Neurotransmitter Release

We now return to the synaptic terminal for a more detailed examination of the mechanism of neurotransmitter release. Acetylcholine is released from the motor nerve terminal in quanta consisting of many molecules. Thus, the basic unit of release is not a single molecule of ACh, but the quantum. At the neuro-muscular junction, it is estimated that a single quantum of ACh contains about 10,000 molecules. An individual quantum is either released all together or not released at all. The release of ACh during neuromuscular transmission can be thought of as the sudden appearance of a "puff" of ACh molecules in the extracellular space as the entire contents of a quantum is released. A single pre-synaptic action potential normally causes the release of more than a hundred quanta from the synaptic terminal.

The original suggestion that ACh is released in multimolecular quanta was made on the basis of a statistical analysis of the response of the postsynaptic muscle cell to action potentials in the presynaptic motor neuron. This analysis was first carried out by P. Fatt and B. Katz, and it initiated a series of studies by Katz and coworkers that gave rise to the basic scheme for chemical neurotransmission presented in this chapter. Experimentally, the analysis was accomplished by reducing the extracellular calcium concentration to the point where the influx of calcium ions into the synaptic terminal during an action potential was much less than usual. Under these conditions, a single presynaptic action potential released on average only one or two quanta of ACh instead of more than a hundred. Examples of end-plate potentials recorded in a muscle cell in response to a series of presynaptic action potentials are shown in Figure 8-7. Because only a small number of quanta are released per action potential, the

( PNa

= 1 Unit \ PNa/PK = 0 02

1 Pk

50 units J Em = -71 mV

+ ACh (add 50 units

each of PNa & Pk )

\

/PNa =

51 Units\ PNa/PK = 0.51

PK =

100 units Em a -17 mV

Figure 8-5 Opening a channel that allows both potassium and sodium to cross the membrane results in a higher value for pNa/pK and causes depolarization.

Figure 8-5 Opening a channel that allows both potassium and sodium to cross the membrane results in a higher value for pNa/pK and causes depolarization.

Figure 8-6 (a) A view through the electron microscope at the face of the postsynaptic membrane of the electric organ of the electrical skate, Torpedo. This organ, which is a rich source of ACh receptors for biochemical study, is a specialized type of muscle tissue. The membrane particles are the ACh-activated channels of the postsynaptic membrane. (b) Several views of individual ACh receptors that have been chemically isolated from preparations like that in (a), then placed in artificial membranes. (Courtesy of J. Cartaud of the Institut Jacques Monod, CNRS/ Universitè Paris 7, France.)

end-plate potentials in the reduced calcium ECF are much smaller than usual and do not reach threshold for generating an action potential in the muscle cell. Notice that the amplitude of the depolarization of the muscle cell fluctuates considerably over the series of presynaptic action potentials: sometimes there was a large response and other times there was no response at all. Fatt and Katz measured a large number of such responses and found that the amplitudes clustered around particular values that were integral multiples of the smallest observed response. For example, as shown in Figure 8-7b, there might be a cluster of responses that were 1 mV in amplitude, another cluster at 2 mV, and another at 3 mV. This indicates that the response was quantized in irreducible units of 1 mV, and that the presynaptic action potential released ACh in corresponding quantal units. Thus, a given presynaptic action potential might release three, two, one, or no quanta, but not 0.5 or 1.5 quanta.

Fatt and Katz also observed occasional, small depolarizations that occurred in the absence of any presynaptic action potentials. These spontaneous

Size of depolarization (mV)

Figure 8-7 Quantized responses of muscle cell to action potentials in the presynaptic motor neuron. Arrows give timing of the presynaptic action potentials. (b) The graph shows the peak response amplitudes recorded in response to a series of several hundred presynaptic action potentials like those shown in (a).

Size of depolarization (mV)

Figure 8-7 Quantized responses of muscle cell to action potentials in the presynaptic motor neuron. Arrows give timing of the presynaptic action potentials. (b) The graph shows the peak response amplitudes recorded in response to a series of several hundred presynaptic action potentials like those shown in (a).

depolarizations had approximately the same amplitude as the single quantum response produced by presynaptic action potentials in low-calcium ECF. That is, if the irreducible unit of evoked muscle depolarization was 1 mV, then the spontaneous events also were about 1 mV in amplitude. Figure 8-8 shows several of these spontaneous depolarizations recorded inside a muscle cell. These events are called miniature end-plate potentials, and they are assumed to result from spontaneous release of single quanta of ACh from the synaptic terminal. Under normal conditions, these spontaneous events occur at a low rate about 1 or 2 per second; however, any manipulation that depolarizes the nerve terminal increases their rate of occurrence, confirming that their source is the process that couples depolarization to quantal ACh release during the normal functioning of the nerve terminal.

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