Central Analgesic Activity

Many diseases have pain as a classical symptom requiring treatment with analgesics. Primary cancer and secondary metastases can lead to severe pain, which needs to be managed with strong analgesics, like opioid drugs. Opioids usually are safe when they are used correctly, but people who misuse opioids can become addicted. The addiction liability of opioids led to intensive research for compounds without this side effect. Many approaches have been used to differentiate the various types of actions of strong analgesics by developing animal models not only to study for analgesic activity but also to understand addiction liability. Several types of opioid receptors have been identified in the brain allowing in vitro binding tests. However, the in vitro tests can only partially substitute for animal experiments involving pain. Pain is a common phenomenon seen in all animals, at least in vertebral animals, similar to that felt by man. Analgesic effects in animals are comparable with the therapeutic effects in man. Needless to say, in every instance, painful stimuli to animals must be restricted as much as possible. Painful stimuli can be elicited due to direct stimulation of the efferent sensory nerves or stimulation of pain receptors by various means such as heat or pressure. The role of endogenous peptides such as enkephalins and endorphins gives more insight into how do brain processes pain and the action of central analgesics. Endorphin and enkephalin are the body’s natural painkillers. We need to consider how an understanding of the opioid system affects clinical responses to opiate administration and what the future may hold for improved pain relief.