eptor subtypes in the type opioids exist defining receptor mutiplicity, it means a class of receptors say adrenergic has subtypes within that class. These adrenergic subtypes allow endogenous noradrenaline to bind and activate the receptor resulting in a biological change, but specific ligands can also activate a selective subtype in that class of receptors without affecting the other subtypes for example salbutamol is an agonist of the 2 adrenoceptor only and produces bronchodilation, it is used in the treatment of asthma and produces little side affects due to it’s selectivity.Different subtypes of the same receptor class can also mediate different biological responses at different locations in the body, the nicotinic cholinergic receptor for example, when antagonised in the ganglia to control blood pressure does not affect the nAChR in skeletal muscle paralysing it. Other agents such as tubocurarine has the ability to antagonize the action of acetylcholine at the neuromuscular junction and is confined to this location. These subtypes of receptor are analogous to tissue specific isozymes of an enzyme.There are two main mechanisms of receptor G –protein coupled receptors and ligand gated ion channels. These two types can occur in the same family for example the serotonin receptor class are all G protein coupled receptors apart from 5HT3 which is a ligand gated ion channel. The differences in these two type will be explained later on(Figs. 1 and 2). The mechanisms of action of some receptor subtypes may be very similar, differing only slightly in kinetics or regulatory activity, but other subtypes the G protein coupled receptors display fundamental differences in their biochemical and cellular regulatory activities. For example 1 and 2 adrenergic and M1 and M2 muscarinic cholinergic subtypes. All four subtypes regulate G proteins, the 1 Figure 3adrenergic and the M1 and M3 muscarin...
