Many psychopharmaceutical drugs change synapse chemistry or directly agonize neuroreceptors in the brain. For example, cabergoline is a D2 agonist.
How do these compounds enter the physical synapses? Are they simply lipid soluble and diffuse throughout all cells?
The answer is that cabergoline does not need to enter a cell in order to function.
Cabergoline (as well as most (possibly all?) other drugs that target synapses) interact with proteins on the surfaces of the synapses. For example, the D2 receptor that you mention is a dopamine receptor which in its active form is expressed on the surface of neurons near the synaptic cleft.
