Question

The \(\mu\) (Mu) opioid receptor is responsible for the following effects except

• A. Miosis
• B. Bradycardia
• C. Hypothermia
• D. Bronchodilation

Hint:

Mu receptor activation gives miosis, bradycardia, and hypothermia; airway smooth muscle is not relaxed by opioids, and morphine can even trigger bronchospasm through histamine release.

Answer 1

The Correct Option is D

Step 1: Recall where mu receptors sit and what they usually do.
Mu opioid receptors are spread through the brainstem, midbrain, spinal cord, and autonomic centres. Along with pain relief, their activation produces a set of classic side effects that examiners like to test.

Step 2: Walk through the true mu effects.
Miosis, pupil constriction, comes from stimulation of the Edinger-Westphal nucleus. Bradycardia comes from increased vagal tone at the brainstem cardiovascular centres. Hypothermia comes from a mu mediated reset of the hypothalamic temperature set point, so opioids lower core body temperature. All three of these are well documented mu receptor actions.

Step 3: Check the airway effect.
Opioids, morphine in particular, are known to release histamine from mast cells and can trigger bronchoconstriction and even wheeze in susceptible patients. This is the opposite of bronchodilation. There is no accepted mu receptor pathway that relaxes bronchial smooth muscle.

Step 4: Match this to the question.
The question asks for the one effect that is NOT produced by mu receptor activation. Since bronchodilation is not on the list of genuine mu effects, and if anything the opposite can occur, this is the odd one out.

Final Answer:
\[ \boxed{\text{Bronchodilation}} \]