Apparent authority can legally be found, even if actual authority has not been given. There must be some act or some knowing omission on the part of the principal-if the agent alone acts to give the third party this false impression, then the principal is not bound.
An endogenous agonist for a particular receptor is a compound naturally produced by the body that binds to and activates that receptor. For example, the endogenous agonist for serotonin receptors is serotonin, and the endogenous agonist for dopamine receptors is dopamine. Full agonists bind to and activate a receptor with the maximum response that an agonist can elicit at the receptor. One example of a drug that can act as a full agonist is isoproterenol, which mimics the action of adrenaline at Î² adrenoreceptors.
A receptor is basically a protein in a cell (usually in a cell membrane, but doesn't have to be (eg. NO and steroid receptors). When a ligand (an agonist) meets the receptor, the protein changes its physical conformation, making it more or less likely to do something else, eg. catalyze something, open an ion channel. Antagonists bind to receptors like agonists, but without doing anything, effectively blocking them.