The mood stabilizer Lithium has many effects, including changes in second-messengers, probably leading to changes in gene expression and receptor density.

Lithium

Some drugs can impact postsynaptic transmitters by directly altering intracellular signals such as the activation of second messengers, activation of genes, and the production of various proteins.

Bowdoin College

When they bind to their matching receptor membranes, neurotransmitters can stimulate a variety of changes within the postsynaptic cell. Examples of this include: 
- regualtion of the number of postsynaptic receptors 
- modulation of intracellular signals

Drugs effects on cellular processes postsynaptically 

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

Behavioral Neuroscience 8th Edition

Drugs can impact the postsynaptic cell by regulating the number of postsynaptic receptors. These drugs can up- or down-regulate the number of receptors for transmitters, this changing the sensitivity of the synapse.

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