Drugs enter the bloodstream and carried through capillaries (small blood vessels) There is a blood-brain barrier (lining that allows nutrients to pass through so that neurons can function), but some drugs manage to get through as well. Agonist a drug that increases the activity of a neurotransmitter: facilitates synaptic transmission, may enhance the production, storage, or release of a neurotransmitter, e. g. opiates (activate endorphins), amphetamines (activate dopamine and norepinephrine) Antagonistic a drug that inhibits or decreases the actions of a neurotransmitter: may reduce the synthesis, storage, or release of a neurotransmitter, e. g. antipsychotics (bind to dopamine) Synthesis, storage, release: agonists cause neurons to synthesize more neurotransmitters, store them safely, or release them. Reuptake: agonist blocks reuptake of neurotransmitter into the presynaptic neuron more neurotransmitters remain in the synapse and are available to activate the postsynaptic neuron. As a drug is used more, intensity of effects produced decrease called tolerance. Must take larger doses to receive same effects as before.