Morphine Pharmacology

Morphine structure and chemistry

Morphine is an opiate found in opium, the juice secreted by the seedpods of poppies. It is a potent pain reliever and is similar in structure to other opiate analgesics.

Mechanism of action

Opioid receptors

As morphine binds to opioid receptors, molecular signalling activates the receptors to mediate certain actions.

There are three important classes of opioid receptors and these are:

• μ receptor or Mu receptors - There are three subtypes of this receptor, the μ1, μ2 and μ3 receptors. Present in the brainstem and the thalamus, activation of these receptors can result in pain relief, sedation and euphoria as well as respiratory depression, constipation and physical dependence.
• κ receptor or kappa receptor - This receptor is present in the limbic system, part of the forebrain called the diencephalon, the brain stem and spinal cord. Activation of this receptor causes pain relief, sedation, loss of breath and dependence.
• δ receptor or delta - This receptor is widely distributed in the brain and also present in the spinal cord and digestive tract. Stimulation of this receptor leads to analgesic as well as antidepressant effects but may also cause respiratory depression.

Pharmacokinetics

Morphine can be administered orally, intravenously, rectally, subcutaneously, through spinal injection (e.g. epidural) as well as through inhalation or snorting. The drug has a significant amount of first-pass metabolism in the liver with only around 40 to 50% of the amount absorbed actually reaching the nervous system. Most of the morphine is processed in the kidneys and eliminated from the body in urine.

Adverse effects or side effects of morphine

Morphine has many side effects. Some of the more common and more dangerous ones include:

• Nausea, vomiting and abdominal cramps
• Constipation
• Sedation and drowsiness
• Itching and allergic skin reactions causing warmth and flushing
• Shrinking of the pupils to pin points
• Respiratory depression or suppressed breathing
• Initial doses lead to euphoria but higher doses cause unpleasant symptoms such as hallucinations, delirium, dizziness and confusion
• Formation of physical or psychological dependence and development of withdrawal symptoms when use of the drug is stopped
• Development of tolerance and the need to increase dose to achieve the same degree of effects as before
• Risk of overdose and poisoning
• Transmission of HIV/AIDS and hepatitis B and C among needle users.

Morphine dose

Morphine sulphate used for pain relief may be injected intramuscularly in doses of 100-150 mcg/kg every 2 hours as required. For operations and anesthesia, the dose of morphine that needs to be administered depends on the duration of surgery and the severity of pain anticipated.