1.3. Describe the role of receptors as targets for drug action

This guide will help you answer 1.3. Describe the role of receptors as targets for drug action.

Drugs interact with the body in various ways to treat illness, relieve symptoms, or modify how the body functions. A major way that drugs work is by targeting specific receptors found in cells. Understanding how receptors function and their role in drug action is key to knowing how medicines affect the body.

What Are Receptors?

Receptors are specialised proteins found in or on the surface of cells. Their primary function is to receive and respond to signals from the body’s internal or external environment. These signals can come from hormones, neurotransmitters, or other molecules. Receptors act as communication points between cells and their surroundings.

They work like locks, and the molecules that bind to them act like keys. When the right molecule (the “key”) attaches to the receptor (the “lock”), it causes a response in the cell. This response can activate or inhibit certain functions inside the cell. For example, it might encourage the cell to divide, produce a substance, or stop performing a particular function.

Types of Receptors

Receptors can be classified into several types depending on their location and how they work in the cell. The most common types involved in drug action include:

1. G Protein-Coupled Receptors (GPCRs)

These are the most common receptors targeted by drugs. They sit on the cell surface and span the cell membrane. GPCRs play an essential role in transmitting signals from outside the cell to the inside. When a molecule binds to a GPCR, it sets off a chain of events that influence the cell’s behaviour. For example, GPCRs are involved in regulating the heart rate, digestion, and mood.

2. Ion Channel Receptors

These control the flow of ions (such as sodium, potassium, or calcium) into or out of the cells. Ions are charged molecules that help the cell communicate or perform specific actions. Ion channel receptors only open or close when the correct molecule binds to them. This mechanism is vital for processes like nerve signal transmission and muscle contraction.

3. Enzyme-Linked Receptors

These receptors are connected to enzymes on the cell surface. When a molecule binds, it activates the enzyme, which then triggers a chemical reaction within the cell. This type of receptor is important for controlling processes like cell growth, division, and repair.

4. Intracellular Receptors

These receptors are located inside the cell rather than on its surface. Molecules that target intracellular receptors must cross the cell membrane. Steroid hormones like cortisol and oestrogen often work through these receptors. Once activated, the receptor influences gene expression and guides the cell’s longer-term activities.

How Receptors Act as Targets for Drugs

Many drugs work by interacting with receptors. The goal is usually to either mimic or block the body’s natural molecules that activate the receptor. By doing so, the drug can increase or decrease the activity of the specific pathway the receptor controls.

Drugs interact with receptors in two main ways:

1. Agonists

Agonists are drugs that mimic a natural molecule. When they bind to the receptor, they activate it, just like the body’s own molecules would. This type of drug enhances the receptor’s normal response.

For example:

• Salbutamol, an asthma medication, is an agonist that targets receptors in the lungs. It relaxes the airways by mimicking the action of adrenaline, making breathing easier.
• Morphine is another example of an agonist. It binds to opioid receptors in the brain, relieving pain by mimicking natural pain-relieving molecules.

2. Antagonists

Antagonists are drugs that block receptors. These drugs prevent the body’s natural molecules from binding to the receptor. They effectively stop the receptor from signalling.

For example:

• Beta-blockers, like propranolol, are antagonists that block receptors in the heart. This can reduce heart rate and blood pressure.
• Antihistamines, used to treat allergies, block histamine receptors. This stops the allergic reaction from causing symptoms like itching or swelling.

The strength and duration of these interactions depend on the drug’s affinity (how strongly it binds to the receptor) and its concentration in the body.

Drug-Receptor Interactions and Selectivity

Drugs are designed to target specific receptors to minimise side effects. A drug’s selectivity refers to its ability to bind only to the intended receptor. High selectivity means the drug is less likely to affect other parts of the body.

However, no drug is 100% selective. For instance, some medications may interact with unintended receptors. This can cause unwanted side effects.

Example:

• Antidepressants, like SSRIs, primarily affect serotonin receptors. However, they may also influence other neurotransmitter systems, leading to effects like nausea or fatigue.

Different drugs can compete for the same receptor. For instance, if two drugs bind to the same receptor, one may block the other’s effect. This is an important consideration when combining medicines.

Receptor Regulation and Drug Tolerance

Receptors do not remain static—cells can increase or decrease the number of receptors to adapt to their environment. This is called receptor regulation.

For example:

• If a receptor is overactivated by a drug, the body may reduce the number of receptors available. This process is called downregulation and often leads to drug tolerance. Someone taking a drug for a long period may find it becomes less effective because there are fewer receptors available for it to target.
• Conversely, if a receptor is underused, the body may increase the number of receptors. This is called upregulation. It can make the body more sensitive to certain drugs when they are introduced.

Drug designers need to consider how prolonged use could affect receptor regulation and modify their approach accordingly.

The Limitations of Receptor Targeting

While drug-receptor interactions are highly effective in many cases, there are limitations:

• Some diseases do not have identifiable receptor targets. In these cases, alternative approaches are required.
• Not all receptors behave predictably. Some might trigger multiple responses, some of which are unwanted.
• Receptor targeting is not a cure for chronic conditions. It may only manage the symptoms.

Conclusion

Receptors are central to how drugs work in the body. They act as gateways for influencing cells’ behaviour. Drugs can either mimic natural molecules (agonists) or block them (antagonists). Their effectiveness depends on how well they interact with receptors and their ability to target the right structures.

Understanding receptors allows health professionals to use medications more effectively. Tailored drug treatments advance the ability to help individuals live healthier lives with fewer side effects.