Neurotransmitter receptors come in variety of different subtypes: for example, the receptor for dopamine has 5 different subtypes (the dopamine receptor D1, D2, D3, D4 and D5); and the receptor for serotonin has 14 different known subtypes (5-HT1a, 5-HT1b, 5-HT2a, 5-HT2b, 5-HT3, 5-HT4, etc). Each receptor subtype is often located in specific parts of the brain. This is actually very useful from the pharmacological point of view, since drugs can be developed which target and act upon specific receptor subtypes in specific areas of the brain, and in that way, you can develop drugs that operate in particular brain regions. Here for example is the distribution of the various dopamine receptor subtypes in the brain: Source: Distribution of dopamine D1–D5 receptors in normal brain However, as far as I am aware, this selective targeting of receptor subtypes does not actual occur in normal human biology: each human neurotransmitter will activate all the subtypes of its receptor. For example, the neurotransmitter dopamine will activate each of the 5 dopamine receptors. So my questions is: What is the biological purpose for having different neurotransmitter receptor subtypes? As far as I am aware, normal biology does not make use of the actual differences in these receptor subtypes, so why are these differences there? Certainly they are very handy from the perspective of creating drugs that target specific brain regions. But what use does nature make of the differences in these receptor subtypes? Are these different receptor subtypes (which are each represented by the own gene) just there as a random (but pharmacologically fortunate) evolutionary accident? @Jonathan Edwards, perhaps you will know the answer to this question.