CN2010 Exam 2 KEY - 1. 10 pts (SM). Synaptic physiologists...

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1. 10 pts (SM). Synaptic physiologists have studied synapses that employ postsynaptic ligand- gated channels for many decades because the readout is (mostly) linear with respect to the amount of transmitter release. Discuss technical limitations and biological realities that limit the linearity between the amount of transmitter release and the size of the postsynaptic signal at glutamate synapses that employ AMPA receptors for transmission. Discuss both EPSP measurements and EPSC measurements. Consider baseline transmission only (not plasticity). When possible, suggest a way of overcoming the limitations with experimental manipulations or by choice of experimental preparation. Sample answers: Non-linear summation when studying EPSPs (voltage clamp). Receptor saturation (rapidly dissociating antagonist). Quantal variability (calyx of Held seems to have normal distribution), spatial filtering (somatic synapse with no dendritic synapses). Receptor desensitization (applicable to calyx and other large synapses). 2. 10 pts (SM). A small molecular buffer A for calcium has a binding rate constant for Ca 2+ of 5 x 10 8 M -1 s -1 ; Its dissociation rate constant is 80 s -1 . Another buffer B has a binding rate constant for Ca 2+ of 2.7 x 10 6 M -1 s -1 and a dissociation rate constant of 0.43 s -1 . A. Calculate the Kd for buffer A and the Kd for buffer B. You decide to investigate the Ca 2+ dependence of transmitter release at the squid giant synapse. You are puzzled to find that when injected into the presynaptic terminal, buffer B has no effect on action potential-driven transmitter release at concentrations up to 5 mM. In desperation, you try buffer A and find that it abolishes transmitter release at this same concentration. B. Given what you know about events driving transmitter release, explain why there is a difference in buffer actions. In Aim 2 of your dissertation work, you explore another synapse, the sea turtle giant synapse, with the same two buffers. When buffer B is introduced into this presynaptic terminal, transmitter release is reduced by 75%. Buffer A again abolishes release. C. Offer a hypothesis to explain the different effectiveness of buffer B at the squid synapse versus the sea turtle synapse. Answer 0.16 µM (160 nM), so equilibrium affinity of the two buffers is similar. Actions of buffer are not in equilibrium with calcium. Relevant [Ca] for vesicle fusion is at the mouth of the Ca channel. Slow buffer on rate is not sufficient to capture, effectively compete with Ca binding the sensor. On the other hand, fast-on buffer can effectively compete. So despite similar ss binding affinities, buffer A, the faster buffer, is the only one that can effectively compete with the sensor under non-equilibrium conditions. At the sea turtle synapse, calcium channels may be further from the vesicles.
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This note was uploaded on 02/27/2012 for the course BIO 5571 taught by Professor P.taghert during the Fall '11 term at Washington University in St. Louis.

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CN2010 Exam 2 KEY - 1. 10 pts (SM). Synaptic physiologists...

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