BIOPHYSICAL MECHANISMS OF SINGLE PHOTON DETECTION

描述(改编自申请者的摘要)： 用于计算光子吸收的暗适应视觉系统已为人所知 然而，使光子探测的生物物理机制 以及可能的计数还不清楚。杆状视觉的大部分发生在 很少发生光子吸收的光级别；因此，这些 生物物理过程将严重损害杆状视力。产品的可靠性 光子计数是指杆体外段的传导过程 必须对O、1或2个光子的吸收产生可区分的响应， 并且这些信号可以可靠地从杆子传输到其余部分 视觉系统。目前对信号转导和信号转导的理解 突触传递不能解释这种水平的表现。 这里提出的实验将确定可靠的调解机制 单光子吸收的传输及其可靠传输 对二次细胞的反应。 对光传导的研究将确定每个吸收的光子是如何 产生几乎相同的电流响应。这种重现性是 对于可靠的光子计数是必不可少的，但考虑到预期的 所涉及的少量分子的统计波动， 尤其是在光激视紫红质的关闭过程中。中的元素 转导级联将被选择性地改变，以确定它们是否 对于可重现的单光子响应是必要的。 对突触传递的研究将决定单光子反应 被可靠地传输到视网膜中的二级细胞，以及如何 突触前机制将光信号与光感受器噪音分开。 穿过突触的单光子信号和产生的噪声 将通过监测突触后突触来研究突触传递 在控制棒电压的同时，双极电池中的电流。讯号 转移到双极细胞也将光信号从光感受器中分离出来 噪音。调节这种突触过滤的一个可能的机制是控制 突触终末的钙浓度。为了测试这一想法， 终末钙变化的动力学将用荧光法测量 并与信号传递动力学指标进行比较。进一步的考验 将通过确定信号传递的动力学如何改变来实现 当终端的缓存容量发生变化时。

DESCRIPTION (Adapted from applicant's abstract): The ability of the dark-adapted visual system to count photon absorptions has been known for many years; however, the biophysical mechanisms which make photon detection and counting possible are not understood. Much of rod vision occurs at light levels where photon absorptions occur rarely; thus, failure of these biophysical processes will severely impair rod vision. The reliability of photon counting means that the transduction process in the rod outer segment must produce distinguishable responses to absorption of O, 1, or 2 photons, and that these signals are reliably transmitted from the rod to the rest of the visual system. Current understanding of signal transduction and synaptic transmission cannot account for this level of performance. Experiments proposed here will determine the mechanisms mediating reliable transduction of single photon absorptions and reliable transmission of these responses to second order cells. Studies of phototransduction will determine how each absorbed photon produces a nearly identical current response. This reproducibility is essential for reliable photon counting, but is surprising given the expected statistical fluctuations in the small number of molecules involved, particularly in the shutoff of photoexcited rhodopsin. Elements of the transduction cascade will be selectively altered to determine if they are necessary for reproducible single photon responses. Studies of synaptic transmission will determine how single photon responses are reliably transmitted to second order cells in the retina, and how pre-synaptic mechanisms separate light signals from photoreceptor noise. The single photon signal traversing the synapse and the noise generated in synaptic transmission will be studied by monitoring the post-synaptic current in a bipolar cell while controlling the rod voltage. Signal transfer to bipolar cells also separates light signals from photoreceptor noise. A likely mechanism mediating this synaptic filtering is the control of the Ca2+ concentration in the synaptic terminal. To test this idea, the kinetics of Ca2+ changes in the terminal will be measured with fluorescent indicators and compared to the kinetics of signal transfer. A further test will be made by determining how the kinetics of signal transfer are altered when the buffering capacity of the terminal is changed.