Why do we love receptors? Because if we know what the receptor does, we know what the drug does! Seriously:
To know a drug you MUST know the receptor.
This will save you many, many sympathetic responses. Trust me.
Receptors are usually proteins that are embedded in the cell membrane, floating in the cytoplasm, or part of the nuclear envelope. There are many subtypes that are very important to know. For now, let’s look at the main categories so you get an idea how they work.
There are a few different categories of receptors. It’s not necessary to know these in ridiculous detail to get how drugs work, but here’s a little bit of an overview to familiarize yourself with these important characters:
Ligand-gated Ion Channels: These are protein channels that are activated to open or close when a certain type of molecule attaches to it. These molecules, called ligands, work a little bit like keys activating a lock. This activation causes a change in the shape of the protein, which allows ions (usually sodium, potassium, chloride or calcium) to flow along their concentration gradient. This produces a change in the cell!
Voltage-gated Protein Channels: Without these guys, we really couldn’t do anything. This is the type of receptor that activates an action potential–the firing of a neuron–and also the type involved in muscle contractions. VGPC’s act by responding to a change in the positive-negative electricity balance on either side of the cell membrane. When the voltage switches from positive to negative, the channels open. These are often responses to the initial actions of a ligand-gated channel!
G-Protein Coupled Receptors: A great many receptors are of this type. When a molecule binds to the receptor, the receptor actually changes its shape. This change causes a series of proteins to be activated in what is called a signalling cascade. This chain reaction causes a certain process to occur in the cell, thus causing the cell to do or not do something.
Transcription Factors: What? Transcription? Yes, it’s true. There are these little guys floating around in the cytoplasm, waiting for a hormone or other uncharged lipid-soluble drug to scream through the cell membrane! Certain drugs go inside the cell and activate transcription factors, thereby executing a direct effect on DNA transcription. Crazy!
Transport Proteins: There are some drugs that act within the cells that just cannot be formulated to either diffuse across the cell membrane or fit through an existing channel by themselves. This is usually because they are too big, move against the concentration gradient, or not lipid-soluble. Sigh. Luckily, there are transport proteins! Not only do these proteins help get drugs across the cell membrane (or even the nuclear envelope!), but they also help kick toxins (and medications, before they become toxic) out of the cell.
There are other categories of receptors, but this should give you a good start.