Reserpine blocks the packaging of transmitter molecules within vesicles, thereby allowing the transmitter to be broken down by enzymes.

Reserpine

Drugs that do not prevent the production of transmitter molecules but instead store transmitters in synaptic vesicles for later release or they can prevent the packaging of transmitter molecules within vesicles and allow them to be broken down by enzymes. Both ways deplete transmitters at synapses.

Bowdoin College

For a presynaptic neuron to produce neurotransmitters, the axon terminal must receive a steady supply of raw materials and the enzymes that use them to synthesize transmitter molecules, vesicles, and other important components. Some drugs alter this process by: 
- inhibition of transmitter synthesis
- blockade of azonal transport 
- interference with the storage of transmitters

Drugs affects on presynaptic transmitter production

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

Behavioral Neuroscience 8th Edition

Inhibition of transmitter synthesis by a drug may happen in many ways. For example, a drug may inhibit the enzymes that neurons use to synthesize a particular neurotransmitter, resulting in depletion of that transmitter.

Drug inhibition of transmitter synthesis

Drugs that block axonal transport prevent materials from reaching the axon terminals, therefore causing the terminals to run out of neurotransmitters.

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