Many drugs used to treat the nervous system act by modulating specific synaptic interactions. Drugs that mimic or potentiate the effect of a neurotransmitter are called agonists; those that block the actions of neurotransmitters are called antagonists. For example, morphine is an agonist at the endorphin receptor and therefore blocks pain. Propranolol, a widely used beta blocker, is an antagonist of receptors in the fight-or-flight response and therefore decreases panic attacks and anxiety. A major emphasis in neurobiology is to identify neurotransmitter receptor subtypes and design drugs that selectively bind to them to have highly specific effects on nervous system activity.