Phenothiazines are a class of drugs used in psychiatric medi

Phenothiazines are a class of drugs used in psychiatric medicine. These drugs are alpha 1 blocking agents. Their effects on the CNS are useful to treat various psychoses. However, they also have peripheral side effects, including changes in pupil diameter and blood pressure. Remembering that constricting blood vessels tends to increase blood pressure and dilating blood vessels tends to reduce blood pressure, explain these side effects by answering the following questions: What effect does epinephrine/norepinephrine have on the alpha receptors of the iris of the eye or the skeletal muscle vasculature? What would be the result in each of those places if the alpha 1 receptors were blocked? That explains why you can often tell which types of medication people are taking by looking at their pupils! Many illegal drugs are sympathetic agonists. What would the pupils of someone taking such sympathomimetic drugs look like?

Solution

Basically adrenergic receptors are of 2 categories -

Alpha recepetors -

Beta receptors -

The above question purely concentrates on alpha1 and beta2 receptors.

Adrenergic recpetors normally activates the sympathetic nervous system giving rise to the fright, flight and fight response.

Catecholamins are the main class of compounds that target there receptors. Epinephrine and Norepinephrine are catecholamines. Both tend to give similar response (i.e., as their names might suggest, epinephrine and norepinephrine, they are not antagonists).

Answer a)

Alpha1 receptors are present on the muscles of the iris of the eye and beta2 on the skeletal muscle vasculature. When activated, alpha1 receptors increases the activity of the cells they are associated with and beta2 decreases the activity of the cells they are associated with. These responses are required for dealing with flight, fight or fright situations.

Answer b)

If the alpha1 recpetors were blocked on the dialator muscles of the iris - vision remains normal.

If the beta2 receptors were blocked on the skeletal muscle vasculature - blood flow to the skeletal muscles remains normal.

Both these responses make the person less able to combat a flight, fright or fight situation.

Answer c)

Sympathetic agonists are those compounds that have similar effect as those compounds that induce sympathetic response (like the class of drug mentioned in the question - Phenothiazine).

Sympathetic responses are the flight, fright and fight responses. Thus blood pressure tend to increase, heart rate increases, pupild enlarge, etc in these reponses.

Therefore, the pupils of a person on sympathomimetic drugs (drugs that mimic the action of compounds causing sympathetic response), would be enlarged due to the activation of alpha1 receptors on the dialator muscles of iris.