The capsaicinoids in chilli bind to a receptor in the lining of the mouth. This is the same receptor that registers pain from heat, thus the effect is a burning feeling. Repeated exposure to capsaicinoids depletes these receptors, enabling you to eat hotter chillis and feel the same effect. The pain caused by this leads to the release of endorphins, the body’s natural painkillers.
Endorphins are a class of neurotransmitters produced by the body and used internally as a painkiller. This class of compounds are similar in their action to opiates, attaching to some of the same receptors in the brain. They are a strong analgesic, and give a pervasive sense of happiness and well-being and are even said to increase the libido!. They are proteins, and due to their many different types and their complexity, the release of endorphins lowers the blood pressure. Endorphins are best known to those who exercise a lot, and give rise to what is known as the ‘runner’s high’. Their release is caused by all pain, including that caused by chillies. Thus a dose of hot chillies will cause the release of these compounds, without any permanent harm.
Fatman chilli – http://fatmanchilli.webs.com/whychillies.htm