Pain serves a vital biological defensive function, often associated with other psychological and central disturbances. It is a physiological condition, which is detected by refined receptors within the damaged body tissues. It has two components: the motivational-affective (emotional) component and the sensory-discriminative component. Nociception and pain are not necessarily analogous. Nociception is the term applied to perception of nociceptor activation by noxious stimuli, whereas pain refers to a subjective response. One pain classification describes: physiological, inflammatory (from tissue damage) and neuropathic pain (from changes in nerves) with peripheral and central nervous system (PNS and CNS, respectively) changes some of which may be permanent, altering the brain's future perspective of pain. We describe basic receptor pharma cology and then some of the more important and unusual receptors in the ‘pain pathway’.

General terminology

Some of the terms used in receptor pharmacology are described below:

• Ligand: General term for a molecule (peptidic, chemical, ionic or synthetic), which binds at a receptor site.

• Agonist: Ligand that binds at, and activates, a regulatory receptor to produce a pharmacological response (can be a full or partial response).

• Antagonist: Ligand that binds at a receptor site but does not produce a response. Binding is generally, but not exclusively, reversible (competitive) and attenuates the effect of an agonist.

• […]