Although the naturally occurring neurotransmitter binds to all of its corresponding receptor types, the different structure of these receptors means that some drugs bind more readily to one type of receptor than another. This is ideal for achieving one specific drug action, whilst minimising side effects.

For example, salbutamol is a drug that mimics the action of adrenaline and noradrenaline in the sympathetic nervous system. However, it has a much greater affinity for the B-receptors, and in particular the B2 - receptors (found in the lungs), rather than the B1 - receptors (found in the heart). This means that it is good for dilating the airways in the treatment of asthma, whilst having a much weaker effect on the heart, and so minimising unwanted effects such as tachycardia (increased heart rate).

Other examples of different receptors for the same neurotransmitter are the receptors for acetylcholine in the peripheral nervous system. There are two main types of receptor here:

• Nicotinic receptors, which are located on the post-ganglionic cell membranes in both the sympathetic and parasympathetic division of the autonomic nervous system, as well as the skeletal neuromuscular junctions:

In the sympathetic nervous system, on the post-ganglionic neurones

In the somatic nervous system, at the neuromuscular junction

For further information on location and function of different receptors:

Merck Manual On-line http://www.merck.com/pubs/mmanual/section14/chapter166/166a.htm [accessed July 2002]