Caffeine acts by blocking the auto receptor effect of the endogenous ligand, adenosine. This in turn increase catecholamine release, resulting in arousal

Caffeine

One way drugs can effect presynaptic transmitter release is through modifying the systems that the neuron uses to monitor and regulate its own transmitter release. This includes altering the auto-receptor that monitors how much transmitter has been release presynaptically. 

Bowdoin College

Even with an adequate supply of transmitters in vesicles in the presynaptic terminal, various agents or conditions can prevent the release if transmitters when an action potential reaches the terminal. Examples of this include: 
- The prevention of synaptic transmission 
- Alteration of synaptic transmitter release
- Alteration of transmitter release through modulation of presynaptic activity 

Drugs effects presynaptically on transmitter release

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

Behavioral Neuroscience 8th Edition

Drugs can work to prevent axons from firing action potentials which prevents synaptic transmission. This can occur in many ways.

Drugs preventing synaptic transmission 

Some drugs are able to alter synaptic transmitter release. This can be done through many ways but include the over stimulation and/or prevention of the release of transmitters. Some are even able to target specific transmitters. 

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