release, or whether the Ca2+ acts on yet another sensor to modulate this release.
Writing in the series Science for All in the late 1800s (6), Andrew Wilson talks of exercising our senseswhatever these may prove to be. Wouldnt he and his contemporaries have enjoyed fiddling with these new tools?
Susan Grammer is a science writer living in Houston. Comments and questions for the author can be addressed to the Editorial Office by e-mail at email@example.com, by fax at 202-776-8166 or by post at 1155 16th Street, NW; Washington, DC 20036.