lect20-08 - Zoo/Neuro/Psych 523 Reading for lect 20 is pp...

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Zoo/Neuro/Psych 523 Lecture 20 2008 Reading for lect 20 is pp 200 - 206. The Role of Calcium in Release The most important experiments on this topic were carried out on the squid giant synapse. This preparation has the advantage that, unlike the frog neuromuscular junction, the presynaptic terminal is large enough to penetrate with microelectrodes, so that the membrane potential can be measured and controlled; also various interesting things can be injected into the nerve terminal directly from an intracellular electrode. This preparation was used to answer several important questions, for example are action potential currents that flow into the terminal necessary for transmitter release. This answer is NO! (If INa is blocked with TTX, and the terminal is depolarized by passing current from an intracellular electrode, release still occurs. Similarly, if IK is blocked with TEA or 4-AP, depolarization causes release. Therefore neither INa nor IK are needed for release). However if I Ca is blocked by lowering external [Ca++] to zero, or by using a specific Ca channel blocker, release is blocked. The role of the action potential seems to be merely to depolarize the cell, and this depolarization produces an increase in g Ca , so that Ca++ enters and release occurs. If this hypothesis is correct, then the following should be true: 1. If Ca++ is injected directly into the terminal, transmitter release should be stimulated. Miledi showed that this worked. 2. There should be a voltage sensitive Ca++ channel in the terminal. Katz and Miledi poisoned the squid synapse with TTX and TEA to suppresses INa and IK, and found that when they depolarized the terminal, a regenerative depolarizing current could be measured. Its amplitude depended on the external Ca++ concentration, and on the amount of depolarization up to a certain point and then decreased if the depolarization was increased even more. Finally if they went all the way to 140 mV, no release occurred. Why? E Ca = 140 mV of course. If they gave the terminal a 140 mV depolarization, no release occurred, but when the pulse was turned off, an enormous PSP occurred. This was because on going to 140 mV, g Ca is turned on, but there is no I Ca (driving force is zero). On turning the pulse off, the Ca++ channels do not close immediately, so a calcium current flows into the terminal and transmitter is released. There is a
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lect20-08 - Zoo/Neuro/Psych 523 Reading for lect 20 is pp...

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