Type # 1. Receptors for Fast Neurotransmitters:
These are coupled directly to an ion- channel. These receptors are involved mainly in fast synaptic transmission. Here the drug binding and channel opening occur on a millisecond time scale e.g., the nicotinic acetylcholine receptor (nAChR), the GABAA receptor, the glutamate receptor.
Type # 2. Receptor for many Hormones and Slow Transmitters:
These are coupled to effector systems via a G-protein. The G-protein coupled receptor family comprises most of the receptors that are familiar to pharmacologist for example muscarinic acetylcholine receptor (mAChR), adrenergic receptors, dopamine receptor, serotonin receptors and many others.
Type # 3. Receptors for Insulin and Various Growth-Factors:
These are directly linked to tyrosine kinase and are involved mainly in events controlling gene transcription. They cause phosphorylation of tyrosine residues in various proteins and stimulate transcription of particular sections of the genome.
Type # 4. Steroid Receptors:
These are soluble cytosolic or nuclear proteins, to which the steroid molecule binds after crossing the cell membrane. This complex is believed to unfold and expose the normally buried DNA binding domain, which recognises specific base sequences, thus activating particular genes.
The effects produced are as a result of increased protein synthesis, and thus are slow in onset. The different steroid hormones are able to activate different genes, and thus initiate completely different pattern of protein synthesis, and produce different physiological effects.
Thyroid hormones, glucocorticoids, mineralo-corticoids etc.