A molecular structure on a cell that allows binding with a biochemical such as a neu-ROTRANSMITTER or a hormone. Most receptors are “open” to just one such substance; some can bind with multiple substances. Some receptors in the brain can bind with either norepinephrine or dopamine, for example. Generally multiple bindings involve substances in the same chemical family; norepinephrine is a dopamine precursor (the body can convert it into dopamine). Binding either activates or inhibits a function within the cell. Many anti-parkinson’s medications either block binding, as do anticholinergic medications, or mimic the natural biochemical and bind in its place, as do dopamine agonist medications.